**4. In-situ codeposition**

Until this section some of the details, advantages and additional performance criterias, related to the properties, were given about electo codeposition technique. When considering this technique, which is applied to metal-ceramic couples for some decades, researchers are tend to deposit the reinforcement and the metal directly. This technique is called *ex-situ codeposi‐ tion* method. In this method, ceramic particles are suspended in the electrolyte and codeposited together with matrix metal directly. Of course, it brings some disadvantageous situations together because of the surface properties of the particles.

Some of the research groups improved ex-situ codeposition technique in last few years to eliminate the disadvantages. The new technique modified by researchers is called *in-situ codeposition* method and identifies the phase transformation of the reinforcement in the composite structure by various additional treatments such as heat-treatments between elements and/or compounds. Using this approach it is possible to obtain composite structures with wide range matrix metal (aluminium, titanium, iron, nickel, chromium and copper) and second phase particles (borides, nitrides, carbides, oxides and mixture of them) [12].

#### **4.1. Comparison of ex-situ and in-situ methods**

There are some advantages of in-situ codeposition technique when compared to the conven‐ tional codeposition (ex-situ method) route. These are:


**•** Additionally, thermally stable ceramic particles transformed in the composite structure can be distributed homogenously by this method. It is well known that homogeneity in the composite materials bring enhanced mechanical properties with itself.

All the advantageous and disadvantageous are summerized in Table 2.


**Table 2.** Comparison summary of two methods

*3.2.8. Residual stress*

72 Electrodeposition of Composite Materials

**4. In-situ codeposition**

Stress in coatings also adversely affects properties. Nowadays, a variety of options are available for stress reduction of coatings. These include: choice of substrate plating solution, use of additives, and higher plating temperatures. A number of theories have been assumed regarding the origins of stress, however, none of them covers every single situation. Several methods for stress measurement vary from the simple rigid strip technique to complicated methods. Both phase transformations in the composite and additional post-treatment of the

Until this section some of the details, advantages and additional performance criterias, related to the properties, were given about electo codeposition technique. When considering this technique, which is applied to metal-ceramic couples for some decades, researchers are tend to deposit the reinforcement and the metal directly. This technique is called *ex-situ codeposi‐ tion* method. In this method, ceramic particles are suspended in the electrolyte and codeposited together with matrix metal directly. Of course, it brings some disadvantageous situations

Some of the research groups improved ex-situ codeposition technique in last few years to eliminate the disadvantages. The new technique modified by researchers is called *in-situ codeposition* method and identifies the phase transformation of the reinforcement in the composite structure by various additional treatments such as heat-treatments between elements and/or compounds. Using this approach it is possible to obtain composite structures with wide range matrix metal (aluminium, titanium, iron, nickel, chromium and copper) and

There are some advantages of in-situ codeposition technique when compared to the conven‐

**•** Reinforcement should be fabricated separately before electro deposition process in ex-situ production method. In this situation, the size of the reinforcement particles limited by the initial particle size and for practical applications size of the particles are rarely in sub-micron

**•** Another advantage of in-situ method is particle-matrix interface. Surface structure of second phase particles and contaminants can cause a weak wettability between the particles and the matrix phase and this would be effective for the final properties of the composite structure. Especially the mechanical properties. On the other hand, coherently developed interface between the reinforcement and the matrix can be obtained strongly in in-situ

second phase particles (borides, nitrides, carbides, oxides and mixture of them) [12].

scale because of the economic reasons of nano particle production routes.

coatings can decrease residual stress value in a composite coating [12].

together because of the surface properties of the particles.

**4.1. Comparison of ex-situ and in-situ methods**

production method.

tional codeposition (ex-situ method) route. These are:

#### **4.2. Post-treatment and transformations**

In-situ codeposition method includes phase transformations and these come from additional post-treatments (heat-treatment) generally. There is a major point to be known about heattreatment processes. It is called the stability of oxidation products such as carbides, nitrides, and oxides under different temperature and atmosphere conditions. Scientific reality that lies down this phenomenon is the change (decrease and increase) in the free energy of these components as a function of temperature (and aslo partial gas pressure). The tendency is important to predict the way of phase transformations according to the heat-treatment processes and defined by using special diagrams called Ellingham Diagrams which show the free energy change versus temperature [35].

