3. Properties and applications of Pb1�xGexTe

However, if the conventional PbTe materials are used as the evaporants, which are prepared from the stoichiometric proportions of pure constituents, a strong n-type Pb-rich layer will be deposited even at a very low substrate temperature, for example, 100C. Because an excess of nonstoichiometric carrier absorption emerges, these Pb-rich layers are completely opaque beyond 12 μm. Therefore, in order to obtain good-quality PbTe layers, a compensative process is required, which is commonly carried out either by introducing oxygen into the evaporation chamber in the course of the deposition of a PbTe layer or by baking the layers in air after deposition has been finished. However, the practice of postdeposition annealing is not ideal when the requirements for precise and reproducible spectral positioning and shape of a required filter profile are tightly specified and the introduction of oxygen raises a complexity in the technological process. In addition, both oxidizing processes cause the presence of lead

Figure 2. The space and astronomical research projects in the recent 5 years, in which infrared thin-film interference filters

were manufactured in infrared multilayer laboratory using PbTe as the infrared high-index coating materials.

Since the partial pressures of Pb and Te2 strongly depend on the properties of the evaporants, it is possible to shift the characteristics of the deposited layers by using a PbTe material with a Te dopant. Therefore, a kind of evaporable PbTe material with "mild" characteristics has been developed in Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, China [21, 22]. By "mild," we mean that, in a rather broad region of substrate-temperature, the

oxide on the surface of the layer [20].

32 Coatings and Thin-Film Technologies

Figure 3. Some products of evaporation materials of "mild" PbTe.

Lead germanium telluride (Pb1�xGexTe) is a pseudo-binary alloy of IV-VI narrow-gap semiconductor compounds, PbTe and GeTe [23].

Like some IV–VI compound semiconductors, for example, the tellurides of Sn and Ge and their alloys, Pb1�xGexTe shows also a ferroelectric phase transition from a high-temperature cubic, rock salt (Oh) structure above a Curie temperature TC to a low-temperature rhombohedral, arsenic-like (C3v) phase. The rhombohedral structure originates from a displacement of two sublattices along a <111> direction that becomes the c axis. In particular, for Pb1�xGexTe, the Curie temperature TC increases steeply with increasing Ge concentration. The phase transition is driven by off-center site occupation of Pb ion sites by Ge ions. Anomalies happen in the electrical resistivity and specific heat of Pb1�xGexTe alloys corresponding to the ferroelectric phase transition [24–32].

In this chapter, some investigations into the optical and mechanical properties of the layers of Pb1�xGexTe, which have been carried out in Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, China, were demonstrated; furthermore, some applications of Pb1�xGexTe as the infrared high-index coating materials were also exhibited.

## 3.1. Low-temperature dependence of mid-infrared optical constants of layers of Pb1�xGexTe

Although many investigations, both theoretical and experimental, have been carried out on the mechanism of ferroelectric phase transition of Pb1�xGexTe, the investigation into the optical constants (refractive index n and absorption coefficient k) of the layers of Pb1�xGexTe as a function of temperature remains to be done [33].

In our investigation, a layer of Pb1�xGexTe was deposited on a silicon wafer using molybdenum boat heating the ingot of Pb1�xGexTe (x = 0.12), of which composition was analyzed using proton-induced X-ray emission (PIXE) at the NEC 9SDH-2 pelletron tandem accelerator and can be represented with Pb0.94Ge0.06Te. The optical transmission spectra of the layer were measured using a Fourier-transform infrared spectrometer (BIO-RAD, FTS-40) in the range of 4000–400 cm�<sup>1</sup> at normal incidence between 80 and 300 K accompanied by using a bath cryostat (Oxford, DN1704). The optical constants of the layer were determined through the fitting of transmission spectra recorded at different temperature using the Lorentz-oscillator model as the dispersion model for the complex frequency dependent dielectric functions.

As a consequence, the temperature dependence of optical constants can be obtained at lowtemperature in the spectral range of 2.5–8.5 μm. It can be found that the layer of Pb1�xGexTe has a refractive index with a value of 5.350–6.000 corresponding to different measured temperatures in the spectral range of 4.0–8.5 μm, in which dispersion originated from the Reststrahlen and the absorption edge can be negligible. At room temperature, the layers of Pb1�xGexTe have a value of refractive index approaching to that of layers of PbTe. A conclusion can be drawn that Pb1�xGexTe is also an infrared high-index coating material.

In Figure 4(a), the change of refractive index of the layers of Pb0.94Ge0.06Te as a function of both wavelength and temperature was shown. It can be seen that the maximum value of refractive index occurs at 150 K, which can be regarded as the results of increased lattice polarizability that is an indication of the ferroelectric nature of the phase transition. Therefore, a conclusion can be drawn that anomalies in the refractive index, similar to those in the electrical resistivity and specific heat, emerge at the Curie temperature TC of the layers of Pb0.94Ge0.06Te. In Figure 4(b), the temperature coefficient of the refractive index, dn/dT, of the layers of Pb0.94Ge0.06Te is given, from which one can find that the value of dn/dT is 20.006–0.002 K�<sup>1</sup> in the spectral range of 3.0– 8.5 μm at all measured temperatures.

An empirical formula for the temperature coefficient of refractive index in the spectral range of 4.0–8.5 μm can be expressed as Eq. (3):

$$\frac{d\mathbf{n}}{dT} = f(\boldsymbol{\lambda}, T) = A(T) + \mathcal{B}(T)\boldsymbol{\lambda}^{-\mathcal{C}(T)} \tag{3}$$

3.2. The stable narrow bandpass interference filters without temperature induced

When the ambient temperature varies, the performance of an optical thin-film interference filter will be also changed, such as the shift of center wavelength and the deterioration of peak transmission [34–36]. In particular, when a long wavelength infrared narrow bandpass filter is used in the spaceborne remote sensing instruments, the change of its spectral characteristics, which will lead to the difficulty to sustain the precision radiometric measurements from space, is not acceptable [37]. Addition of an auxiliary temperature control to the filters is not practical in order to maintain its stable optical performance in spaceborne remote sensing systems.

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There are two factors that cause the shift of wavelength accompanied by the change of ambient temperature. One is the temperature-induced variations in the indices of refraction of the layers, and another is the variations in the physical thicknesses of the layers. Since the bulk temperature coefficients of linear expansion are an order of magnitude smaller than the temperature coefficients of the indices of refraction for substances similar to those usually employed for thin-film interference filters, it may be speculated that the shift of wavelength should be ascribed to the

As far as the infrared filters employed in the spaceborne remote sensing systems are concerned, the convenient materials used for the low-index layers are either ZnS (2.2) or ZnSe (2.3). Moreover, Seeley et al. [39] modeled the sensitivity of the narrow bandpass filter to the change of temperature, showing that the spacer and the next two adjacent layers are dominant contributors relative to the other layers (and stacks). Therefore, when a negative shift in PbTe resulting from the negative temperature coefficient of refractive index is suitably combined with a positive shift in ZnS (or ZnSe) from its positive coefficient, temperature-invariant compensation becomes

However, as a matter of fact, the temperature coefficient of refractive index of PbTe cannot exactly compensate for that of ZnS (ZnSe). Therefore, another solution to the problem is to seek a coating material of which the temperature coefficient of refractive index can be tuned.

Since the maximum value of refractive index of layers of Pb1�xGexTe occurs corresponding to the structural phase transition, as a consequence, at the designated low-temperature, the temperature coefficient of refractive index of layers of Pb1�xGexTe can be tuned from negative to positive by varying the Ge composition, that is, the layers of Pb1�xGexTe with the specific

Since the component elements in a multicomponent alloy system will evaporate at a different rate, which causes changes in compositions of layers relative to the evaporants, and it is in great necessity to designate directly the compositions in evaporated layers of Pb1�xGexTe. For example, the corresponding stoichiometry of evaporated layers can be expressed by the for-

In Figure 5, the spectral characteristics of a simple one-cavity Fabry-Perot filter on a Ge substrate was demonstrated in the temperature range of 85–300 K, which was designed with

composition may be used as the high-index layers in the thin-film interference filters.

variations of temperature coefficients of the indices of refraction of the layers [38].

possible; namely, to achieve a negligible wavelength shift with temperature.

wavelength shift

mula (Pb1�xGex)1-yTey.

where

$$\begin{aligned} \mathbf{A}(T) &= -0.05964 + 0.00156T - 1.41679 \times 10^{-5}T^2 + 5.24258 \times 10^{-8}T^3 - 6.76599 \times 10^{-11}T^4; \\ \mathbf{B}(T) &= 247.15385 - 6.75508T + 0.07137T^2 - 3.69672 \times 10^{-4}T^3 + 9.40269 \times 10^{-7}T^4 \\ &- 9.37957 \times 10^{-10}T^5; \end{aligned}$$

<sup>C</sup>ð Þ¼ <sup>T</sup> <sup>156</sup>:<sup>18266</sup> � <sup>4</sup>:49072<sup>T</sup> <sup>þ</sup> <sup>0</sup>:05023T<sup>2</sup> � <sup>2</sup>:<sup>65423</sup> � <sup>10</sup>�<sup>4</sup> <sup>T</sup><sup>3</sup> <sup>þ</sup> <sup>6</sup>:<sup>6734</sup> � <sup>10</sup>�<sup>7</sup> T4 �6:<sup>44758</sup> � <sup>10</sup>�<sup>10</sup>T<sup>5</sup> .

Figure 4. (a) The change of refractive index of the layers of Pb0.94Ge0.06Te as a function of both wavelength and temperature and (b) the temperature coefficient of the refractive index, dn/dT, of the layers of Pb0.94Ge0.06Te (ref. [33], reuse permission obtained from AIP).

### 3.2. The stable narrow bandpass interference filters without temperature induced wavelength shift

temperatures in the spectral range of 4.0–8.5 μm, in which dispersion originated from the Reststrahlen and the absorption edge can be negligible. At room temperature, the layers of Pb1�xGexTe have a value of refractive index approaching to that of layers of PbTe. A conclu-

In Figure 4(a), the change of refractive index of the layers of Pb0.94Ge0.06Te as a function of both wavelength and temperature was shown. It can be seen that the maximum value of refractive index occurs at 150 K, which can be regarded as the results of increased lattice polarizability that is an indication of the ferroelectric nature of the phase transition. Therefore, a conclusion can be drawn that anomalies in the refractive index, similar to those in the electrical resistivity and specific heat, emerge at the Curie temperature TC of the layers of Pb0.94Ge0.06Te. In Figure 4(b), the temperature coefficient of the refractive index, dn/dT, of the layers of Pb0.94Ge0.06Te is given, from which one can find that the value of dn/dT is 20.006–0.002 K�<sup>1</sup> in the spectral range of 3.0–

An empirical formula for the temperature coefficient of refractive index in the spectral range of

Figure 4. (a) The change of refractive index of the layers of Pb0.94Ge0.06Te as a function of both wavelength and temperature and (b) the temperature coefficient of the refractive index, dn/dT, of the layers of Pb0.94Ge0.06Te (ref. [33],

dT <sup>¼</sup> <sup>f</sup>ð Þ¼ <sup>λ</sup>; <sup>T</sup> A Tð Þþ B Tð Þλ�C Tð Þ (3)

<sup>T</sup><sup>3</sup> � <sup>6</sup>:<sup>76599</sup> � <sup>10</sup>�<sup>11</sup>T<sup>4</sup>

T4

T4

<sup>T</sup><sup>3</sup> <sup>þ</sup> <sup>9</sup>:<sup>40269</sup> � <sup>10</sup>�<sup>7</sup>

<sup>T</sup><sup>3</sup> <sup>þ</sup> <sup>6</sup>:<sup>6734</sup> � <sup>10</sup>�<sup>7</sup>

;

<sup>T</sup><sup>2</sup> <sup>þ</sup> <sup>5</sup>:<sup>24258</sup> � <sup>10</sup>�<sup>8</sup>

sion can be drawn that Pb1�xGexTe is also an infrared high-index coating material.

8.5 μm at all measured temperatures.

34 Coatings and Thin-Film Technologies

4.0–8.5 μm can be expressed as Eq. (3):

�9:<sup>37957</sup> � <sup>10</sup>�<sup>10</sup>T<sup>5</sup>

�6:<sup>44758</sup> � <sup>10</sup>�<sup>10</sup>T<sup>5</sup>

reuse permission obtained from AIP).

<sup>A</sup>ð Þ¼� <sup>T</sup> <sup>0</sup>:<sup>05964</sup> <sup>þ</sup> <sup>0</sup>:00156<sup>T</sup> � <sup>1</sup>:<sup>41679</sup> � <sup>10</sup>�<sup>5</sup>

where

dn

<sup>B</sup>ð Þ¼ <sup>T</sup> <sup>247</sup>:<sup>15385</sup> � <sup>6</sup>:75508<sup>T</sup> <sup>þ</sup> <sup>0</sup>:07137T<sup>2</sup> � <sup>3</sup>:<sup>69672</sup> � <sup>10</sup>�<sup>4</sup>

; <sup>C</sup>ð Þ¼ <sup>T</sup> <sup>156</sup>:<sup>18266</sup> � <sup>4</sup>:49072<sup>T</sup> <sup>þ</sup> <sup>0</sup>:05023T<sup>2</sup> � <sup>2</sup>:<sup>65423</sup> � <sup>10</sup>�<sup>4</sup>

.

When the ambient temperature varies, the performance of an optical thin-film interference filter will be also changed, such as the shift of center wavelength and the deterioration of peak transmission [34–36]. In particular, when a long wavelength infrared narrow bandpass filter is used in the spaceborne remote sensing instruments, the change of its spectral characteristics, which will lead to the difficulty to sustain the precision radiometric measurements from space, is not acceptable [37]. Addition of an auxiliary temperature control to the filters is not practical in order to maintain its stable optical performance in spaceborne remote sensing systems.

There are two factors that cause the shift of wavelength accompanied by the change of ambient temperature. One is the temperature-induced variations in the indices of refraction of the layers, and another is the variations in the physical thicknesses of the layers. Since the bulk temperature coefficients of linear expansion are an order of magnitude smaller than the temperature coefficients of the indices of refraction for substances similar to those usually employed for thin-film interference filters, it may be speculated that the shift of wavelength should be ascribed to the variations of temperature coefficients of the indices of refraction of the layers [38].

As far as the infrared filters employed in the spaceborne remote sensing systems are concerned, the convenient materials used for the low-index layers are either ZnS (2.2) or ZnSe (2.3). Moreover, Seeley et al. [39] modeled the sensitivity of the narrow bandpass filter to the change of temperature, showing that the spacer and the next two adjacent layers are dominant contributors relative to the other layers (and stacks). Therefore, when a negative shift in PbTe resulting from the negative temperature coefficient of refractive index is suitably combined with a positive shift in ZnS (or ZnSe) from its positive coefficient, temperature-invariant compensation becomes possible; namely, to achieve a negligible wavelength shift with temperature.

However, as a matter of fact, the temperature coefficient of refractive index of PbTe cannot exactly compensate for that of ZnS (ZnSe). Therefore, another solution to the problem is to seek a coating material of which the temperature coefficient of refractive index can be tuned.

Since the maximum value of refractive index of layers of Pb1�xGexTe occurs corresponding to the structural phase transition, as a consequence, at the designated low-temperature, the temperature coefficient of refractive index of layers of Pb1�xGexTe can be tuned from negative to positive by varying the Ge composition, that is, the layers of Pb1�xGexTe with the specific composition may be used as the high-index layers in the thin-film interference filters.

Since the component elements in a multicomponent alloy system will evaporate at a different rate, which causes changes in compositions of layers relative to the evaporants, and it is in great necessity to designate directly the compositions in evaporated layers of Pb1�xGexTe. For example, the corresponding stoichiometry of evaporated layers can be expressed by the formula (Pb1�xGex)1-yTey.

In Figure 5, the spectral characteristics of a simple one-cavity Fabry-Perot filter on a Ge substrate was demonstrated in the temperature range of 85–300 K, which was designed with

The results from our investigation are not in agreement with those reported by Partin. In Figure 6, a representative compositional depth profile was illustrated for an evaporated layer of Pb1xGexTe, of which the measured elemental concentrations are 42.92 0.55 for Pb, 54.96 0.90 for Te, and 2.12 0.74 for Ge, respectively. It can be observed that the Ge concentration at the surface is lower than that of the stoichiometry, meanwhile, the Pb concentration was distinctly higher. However, the Ge concentration increases from a Ge-deficient state at near surface to the Ge-rich one at near substrate. At same depth, all concentrations of elements remain balanced, that is, the increase of the Ge concentration must be accompanied with a decrease of the Pb concentration. After removal of the upper layers with a thickness of about 300 nm, the layer of Pb1xGexTe transforms to the Te-deficient characteristic from the Te-rich one. In addition, a stepwise change of elemental concentrations in the depth profile cannot be

Infrared High-Index Coating Materials, PbTe and Pb1−xGexTe: Properties and Applications

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37

In fact, the mechanism of evaporating from an alloy is much more complicated than that from a single metal because of the different vapor pressures of their individual components. It is much more possible that Ge partial pressure will change with the increasing of deposition time because Ge is more volatile than other components, Pb and Te, in the Pb1xGexTe alloy. Thereby, Ge concentration in the evaporants will be gradually depleted and the deposition process will result in a compositional gradient in the layers. As a consequence, the results obtained in the compositional depth profiles in evaporated layers of Pb1xGexTe can be rea-

Because it is often problematic to maintain a desired alloy in layers and over larger substrate surfaces, therefore, an empirical procedure may be used to determine how to control the

Figure 6. A representative compositional depth profile for an evaporated layer, of which the measured elemental concentrations are 42.92 0.55 for Pb, 54.96 0.90 for Te, and 2.12 0.74 for Ge, respectively (ref. [40], reuse permission

composition of the layers by adjusting the composition of the bulk alloy.

detected in the layer.

sonably explained.

obtained from SPIE).

Figure 5. The spectral characteristics of a simple one-cavity Fabry-Perot filter measured in the temperature range of 85–300 K, which was fabricated using Pb0.79Ge0.21Te as high-index evaporation material and ZnSe as low-index layers (ref. [35], reuse permission obtained from OSA).

peak wavelength of 11.30 μm and fabricated using ZnSe as the low-index layer. For its highindex layers, the ingots of Pb1�xGexTe (x = 0.21) were used as the evaporants, from which the layers with corresponding stoichiometry of (Pb0.86Ge0.14)0.46Te0.54 can be obtained using molybdenum boat evaporation. It can be observed that when ambient temperature changes from 300 to 85 K, a shift of peak wavelength of 0.05935 nm K�<sup>1</sup> has been achieved for this narrow bandpass interference filter.

### 3.3. Homogeneity of composition in evaporated layers of Pb1�xGexTe

It is commonly believed that the existence of inhomogeneity of composition in layers of Pb1�xGex Te will have a disadvantageous influence on the performance of thin-film interference filters, because the existence of graded Ge concentration profile in Pb1�xGexTe layers will lead into the coexistence of ferroelectric and paraelectric phases at a fixed temperature associated with phase transition as a function of Ge concentration and temperature [40]. Although Partin [41] observed the Ge concentration profile in the layers of Pb1�xGexTe grown on (100) oriented PbTe single crystal by molecular beam epitaxy from PbTe, GeTe, and Te source ovens, to the author's best knowledge, it has still not been clarified whether or not a Ge concentration gradient exists in the layers of Pb1�xGexTe evaporated directly from bulk alloys. Therefore, the investigation on compositional depth profile in evaporated layers of Pb1�xGexTe is of a great significance.

In our investigation, the layers were deposited on silicon wafers using molybdenum boat heating the ingots of Pb1�xGexTe, of which compositions were analyzed using energydispersive X-ray analysis (EDAX) in a Hitachi S-520 scanning electron microscope. Depth distribution of elements was measured by using a Microlab 301F Scanning Auger Microprobe (SAM) system combined with a discontinuous ion sputtering mode at a base pressure below 8.0 � <sup>10</sup>�<sup>8</sup> Pa.

The results from our investigation are not in agreement with those reported by Partin. In Figure 6, a representative compositional depth profile was illustrated for an evaporated layer of Pb1xGexTe, of which the measured elemental concentrations are 42.92 0.55 for Pb, 54.96 0.90 for Te, and 2.12 0.74 for Ge, respectively. It can be observed that the Ge concentration at the surface is lower than that of the stoichiometry, meanwhile, the Pb concentration was distinctly higher. However, the Ge concentration increases from a Ge-deficient state at near surface to the Ge-rich one at near substrate. At same depth, all concentrations of elements remain balanced, that is, the increase of the Ge concentration must be accompanied with a decrease of the Pb concentration. After removal of the upper layers with a thickness of about 300 nm, the layer of Pb1xGexTe transforms to the Te-deficient characteristic from the Te-rich one. In addition, a stepwise change of elemental concentrations in the depth profile cannot be detected in the layer.

In fact, the mechanism of evaporating from an alloy is much more complicated than that from a single metal because of the different vapor pressures of their individual components. It is much more possible that Ge partial pressure will change with the increasing of deposition time because Ge is more volatile than other components, Pb and Te, in the Pb1xGexTe alloy. Thereby, Ge concentration in the evaporants will be gradually depleted and the deposition process will result in a compositional gradient in the layers. As a consequence, the results obtained in the compositional depth profiles in evaporated layers of Pb1xGexTe can be reasonably explained.

peak wavelength of 11.30 μm and fabricated using ZnSe as the low-index layer. For its highindex layers, the ingots of Pb1�xGexTe (x = 0.21) were used as the evaporants, from which the layers with corresponding stoichiometry of (Pb0.86Ge0.14)0.46Te0.54 can be obtained using molybdenum boat evaporation. It can be observed that when ambient temperature changes from 300 to 85 K, a shift of peak wavelength of 0.05935 nm K�<sup>1</sup> has been achieved for this

Figure 5. The spectral characteristics of a simple one-cavity Fabry-Perot filter measured in the temperature range of 85–300 K, which was fabricated using Pb0.79Ge0.21Te as high-index evaporation material and ZnSe as low-index layers (ref. [35], reuse

It is commonly believed that the existence of inhomogeneity of composition in layers of Pb1�xGex Te will have a disadvantageous influence on the performance of thin-film interference filters, because the existence of graded Ge concentration profile in Pb1�xGexTe layers will lead into the coexistence of ferroelectric and paraelectric phases at a fixed temperature associated with phase transition as a function of Ge concentration and temperature [40]. Although Partin [41] observed the Ge concentration profile in the layers of Pb1�xGexTe grown on (100) oriented PbTe single crystal by molecular beam epitaxy from PbTe, GeTe, and Te source ovens, to the author's best knowledge, it has still not been clarified whether or not a Ge concentration gradient exists in the layers of Pb1�xGexTe evaporated directly from bulk alloys. Therefore, the investigation on com-

In our investigation, the layers were deposited on silicon wafers using molybdenum boat heating the ingots of Pb1�xGexTe, of which compositions were analyzed using energydispersive X-ray analysis (EDAX) in a Hitachi S-520 scanning electron microscope. Depth distribution of elements was measured by using a Microlab 301F Scanning Auger Microprobe (SAM) system combined with a discontinuous ion sputtering mode at a base pressure below

positional depth profile in evaporated layers of Pb1�xGexTe is of a great significance.

3.3. Homogeneity of composition in evaporated layers of Pb1�xGexTe

narrow bandpass interference filter.

permission obtained from OSA).

36 Coatings and Thin-Film Technologies

8.0 � <sup>10</sup>�<sup>8</sup> Pa.

Because it is often problematic to maintain a desired alloy in layers and over larger substrate surfaces, therefore, an empirical procedure may be used to determine how to control the composition of the layers by adjusting the composition of the bulk alloy.

Figure 6. A representative compositional depth profile for an evaporated layer, of which the measured elemental concentrations are 42.92 0.55 for Pb, 54.96 0.90 for Te, and 2.12 0.74 for Ge, respectively (ref. [40], reuse permission obtained from SPIE).

### 3.4. Compositional dependence of absorption edges in evaporated layers of Pb1xGexTe and tunable infrared short wavelength cutoff filters

An ideal cutoff filter should have small losses in the transmission region and high attenuation or reflectance in the rejection region over an extended spectral range, which can be carried out depending on interference or absorption [42]. Therefore, a cutoff filter may take a number of different forms, such as interference cutoff filters and thin-film absorption filters.

A thin-film absorption filter usually has very high rejection in the stop region and consists of a layer of material which has an absorption edge at the required wavelength. It is usually short wavelength cutoff in character. A layer of semiconductor that exhibits a very rapid transition from opacity to transparency at the intrinsic absorption edge is a good example to make an excellent thin-film short wavelength cutoff absorption filter. Nevertheless, as far as a layer of a certain semiconductor, such as Ge or PbTe, is concerned, the absorption filter is inflexible in character and the cutoff wavelength cannot be tuned, because of their fundamental optical properties. In order to take advantage of the characteristic of deep rejection of thin-film absorption filters, it will be of great significance to find out a semiconductor material, of which the absorption edge position can be tuned by means of controlling the composition of its layers, to fabricate the absorption filter.

the enormous number of existing deposition systems [3]. However, it may be undesirable from a practical viewpoint to evaporate compound semiconductors from a single source because the vapor compositions of compound semiconductors are usually different from their nominal compositions. As a consequence, the stoichiometry of the layers will differ generally from the

Figure 7. (a) An example to illustrate that the absorption edges will shift toward short wavelength with the increase of Ge concentration x in layers and (b) an example for the edges will also shift toward the short wavelength with Te concentra-

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39

tion approach to stoichiometry (ref. [42], reuse permission obtained from SPIE).

In our investigation, the ingots with different Ge concentrations were chosen and evaporated using the molybdenum boat heating; meanwhile, the ingots with five Ge concentrations, x = 0.10, 0.14, 0.17, 0.20, and 0.22, together with PbTe, were evaporated using electron-beam heating. The stoichiometry of the evaporated layers was determined using the energy-dispersive X-ray analysis from a Horiba EX-220 energy dispersive X-ray microanalyzer (model 6853-H) attached to a Hitachi S-4300 cold field emission scanning electron microscope (FE-SEM) without coating the

In Figure 8(a), for the layers of Pb1�xGexTe evaporated using electron beam heating, the change of Ge and Te concentration, that is, y and z, with the increasing of Ge concentration in evaporants, x, was illustrated. Similarly, in order to make a clear comparison, for layers evaporated using molybdenum boat heating, the dependence of y and z on x, was also

It can be observed that Ge concentration y in layers evaporated using electron beam heating is approaching to Ge concentration x of the ingots. A green line that designates an exact linear relation y = x serves as a guideline for the eye in the figure. In comparison with the compositional dependence presented in Figure 8(b) for the layers evaporated using molybdenum boat heating, therefore, it can be concluded that electron beam evaporation is a more effective congruent-transfer technique to deposit the layers of Pb1�xGexTe directly from the original

Furthermore, with an increasing of Ge concentration, it can be obviously observed that concentration of tellurium z gradually decreases in the layers evaporated using both electron beam

evaporants [43].

surfaces.

described in Figure 8(b).

Pb1�xGexTe evaporants.

In our investigation, the layers were deposited on silicon wafers using molybdenum boat heating the ingots of Pb1xGexTe, of which compositions were analyzed using energydispersive X-ray analysis (EDAX) in a Hitachi S-520 scanning electron microscope. The optical transmission spectra of the layers were measured in the spectral range of 2.5–25 μm using a Perkin Elmer Spectrum GX Fourier-Transform Infrared Spectrometer with a resolution of 8 cm<sup>1</sup> at normal incidence. The crystallographic structures of the layers were investigated by X-ray-diffraction (XRD) using Cu K<sup>α</sup> radiation on a D/max 2550 V diffractometer with an accuracy of 0.02. The single-phase nature and polycrystalline of the layers were revealed. The composition dependence of positions of the fundamental absorption edges in the evaporated layers of Pb1xGexTe was explored. The aim is to elucidate that the tunability of the cutoff wavelength in thin-film absorption filters can be reached if the controllability of the composition of constituent layers can be carried out.

It is revealed that for the evaporated layers of Pb1xGexTe with an identical Te concentration, the absorption edges will shift toward short wavelength with the increase of Ge concentration x in layers, an example was illustrated in Figure 7(a); furthermore, for those with a similar Ge concentration within a small range of deviation, the edges will also shift toward the short wavelength with Te concentration approach to stoichiometry, an example was illustrated in Figure 7(b).

Our investigation indicates that if the controllability of the composition of evaporated layers of Pb1xGexTe can be carried out, it will be possible to fabricate an infrared single-layer thin-film absorption filter with the short wavelength cutoff at the required wavelength.

### 3.5. Compositional congruency, correlation and high-pressure polymorphism in electronbeam evaporated layers of Pb1xGexTe

Currently, evaporation, as a physical vapor deposition process, is still principally used in optical coating industry, because of its simplicity, flexibility and relatively low cost; moreover,

Infrared High-Index Coating Materials, PbTe and Pb1−xGexTe: Properties and Applications http://dx.doi.org/10.5772/intechopen.79272 39

3.4. Compositional dependence of absorption edges in evaporated layers of Pb1xGexTe

different forms, such as interference cutoff filters and thin-film absorption filters.

An ideal cutoff filter should have small losses in the transmission region and high attenuation or reflectance in the rejection region over an extended spectral range, which can be carried out depending on interference or absorption [42]. Therefore, a cutoff filter may take a number of

A thin-film absorption filter usually has very high rejection in the stop region and consists of a layer of material which has an absorption edge at the required wavelength. It is usually short wavelength cutoff in character. A layer of semiconductor that exhibits a very rapid transition from opacity to transparency at the intrinsic absorption edge is a good example to make an excellent thin-film short wavelength cutoff absorption filter. Nevertheless, as far as a layer of a certain semiconductor, such as Ge or PbTe, is concerned, the absorption filter is inflexible in character and the cutoff wavelength cannot be tuned, because of their fundamental optical properties. In order to take advantage of the characteristic of deep rejection of thin-film absorption filters, it will be of great significance to find out a semiconductor material, of which the absorption edge position can be tuned by means of controlling the composition of its layers, to fabricate the absorption filter.

In our investigation, the layers were deposited on silicon wafers using molybdenum boat heating the ingots of Pb1xGexTe, of which compositions were analyzed using energydispersive X-ray analysis (EDAX) in a Hitachi S-520 scanning electron microscope. The optical transmission spectra of the layers were measured in the spectral range of 2.5–25 μm using a Perkin Elmer Spectrum GX Fourier-Transform Infrared Spectrometer with a resolution of 8 cm<sup>1</sup> at normal incidence. The crystallographic structures of the layers were investigated by X-ray-diffraction (XRD) using Cu K<sup>α</sup> radiation on a D/max 2550 V diffractometer with an accuracy of 0.02. The single-phase nature and polycrystalline of the layers were revealed. The composition dependence of positions of the fundamental absorption edges in the evaporated layers of Pb1xGexTe was explored. The aim is to elucidate that the tunability of the cutoff wavelength in thin-film absorption filters can be reached if the controllability of the

It is revealed that for the evaporated layers of Pb1xGexTe with an identical Te concentration, the absorption edges will shift toward short wavelength with the increase of Ge concentration x in layers, an example was illustrated in Figure 7(a); furthermore, for those with a similar Ge concentration within a small range of deviation, the edges will also shift toward the short wavelength with Te concentration approach to stoichiometry, an example was illustrated in Figure 7(b).

