**3. Formation of insulating coatings by plasma chemical vapor deposition (CVD)**

The ion-plasma sputtering as one of the CVD technique methods was chosen as an alternative method of synthesizing protective insulating coatings on stainless steels. As materials for the sputtering, it is advisable to use pure compounds based on nitrides and oxides of metals, which have a wide range of functional properties [27–31].

The most optimal materials for application to the surfaces of ferritic and ferriticmartensitic stainless steels are coatings of magnesia (MgO), alumina (Al2O3), titanium dioxide (TiO2), and aluminum nitride (AlN) (**Table 6**). It should be noted that the thermal conductivity of beryllium oxide (BeO) is nine times higher than of the listed materials. Thus, it can significantly increase the power of FHE on stainless steel substrates. However, the use of BeO is hampered by its high cost and toxicity during processing. Traditionally, this or that type of ceramics is obtained by sintering oxide or nitride powders at various temperatures. However, high-energy methods of surface engineering are promising when forming dielectric layers on the surfaces of various structural materials. The properties of the layers synthesized in this way, which are given in various literary sources, differ significantly and depend on a method used to form these layers.

Magnesium oxide is the only oxygen compound of magnesium. It exists in only one modification and crystallizes in a cubic system. The crystalline form of MgO is called periclase. It has a potassium salt lattice with a parameter of 0.42 nm. By its chemical properties, MgO is a basic oxide, and, as a consequence, it combines with

**2.2 X-ray structural studies of functional coatings based on glass crystalline**

confirms the fact of the formation of a crystalline structure during synthesis. It shows pronounced peaks corresponding to lead and zinc as pure chemical elements. The shape and height of the peaks indicate the presence of crystalline phases in the

To determine the chemical homogeneity of the coatings, a local EDS microanalysis was performed. It was detected lead, zinc, and boron oxides in all types of synthesized coatings in various proportions depending on the location of the analysis. In the coatings based on SC 90-1, SC 88, and SC 90 powders, BaO and SiO2 oxides were found. Due to the low accuracy of the EDS method when detecting light elements, including boron oxide, the volume fraction of each of the oxides in

X-ray photoelectron spectroscopy (XPS) was used to determine the qualitative and quantitative chemical composition of coatings, as it allows doing this more accurately and detecting all chemical elements except hydrogen and helium. This technique is based on obtaining XPS spectra by irradiating a material with a beam of X-rays (Al Kα) while simultaneously measuring the kinetic energy and number of electrons that escape from the top 0 to 10 nm of the coating being analyzed. To improve the reliability of the results obtained, the evaluation of the chemical composition was performed in three different areas on the sample surfaces. According to the investigation results, experimental diffraction patterns were constructed

It was revealed that the elemental compositions of the synthesized coatings correspond to the compositions of the original powders within the error of the device (2%). As an example, the chemical composition of the synthesized coating based on sital cement of SC 90-1 grade is as follows: 75.3 wt% PbO, 11.6 wt% ZnO, 8.5 wt% B2O3, 2.1 wt% SiO2, 0.8 wt% Al2O3, and 1.7 wt% BaO. Besides, unwanted phases of 0.15% BaSO4, 1.02 % (ZnS + ZnF2), and 0.28% B4C, which could be formed during prolonged isothermal holding and interaction of powder particles with residues of nitrocellulose lacquer, were detected. To avoid the formation of these phases, it is necessary to increase the annealing temperature to accelerate the crystallization process and simultaneously reduce the holding time. However, it

*X-ray pattern of the surface of the coating based on the glass crystalline material of SC 100-1 grade.*

the coating structure could not be estimated correctly.

The diffraction pattern of the coating based on the SC 100-1 powder (**Figure 10**)

**materials**

*Engineering Steels and High Entropy-Alloys*

volume of the coating.

(**Figure 11**).

**Figure 10.**

**66**


#### **Table 6.**

*Physical properties of materials of the dielectric layer.*

all acidic oxides and dissolves in inorganic acids (partially in water). Hydration of MgO limits the possibility of its fine fragmentation in water, as it increases greatly with increasing both fragmentation and temperature. MgO sintering is facilitated by TiO2, ZrO2, Al2O3, and Fe2O3 impurities. Magnesium oxide is a good insulator. Its crystals have ionic conductivity. The work [32] presents the study of the electrical properties of magnesium oxide layers formed by magnetron sputtering of magnesium followed by annealing in air. It is found out that, depending on the annealing temperature, the layer resistivity ranges from 1.7 <sup>10</sup><sup>7</sup> to 2.81 <sup>10</sup><sup>12</sup> mOhm cm. The work [33] presents the study of MgO layers formed by the sol-gel method. The obtained results indicate the weak crystallinity and the disorientation of the magnesium oxide phase. The breakdown voltage ranges from 5 to 78 MW/cm. Leakage currents when heated to 250°C increase within 10<sup>9</sup> –10<sup>3</sup> A/cm<sup>2</sup> . In the work [34], using the method of pulsed laser deposition, MgO polycrystalline films with various crystallographic orientations were obtained. The value of the dielectric constant was 9.67 for a density near the band gap of 4.5 1011 eV<sup>1</sup> cm<sup>2</sup> . In some works [32–35], magnesia ceramics are characterized by a dense fine-crystalline structure with perfect physical, mechanical, and electrophysical properties.

that delay the growth of crystals, the oxides TiO2, MnO2, and Fe2O3 should be noted. The most effective is the addition of TiO2, forming a solid solution and reducing the sintering temperature of corundum to 1500–1550°C. The introduction of the Mn4+ ion, whose ion radius is 0.052 nm, should also lead to the formation of an interstitial solid solution. Another group of additives affects the growth of crystals during annealing. The addition of such compounds results in crystal growth, sometimes very intense, and the sintering temperature can either decrease or remain unchanged. Thus, there are additives that simultaneously reduce the