#### **4.3. Properties and performance**

Traditionally, in electro codeposition technique for fabrication of metal-ceramic composites, ex-situ method is used [36-39]. In this method, ceramic particles are codeposited on cathode directly from the electrolyte (ceramic particle containing suspension). During the last decade, much attention has been paid to the preparation of metal matrix composites by a method that combines several techniques with an in-situ reaction method providing some advantages [40]. Differently, phase transformation of codeposited particles, such as carbon black with an additional heat treatment step has its special attention in this specific production route. It is because carbon is the unique element to compose carbide phase by in-situ transformation. From this perspective, the production route for this kind research includes both the codepo‐ sition of less studied carbon structures and in-situ phase transformation of metal matrix, such as chromium and various size of carbon black particles. Additionally, many different carbon sources in in-situ method codeposition can be used. To give some examples, they can be directly carbon based particles, such as carbon black and graphite, or steel substrate itself, and etc.

Based on some former studies [14] the following section discusses fabrication of Cr-C compo‐ site coatings on steel substrates by electro codeposition technique and transformation of the layers into chromium matrix, carbide, and/or nitride reinforced composite phase structures with an additional heat treatment process for enhanced corrosive and mechanical applications. The discussion is based on the comparison of the similar research in the sphere.

The present section includes the production of functionally modified hard chromium coatings with a new approach. In addition to this, the observations of the property-performance relation for a specific Cr-C composite coating fabrication are provided. According to the experimental studies researchers fabricated three different coating groups. The identification of these groups are given in Table 3.


**Table 3.** Identification of the samples [15]

XRD patterns are shown by annealing as-deposited Cr–C coatings, crystallization into a crystalline structure would occur. In the same results, it is clearly identified that for the Cr-C composite coatings give peaks that belong to carbide (Cr23C6), nitride (Cr2N), oxide (Cr2O3) and pure metallic Cr phases after annealing (800o C/3h N2 or Ar). The results showed that a small amount of Cr2O3 and Cr23C6 phases transformed under inert gas atmosphere (Ar) in some regions. On the other hand, change in heat treatment atmosphere (N2) formed a new phase structure with the form of Cr2N.

When compared the surface morphologies micro-cracked structure is visible along the surface for metallic chromium (fabricated by conventional methods). The similar surface structure with cracks is seen for the sample heat treated under Ar atmosphere with an increased crack width and density against the reference sample. Nevertheless, nitride phase formation modifies the surface denser and makes it crack-free with respect to the reference coating (see Figure 9 for details) [15].

Researchers have revealed that nitride formation takes place near the cracks and enlargens nitride-gas interface by the oxidation mechanism as expected [35]. Similarly, an interlayer is formed between the interfaces of coating and substrate. The cross-section of Sample-A gives the same interlayer lying between the coating-substrate interfaces (dark grey contrast). Since atomic carbon in the Cr-C layer can diffuse faster than chromium, the excess carbon atom in the Cr-C layer tends to form carbide phases [41]. The reason for this phenomena is the diffusion of C atoms from the steel substrate to the coating. Carbon atoms diffused from then transform to carbide phase due to the high chemical affinity of C atoms to the Cr atoms [42].

directly carbon based particles, such as carbon black and graphite, or steel substrate itself, and

Based on some former studies [14] the following section discusses fabrication of Cr-C compo‐ site coatings on steel substrates by electro codeposition technique and transformation of the layers into chromium matrix, carbide, and/or nitride reinforced composite phase structures with an additional heat treatment process for enhanced corrosive and mechanical applications.

The present section includes the production of functionally modified hard chromium coatings with a new approach. In addition to this, the observations of the property-performance relation for a specific Cr-C composite coating fabrication are provided. According to the experimental studies researchers fabricated three different coating groups. The identification of these groups

XRD patterns are shown by annealing as-deposited Cr–C coatings, crystallization into a crystalline structure would occur. In the same results, it is clearly identified that for the Cr-C composite coatings give peaks that belong to carbide (Cr23C6), nitride (Cr2N), oxide (Cr2O3) and

amount of Cr2O3 and Cr23C6 phases transformed under inert gas atmosphere (Ar) in some regions. On the other hand, change in heat treatment atmosphere (N2) formed a new phase

When compared the surface morphologies micro-cracked structure is visible along the surface for metallic chromium (fabricated by conventional methods). The similar surface structure with cracks is seen for the sample heat treated under Ar atmosphere with an increased crack width and density against the reference sample. Nevertheless, nitride phase formation modifies the surface denser and makes it crack-free with respect to the reference coating (see