Our investigation indicates that if the controllability of the composition of evaporated layers of Pb1xGexTe can be carried out, it will be possible to fabricate an infrared single-layer thin-film

3.5. Compositional congruency, correlation and high-pressure polymorphism in electron-

Currently, evaporation, as a physical vapor deposition process, is still principally used in optical coating industry, because of its simplicity, flexibility and relatively low cost; moreover,

absorption filter with the short wavelength cutoff at the required wavelength.

and tunable infrared short wavelength cutoff filters

38 Coatings and Thin-Film Technologies

composition of constituent layers can be carried out.

beam evaporated layers of Pb1xGexTe

Figure 7. (a) An example to illustrate that the absorption edges will shift toward short wavelength with the increase of Ge concentration x in layers and (b) an example for the edges will also shift toward the short wavelength with Te concentration approach to stoichiometry (ref. [42], reuse permission obtained from SPIE).

the enormous number of existing deposition systems [3]. However, it may be undesirable from a practical viewpoint to evaporate compound semiconductors from a single source because the vapor compositions of compound semiconductors are usually different from their nominal compositions. As a consequence, the stoichiometry of the layers will differ generally from the evaporants [43].

In our investigation, the ingots with different Ge concentrations were chosen and evaporated using the molybdenum boat heating; meanwhile, the ingots with five Ge concentrations, x = 0.10, 0.14, 0.17, 0.20, and 0.22, together with PbTe, were evaporated using electron-beam heating. The stoichiometry of the evaporated layers was determined using the energy-dispersive X-ray analysis from a Horiba EX-220 energy dispersive X-ray microanalyzer (model 6853-H) attached to a Hitachi S-4300 cold field emission scanning electron microscope (FE-SEM) without coating the surfaces.

In Figure 8(a), for the layers of Pb1�xGexTe evaporated using electron beam heating, the change of Ge and Te concentration, that is, y and z, with the increasing of Ge concentration in evaporants, x, was illustrated. Similarly, in order to make a clear comparison, for layers evaporated using molybdenum boat heating, the dependence of y and z on x, was also described in Figure 8(b).

It can be observed that Ge concentration y in layers evaporated using electron beam heating is approaching to Ge concentration x of the ingots. A green line that designates an exact linear relation y = x serves as a guideline for the eye in the figure. In comparison with the compositional dependence presented in Figure 8(b) for the layers evaporated using molybdenum boat heating, therefore, it can be concluded that electron beam evaporation is a more effective congruent-transfer technique to deposit the layers of Pb1�xGexTe directly from the original Pb1�xGexTe evaporants.

Furthermore, with an increasing of Ge concentration, it can be obviously observed that concentration of tellurium z gradually decreases in the layers evaporated using both electron beam

types of nearest-neighbor bonds, Pb—Te and Ge—Te, must be concerned. Thus, if only the strength of a Ge—Te bond is weaker than that of Pb—Te bond, the amount of Te ions which are incorporated into the system of Pb1�xGexTe will decrease with the increasing of Ge concentration

Infrared High-Index Coating Materials, PbTe and Pb1−xGexTe: Properties and Applications

http://dx.doi.org/10.5772/intechopen.79272

Our assumption can be supported from the experimental data of bond energies reported by Rao and Mohan [45]. It is pointed out that Ge—Te has a bond energy of 1.87 eV, whereas Pb—Te has a bond energy of 1.90 eV; therefore, Ge ions cannot "hold" Te ions as tight as Pb ions do. When the more Ge ions are placed on the Pb site, the more Te ions will "escape." Therefore, the gradual decreasing of Te concentration will make a Te-rich characteristic in

In Figure 9, the patterns of XRD analysis were demonstrated for the layers evaporated using electron beam heating the ingots with Ge concentration x = 0.10, 0.14, 0.17, 0.20, and 0.22, respectively. It is worthwhile to note that the results for the layers evaporated from ingots with x = 0.22 have some important features. First of all, a new peak can be found, which corresponds to the strongest (111) reflection of rock salt-type structure of GeTe (space-group o<sup>5</sup>

referred in JCPDS card number 65-0315. Although the intensity is weak, it can be revealed that

Figure 9. The patterns of XRD analysis for layers evaporated using electron beam heating from ingots with Ge concen-

tration x = 0.10, 0.14, 0.17, 0.20, and 0.22 (ref. [43], reuse permission obtained from Elsevier).

<sup>h</sup>) as

41

in the layers.

layers shift into a Te-deficient one.

Figure 8. The change of Ge concentration y and Te concentration z in the layers with the increasing Ge concentration in evaporants x, (a) evaporated using electron beam heating, a green line that designates an exact linear relation y = x serves as a guideline for the eye; (b) evaporated using molybdenum boat heating, a green line that designates an exact linear relation y = x, and a blue line that represents an exact stoichiometry z = 50 for Te concentration in thin films serves as guideline for the eyes, respectively (ref. [43], reuse permission obtained from Elsevier).

and resistance heating. As a consequence, the Te-rich characteristics presented in the layers will shift into the Te-deficient one with the increasing of Ge concentration.

Perhaps, the congruency in the process of evaporation of Pb1�xGexTe using electron beam heating of ingots can be attributed to the "ablation" characteristics of electron beam evaporation and lower thermal conductivity of the PbTe-based alloy. A significant feature of an electron beam evaporator is its ability to concentrate a large amount of power onto a small area of the surface of evaporants, independent of materials being heated. Usually when the evaporants are heated, water cooling is supplied to the crucible to remove the heat which escapes by conduction through the evaporants and liner. In addition, it has been well accepted that PbTe-based alloys constitute a category of materials with excellent thermoelectric figure of merit, zT, because PbTe has a rather lower value of thermal conductivity (2.3 W m�<sup>1</sup> K�<sup>1</sup> ) [44]. Therefore, when electron beam is focused on the surface of the ingots of Pb1�xGexTe, the heat is hardly conducted out and a great temperature gradient is established in the ingots accompanied by the water-cooled crucible. As a consequence, due to the absorption of high energy density by only a small fraction of the ingots irradiated by electron beam, the evaporation behaves like "ablation" with a nonequilibrium nature, at which energy absorbed is much higher than that needed for evaporation, namely, vaporization is independent on the vapor pressures of the constituents.

Furthermore, in our investigation, an assumption can also be proposed to explain the compositional correlation observed in the layers of Pb1�xGexTe. It has been well known that the ionic radii of Ge and Pb are 0.73 and 1.2 Å, respectively; therefore, Pb1�xGexTe belongs to a class of alloys in which a substitutional atom has a size significantly smaller than that of the host atom it replaces. In such a "diluted" alloy, the addition of even a very small number of substitutional atoms will lead to a substantial change in their physical properties. For example, although PbTe itself is not ferroelectric, the addition of even 0.05% Ge to PbTe will induce a structural transition [24]. It is obvious that in such a ternary alloy, due to the substitution of Ge ions for Pb ions, two types of nearest-neighbor bonds, Pb—Te and Ge—Te, must be concerned. Thus, if only the strength of a Ge—Te bond is weaker than that of Pb—Te bond, the amount of Te ions which are incorporated into the system of Pb1�xGexTe will decrease with the increasing of Ge concentration in the layers.

Our assumption can be supported from the experimental data of bond energies reported by Rao and Mohan [45]. It is pointed out that Ge—Te has a bond energy of 1.87 eV, whereas Pb—Te has a bond energy of 1.90 eV; therefore, Ge ions cannot "hold" Te ions as tight as Pb ions do. When the more Ge ions are placed on the Pb site, the more Te ions will "escape." Therefore, the gradual decreasing of Te concentration will make a Te-rich characteristic in layers shift into a Te-deficient one.

In Figure 9, the patterns of XRD analysis were demonstrated for the layers evaporated using electron beam heating the ingots with Ge concentration x = 0.10, 0.14, 0.17, 0.20, and 0.22, respectively. It is worthwhile to note that the results for the layers evaporated from ingots with x = 0.22 have some important features. First of all, a new peak can be found, which corresponds to the strongest (111) reflection of rock salt-type structure of GeTe (space-group o<sup>5</sup> <sup>h</sup>) as referred in JCPDS card number 65-0315. Although the intensity is weak, it can be revealed that

and resistance heating. As a consequence, the Te-rich characteristics presented in the layers

Figure 8. The change of Ge concentration y and Te concentration z in the layers with the increasing Ge concentration in evaporants x, (a) evaporated using electron beam heating, a green line that designates an exact linear relation y = x serves as a guideline for the eye; (b) evaporated using molybdenum boat heating, a green line that designates an exact linear relation y = x, and a blue line that represents an exact stoichiometry z = 50 for Te concentration in thin films serves as

Perhaps, the congruency in the process of evaporation of Pb1�xGexTe using electron beam heating of ingots can be attributed to the "ablation" characteristics of electron beam evaporation and lower thermal conductivity of the PbTe-based alloy. A significant feature of an electron beam evaporator is its ability to concentrate a large amount of power onto a small area of the surface of evaporants, independent of materials being heated. Usually when the evaporants are heated, water cooling is supplied to the crucible to remove the heat which escapes by conduction through the evaporants and liner. In addition, it has been well accepted that PbTe-based alloys constitute a category of materials with excellent thermoelectric figure of merit, zT, because PbTe has a rather lower value of thermal conductivity (2.3 W m�<sup>1</sup> K�<sup>1</sup>

Therefore, when electron beam is focused on the surface of the ingots of Pb1�xGexTe, the heat is hardly conducted out and a great temperature gradient is established in the ingots accompanied by the water-cooled crucible. As a consequence, due to the absorption of high energy density by only a small fraction of the ingots irradiated by electron beam, the evaporation behaves like "ablation" with a nonequilibrium nature, at which energy absorbed is much higher than that needed for evaporation, namely, vaporization is independent on the vapor

Furthermore, in our investigation, an assumption can also be proposed to explain the compositional correlation observed in the layers of Pb1�xGexTe. It has been well known that the ionic radii of Ge and Pb are 0.73 and 1.2 Å, respectively; therefore, Pb1�xGexTe belongs to a class of alloys in which a substitutional atom has a size significantly smaller than that of the host atom it replaces. In such a "diluted" alloy, the addition of even a very small number of substitutional atoms will lead to a substantial change in their physical properties. For example, although PbTe itself is not ferroelectric, the addition of even 0.05% Ge to PbTe will induce a structural transition [24]. It is obvious that in such a ternary alloy, due to the substitution of Ge ions for Pb ions, two

) [44].

will shift into the Te-deficient one with the increasing of Ge concentration.

guideline for the eyes, respectively (ref. [43], reuse permission obtained from Elsevier).

pressures of the constituents.

40 Coatings and Thin-Film Technologies

Figure 9. The patterns of XRD analysis for layers evaporated using electron beam heating from ingots with Ge concentration x = 0.10, 0.14, 0.17, 0.20, and 0.22 (ref. [43], reuse permission obtained from Elsevier).

a secondary phase GeTe emerges in the layers. Furthermore, the strongest reflection is different from that in the layers of Pb1xGexTe with a high temperature paraelectric phase. A strongest peak is expected to occur at 2θ = 27.58, which stands for the (200) plane in PbTe and discloses a highly textured with (001) plane parallel to the silicon substrate. However, the strongest reflection can be attributed to a (111) reflection of substrate silicon (space-group o<sup>5</sup> <sup>h</sup>) and (104) reflection of a highly symmetric body-centered cubic (bcc) structure Te-V (space-group o<sup>9</sup> <sup>h</sup>). In order to make a clear elucidation, the position and intensity of reflections given in the JCPDS cards for rock salt-type PbTe and bcc structure Te-V were also added in Figure 9, respectively. Therefore, it can be concluded that high-pressure phases for GeTe and Te compounds are also presented in evaporated layers of Pb1xGexTe, which are commonly generated at the higher pressure applying hydrostatic pressure (such as diamond anvil cell) or shock loaded techniques. Perhaps, the transition from the disorder to the order in the system of Pb1xGexTe, which is responsible for ferroelectric phase transition, induces high pressure polymorphism in evaporated layers. Of course, more evidence is furthermore needed.

### 3.6. The mechanical properties of evaporated layers of Pb1xGexTe

As far as the single-crystal of PbTe is concerned, the microhardness is relatively a constant of 30 HV for the various carrier concentrations [46, 47]. To the author's best knowledge, no data is reported on hardness of layer of PbTe. However, a layer of PbTe is so soft that it can be scratched easily. As a consequence, an infrared thin-film interference filters consisting of the layers of PbTe is not robust enough to withstand the damage originated from standard wafer dicing processing, such as "from wafer to chips," even if more robust low-index materials, like ZnSe or ZnS, are chosen as an outermost layer.

the hardness of the layer of Pb0.83Ge0.17Te is three times as great as that of PbTe; meanwhile, Young's modulus is twice greater than that of PbTe, as seen in Figure 10(b). Therefore, a conclusion can be drawn that a mechanically robust infrared high-index layer can be obtained

Figure 10. A comparison of the hardness and Young's modulus of the layers evaporated from ingots with three Ge concentrations x, 0.10, 0.17, and 0.22, to those of PbTe: (a) the hardness and (b) the Young's modulus (ref. [46], reuse

Infrared High-Index Coating Materials, PbTe and Pb1−xGexTe: Properties and Applications

http://dx.doi.org/10.5772/intechopen.79272

43

These mechanical behaviors of layers of Pb1�xGexTe can be linked to the ferroelectric phase transition. Moreover, the strength loss in the layers can be also explained in light of strong

Since Seeley et al. begun the employment of PbTe into the design and manufacture of infrared thin-film interference filters in Infrared Multilayer Laboratory at the University of Reading in 1960s, half of a century has passed. Nowadays, PbTe is still the first choice for the design of infrared thin-film interference filters operating in the long wavelength infrared both at room and cryogenic temperature. In the beginning of this century, aiming at the further improvement of the performance of PbTe, the investigations into Pb1�xGexTe were started up in Shanghai Institute of Technical Physics, Chinese Academy of Sciences. Nowadays, many fruits

First of all, it can be concluded that the electron beam evaporation can prove itself a promising powerful tool to make sure the congruent-deposition of the layers of Pb1�xGexTe directly from original Pb1�xGexTe evaporants. Therefore, because the controllability of the composition of evaporated layers of Pb1�xGexTe can be carried out, Pb1�xGexTe will be a prospective infrared high-index material in thin-film interference filters, due to its tunable optical properties corresponding to its intrinsic ferroelectric phase transition, such as temperature coefficient of refractive index and fundamental absorption edge. Furthermore, because the layers of Pb1�xGexTe

using Pb1�xGexTe as evaporation materials.

permission obtained from Springer).

4. Conclusion

localized elastic-strain fields in concentrated solid solutions.

have been harvested after more than a decade passed.

Therefore, as a solution to the problem in integrating of the standard semiconductor process into the mass-production of infrared thin-film interference filters, a new infrared high-index coating material is needed to deposit the more robust high-index layers to withstand the damages in wafer dicing processing. It is necessary to investigate the mechanical properties of evaporated layers of Pb1xGexTe.

In our investigation, the layers of Pb1xGexTe were deposited on silicon wafers using electron beam evaporation, of which compositions were analyzed using energy-dispersive X-ray analysis (EDAX) in a Horiba EX-220 energy-dispersive X-ray microanalyzer (model 6853-H) attached to the FE-SEM without coating the surfaces of the layers. Nanoindentation measurements were performed using a Nano Indenter G200 with a three-side pyramidal Berkovich diamond indenter of 50 nm radius under the continuous stiffness measurement (CSM) option. At least 10 indents were performed on each layer with a maximum load of 13 mN, accompanied with a corresponding indentation depth no more than 500 nm. Following the analytic method proposed by Oliver and Pharr [48], the average values and standard deviations of the hardness and Young's modulus of thin films were extracted from the load–displacement results.

It can be revealed that the layers of Pb1xGexTe have greater values of hardness and Young's modulus compared with those of PbTe. For example, from Figure 10(a), it can be found that Infrared High-Index Coating Materials, PbTe and Pb1−xGexTe: Properties and Applications http://dx.doi.org/10.5772/intechopen.79272 43

Figure 10. A comparison of the hardness and Young's modulus of the layers evaporated from ingots with three Ge concentrations x, 0.10, 0.17, and 0.22, to those of PbTe: (a) the hardness and (b) the Young's modulus (ref. [46], reuse permission obtained from Springer).

the hardness of the layer of Pb0.83Ge0.17Te is three times as great as that of PbTe; meanwhile, Young's modulus is twice greater than that of PbTe, as seen in Figure 10(b). Therefore, a conclusion can be drawn that a mechanically robust infrared high-index layer can be obtained using Pb1�xGexTe as evaporation materials.

These mechanical behaviors of layers of Pb1�xGexTe can be linked to the ferroelectric phase transition. Moreover, the strength loss in the layers can be also explained in light of strong localized elastic-strain fields in concentrated solid solutions.

### 4. Conclusion

a secondary phase GeTe emerges in the layers. Furthermore, the strongest reflection is different from that in the layers of Pb1xGexTe with a high temperature paraelectric phase. A strongest peak is expected to occur at 2θ = 27.58, which stands for the (200) plane in PbTe and discloses a highly textured with (001) plane parallel to the silicon substrate. However, the strongest

<sup>h</sup>) and (104)

<sup>h</sup>). In

reflection can be attributed to a (111) reflection of substrate silicon (space-group o<sup>5</sup>

evaporated layers. Of course, more evidence is furthermore needed.

3.6. The mechanical properties of evaporated layers of Pb1xGexTe

ZnSe or ZnS, are chosen as an outermost layer.

evaporated layers of Pb1xGexTe.

42 Coatings and Thin-Film Technologies

results.

reflection of a highly symmetric body-centered cubic (bcc) structure Te-V (space-group o<sup>9</sup>

order to make a clear elucidation, the position and intensity of reflections given in the JCPDS cards for rock salt-type PbTe and bcc structure Te-V were also added in Figure 9, respectively. Therefore, it can be concluded that high-pressure phases for GeTe and Te compounds are also presented in evaporated layers of Pb1xGexTe, which are commonly generated at the higher pressure applying hydrostatic pressure (such as diamond anvil cell) or shock loaded techniques. Perhaps, the transition from the disorder to the order in the system of Pb1xGexTe, which is responsible for ferroelectric phase transition, induces high pressure polymorphism in

As far as the single-crystal of PbTe is concerned, the microhardness is relatively a constant of 30 HV for the various carrier concentrations [46, 47]. To the author's best knowledge, no data is reported on hardness of layer of PbTe. However, a layer of PbTe is so soft that it can be scratched easily. As a consequence, an infrared thin-film interference filters consisting of the layers of PbTe is not robust enough to withstand the damage originated from standard wafer dicing processing, such as "from wafer to chips," even if more robust low-index materials, like

Therefore, as a solution to the problem in integrating of the standard semiconductor process into the mass-production of infrared thin-film interference filters, a new infrared high-index coating material is needed to deposit the more robust high-index layers to withstand the damages in wafer dicing processing. It is necessary to investigate the mechanical properties of

In our investigation, the layers of Pb1xGexTe were deposited on silicon wafers using electron beam evaporation, of which compositions were analyzed using energy-dispersive X-ray analysis (EDAX) in a Horiba EX-220 energy-dispersive X-ray microanalyzer (model 6853-H) attached to the FE-SEM without coating the surfaces of the layers. Nanoindentation measurements were performed using a Nano Indenter G200 with a three-side pyramidal Berkovich diamond indenter of 50 nm radius under the continuous stiffness measurement (CSM) option. At least 10 indents were performed on each layer with a maximum load of 13 mN, accompanied with a corresponding indentation depth no more than 500 nm. Following the analytic method proposed by Oliver and Pharr [48], the average values and standard deviations of the hardness and Young's modulus of thin films were extracted from the load–displacement

It can be revealed that the layers of Pb1xGexTe have greater values of hardness and Young's modulus compared with those of PbTe. For example, from Figure 10(a), it can be found that Since Seeley et al. begun the employment of PbTe into the design and manufacture of infrared thin-film interference filters in Infrared Multilayer Laboratory at the University of Reading in 1960s, half of a century has passed. Nowadays, PbTe is still the first choice for the design of infrared thin-film interference filters operating in the long wavelength infrared both at room and cryogenic temperature. In the beginning of this century, aiming at the further improvement of the performance of PbTe, the investigations into Pb1�xGexTe were started up in Shanghai Institute of Technical Physics, Chinese Academy of Sciences. Nowadays, many fruits have been harvested after more than a decade passed.

First of all, it can be concluded that the electron beam evaporation can prove itself a promising powerful tool to make sure the congruent-deposition of the layers of Pb1�xGexTe directly from original Pb1�xGexTe evaporants. Therefore, because the controllability of the composition of evaporated layers of Pb1�xGexTe can be carried out, Pb1�xGexTe will be a prospective infrared high-index material in thin-film interference filters, due to its tunable optical properties corresponding to its intrinsic ferroelectric phase transition, such as temperature coefficient of refractive index and fundamental absorption edge. Furthermore, because the layers of Pb1�xGexTe have superior mechanical properties, such as the hardness and Young's modulus, to those of PbTe, an infrared thin-film interference filters consisting of them will be robust enough to withstand the damage originated from standard wafer dicing processing. As a consequence, the integration of the standard semiconductor process into the mass-production of infrared thin-film interference filters can be also realized.

References

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DOI: 10.1007/s00340-008-3134-z

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In addition, one main challenge that needs to be addressed is the toxicity of lead and tellurium. In particular, some issues are concerned regarding the massive use of them in technology due to the toxicity, high costs, and scarcity. In fact, as far as an infrared thin-film interference filter is concerned, the low-index materials, like ZnSe or ZnS, are chosen as an outermost layer so that the layers of PbTe or Pb1�xGexTe are extremely well encapsulated between two adjacent layers of ZnSe or ZnS, followed by careful edge sealing, in order to reduce the hazards of Pb and Te exposure. Furthermore, at the end of the module lifetime, it is important to ensure that all materials be recycled, as already happens for all of the products of infrared thin-film interference filters. However, it would be desirable to find alternatives which retain the unique optical properties of PbTe and Pb1�xGexTe. Currently, an investigation is in progress in Shanghai Institute of Technical Physics, Chinese Academy of Sciences to seek an environmentallyfriendly, cost-efficient alternative to PbTe-based infrared high-index coating materials.

### Acknowledgements

All authors are sincerely grateful to contributions from Prof. Fengshan Zhang and Mr. Ling Zhang (Shanghai Institute of Technical Physics, Chinese Academy of Sciences), Dr. Bin Fan (Optorun Co., Ltd.) and Prof. Jinchun Jiang (East China Normal University). Many thanks to Mrs. Ling Yu (Shanghai Institute of Ceramics, Chinese Academy of Sciences) for the AES analyzes; to Mr. Yuehong Hong and Dr. Guofeng Cheng (Shanghai Institute of Ceramics, Chinese Academy of Sciences) for the XRD analyzes; to Prof. Xianghua Nan (Shanghai Jiao Tong University) and Prof. Xiangming Meng (Technical Institute of Physics and Chemistry, Chinese Academy of Sciences) for the EDAX analyzes; to Dr. Jinlong Li (Ningbo Institute of Industrial Technology, Chinese Academy of Sciences) for his support in the nanoindentation measurements.

### Conflict of interest

We declare that we have no conflict of interest.

### Author details

Bin Li\*, Ping Xie, Suying Zhang and Dingquan Liu

\*Address all correspondence to: binli@mail.sitp.ac.cn

Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, China

### References

have superior mechanical properties, such as the hardness and Young's modulus, to those of PbTe, an infrared thin-film interference filters consisting of them will be robust enough to withstand the damage originated from standard wafer dicing processing. As a consequence, the integration of the standard semiconductor process into the mass-production of infrared thin-film

In addition, one main challenge that needs to be addressed is the toxicity of lead and tellurium. In particular, some issues are concerned regarding the massive use of them in technology due to the toxicity, high costs, and scarcity. In fact, as far as an infrared thin-film interference filter is concerned, the low-index materials, like ZnSe or ZnS, are chosen as an outermost layer so that the layers of PbTe or Pb1�xGexTe are extremely well encapsulated between two adjacent layers of ZnSe or ZnS, followed by careful edge sealing, in order to reduce the hazards of Pb and Te exposure. Furthermore, at the end of the module lifetime, it is important to ensure that all materials be recycled, as already happens for all of the products of infrared thin-film interference filters. However, it would be desirable to find alternatives which retain the unique optical properties of PbTe and Pb1�xGexTe. Currently, an investigation is in progress in Shanghai Institute of Technical Physics, Chinese Academy of Sciences to seek an environmentally-

friendly, cost-efficient alternative to PbTe-based infrared high-index coating materials.

Chinese Academy of Sciences) for his support in the nanoindentation measurements.

Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, China

All authors are sincerely grateful to contributions from Prof. Fengshan Zhang and Mr. Ling Zhang (Shanghai Institute of Technical Physics, Chinese Academy of Sciences), Dr. Bin Fan (Optorun Co., Ltd.) and Prof. Jinchun Jiang (East China Normal University). Many thanks to Mrs. Ling Yu (Shanghai Institute of Ceramics, Chinese Academy of Sciences) for the AES analyzes; to Mr. Yuehong Hong and Dr. Guofeng Cheng (Shanghai Institute of Ceramics, Chinese Academy of Sciences) for the XRD analyzes; to Prof. Xianghua Nan (Shanghai Jiao Tong University) and Prof. Xiangming Meng (Technical Institute of Physics and Chemistry, Chinese Academy of Sciences) for the EDAX analyzes; to Dr. Jinlong Li (Ningbo Institute of Industrial Technology,

interference filters can be also realized.

44 Coatings and Thin-Film Technologies

Acknowledgements

Conflict of interest

Author details

We declare that we have no conflict of interest.

Bin Li\*, Ping Xie, Suying Zhang and Dingquan Liu \*Address all correspondence to: binli@mail.sitp.ac.cn


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[28] Takano S, Kumashiro Y, Tsuji K. Resistivity anomalies in Pb1�xGexTe at low temperatures. Journal of the Physical Society of Japan. 1984;53:4309-4314. DOI: 10.1143/JPSJ.53.4309 [29] Bangert E, Bauer G, Fantner EJ, Pascher H. Magneto-optical investigations of phasetransition-induced band-structure changes of Pb1�xGexTe. Physical Review B. 1985;31:

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[32] Sariel J, Dahan I, Gelbstein Y. Rhombohedral-cubic phase transition characterization of (Pb, Ge)Te using high-temperature XRD. Powder Diffraction. 2008;23:137-140. DOI: 10.1154/

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**Chapter 3**

**Provisional chapter**

**Electrochemical Evaluation Technologies of Organic**

**Electrochemical Evaluation Technologies of Organic** 

Organic coatings are widely utilized to protect metals from corrosion and inevitably suffer degradation due to exposure to surroundings. Electrochemical technology is suitable for evaluating the protective performance of organic coatings since it has the advantages in rapidity and in-situ measurement. In this chapter, several electrochemical measurement technologies including open circuit potential (OCP), linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS) as well as electrochemical noise (EN) are introduced as ideal methods for acquiring mechanistic information about the failure behavior of the painted metal. The research status on measuring configurations, choosing data acquisition parameters and analytical methods are also discussed. **Keywords:** organic coatings, open circuit potential, electrochemical impedance

Organic coatings mostly have dual uses of protecting the substrate and being decorative. Concerning the engineering purposes, organic coating is presumably only for the function of preventing metal corrosion, which is an effective means for the corrosion protection of marine, pipeline, bridge and so on [1]. However, coating degradation is always inevitable, due to the inherent nature and the preparation process of organic coatings. The protective function loses gradually when exposed to the corrosion environment, and it often cannot be detected in time, resulting in undetectable corrosion destruction of the metal beneath the coating. Therefore, developing an in-situ evaluation technique for the protective performance

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.79736

**Coatings**

**Coatings**

Fandi Meng and Li Liu

Fandi Meng and Li Liu

**Abstract**

**1. Introduction**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79736

spectroscopy, electrochemical noise

of coatings is an urgent demand presently.


#### **Electrochemical Evaluation Technologies of Organic Coatings Electrochemical Evaluation Technologies of Organic Coatings**

DOI: 10.5772/intechopen.79736

Fandi Meng and Li Liu Fandi Meng and Li Liu

[41] Partin DL. Growth of lead germanium telluride thin film structures by molecular beam epitaxy. Journal of Vacuum Science and Technology. 1982;21:1-5. DOI: 10.1116/1.571714

[42] Li B, Zhang SY, Xie P, Liu DQ. Compositional dependence of absorption edges in evaporated Pb1�xGexTe thin films as infrared short-wavelength cutoff filters. Proceedings of SPIE. Advanced Optical Manufacturing Technologies. 2009;7082:70822L. DOI: 10.1117/

[43] Li B, Xie P, Zhang SY, Liu DQ. Compositional congruency, correlation and high pressure polymorphism in electron-beam evaporated Pb1�xGexTe thin films. The Journal of Alloys

[44] Nolas GS, Goldsmid HJ. Thermal conductivity of semiconductors. In: Tritt TM, editor. Thermal Conductivity: Theory, Properties and Applications. New York: Kluwer Academic;

[45] Rao KJ, Mohan R. Chemical bond approach to determining conductivity band gaps in amorphous chalcogenides and pnictides. Solid State Communications. 1981;39:1065-1068.

[46] Li B, Xie P, Zhang SY, Liu DQ. Lead germanium telluride: A mechanically robust infrared high-index layer. Journal of Materials Science. 2011;46:4000-4004. DOI: 10.1007/s10853-

[47] Crocker AJ, Wilson M. Microhardness in PbTe and related alloys. Journal of Materials

[48] Oliver WC, Pharr GM. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. Journal of Materials

and Compounds. 2014;589:109-114. DOI: 10.1016/j.jallcom.2013.11.179

12.830991

48 Coatings and Thin-Film Technologies

2004. pp. 105-122

011-5327-9

DOI: 10.1016/0038-1098(81)90209-X

Science. 1978;13:833-842. DOI: 10.1007/BF00570520

Research. 1992;7:1564-1583. DOI: 10.1557/JMR.1992.1564

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79736

#### **Abstract**

Organic coatings are widely utilized to protect metals from corrosion and inevitably suffer degradation due to exposure to surroundings. Electrochemical technology is suitable for evaluating the protective performance of organic coatings since it has the advantages in rapidity and in-situ measurement. In this chapter, several electrochemical measurement technologies including open circuit potential (OCP), linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS) as well as electrochemical noise (EN) are introduced as ideal methods for acquiring mechanistic information about the failure behavior of the painted metal. The research status on measuring configurations, choosing data acquisition parameters and analytical methods are also discussed.