The formation of coatings has many technological difficulties related to the provision of the required stoichiometry, phase composition, crystalline structure, impurity level, porosity, etc. In the work [5], the influence of the parameters of the Al2O3 film forming process in high-frequency magnetron discharge on the structure, phase composition, porosity, and electrical strength of the films was investigated. It is shown that under appropriate conditions, multiphase polycrystalline coatings with stable α-structure are formed. Their electrical strength varies in the range of 0.4–1.6 V/m. The high chemical and thermal resistances of aluminum

Aluminum nitride is the only compound of nitrogen with aluminum that has high chemical resistance. It crystallizes to form a hexagonal lattice of the wurtzite type with parameters a = 0.31–0.313 nm and c = 0.493–0.498 nm. Aluminum nitride has no modification, which simplifies the technology of manufacturing products. At 1900–2000°C, AlN decomposes. AlN ceramics can be used in an inert environment up to 1800°C, in vacuum up to 1600°C, in air up to 1300–1400°C. The high thermal

and the heat resistance make these ceramics promising for use in conditions with a sharp change in temperature. In particular, the thermophysical properties of aluminum nitride ceramics were investigated in. The heat capacity of *Cp* was measured using the standard adiabatic calorimeter method. The thermal conductivity *χ* was determined by the absolute stationary longitudinal heat flux method. It was found

), which is a characteristic of aluminum nitride,

. The CLTE was

, *χ* = 123 W m<sup>1</sup> K<sup>1</sup>

. Aluminum nitride has also been used as a protective coating

oxide make it possible to use it in the manufacture of heating devices.

sintering temperature and affect the growth of crystals.

*Technologies of High-Temperature Insulating Coatings on Stainless Steels*

*DOI: http://dx.doi.org/10.5772/intechopen.91334*

*Coordination polyhedra in α-Al2O3 structure [36].*

conductivity (140–280 W m<sup>1</sup> K<sup>1</sup>

(2.9–3.4) <sup>10</sup><sup>6</sup> <sup>K</sup><sup>1</sup>

**69**

**Figure 12.**

that at room temperature *Cp* = 25 J mol<sup>1</sup> K<sup>1</sup>

Another promising material widely used in radio and microelectronic technology is aluminum oxide [33–35]. It is known for its nine crystallographic modifications, the most important for the industry being α-modification. The corundum structure (**Figure 12**) can be considered as a hexagonal dense packing of O<sup>2</sup> ions in which 2/3 of the octahedral gaps are occupied by Al3+.

The sintering temperature of α-Al2O3 of technical purity (99–99.5) and the fraction grade of 1–2 μm without additives are 1700–1750°C. At this temperature, a density of 3.75–3.85 g/cm<sup>3</sup> is reached. The porosity of the sintered corundum is mainly closed and internal, and the pore shape is circular. The size of initial Al2O3 particles has a decisive influence on the sintering temperature. The maximum size of Al2O3 crystallites capable of active solid phase sintering should not exceed 3–5 μm. By adding to Al2O3 powder some substances in the form of oxides or salts called mineralizers, it is possible to reduce the sintering temperature of corundum by 150–200°C, and the character of crystallization can become directional because the delay or growth of crystals in certain directions is ensured. Among the additives

*Technologies of High-Temperature Insulating Coatings on Stainless Steels DOI: http://dx.doi.org/10.5772/intechopen.91334*

**Figure 12.** *Coordination polyhedra in α-Al2O3 structure [36].*

that delay the growth of crystals, the oxides TiO2, MnO2, and Fe2O3 should be noted. The most effective is the addition of TiO2, forming a solid solution and reducing the sintering temperature of corundum to 1500–1550°C. The introduction of the Mn4+ ion, whose ion radius is 0.052 nm, should also lead to the formation of an interstitial solid solution. Another group of additives affects the growth of crystals during annealing. The addition of such compounds results in crystal growth, sometimes very intense, and the sintering temperature can either decrease or remain unchanged. Thus, there are additives that simultaneously reduce the sintering temperature and affect the growth of crystals.

The formation of coatings has many technological difficulties related to the provision of the required stoichiometry, phase composition, crystalline structure, impurity level, porosity, etc. In the work [5], the influence of the parameters of the Al2O3 film forming process in high-frequency magnetron discharge on the structure, phase composition, porosity, and electrical strength of the films was investigated. It is shown that under appropriate conditions, multiphase polycrystalline coatings with stable α-structure are formed. Their electrical strength varies in the range of 0.4–1.6 V/m. The high chemical and thermal resistances of aluminum oxide make it possible to use it in the manufacture of heating devices.

Aluminum nitride is the only compound of nitrogen with aluminum that has high chemical resistance. It crystallizes to form a hexagonal lattice of the wurtzite type with parameters a = 0.31–0.313 nm and c = 0.493–0.498 nm. Aluminum nitride has no modification, which simplifies the technology of manufacturing products. At 1900–2000°C, AlN decomposes. AlN ceramics can be used in an inert environment up to 1800°C, in vacuum up to 1600°C, in air up to 1300–1400°C. The high thermal conductivity (140–280 W m<sup>1</sup> K<sup>1</sup> ), which is a characteristic of aluminum nitride, and the heat resistance make these ceramics promising for use in conditions with a sharp change in temperature. In particular, the thermophysical properties of aluminum nitride ceramics were investigated in. The heat capacity of *Cp* was measured using the standard adiabatic calorimeter method. The thermal conductivity *χ* was determined by the absolute stationary longitudinal heat flux method. It was found that at room temperature *Cp* = 25 J mol<sup>1</sup> K<sup>1</sup> , *χ* = 123 W m<sup>1</sup> K<sup>1</sup> . The CLTE was (2.9–3.4) <sup>10</sup><sup>6</sup> <sup>K</sup><sup>1</sup> . Aluminum nitride has also been used as a protective coating

all acidic oxides and dissolves in inorganic acids (partially in water). Hydration of MgO limits the possibility of its fine fragmentation in water, as it increases greatly with increasing both fragmentation and temperature. MgO sintering is facilitated by TiO2, ZrO2, Al2O3, and Fe2O3 impurities. Magnesium oxide is a good insulator. Its crystals have ionic conductivity. The work [32] presents the study of the electrical properties of magnesium oxide layers formed by magnetron sputtering of mag-