Researchers have revealed that nitride formation takes place near the cracks and enlargens nitride-gas interface by the oxidation mechanism as expected [35]. Similarly, an interlayer is formed between the interfaces of coating and substrate. The cross-section of Sample-A gives the same interlayer lying between the coating-substrate interfaces (dark grey contrast). Since atomic carbon in the Cr-C layer can diffuse faster than chromium, the excess carbon atom in the Cr-C layer tends to form carbide phases [41]. The reason for this phenomena is the diffusion of C atoms from the steel substrate to the coating. Carbon atoms diffused from then transform

to carbide phase due to the high chemical affinity of C atoms to the Cr atoms [42].

C/3h N2 or Ar). The results showed that a small

The discussion is based on the comparison of the similar research in the sphere.

**Sample-N** Electrodeposited Cr/Cmicro composite (heat-treated under N2 atmosphere) **Sample-A** Electrodeposited Cr/C micro composite (heat-treated under Ar atmosphere)

etc.

are given in Table 3.

74 Electrodeposition of Composite Materials

**Table 3.** Identification of the samples [15]

structure with the form of Cr2N.

Figure 9 for details) [15].

**Sample-R** Traditional hard Cr coating (reference coating)

pure metallic Cr phases after annealing (800o

**Figure 9.** SEM micrographs of Sample-R, -N, and -A from the cross section and the surface

According to the EDS analysis taken, it was assumed that the dark regions are nitride phase for Sample-N and carbide formation occurred in the matrix and also along the interface for the both samples (Sample-N and Sample-A) [42]. Additionally, some of the zones are identified as nitride containing field and some not only nitride phase but also carbide. This is explained by the N atoms diffusion from atmosphere into the and finally nitride phase formation in the micro-cracks [35].

Until this point, the details which render the study specificly are given about phase structure, transformations, and morphological structure of the composite coatings fabricated by this new method. The technique repeatedly shows its importance when considering the experimental results on both corrosion and mechanical behaviour.

Here are the results summarizing this specific study of the corrosion behaviour of coatings mentioned. Open-circuit potentials and cyclic polarization curves of steel substrate (1040) and, Sample-R, -N, and -A in aerated 3.5wt% NaCl solution are demonstrated in Figure 10 and some quantitative corrosion results are given in Table 4 in details.

**Figure 10.** Open-circuit potentials (left) and cyclic polarization curves (right) of steel substrate (1040), and Sample-R, - N, and -A in aerated 3.5wt% NaCl solution [15]

Corrosion test results reveale that the passive current density of the heat-treated samples are lower than those of the untreated samples. That means it is possible to change the corrosion behaviors of the coatings by atmosphere controlled heat-treatments. In addition, Sample-N exhibits the higher polarization resistance and the lower corrosion current density as compared to the others.


**Table 4.** Quantitative corrosion data of steel substrate and, Sample-R, -N, and -A [15]

It is clearly seen from the SEM micrographs of the coatings that countless number of superficial cracks exist all over the surface. Some of them lying from the coating surface through the crosssection. According to the microstructures it becomes clear that the presence of micro-cracks in Cr coating is the main reason on the corrosion resistance decrease. When it comes to the heattreated samples, it is improved by the crack-filling effect of Cr2N phase formation as seen in the cross-sectional images.

After the corrosion property, hardness and elastic modulus values are investigated for this study due to the heat-treatment effect on the in-situ phase transformations. It is shown that these phase formations are in a competition with stress relaxation on the hardness of composite coatings. While the relaxation decreases the hardness value, phase formations support the increase in the total hardness. The mentioned hardness values were decreased up to 480Hv level for only carbide formed composite coatings (see Table 5).