**Keywords:** organic coatings, open circuit potential, electrochemical impedance spectroscopy, electrochemical noise

### **1. Introduction**

Organic coatings mostly have dual uses of protecting the substrate and being decorative. Concerning the engineering purposes, organic coating is presumably only for the function of preventing metal corrosion, which is an effective means for the corrosion protection of marine, pipeline, bridge and so on [1]. However, coating degradation is always inevitable, due to the inherent nature and the preparation process of organic coatings. The protective function loses gradually when exposed to the corrosion environment, and it often cannot be detected in time, resulting in undetectable corrosion destruction of the metal beneath the coating. Therefore, developing an in-situ evaluation technique for the protective performance of coatings is an urgent demand presently.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The principle of corrosion of bare metal (active dissolution) in the electrolyte aqueous solution or in the humid environment is the balancing electrochemical reactions, i.e., the anodic reaction, e.g., metal dissolution (M → Mn+ + *ne*<sup>−</sup> ) and the associated cathodic reaction, e.g., oxygen reduction (O2 + 2H2 O + 4*e*<sup>−</sup> → 4OH<sup>−</sup> ). When it comes to the metal coated with an organic coating, the above electrochemical corrosion also occurs. Firstly, water permeates into the coating along the pores and defects of fillers/binder interface. Subsequently, the oxygen and the dissolved ions penetrate into the coating; thus, the corrosive medium solution forms on the coating/metal interface, resulting in the occurrence of electrochemical reactions [2]. The rate determining step in controlling the electrochemical reaction should be the diffusion rate of ions through the coating [3]. The locations of anode and cathode areas are separate on the surface of metal substrate, due to the inhomogeneity of transport of the corrosive medium (**Figure 1**). In conclusion, the protective performance of organic coating can be reflected by the corrosion of metal substrate. This is the reason that electrochemical measurement technologies are valid for the evaluation of the coated metals.

**2. Open circuit potential method**

cathode (for example, rising concentrations of Fe2+ and OH<sup>−</sup>

OCP is a simple but important parameter in the research of corrosion and protection, which is the potential of a working electrode (WE) when no external current is applied to the circuit [5]. As a fundamental electrochemical method for assessing the anti-corrosion performance of coating/metal system, it is generally recognized that the OCP of coated metal is more positive than that of the bare metal [6]. Gowri and Balakrishnan [7] confirmed that their greatest corrosion resistant specimen showed a more positive value in potential than other coated samples on general levels. There are several factors that affect the potential of coated metals, and the resistance of the film is the most significant one. Besides, there are corrosion products of local anode and

cathodic protection by some active pigments, etc. Deya et al. [8] showed that the OCP of alkyd coating varied jointly with ionic resistance and changed toward less negative values in cases of coatings with high ionic resistance. Liu et al. [9] measured the OCP of aluminum alloy specimens coated with 2 wt.% polyaniline (PANI) epoxy coating over 40 h (**Figure 2**). During the initial 20 h, the potential rapidly became negative. After this period, the potential gradually increased up to 48 h. It concluded that the protection mechanism of coating changed from barrier inhibition to

A tracking measurement of OCP can reflect the corrosion process of metal substrate. Murray [6] reported the typical epoxy coated sample potential-time data through 3000 h of exposure to the ASTM-D-1141 substitute ocean water test solution. **Figure 3** shows the OCP curve of epoxy mica coating/steel system in 3.5 wt.% NaCl solution under alternating hydrostatic pressure (AHP) condition [10]. OCP of the coating as a whole has a negative relationship with immersion time. Three stages are found in the measurement. First, there is a large fluctuation appearing in the first 24 h, the OCP decreases to the minimum point from 0.52 V (vs. Ag/ AgCl) to −0.41 V (vs. Ag/AgCl), and then rebounds to about 0.2 V (vs. Ag/AgCl) rapidly.

ionic resistance by the formation of a complete oxide layer during immersion time.

**Figure 2.** OCP curve for 2 wt.% PANI coated aluminum in 3.5% NaCl solution for 48 h [8].

due to their slow dispersion), the

Electrochemical Evaluation Technologies of Organic Coatings

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51

Compared with the routine test methods for coating evaluation, electrochemical measurement technologies have many unique advantages [4]. First, the measuring process is fast, and the instruments are relatively simple. Second, electrochemical methods achieve the quantitative or semi-quantitative evaluation for the protection level. The accurate results of the analysis are superior to other performance tests. More importantly, the in-situ examination of organic coatings makes it possible for continuous monitoring in the field. In this chapter, several electrochemical measurement technologies including open circuit potential (OCP), linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS) as well as electrochemical noise (EN), are introduced for assessing the performance and acquiring mechanistic information on the failure behavior of the painted metal. Although application of these technologies is relatively mature in the lab, lots of significant challenges still exist in the field evaluation, and the corresponding considerations are required. The aim of this review is to summarize the specific characteristics of electrochemical technologies, the data parameters and the analytical methods, which can assist the application for anti-corrosive evaluation of organic coatings.

**Figure 1.** (a) Illustration diagram of electrochemical corrosion process of coating/metal system and (b) optical picture of the corrosion morphology of coated metal.

### **2. Open circuit potential method**

The principle of corrosion of bare metal (active dissolution) in the electrolyte aqueous solution or in the humid environment is the balancing electrochemical reactions, i.e., the anodic

organic coating, the above electrochemical corrosion also occurs. Firstly, water permeates into the coating along the pores and defects of fillers/binder interface. Subsequently, the oxygen and the dissolved ions penetrate into the coating; thus, the corrosive medium solution forms on the coating/metal interface, resulting in the occurrence of electrochemical reactions [2]. The rate determining step in controlling the electrochemical reaction should be the diffusion rate of ions through the coating [3]. The locations of anode and cathode areas are separate on the surface of metal substrate, due to the inhomogeneity of transport of the corrosive medium (**Figure 1**). In conclusion, the protective performance of organic coating can be reflected by the corrosion of metal substrate. This is the reason that electrochemical measurement technolo-

Compared with the routine test methods for coating evaluation, electrochemical measurement technologies have many unique advantages [4]. First, the measuring process is fast, and the instruments are relatively simple. Second, electrochemical methods achieve the quantitative or semi-quantitative evaluation for the protection level. The accurate results of the analysis are superior to other performance tests. More importantly, the in-situ examination of organic coatings makes it possible for continuous monitoring in the field. In this chapter, several electrochemical measurement technologies including open circuit potential (OCP), linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS) as well as electrochemical noise (EN), are introduced for assessing the performance and acquiring mechanistic information on the failure behavior of the painted metal. Although application of these technologies is relatively mature in the lab, lots of significant challenges still exist in the field evaluation, and the corresponding considerations are required. The aim of this review is to summarize the specific characteristics of electrochemical technologies, the data parameters and the analytical methods, which can assist the application for anti-corrosive evaluation of

**Figure 1.** (a) Illustration diagram of electrochemical corrosion process of coating/metal system and (b) optical picture of

O + 4*e*<sup>−</sup> → 4OH<sup>−</sup>

) and the associated cathodic reaction, e.g.,

). When it comes to the metal coated with an

reaction, e.g., metal dissolution (M → Mn+ + *ne*<sup>−</sup>

gies are valid for the evaluation of the coated metals.

oxygen reduction (O2 + 2H2

50 Coatings and Thin-Film Technologies

organic coatings.

the corrosion morphology of coated metal.

OCP is a simple but important parameter in the research of corrosion and protection, which is the potential of a working electrode (WE) when no external current is applied to the circuit [5]. As a fundamental electrochemical method for assessing the anti-corrosion performance of coating/metal system, it is generally recognized that the OCP of coated metal is more positive than that of the bare metal [6]. Gowri and Balakrishnan [7] confirmed that their greatest corrosion resistant specimen showed a more positive value in potential than other coated samples on general levels. There are several factors that affect the potential of coated metals, and the resistance of the film is the most significant one. Besides, there are corrosion products of local anode and cathode (for example, rising concentrations of Fe2+ and OH<sup>−</sup> due to their slow dispersion), the cathodic protection by some active pigments, etc. Deya et al. [8] showed that the OCP of alkyd coating varied jointly with ionic resistance and changed toward less negative values in cases of coatings with high ionic resistance. Liu et al. [9] measured the OCP of aluminum alloy specimens coated with 2 wt.% polyaniline (PANI) epoxy coating over 40 h (**Figure 2**). During the initial 20 h, the potential rapidly became negative. After this period, the potential gradually increased up to 48 h. It concluded that the protection mechanism of coating changed from barrier inhibition to ionic resistance by the formation of a complete oxide layer during immersion time.

A tracking measurement of OCP can reflect the corrosion process of metal substrate. Murray [6] reported the typical epoxy coated sample potential-time data through 3000 h of exposure to the ASTM-D-1141 substitute ocean water test solution. **Figure 3** shows the OCP curve of epoxy mica coating/steel system in 3.5 wt.% NaCl solution under alternating hydrostatic pressure (AHP) condition [10]. OCP of the coating as a whole has a negative relationship with immersion time. Three stages are found in the measurement. First, there is a large fluctuation appearing in the first 24 h, the OCP decreases to the minimum point from 0.52 V (vs. Ag/ AgCl) to −0.41 V (vs. Ag/AgCl), and then rebounds to about 0.2 V (vs. Ag/AgCl) rapidly.

**Figure 2.** OCP curve for 2 wt.% PANI coated aluminum in 3.5% NaCl solution for 48 h [8].

applying current polarization on the WE, the electrode potential of WE would change near the self-corrosion potential (about ±20 mV); thus, a linear relationship between Δ*E* and Δ*I* can be obtained at this point according to the Stern and Geary theory [11]. In the active corrosion

> <sup>Δ</sup>*<sup>I</sup>* <sup>=</sup> *ba <sup>b</sup>* \_\_\_\_\_\_\_\_\_\_ *<sup>c</sup>* 2.303(*ba* <sup>+</sup> *bc*) <sup>×</sup> \_\_1

where *R*p is the polarization resistance, Δ*E* and Δ*I* are the polarization potential and polarization

The higher value of the resistance, the smaller corrosion rate, thus the corrosion resistance of the coating can be evaluated by *R*p value. In general, the measured *R* values of the coating/metal system actually contain the polarization resistance, coating resistance, resistance of the lead, film resistance of the substrate and solution resistance, which are comprehensive test results. Therefore, the LP method provides comprehensive information for the evaluation of the coating/metal system, which can be used as a reference. It should be pointed out that a lot of heavyduty corrosion protective coatings tend to reach hundreds of micrometers thick. The LP method

active pigment contained coatings, such as zinc-rich powder coatings [12]. The researches on long range change in the removal of current from the coating under cathodic protection conditions and short range currents measured after pulsing the sample also have been reported [6].

As early as 1980s, researchers have started using EIS to investigate the protective properties and deterioration of organic coatings. EIS can get the information of coating/metal systems in different frequency bands. According to the calculation of coating capacitance and coating resistance, information of coating body can be quantitatively acquired. The double-layer capacitance and charge-transfer resistance also reflect the corrosion process of metal substrate. Therefore, EIS becomes the main method to assess coating performance among the electrochemical techniques. The electrochemical behavior of measured coating/metal system (i.e., the coated metal electrode) is different compared with that of bare metal. Due to a wider linear response region of coating/metal system, EIS tests were usually performed in the frequency range from 100 kHz to 10 mHz. In addition, organic coating is often a high impedance system, the impedance

than that of bare metal, to avoid the errors caused by potential drift and improve the signalnoise ratio. Generally, 20 mV (rms) amplitude coupled with OCP is enough for coating system. When the coating has a higher impedance, a higher sinusoidal perturbance should be used. No more than 50 mV (rms) is accepted, otherwise the electrochemical process will be artificially changed. For the continued EIS tests, a special flat plate test cell was developed, which consists of a horizontally positioned coated-flat plate specimen, a clamped, O-ring seal and a glass tube [13]. A Faraday cage is often utilized to effectively reduce the instrumentation

and ambience interferences when EIS is applied in the lab and in the field [14].

is the self-corrosion current density, ba

*i c*

and bc

Electrochemical Evaluation Technologies of Organic Coatings

http://dx.doi.org/10.5772/intechopen.79736

. Thus, a larger value in sinusoidal voltage is applied

(1)

53

are Tafel constants.

), but except some

ΔE

may not work very well due to a large coating resistance (above 106 Ω·cm<sup>2</sup>

**4. Electrochemical impedance spectroscopy method**

system, there is a mathematical relation as follows:

c

Rp <sup>=</sup> \_\_\_

modulus of coating can reach 1011 Ω·cm<sup>2</sup>

current density, respectively; *i*

**Figure 3.** OCP curve for epoxy mica coating/steel system under AHP [9].

This may be attributed to the water permeation at this time, and the conductive paths for electrolyte diffusion have been created. From 25 h to about 130 h, the OCP shows decrease tendency in fluctuations. It suggested that the coating starts to deteriorate, and the substrate under coating is corroding at a low rate. However, a significant decline in the value of OCP can be obtained after 130 h, and the OCP reaches to −0.53 V (vs. Ag/AgCl). This implied that the steel suffered from serious corrosion and the coating obviously degenerated. The rapid changes in OCP are much earlier than visible corrosion products on the surface of substrate.

Sometimes OCP is observed to fluctuate up and down in the initial time. This can attribute to a change in the ratio of local anodic area to cathodic area. Mayne described OCP fluctuations of coated steel by the concept of an *iR* drop across the coating. It suggested that an *iR* drop at the anodic and cathodic areas is a criterion for determination of the corrosion potential of coated specimens [5]. In general, anodic areas are quite smaller than cathodic areas at the early stage. Thus, the potential is more positive. However, with an increase in anodic sites during immersion, the corrosion potential becomes more negative. In short, OCP is an effective semi-quantitative method for coating evaluation, which is also utilized to confirm the results of EIS in many researches.

### **3. Linear polarization method**

Linear polarization (LP) is one of the most commonly used electrochemical methods for the rapid test of metal corrosion rate. The characteristics of LP are sensitive and fast, which are suitable for the corrosion system in any electrolytes. The surface state of the sample would not be damaged due to a small polarization current, which is appropriate for the measurement of the anti-corrosion properties of the coated metals. The principle of LP technique is applying current polarization on the WE, the electrode potential of WE would change near the self-corrosion potential (about ±20 mV); thus, a linear relationship between Δ*E* and Δ*I* can be obtained at this point according to the Stern and Geary theory [11]. In the active corrosion system, there is a mathematical relation as follows:

$$\mathbf{R}\_p = \frac{\Delta \mathbf{E}}{\Delta I} = \frac{b\_s b\_c}{2.303(b\_s + b\_c)} \times \frac{1}{\bar{l}\_c} \tag{1}$$

where *R*p is the polarization resistance, Δ*E* and Δ*I* are the polarization potential and polarization current density, respectively; *i* c is the self-corrosion current density, ba and bc are Tafel constants. The higher value of the resistance, the smaller corrosion rate, thus the corrosion resistance of the coating can be evaluated by *R*p value. In general, the measured *R* values of the coating/metal system actually contain the polarization resistance, coating resistance, resistance of the lead, film resistance of the substrate and solution resistance, which are comprehensive test results. Therefore, the LP method provides comprehensive information for the evaluation of the coating/metal system, which can be used as a reference. It should be pointed out that a lot of heavyduty corrosion protective coatings tend to reach hundreds of micrometers thick. The LP method may not work very well due to a large coating resistance (above 106 Ω·cm<sup>2</sup> ), but except some active pigment contained coatings, such as zinc-rich powder coatings [12]. The researches on long range change in the removal of current from the coating under cathodic protection conditions and short range currents measured after pulsing the sample also have been reported [6].

### **4. Electrochemical impedance spectroscopy method**

This may be attributed to the water permeation at this time, and the conductive paths for electrolyte diffusion have been created. From 25 h to about 130 h, the OCP shows decrease tendency in fluctuations. It suggested that the coating starts to deteriorate, and the substrate under coating is corroding at a low rate. However, a significant decline in the value of OCP can be obtained after 130 h, and the OCP reaches to −0.53 V (vs. Ag/AgCl). This implied that the steel suffered from serious corrosion and the coating obviously degenerated. The rapid changes in OCP are much earlier than visible corrosion products on the surface of substrate. Sometimes OCP is observed to fluctuate up and down in the initial time. This can attribute to a change in the ratio of local anodic area to cathodic area. Mayne described OCP fluctuations of coated steel by the concept of an *iR* drop across the coating. It suggested that an *iR* drop at the anodic and cathodic areas is a criterion for determination of the corrosion potential of coated specimens [5]. In general, anodic areas are quite smaller than cathodic areas at the early stage. Thus, the potential is more positive. However, with an increase in anodic sites during immersion, the corrosion potential becomes more negative. In short, OCP is an effective semi-quantitative method for coating evaluation, which is also utilized to confirm the

**Figure 3.** OCP curve for epoxy mica coating/steel system under AHP [9].

Linear polarization (LP) is one of the most commonly used electrochemical methods for the rapid test of metal corrosion rate. The characteristics of LP are sensitive and fast, which are suitable for the corrosion system in any electrolytes. The surface state of the sample would not be damaged due to a small polarization current, which is appropriate for the measurement of the anti-corrosion properties of the coated metals. The principle of LP technique is

results of EIS in many researches.

52 Coatings and Thin-Film Technologies

**3. Linear polarization method**

As early as 1980s, researchers have started using EIS to investigate the protective properties and deterioration of organic coatings. EIS can get the information of coating/metal systems in different frequency bands. According to the calculation of coating capacitance and coating resistance, information of coating body can be quantitatively acquired. The double-layer capacitance and charge-transfer resistance also reflect the corrosion process of metal substrate. Therefore, EIS becomes the main method to assess coating performance among the electrochemical techniques.

The electrochemical behavior of measured coating/metal system (i.e., the coated metal electrode) is different compared with that of bare metal. Due to a wider linear response region of coating/metal system, EIS tests were usually performed in the frequency range from 100 kHz to 10 mHz. In addition, organic coating is often a high impedance system, the impedance modulus of coating can reach 1011 Ω·cm<sup>2</sup> . Thus, a larger value in sinusoidal voltage is applied than that of bare metal, to avoid the errors caused by potential drift and improve the signalnoise ratio. Generally, 20 mV (rms) amplitude coupled with OCP is enough for coating system. When the coating has a higher impedance, a higher sinusoidal perturbance should be used. No more than 50 mV (rms) is accepted, otherwise the electrochemical process will be artificially changed. For the continued EIS tests, a special flat plate test cell was developed, which consists of a horizontally positioned coated-flat plate specimen, a clamped, O-ring seal and a glass tube [13]. A Faraday cage is often utilized to effectively reduce the instrumentation and ambience interferences when EIS is applied in the lab and in the field [14].

### **4.1. Physical model of EIS and its evolution in the failure process of coating/metal system**

There are two basic purposes of EIS measurement for organic coatings. One is the equivalent electrical circuit (EEC) model by fitting analysis. Furthermore, the evolution of failure process of coating/metal system can be reflected by different equivalent circuit models. Another purpose is to obtain some of the electrical parameters for evaluating on the protective performance of the coatings. The choice of physical model should follow the features of EIS plots and coating structure. Different kinds of organic coatings (involving the binder and even pigment) or the same coating in different service environments may have disparate EIS plots. Therefore, the physical models are impossible to have only a few fixed forms. Nonetheless, some typical stages in the coating failure process can be concluded by EIS analysis. It is generally accepted that the electrochemical behavior of coated metals during their exposure to aqueous solution at ambient temperature involves the penetration of water electrolyte, electrochemical reaction on the interface and further deterioration of the protectiveness (formation of under film corrosion, growth of blisters, delamination of paint film and so on), which finally culminates in the complete failure of the coatings [15]. Liu et al. [16] measured the EIS of epoxy varnish coating at different immersion time. The result indicated that there were two basic stages of coating failure, i.e., the single capacitance arc stage and double capacitance arc stage, which implied that water was absorbed into the coating after initial immersion (the single capacitance arc stage), and then electrochemical corrosion started when water arrived at the interface between the coating and the steel (double capacitance arcs stage). During the second stage, the reaction rate determining step was controlled by corrosion.

(CR) model in **Figure 4c**, the water should uniformly permeate into the coating, such as the zinc-rich coating. Two time-constants represent the dielectric properties of polymer and the corrosion of zinc particles, respectively. Sometimes, the mass transfer of reactive particles is postponed due to the addition of pigments and fillers, resulting in diffusion characteristics of EIS, such as the Warburg impedance. Two typical EEC models with Warburg impedance are given. In **Figure 4d**, the R(C(RW(CR))) model is commonly used in the medium-term immersion, because the electrolyte diffusion occurs in the gaps among the fillers. When the diffusion region is next to the coating/metal interface, the R(C(R(C(RW)))) model in **Figure 4e** is appropriate. In later stage of immersion, the R(C(RW)) model in **Figure 4f** is often used, because macroscopic pores and blisters make the coating ineffective, and the diffusion pro-

**Figure 4.** Several typical equivalent circuit models applied in the evaluation of organic coatings: (a) general model used at the first stage; (b) and (c) models with two time-constants; (d) and (e) models with two time-constants and

characteristics of Warburg impedance; (f) model with Warburg impedance at the later stage of immersion.

When organic coating applies to a special environment, the fitting results may be different. Meng et al. [10] investigated the failure behavior of epoxy mica (EM) coating under AHP environment by EIS, which could be a typical example of organic coating with inert pigment. Four distinct stages of the coating deterioration were determined according to the evolution of EIS plots and the fitting results of EEC models (**Figure 5a**–**d**). At the first stage from 0 to 15 h (**Figure 5a**), the Nyquist plot reveals one capacitive characteristic, and the impedance modulus

acted as a barrier layer with a parallel connection of a high-value coating resistance and a lowvalue coating capacitance. The corresponding equivalent circuit A (in **Figure 5a**) was used to fit

element, due to the "scattering effect" arising from the heterogeneity of the coating surface [18]. The gradually reduced capacitance arc suggested that water permeated the coating rapidly.

during the initial periods of immersion. It demonstrated that the coating

. The constant phase element (CPE) was used to replace the capacitance

, the coating capacitance *C*<sup>c</sup>

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and

cess is mainly determined by the corrosion reactions of metal substrate.

the impedance data, which included the solution resistance *R*<sup>s</sup>

reaches 1011 Ω·cm<sup>2</sup>

the coating resistance *R*<sup>c</sup>

Although the EEC models of coating systems are quite different, the failure process of coating can be determined according to some typical parameters, such as the time-constant, coating capacitance and coating resistance. At the initial period of immersion, the electrolyte permeates in the coating but has not reached the coating/metal interface yet. The EEC model with only one time-constant can be used due to the great barrier properties of coating. The coating capacitance increases meanwhile the coating resistance decreases with increasing immersion time. In the medium term, water diffuses to the coating/metal interface, and then the electrochemical reactions occur, resulting in the appearance of two time-constants in the EIS plots or EEC models. There is no macroscopic corrosion product on the coating surface at this stage. When the corrosion products can be observed by naked eyes, the EEC models often return to the characteristic of one time-constant, which defines as the final stage. The coating capacitance reaches a steady value, which indicates that the water absorption of the coating gets a saturated state, and the coating losses its barrier function against electrolyte permeation [13, 17].

According to the above analysis of coating failure, several typical equivalent circuit models for EIS results of organic coatings are summarized. As shown in **Figure 4a**, the R(CR) model is often used in the initial immersion or for an intact coating, which means that the coating can act as an isolation layer and provide a good protective performance. When the electrolyte reaches the coating/metal interface, EEC models in **Figure 4b** and **c** may be selected. The model R(C(R(CR))) in **Figure 4b** is suitable for the majority of organic coatings, because the blisters or corrosion at the coating/metal interface are often localized. Concerning the R(CR)

**4.1. Physical model of EIS and its evolution in the failure process of coating/metal** 

second stage, the reaction rate determining step was controlled by corrosion.

Although the EEC models of coating systems are quite different, the failure process of coating can be determined according to some typical parameters, such as the time-constant, coating capacitance and coating resistance. At the initial period of immersion, the electrolyte permeates in the coating but has not reached the coating/metal interface yet. The EEC model with only one time-constant can be used due to the great barrier properties of coating. The coating capacitance increases meanwhile the coating resistance decreases with increasing immersion time. In the medium term, water diffuses to the coating/metal interface, and then the electrochemical reactions occur, resulting in the appearance of two time-constants in the EIS plots or EEC models. There is no macroscopic corrosion product on the coating surface at this stage. When the corrosion products can be observed by naked eyes, the EEC models often return to the characteristic of one time-constant, which defines as the final stage. The coating capacitance reaches a steady value, which indicates that the water absorption of the coating gets a saturated

state, and the coating losses its barrier function against electrolyte permeation [13, 17].

According to the above analysis of coating failure, several typical equivalent circuit models for EIS results of organic coatings are summarized. As shown in **Figure 4a**, the R(CR) model is often used in the initial immersion or for an intact coating, which means that the coating can act as an isolation layer and provide a good protective performance. When the electrolyte reaches the coating/metal interface, EEC models in **Figure 4b** and **c** may be selected. The model R(C(R(CR))) in **Figure 4b** is suitable for the majority of organic coatings, because the blisters or corrosion at the coating/metal interface are often localized. Concerning the R(CR)

There are two basic purposes of EIS measurement for organic coatings. One is the equivalent electrical circuit (EEC) model by fitting analysis. Furthermore, the evolution of failure process of coating/metal system can be reflected by different equivalent circuit models. Another purpose is to obtain some of the electrical parameters for evaluating on the protective performance of the coatings. The choice of physical model should follow the features of EIS plots and coating structure. Different kinds of organic coatings (involving the binder and even pigment) or the same coating in different service environments may have disparate EIS plots. Therefore, the physical models are impossible to have only a few fixed forms. Nonetheless, some typical stages in the coating failure process can be concluded by EIS analysis. It is generally accepted that the electrochemical behavior of coated metals during their exposure to aqueous solution at ambient temperature involves the penetration of water electrolyte, electrochemical reaction on the interface and further deterioration of the protectiveness (formation of under film corrosion, growth of blisters, delamination of paint film and so on), which finally culminates in the complete failure of the coatings [15]. Liu et al. [16] measured the EIS of epoxy varnish coating at different immersion time. The result indicated that there were two basic stages of coating failure, i.e., the single capacitance arc stage and double capacitance arc stage, which implied that water was absorbed into the coating after initial immersion (the single capacitance arc stage), and then electrochemical corrosion started when water arrived at the interface between the coating and the steel (double capacitance arcs stage). During the

**system**

54 Coatings and Thin-Film Technologies

**Figure 4.** Several typical equivalent circuit models applied in the evaluation of organic coatings: (a) general model used at the first stage; (b) and (c) models with two time-constants; (d) and (e) models with two time-constants and characteristics of Warburg impedance; (f) model with Warburg impedance at the later stage of immersion.

(CR) model in **Figure 4c**, the water should uniformly permeate into the coating, such as the zinc-rich coating. Two time-constants represent the dielectric properties of polymer and the corrosion of zinc particles, respectively. Sometimes, the mass transfer of reactive particles is postponed due to the addition of pigments and fillers, resulting in diffusion characteristics of EIS, such as the Warburg impedance. Two typical EEC models with Warburg impedance are given. In **Figure 4d**, the R(C(RW(CR))) model is commonly used in the medium-term immersion, because the electrolyte diffusion occurs in the gaps among the fillers. When the diffusion region is next to the coating/metal interface, the R(C(R(C(RW)))) model in **Figure 4e** is appropriate. In later stage of immersion, the R(C(RW)) model in **Figure 4f** is often used, because macroscopic pores and blisters make the coating ineffective, and the diffusion process is mainly determined by the corrosion reactions of metal substrate.

When organic coating applies to a special environment, the fitting results may be different. Meng et al. [10] investigated the failure behavior of epoxy mica (EM) coating under AHP environment by EIS, which could be a typical example of organic coating with inert pigment. Four distinct stages of the coating deterioration were determined according to the evolution of EIS plots and the fitting results of EEC models (**Figure 5a**–**d**). At the first stage from 0 to 15 h (**Figure 5a**), the Nyquist plot reveals one capacitive characteristic, and the impedance modulus reaches 1011 Ω·cm<sup>2</sup> during the initial periods of immersion. It demonstrated that the coating acted as a barrier layer with a parallel connection of a high-value coating resistance and a lowvalue coating capacitance. The corresponding equivalent circuit A (in **Figure 5a**) was used to fit the impedance data, which included the solution resistance *R*<sup>s</sup> , the coating capacitance *C*<sup>c</sup> and the coating resistance *R*<sup>c</sup> . The constant phase element (CPE) was used to replace the capacitance element, due to the "scattering effect" arising from the heterogeneity of the coating surface [18]. The gradually reduced capacitance arc suggested that water permeated the coating rapidly.

electrochemical reactions started at the coating/steel interface and developed with the water diffusion through the coating. Only one capacitive characteristic is shown in **Figure 5b**, it could be that the electrochemical reactions were quite weak during the immersion period, the order of magnitude of time-constant of the electrochemical reaction impedance is the same as

After a period of immersion time under AHP, the diffusion character was added to the plot at low frequency from 96 to 150 h. The third stage shown in **Figure 5c** indicated that the corrosion behavior of the coated steel has been altered. At this moment, corrosion products were visible by the naked eyes at the surface of steel. It is probably that the corrosion of the steel substrate was accelerated at the interface area, a new diffusion field appeared around the substrate. As a result, model C (in **Figure 5c**) containing a diffusion component was applied

At the final stage (until 240 h), a capacitive loop with a characteristic of Warburg impedance arc is observed (**Figure 5d**). Water was mainly responsible for the measured diffusion in the EIS response at the initial stage. Since water absorption approached or reached saturation, the ions or products involved in corrosion process of coating/metal interface were mainly responsible for the diffusion in the EIS response. The impedance modulus (|Z|) has dropped

pening under AHP. The electrolyte has reached the surface of metal, and obvious corrosion

As a multi-interface corrosion system, the coating/metal system often has complex failure process, due to inhomogeneous physical and chemical properties of binder/pigment and coating/metal interfaces [19]. Among several sub-processes of coating failure, there are two critical steps: the water diffusion process and the following electrochemical reactions happened at the coating/metal interface [20]. In order to investigate how electrical parameters of EIS quantitatively evaluate the protective performance of organic coating, the physical meaning of parameters and their correlation to two critical steps of coating failure behavior have been

The process of water diffusion, i.e., the water impermeability of coatings is the key perfor-

eters can be used to indicate the anti-corrosion performance of the coatings. Tian et al. [21]

and AHP environments (**Figure 6a**). Because the electrical resistance of electrolyte (such as 3.5 wt.% NaCl solution, 101 Ω order of magnitude at room temperature) is fairly smaller than

–1011 Ω), thus the variation of *R*<sup>c</sup>

to 3.1 × 106 Ω·cm<sup>2</sup>

tence of initial micro-pores in the coating. For AP, the value of *R*<sup>c</sup>

and *R*<sup>c</sup>

of the epoxy glass flake coating under atmospheric pressure (AP)

suggested that water permeated into coatings quickly, due to the exis-

after being immersed for 240 h. Meanwhile, the value of *R*<sup>c</sup>

**4.2. EIS analysis on water diffusion process and coating/steel interfacial reaction**

. By this time the epoxy coating cannot prevent the corrosion of metals from hap-

–107 Ω·cm<sup>2</sup>

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57

, which was chosen as an

. Therefore, the two important param-

mainly depends on water permeation

under AHP, which is smaller than that under

decreased gradually from

that of the coating impedance [18].

to fit the experimental data.

discussed in details as follow.

compared the fitted *R*<sup>c</sup>

that of the coatings (106

[18]. The decline of *R*<sup>c</sup>

about 3.6 × 1010 to 1.4 × 108 Ω·cm<sup>2</sup>

declined from about 3.9 × 109

mance indicators, which is closely related to *C*<sup>c</sup>

has been observed when epoxy coating fell below 106

indicator of poor protective performance empirically.

to 107 Ω·cm<sup>2</sup>

**Figure 5.** Nyquist plots of EM coating/steel system at different immersion time under AHP: (a) 0–15 h; (b) 16–95 h; (c) 96–150 h; (d) 151–240 h (scatter points: experimental data, solid lines: fitting results according to corresponding EEC models); (e) 16 h experimental data and fitting results of model A and B [9].