**Parameter MgO AlN Al2O3 TiO2** Density, g/cm<sup>3</sup> 3.58 3.2–3.24 3.6 3.9–4.3 Dielectric constant 8–9 8.5 12.0 31–173 (114 for

Relative density 0.95–0.96 0.96–0.98 0.94–0.98 - Melting point, °С 2800 2400 2050 1855

Sintering temperature, °С 1600 - 1700–1750 -

Up to 2000 in air

Thermal conductivity, W m<sup>1</sup> K<sup>1</sup> 28.0 66 24–29

а = 0.42 nm

Operating temperature range for

*Engineering Steels and High Entropy-Alloys*

Coefficient of linear thermal expansion <sup>α</sup>, 106 <sup>К</sup><sup>1</sup>

Modification Cubic

*Physical properties of materials of the dielectric layer.*

an atmosphere, °С

**Table 6.**

**68**

work [34], using the method of pulsed laser deposition, MgO polycrystalline films with various crystallographic orientations were obtained. The value of the dielectric

Another promising material widely used in radio and microelectronic technology is aluminum oxide [33–35]. It is known for its nine crystallographic modifications, the most important for the industry being α-modification. The corundum structure (**Figure 12**) can be considered as a hexagonal dense packing of O<sup>2</sup> ions in

The sintering temperature of α-Al2O3 of technical purity (99–99.5) and the fraction grade of 1–2 μm without additives are 1700–1750°C. At this temperature, a density of 3.75–3.85 g/cm<sup>3</sup> is reached. The porosity of the sintered corundum is mainly closed and internal, and the pore shape is circular. The size of initial Al2O3 particles has a decisive influence on the sintering temperature. The maximum size of Al2O3 crystallites capable of active solid phase sintering should not exceed 3–5 μm. By adding to Al2O3 powder some substances in the form of oxides or salts called mineralizers, it is possible to reduce the sintering temperature of corundum by 150–200°C, and the character of crystallization can become directional because the delay or growth of crystals in certain directions is ensured. Among the additives

–10<sup>3</sup> A/cm<sup>2</sup>

. In the

rutile)

Anatase Brookite Rutile

1300–1400 1600–1700 -

α-Trigonal β-Hexagonal γ-Cubic

11.7–14.2 4.8 5-6.7 9.8–10.8

Hexagonal a = 0.31–0.313 nm c = 0.493–0.498 nm

. In some

nesium followed by annealing in air. It is found out that, depending on the annealing temperature, the layer resistivity ranges from 1.7 <sup>10</sup><sup>7</sup> to 2.81 <sup>10</sup><sup>12</sup> mOhm cm. The work [33] presents the study of MgO layers formed by the sol-gel method. The obtained results indicate the weak crystallinity and the disorientation of the magnesium oxide phase. The breakdown voltage ranges from 5 to 78 MW/cm.

constant was 9.67 for a density near the band gap of 4.5 1011 eV<sup>1</sup> cm<sup>2</sup>

works [32–35], magnesia ceramics are characterized by a dense fine-crystalline structure with perfect physical, mechanical, and electrophysical properties.

Leakage currents when heated to 250°C increase within 10<sup>9</sup>

which 2/3 of the octahedral gaps are occupied by Al3+.

for electronic components under the influence of percussion mechanical loads, which is associated with its high hardness (7–8 on the Mohs scale) [2, 5].

0.15 A was performed. During the purification, argon ions bombarded the substrate, removing residues of contaminants and impurities from its surface [1, 5]. The ion-plasma discharge device is designed to form nanoscale structure elements [32–37] and, due to its low operating temperatures, allows to apply functional layers to virtually any substrate (metal, plastic, ceramics, etc.). The plasma discharge chamber consists of three process modules (helicon source discharge chamber, drift chamber, treatment chamber), which are connected by flanges using vacuum seals. The scheme of the discharge plasma chamber is shown in **Figure 13**. A working chamber is connected directly to a flange of the vacuum system. A table with the substrate is located in this chamber. A drift chamber with docked vacuum arc accelerators is located above the working chamber. The drift chamber is

*Technologies of High-Temperature Insulating Coatings on Stainless Steels*

*DOI: http://dx.doi.org/10.5772/intechopen.91334*

connected to the helicon source discharge camera. Its top is covered by quartz glass,

The magnetic coil located in the working chamber provides the plasma stream from the helicon source to the substrate. Besides, it makes it possible to direct the plasma stream from the plasma-arc accelerator toward the substrate, which significantly reduces the amount of transferred micro-droplet fraction that is formed

For the formation of high-quality insulating and resistive layers, it is necessary to constantly monitor the state of the substrate surface in all stages of the technological process. Only in this case can the reproduction of the results, the stability of the process, and the high level of quality of coatings on the surfaces of stainless steels be ensured. When using standard high-vacuum equipment, the surface of the substrate always contains several adsorbed monolayers with residual components of the atmosphere. A plasma helicon source and plasma-arc accelerators ensure constant action of an argon ion flux on the substrate in all stages of the process. In this case, the density and energy of the ion flux are sufficient to effectively remove the adsorbed light residual components of the atmosphere from the substrate surface

*The scheme of the ion-plasma discharge chamber [2, 37]: (1) discharge chamber, (2) drift chamber, (3) table, (4) substrate, (5) plasma-arc accelerator, (6) dielectric window, (7) antenna, (8) coordination unit, (9) HF generator, (10) permanent magnet or an electromagnet, (11) heating system, (12) optical spectrometer,*

through which high-frequency energy is supplied during discharge using an antenna connected via a coordination unit to a high-frequency generator.

when working with fusible materials (Tm < 2000 K).