**Table 5.** Hardness and elastic modulus data of Sample-R, -N, and -A [12]

by the N atoms diffusion from atmosphere into the and finally nitride phase formation in the

Until this point, the details which render the study specificly are given about phase structure, transformations, and morphological structure of the composite coatings fabricated by this new method. The technique repeatedly shows its importance when considering the experimental

Here are the results summarizing this specific study of the corrosion behaviour of coatings mentioned. Open-circuit potentials and cyclic polarization curves of steel substrate (1040) and, Sample-R, -N, and -A in aerated 3.5wt% NaCl solution are demonstrated in Figure 10 and some

**Figure 10.** Open-circuit potentials (left) and cyclic polarization curves (right) of steel substrate (1040), and Sample-R, -

Corrosion test results reveale that the passive current density of the heat-treated samples are lower than those of the untreated samples. That means it is possible to change the corrosion behaviors of the coatings by atmosphere controlled heat-treatments. In addition, Sample-N exhibits the higher polarization resistance and the lower corrosion current density as compared

*Ecorr (mV) Icorr (A) Rp (Ω)*

Sample-S -635 5.65x10-6 1.99 Sample-R -587 1.53x10-6 21.40 Sample-A -394 4.85x10-7 42.28 Sample-N -124 2.90x10-8 538.77

**Table 4.** Quantitative corrosion data of steel substrate and, Sample-R, -N, and -A [15]

micro-cracks [35].

76 Electrodeposition of Composite Materials

results on both corrosion and mechanical behaviour.

N, and -A in aerated 3.5wt% NaCl solution [15]

to the others.

quantitative corrosion results are given in Table 4 in details.

Nevertheless, this value is about 600Hv for the composite coatings which have both carbide and nitride phase. This result can be explained solely by the in-situ nitride formation with a crack filling effect. Similar results are explained on the elastic modulus increase for this specific research.

Two ways to improve the wear resistance of a composite coating were explained previously. One of them is to decrease the friction coefficient. The results of this study showed that the friction coefficient decrease was found to be about 50% for the samples in which nitride formation occurred. As a result of this effect, it is possible to suggest that the need of lubrication, vibrations and over heating can be decreased for engineering machinery applications by using these type functionally modified surfaces [12].

To take into account the adhesion behaviour of these composite coatings, there are no adhesive damage observed up to 30N maximum load for these samples (Sample-N, and -A). Only a little amount of cohesive damage occurred. In contrast, for the nitride formed samples this effect was seen much less. The coherence between the coating and the substrate is explained and supported by the diffusion of C atoms from the substrate to the coating and the formation of carbide interface for the samples.

Finally, as another mechanical result, residual stress values of the coatings were measured for these electro codeposited composites in this research. It is stated that the reference sample, which has only the pure metallic chromium phase, showing tensile stress in a high level as it was expected. Against, residual stresses are compressive and found to be about -380MPa and details) [12].

the formation of carbide interface for the samples.


Only a little amount of cohesive damage occurred. In contrast, for the nitride formed samples this effect was seen much less. The coherence between the coating and the substrate is explained and supported by the diffusion of C atoms from the substrate to the coating and

Finally, as another mechanical result, residual stress values of the coatings were measured

sample, which has only the pure metallic chromium phase, showing tensile stress in a high level as it was expected. Against, residual stresses are compressive and found to be about ‐

Figure 11. Intensity‐angle, intensity‐sin2(Psi) fit curves residual stress values calculated from **Figure 11.** Intensity-angle, intensity-sin2 (Psi) fit curves residual stress values calculated from

The coatings are compared with respect to the conventional hard chromium coatings. In regard to the electrochemical behavior, the chromium‐carbon black composite coatings heat‐ treated under nitrogen atmosphere showing a rehabilitated crack‐free microstructure, exhibited better corrosion resistance than the conventional hard chromium structures. Therefore, the increase in corrosion potential suggests improvement of corrosion resistance due to the formation of carbide/nitride. Characteristic properties such as hardness and The coatings are compared with respect to the conventional hard chromium coatings. In regard to the electrochemical behavior, the chromium-carbon black composite coatings heat-treated under nitrogen atmosphere showing a rehabilitated crack-free microstructure, exhibited better corrosion resistance than the conventional hard chromium structures. Therefore, the increase in corrosion potential suggests improvement of corrosion resistance due to the formation of carbide/nitride. Characteristic properties, such as hardness, and modulus of elasticity are determined for carbide and nitride formed composite coatings. It is observed that the following phase transformations support the recovery of friction-wear characteristics and accordance of substrate-coating interface belonging to the material. According to the detailed inspections, it is assigned that the definite results directly correlate with both the magnitude and the direction of the residual stresses. As a result, the corrosion behaviour and the mechanical properties of the in-situ electro codeposited coatings are believed to be controlled by microstructure and surface properties of the metallic chromium layer, which is modified by the formation of carbide and/or nitride phase, and can be used for many engineering applications instead of traditional metallic coatings.