The second stage (16–95 h) was identified by the fitting results of EEC. As the immersion time increased to 16 h, the EIS data is no longer satisfactorily fitted by model A. Obvious deviation is visible in the plot (**Figure 5e**). Considering the water and oxygen molecules reached the substrate surface through micro-pores in the coating, model B (see **Figure 5b**) was applied which added the double-layer capacitance CPEdl and the charge-transfer resistance *R*ct to fit the experimental data. A better fit was obtained (**Figure 5e**), it can reveal that the electrochemical reactions started at the coating/steel interface and developed with the water diffusion through the coating. Only one capacitive characteristic is shown in **Figure 5b**, it could be that the electrochemical reactions were quite weak during the immersion period, the order of magnitude of time-constant of the electrochemical reaction impedance is the same as that of the coating impedance [18].

After a period of immersion time under AHP, the diffusion character was added to the plot at low frequency from 96 to 150 h. The third stage shown in **Figure 5c** indicated that the corrosion behavior of the coated steel has been altered. At this moment, corrosion products were visible by the naked eyes at the surface of steel. It is probably that the corrosion of the steel substrate was accelerated at the interface area, a new diffusion field appeared around the substrate. As a result, model C (in **Figure 5c**) containing a diffusion component was applied to fit the experimental data.

At the final stage (until 240 h), a capacitive loop with a characteristic of Warburg impedance arc is observed (**Figure 5d**). Water was mainly responsible for the measured diffusion in the EIS response at the initial stage. Since water absorption approached or reached saturation, the ions or products involved in corrosion process of coating/metal interface were mainly responsible for the diffusion in the EIS response. The impedance modulus (|Z|) has dropped to 107 Ω·cm<sup>2</sup> . By this time the epoxy coating cannot prevent the corrosion of metals from happening under AHP. The electrolyte has reached the surface of metal, and obvious corrosion has been observed when epoxy coating fell below 106 –107 Ω·cm<sup>2</sup> , which was chosen as an indicator of poor protective performance empirically.

### **4.2. EIS analysis on water diffusion process and coating/steel interfacial reaction**

As a multi-interface corrosion system, the coating/metal system often has complex failure process, due to inhomogeneous physical and chemical properties of binder/pigment and coating/metal interfaces [19]. Among several sub-processes of coating failure, there are two critical steps: the water diffusion process and the following electrochemical reactions happened at the coating/metal interface [20]. In order to investigate how electrical parameters of EIS quantitatively evaluate the protective performance of organic coating, the physical meaning of parameters and their correlation to two critical steps of coating failure behavior have been discussed in details as follow.

The process of water diffusion, i.e., the water impermeability of coatings is the key performance indicators, which is closely related to *C*<sup>c</sup> and *R*<sup>c</sup> . Therefore, the two important parameters can be used to indicate the anti-corrosion performance of the coatings. Tian et al. [21] compared the fitted *R*<sup>c</sup> of the epoxy glass flake coating under atmospheric pressure (AP) and AHP environments (**Figure 6a**). Because the electrical resistance of electrolyte (such as 3.5 wt.% NaCl solution, 101 Ω order of magnitude at room temperature) is fairly smaller than that of the coatings (106 –1011 Ω), thus the variation of *R*<sup>c</sup> mainly depends on water permeation [18]. The decline of *R*<sup>c</sup> suggested that water permeated into coatings quickly, due to the existence of initial micro-pores in the coating. For AP, the value of *R*<sup>c</sup> decreased gradually from about 3.6 × 1010 to 1.4 × 108 Ω·cm<sup>2</sup> after being immersed for 240 h. Meanwhile, the value of *R*<sup>c</sup> declined from about 3.9 × 109 to 3.1 × 106 Ω·cm<sup>2</sup> under AHP, which is smaller than that under

The second stage (16–95 h) was identified by the fitting results of EEC. As the immersion time increased to 16 h, the EIS data is no longer satisfactorily fitted by model A. Obvious deviation is visible in the plot (**Figure 5e**). Considering the water and oxygen molecules reached the substrate surface through micro-pores in the coating, model B (see **Figure 5b**) was applied which added the double-layer capacitance CPEdl and the charge-transfer resistance *R*ct to fit the experimental data. A better fit was obtained (**Figure 5e**), it can reveal that the

**Figure 5.** Nyquist plots of EM coating/steel system at different immersion time under AHP: (a) 0–15 h; (b) 16–95 h; (c) 96–150 h; (d) 151–240 h (scatter points: experimental data, solid lines: fitting results according to corresponding EEC

models); (e) 16 h experimental data and fitting results of model A and B [9].

56 Coatings and Thin-Film Technologies

**Figure 6.** (a) *R*<sup>c</sup> and (b) *C*<sup>c</sup> as a function of immersion time under AP and AHP [18].

AP by two orders of magnitude. Obviously, the changes in *R*<sup>c</sup> reflected the water permeation differences under two environments. On the other hand, the coating capacitance *C*<sup>c</sup> is also quite useful to evaluate the water uptake in organic coatings, because water diffusion can modify the dielectric constant of the polymer. When the water diffusion meets law of Fick diffusion, the diffusion coefficient can be calculated according to Eq. (2).

$$\frac{\log \text{C}\_{\text{i}} \cdot \log \text{C}\_{\text{o}}}{\log \text{C}\_{\text{u}} \cdot \log \text{C}\_{\text{o}}} = \frac{2}{I} \sqrt{\frac{D}{\pi}} \sqrt{t} \tag{2}$$

researchers are hoping to utilize a nondestructive electrochemical measurement technique in the field. EN measurement is such an appropriate technique for the coating evaluation, which is expected to be comparable to EIS [22, 23]. The spontaneous fluctuations of electrode potential and current during electrochemical reactions are known as EN. High sensitivity to high resistance system and detection of localized corrosion are additional advantages of EN. Consequently, the application of EN has gained growing attention in corrosion research [24, 25]. In recent decades, studies on EN analysis of polymer coated metals also have been reported widely [3, 26, 27]. However, two issues restrict its application in the field. First, the on-site EN configuration and its reliability remain to be identified. Second, a quick and accu-

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Theoretically, the measuring instruments for EN are very simple. While EN configurations are complex in practical applications. Since EN research began in 1968 [28], the measuring mode and system of EN have experienced several stages of evolution. Two-electrode (working electrode and reference electrode) system was first adopted to measure potential noise under OCP or a certain constant current [28]. This simple configuration is mainly applied in the field of electro-deposition now. Then classical three-electrode system was applied so that the potential and current signals of WE can be separately measured through reference electrode (RE) and counter electrode (CE). Due to the respective measurement, however, some correlation analyses between potential and current noise cannot be made, such as noise resistance [29]. In order to overcome this, the three-electrode system was improved by researchers. Zero resistance amperometer (ZRA) was connected between two working electrodes (WE 1 and WE 2), which were made of the same material to avoid polarization [24]. Consequently, the current noise was measured as galvanic coupling current between two WEs, the potential noise between coupled WEs and RE was measured simultaneously. This electrode system has become the basic approach for EN measurement until now [30]. As for organic coatings, the EN three-electrode configuration can also be utilized to evaluate the performance of organic

Based on the principle of EN measurement above, many attempts on the improvement of EN measuring methods for in-situ monitoring have been made in recent years. Mabbutt et al. [31] firstly designed a nonstandard EN configuration. Two saturated calomel electrodes (SCEs) were utilized as the WEs, and the substrate served as the RE. Jamali et al. [32] also proposed the so-called "single cell" configuration, since the SCE served as RE and CE consecutively but not simultaneously. It is noteworthy that this configuration belongs to asymmetric electrode configuration, and further investigation on the asymmetry of electrodes is still required to clarify. In addition, some in-situ test electrode techniques, such as microelectrodes or multiple electrodes, have been used for the application of EN in coating evaluation in the field or under some specific conditions. Bierwagen et al. [33] have used the microelectrodes to study EN measurement of the coatings in a cyclic salt fog test chamber. Simpson et al. [34] made a thin sheet of gold by electron-beam-deposition, which was deposited on the painted steel and served as the microelectrode. The coating degradation was tested by the electrochemical method in an atmospheric exposure chamber. Tan [35] measured the EN of various corrosion systems by the wire beam electrode, which not only detects noise signatures and noise resistance, but also provides unprecedented spatial and temporal information on localized

rate EN analysis method is needed for the in-situ coating evaluation.

coatings by measuring the corrosion reactions of substrate metal [22].

**5.1. Measuring methods and configurations for EN**

where *C*<sup>t</sup> , *C*<sup>0</sup> , *C*∞ are the coating capacitance at immersion time *t*, before immersion, at saturation, respectively. *D* is the diffusion coefficient, *l* is the coating thickness. The dependence of the fitted coating capacitance *C*<sup>c</sup> in [21] is presented in **Figure 6b**. It can be seen that *C*<sup>c</sup> under AP increased from 1.1 × 10−10 to 1.7 × 10−10 F·cm−2 gradually with immersion time. For that under AHP, it shows two stages: (1) a rapid increase from 1.04 × 10−10 to 1.42 × 10−10 F·cm−2, which showed a rapid rise and a quasi-stable stage (within 24 h); (2) a fluctuation stage (after 24 h of immersion). The rapid increase of *C*<sup>c</sup> suggested that water penetrated into the coating rapidly due to increase in pressure.

After a period of immersion, the interfacial reactions between coating and steel may occur. From the EIS data fitted by EEC, the interfacial reaction is mainly electrochemical reaction, while for that under AHP [21], the interfacial reaction includes water spread at the coating/ steel interface and electrochemical reaction. The charge-transfer resistance, *R*ct, is an indicator for the electrochemical reaction at coating/steel interface. The higher value of *R*ct implies the greater corrosion resistance and the slower development of corrosion under the coatings. The appearance of *R*ct under AHP is faster than that under AP, suggesting the earlier appearance of electrochemical corrosion at the coating/steel interface.

### **5. Electrochemical noise method**

Although EIS is a well-established method for corrosion monitoring, applying artificial disturbance to the measured system may affect the process of electrochemical reaction. Thus, researchers are hoping to utilize a nondestructive electrochemical measurement technique in the field. EN measurement is such an appropriate technique for the coating evaluation, which is expected to be comparable to EIS [22, 23]. The spontaneous fluctuations of electrode potential and current during electrochemical reactions are known as EN. High sensitivity to high resistance system and detection of localized corrosion are additional advantages of EN. Consequently, the application of EN has gained growing attention in corrosion research [24, 25]. In recent decades, studies on EN analysis of polymer coated metals also have been reported widely [3, 26, 27]. However, two issues restrict its application in the field. First, the on-site EN configuration and its reliability remain to be identified. Second, a quick and accurate EN analysis method is needed for the in-situ coating evaluation.

### **5.1. Measuring methods and configurations for EN**

AP by two orders of magnitude. Obviously, the changes in *R*<sup>c</sup>

logCt \_\_\_\_\_\_\_\_\_\_ ‐ logC0

24 h of immersion). The rapid increase of *C*<sup>c</sup>

**5. Electrochemical noise method**

of electrochemical corrosion at the coating/steel interface.

rapidly due to increase in pressure.

where *C*<sup>t</sup>

**Figure 6.** (a) *R*<sup>c</sup>

, *C*<sup>0</sup>

the fitted coating capacitance *C*<sup>c</sup>

and (b) *C*<sup>c</sup>

58 Coatings and Thin-Film Technologies

diffusion, the diffusion coefficient can be calculated according to Eq. (2).

differences under two environments. On the other hand, the coating capacitance *C*<sup>c</sup>

as a function of immersion time under AP and AHP [18].

logC<sup>∞</sup> ‐ logC0

quite useful to evaluate the water uptake in organic coatings, because water diffusion can modify the dielectric constant of the polymer. When the water diffusion meets law of Fick

tion, respectively. *D* is the diffusion coefficient, *l* is the coating thickness. The dependence of

AP increased from 1.1 × 10−10 to 1.7 × 10−10 F·cm−2 gradually with immersion time. For that under AHP, it shows two stages: (1) a rapid increase from 1.04 × 10−10 to 1.42 × 10−10 F·cm−2, which showed a rapid rise and a quasi-stable stage (within 24 h); (2) a fluctuation stage (after

After a period of immersion, the interfacial reactions between coating and steel may occur. From the EIS data fitted by EEC, the interfacial reaction is mainly electrochemical reaction, while for that under AHP [21], the interfacial reaction includes water spread at the coating/ steel interface and electrochemical reaction. The charge-transfer resistance, *R*ct, is an indicator for the electrochemical reaction at coating/steel interface. The higher value of *R*ct implies the greater corrosion resistance and the slower development of corrosion under the coatings. The appearance of *R*ct under AHP is faster than that under AP, suggesting the earlier appearance

Although EIS is a well-established method for corrosion monitoring, applying artificial disturbance to the measured system may affect the process of electrochemical reaction. Thus,

= \_\_2 *l* √ \_\_ \_\_ *D π* √ \_

, *C*∞ are the coating capacitance at immersion time *t*, before immersion, at satura-

in [21] is presented in **Figure 6b**. It can be seen that *C*<sup>c</sup>

reflected the water permeation

*t* (2)

suggested that water penetrated into the coating

is also

under

Theoretically, the measuring instruments for EN are very simple. While EN configurations are complex in practical applications. Since EN research began in 1968 [28], the measuring mode and system of EN have experienced several stages of evolution. Two-electrode (working electrode and reference electrode) system was first adopted to measure potential noise under OCP or a certain constant current [28]. This simple configuration is mainly applied in the field of electro-deposition now. Then classical three-electrode system was applied so that the potential and current signals of WE can be separately measured through reference electrode (RE) and counter electrode (CE). Due to the respective measurement, however, some correlation analyses between potential and current noise cannot be made, such as noise resistance [29]. In order to overcome this, the three-electrode system was improved by researchers. Zero resistance amperometer (ZRA) was connected between two working electrodes (WE 1 and WE 2), which were made of the same material to avoid polarization [24]. Consequently, the current noise was measured as galvanic coupling current between two WEs, the potential noise between coupled WEs and RE was measured simultaneously. This electrode system has become the basic approach for EN measurement until now [30]. As for organic coatings, the EN three-electrode configuration can also be utilized to evaluate the performance of organic coatings by measuring the corrosion reactions of substrate metal [22].

Based on the principle of EN measurement above, many attempts on the improvement of EN measuring methods for in-situ monitoring have been made in recent years. Mabbutt et al. [31] firstly designed a nonstandard EN configuration. Two saturated calomel electrodes (SCEs) were utilized as the WEs, and the substrate served as the RE. Jamali et al. [32] also proposed the so-called "single cell" configuration, since the SCE served as RE and CE consecutively but not simultaneously. It is noteworthy that this configuration belongs to asymmetric electrode configuration, and further investigation on the asymmetry of electrodes is still required to clarify. In addition, some in-situ test electrode techniques, such as microelectrodes or multiple electrodes, have been used for the application of EN in coating evaluation in the field or under some specific conditions. Bierwagen et al. [33] have used the microelectrodes to study EN measurement of the coatings in a cyclic salt fog test chamber. Simpson et al. [34] made a thin sheet of gold by electron-beam-deposition, which was deposited on the painted steel and served as the microelectrode. The coating degradation was tested by the electrochemical method in an atmospheric exposure chamber. Tan [35] measured the EN of various corrosion systems by the wire beam electrode, which not only detects noise signatures and noise resistance, but also provides unprecedented spatial and temporal information on localized corrosion. In a word, further improvement of the EN configuration is still needed, particularly in the case of asymmetric electrode and the electrode with complicated shapes, since two identical coating/metal WEs tend to impractical in the field.

In the actual measurement of EN, it is also very important to select an appropriate sampling frequency, which is directly related to the reliability of the results. An excessively high sampling frequency leads to the lower power spectral density of EN, which is close to the white noise and produces difficulty in the data analysis. If the frequency is excessively low, some useful information will be lost. The sampling frequencies at 0.5, 1 and 2 Hz are commonly used and suitable for the general corrosion system, the specific value should be determined according to the EN sources of the tested system. Regarding the coating/metal system, EN data are usually recorded with a data-sampling interval of 0.25 or 0.5 s.

For the original EN signals, data preprocessing before various theoretical analyzing is necessary. The original data is composed of the real EN signals and the direct current (DC) drift signals. The drift can significantly affect the analysis results from time domain and frequency domain [36]. Considering that the generation of EN is a stable process, thus DC drift must be removed. At present, the DC drift removal method is still being explored. Tan et al. proposed the moving average removal (MAR) method to eliminate the DC drift of EN [37]. However, improper selection of filter window will make an obvious erroneous result [38]. In 2001, Mansfield proposed the linear fitting removal method and obtained satisfactory results in some corrosion systems [39]. This method is only suitable for the condition of DC drift with a linear feature. Meanwhile, Bertocci et al. proposed the polynomial method [38], which had satisfactory results as well as a wide range of application, despite the physical meaning of choosing polynomial exponent is still not clear.

changes of the corrosion process. The standard deviation can be used to describe noise inten-

**Figure 7.** Electrochemical noise records of epoxy coating in time domain after direct current trend removal [37].

more serious or more localized. From **Figure 8a** it could be seen that both E¯ and σE exhibit high amplitude fluctuations at the beginning. After about 36 h, the absolute value of E¯ is steady at around 0 V with a sudden decrease and a slow recovery at each moment of adding pressure or releasing pressure; the value of σE is close to zero with sudden increases during the change of pressure. On the other hand, I¯ is relatively stable until around 132 h; after 132 h, the value has

edly after about 126 h, as shown in **Figure 8b**. Therefore, it can be concluded that the early stage should be the water permeation period while 36–132 h should be the corrosion occurrence and development period, because the observable pits were found on the steel surface after 132 h. After 132 h, it should be the serious corrosion period. *R*n is one of the most common used noise indicators, which defined as the ratio of a standard deviation of the potential to that of the current noise [45], as shown in Eq. (3). It is generally considered that *R*n is inversely proportional to

σI

Wavelet analysis is the development and continuation of Fourier analytical method. For a large number of unsteady signals, the FFT transformation is not particularly appropriate. The wavelet transform is a time-scale analytical method of signal, which has the characteristics of multi-resolution analysis, and has the ability to characterize the local characteristics of the signal in time domain and frequency domain. Therefore, People tend to use wavelet transform to extract useful information of EN. In [44], wavelet analysis was applied and energy distribution plots (EDPs) of current noise at different immersion time were provided (**Figure 9**), to figure out the correlation between corrosion states of substrate and EN results. In **Figure 9a** (0–31 h immersion), the vast majority of energy distribution (ED) is in the crystal d7, d8. The scale range of time-constants of d7 and d8 is 32–128 s, it is considered that the diffusion

will increase as the corrosion becomes

Electrochemical Evaluation Technologies of Organic Coatings

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61

(3)

increase gradually at a low level and then fluctuate mark-

sity. It is reasonable to expect that the values of σE and σ<sup>I</sup>

corr.

Rn <sup>=</sup> <sup>σ</sup>\_\_E

increased suddenly. The values of σ<sup>I</sup>

the corrosion current density *i*

### **5.2. EN data analysis and parameter acquisition for coating/metal system**

Presently, a variety of analysis methods have been developed for the processing of EN signals, which include statistical analysis [40], spectral analysis [41], wavelet analysis [42], fractal analysis [43] and so on. These methods have successfully been applied to analyze many corrosion or degradation mechanisms of coatings.

Meng et al. [44] measured EN of an epoxy coating/steel system under AHP condition. The characteristics of EN time records are related with different corrosion states of the substrate steel. Three distinct stages can be divided with different characteristics (**Figure 7**). In the first stage, the potential signal exhibited strong and stochastic fluctuations. The current signal displayed the characteristic of white noise with a narrow range of fluctuations, which always occurs in 0–36 h. In the second stage, the EN transients began to appear simultaneously in potential and current signals from about 37 to about 130 h. The transients may be caused by the localized corrosion which occurs on the surface of metal. Finally, a typical example of potential and current signals after 130 h is shown in the figure, which fluctuated in larger amplitude with quick ascending and slow recovery pattern.

Statistical analysis has several commonly used parameters [36], such as E¯, I¯, σE, σ<sup>I</sup> and noise resistance *R*n. Regarding the physical meanings of these parameters, it is pointed out that the fluctuation of the average corrosion potential over the longer term may be directly related to the

corrosion. In a word, further improvement of the EN configuration is still needed, particularly in the case of asymmetric electrode and the electrode with complicated shapes, since two

In the actual measurement of EN, it is also very important to select an appropriate sampling frequency, which is directly related to the reliability of the results. An excessively high sampling frequency leads to the lower power spectral density of EN, which is close to the white noise and produces difficulty in the data analysis. If the frequency is excessively low, some useful information will be lost. The sampling frequencies at 0.5, 1 and 2 Hz are commonly used and suitable for the general corrosion system, the specific value should be determined according to the EN sources of the tested system. Regarding the coating/metal system, EN

For the original EN signals, data preprocessing before various theoretical analyzing is necessary. The original data is composed of the real EN signals and the direct current (DC) drift signals. The drift can significantly affect the analysis results from time domain and frequency domain [36]. Considering that the generation of EN is a stable process, thus DC drift must be removed. At present, the DC drift removal method is still being explored. Tan et al. proposed the moving average removal (MAR) method to eliminate the DC drift of EN [37]. However, improper selection of filter window will make an obvious erroneous result [38]. In 2001, Mansfield proposed the linear fitting removal method and obtained satisfactory results in some corrosion systems [39]. This method is only suitable for the condition of DC drift with a linear feature. Meanwhile, Bertocci et al. proposed the polynomial method [38], which had satisfactory results as well as a wide range of application, despite the physical meaning of

Presently, a variety of analysis methods have been developed for the processing of EN signals, which include statistical analysis [40], spectral analysis [41], wavelet analysis [42], fractal analysis [43] and so on. These methods have successfully been applied to analyze many cor-

Meng et al. [44] measured EN of an epoxy coating/steel system under AHP condition. The characteristics of EN time records are related with different corrosion states of the substrate steel. Three distinct stages can be divided with different characteristics (**Figure 7**). In the first stage, the potential signal exhibited strong and stochastic fluctuations. The current signal displayed the characteristic of white noise with a narrow range of fluctuations, which always occurs in 0–36 h. In the second stage, the EN transients began to appear simultaneously in potential and current signals from about 37 to about 130 h. The transients may be caused by the localized corrosion which occurs on the surface of metal. Finally, a typical example of potential and current signals after 130 h is shown in the figure, which fluctuated in larger

Statistical analysis has several commonly used parameters [36], such as E¯, I¯, σE, σ<sup>I</sup>

resistance *R*n. Regarding the physical meanings of these parameters, it is pointed out that the fluctuation of the average corrosion potential over the longer term may be directly related to the

and noise

identical coating/metal WEs tend to impractical in the field.

60 Coatings and Thin-Film Technologies

choosing polynomial exponent is still not clear.

rosion or degradation mechanisms of coatings.

amplitude with quick ascending and slow recovery pattern.

data are usually recorded with a data-sampling interval of 0.25 or 0.5 s.

**5.2. EN data analysis and parameter acquisition for coating/metal system**

**Figure 7.** Electrochemical noise records of epoxy coating in time domain after direct current trend removal [37].

changes of the corrosion process. The standard deviation can be used to describe noise intensity. It is reasonable to expect that the values of σE and σ<sup>I</sup> will increase as the corrosion becomes more serious or more localized. From **Figure 8a** it could be seen that both E¯ and σE exhibit high amplitude fluctuations at the beginning. After about 36 h, the absolute value of E¯ is steady at around 0 V with a sudden decrease and a slow recovery at each moment of adding pressure or releasing pressure; the value of σE is close to zero with sudden increases during the change of pressure. On the other hand, I¯ is relatively stable until around 132 h; after 132 h, the value has increased suddenly. The values of σ<sup>I</sup> increase gradually at a low level and then fluctuate markedly after about 126 h, as shown in **Figure 8b**. Therefore, it can be concluded that the early stage should be the water permeation period while 36–132 h should be the corrosion occurrence and development period, because the observable pits were found on the steel surface after 132 h. After 132 h, it should be the serious corrosion period. *R*n is one of the most common used noise indicators, which defined as the ratio of a standard deviation of the potential to that of the current noise [45], as shown in Eq. (3). It is generally considered that *R*n is inversely proportional to the corrosion current density *i* corr.

$$\mathbf{R}\_{\mathbf{n}} = \frac{\sigma\_{\mathbf{e}}}{\sigma\_{\mathbf{i}}} \tag{3}$$

Wavelet analysis is the development and continuation of Fourier analytical method. For a large number of unsteady signals, the FFT transformation is not particularly appropriate. The wavelet transform is a time-scale analytical method of signal, which has the characteristics of multi-resolution analysis, and has the ability to characterize the local characteristics of the signal in time domain and frequency domain. Therefore, People tend to use wavelet transform to extract useful information of EN. In [44], wavelet analysis was applied and energy distribution plots (EDPs) of current noise at different immersion time were provided (**Figure 9**), to figure out the correlation between corrosion states of substrate and EN results. In **Figure 9a** (0–31 h immersion), the vast majority of energy distribution (ED) is in the crystal d7, d8. The scale range of time-constants of d7 and d8 is 32–128 s, it is considered that the diffusion

pitting nucleation and metastable pitting process [46]. It indicated that the corrosion of metal occurred at this time, the failure behavior was dominated by the mixed mechanisms of water transport and charge-transfer reaction. In the third stage, the energy was stored predominantly in the crystal d1 and d3 (**Figure 9c**), which might indicate the fast corrosion process in this period, and the charge-transfer mechanism was the dominant process. It could be found that the EDPs were not only in accordance with, but also reflect more information about the

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63

To enable the rapid and automatic monitoring of the coatings by EN, the combination with artificial intelligence method and theory of nonlinear mathematics should be explored. For example, pattern recognition (PR) is an important method of information science that focuses on the recognition of regularities in data and the data classification [47]. EN signals show different distribution characteristics in different stages of corrosion, thus the similarity and the difference of EN waveform features could be categorized by PR. A close correlation is expected to be established between the classification results and the corrosion states. Huang et al. [48] applied some PR procedures for identifying different pitting states for Q235 carbon steel in NaHCO3 + NaCl solutions. Meng et al. [44] applied PR method to the establishment of an evaluation model for EN statistical parameters. For the painted steel system, different failure stages can be conveniently identified. The unique role of new analytical method will

Since water penetrates to the coating/steel interface, the delamination of coating from steel and the corrosion of steel substrate lead to obvious changes in electrochemical signals. Therefore, for the state of coating/steel interface, electrochemical evaluation technique can get the first-hand information. In addition, defects in the coating body, such as pores and cracks of pigment/binder interfaces, are gradually increasing by the erosion environment. Enlarged conductive paths for electrolyte cause a decrease in permeability resistance of coating, which can be detected by electrochemical methods consequently. In short, coating adhesion and compactness are two critical parts in failure process of organic coating, which can be used to achieve in-situ evaluation by electrochemical measurement methods. As to the further investigation of electrochemical evaluation, the application of artificial intelligence analysis [49, 50]

The investigation is supported by the National Natural Science Fund of China under the Contract No. 51622106, and the Fundamental Research Funds for the Central Universities

corrosion mechanisms.

be more pronounced in the future.

**6. Conclusion**

may be an essential trend.

**Acknowledgements**

under the Contract No. N170203005 and No. N170212021.

**Figure 8.** Variation of (a) E ¯ and σE as well as (b) I ¯ and σ<sup>I</sup> of EN for epoxy coating/steel system under AHP [37].

process often occurs during this period. Therefore, the first stage should be the water permeation period, which is in agreement with the other analysis results. In the second stage (**Figure 9b**), ED of d8 decreased and ED of d1, d3 increased obviously. The crystal d1, d3 (scale range 0.25–0.5 s and 1–2 s) represent fast corrosion process, which could be attributed to

**Figure 9.** EDPs of current noise of coating/steel system at different immersion time: (a) ED in the first stage; (b) ED in the second stage and (c) ED in the later stage [37].

pitting nucleation and metastable pitting process [46]. It indicated that the corrosion of metal occurred at this time, the failure behavior was dominated by the mixed mechanisms of water transport and charge-transfer reaction. In the third stage, the energy was stored predominantly in the crystal d1 and d3 (**Figure 9c**), which might indicate the fast corrosion process in this period, and the charge-transfer mechanism was the dominant process. It could be found that the EDPs were not only in accordance with, but also reflect more information about the corrosion mechanisms.

To enable the rapid and automatic monitoring of the coatings by EN, the combination with artificial intelligence method and theory of nonlinear mathematics should be explored. For example, pattern recognition (PR) is an important method of information science that focuses on the recognition of regularities in data and the data classification [47]. EN signals show different distribution characteristics in different stages of corrosion, thus the similarity and the difference of EN waveform features could be categorized by PR. A close correlation is expected to be established between the classification results and the corrosion states. Huang et al. [48] applied some PR procedures for identifying different pitting states for Q235 carbon steel in NaHCO3 + NaCl solutions. Meng et al. [44] applied PR method to the establishment of an evaluation model for EN statistical parameters. For the painted steel system, different failure stages can be conveniently identified. The unique role of new analytical method will be more pronounced in the future.

### **6. Conclusion**

process often occurs during this period. Therefore, the first stage should be the water permeation period, which is in agreement with the other analysis results. In the second stage (**Figure 9b**), ED of d8 decreased and ED of d1, d3 increased obviously. The crystal d1, d3 (scale range 0.25–0.5 s and 1–2 s) represent fast corrosion process, which could be attributed to

of EN for epoxy coating/steel system under AHP [37].

**Figure 9.** EDPs of current noise of coating/steel system at different immersion time: (a) ED in the first stage; (b) ED in the

second stage and (c) ED in the later stage [37].

**Figure 8.** Variation of (a) E ¯ and σE as well as (b) I ¯ and σ<sup>I</sup>

62 Coatings and Thin-Film Technologies

Since water penetrates to the coating/steel interface, the delamination of coating from steel and the corrosion of steel substrate lead to obvious changes in electrochemical signals. Therefore, for the state of coating/steel interface, electrochemical evaluation technique can get the first-hand information. In addition, defects in the coating body, such as pores and cracks of pigment/binder interfaces, are gradually increasing by the erosion environment. Enlarged conductive paths for electrolyte cause a decrease in permeability resistance of coating, which can be detected by electrochemical methods consequently. In short, coating adhesion and compactness are two critical parts in failure process of organic coating, which can be used to achieve in-situ evaluation by electrochemical measurement methods. As to the further investigation of electrochemical evaluation, the application of artificial intelligence analysis [49, 50] may be an essential trend.