**Figure 13.**

**71**

*(13) displacement probe, and (14) gas inlet.*

Ceramics, the crystalline base of which is titanium dioxide, are considered as a separate grade of technical ceramics since this compound has a high value of relative permittivity as compared to other ceramic materials (**Table 6**). Titanium oxide films, due to their physicochemical properties, are widely used as protective and optically sensitive coatings in gas sensors and during photocatalysis. They are also used in microelectronics (dynamic memory, field-effect transistors, ferromagnetic materials). Besides, TiO2 is used in electronic engineering, in particular for the manufacture of capacitors. Titanium dioxide does not occur in nature in pure form. It is obtained by chemical processing of titanium ores FeTiO3, CaTiSiO5, CaTiO3, and others. Titanium dioxide is available in three modifications: anatase, brookite, and rutile. Rutile is a stable high-temperature form of titanium dioxide, and the other two transforms irreversibly into this. Anatase modification of titanium dioxide is used as a catalyst and component of solar cells. Due to its high reflectance, it is used to protect spacecraft from the sun's radiation. For the ceramics industry, they produce a special brand of TiO2 called "capacitor."

Thus, the properties of MgO, Al2O3, TiO2, and AlN ceramics depend strongly on the method of their production and a set of certain parameters of the initial powders. Of the four dielectric materials presented, all to a greater or lesser extent satisfy the requirements relating to insulating layers of film heating elements based on a stainless steel substrate. Thus, titanium oxide has high relative permittivity, while aluminum nitride has high thermal conductivity. Both magnesium and aluminum oxides have satisfactory electrical strength at low CLTE. The combination of such dielectric properties with the verified technology of forming ceramic coatings on the surfaces of structural materials can serve effectively for the replacement of FHE constituents.

Among the high-energy surface engineering technologies, the ion-plasma sputtering method is the most promising for the formation of dielectric and resistive layers on the surfaces of stainless steels [37]. Therefore, the functional layers were formed by this method using a multifunctional ion-plasma discharge system. To create the dielectric layer by ion-plasma sputtering, a number of materials were analyzed that would provide the dielectric properties of the FHE and meet technological, economic, and environmental requirements. Oxides and nitrides of Mg, Ti, and Al, which corresponded to the abovementioned dielectric properties of FHEs [38], were selected for the application.

The stainless steel substrates were placed in the substrate holder of a reaction chamber at a distance of 40–50 cm from the cathode. The cathodes were made of aluminum of A1 99.7 grade (for AlN and Al2O3), magnesium of Mg 98 grade (for MgO), titanium of VT1-00 grade (for TiO2), and Cr20Ni80-H nichrome alloy (for the resistive layer). They were manufactured in the form of rods with a diameter of 40 mm. The sputtering was performed under various modes in oxygen, nitrogen, or vacuum atmosphere. Each mode was characterized by certain parameters such as the gas pressure in the reaction chamber, the substrate potential, the current, and the sputtering time. To increase the adhesion strength of the layer, the substrate was heated using a furnace placed directly in the reaction chamber of the ion-plasma device. The plasma stream was directed from the cathode region toward the substrate. The beam divergence angle was about 20°. This provided optimum ion density in the plasma stream [37].

After the samples are uploaded, the reaction chamber of the hybrid ion-plasma device was evacuated. Then, using the helicon source in column mode, the final purification of the substrates in an argon plasma stream for 30 min at a pressure of 0.93 Pa, the potential on the substrate E = 100 V, and the current on the sample of

#### *Technologies of High-Temperature Insulating Coatings on Stainless Steels DOI: http://dx.doi.org/10.5772/intechopen.91334*

0.15 A was performed. During the purification, argon ions bombarded the substrate, removing residues of contaminants and impurities from its surface [1, 5].

The ion-plasma discharge device is designed to form nanoscale structure elements [32–37] and, due to its low operating temperatures, allows to apply functional layers to virtually any substrate (metal, plastic, ceramics, etc.). The plasma discharge chamber consists of three process modules (helicon source discharge chamber, drift chamber, treatment chamber), which are connected by flanges using vacuum seals. The scheme of the discharge plasma chamber is shown in **Figure 13**. A working chamber is connected directly to a flange of the vacuum system. A table with the substrate is located in this chamber. A drift chamber with docked vacuum arc accelerators is located above the working chamber. The drift chamber is connected to the helicon source discharge camera. Its top is covered by quartz glass, through which high-frequency energy is supplied during discharge using an antenna connected via a coordination unit to a high-frequency generator.

The magnetic coil located in the working chamber provides the plasma stream from the helicon source to the substrate. Besides, it makes it possible to direct the plasma stream from the plasma-arc accelerator toward the substrate, which significantly reduces the amount of transferred micro-droplet fraction that is formed when working with fusible materials (Tm < 2000 K).

For the formation of high-quality insulating and resistive layers, it is necessary to constantly monitor the state of the substrate surface in all stages of the technological process. Only in this case can the reproduction of the results, the stability of the process, and the high level of quality of coatings on the surfaces of stainless steels be ensured. When using standard high-vacuum equipment, the surface of the substrate always contains several adsorbed monolayers with residual components of the atmosphere. A plasma helicon source and plasma-arc accelerators ensure constant action of an argon ion flux on the substrate in all stages of the process. In this case, the density and energy of the ion flux are sufficient to effectively remove the adsorbed light residual components of the atmosphere from the substrate surface

#### **Figure 13.**

*The scheme of the ion-plasma discharge chamber [2, 37]: (1) discharge chamber, (2) drift chamber, (3) table, (4) substrate, (5) plasma-arc accelerator, (6) dielectric window, (7) antenna, (8) coordination unit, (9) HF generator, (10) permanent magnet or an electromagnet, (11) heating system, (12) optical spectrometer, (13) displacement probe, and (14) gas inlet.*

for electronic components under the influence of percussion mechanical loads, which is associated with its high hardness (7–8 on the Mohs scale) [2, 5].

separate grade of technical ceramics since this compound has a high value of relative permittivity as compared to other ceramic materials (**Table 6**). Titanium oxide films, due to their physicochemical properties, are widely used as protective and optically sensitive coatings in gas sensors and during photocatalysis. They are also used in microelectronics (dynamic memory, field-effect transistors, ferromagnetic materials). Besides, TiO2 is used in electronic engineering, in particular for the manufacture of capacitors. Titanium dioxide does not occur in nature in pure form. It is obtained by chemical processing of titanium ores FeTiO3, CaTiSiO5, CaTiO3, and others. Titanium dioxide is available in three modifications: anatase, brookite, and rutile. Rutile is a stable high-temperature form of titanium dioxide, and the other two transforms irreversibly into this. Anatase modification of titanium dioxide is used as a catalyst and component of solar cells. Due to its high reflectance, it is used to protect spacecraft from the sun's radiation. For the ceramics industry, they

produce a special brand of TiO2 called "capacitor."

*Engineering Steels and High Entropy-Alloys*

FHE constituents.

[38], were selected for the application.

density in the plasma stream [37].

**70**

Ceramics, the crystalline base of which is titanium dioxide, are considered as a

Thus, the properties of MgO, Al2O3, TiO2, and AlN ceramics depend strongly on the method of their production and a set of certain parameters of the initial powders. Of the four dielectric materials presented, all to a greater or lesser extent satisfy the requirements relating to insulating layers of film heating elements based on a stainless steel substrate. Thus, titanium oxide has high relative permittivity, while aluminum nitride has high thermal conductivity. Both magnesium and aluminum oxides have satisfactory electrical strength at low CLTE. The combination of such dielectric properties with the verified technology of forming ceramic coatings on the surfaces of structural materials can serve effectively for the replacement of

Among the high-energy surface engineering technologies, the ion-plasma sputtering method is the most promising for the formation of dielectric and resistive layers on the surfaces of stainless steels [37]. Therefore, the functional layers were formed by this method using a multifunctional ion-plasma discharge system. To create the dielectric layer by ion-plasma sputtering, a number of materials were analyzed that would provide the dielectric properties of the FHE and meet technological, economic, and environmental requirements. Oxides and nitrides of Mg, Ti, and Al, which corresponded to the abovementioned dielectric properties of FHEs

The stainless steel substrates were placed in the substrate holder of a reaction chamber at a distance of 40–50 cm from the cathode. The cathodes were made of aluminum of A1 99.7 grade (for AlN and Al2O3), magnesium of Mg 98 grade (for MgO), titanium of VT1-00 grade (for TiO2), and Cr20Ni80-H nichrome alloy (for the resistive layer). They were manufactured in the form of rods with a diameter of 40 mm. The sputtering was performed under various modes in oxygen, nitrogen, or vacuum atmosphere. Each mode was characterized by certain parameters such as the gas pressure in the reaction chamber, the substrate potential, the current, and the sputtering time. To increase the adhesion strength of the layer, the substrate was heated using a furnace placed directly in the reaction chamber of the ion-plasma device. The plasma stream was directed from the cathode region toward the substrate. The beam divergence angle was about 20°. This provided optimum ion

After the samples are uploaded, the reaction chamber of the hybrid ion-plasma device was evacuated. Then, using the helicon source in column mode, the final purification of the substrates in an argon plasma stream for 30 min at a pressure of 0.93 Pa, the potential on the substrate E = 100 V, and the current on the sample of layer by the selective sputtering mechanism and do not interfere with the basic technological operations [37].

Functional dielectric coatings based on oxides of Mg, Ti, and Al for FHEs on stainless steels were obtained by ion-plasma sputtering and were investigated by

• Study of peculiarities of formation of structure and phase composition of

The study of the surface morphology of the formed layers was carried out using an electron microscope JSM-6490LV (JEOL, Japan), equipped with an analytical setup for elemental analysis (spectrometer with energy dispersion INCA Energy+ Oxford). Determination of thickness and the structure and elemental composition analyses were performed using a scanning electron microscope with a PEMMA-102-02 microanalyzer. Surface topography was investigated using a Solver P47-PRO

The surface morphology of the formed dielectric layer with MgO is shown in **Figure 15**. The grain sizes varied from 20 to 70 nm. Grain growth is likely carried out by an islet mechanism when the formed grain becomes the basis for the formation of nuclei of grains and their subsequent growth (**Figure 15**a). Thus, the newly formed grains are combined into clusters of various sizes. Due to the specific structure, the grains are characterized by a large specific surface, which increases because of petals formed in the perpendicular direction (**Figure 15b**). The thickness of the identified petals is in the range of 10–20 nm. This mechanism of the layer structure formation and a significant difference between the sizes of the grain clusters cause a substantially high porosity of the dielectric layer, which can signif-

To study the microtopography of the TiO2 layer surface, SEM and local EDS analyses were performed (**Figure 16**). The surface of the formed layer is continuous and homogeneous, with a small porosity. In some parts of the surface layer, elliptical inclusions of size from 0.08 to 1 μm are observed (**Figure 16b**). Elemental analysis indicates that the bulk of the layer contains titanium and oxygen, the quantitative ratio of which corresponds to the TiO2 compound of stoichiometric composition. Besides, impurity atoms of iron and aluminum were identified, the

*SEM surface microtopography of the MgO dielectric layer [2]. Magnifications: (а) 900 and (b) 2400.*

• Measurement of electrophysical characteristics of functional layers

• Study of adhesion and micromechanical properties

*Technologies of High-Temperature Insulating Coatings on Stainless Steels*

various methods, namely:

*DOI: http://dx.doi.org/10.5772/intechopen.91334*

dielectric layers

atomic force microscope.

**Figure 15.**

**73**

icantly increase its hygroscopicity [1].

presence of which was caused by contamination [2].

The ion-plasma device consists of a high-frequency plasma source based on helicon discharge, for the implementation of the CVD process, and plasma-arc accelerators. The high degree of ionization of the plasma stream (more than 80 %) formed by the plasma-arc accelerator allows controlling the thickness of the layers during their application by the value of the total charge transferred to the substrate. **Figure 14** shows a general view of the discharge chamber of an ion-plasma device [2, 37].