### **Acknowledgements**

The investigation is supported by the National Natural Science Fund of China under the Contract No. 51622106, and the Fundamental Research Funds for the Central Universities under the Contract No. N170203005 and No. N170212021.

### **Conflict of interest**

The authors declare no conflict of interest.

### **Author details**

Fandi Meng and Li Liu\*

\*Address all correspondence to: liliu@mail.neu.edu.cn

Corrosion and Protection Division, Shenyang National Laboratory for Materials Science, Northeastern University, Shenyang, China

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[27] Mansfeld F, Sun Z, Hsu CH. Electrochemical noise analysis (ENA) for active and passive systems in chloride media. Electrochimica Acta. 2001;**46**:3651-3664. DOI: 10.1016/S0013-4

[28] Iverson WP. Transient voltage changes produced in corroding metals and alloys. Journal of the Electrochemical Society. 1968;**115**:617-625. DOI: 10.1149/1.2411362

[29] Blanc G, Gabrielli C, Keddam M. Measurement of electrochemical noise by a cross-correlation method. Electrochimica Acta. 1975;**20**:687-689. DOI: 10.1016/0013-4686(75)90069-9

[30] Bosch RW, Cottis RA, Csecs K, Dorsch T, Dunbar L, Heyn A, Huet F, Hyokyvirta O, Kerner Z, Kobzova A, Macak J, Novotny R, Oijerholm J, Piippo J, Richner R, Ritter S, Sanchez-Amaya JM, Somogyi A, Vaisanen S, Zhang WZ. Reliability of electrochemical noise measurements: Results of round-robin testing on electrochemical noise. Electrochimica Acta.

[31] Mills DJ, Mabbutt S. Investigation of defects in organic anti-corrosive coatings using electrochemical noise measurement. Progress in Organic Coating. 2000;**39**:41-48. DOI:

[32] Jamali SS, Mills DJ, Sykes JM. Measuring electrochemical noise of a single working electrode for assessing corrosion resistance of polymer coated metals. Progress in Organic

[33] Bierwagen GP, Allahar KN, Su Q, Gelling VJ. Electrochemically characterizing the AC-DC-AC accelerated test method using embedded electrodes. Corrosion Science. 2009;**51**:

[34] Simpson TC, Moran PJ, Hampel H, Davis GD, Shaw BA, Arah CO, Fritz TL, Zankel K. Electrochemical monitoring of organic coating degradation during atmospheric or

[35] Zhang Y, Yu B, Lu S, Meng X, Zhao X, Ji Y, Wang Y, Fu C, Liu X, Li X, Sui Y, Lang J, Yang

[37] Tan YJ, Bailey S, Kinsella B. The monitoring of the formation and destruction of corrosion inhibitor films using electrochemical noise analysis (ENA). Corrosion Science.

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Coating. 2014;**77**:733-741


**Chapter 4**

**Provisional chapter**

**Surface Modification of Polystyrene by Nitrogen**

**Surface Modification of Polystyrene by Nitrogen** 

DOI: 10.5772/intechopen.79716

Polystyrene has been utilized in biomedical purposes, interacting with various biological molecules. The interactions can be physical adsorption or a long-lasting chemical bonding, depending on the surface characteristic and behaviors. The characteristic can be designed related to the targeted interactions with the molecules by creating certain roughness, morphology, and patterns of the surface. Original characteristics of the material were usually enhanced by its surface modifications. Plasma treatments have been used to modify the polymer surfaces, resulting in a specifically targeted behavior such as hydrophobicity and molecule selectivity through the physical adsorption. A nitrogen plasma treatment is one of the effective and economical surface modification processes. The nitrogen gas is abundant in the atmosphere and generates nontoxic active plasma species for the polymer surface modification. The plasma treatment effectively changes

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

**Plasma Treatment**

**Abstract**

**1. Introduction**

**Plasma Treatment**

Masruroh and Dionysius J.D.H. Santjojo

Masruroh and Dionysius J.D.H. Santjojo

http://dx.doi.org/10.5772/intechopen.79716

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

the hydrophobicity and adsorption of the surface.

**Keywords:** nitrogen plasma, surface modification, adsorption, polystyrene

Polystyrene (PS) has long been recognized and widespread in the world. It is a thermoplastic aromatic polymer. The polystyrene is conventionally utilized in a wide range of application such as packaging, laboratory ware, house items, building materials, and so on [1]. However, it is also known that PS has been developed as functional materials in the form of a thin film and a microsphere [2]. In the field of biomedical, PS is extensively utilized in vitro as cells or bacteria storage [3] or in vivo as a drug-delivery system [4]. In fact, the polystyrene has also been used in biosensors [5–7]. Most of the applications require a hydrophilic surface to

#### **Surface Modification of Polystyrene by Nitrogen Plasma Treatment Surface Modification of Polystyrene by Nitrogen Plasma Treatment**

DOI: 10.5772/intechopen.79716

Masruroh and Dionysius J.D.H. Santjojo Masruroh and Dionysius J.D.H. Santjojo

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79716

#### **Abstract**

Polystyrene has been utilized in biomedical purposes, interacting with various biological molecules. The interactions can be physical adsorption or a long-lasting chemical bonding, depending on the surface characteristic and behaviors. The characteristic can be designed related to the targeted interactions with the molecules by creating certain roughness, morphology, and patterns of the surface. Original characteristics of the material were usually enhanced by its surface modifications. Plasma treatments have been used to modify the polymer surfaces, resulting in a specifically targeted behavior such as hydrophobicity and molecule selectivity through the physical adsorption. A nitrogen plasma treatment is one of the effective and economical surface modification processes. The nitrogen gas is abundant in the atmosphere and generates nontoxic active plasma species for the polymer surface modification. The plasma treatment effectively changes the hydrophobicity and adsorption of the surface.

**Keywords:** nitrogen plasma, surface modification, adsorption, polystyrene

### **1. Introduction**

Polystyrene (PS) has long been recognized and widespread in the world. It is a thermoplastic aromatic polymer. The polystyrene is conventionally utilized in a wide range of application such as packaging, laboratory ware, house items, building materials, and so on [1]. However, it is also known that PS has been developed as functional materials in the form of a thin film and a microsphere [2]. In the field of biomedical, PS is extensively utilized in vitro as cells or bacteria storage [3] or in vivo as a drug-delivery system [4]. In fact, the polystyrene has also been used in biosensors [5–7]. Most of the applications require a hydrophilic surface to

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

improve the interaction of the surface with proteins. Physical adsorption of protein on the surface is the key to how the PS will be utilized. In a biosensor, for example, selective adsorption is needed to immobilize a specific kind of biomolecule or protein [6]. The adsorption phenomenon is controlled by surface forces such as the weak van der Waals force and surface ionic force, hydrogen bonding, or surface wettability. In some cases, a long-lasting bonding is needed to immobilize the biomolecules involving stronger chemical covalent bonds. A crosslinker, which is incorporated in the functional layer, provides a covalent bonding between the surface and the molecules. The cross-linker can also be produced by the chemical modification of the functional surface [8]. The adsorption of the polystyrene depends on its surface properties that can be designed widely to immobilize macromolecules such as proteins and enzymes.

used in the polymer plasma treatment including Ar, N2

affect the polymer's surface roughness [4].

roughness, morphology, and microstructure.

**2. Experimental**

, O2, and NH3

Surface Modification of Polystyrene by Nitrogen Plasma Treatment

selected related to its plasma-state characteristics required for the intended process. Foerch and Hunter, for example, showed that nitrogen plasma treatment resulted in additional nitrogen atoms in the hydrocarbon network of the treated polymer [14]. Another researcher used

Plasma treatment for the modification and functionalization of polymer surface is very effective since the plasma interacts physically and chemically with the surface. The short time shot for every treatment makes the technique more efficient than other surface treatment processes. Depending on the property and character of the plasma, the treatment is able to produce group functionalization, graft polymerization, or molecules cross-linking which can

This chapter discusses surface modification of polystyrene. Nitrogen plasma is utilized to control the wettability of the polystyrene surface. The wettability is related to the hydrophilic or hydrophobic character of the surface. Research has shown that hydrophilic surface can adsorb protein molecules twice as much as hydrophobic surface [15]. The wettability of the surface can be evaluated or determined by surface-water contact measurement or surface contact measurement in short. In general, the wettability of the surface depends on the surface

The polystyrene layer is a thin film produced by means of a spin-coating technique. A number of aspects of the procedure affect the properties of the thin film. Two aspects, which are solvent and raw polystyrene molecular weight, were considered importantly related to the nitrogen plasma treatment process and results. The plasma character was diagnosed using optical emission spectroscopy (OES). Specimen characterizations were intended to determine

The polystyrene thin film was synthesized and deposited by a spin-coating method. Polystyrene raw material was dissolved into a polystyrene solution. Three raw materials, which were obtained from Sigma Aldrich with different molecular weights (MW), were utilized in this work, that is, 35, 129, and 280 kDa. This work utilized four different solvents to investigate the relation between the solvent and the effect of the plasma treatments. The solvents, that is, chloroform, toluene, xylene, and tetrahydrofuran, were also obtained from Sigma Aldrich. Each of the solutions was then deposited on a substrate by the spin-coating method. The specific quantity of the solution drop and rotation per minute was optimized and set for all the specimens. The polystyrene films were then heated in an oven after the deposition. The treatment was carried out to dry the excess solvent and to enhance interfacial

the surface roughness, surface wettability, and surface microstructure.

**2.1. Synthesis and deposition of polystyrene thin film**

bonding between the film and the substrate.

oxygen plasma to modify a hydrophobic polymer into a hydrophilic polymer [13].

. The processing gas is

71

http://dx.doi.org/10.5772/intechopen.79716

However, PS originally has a low-surface energy and poor polarizability, resulting in a hydrophobic surface. On the polystyrene surface, the adsorption is caused by mainly the intermolecular attraction forces. The forces often referred to the van der Waals forces originated from intramolecular electric polarities in the polymer chains. Two types of polarities exist, that is, alternating polarities and stationary polarities. The latter is often called the dipoles. The alternating polarities emerge when molecules come close to each other causing disruptions in the electron clouds. The alternating polarities are the solution of the disruptions, resulting in a kind of a molecular bond. Unlike the dipoles, the attraction forces due to the alternating polarities that drastically decrease with the increase of intermolecular distance. The hydrophobicity and protein adsorption are also affected by the combination of two polarities. The more alternating polarities result in the more hydrophobic surface and vice versa. Chemical functional groups such as –OH, =O, –NH2 , =NH, and ≡N produce stationary polarities, resulting in hydrophilic property [9].

One way of designing the appropriate adsorption is controlling the surface topography and morphology of the polymer. The performance of a biosensor can be enhanced by optimizing the surface roughness of its functional polymer [6]. Surface roughness is the topographical material's characteristic related to micro-profile of its surface. A higher surface roughness usually resulted in the wider contact area with other materials such as biomolecules on the surface. If the surface is adhesive to the biomolecules, the rougher the surface will attract more biomolecules. However, the physical adsorption is relatively weak, limiting the net attracted biomolecules by resorption (re-desorption) process [10]. The other way of controlling the adsorption was by surface activation [11].

In recent years, plasma treatment has been a common method to modify, activate, and functionalize the surface of polymers [12]. The plasma is the fourth state of a material, consisting of energetic atoms, ions, molecules, and radicals. Free electrons in the plasma maintain its quasi-neutral and equilibrium conditions. However, internally, the plasma has unique properties which can be designed and controlled for many purposes. Plasma surface treatment for polymers generally utilizes low-temperature plasma which is non-destructive to the bulk of the polymers. The low-temperature plasma is generated at low vacuum or better in the atmospheric environment. Another technique of lowering the temperature is by utilizing a low-frequency plasma, for example, a 40-kHz plasma [13]. A number of gases are usually used in the polymer plasma treatment including Ar, N2 , O2, and NH3 . The processing gas is selected related to its plasma-state characteristics required for the intended process. Foerch and Hunter, for example, showed that nitrogen plasma treatment resulted in additional nitrogen atoms in the hydrocarbon network of the treated polymer [14]. Another researcher used oxygen plasma to modify a hydrophobic polymer into a hydrophilic polymer [13].

Plasma treatment for the modification and functionalization of polymer surface is very effective since the plasma interacts physically and chemically with the surface. The short time shot for every treatment makes the technique more efficient than other surface treatment processes. Depending on the property and character of the plasma, the treatment is able to produce group functionalization, graft polymerization, or molecules cross-linking which can affect the polymer's surface roughness [4].

This chapter discusses surface modification of polystyrene. Nitrogen plasma is utilized to control the wettability of the polystyrene surface. The wettability is related to the hydrophilic or hydrophobic character of the surface. Research has shown that hydrophilic surface can adsorb protein molecules twice as much as hydrophobic surface [15]. The wettability of the surface can be evaluated or determined by surface-water contact measurement or surface contact measurement in short. In general, the wettability of the surface depends on the surface roughness, morphology, and microstructure.

The polystyrene layer is a thin film produced by means of a spin-coating technique. A number of aspects of the procedure affect the properties of the thin film. Two aspects, which are solvent and raw polystyrene molecular weight, were considered importantly related to the nitrogen plasma treatment process and results. The plasma character was diagnosed using optical emission spectroscopy (OES). Specimen characterizations were intended to determine the surface roughness, surface wettability, and surface microstructure.

### **2. Experimental**

improve the interaction of the surface with proteins. Physical adsorption of protein on the surface is the key to how the PS will be utilized. In a biosensor, for example, selective adsorption is needed to immobilize a specific kind of biomolecule or protein [6]. The adsorption phenomenon is controlled by surface forces such as the weak van der Waals force and surface ionic force, hydrogen bonding, or surface wettability. In some cases, a long-lasting bonding is needed to immobilize the biomolecules involving stronger chemical covalent bonds. A crosslinker, which is incorporated in the functional layer, provides a covalent bonding between the surface and the molecules. The cross-linker can also be produced by the chemical modification of the functional surface [8]. The adsorption of the polystyrene depends on its surface properties that can be designed widely to immobilize macromolecules such as proteins and enzymes. However, PS originally has a low-surface energy and poor polarizability, resulting in a hydrophobic surface. On the polystyrene surface, the adsorption is caused by mainly the intermolecular attraction forces. The forces often referred to the van der Waals forces originated from intramolecular electric polarities in the polymer chains. Two types of polarities exist, that is, alternating polarities and stationary polarities. The latter is often called the dipoles. The alternating polarities emerge when molecules come close to each other causing disruptions in the electron clouds. The alternating polarities are the solution of the disruptions, resulting in a kind of a molecular bond. Unlike the dipoles, the attraction forces due to the alternating polarities that drastically decrease with the increase of intermolecular distance. The hydrophobicity and protein adsorption are also affected by the combination of two polarities. The more alternating polarities result in the more hydrophobic surface and vice versa. Chemical

One way of designing the appropriate adsorption is controlling the surface topography and morphology of the polymer. The performance of a biosensor can be enhanced by optimizing the surface roughness of its functional polymer [6]. Surface roughness is the topographical material's characteristic related to micro-profile of its surface. A higher surface roughness usually resulted in the wider contact area with other materials such as biomolecules on the surface. If the surface is adhesive to the biomolecules, the rougher the surface will attract more biomolecules. However, the physical adsorption is relatively weak, limiting the net attracted biomolecules by resorption (re-desorption) process [10]. The other way of controlling the

In recent years, plasma treatment has been a common method to modify, activate, and functionalize the surface of polymers [12]. The plasma is the fourth state of a material, consisting of energetic atoms, ions, molecules, and radicals. Free electrons in the plasma maintain its quasi-neutral and equilibrium conditions. However, internally, the plasma has unique properties which can be designed and controlled for many purposes. Plasma surface treatment for polymers generally utilizes low-temperature plasma which is non-destructive to the bulk of the polymers. The low-temperature plasma is generated at low vacuum or better in the atmospheric environment. Another technique of lowering the temperature is by utilizing a low-frequency plasma, for example, a 40-kHz plasma [13]. A number of gases are usually

, =NH, and ≡N produce stationary polarities, result-

functional groups such as –OH, =O, –NH2

adsorption was by surface activation [11].

ing in hydrophilic property [9].

70 Coatings and Thin-Film Technologies

### **2.1. Synthesis and deposition of polystyrene thin film**

The polystyrene thin film was synthesized and deposited by a spin-coating method. Polystyrene raw material was dissolved into a polystyrene solution. Three raw materials, which were obtained from Sigma Aldrich with different molecular weights (MW), were utilized in this work, that is, 35, 129, and 280 kDa. This work utilized four different solvents to investigate the relation between the solvent and the effect of the plasma treatments. The solvents, that is, chloroform, toluene, xylene, and tetrahydrofuran, were also obtained from Sigma Aldrich. Each of the solutions was then deposited on a substrate by the spin-coating method. The specific quantity of the solution drop and rotation per minute was optimized and set for all the specimens. The polystyrene films were then heated in an oven after the deposition. The treatment was carried out to dry the excess solvent and to enhance interfacial bonding between the film and the substrate.

### **2.2. Plasma treatments**

A mini plasma reactor was utilized to treat the surface of the deposited film. Nitrogen gas was introduced into the reactor after evacuation procedure. A flow meter controlled the quantity of the gas during the plasma treatment. The plasma was generated by a 40-kHz AC power source. Only a small power of 40 watts was set for this 2-min' process. The schematic design of the plasma system is shown in **Figure 1**.

splitting in the Michelson interferometer [16]. Unlike the conventional profiler relying on a stylus which results in edging errors and damages, the interferometric system scans the surface vertically and records the data without mechanical contact. The interference patterns are produced when the differences in the path lengths between the measurement beam and the reference beam are nearly zero. During the vertical scanning, then a correlogram is recorded at each pixel in the camera. The correlogram or the interference signal is then processed into the three-dimensional surface profile. Based on the profile metrology, surface roughness,

Surface Modification of Polystyrene by Nitrogen Plasma Treatment

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73

Finally, the wettability of the specimens was observed and characterized by contact angle measurement. The specimen was placed on the observation stage of the system. The stage can be adjusted so that the surface of the specimen is perfectly aligned horizontally. A small droplet (30 μL) of pure water is usually used in the measurement procedure. A camera captures

Wettability is a critical property in the development of biosensors, especially during immobilization of biomolecules. The immobilization of the molecules by physical adsorption can be realized by a thin film of polystyrene. However, optimization should be performed since the thin film serves the immobilization as well as mechanical interfacing. Both the functions strongly affect the overall performance of the biosensor. Modification of the thin film's surface is one of the methods to enhance the immobilization. Controlling the wettability means designing the surface roughness, morphology, and microstructure. It is obvious that hydro-

This chapter discusses a technique of the plasma treatment on polystyrene thin film. It is desirable that the modification process does not affect the bulk of the thin film. Nitrogen plasma was utilized to modify the polystyrene surface. The characteristics and states of the plasma strongly control the reactions on the surface. Furthermore, the final result of the treatment depends also on the compositions, microstructures, and characteristics of the original material. The effects of the nitrogen plasma treatment on the polystyrene surface will be examined related to the original material produced with different raw materials' molecular weight and

**3.1. The effect of solvent during deposition process on the polystyrene surface and** 

The polystyrene film was deposited by means of the spin-coating method. Polystyrene raw material was in the form of chips or granules. The raw material was dissolved into a polymer solution before spin-coated on the substrates. The solvents used to make the solution were chosen by considering the Hansen solubility parameters (HSPs) for the polymer and the solvents. The HSP considers three interactions, that is, non-polar or dispersive interaction, polar

irregularity, and other topographic features can be determined.

the image of the droplet and sends it to a computer.

phobicity of surfaces is directly related to the roughness.

with the different solvents used during the deposition.

**microstructure**

**3. Results and discussions**

The character of the plasma was diagnosed with a spectrometer which has a range of 200–900 nm. Optical emission from the plasma was detected through a quartz window of the chamber. A fiber optic picks up and delivers the light to the spectrometer. The spectrometer was connected to a computer for data acquisition and processing.

### **2.3. Characterizations**

Observations and characterizations of the specimens were conducted before and after the plasma treatment. First, the specimen was characterized by means of a Fourier transform infrared (FTIR) spectrometer (*Aurora 4000*). The characterization identified functional groups of bondings, which specify the polystyrene microstructures.

The second measurement was the topographical measurement which was performed by an interferometric surface micro-profiler (*TMS 1200 Polytech TopMap-μLab*). The topographical measurement system provides non-destructive observation and measurement. The interferometric system employed a Mirau objective. The Mirau objective is basically a combination of a microscope objective lenses and a special interferometer called Mirau interferometer. The interferometer is a modification of the Michelson interferometer for the practical function in the objectives of the microscope. The key difference between the two interferometers is that the Mirau exploits parallel arrangement of beam splitting rather than perpendicular beam

**Figure 1.** A schematic design of plasma reactor for nitrogen plasma treatment.

splitting in the Michelson interferometer [16]. Unlike the conventional profiler relying on a stylus which results in edging errors and damages, the interferometric system scans the surface vertically and records the data without mechanical contact. The interference patterns are produced when the differences in the path lengths between the measurement beam and the reference beam are nearly zero. During the vertical scanning, then a correlogram is recorded at each pixel in the camera. The correlogram or the interference signal is then processed into the three-dimensional surface profile. Based on the profile metrology, surface roughness, irregularity, and other topographic features can be determined.

Finally, the wettability of the specimens was observed and characterized by contact angle measurement. The specimen was placed on the observation stage of the system. The stage can be adjusted so that the surface of the specimen is perfectly aligned horizontally. A small droplet (30 μL) of pure water is usually used in the measurement procedure. A camera captures the image of the droplet and sends it to a computer.

### **3. Results and discussions**

**2.2. Plasma treatments**

72 Coatings and Thin-Film Technologies

**2.3. Characterizations**

of the plasma system is shown in **Figure 1**.

was connected to a computer for data acquisition and processing.

of bondings, which specify the polystyrene microstructures.

**Figure 1.** A schematic design of plasma reactor for nitrogen plasma treatment.

A mini plasma reactor was utilized to treat the surface of the deposited film. Nitrogen gas was introduced into the reactor after evacuation procedure. A flow meter controlled the quantity of the gas during the plasma treatment. The plasma was generated by a 40-kHz AC power source. Only a small power of 40 watts was set for this 2-min' process. The schematic design

The character of the plasma was diagnosed with a spectrometer which has a range of 200–900 nm. Optical emission from the plasma was detected through a quartz window of the chamber. A fiber optic picks up and delivers the light to the spectrometer. The spectrometer

Observations and characterizations of the specimens were conducted before and after the plasma treatment. First, the specimen was characterized by means of a Fourier transform infrared (FTIR) spectrometer (*Aurora 4000*). The characterization identified functional groups

The second measurement was the topographical measurement which was performed by an interferometric surface micro-profiler (*TMS 1200 Polytech TopMap-μLab*). The topographical measurement system provides non-destructive observation and measurement. The interferometric system employed a Mirau objective. The Mirau objective is basically a combination of a microscope objective lenses and a special interferometer called Mirau interferometer. The interferometer is a modification of the Michelson interferometer for the practical function in the objectives of the microscope. The key difference between the two interferometers is that the Mirau exploits parallel arrangement of beam splitting rather than perpendicular beam Wettability is a critical property in the development of biosensors, especially during immobilization of biomolecules. The immobilization of the molecules by physical adsorption can be realized by a thin film of polystyrene. However, optimization should be performed since the thin film serves the immobilization as well as mechanical interfacing. Both the functions strongly affect the overall performance of the biosensor. Modification of the thin film's surface is one of the methods to enhance the immobilization. Controlling the wettability means designing the surface roughness, morphology, and microstructure. It is obvious that hydrophobicity of surfaces is directly related to the roughness.

This chapter discusses a technique of the plasma treatment on polystyrene thin film. It is desirable that the modification process does not affect the bulk of the thin film. Nitrogen plasma was utilized to modify the polystyrene surface. The characteristics and states of the plasma strongly control the reactions on the surface. Furthermore, the final result of the treatment depends also on the compositions, microstructures, and characteristics of the original material. The effects of the nitrogen plasma treatment on the polystyrene surface will be examined related to the original material produced with different raw materials' molecular weight and with the different solvents used during the deposition.

### **3.1. The effect of solvent during deposition process on the polystyrene surface and microstructure**

The polystyrene film was deposited by means of the spin-coating method. Polystyrene raw material was in the form of chips or granules. The raw material was dissolved into a polymer solution before spin-coated on the substrates. The solvents used to make the solution were chosen by considering the Hansen solubility parameters (HSPs) for the polymer and the solvents. The HSP considers three interactions, that is, non-polar or dispersive interaction, polar or dipole-dipole interaction, and hydrogen bonding interaction. The three parameters for the solvent and the solute are used to calculate the polymer solubility sphere, Ra <sup>4</sup> (δ<sup>D</sup> <sup>s</sup> <sup>−</sup> <sup>δ</sup><sup>D</sup> p)2 <sup>+</sup> (δ<sup>P</sup> <sup>s</sup> <sup>−</sup> <sup>δ</sup><sup>P</sup> p)2 <sup>+</sup> (δ<sup>H</sup> <sup>s</sup> <sup>−</sup> <sup>δ</sup><sup>H</sup> p)2 (1)

$$\mathbf{Ra} = \sqrt{4\left(\mathbf{\dot{\delta}\_{\rm D}}\mathbf{s} - \mathbf{\dot{\delta}\_{\rm D}}\mathbf{p}\right)^2 + \left(\mathbf{\dot{\delta}\_{\rm p}}\mathbf{s} - \mathbf{\dot{\delta}\_{\rm p}}\mathbf{p}\right)^2 + \left(\mathbf{\dot{\delta}\_{\rm H}}\mathbf{s} - \mathbf{\dot{\delta}\_{\rm H}}\mathbf{p}\right)^2} \tag{1}$$

where *δ<sup>D</sup> <sup>s</sup>* is the HSP for dispersive interaction of the solvent, *δ<sup>D</sup> <sup>p</sup>* is the HSP for dispersive interaction of the polymer, *δ<sup>P</sup> <sup>s</sup>* is the HSP for polar interaction of the solvent, *δ<sup>P</sup> <sup>p</sup>* is the HSP for polar interaction of the polymer, *δ<sup>H</sup> <sup>s</sup>* is the HSP for hydrogen bonding interaction of the solvent, and *δ<sup>H</sup> <sup>p</sup>* is the HSP for hydrogen bonding interaction of the polymer.

The possibility of the polymer to be dissolved in the solvent is predicted by comparing the Ra with the radius of the interaction of the polymer, Ro. The ratio (Ra/Ro), which is often called RED affinity number, should be less than 1. Smaller RED means that the polymer is easier to be dissolved in the solvent [17]. **Table 1** shows the HSP for the polystyrene and the four solvents [18]. The two last columns show the result of Ra and RED calculations based on the parameters and Ro = 12.7 for the polymer.

As previously described, this work examined four solvents. The results of spin-coating deposition of the polystyrene on a glass substrate with the variation of solvent were observed using SEM imaging as shown in **Figure 2**. The SEM micrograph of the deposited films noticeably shows that the different solvent resulted in the different morphology of the film. The difference is caused by variation in the solvent evaporation during the spin-coating process. The mechanism of the solvent evaporation is controlled by the solubility parameters and vapor pressure [19]. Koenhen and Smolders found that the solubility parameter was proportional to the vapor pressure [20]. Since the solubility parameter represents the cohesive energy, the higher the cohesive energy results in higher vapor pressure or vice versa. The solubility parameters of the solvents, that is, chloroform, tetrahydrofuran, toluene, and xylene, are 18.7, 18.5, 18.3, and 18.2 MPa1/2, respectively [21], while the pressure vapor of the above solvents is 669, 637, 577, and 562 mmHg, respectively. Measurements of the surface roughness of the polystyrene thin film produced using different solvents are shown in **Figure 3**. The axis represents different solvents sorted from its lowest pressure vapor to the highest one.


The result shows that all of the polystyrene specimens were hydrophobic since their contact

**Figure 3.** Graphical relationship between the solvent vapor pressure and surface roughness of the resulted polystyrene

**Figure 2.** SEM micrograph of polystyrene film produced with different solvents: (a) chloroform, (b) tetrahydrofuran, (c)

The molecule is non-polar which has a small electronegativity. This makes sense since the C and H atoms have similar electronegativity, that is, C = 2.25 and H = 2.20. On the other hand,

bond interconnected to a benzene ring (C6

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75

H5 ).

angles are above 90°. Polystyrene has a CH=CH<sup>2</sup>

toluene, and (d) xylene.

film.

The effect of the solvent on the wettability of the surface of the deposited polystyrene was observed by contact angle measurement. The results are shown in **Figure 4**.

**Table 1.** Relation of HSP and the RED number of solvents.

Surface Modification of Polystyrene by Nitrogen Plasma Treatment http://dx.doi.org/10.5772/intechopen.79716 75

or dipole-dipole interaction, and hydrogen bonding interaction. The three parameters for the

where *δ<sup>D</sup> <sup>s</sup>* is the HSP for dispersive interaction of the solvent, *δ<sup>D</sup> <sup>p</sup>* is the HSP for dispersive interaction of the polymer, *δ<sup>P</sup> <sup>s</sup>* is the HSP for polar interaction of the solvent, *δ<sup>P</sup> <sup>p</sup>* is the HSP for polar interaction of the polymer, *δ<sup>H</sup> <sup>s</sup>* is the HSP for hydrogen bonding interaction of the

The possibility of the polymer to be dissolved in the solvent is predicted by comparing the Ra with the radius of the interaction of the polymer, Ro. The ratio (Ra/Ro), which is often called RED affinity number, should be less than 1. Smaller RED means that the polymer is easier to be dissolved in the solvent [17]. **Table 1** shows the HSP for the polystyrene and the four solvents [18]. The two last columns show the result of Ra and RED calculations based on the

As previously described, this work examined four solvents. The results of spin-coating deposition of the polystyrene on a glass substrate with the variation of solvent were observed using SEM imaging as shown in **Figure 2**. The SEM micrograph of the deposited films noticeably shows that the different solvent resulted in the different morphology of the film. The difference is caused by variation in the solvent evaporation during the spin-coating process. The mechanism of the solvent evaporation is controlled by the solubility parameters and vapor pressure [19]. Koenhen and Smolders found that the solubility parameter was proportional to the vapor pressure [20]. Since the solubility parameter represents the cohesive energy, the higher the cohesive energy results in higher vapor pressure or vice versa. The solubility parameters of the solvents, that is, chloroform, tetrahydrofuran, toluene, and xylene, are 18.7, 18.5, 18.3, and 18.2 MPa1/2, respectively [21], while the pressure vapor of the above solvents is 669, 637, 577, and 562 mmHg, respectively. Measurements of the surface roughness of the polystyrene thin film produced using different solvents are shown in **Figure 3**. The axis rep-

resents different solvents sorted from its lowest pressure vapor to the highest one.

observed by contact angle measurement. The results are shown in **Figure 4**.

Polystyrene 21.3 5.8 4.3 —

Xylene 17.8 1.0 3.1 8.57 0.67 Toluene 18.0 1.4 2.0 8.26 0.65 THF 16.8 5.7 8.0 9.73 0.77 Chloroform 17.8 3.1 5.7 7.63 0.60

**(MPa)1/2 δ***<sup>p</sup>*

**Table 1.** Relation of HSP and the RED number of solvents.

The effect of the solvent on the wettability of the surface of the deposited polystyrene was

**(MPa)1/2 δ***<sup>h</sup>*

**(MPa)1/2 Ra RED**

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

<sup>4</sup> (δ<sup>D</sup> <sup>s</sup> <sup>−</sup> <sup>δ</sup><sup>D</sup> p)2 <sup>+</sup> (δ<sup>P</sup> <sup>s</sup> <sup>−</sup> <sup>δ</sup><sup>P</sup> p)2 <sup>+</sup> (δ<sup>H</sup> <sup>s</sup> <sup>−</sup> <sup>δ</sup><sup>H</sup> p)2 (1)

solvent and the solute are used to calculate the polymer solubility sphere, Ra

solvent, and *δ<sup>H</sup> <sup>p</sup>* is the HSP for hydrogen bonding interaction of the polymer.

Ra = √

74 Coatings and Thin-Film Technologies

parameters and Ro = 12.7 for the polymer.

**Material δ***<sup>d</sup>*

**Figure 2.** SEM micrograph of polystyrene film produced with different solvents: (a) chloroform, (b) tetrahydrofuran, (c) toluene, and (d) xylene.

**Figure 3.** Graphical relationship between the solvent vapor pressure and surface roughness of the resulted polystyrene film.

The result shows that all of the polystyrene specimens were hydrophobic since their contact angles are above 90°. Polystyrene has a CH=CH<sup>2</sup> bond interconnected to a benzene ring (C6 H5 ). The molecule is non-polar which has a small electronegativity. This makes sense since the C and H atoms have similar electronegativity, that is, C = 2.25 and H = 2.20. On the other hand,

Both of the two processes can result in modification of the surface roughness. The changes of the surface roughness depend on the initial state of the specimen which was controlled by the solvent used in the deposition process. **Figure 6** shows the topographical measurement of the polystyrene specimen produced with THF solvent before and after the plasma treatment. The surface roughness of the polystyrene specimen before and after treatment was 637 and 535 nm. Evidently, the plasma treatment decreases the surface roughness of all the specimens

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77

It can be seen from **Figure 7** that the effect of the treatment greatly reduces the surface roughness by 107 nm in average except for the polystyrene produced with xylene solvent. The changes of the surface were firstly caused by the physical process, where ion and atom bombardments eroded the polystyrene surface. The threshed particles were then redeposited on

**Figure 6.** Surface profile of polystyrene thin film produced with THF solvent before and after the plasma treatment.

the surface filling pits. Illustratively, the mechanism is described in **Figure 8**.

produced with different solvents as shown in **Figure 7**.

**Figure 5.** Nitrogen plasma spectrum.

**Figure 4.** The effect of solvent on wettability represented by contact angle.

the polar water (H2 O) molecule results in strong cohesive forces. The adhesive interaction between the polystyrene surface and water then is weaker than the cohesive water interaction.

Although the specimens were hydrophobic, the contact angle measurements showed differences related to the variation of solvent utilized during the deposition process. Considering the phenomena described in **Figure 3** where the variation of the solvent significantly controlled the surface roughness, it can be seen that the difference of the wettability of the polystyrene depends on the surface roughness. Polystyrene thin film which produced using chloroform showed the highest surface roughness and surface contact angle, while the one produced using xylene showed the lowest.

### **3.2. The effect of nitrogen plasma treatment on the surface roughness and wettability of polystyrene produced with different solvents**

The original hydrophobic property of the polystyrene film can be modified by a plasma treatment. This section discusses the effect of the treatment on the surface roughness and wettability of the polystyrene specimens produced with various solvents. As discussed earlier, plasma treatment is one of the effective methods to modify polymer surfaces. The plasma used in this work is the nitrogen plasma. The generated plasma was monitored using optical emission spectroscopy (OES) technique. **Figure 5** shows a nitrogen plasma spectrum generated by our system as discussed in the previous section.

Typically, the plasma consists of some species such as an N2 + ion, energetic N2, and NH radical as identified in the spectrum [22]. The existence of the NH was due to the small percentage of hydrogen gas left during the evacuation procedure. The surface modification can be controlled by physical and/or chemical processes. The physical process is effected by ion and atomic bombardment without any formation of new compounds. The ions and atoms will bounce back accompanied by some ejected ions and atoms from the thin film. On the other hand, the chemical processes are initiated by ions or radicals which significantly change the microstructure of the polystyrene.

Both of the two processes can result in modification of the surface roughness. The changes of the surface roughness depend on the initial state of the specimen which was controlled by the solvent used in the deposition process. **Figure 6** shows the topographical measurement of the polystyrene specimen produced with THF solvent before and after the plasma treatment.

The surface roughness of the polystyrene specimen before and after treatment was 637 and 535 nm. Evidently, the plasma treatment decreases the surface roughness of all the specimens produced with different solvents as shown in **Figure 7**.

It can be seen from **Figure 7** that the effect of the treatment greatly reduces the surface roughness by 107 nm in average except for the polystyrene produced with xylene solvent. The changes of the surface were firstly caused by the physical process, where ion and atom bombardments eroded the polystyrene surface. The threshed particles were then redeposited on the surface filling pits. Illustratively, the mechanism is described in **Figure 8**.

**Figure 5.** Nitrogen plasma spectrum.

the polar water (H2

76 Coatings and Thin-Film Technologies

using xylene showed the lowest.

system as discussed in the previous section.

microstructure of the polystyrene.

O) molecule results in strong cohesive forces. The adhesive interaction

+

ion, energetic N2, and NH radical

between the polystyrene surface and water then is weaker than the cohesive water interaction.

Although the specimens were hydrophobic, the contact angle measurements showed differences related to the variation of solvent utilized during the deposition process. Considering the phenomena described in **Figure 3** where the variation of the solvent significantly controlled the surface roughness, it can be seen that the difference of the wettability of the polystyrene depends on the surface roughness. Polystyrene thin film which produced using chloroform showed the highest surface roughness and surface contact angle, while the one produced

The original hydrophobic property of the polystyrene film can be modified by a plasma treatment. This section discusses the effect of the treatment on the surface roughness and wettability of the polystyrene specimens produced with various solvents. As discussed earlier, plasma treatment is one of the effective methods to modify polymer surfaces. The plasma used in this work is the nitrogen plasma. The generated plasma was monitored using optical emission spectroscopy (OES) technique. **Figure 5** shows a nitrogen plasma spectrum generated by our

as identified in the spectrum [22]. The existence of the NH was due to the small percentage of hydrogen gas left during the evacuation procedure. The surface modification can be controlled by physical and/or chemical processes. The physical process is effected by ion and atomic bombardment without any formation of new compounds. The ions and atoms will bounce back accompanied by some ejected ions and atoms from the thin film. On the other hand, the chemical processes are initiated by ions or radicals which significantly change the

**3.2. The effect of nitrogen plasma treatment on the surface roughness and** 

**wettability of polystyrene produced with different solvents**

**Figure 4.** The effect of solvent on wettability represented by contact angle.

Typically, the plasma consists of some species such as an N2

**Figure 6.** Surface profile of polystyrene thin film produced with THF solvent before and after the plasma treatment.

cos*θ<sup>m</sup>* = *rcos θ<sup>y</sup>* (2)

equation. The roughness parameter *r*, which is larger than 1, can be calculated by taking the

Results of the contact angle measurements of the polystyrene film specimens show that all of them became hydrophilic after the plasma treatment. The relation between the contact angle and the variation of solvent is shown in **Figure 9**. It can be seen from **Figures 7** and **9** that the decrease in surface roughness decreases the contact angle. The variation of solvent during the thin film production clearly affects the change of the wettability. The observation indicates that the effect of the solvent on the treatment and the resulted surface wettability were also influenced by the chemical process. Nitrogen radicals in the plasma induce reactions on the surface of polymers. The reactions break some hydrogen bonds of the polystyrene molecules, resulting in the reactive surface. The neighboring reactive sites may produce new functional groups. Indeed, the functional group was observed and identified by means of FTIR spectroscopy (**Figure 10**). The infrared spectra in **Figure 10** show the effect of the plasma treatment noticeably at wave number between 2300 and 2400 cm−1. Two strong peaks appear after the plasma treatment. The two peaks were related to a complex C ≡ N bond [24]. The existence of the two stretch vibrations reflects the effect of the carbon or hydrogen environment to the carbon-nitrogen bonding. The carbon-nitrogen bonding is polar, which contributes to the wettability of the

ratio of the actual area of the polymer surface to the normally projected area.

**3.3. The effect of nitrogen plasma treatment on the surface roughness and** 

**Figure 9.** Changes of the contact angle of specimens before and after plasma treatment.

**wettability of polystyrene produced with different raw materials' molecular weight**

Further study, related to the contributions of the surface roughness and the functional group to the wettability due to plasma treatment, was carried out by observing the phenomena on

is the contact angle calculated from the Young

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79

Surface Modification of Polystyrene by Nitrogen Plasma Treatment

where *θ***m** is the effective contact angle and *θ***<sup>y</sup>**

polystyrene surface by polar-to-polar interaction.

**Figure 7.** Comparison of the decrease in surface roughness of the polystyrene produced with different solvents before and after the plasma treatment.

**Figure 8.** Illustration of the surface process during the plasma treatment of the polystyrene thin film.

The effects of surface roughness on the wettability have been studied for long. Wenzel's law, for example, concludes that the surface roughness amplifies the surface wetting property where the hydrophobic surface becomes more hydrophobic and hydrophilic surface becomes more hydrophilic [23]. The law was constructed by relating the macroscopic Young's and the effective contact angles. Furthermore, it should be assumed that the liquid has full contact with the rough surface. The expression of Wenzel's law is written with the equation:

$$\cos \theta\_w = r \cos \theta\_y \tag{2}$$

where *θ***m** is the effective contact angle and *θ***<sup>y</sup>** is the contact angle calculated from the Young equation. The roughness parameter *r*, which is larger than 1, can be calculated by taking the ratio of the actual area of the polymer surface to the normally projected area.

Results of the contact angle measurements of the polystyrene film specimens show that all of them became hydrophilic after the plasma treatment. The relation between the contact angle and the variation of solvent is shown in **Figure 9**. It can be seen from **Figures 7** and **9** that the decrease in surface roughness decreases the contact angle. The variation of solvent during the thin film production clearly affects the change of the wettability. The observation indicates that the effect of the solvent on the treatment and the resulted surface wettability were also influenced by the chemical process. Nitrogen radicals in the plasma induce reactions on the surface of polymers. The reactions break some hydrogen bonds of the polystyrene molecules, resulting in the reactive surface. The neighboring reactive sites may produce new functional groups. Indeed, the functional group was observed and identified by means of FTIR spectroscopy (**Figure 10**).

The infrared spectra in **Figure 10** show the effect of the plasma treatment noticeably at wave number between 2300 and 2400 cm−1. Two strong peaks appear after the plasma treatment. The two peaks were related to a complex C ≡ N bond [24]. The existence of the two stretch vibrations reflects the effect of the carbon or hydrogen environment to the carbon-nitrogen bonding. The carbon-nitrogen bonding is polar, which contributes to the wettability of the polystyrene surface by polar-to-polar interaction.

### **3.3. The effect of nitrogen plasma treatment on the surface roughness and wettability of polystyrene produced with different raw materials' molecular weight**

Further study, related to the contributions of the surface roughness and the functional group to the wettability due to plasma treatment, was carried out by observing the phenomena on

**Figure 9.** Changes of the contact angle of specimens before and after plasma treatment.

The effects of surface roughness on the wettability have been studied for long. Wenzel's law, for example, concludes that the surface roughness amplifies the surface wetting property where the hydrophobic surface becomes more hydrophobic and hydrophilic surface becomes more hydrophilic [23]. The law was constructed by relating the macroscopic Young's and the effective contact angles. Furthermore, it should be assumed that the liquid has full contact

**Figure 7.** Comparison of the decrease in surface roughness of the polystyrene produced with different solvents before

and after the plasma treatment.

78 Coatings and Thin-Film Technologies

with the rough surface. The expression of Wenzel's law is written with the equation:

**Figure 8.** Illustration of the surface process during the plasma treatment of the polystyrene thin film.

**Figure 10.** Infrared spectra of untreated (red) and plasma-treated (blue) polystyrene thin film.

polystyrene thin film deposited with different raw materials' molecular weight. Polystyrene has various molecular weights (MW) and its related polymerization degree [25]. The variation of the polystyrene's molecule weight used in the thin-film production produces various surface properties as well. The surface roughness and the wettability will also be affected. During this study, three kinds of raw materials, which have different molecular weights, that is, 35, 129, and 208 kDa, were used to produce the polystyrene thin-film specimen. Surface profiles of the specimens, which were characterized using the topographical measurement, are shown in **Figure 11**.

After the plasma treatment, the surface roughness and the contact angle were greatly decreased. The decrease of the surface roughness was larger for the larger Mw, but the decrease of the contact angle was smaller for the larger Mw. The plasma treatment has less effect on changing the wettability of the larger Mw. While the physical ablation greatly changes the surface roughness, the chemical surface reactions seem to control the change of morphology and

**Figure 11.** Surface profile of polystyrene with different molecular weights before (a) and after (b) plasma treatment.

FTIR characterization of the plasma-treated specimens, which are shown in **Figure 13**, revealed the change of the two peaks between 2300 and 2400 cm−1. The peaks correspond to the polar C≡N complex group. The absorbance and the integrated area of the peaks increase with the increase of the Mw. The integrated area shows that the surface concentration of the complex or nitrile complex of the polystyrene thin film increases related to the higher MW. The breaking of polymer chains and of the hydrogen bonds attracts the nitrogen ions to the

Beside the FTIR spectra, the OES data can also be used to confirm the surface reactions by observing the change of the plasma state and species [27]. The plasma generator used in this work is a low-frequency 40-kHz AC. There have been a number of researchers who studied the application of the low-frequency plasma, especially in polymer treatment [13]. Lowerfrequency plasma produces a higher ion density and a lower temperature [28]. This condition is beneficial for polymers having a low melting point. **Figure 14** shows the optical emission

+

It can be seen from **Figure 15** that the intensities of the peaks at 387.8 and 424.1 nm decrease with the increase of the polystyrene Mw. The peaks are associated to the existence of the N2

intensities change with the variation of MW.

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81

+

microstructure of the surface.

structure and creates new microstructures.

of the nitrogen plasma at the range where the N2

Observations during the film production showed that larger molecular weight polystyrene made the polymer precursor solution more viscous. **Figure 11a** shows that larger MW resulted in a higher surface roughness. The thicker film was also apparently related to the higher roughness. The molecular weight affects the kinetic stabilization rate of the polystyrene during the deposition [26]. Together with the vaporization rate, the deposition mechanism results in various morphologies and microstructures.

As described in the previous section, the plasma treatment induced the physical and chemical reactions on the surface of the polystyrene. The physical ablation of the surface has a different effect on different weight molecules. The effect of the plasma treatment on the surface roughness is shown in **Figure 11b**. A greater effect was observed on the specimen with a larger molecular weight, that is, 97 nm.

The wettability represented by the contact angle of the untreated and plasma-treated specimens with different MW is shown in **Figure 12**.

The contact angle measurement on untreated specimens shows that the increase in surface roughness due to the molecular weight is followed by only a small change in the contact angle. This indicates that the variation of surface roughness in the range of 300 to around 500 nm has a small effect on the wettability.

Surface Modification of Polystyrene by Nitrogen Plasma Treatment http://dx.doi.org/10.5772/intechopen.79716 81

**Figure 11.** Surface profile of polystyrene with different molecular weights before (a) and after (b) plasma treatment.

polystyrene thin film deposited with different raw materials' molecular weight. Polystyrene has various molecular weights (MW) and its related polymerization degree [25]. The variation of the polystyrene's molecule weight used in the thin-film production produces various surface properties as well. The surface roughness and the wettability will also be affected. During this study, three kinds of raw materials, which have different molecular weights, that is, 35, 129, and 208 kDa, were used to produce the polystyrene thin-film specimen. Surface profiles of the specimens, which were characterized using the topographical measurement,

**Figure 10.** Infrared spectra of untreated (red) and plasma-treated (blue) polystyrene thin film.

Observations during the film production showed that larger molecular weight polystyrene made the polymer precursor solution more viscous. **Figure 11a** shows that larger MW resulted in a higher surface roughness. The thicker film was also apparently related to the higher roughness. The molecular weight affects the kinetic stabilization rate of the polystyrene during the deposition [26]. Together with the vaporization rate, the deposition mechanism results

As described in the previous section, the plasma treatment induced the physical and chemical reactions on the surface of the polystyrene. The physical ablation of the surface has a different effect on different weight molecules. The effect of the plasma treatment on the surface roughness is shown in **Figure 11b**. A greater effect was observed on the specimen with a larger

The wettability represented by the contact angle of the untreated and plasma-treated speci-

The contact angle measurement on untreated specimens shows that the increase in surface roughness due to the molecular weight is followed by only a small change in the contact angle. This indicates that the variation of surface roughness in the range of 300 to around

are shown in **Figure 11**.

80 Coatings and Thin-Film Technologies

in various morphologies and microstructures.

mens with different MW is shown in **Figure 12**.

500 nm has a small effect on the wettability.

molecular weight, that is, 97 nm.

After the plasma treatment, the surface roughness and the contact angle were greatly decreased. The decrease of the surface roughness was larger for the larger Mw, but the decrease of the contact angle was smaller for the larger Mw. The plasma treatment has less effect on changing the wettability of the larger Mw. While the physical ablation greatly changes the surface roughness, the chemical surface reactions seem to control the change of morphology and microstructure of the surface.

FTIR characterization of the plasma-treated specimens, which are shown in **Figure 13**, revealed the change of the two peaks between 2300 and 2400 cm−1. The peaks correspond to the polar C≡N complex group. The absorbance and the integrated area of the peaks increase with the increase of the Mw. The integrated area shows that the surface concentration of the complex or nitrile complex of the polystyrene thin film increases related to the higher MW. The breaking of polymer chains and of the hydrogen bonds attracts the nitrogen ions to the structure and creates new microstructures.

Beside the FTIR spectra, the OES data can also be used to confirm the surface reactions by observing the change of the plasma state and species [27]. The plasma generator used in this work is a low-frequency 40-kHz AC. There have been a number of researchers who studied the application of the low-frequency plasma, especially in polymer treatment [13]. Lowerfrequency plasma produces a higher ion density and a lower temperature [28]. This condition is beneficial for polymers having a low melting point. **Figure 14** shows the optical emission of the nitrogen plasma at the range where the N2 + intensities change with the variation of MW.

It can be seen from **Figure 15** that the intensities of the peaks at 387.8 and 424.1 nm decrease with the increase of the polystyrene Mw. The peaks are associated to the existence of the N2 +

ions which undergo B<sup>2</sup>

e + *N*<sup>2</sup>

which originally come from the N2

the concentration of the N2

CH=CH<sup>2</sup>

plasma treatment.

**4. Conclusions**

Σu→X2

+

Σu transitions (Δν = 0 at 387.8 nm and Δν = 1 at 424.1 nm). In the

Surface Modification of Polystyrene by Nitrogen Plasma Treatment

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ions, have to be withdrawn from the plasma. This makes

) (3)

H5

+

+

ions in the

83

ions leads to


. Depending on the

<sup>+</sup> (B, ν′

ions to decrease with the increase of the polystyrene Mw.

plasma, the ions are firstly produced by a complex ionization and excitation, resulting in the

(X, 0) → e + e + *N*<sup>2</sup>

the incorporation of the nitrogen into the polystyrene network. The larger molecular weight of the polystyrene, the more nitrogen atoms are needed in the network. The nitrogen atoms,

Based on the FTIR and the OES spectra, a hypothetical model to predict the bond breaking

When the nitrogen ions or radicals reach the surface of the polystyrene, a number of reactions

ions and radicals energy, the substitution reactions can either break or combine the polymer chain. The larger MW polystyrene results in the larger number of nitrogen incorporated by the

This work concludes the possibility of nitrogen plasma treatment to control the wettability of polystyrene surface. In general, the plasma effectively reduced the contact angle and hence increased the wettability or reduced the hydrophobicity of the surface. The solvents utilized during the deposition procedure affected the surface roughness of the resulted thin film. A solvent with a higher HSP or a vapor pressure produced the film with a higher surface roughness. The plasma treatment greatly reduced the surface roughness by 107 nm in average. The contact angle of the surface after the treatment drastically reduced, depending on the surface

. The polymerization separates the double bond into –CH2, resulting in a chain of the

first negative band transitions. The simplified reaction can be seen as follows:

**Figure 15.** A hypothetical model of the molecular restructuring of polystyrene by nitrogen incorporation.

The decrease of the peak indicates the reduction of the concentration of the N2

plasma which is related to the surface reaction. The reaction controlled by the N2

+

and the restructuring of the polystyrene surface is shown in **Figure 15**.

can take place. The styrene monomer is composed of an aromatic ring of C6

polystyrene. The energetic nitrogen ions or radicals substitute the –CH2

**Figure 12.** Contact angle of untreated and plasma treated polystyrene surface with different molecular weights.

**Figure 13.** Stretching band of C≡N complex appearing after the plasma treatment.

**Figure 14.** The effect of polystyrene Mw on the plasma optical emission spectrum during the treatment.

**Figure 15.** A hypothetical model of the molecular restructuring of polystyrene by nitrogen incorporation.

ions which undergo B<sup>2</sup> Σu→X2 Σu transitions (Δν = 0 at 387.8 nm and Δν = 1 at 424.1 nm). In the plasma, the ions are firstly produced by a complex ionization and excitation, resulting in the first negative band transitions. The simplified reaction can be seen as follows:

$$\mathbf{e} \star \mathrm{N}\_{\mathrm{2}}(\mathrm{X}, \mathrm{0}) \rightarrow \mathbf{e} \star \mathbf{e} \star \mathrm{N}\_{\mathrm{2}}^{\*}(\mathrm{B}, \mathrm{v} \big) \tag{3}$$

The decrease of the peak indicates the reduction of the concentration of the N2 + ions in the plasma which is related to the surface reaction. The reaction controlled by the N2 + ions leads to the incorporation of the nitrogen into the polystyrene network. The larger molecular weight of the polystyrene, the more nitrogen atoms are needed in the network. The nitrogen atoms, which originally come from the N2 + ions, have to be withdrawn from the plasma. This makes the concentration of the N2 + ions to decrease with the increase of the polystyrene Mw.

Based on the FTIR and the OES spectra, a hypothetical model to predict the bond breaking and the restructuring of the polystyrene surface is shown in **Figure 15**.

When the nitrogen ions or radicals reach the surface of the polystyrene, a number of reactions can take place. The styrene monomer is composed of an aromatic ring of C6 H5 - and a vinyl – CH=CH<sup>2</sup> . The polymerization separates the double bond into –CH2, resulting in a chain of the polystyrene. The energetic nitrogen ions or radicals substitute the –CH2 . Depending on the ions and radicals energy, the substitution reactions can either break or combine the polymer chain. The larger MW polystyrene results in the larger number of nitrogen incorporated by the plasma treatment.

### **4. Conclusions**

**Figure 14.** The effect of polystyrene Mw on the plasma optical emission spectrum during the treatment.

**Figure 13.** Stretching band of C≡N complex appearing after the plasma treatment.

82 Coatings and Thin-Film Technologies

**Figure 12.** Contact angle of untreated and plasma treated polystyrene surface with different molecular weights.

This work concludes the possibility of nitrogen plasma treatment to control the wettability of polystyrene surface. In general, the plasma effectively reduced the contact angle and hence increased the wettability or reduced the hydrophobicity of the surface. The solvents utilized during the deposition procedure affected the surface roughness of the resulted thin film. A solvent with a higher HSP or a vapor pressure produced the film with a higher surface roughness. The plasma treatment greatly reduced the surface roughness by 107 nm in average. The contact angle of the surface after the treatment drastically reduced, depending on the surface roughness before treatment. Further results of the plasma treatment on the film produced with various molecular weights revealed that the plasma treatment has more effect on changing the surface roughness but less effect on changing the wettability of the larger molecular weight. This indicates that the plasma treatment resulted in the modification of the surface microstructure and morphology beside the surface roughness. The modification was controlled by the incorporation of the nitrogen from the plasma into the polystyrene's molecules.

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### **Acknowledgements**

We would like to thank Dr. Ing. Setyawan P. Sakti of Sensors Laboratory at Brawijaya University for providing us the contact angle measurement. We are also grateful to members of the Collaborative Research Center for Advanced Systems and Material Technology at Brawijaya for laboratory works and fruitful discussions.

### **Author details**

Masruroh\* and Dionysius J.D.H. Santjojo

\*Address all correspondence to: ruroh@ub.ac.id

Department of Physics, Brawijaya University, Malang, Indonesia

### **References**


roughness before treatment. Further results of the plasma treatment on the film produced with various molecular weights revealed that the plasma treatment has more effect on changing the surface roughness but less effect on changing the wettability of the larger molecular weight. This indicates that the plasma treatment resulted in the modification of the surface microstructure and morphology beside the surface roughness. The modification was controlled by the incorporation of the nitrogen from the plasma into the polystyrene's molecules.

We would like to thank Dr. Ing. Setyawan P. Sakti of Sensors Laboratory at Brawijaya University for providing us the contact angle measurement. We are also grateful to members of the Collaborative Research Center for Advanced Systems and Material Technology at

**Acknowledgements**

84 Coatings and Thin-Film Technologies

**Author details**

**References**

Brawijaya for laboratory works and fruitful discussions.

Masruroh\* and Dionysius J.D.H. Santjojo

\*Address all correspondence to: ruroh@ub.ac.id

and Coatings Technology. 2013;**233**:99-107

**16**(9-12):1101-1108

**03**(03):3-7

Department of Physics, Brawijaya University, Malang, Indonesia

[1] Lynwood C. Polystyrene: Synthesis, Characteristics and Applications; 2014

[2] Mikac L et al. Surface-enhanced Raman spectroscopy substrate based on Ag-coated selfassembled polystyrene spheres. Journal of Molecular Structure. 2017;**1146**:530-535 [3] Browne MM, Lubarsky GV, Davidson MR, Bradley RH. Protein adsorption onto polystyrene surfaces studied by XPS and AFM. Surface Science. 2004;**553**(1-3):155-167

[4] Yoshida S, Hagiwara K, Hasebe T, Hotta A. Surface modification of polymers by plasma treatments for the enhancement of biocompatibility and controlled drug release. Surface

[5] Sakti SP, Lucklum R, Hauptmann P, Bühling F, Ansorge S. Disposable TSM-biosensor based on viscosity changes of the contacting medium. Biosensors & Bioelectronics. 2001;

[6] Sakti SP, Santjojo DJDH. Improvement of biomolecule immobilization on polystyrene surface by increasing surface roughness. Journal of Biosensors and Bioelectronics. 2012;


[21] Burke J. Solubility Parameters: Theory and Application. The Book and Paper Group: Annual; 1984. [Online]. Available from: http://cool.conservation-us.org/coolaic/sg/bpg/ annual/v03/bp03-04.html [Accessed: 16-Apr-2018]

**Chapter 5**

**Provisional chapter**

**Crack Resistance of Paint Coatings, Cement Concretes**

Information on the method for assessing the crack resistance of paint and varnish coatings is given. A method is proposed, based on the ratio between the length of the crack, the Vickers indenter imprint, and the fracture toughness. Numerical values and stress intensity factor in coatings are given, depending on the type and duration of aging, porosity of the cement substrate, and the quality of the appearance of the coatings. It is established that with an increase in the roughness of the coating surface, the value of the stress intensity coefficient is increased. The increase in the moisture content of the substrate at the time of application of the paint composition leads to the appearance of a more defective structure of the contact layer "coating-substrate" and a greater propensity to cracking. It is revealed that there is a value of the optimum substrate moisture, for each particular coating in terms of its fracture toughness. It is shown that during the aging of protective and decorative coatings of the exterior walls of buildings, a mechanism of their

**Keywords:** coatings, crack resistance fracture intensity factor, failure mechanism

Building and maintaining the working condition of buildings and structures require a large number of paint and varnish compositions. Growing competition in the market of finishing materials, increasing demands of consumers require manufacturers to obtain high-quality painted surfaces. However, the practice of finishing works shows that often the quality of the

Coatings for finishing facades of buildings must have a high-quality appearance. The conditions for obtaining and the quality of the appearance of cured inking coatings largely depend

finish is bad. It leads to premature unscheduled repairs and additional costs.

**Crack Resistance of Paint Coatings, Cement Concretes**

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.78537

Valentina Loganina

Valentina Loganina

**Abstract**

**1. Introduction**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

destruction from elastic ductile to brittle changes occurs.

http://dx.doi.org/10.5772/intechopen.78537


#### **Crack Resistance of Paint Coatings, Cement Concretes Crack Resistance of Paint Coatings, Cement Concretes**

DOI: 10.5772/intechopen.78537

#### Valentina Loganina Valentina Loganina

[21] Burke J. Solubility Parameters: Theory and Application. The Book and Paper Group: Annual; 1984. [Online]. Available from: http://cool.conservation-us.org/coolaic/sg/bpg/

[22] Shah M, Ahmad R, Ikhlaq U, Saleem S. Characterization of pulsed DC nitrogen plasma using optical emission and Langmuir probe. Journal of Natural Sciences and Mathe-

[23] Wenzel RN. Resistance of solid surfaces to wetting by water. Industrial and Engineering

[24] Liu X, Zhao J, Yang R, Iervolino R, Barbera S. A novel in-situ aging evaluation method by FTIR and the application to thermal oxidized nitrile rubber. Polymer Degradation and

[25] Gao H, Konstantinov IA, Arturo SG, Broadbelt LJ. On the modeling of number and weight average molecular weight of polymers. Chemical Engineering Journal. 2017;**327**:906-913

[26] Luo S et al. Molecular weight and interfacial effect on the kinetic stabilization of ultra-

[27] Thiry D, Konstantinidis S, Cornil J, Snyders R. Plasma diagnostics for the low-pressure plasma polymerization process: A critical review. Thin Solid Films. 2016;**606**:19-44 [28] Xu X, Ge H, Wang S, Dai Z, Wang Y, Zhu A. Influence of the low-frequency source parameters on the plasma characteristics in a dual frequency capacitively coupled plasma reactor: Two dimensional simulations. Progress in Natural Science. 2009;**19**(6):677-684

thin polystyrene films. Polymer. (United Kingdom). 2018;**134**:204-210

annual/v03/bp03-04.html [Accessed: 16-Apr-2018]

matics. 2014;**53**(2013):1-12

86 Coatings and Thin-Film Technologies

Stability. 2016;**128**:99-106

Chemistry. 1936;**28**(8):988-994

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.78537

**Abstract**

Information on the method for assessing the crack resistance of paint and varnish coatings is given. A method is proposed, based on the ratio between the length of the crack, the Vickers indenter imprint, and the fracture toughness. Numerical values and stress intensity factor in coatings are given, depending on the type and duration of aging, porosity of the cement substrate, and the quality of the appearance of the coatings. It is established that with an increase in the roughness of the coating surface, the value of the stress intensity coefficient is increased. The increase in the moisture content of the substrate at the time of application of the paint composition leads to the appearance of a more defective structure of the contact layer "coating-substrate" and a greater propensity to cracking. It is revealed that there is a value of the optimum substrate moisture, for each particular coating in terms of its fracture toughness. It is shown that during the aging of protective and decorative coatings of the exterior walls of buildings, a mechanism of their destruction from elastic ductile to brittle changes occurs.