Plasma helicon sources are non-electrode high-frequency induction magnetic field sources capable of generating dense (*n* = 1011–10<sup>13</sup> cm<sup>3</sup> ) low-temperature (*T*<sup>e</sup> = 2–10 eV) plasma over wide ranges of the operating gas pressure (*P* = 0.5–100 mTorr) and magnetic fields (*B* = 10–2000 Hs). These sources range in size from a few centimeters to several meters. They are excited by simple antennas over a wide frequency range (*f* = 7–100 MHz) and effectively generate plasma at relatively low specific HF power (*P*rf ≥ 50–100 mW/cm<sup>3</sup> ) [37].

In general, this method is not effective in applying dielectric and resistive layers under conditions of mass production of FHEs, because of the high cost of the device for magnetron sputtering; the small size of the camera, which does not allow sputtering on a large number of samples simultaneously; and the lack of quality cleaning of the working chamber when replacing sputtering cathodes (MgO, Al2O3, nichrome, etc.). The application of this method is also limited since the obtained dielectric layers have a high porosity, which increases the probability of electrical breakdown of the coating and failure of the heating device as a whole.

**Figure 14.** *General view of the discharge chamber of an ion-plasma device [2].*

Functional dielectric coatings based on oxides of Mg, Ti, and Al for FHEs on stainless steels were obtained by ion-plasma sputtering and were investigated by various methods, namely:


The study of the surface morphology of the formed layers was carried out using an electron microscope JSM-6490LV (JEOL, Japan), equipped with an analytical setup for elemental analysis (spectrometer with energy dispersion INCA Energy+ Oxford). Determination of thickness and the structure and elemental composition analyses were performed using a scanning electron microscope with a PEMMA-102-02 microanalyzer. Surface topography was investigated using a Solver P47-PRO atomic force microscope.

The surface morphology of the formed dielectric layer with MgO is shown in **Figure 15**. The grain sizes varied from 20 to 70 nm. Grain growth is likely carried out by an islet mechanism when the formed grain becomes the basis for the formation of nuclei of grains and their subsequent growth (**Figure 15**a). Thus, the newly formed grains are combined into clusters of various sizes. Due to the specific structure, the grains are characterized by a large specific surface, which increases because of petals formed in the perpendicular direction (**Figure 15b**). The thickness of the identified petals is in the range of 10–20 nm. This mechanism of the layer structure formation and a significant difference between the sizes of the grain clusters cause a substantially high porosity of the dielectric layer, which can significantly increase its hygroscopicity [1].

To study the microtopography of the TiO2 layer surface, SEM and local EDS analyses were performed (**Figure 16**). The surface of the formed layer is continuous and homogeneous, with a small porosity. In some parts of the surface layer, elliptical inclusions of size from 0.08 to 1 μm are observed (**Figure 16b**). Elemental analysis indicates that the bulk of the layer contains titanium and oxygen, the quantitative ratio of which corresponds to the TiO2 compound of stoichiometric composition. Besides, impurity atoms of iron and aluminum were identified, the presence of which was caused by contamination [2].

**Figure 15.** *SEM surface microtopography of the MgO dielectric layer [2]. Magnifications: (а) 900 and (b) 2400.*

layer by the selective sputtering mechanism and do not interfere with the basic

The ion-plasma device consists of a high-frequency plasma source based on helicon discharge, for the implementation of the CVD process, and plasma-arc accelerators. The high degree of ionization of the plasma stream (more than 80 %) formed by the plasma-arc accelerator allows controlling the thickness of the layers during their application by the value of the total charge transferred to the substrate. **Figure 14** shows a general view of the discharge chamber of an ion-plasma device

Plasma helicon sources are non-electrode high-frequency induction magnetic

(*T*<sup>e</sup> = 2–10 eV) plasma over wide ranges of the operating gas pressure (*P* = 0.5–100 mTorr) and magnetic fields (*B* = 10–2000 Hs). These sources range in size from a few centimeters to several meters. They are excited by simple antennas over a wide frequency range (*f* = 7–100 MHz) and effectively generate plasma at relatively low

) [37]. In general, this method is not effective in applying dielectric and resistive layers under conditions of mass production of FHEs, because of the high cost of the device for magnetron sputtering; the small size of the camera, which does not allow sputtering on a large number of samples simultaneously; and the lack of quality cleaning of the working chamber when replacing sputtering cathodes (MgO, Al2O3, nichrome, etc.). The application of this method is also limited since the obtained dielectric layers have a high porosity, which increases the probability of electrical

) low-temperature

field sources capable of generating dense (*n* = 1011–10<sup>13</sup> cm<sup>3</sup>

breakdown of the coating and failure of the heating device as a whole.

specific HF power (*P*rf ≥ 50–100 mW/cm<sup>3</sup>

technological operations [37].

*Engineering Steels and High Entropy-Alloys*

[2, 37].

**Figure 14.**

**72**

*General view of the discharge chamber of an ion-plasma device [2].*

**Figure 16.**

*SEM surface microtopography of the TiO2 dielectric layer [2]. Magnifications: (а) 900 and (b) 2400.*

#### **Figure 17.** *SEM surface microtopography of the Al2O3 dielectric layer [2]. Magnifications: (а) 100 and (b) 1000.*

The Al2O3 layer was formed by ion-plasma deposition of aluminum onto a substrate of the AMg2 aluminum magnesium alloy in an oxygen atmosphere [5]. The result is a layer with a solid, smooth, visually nonporous surface topography (**Figure 17a**). The predominant orientation of individual elements of the surface structure in the form of projections of elongated shape was observed (**Figure 17b**) [1]. clusters along with the individual grains. Each cluster of the size of 200–400 nm is separated by smaller grains (60–150 nm). Similarly to the case of MgO, the outer TiO2 sublayer has a developed surface with well-defined projections, gaps, and grain boundaries (**Figure 19b**). The TiO2 surface is characterized by a spheroidal, row-like structure that could have been formed as a result of local instability of the plasma combustion. The disorientation of TiO2 grains may be due to the high concentration of plasma on the substrate surface and the intensity of heat removal from the substrate. Grain clusters of the size of 50–100 nm in the structure of the TiO2 sublayer are aggregated into packages up to 500 nm in size. More precise studies of the topology of the outer Al2O3 sublayer made it possible to clearly identify grain boundaries and quantify grain size gradation (**Figure 19c**). The microrelief of the Al2O3 surface has a dimple structure which is oriented