**Keywords:** coatings, crack resistance fracture intensity factor, failure mechanism

### **1. Introduction**

Building and maintaining the working condition of buildings and structures require a large number of paint and varnish compositions. Growing competition in the market of finishing materials, increasing demands of consumers require manufacturers to obtain high-quality painted surfaces. However, the practice of finishing works shows that often the quality of the finish is bad. It leads to premature unscheduled repairs and additional costs.

Coatings for finishing facades of buildings must have a high-quality appearance. The conditions for obtaining and the quality of the appearance of cured inking coatings largely depend

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

on the rheological properties of paint and are determined by the processes of wetting and application of paint. In literature, the quality of the appearance of coatings on metal substrates is studied. When selecting paint systems for concrete, the features of this material (strength, roughness, humidity, alkalinity, etc.) should be taken into account. Features of concrete affect the quality coating.

In this regard, the development of a methodology for ensuring the quality of the painted surface of building products and structures and control methods is an important scientific, technical, and economic problem. The solution of this problem as a whole will contribute to an increase in the service life of protective and decorative coatings. One of the common types of destruction of coatings of cement concrete is the appearance of cracks. There are known works [1–8] in which the regularities of the appearance of cracks in coatings on metal substrates are described. However, the resistance of coatings of cement concrete has not been studied sufficiently.

### **2. Stress state of coatings under exposure operational factors**

For analysis, the reasons for the destruction of the paint and varnish facades of buildings were used, the Pareto diagram, allowing for a variety of existing defects to distinguish those that make a significant contribution to the assessment of the quality of the appearance of coatings. When analyzing the Pareto chart, the 80/20 rule was used. For example, priority factors were identified, which fall into 80% of the cumulative curve [9–16]. The survey was carried out on residential 5-storey houses, which are located in accordance with GOST 9.039–74 "Corrosive aggressiveness of the environment" in the climatic region IY (moderately cold) and GOST 16350–80 "Climate of the USSR. Regionalizing and statistical parameters of climatic factors for technical purposes." The facades were painted with calcareous and cement per chlorinated vinyl CPCV paints. Colorful compositions were applied to the plaster thickness of 1.5–2 cm. When inspecting the painted surface, the following types of defects were detected: cracking, flaking, weathering, dirt retention of coatings, wet spots, and different tonality. The number and types of defects were determined visually (GOST 9.407–2015 Unified system of protection against corrosion and aging. PAINTWOOD COATINGS. Appraisal method). The names of the types of defects and the number of their appearances for calcareous and CPCV coatings that take after different service lives are given in **Table 1**.

Pareto diagrams with all types of defects for their calcareous CPCV coatings are shown in **Figures 1**–**3**. Analysis of survey results indicates that the list of defects in coatings, constituting 80% of the cumulative curve, consists mainly of cracks along the vertical joint at the end of the building, different tonality of color, and peeling. All of these factors remain constant for both coatings. This allows us to consider them a source of "failure" regardless of the type of coating. In this case, such a defect as cracks in the coating along the vertical joint of the panels goes in the Pareto diagram in the first place and is 22.6–66.6% of the total number of defects, depending on the type of coverage and exploitation term.

after 3 years of operation of the calcareous coating, defects such as flaking of the coatings at the base of the metal roofs of the porch and at the ends of the fencing panels of the loggias are observed, the total specific gravity of which is 41.4%. The priority defects after 5 years of operation include cracks in the coating, along the vertical joint at the end of the building, exfoliation coating, and different tonality of color. A similar list of defects is also characteristic for CPCV coatings. Of this follows that the efforts of all specialists in developing the formulation of building paint compositions, the technology of their application should be aimed at increasing the crack resistance of protective-decorative coatings, since this type of defect is the

most characteristic and widespread.

**Figure 1.** Pareto chart for lime coating (1 year of operation).

**№ defect**

1 Crack in the coating along vertical joint at the end of the building

2 Flaking of coatings on the ends enclosing panels of loggias

3 Flaking of coatings at the base of metal roofs of entrances

6 Wet spots at the bottom joint of balconies with panels

**Defect denomination Numeric appearance of defects**

4 Flaking of coatings on the facade 3 72 35 5 Different tonality of coloring 2 31 30

7 Weathering of the color — 25 — 8 Other 11 12 25

**Table 1.** Types and number of defects of protective and decorative coatings.

**Calcareous coating, 1 year of operation**

**Calcareous coating, 5 years of operation**

Crack Resistance of Paint Coatings, Cement Concretes http://dx.doi.org/10.5772/intechopen.78537

80 100 100

21 100 80

13 84 80

— 26 30

**CPCV coating, 5 years of operation** 89

As the coatings age, there is a change in the specific gravity of the priority defects that affect the quality of their appearance, as well as the appearance of new types of defects. Therefore,


**Table 1.** Types and number of defects of protective and decorative coatings.

on the rheological properties of paint and are determined by the processes of wetting and application of paint. In literature, the quality of the appearance of coatings on metal substrates is studied. When selecting paint systems for concrete, the features of this material (strength, roughness, humidity, alkalinity, etc.) should be taken into account. Features of concrete affect

In this regard, the development of a methodology for ensuring the quality of the painted surface of building products and structures and control methods is an important scientific, technical, and economic problem. The solution of this problem as a whole will contribute to an increase in the service life of protective and decorative coatings. One of the common types of destruction of coatings of cement concrete is the appearance of cracks. There are known works [1–8] in which the regularities of the appearance of cracks in coatings on metal substrates are described. However, the resistance of coatings of cement concrete has not been

For analysis, the reasons for the destruction of the paint and varnish facades of buildings were used, the Pareto diagram, allowing for a variety of existing defects to distinguish those that make a significant contribution to the assessment of the quality of the appearance of coatings. When analyzing the Pareto chart, the 80/20 rule was used. For example, priority factors were identified, which fall into 80% of the cumulative curve [9–16]. The survey was carried out on residential 5-storey houses, which are located in accordance with GOST 9.039–74 "Corrosive aggressiveness of the environment" in the climatic region IY (moderately cold) and GOST 16350–80 "Climate of the USSR. Regionalizing and statistical parameters of climatic factors for technical purposes." The facades were painted with calcareous and cement per chlorinated vinyl CPCV paints. Colorful compositions were applied to the plaster thickness of 1.5–2 cm. When inspecting the painted surface, the following types of defects were detected: cracking, flaking, weathering, dirt retention of coatings, wet spots, and different tonality. The number and types of defects were determined visually (GOST 9.407–2015 Unified system of protection against corrosion and aging. PAINTWOOD COATINGS. Appraisal method). The names of the types of defects and the number of their appearances for calcareous and CPCV coatings

Pareto diagrams with all types of defects for their calcareous CPCV coatings are shown in **Figures 1**–**3**. Analysis of survey results indicates that the list of defects in coatings, constituting 80% of the cumulative curve, consists mainly of cracks along the vertical joint at the end of the building, different tonality of color, and peeling. All of these factors remain constant for both coatings. This allows us to consider them a source of "failure" regardless of the type of coating. In this case, such a defect as cracks in the coating along the vertical joint of the panels goes in the Pareto diagram in the first place and is 22.6–66.6% of the total number of defects,

As the coatings age, there is a change in the specific gravity of the priority defects that affect the quality of their appearance, as well as the appearance of new types of defects. Therefore,

**2. Stress state of coatings under exposure operational factors**

that take after different service lives are given in **Table 1**.

depending on the type of coverage and exploitation term.

the quality coating.

88 Coatings and Thin-Film Technologies

studied sufficiently.

**Figure 1.** Pareto chart for lime coating (1 year of operation).

after 3 years of operation of the calcareous coating, defects such as flaking of the coatings at the base of the metal roofs of the porch and at the ends of the fencing panels of the loggias are observed, the total specific gravity of which is 41.4%. The priority defects after 5 years of operation include cracks in the coating, along the vertical joint at the end of the building, exfoliation coating, and different tonality of color. A similar list of defects is also characteristic for CPCV coatings. Of this follows that the efforts of all specialists in developing the formulation of building paint compositions, the technology of their application should be aimed at increasing the crack resistance of protective-decorative coatings, since this type of defect is the most characteristic and widespread.

**Figure 2.** Pareto chart for the calcareous coating (5 years of operation).

**Figure 3.** Pareto chart for CPCV coating (5 years of operation).

Results of the experimental researches testify that in use of protective-decorative coatings of external walls of buildings, there is a change of the mechanism of their destruction from elastic-plastic to friable, that is, "embrittlement" of coatings is observed. According to the linear mechanics of destruction, a cracking fissuring of coatings happens [17, 18], if

$$K\_{\mathbf{i}} \cong K\_{\mathbf{i}\iota} \tag{1}$$

us according to the technique [23] based on a ratio between the crack length, a print of the Vickers indenter, and viscosity of destruction. We are invited to assess the cracking of polymer coatings using the technique, based on the ratio between the crack length, the Vickers indenter print, and the fracture toughness. This technique is successfully used to assess the

In this method, the value of the stress intensity factor K1c is determined from the length of the radial cracks formed in brittle materials from the corners of the Vickers indentation. To obtain a semi-empirical dependence of the length of radial cracks on crack resistance and hardness, the approach proposed by A. Evans and E. Charles is used. Dependence "normalized crack

*К*1*<sup>с</sup>* = *АН*1/2 (*Е*/*Н*)1/2 (*С*/*α*)−*<sup>В</sup>*. (3)

If the value of hardness does not depend on the load on the indenter, then Eq. (3) can be

*К*1*<sup>с</sup>* = *const* (*E*/*H*)1/2 *P*/*CB*, (4)

*К*1*<sup>с</sup>* = 0.028 *Н*1/2 (*E*/*H*)0.5 (*С*/*α*)−1.5 (5)

К1с = 0.028 На0.5 (Е/Н)0.5 (С/а)−1.5 (6)

/*Н*1/2

The most common semi-empirical equation of this type is given as follows:

The critical coefficient of intensity of tensions was determined by a formula:

where H is Vickers hardness; A and B are empirical coefficients.

*/Нα*1/2*)*⋅*(Н/Е)*1/2-normalized length of radial cracks *С/α*" is described by the fol-

) ⋅ (*Н*/*Е*)1/2 = *А* ⋅ (*С*/*α*)−*<sup>В</sup>*, (2)

Crack Resistance of Paint Coatings, Cement Concretes http://dx.doi.org/10.5772/intechopen.78537 91

fracture toughness (K1c) of ceramics.

(*К*1*<sup>с</sup>*

From expression (2), we obtain:

where H is Vickers hardness;

E: the modulus of elasticity;

where hardness by Vickers;

Р: loading on indents;

C: half-length of radial cracks;

*α:* half-length of the diagonal of the print.

where P is the load on the Vickers indenter.

resistance *(К*1*<sup>с</sup>*

lowing equation:

written in the form:

*К*1 -coefficient of intensity of tensions,

*К*1*с* -critical value of coefficient of intensity of tensions.

Considering that a main type of destruction of protective-decorative coatings is cracking fissuring, it is of practical interest to estimate parameters of crack formation of coatings during an aging. At present, there are several methods for assessing the crack resistance of coatings [19–22]. The determination of critical coefficient of intensity of tensions *К*1*<sup>с</sup>* was carried out by us according to the technique [23] based on a ratio between the crack length, a print of the Vickers indenter, and viscosity of destruction. We are invited to assess the cracking of polymer coatings using the technique, based on the ratio between the crack length, the Vickers indenter print, and the fracture toughness. This technique is successfully used to assess the fracture toughness (K1c) of ceramics.

In this method, the value of the stress intensity factor K1c is determined from the length of the radial cracks formed in brittle materials from the corners of the Vickers indentation. To obtain a semi-empirical dependence of the length of radial cracks on crack resistance and hardness, the approach proposed by A. Evans and E. Charles is used. Dependence "normalized crack resistance *(К*1*<sup>с</sup> /Нα*1/2*)*⋅*(Н/Е)*1/2-normalized length of radial cracks *С/α*" is described by the following equation:

$$\left\{\mathcal{K}\_{\text{tr}}/Ha^{1/2}\right\}\cdot\left\{\mathcal{H}/\mathcal{E}\right\}^{1/2} = A\cdot\left\{\mathcal{C}/a\right\}^{-\theta},\tag{2}$$

where H is Vickers hardness; A and B are empirical coefficients.

From expression (2), we obtain:

$$\mathcal{K}\_{\mathfrak{u}} = A H a^{\mathfrak{u}2} \{\mathcal{E}/H\}^{\mathfrak{u}2} \{\mathcal{C}/a\}^{-\mathfrak{s}}.\tag{3}$$

If the value of hardness does not depend on the load on the indenter, then Eq. (3) can be written in the form:

$$K\_{\rm loc} = \text{const} \, \text{(E/H)}^{12} \, ^\circ \text{P/C}^\circ \text{ } \tag{4}$$

where P is the load on the Vickers indenter.

The most common semi-empirical equation of this type is given as follows:

$$K\_{\rm \nu} = 0.028 \, Hz^{\rm l2} \, \text{(E/H)}^{0.5} \text{(C/}a\text{)}^{-1.5} \tag{5}$$

where H is Vickers hardness;

Results of the experimental researches testify that in use of protective-decorative coatings of external walls of buildings, there is a change of the mechanism of their destruction from elastic-plastic to friable, that is, "embrittlement" of coatings is observed. According to the

*К*<sup>1</sup> ≥ *К*1*<sup>с</sup>* (1)

Considering that a main type of destruction of protective-decorative coatings is cracking fissuring, it is of practical interest to estimate parameters of crack formation of coatings during an aging. At present, there are several methods for assessing the crack resistance of coatings

was carried out by

linear mechanics of destruction, a cracking fissuring of coatings happens [17, 18], if

[19–22]. The determination of critical coefficient of intensity of tensions *К*1*<sup>с</sup>*

*К*1

*К*1*с*



**Figure 3.** Pareto chart for CPCV coating (5 years of operation).

**Figure 2.** Pareto chart for the calcareous coating (5 years of operation).

90 Coatings and Thin-Film Technologies

E: the modulus of elasticity;

C: half-length of radial cracks;

*α:* half-length of the diagonal of the print.

The critical coefficient of intensity of tensions was determined by a formula:

$$\mathbf{K}\_{\rm 1c} = 0.028 \,\mathrm{Hz}^{0.5} \,\mathrm{(E/H)^{0.5} (C/a)^{-1.5}} \tag{6}$$

where hardness by Vickers;

Р: loading on indents;

С: semi-length of radial cracks;

a: semi-length of print diagonal.

As colorful structures in the work, polyvinyl acetate cement PVAC and polymer-calcareous paints were applied, and as a substrate—cement and sand solution. After curing, the painted solution exemplars were subjected to alternate freezing and thawing and also to humidification and a thermal aging. During tests with the help of Vickers indenter, we measured a print diameter and length of radial cracks which are formed on both sides from a print. Hardness by Vickers was calculated on a formula:

$$H = \frac{2P}{d^2} \sin \frac{a}{2'} \tag{7}$$

For coatings KO-168

For the PVAC coatings

For polymer-calcareous coatings

For coatings based on oil paint

For polymer-calcareous coatings

For the PVAC coatings

For coatings KO-168

obtained:

*а* = 0.141 *Р*0.5. (13)

Crack Resistance of Paint Coatings, Cement Concretes http://dx.doi.org/10.5772/intechopen.78537 93

Eqs. (10)–(13) indicate that the exponent a is a = 0.5, that is, this should be the case if the hardness is independent of the load in accordance with Eq. (8). After a certain duration of thermostating of the coatings, a decrease in coefficient A in Eq. (8) indicates an increase in their hardness. So, for example, after 100 h of thermostating, the following dependences were

*а* = 0.065 *Р*0.5**.** (14)

*а* = 0.085 *Р*0.5**.** (15)

*а* = 0.073 *Р*0.5**.** (16)

The analysis of the obtained results testifies that the dependence of the half-length of radial

С = 0.0327 *Р*0.67**.** (17)

С = 0.042*Р* 00.67**.** (18)

С = 0.252 Р0.67**.** (19)

The results of experimental studies indicate that in the process of aging protective-decorative coatings of external walls of buildings, there is a change in the mechanism of their destruction from elastic-plastic to brittle, that is, "embrittlement" of coatings is observed [24–29]. Test results are provided in **Table 3**. It was established that in PVAC and the polymer-calcareous coatings on a solution substrate, the "embrittlement" occurs after a particular duration of impact of alternate freezing and thawing. The presence of a crack was determined visually

cracks C on the load P in the coatings is approximated by the equations:

In the work applied, the following types of paints are listed in **Table 2**.

and also with the aid of a magnifier at a magnification of 30 times.

where d: diameter of a print;

*α*: angle at indenter top.

The dependence of the size of the semi-diagonal of the imprint on the load on the indenter is described by the equation:

$$a = A \cdot P^{0.5}.\tag{8}$$

The exponent is 0.5, as it should be when the hardness of the material does not depend on the magnitude of the load. The dependence of the length of radial cracks on the load is fairly accurately approximated by an equation of the form:

$$
\mathbf{C} = \mathbf{B} \cdot \mathbf{P}^{0.67}.\tag{9}
$$

In order that the critical value of the intensity factor *К*1*<sup>с</sup>* , measured by the method of indentation, did not depend on the load, exponent must be equal to 2/3 (≈ 0.67). The results of previous research work show that most protective and decorative coatings have a fragile character of destruction, which gives grounds to apply this technique to assess the crack resistance of paint and varnish coatings. From the experimental data obtained, a correlation was established between the semi-diagonal of the imprint and the load on the indenter P, which is described by expression (8).

For the PVAC coatings tested after curing obtained dependence

$$a = 0.087 \, P^{0.8}.\tag{10}$$

For polymer-calcareous coatings

$$a = 0.124 \, P^{0.5}.\tag{11}$$

For coatings based on oil paint

$$a = 0.097 \, P^{0.5} \, \text{.} \tag{12}$$

Crack Resistance of Paint Coatings, Cement Concretes http://dx.doi.org/10.5772/intechopen.78537 93

For coatings KO-168

С: semi-length of radial cracks; a: semi-length of print diagonal.

92 Coatings and Thin-Film Technologies

by Vickers was calculated on a formula:

where d: diameter of a print;

*α*: angle at indenter top.

described by the equation:

described by expression (8).

For polymer-calcareous coatings

For coatings based on oil paint

*Н* = \_\_\_ <sup>2</sup>*<sup>P</sup>*

accurately approximated by an equation of the form:

In order that the critical value of the intensity factor *К*1*<sup>с</sup>*

For the PVAC coatings tested after curing obtained dependence

As colorful structures in the work, polyvinyl acetate cement PVAC and polymer-calcareous paints were applied, and as a substrate—cement and sand solution. After curing, the painted solution exemplars were subjected to alternate freezing and thawing and also to humidification and a thermal aging. During tests with the help of Vickers indenter, we measured a print diameter and length of radial cracks which are formed on both sides from a print. Hardness

> *<sup>d</sup>*<sup>2</sup> sin \_\_ *α*

The dependence of the size of the semi-diagonal of the imprint on the load on the indenter is

*а* = *А* · *Р*0.5. (8)

The exponent is 0.5, as it should be when the hardness of the material does not depend on the magnitude of the load. The dependence of the length of radial cracks on the load is fairly

*С* = *В* · *Р*0.67. (9)

tion, did not depend on the load, exponent must be equal to 2/3 (≈ 0.67). The results of previous research work show that most protective and decorative coatings have a fragile character of destruction, which gives grounds to apply this technique to assess the crack resistance of paint and varnish coatings. From the experimental data obtained, a correlation was established between the semi-diagonal of the imprint and the load on the indenter P, which is

*а* = 0.087 *Р*0.5. (10)

*а* = 0.124 *Р*0.5. (11)

*а* = 0.097 *Р*0.5**.** (12)

<sup>2</sup>, (7)

, measured by the method of indenta-

$$a = 0.141 \, P^{0.5}.\tag{13}$$

Eqs. (10)–(13) indicate that the exponent a is a = 0.5, that is, this should be the case if the hardness is independent of the load in accordance with Eq. (8). After a certain duration of thermostating of the coatings, a decrease in coefficient A in Eq. (8) indicates an increase in their hardness. So, for example, after 100 h of thermostating, the following dependences were obtained:

For the PVAC coatings

$$a = 0.065 \, P^{0.5} \, \text{.} \tag{14}$$

For polymer-calcareous coatings

$$a \approx 0.085 \, P^{0.5} \,\text{.}\tag{15}$$

For coatings based on oil paint

$$a = 0.073 \, P^{0.8} \, \text{.} \tag{16}$$

The analysis of the obtained results testifies that the dependence of the half-length of radial cracks C on the load P in the coatings is approximated by the equations:

For polymer-calcareous coatings

$$\mathbf{C} = 0.0327 \, P^{0.87} \, . \tag{17}$$

For the PVAC coatings

$$\mathbf{C} = 0.042P\,\mathrm{0}^{0.67}.\tag{18}$$

For coatings KO-168

$$\mathbf{C} = 0.252 \text{ P}^{0.67}.\tag{19}$$

In the work applied, the following types of paints are listed in **Table 2**.

The results of experimental studies indicate that in the process of aging protective-decorative coatings of external walls of buildings, there is a change in the mechanism of their destruction from elastic-plastic to brittle, that is, "embrittlement" of coatings is observed [24–29]. Test results are provided in **Table 3**. It was established that in PVAC and the polymer-calcareous coatings on a solution substrate, the "embrittlement" occurs after a particular duration of impact of alternate freezing and thawing. The presence of a crack was determined visually and also with the aid of a magnifier at a magnification of 30 times.


**Table 2.** Technical characteristics of the paints.

The critical coefficient of intensity of tensions was determined by formula (6). The size of the diagonal of the Vickers indenter print was measured using a magnifier. Cracks in coatings at cave-in of an indenter Vickers appear only after 15–20 testing cycles. The value of critical coefficient of intensity of tensions of PVAC coating is equal to K1c = 0.088 MH/m3/2, and for polymer-calcareous coating, K1c = 0.069 MH/m3/2.

Introduction into a compounding of PVAC paint, the fibrous asbestos increases crack resistance of coatings. Thus, even after 20 cycles of alternate freezing-thawing, the "embrittlement" of coating is not observed. The comparative analysis of data shows that at the same intensity of influences of the environment coatings with a fibrous asbestos possess the smaller value of coefficient of intensity of tensions, after eight cycles of alternate freezing-thawing K1 (PVAC) = 0.078 MH/m3/2 and K1 (PVAC with 1% of asbestos) =0.073 MH/m3/2.

Humidification of coatings leads to a decrease of an elastic modulus and hardness of coatings that reduces a danger of crack formation at deformation of a wall construction. Humidification of coatings for 30 days does not cause crack fissuring of coatings. The coefficient of intensity of tensions of PVAC coating after curing is equal to K1c = 0.06 MH/m3/2, and after humidification K1c = 0.054 MH/m3/2. Similar data are obtained and for the polymer-calcareous coatings.

At studying a thermo aging, it was recorded that the increase of time of thermoaging leads to natural increase of value of coefficient of intensity of tensions. For example, after a thermoaging of polymer-calcareous coatings for a 100-h increase, the value of coefficient of intensity of tensions is observed from K1c = 0.044 MH/m3/2 (after curing) to KK1c = 0.053 MH/m3/2, and after 200 h value makes KK1c = 0.0546 MH/mm3/2. Considering that properties of a protective-decorative coatings are defined among other factors by properties of the painted construction and are heterogeneous on an extension, we followed up and carried out the calculation of tensions arising in coatings as a result of the influence of various factors according to a technique [30]. It will allow approaching the choice of materials research factors of the increase of crack resistance of coatings more reasonably. The conducted researches are justification for recommendations at developing the compounding of colorful structures, at carrying out research works with the use of technique of an assessment of crack resistance of coatings according to the offered scheme. It will allow to more reasonably predict firmness of coatings and also to optimize finishing structures for the purpose of receiving coatings with a complex of the given properties.

**3. Persistence of varnish-and-lacquer coatings to cracking in the** 

**Table 3.** Parameters of a crack for the formation of a protective-decorative coating.

In accordance with the statistical theory of the strength of solids, the probability of destruction of coatings is determined by the presence and concentration of defects, including on the surface of the coatings. Thus, the quality of the appearance of the coatings among other factors

**process of environmental impact**

**Type of coatings Type of influence Loading** 

Three cycles of freezing-thawing

Eight cycles of freezing-thawing

15 cycles of freezing-thawing

Three cycles of freezing-thawing

Three cycles of freezing-thawing

Eight cycles of freezing-thawing

20 cycles of freezing-thawing

Critical coefficient of intensity of tensions.

20 cycles of freezing-thawing

PVAC (1% asbestos by weight of cement)

\*

**P, Н**

PVAC After curing 47.39 61 1 0.06

Polymer- calcareous After curing 47.39 27 1 0.044

**Hardness of coating, Н, Н/мм<sup>2</sup>**

Humidification 15 days 49 1 0.058 Humidification 30 days 37 1 0.054 Thermoaging 100 h 85 1 0.065 Thermoaging 200 h 104 1 0.068

Humidification 15 days 23 1 0.055 Humidification 30 days 16 1 0.040 Thermoaging 100 h 55 1 0.053 Thermoaging 200 h 62 1 0.0546

After curing 47.39 44 1 0.055

**The relation of crack semi-length C to the size of semi-diagonal of** 

Crack Resistance of Paint Coatings, Cement Concretes http://dx.doi.org/10.5772/intechopen.78537

> **Coefficient of intensity of tension, К1с, МН/м3/2**

95

**print**

137 1 0.075

164 1 0.078

179 1.2 0.088\*

45 1 0.05

70 1.54 0.069\*

130 1 0.073

130 1 0.073

133 1 0.074


**Table 3.** Parameters of a crack for the formation of a protective-decorative coating.

The critical coefficient of intensity of tensions was determined by formula (6). The size of the diagonal of the Vickers indenter print was measured using a magnifier. Cracks in coatings at cave-in of an indenter Vickers appear only after 15–20 testing cycles. The value of critical coefficient of intensity of tensions of PVAC coating is equal to K1c = 0.088 MH/m3/2, and for

PF-115 60–120 24 1 5

PVAC 40–100 3 1 5 Water dispersive (facade) 40–45 1 5 Nitrocellulose NC-123 60–100 3 1 3 Polymer-calcareous 40–60 24 2 5 Acrylate class "Universal" 40–60 3

Introduction into a compounding of PVAC paint, the fibrous asbestos increases crack resistance of coatings. Thus, even after 20 cycles of alternate freezing-thawing, the "embrittlement" of coating is not observed. The comparative analysis of data shows that at the same intensity of influences of the environment coatings with a fibrous asbestos possess the smaller value of coefficient of intensity of tensions, after eight cycles of alternate freezing-thawing

Humidification of coatings leads to a decrease of an elastic modulus and hardness of coatings that reduces a danger of crack formation at deformation of a wall construction. Humidification of coatings for 30 days does not cause crack fissuring of coatings. The coefficient of intensity of tensions of PVAC coating after curing is equal to K1c = 0.06 MH/m3/2, and after humidification K1c = 0.054 MH/m3/2. Similar data are obtained and for the polymer-calcareous coatings.

At studying a thermo aging, it was recorded that the increase of time of thermoaging leads to natural increase of value of coefficient of intensity of tensions. For example, after a thermoaging of polymer-calcareous coatings for a 100-h increase, the value of coefficient of intensity of tensions is observed from K1c = 0.044 MH/m3/2 (after curing) to KK1c = 0.053 MH/m3/2, and after 200 h value makes KK1c = 0.0546 MH/mm3/2. Considering that properties of a protective-decorative coatings are defined among other factors by properties of the painted construction and are heterogeneous on an extension, we followed up and carried out the calculation of tensions arising in coatings as a result of the influence of various factors according to a technique [30]. It will allow approaching the choice of materials research factors of the increase of crack resistance of coatings more reasonably. The conducted researches are justification for recommendations at developing the compounding of colorful structures, at carrying out research works with the use of technique of an assessment of crack resistance of coatings according to the offered scheme. It will allow to more reasonably predict firmness of coatings and also to optimize finishing

structures for the purpose of receiving coatings with a complex of the given properties.

(PVAC with 1% of asbestos) =0.073 MH/m3/2.

**Viscosity, s Drying time, no less. h Adhesion, point Life, year**

polymer-calcareous coating, K1c = 0.069 MH/m3/2.

**Type of paint Name of indicators**

94 Coatings and Thin-Film Technologies

MA-15 64–140 24

(PVAC) = 0.078 MH/m3/2 and K1

**Table 2.** Technical characteristics of the paints.

K1

### **3. Persistence of varnish-and-lacquer coatings to cracking in the process of environmental impact**

In accordance with the statistical theory of the strength of solids, the probability of destruction of coatings is determined by the presence and concentration of defects, including on the surface of the coatings. Thus, the quality of the appearance of the coatings among other factors


determines the resistance of the coatings to failure, in particular, to cracking. To establish the connection between the fracture toughness indexes of coatings and the quality of their external type in the process of a corrosive environmental impact, we conducted the following

**Table 4.** Crack resistance of coatings depending on the quality of their appearance in the process of freezing-thawing.

**Number of defects Surface roughness,** 

Hardening 192 0.44 0.01688

Five freeze-thaw cycles 229 0.89 0.02103

10 freeze-thaw cycles 232 3.01 0.02305

13 freeze-thaw cycles 233 3.65 0.02636

15 freeze-thaw cycles 247 3.80 0.02752

Hardening 160 0.24 0.01283

Five freeze-thaw cycles 210 0.74 0.01716

10 freeze-thaw cycles 215 2.40 0.02146

13 freeze-thaw cycles 223 2.62 0.02342

Freeze-thaw cycles 228 3.30 0.25631

**Ra, мкм**

113 0.34 0.01481 80 0.24 0.01308

135 0.76 0.01799 94 0.70 0.01409

141 2.55 0.01967 97 1.85 0.01503

148 3.10 0.02013 106 1.98 0.01723

153 3.40 0.02432 114 2.80 0.02056

120 0.22 0.01193 86 0.20 0.01137

134 0.60 0.01513 108 0.55 0.01391

150 1.77 0.01692 117 1.44 0.01403

161 1.83 0.01863 123 1.63 0.01608

176 2.31 0.02018 129 1.96 0.01801

**Coefficient of intensity of** 

**, МН/м3/2**

97

**stresses** *К***<sup>1</sup>**

Crack Resistance of Paint Coatings, Cement Concretes http://dx.doi.org/10.5772/intechopen.78537

**Name of paint Type of corrosion attack**

Water dispersive (facade)

Acrylate, class Wagon


**Name of paint Type of corrosion attack**

96 Coatings and Thin-Film Technologies

Alkyd enamel PF-115

Nitrocellulose NC-123

Nitrocellulose NC-123

**Number of defects Surface roughness,** 

Hardening 36 0.12 0.01561

Five freeze-thaw cycles 39 0.47 0.01708

15 freeze-thaw cycles 57 3.26 0.02123

10 freeze-thaw cycles 26 1.69 0.02653

13 freeze-thaw cycles 30 1.83 0.02903

15 freeze-thaw cycles 33 2.1 0.02985

Hardening 27 0.19 0.00986

Five freeze-thaw cycles 30 0.52 0.01377

13 freeze-thaw cycles 31 2.90 0.01802

15 freeze-thaw cycles 28 2.65 0.01925

Continuation of Table 4

10 freeze-thaw cycles There is a cracking of the coating

10 freeze-thaw cycles There is a peeling of the coating

13 freeze-thaw cycles There is a peeling of the coating

Oil paint MA-15 Hardening 29 0.23 0.01855

Five freeze-thaw cycles There is a peeling of the coating

**Ra, мкм**

30 0.10 0.01035 18 0.08 0.01002

32 0.36 0.01677 25 0.23 0.01076

36 2.58 0.01913 31 2.21 0.01846

56 3.10 0.02082

20 0.18 0.01177 10 0.14 0.01170

23 0.59 0.02056 15 0.40 0.01864

20 1.46 0.02014

21 1.63 0.02134

26 1.95 0.02461

19 0.17 0.00984 8 0.14 0.00824

21 0.48 0.01306 12 0.16 0.01061

28 2.78 0.01672 18 2.32 0.01543

24 2.51 0.01701

**Coefficient of intensity of** 

**, МН/м3/2**

**stresses** *К***<sup>1</sup>**

**Table 4.** Crack resistance of coatings depending on the quality of their appearance in the process of freezing-thawing.

determines the resistance of the coatings to failure, in particular, to cracking. To establish the connection between the fracture toughness indexes of coatings and the quality of their external type in the process of a corrosive environmental impact, we conducted the following


"Universal," and acrylic water dispersion paint (facade). Different quality of the appearance of the coatings was created by changing the porosity of the substrate and the rheological properties of the paint compositions. During the tests, the colored solution samples were subjected to various types of corrosion attack, namely alternating freezing-thawing according to the regime: 4 h freezing at a temperature of −18°C, 20 h of thawing, moistening-drying according to the regime: 20 h of moistening at room temperature and 4 h of drying at a temperature of 60°C.

During the experiment, the concentration of defects on the surface of the coating was also

surface roughness was determined by profilograph TR-100 state. The results of the studies are given in **Tables 3** and **4**. It was found that during the test, the cracks appear locally and are formed near the defects on the surface of the coating. In particular, after five test cycles on the surface of the coating based on MA-15 paint, surface roughness, R<sup>a</sup> = 0.23 μm, surface cracks visible to the naked eye appeared, and on the coating with a roughness index Ra = 0.14 μm after 15 test cycles. Similar patterns are also characteristic for other coatings (**Table 4**). It is established that with an increase in the roughness of the coating surface, the value of the stress intensity coefficient is increased. Therefore, for example, the surface roughness of a coating based on PF-115 ink is R<sup>a</sup> = 0.12 μm, the stress intensity factor K<sup>1</sup> = 0.01561 MN/m3/2, a decrease in the roughness of the coating surface Ra = 0.08 μm leads to a decrease in the stress intensity factor to K1 = 0.01002 MN/m3/2. Similar patterns are also characteristic for other types of coatings.

The results (**Table 5**) show that the nature of the destruction of the coatings during the corrosive action of the medium is not the same. Therefore, coatings based on oil and alkyd paint are characterized by peeling, coatings based on acrylate class Universal, nitrocellulose, and water dispersion paint-cracking. Regardless of the type of coating and the corrosive effect of the medium, there is an increase in the roughness of the coating surface and coefficient of

**4. The influence of the porosity of the substrate on crack resistance** 

The operational stability of protective-decorative coatings of the outer walls of buildings is significantly influenced by processes occurring both in the coating itself and at the interface of the "substrate-coating" contact [6]. The strength of the adhesion of protective-decorative coatings to the concrete substrate depends significantly on the quality of the substrate. The quality of the substrate, first of all, is understood as its macro- and microstructure, the degree of its homogeneity, providing the desired solidity contact layer, its density, and porosity. Features of the porous substrate, such as cement concrete, mortar, and so on, have a significant effect on the formation of the structure and properties of the coatings applied. The analysis of the results (**Table 6**) indicates that the porosity of the substrate has a great influence on the nature of the destruction of the protective and decorative coating. At the same time, it should be noted that the nature of the destruction, for example, of PVAC coatings, has a significant difference from polymer-based coatings. Thus, for example, an increase in the porosity of the substrate from 20 to 28% leads to a reduction in the crack resistance of polymer-calcareous coatings.

intensity of stresses.

**of protective-decorative coating**

. The coating

99

Crack Resistance of Paint Coatings, Cement Concretes http://dx.doi.org/10.5772/intechopen.78537

determined. The number of defects was determined on the surface area of 64 cm<sup>2</sup>

**Table 5.** Crack resistance of coatings depending on the quality of their appearance in the process of moistening-drying.

experiment. Colorful compositions were applied using a brush on the solvent substrates in two layers with intermediate drying for 24 h. The following color compositions were used: alkyd grade enamel PF-115, oil paint MA-15, nitrocellulose enamel НЦ-123, paint acryl ate class "Universal," and acrylic water dispersion paint (facade). Different quality of the appearance of the coatings was created by changing the porosity of the substrate and the rheological properties of the paint compositions. During the tests, the colored solution samples were subjected to various types of corrosion attack, namely alternating freezing-thawing according to the regime: 4 h freezing at a temperature of −18°C, 20 h of thawing, moistening-drying according to the regime: 20 h of moistening at room temperature and 4 h of drying at a temperature of 60°C.

During the experiment, the concentration of defects on the surface of the coating was also determined. The number of defects was determined on the surface area of 64 cm<sup>2</sup> . The coating surface roughness was determined by profilograph TR-100 state. The results of the studies are given in **Tables 3** and **4**. It was found that during the test, the cracks appear locally and are formed near the defects on the surface of the coating. In particular, after five test cycles on the surface of the coating based on MA-15 paint, surface roughness, R<sup>a</sup> = 0.23 μm, surface cracks visible to the naked eye appeared, and on the coating with a roughness index Ra = 0.14 μm after 15 test cycles. Similar patterns are also characteristic for other coatings (**Table 4**). It is established that with an increase in the roughness of the coating surface, the value of the stress intensity coefficient is increased. Therefore, for example, the surface roughness of a coating based on PF-115 ink is R<sup>a</sup> = 0.12 μm, the stress intensity factor K<sup>1</sup> = 0.01561 MN/m3/2, a decrease in the roughness of the coating surface Ra = 0.08 μm leads to a decrease in the stress intensity factor to K1 = 0.01002 MN/m3/2. Similar patterns are also characteristic for other types of coatings.

The results (**Table 5**) show that the nature of the destruction of the coatings during the corrosive action of the medium is not the same. Therefore, coatings based on oil and alkyd paint are characterized by peeling, coatings based on acrylate class Universal, nitrocellulose, and water dispersion paint-cracking. Regardless of the type of coating and the corrosive effect of the medium, there is an increase in the roughness of the coating surface and coefficient of intensity of stresses.

### **4. The influence of the porosity of the substrate on crack resistance of protective-decorative coating**

The operational stability of protective-decorative coatings of the outer walls of buildings is significantly influenced by processes occurring both in the coating itself and at the interface of the "substrate-coating" contact [6]. The strength of the adhesion of protective-decorative coatings to the concrete substrate depends significantly on the quality of the substrate. The quality of the substrate, first of all, is understood as its macro- and microstructure, the degree of its homogeneity, providing the desired solidity contact layer, its density, and porosity. Features of the porous substrate, such as cement concrete, mortar, and so on, have a significant effect on the formation of the structure and properties of the coatings applied. The analysis of the results (**Table 6**) indicates that the porosity of the substrate has a great influence on the nature of the destruction of the protective and decorative coating. At the same time, it should be noted that the nature of the destruction, for example, of PVAC coatings, has a significant difference from polymer-based coatings. Thus, for example, an increase in the porosity of the substrate from 20 to 28% leads to a reduction in the crack resistance of polymer-calcareous coatings.

experiment. Colorful compositions were applied using a brush on the solvent substrates in two layers with intermediate drying for 24 h. The following color compositions were used: alkyd grade enamel PF-115, oil paint MA-15, nitrocellulose enamel НЦ-123, paint acryl ate class

**Table 5.** Crack resistance of coatings depending on the quality of their appearance in the process of moistening-drying.

**Name of the paint composition**

Alkyd enamel PF-115 0.01561

98 Coatings and Thin-Film Technologies

Oil paint MA-15 0.01855

Nitrocellulose NC-123

Water dispersive (facade)

Acrylate, class Wagon

**Alter hardening**

0.58

0.01035 0.4

0.01002 0.21

0.8

0.01177 0.69

0.01170 0.46

0.00986 0.78

0.00934 0.6

0.00824 0.32

0.01488 3.01

0.01471 2.55

0.01308 1.85

0.01282 2.4

0.01193 1.77

0.01137 1.51

**The change in coefficient of intensity of stresses К<sup>1</sup>**

0.01804 0.83

0.01581 0.58

0.01264 0.32

0.02004 1.43

0.01658 0.94

0.01342 0.65

0.01564 1.02

0.01268 0.79

0.01194 0.44

0.01568 3.42

0.01496 2.76

0.01386 1.94

0.01462 2.62

0.01266 1.94

0.01204 1.63

**, МН/м3/2**

There is a peeling of the coating

There is a cracking of the coating

0.01968 1.32

0.02393 3.94

0.02363 3.28

0.02186 2.46

0.02238 3.38

0.02113 2.64

0.02073 2.18

There is a cracking of the coating

0.02538 4.16

0.02506 3.34

0.02483 2.61

0.02368 3.52

0.02309 2.83

0.02298 2.31

**Five cycles Eight cycles 11 cycles 13 cycles 15 cycles**

There is a peeling of the coating

There is a peeling of the coating

There is a peeling of the coating

There is a peeling of the coating

There is a peeling of the coating

There is a cracking of the coating

0.01683 1.18

0.02236 3.72

0.02198 3.16

0.02106 2.34

0.02068 3.28

0.02032 2.43

0.01958 1.96

0.01532 0.93

0.01586 1.12

0.01302 0.68

0.01932 3.58

0.01906 2.96

0.01896 2.08

0.01768 3.12

0.01701 2.26

0.01632 1.84


The appearance of cracks in indentation of the Vickers indenter in a polymer-calcareous coating on substrates with a porosity of P = 20% is observed after 20 cycles of freezing-thawing.

P = 28%, the appearance of cracks in indentation of the Vickers indenter occurs in the coatings after 14 cycles of freezing-thawing. This is explained by the appearance of a more inhomogeneous stress state in the coating. For PVAC coatings, it is characteristic that the fracture toughness increases with increasing porosity of the substrate from 20 to 28%. The critical value of

MN/m3/2. In this case, the appearance of cracks in the coating during the introduction of the Vickers indenter was recorded after 11 cycles of freezing-thawing. With a substrate porosity

However, the change in the coefficient of intensity of stresses for PVAC coatings is extreme. So, for example, when using as a substrate of brick samples with a porosity of P = 40%, the fracture toughness of PVAC coatings is significantly reduced. Thus, the presence of cracks at

Preliminary preparation of the substrate surface has a significant effect on the crack resistance of protective and decorative coatings. So, for example, priming the surface of the substrate leads to an increase in the crack resistance of the coatings. At the same time, the appearance of cracks with the indentation of the Vickers indenter in the PVAC coating on a substrate with a porosity of P = 20% was observed after 15 cycles of alternating freezing-thawing. For some types of coatings PF-115 and water dispersive acrylate paint of the "Universal" class, it is very important in advance application of putty to the surface before staining to reduce the porosity of the substrate. This leads to an increase in the crack resistance of these coatings. Thus, the coefficient of the intensity of stresses of the acrylate coating applied to the surface of the substrate after its preliminary preparation was 0.004

It was found that in coatings, "embrittlement" occurs after a certain time of moistening. Thus, cracks in the coatings with the indentation of the Vickers indenter appear on the coating of PF-266 on a substrate with a surface porosity of P = 0% and on a substrate with a surface porosity of P = 6.2% and humidity at the application of paint of 9.9%. For coatings MA-115 on a substrate with P = 6.7% and humidity at the time of application of paint W = 10.2%, peeling of the coating after 2 months of moistening is characteristic. Analysis of the experimental data (**Table 7**) shows that with increasing surface porosity of the substrate, a decrease in the stress intensity factor is observed up to a certain limit. Thus, with a load П = 25.39 N, the

P = 0.13–1.2% after 2 months of moistening is K<sup>c</sup> = 0.0376МН/m1.5, and on a substrate with a surface porosity, P = 10.5%—0.0256 МН/m1.5, and for the MA-15 coating, the values of the

Obviously, this is due to the fact that the pores on the surface of the substrate to some extent "extinguish" internal stresses and reduce the tendency to crack coatings. Increasing the substrate moisture at the time of application of the paint results in a more defective structure of the contact layer "coating-substrate" and a greater propensity to cracking. The stress intensity factor for PF-115 coatings at substrate moisture at the time of application of a paint composition equal to W = 10.4% after 2 months of wetting is K<sup>c</sup> = 0.404 МН/m1.5. Among other factors, the

stress intensity factor are, respectively, 0.0257 МН/м1.5 and 0.02147 МН/m1.5..

for coating PF-115 on a substrate with a surface porosity

coefficient of intensity of stresses for PVAC coatings in substrate porosity of 20% is *K*1*<sup>c</sup>*

of 28%, the appearance of cracks is observed only after 15 cycles of freezing-thawing.

the indentation of Vickers indenter is observed after five cycles of freezing thawing.

= 0.06 MN/m3/2. At porosity of the substrate,

Crack Resistance of Paint Coatings, Cement Concretes http://dx.doi.org/10.5772/intechopen.78537

= 0.088

101

The critical coefficient of intensity of stresses is *K*1*<sup>c</sup>*

MN/m3/2, while without preparation—0.027 MN/m3/2.

value of the stress intensity factor Kc

**Table 6.** Parameters crack education of protective-decorative coatings depending on the porosity of the substrate.

The appearance of cracks in indentation of the Vickers indenter in a polymer-calcareous coating on substrates with a porosity of P = 20% is observed after 20 cycles of freezing-thawing. The critical coefficient of intensity of stresses is *K*1*<sup>c</sup>* = 0.06 MN/m3/2. At porosity of the substrate, P = 28%, the appearance of cracks in indentation of the Vickers indenter occurs in the coatings after 14 cycles of freezing-thawing. This is explained by the appearance of a more inhomogeneous stress state in the coating. For PVAC coatings, it is characteristic that the fracture toughness increases with increasing porosity of the substrate from 20 to 28%. The critical value of coefficient of intensity of stresses for PVAC coatings in substrate porosity of 20% is *K*1*<sup>c</sup>* = 0.088 MN/m3/2. In this case, the appearance of cracks in the coating during the introduction of the Vickers indenter was recorded after 11 cycles of freezing-thawing. With a substrate porosity of 28%, the appearance of cracks is observed only after 15 cycles of freezing-thawing.

**Name of coating**

(P = 20%) PVAC

(P = 28%) PVAC

PVAC on a brick substrate (P = 40%)

(P = 20%) Polymercalcareous

(P = 28%) Polymercalcareous

(P = 20%) Acrylate class "Universal"

\*

Continuation of Table 6

**Impact type Hardness** 

Five freeze-thaw

Five freeze-thaw

10 freeze-thaw cycles

11 freeze-thaw cycles

Five freeze-thaw

10 freeze-thaw cycles

15 freeze-thaw cycles

Five freeze-thaw

Five freeze-thaw

10 freeze-thaw cycles

20 freeze-thaw cycles

Five freeze-thaw

14 freeze-thaw cycles

Curing on preliminary putty surfaces

Critical coefficient of intensity of stresses.

cycles

cycles

cycles

cycles

Eight freezethaw cycles

cycles

100 Coatings and Thin-Film Technologies

cycles

**H, N/mm<sup>2</sup>**

1 2 3 4 5 PVAC on glass Hardening 37 1 0.051

1 2 3 4 5

**The ratio of the half-length of the crack C to the size of the semi-diagonal of the imprint a**

76 1 0.072

85 1.27 0.083\*

125 1 0.08

140 1 0.084

180 1.4 0.088\*

80 1 0.066

102 1 0.08

174 1.23 0.083\*

176 1.3 0.085\*

47.6 1 0.05

52.2 1 0.052

60.8 1.54 0.06\*

38 1 0.04

57 1.2 0.056\*

22 1 0.004

Hardening 85 1 0.065

Hardening 75 1 0.065

Hardening 73 1 0.077

Hardening 21.7 1 0.024

Hardening 27 1 0.029

Hardening 61 1 0.027

**Table 6.** Parameters crack education of protective-decorative coatings depending on the porosity of the substrate.

**Coefficient of intensity of stresses,** *К***<sup>1</sup>**

**, МН/м3/2**

However, the change in the coefficient of intensity of stresses for PVAC coatings is extreme. So, for example, when using as a substrate of brick samples with a porosity of P = 40%, the fracture toughness of PVAC coatings is significantly reduced. Thus, the presence of cracks at the indentation of Vickers indenter is observed after five cycles of freezing thawing.

Preliminary preparation of the substrate surface has a significant effect on the crack resistance of protective and decorative coatings. So, for example, priming the surface of the substrate leads to an increase in the crack resistance of the coatings. At the same time, the appearance of cracks with the indentation of the Vickers indenter in the PVAC coating on a substrate with a porosity of P = 20% was observed after 15 cycles of alternating freezing-thawing. For some types of coatings PF-115 and water dispersive acrylate paint of the "Universal" class, it is very important in advance application of putty to the surface before staining to reduce the porosity of the substrate. This leads to an increase in the crack resistance of these coatings. Thus, the coefficient of the intensity of stresses of the acrylate coating applied to the surface of the substrate after its preliminary preparation was 0.004 MN/m3/2, while without preparation—0.027 MN/m3/2.

It was found that in coatings, "embrittlement" occurs after a certain time of moistening. Thus, cracks in the coatings with the indentation of the Vickers indenter appear on the coating of PF-266 on a substrate with a surface porosity of P = 0% and on a substrate with a surface porosity of P = 6.2% and humidity at the application of paint of 9.9%. For coatings MA-115 on a substrate with P = 6.7% and humidity at the time of application of paint W = 10.2%, peeling of the coating after 2 months of moistening is characteristic. Analysis of the experimental data (**Table 7**) shows that with increasing surface porosity of the substrate, a decrease in the stress intensity factor is observed up to a certain limit. Thus, with a load П = 25.39 N, the value of the stress intensity factor Kc for coating PF-115 on a substrate with a surface porosity P = 0.13–1.2% after 2 months of moistening is K<sup>c</sup> = 0.0376МН/m1.5, and on a substrate with a surface porosity, P = 10.5%—0.0256 МН/m1.5, and for the MA-15 coating, the values of the stress intensity factor are, respectively, 0.0257 МН/м1.5 and 0.02147 МН/m1.5..

Obviously, this is due to the fact that the pores on the surface of the substrate to some extent "extinguish" internal stresses and reduce the tendency to crack coatings. Increasing the substrate moisture at the time of application of the paint results in a more defective structure of the contact layer "coating-substrate" and a greater propensity to cracking. The stress intensity factor for PF-115 coatings at substrate moisture at the time of application of a paint composition equal to W = 10.4% after 2 months of wetting is K<sup>c</sup> = 0.404 МН/m1.5. Among other factors, the


freeze-thaw cycles. The obtained results make it possible to assume that an increase in the moisture content of the substrate leads to a significant decrease in internal stresses in the coating, which is apparently due to a decrease in the adhesion strength due to the presence of moisture on the surface of the substrate. However, for different types of coatings, the optimum moisture content of the substrate is characteristic. Increasing the optimum moisture

Thus, the value of the moisture content of the substrate, established in the regulatory and technical documentation, in particular, W = 8% for water paint, is not correct. There is a value for the optimum moisture content of the substrate for each particular coating in terms of its fracture toughness. It is necessary to conduct extensive research to create a databank on the influence of substrate moisture on the crack resistance of protective and decorative coatings.

content of the substrate leads to a decrease in the crack resistance of coatings.

This will help in the future to develop measures to create crack-resistant coatings.

1 2 3 4 5 6 PVAC 0 Hardening 62 1 0.06 Five freeze-thaw

> 10 freeze-thaw cycles

> 20 freeze-thaw cycles

> 10 freeze-thaw cycles

> 25 freeze-thaw cycles

Five freeze-thaw

10 freeze-thaw cycles

1 2 3 4 5 6

4 Hardening 44 1 0.053

PVAC 1 Hardening 32 1 0.018 Five freeze-thaw

cycles

cycles

cycles

**Impact type Hardness** 

**H, N, mm2**

**The ratio of the half-length of the crack C to the size of the semi-diagonal of the imprint a**

112 1 0.08

125 1 0.084

180 1.1 0.09\*

80 1 0.022

102 1 0.08

112 1.08 0.09\*

80 1 0.066

131 1.2 0.088\*

**Coefficient of intensity of stresses, К1с, МН/**

**m1.5**

Crack Resistance of Paint Coatings, Cement Concretes http://dx.doi.org/10.5772/intechopen.78537 103

**Name of coating Moisture of** 

Continuation of Table 8

**the substrate, W, %**

**Table 7.** Parameters crack education of protective-decorative coatings.

state of the painted surface of the facades of buildings is determined by the time of application of the paint. So, for example, if the paint is applied in April-May, when the moisture of the substrate and the coating is high due to moisture migration from the side of the wall material, this can lead to premature failure of the coating. We evaluated the effect of substrate moisture on the properties of protective and decorative coatings, in particular, on their crack resistance. The analysis of the results (**Table 8**) shows that the substrate moisture at the time of application of paint has a significant effect on the crack resistance of the coatings. For example, when PVAC coating is applied to a dry surface, the appearance of cracks in the coating when Vickers indenter is introduced occurs after 20 cycles of freezing-thawing, the critical value of the stress intensity factor being K1c = 0.09 MN/m3/2. An increase in the initial moisture content of the substrate to W = 1% leads to a significant increase in the fracture toughness of the PVAC coating, with the appearance of cracks in the introduction of the Vickers indenter only after 25 cycles of freezing-thawing. A further increase in the substrate moisture during painting leads to the appearance of a more defective structure of the contact layer "coating-substrate" and a greater tendency of the coating to crack formation. Thus, an increase in substrate moisture to W = 4% led to the appearance of cracks in the introduction of the Vickers indenter after 10 cycles of freezing-thawing.

The results of the analysis of fracture toughness indexes of coatings PF-115 indicate that after 15 cycles of freezing-thawing, the coefficient of intensity of stresses in coatings on absolutely dry substrate is K1 = 0.057 MN/m3/2, whereas for substrate moisture W = 4%— K1 = 0.026 MN/m3/2.

Coatings PF-115 on a dry substrate are characterized by peeling after 15 cycles of freezingthawing, while at substrate moisture content W = 4%, peeling is not observed and after 35 freeze-thaw cycles. The obtained results make it possible to assume that an increase in the moisture content of the substrate leads to a significant decrease in internal stresses in the coating, which is apparently due to a decrease in the adhesion strength due to the presence of moisture on the surface of the substrate. However, for different types of coatings, the optimum moisture content of the substrate is characteristic. Increasing the optimum moisture content of the substrate leads to a decrease in the crack resistance of coatings.

Thus, the value of the moisture content of the substrate, established in the regulatory and technical documentation, in particular, W = 8% for water paint, is not correct. There is a value for the optimum moisture content of the substrate for each particular coating in terms of its fracture toughness. It is necessary to conduct extensive research to create a databank on the influence of substrate moisture on the crack resistance of protective and decorative coatings. This will help in the future to develop measures to create crack-resistant coatings.


state of the painted surface of the facades of buildings is determined by the time of application of the paint. So, for example, if the paint is applied in April-May, when the moisture of the substrate and the coating is high due to moisture migration from the side of the wall material, this can lead to premature failure of the coating. We evaluated the effect of substrate moisture on the properties of protective and decorative coatings, in particular, on their crack resistance. The analysis of the results (**Table 8**) shows that the substrate moisture at the time of application of paint has a significant effect on the crack resistance of the coatings. For example, when PVAC coating is applied to a dry surface, the appearance of cracks in the coating when Vickers indenter is introduced occurs after 20 cycles of freezing-thawing, the critical value of the stress intensity factor being K1c = 0.09 MN/m3/2. An increase in the initial moisture content of the substrate to W = 1% leads to a significant increase in the fracture toughness of the PVAC coating, with the appearance of cracks in the introduction of the Vickers indenter only after 25 cycles of freezing-thawing. A further increase in the substrate moisture during painting leads to the appearance of a more defective structure of the contact layer "coating-substrate" and a greater tendency of the coating to crack formation. Thus, an increase in substrate moisture to W = 4% led to the appearance of cracks in the introduction of the Vickers indenter after 10 cycles of

25.39 0.0379

0.0379 0.0256 0.0404 0.0554 0.0554 0.048 0.054 0.0257 0.02147 Peeling

**Load, N Coefficient of intensity of stresses, К1с, МН/m1.5**

The results of the analysis of fracture toughness indexes of coatings PF-115 indicate that after 15 cycles of freezing-thawing, the coefficient of intensity of stresses in coatings on absolutely dry substrate is K1 = 0.057 MN/m3/2, whereas for substrate moisture W = 4%—

Coatings PF-115 on a dry substrate are characterized by peeling after 15 cycles of freezingthawing, while at substrate moisture content W = 4%, peeling is not observed and after 35

freezing-thawing.

**Kind of colorful composition**

102 Coatings and Thin-Film Technologies

Alkyd PF-115 Alkyd PF-268 Oil MA-15

**Substrate moisture, %**

1 2 3 4 5

**Table 7.** Parameters crack education of protective-decorative coatings.

0.13 0.9 10.5 6.4 0.13 0.9 10.5 6.4 0.33 6.4 6.4

**Porosity of the substrate, %**

K1 = 0.026 MN/m3/2.


imprint, and the fracture toughness that is proposed. The numerical values of the coefficient of intensity of stresses in coatings are given, depending on the type and duration of aging, the

Crack Resistance of Paint Coatings, Cement Concretes http://dx.doi.org/10.5772/intechopen.78537 105

It is established that with an increase in the roughness of the coating surface, the value of the stress intensity coefficient is increased. Increasing the substrate moisture at the time of application of the paint composition results in a more defective structure of the contact layer "coating-substrate" and a greater propensity to cracking. It has been revealed that there exists a value for the optimum substrate moisture for each particular coating in terms of its fracture toughness. It is shown that during the aging of protective and decorative coatings of the exterior walls of buildings, a mechanism of their destruction from elastic-ductile to

The conducted researches are justification for recommendations at developing paints, at carrying out of research works with use of technique of an assessment of crack resistance of coatings according to the offered scheme. It will allow to more reasonably predict firmness of coatings and also to optimize finishing structures for the purpose of receiving coatings with a

[1] Sanzharovsky AT. Physical and Mechanical Properties of Polymer and Paint Coatings.

[2] Karyakina MI. Testing of Paint and Varnish Materials and Coatings. Moscow: Chemistry;

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tings Technology and Research. 2004;**1**(2):117-125

porosity of the cement substrate.

brittle changes occurs.

**Author details**

Valentina Loganina

**References**

1988. p. 272

complex of the given properties.

Address all correspondence to: loganin@mail.ru

Moscow: Chemistry; 1978. 183 p

Penza State University of Architecture and Construction, Penza, Russia

**Table 8.** Parameters crack education of protective-decorative coatings depending on the initial moisture content of the substrate.

### **5. Conclusion**

A method for evaluating the fracture toughness of coatings of cement concrete is proposed. This is a method based on the relationship between the crack length, the Vickers indenter imprint, and the fracture toughness that is proposed. The numerical values of the coefficient of intensity of stresses in coatings are given, depending on the type and duration of aging, the porosity of the cement substrate.

It is established that with an increase in the roughness of the coating surface, the value of the stress intensity coefficient is increased. Increasing the substrate moisture at the time of application of the paint composition results in a more defective structure of the contact layer "coating-substrate" and a greater propensity to cracking. It has been revealed that there exists a value for the optimum substrate moisture for each particular coating in terms of its fracture toughness. It is shown that during the aging of protective and decorative coatings of the exterior walls of buildings, a mechanism of their destruction from elastic-ductile to brittle changes occurs.

The conducted researches are justification for recommendations at developing paints, at carrying out of research works with use of technique of an assessment of crack resistance of coatings according to the offered scheme. It will allow to more reasonably predict firmness of coatings and also to optimize finishing structures for the purpose of receiving coatings with a complex of the given properties.

### **Author details**

Valentina Loganina

Address all correspondence to: loganin@mail.ru

Penza State University of Architecture and Construction, Penza, Russia

### **References**

**5. Conclusion**

Critical coefficient of intensity of stresses.

\*

substrate.

**Name of coating Moisture of** 

104 Coatings and Thin-Film Technologies

**the substrate, W, %**

**Impact type Hardness** 

PF-115 0 Hardening 48 1 0.006 Five freeze-thaw

cycles

10 freeze-thaw cycles

15 freeze-thaw cycles

Five freeze-thaw

15 freeze-thaw cycles

20 freeze-thaw cycles

30 freeze-thaw cycles

35 freeze-thaw cycles

Five freeze-thaw

15 freeze-thaw cycles

20 freeze-thaw cycles

30 freeze-thaw cycles

35 freeze-thaw cycles

cycles

cycles

**H, N, mm2**

1 Hardening 24 1 0.019

4 Hardening 18 1 0.017

**The ratio of the half-length of the crack C to the size of the semi-diagonal of the imprint a**

87 1 0.033

111 1 0.05

142 1 0.057

45 1 0.026

65 1 0.04

99 1 0.049

108 1 0.053

126 1 0.055

37 1 0.024

44 1 0.026

63 1 0.038

86 1 0.045

97 1 0.05

**Coefficient of intensity of stresses, К1с, МН/**

Peeling of the coating is observed

Peeling of the coating is observed

**m1.5**

A method for evaluating the fracture toughness of coatings of cement concrete is proposed. This is a method based on the relationship between the crack length, the Vickers indenter

**Table 8.** Parameters crack education of protective-decorative coatings depending on the initial moisture content of the


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106 Coatings and Thin-Film Technologies


**Section 2**

**Deposition Technologies**