*SEM microstructure of polished transverse microsections by the depth of oxide layers (а) Al2O3, (b) TiO2, and*

*Technologies of High-Temperature Insulating Coatings on Stainless Steels*

*DOI: http://dx.doi.org/10.5772/intechopen.91334*

In order to estimate the increase in the loss current and the breakdown voltage of the dielectric layers, the electrical characteristics were investigated for an alternating current case. To obtain the electrical characteristics of the coating, the upper contact made of foil was pressed against its surface, and an electrode was applied over the foil. As the lower contact, a steel substrate was used. Measurements of capacitance (*C*) and tangent of dielectric loss angle (*tg δ*) at frequencies of 120 Hz, 1 kHz, 10 kHz, and 100 kHz were performed by bridge method using RLC meter in the mode of the parallel equivalent circuit with a measurement error of 0.05%. The electrical conductivity values for Al2O3, MgO, and TiO2 coatings at low frequencies are almost unchanged at room temperature and are approximately

, and 3 <sup>10</sup><sup>8</sup> Ohm<sup>1</sup> <sup>m</sup><sup>1</sup>

, respectively.

, 7 <sup>10</sup><sup>8</sup> Ohm<sup>1</sup> <sup>m</sup><sup>1</sup>

In dielectric layers of Al2O3 (at temperatures above 300°C) and MgO

perpendicularly to the surface [4].

<sup>3</sup> <sup>10</sup><sup>8</sup> Ohm<sup>1</sup> <sup>m</sup><sup>1</sup>

**75**

**Figure 18.**

*(c) MgO [2].*

By studying SEM microstructure of polished transverse microsections by the depth of the oxide layers MgO, TiO2, and Al2O3, it was revealed that all the layers except MgO have a gradient structure and consist of two sublayers (**Figure 18**). The inner Al2O3 sublayer, 1.5–2.5 μm thick, is the boundary section of the substrate layer system and has a highly dispersed structure with a grain size of 90–200 nm, and the outer Al2O3 sublayer of a thickness of 12.5–13.5 μm has a fragmented structure (**Figure 18a**). The outer TiO2 sublayer, 12–13 μm thick, is almost defect-free, uniform, solid, and homogeneous (**Figure 18b**). It has a two-phase structure, with dispersed inclusions of a globular shape, which may belong to one of the TiO2 modifications. The inner TiO2 sublayer, 2–3 μm thick, adjoins the substrate surface and completely reproduces its morphology. By studying SEM microstructure of polished transverse microsections of MgO dielectric layers, it was revealed that their thickness ranges within 25–65 μm. MgO layer continuity disturbances were observed only along the boundaries of the separation of large clusters. At higher magnifications, MgO dendritic crystallites were identified, which are oriented in the direction perpendicular to the substrate surface (**Figure 18c**).

The MgO layer has a developed surface with well-defined elongated grains, which demonstrates the growth direction of individual grains. Based on the surface microtopography of the MgO layer (**Figure 19a**), it is revealed that there exist grain *Technologies of High-Temperature Insulating Coatings on Stainless Steels DOI: http://dx.doi.org/10.5772/intechopen.91334*

**Figure 18.**

The Al2O3 layer was formed by ion-plasma deposition of aluminum onto a substrate of the AMg2 aluminum magnesium alloy in an oxygen atmosphere [5]. The result is a layer with a solid, smooth, visually nonporous surface topography (**Figure 17a**). The predominant orientation of individual elements of the surface structure in the form of projections of elongated shape was observed (**Figure 17b**) [1]. By studying SEM microstructure of polished transverse microsections by the depth of the oxide layers MgO, TiO2, and Al2O3, it was revealed that all the layers except MgO have a gradient structure and consist of two sublayers (**Figure 18**). The inner Al2O3 sublayer, 1.5–2.5 μm thick, is the boundary section of the substrate layer system and has a highly dispersed structure with a grain size of 90–200 nm, and the outer Al2O3 sublayer of a thickness of 12.5–13.5 μm has a fragmented structure (**Figure 18a**). The outer TiO2 sublayer, 12–13 μm thick, is almost defect-free, uniform, solid, and homogeneous (**Figure 18b**). It has a two-phase structure, with dispersed inclusions of a globular shape, which may belong to one of the TiO2 modifications. The inner TiO2 sublayer, 2–3 μm thick, adjoins the substrate surface and completely reproduces its morphology. By studying SEM microstructure of polished transverse microsections of MgO dielectric layers, it was revealed that their thickness ranges within 25–65 μm. MgO layer continuity disturbances were observed only along the boundaries of the separation of large clusters. At higher magnifications, MgO dendritic crystallites were identified, which are oriented in

*SEM surface microtopography of the Al2O3 dielectric layer [2]. Magnifications: (а) 100 and (b) 1000.*

*SEM surface microtopography of the TiO2 dielectric layer [2]. Magnifications: (а) 900 and (b) 2400.*

**Figure 16.**

*Engineering Steels and High Entropy-Alloys*

**Figure 17.**

**74**

the direction perpendicular to the substrate surface (**Figure 18c**).

The MgO layer has a developed surface with well-defined elongated grains, which demonstrates the growth direction of individual grains. Based on the surface microtopography of the MgO layer (**Figure 19a**), it is revealed that there exist grain

*SEM microstructure of polished transverse microsections by the depth of oxide layers (а) Al2O3, (b) TiO2, and (c) MgO [2].*

clusters along with the individual grains. Each cluster of the size of 200–400 nm is separated by smaller grains (60–150 nm). Similarly to the case of MgO, the outer TiO2 sublayer has a developed surface with well-defined projections, gaps, and grain boundaries (**Figure 19b**). The TiO2 surface is characterized by a spheroidal, row-like structure that could have been formed as a result of local instability of the plasma combustion. The disorientation of TiO2 grains may be due to the high concentration of plasma on the substrate surface and the intensity of heat removal from the substrate. Grain clusters of the size of 50–100 nm in the structure of the TiO2 sublayer are aggregated into packages up to 500 nm in size. More precise studies of the topology of the outer Al2O3 sublayer made it possible to clearly identify grain boundaries and quantify grain size gradation (**Figure 19c**). The microrelief of the Al2O3 surface has a dimple structure which is oriented perpendicularly to the surface [4].

In order to estimate the increase in the loss current and the breakdown voltage of the dielectric layers, the electrical characteristics were investigated for an alternating current case. To obtain the electrical characteristics of the coating, the upper contact made of foil was pressed against its surface, and an electrode was applied over the foil. As the lower contact, a steel substrate was used. Measurements of capacitance (*C*) and tangent of dielectric loss angle (*tg δ*) at frequencies of 120 Hz, 1 kHz, 10 kHz, and 100 kHz were performed by bridge method using RLC meter in the mode of the parallel equivalent circuit with a measurement error of 0.05%.

The electrical conductivity values for Al2O3, MgO, and TiO2 coatings at low frequencies are almost unchanged at room temperature and are approximately <sup>3</sup> <sup>10</sup><sup>8</sup> Ohm<sup>1</sup> <sup>m</sup><sup>1</sup> , 7 <sup>10</sup><sup>8</sup> Ohm<sup>1</sup> <sup>m</sup><sup>1</sup> , and 3 <sup>10</sup><sup>8</sup> Ohm<sup>1</sup> <sup>m</sup><sup>1</sup> , respectively. In dielectric layers of Al2O3 (at temperatures above 300°C) and MgO

make it possible to obtain high-quality dielectric coatings for film heating elements with high micromechanical and electrophysical properties. The disadvantage of this method is its low productivity, which makes it impossible to use it in mass production. The relevance of using highly efficient and low-cost methods for the synthesis of functional coatings based on glass-ceramic glass sealants is shown. This method is simpler in execution technology and cheaper considering the use of raw materials, which opens up wide prospects for use in mass production. Given the proportionality of the sizes of coatings formed by both methods, the identity of the structure, microtopography of the surface, and the level of electrophysical properties, it can be argued that the first method can be recommended for applying dielectric coatings on the surface of stainless steels which is used for creating film heating elements.

*Technologies of High-Temperature Insulating Coatings on Stainless Steels*

*DOI: http://dx.doi.org/10.5772/intechopen.91334*

, Volodymyr Kulyk<sup>1</sup>

© 2020 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,

, Andriy Trostianchyn<sup>1</sup>

**Author details**

and Tetiana Tepla1

**77**

Zoia Duriagina1,2\*, Taras Kovbasyuk1

1 Lviv Polytechnic National University, Lviv, Ukraine

\*Address all correspondence to: zduriagina@ukr.net

provided the original work is properly cited.

2 John Paul II Catholic University of Lublin, Lublin, Poland

**Figure 19.** *AFM surface microtopography of the dielectric layers (a) MgO, (b) TiO2, and (c) Al2O3 [2].*

(at temperatures above 150°C), a monotonous increase in conductivity begins, caused by the thermal activation of charge carriers from energy levels of 0.63 eV and 0.35 eV, respectively. The electrical conductivity of TiO2 coatings decreases from 4 <sup>10</sup><sup>8</sup> Ohm<sup>1</sup> <sup>m</sup><sup>1</sup> to 1 <sup>10</sup><sup>10</sup> Ohm<sup>1</sup> <sup>m</sup><sup>1</sup> with increasing temperature from room temperature to 250°C and increases to 1 <sup>10</sup><sup>7</sup> Ohm<sup>1</sup> <sup>m</sup><sup>1</sup> with increasing temperature up to 400°C [2].

As can be seen from the temperature dependences of the electrical conductivity of the dielectric layers for an alternating current case, in all investigated coating systems with increasing frequency, the magnitude of the electrical conductivity increases linearly. The increase in *σ*(*ω*) is due to the delay of slow polarization mechanisms. Moreover, the exponent *n* in the equation *σ*(*ω*) = *ω<sup>n</sup>* for the studied materials differs: for the Al2O3 layers *n* = 0.2, whereas for MgO *n* = 0.5. For titanium oxide coatings, it increases to 0.9.

Thus, the peculiarities of formation of the structure, adhesive, micromechanical, and electrophysical properties of the Al2O3, TiO2, and MgO dielectric layers obtained by ion-plasma sputtering were established. An optimal mode of formation of Al2O3, TiO2, and MgO dielectric layers was also determined: the gas pressure in the range <sup>Р</sup> = (1.5–4) <sup>10</sup><sup>2</sup> mmHg, the arc current in the range I = 30–40 A, the displacement potential on substrate Е = 60 V, and the time τ = 20 min.

When selecting the material for the substrate and dielectric layer during the design of a film heating element, it is necessary to evaluate the type and order of values of the dielectric losses that occur under operating conditions.

#### **4. Conclusion**

The existing methods for producing insulating coatings on the surface of corrosion-resistant steels are analyzed. It is shown that the CVD methods (ionplasma deposition of metal oxide metal) of deposition on stainless steel substrates

## *Technologies of High-Temperature Insulating Coatings on Stainless Steels DOI: http://dx.doi.org/10.5772/intechopen.91334*

make it possible to obtain high-quality dielectric coatings for film heating elements with high micromechanical and electrophysical properties. The disadvantage of this method is its low productivity, which makes it impossible to use it in mass production. The relevance of using highly efficient and low-cost methods for the synthesis of functional coatings based on glass-ceramic glass sealants is shown. This method is simpler in execution technology and cheaper considering the use of raw materials, which opens up wide prospects for use in mass production. Given the proportionality of the sizes of coatings formed by both methods, the identity of the structure, microtopography of the surface, and the level of electrophysical properties, it can be argued that the first method can be recommended for applying dielectric coatings on the surface of stainless steels which is used for creating film heating elements.
