3.1. Morphology of the Surface of PEO Layers Synthesized on Cast Alloys AK9 and AK12 in Different Electrolytes
An analysis of the surface morphology of PEO layers obtained in an electrolyte of their basic composition and with the addition of hydrogen peroxide showed similar features for both alloys. In particular, the synthesis of PEO layers in the basic electrolyte contributed to the cracking of their surface layers (
Figure 2a,b). As a result, cracks appeared along the boundaries of the craters at the exit of the plasma discharge channels to the surface of the PEO layers. They formed a fairly developed network of damage on the surface of the PEO layers of both alloys. As regards the shape and dimensions of the plasma discharge channels in both alloys, they did not differ significantly in general. However, cracks on the surfaces of both alloys could increase the wear of PEO layers with such morphology.
Signs of cracking on the surface of the PEO layers of both alloys during their synthesis in an electrolyte with the addition of hydrogen peroxide to the base composition were practically not observed (
Figure 2c,d). In this case, pores of different sizes became dominant defects. This was especially pronounced on the surface of the PEO layer in the AK9 alloy, on which the pore size reached 5 μm (
Figure 2c). At the same time, smaller pores (up to 1 μm) were observed on the surface of the PEO layer on the AK12 alloy. The craters at the exit of the plasma discharge channels to the surface of the PEO layer were also much smaller in this case. This provided a fairly uniform surface morphology of the PEO layer on the AK12 alloy and its more uniform structure (
Figure 2d). Although microcracks were practically not observed on the surface of the PEO layer on the AK12 alloy (in contrast to the case of synthesis in the electrolyte of the basic composition), a network of tracks from very small pores was observed around the conglomerates from craters. They were considered a factor in the heterogeneity of the surface morphology, which can facilitate wear.
3.3. Structure and Phase Composition of the PEO Layer Synthesized in Different Electrolytes on AK9 Alloy
Figure 4 shows the structure of the oxide-ceramic layer formed on the surface of the AK9 aluminum alloy. First, the uneven growth of the front of the PEO layer into the depth of the substrate is obvious. This causes a significant change in the thickness of the PEO layer at the front of its synthesis. It is clear that this feature is more noticeable at high resolution. Secondly, a rather high homogeneity of the inner part of the PEO layer (near the substrate) and an increased density of pores and cracks in its outer layer were noted. Thirdly, a significant amount of silicon crystals found in the substrate structure practically did not show up in the structure of the PEO layer. This indicates that, during the synthesis of the PEO layer, the initial silicon crystals (observed in the structure of the AK9 alloy) were mainly transformed into oxide phases.
Unlike Al–Cu–Mg alloys, the PEO layer on the AK9 alloy grows unevenly (non-uniformly) into the matrix [
24]. In contrast to Al–Cu–Mg alloys, in which the PEO layer propagates into the substrate with a more or less even front [
26], on the AK9 alloy, the PEO layer is synthesized nonuniformly along its front. It was assumed that the growth of the PEO layer slowed down when silicon crystals were encountered on the way of its propagation deep into the substrate. This is confirmed by the image of the synthesis front of the PEO layer deep into the substrate, which is stopped by a successfully oriented chain of silicon crystals encountered in its path (
Figure 5). Silicon has a high melting point (T-1683 K) and boiling point (T-2628 K) [
27]. It is a semiconductor whose electrical conductivity depends on impurities and temperature. During the synthesis of PEO layers, high-temperature plasma discharges can increase the substrate temperature up to 300 °C. This contributes to an increase in the electrical conductivity of silicon crystals and ensures the passage of electric current through them.
Obviously, during the synthesis of the PEO layer on the AK9 alloy, a high temperature is reached in the discharge channels and around them, which leads to the melting of silicon crystals. In addition, volatile compounds, such as SiO and SiH, can form during the interaction of silicon with electrolyte components in the discharge channels. At high temperatures (more than 2100 °C), these compounds can evaporate and pass both into the electrolyte and into the oxide ceramic layer. Such processes lead to an increase in the number of pores and, consequently, a decrease in the microhardness of the PEO layer.
The XRD patterns of the PEO layer synthesized on the AK9 alloy in 3 g/L KOH + 2 g/L Na
2SiO
3 electrolytes are shown in
Figure 6.
All compounds in the composition of the PEO layer were detected by XRD method (DRON-3.0, in Cu-K
α radiation). The obtained XRD data were refined with the use of the FullProf program [
25]. The refined crystallographic parameters of all phases and their contents are presented in
Table 1.
The oxide ceramic layer mainly consists of г-Al2O3, Al2O3·SiO2, б-Al2O3 and a small amount of SiO2.
Some approaches were developed to increase the growth rate of the layer [
28]. The addition of hydrogen peroxide to a weakly alkaline electrolyte of 3 g/L KOH + 2 g/L Na
2SiO
3 causes an increase in the temperature in the plasma discharge channels and an increase in the amount of oxygen and hydrogen in the discharge channels and their surroundings. This leads to both an increase in the rate of aluminum oxide formation and an increase in the amount of volatile SiO and SiH compounds [
29]. This effect is manifested in the formation of pores and microcracks in the vicinity of silicon inclusions at the border between the PEO layer and the matrix. An additional factor leading to the formation of a network of microcracks is the rapid cooling of the newly formed aluminum oxide by the electrolyte (
Figure 7).
The phase compositions of the PEO layer synthesized in 3 g/L KOH + 2 g/L Na
2SiO
3 electrolyte and in the same electrolyte with the addition of 3 g/L H
2O
2 are given in
Table 1 and
Table 2 respectively. Hydrogen peroxide contributes to the synthesis of a larger amount of the high-temperature б-Al
2O
3 and γ- Al
2O
3 phases in the PEO layer (
Figure 8,
Table 2). However, the main phases of the PEO layer synthesized on silumin AK9 are Al
2O
3·SiO
2 and γ-Al
2O
3. The X-ray pattern of the PEO layer synthesized on the AK9 alloy in a 3 g/L KOH + 2 g/L Na
2SiO
3 + 3 g/L H
2O
2 electrolyte is shown in
Figure 8. The refined crystallographic parameters of all phases and their contents are presented in
Table 2.
A spectral analysis over the area with the distribution of elements in the substrate and PEO layer synthesized on the AK9 alloy in an electrolyte of 3 g/L KOH + 2 g/L Na
2SiO
3 is shown (
Figure 9). It was found that the distribution of oxygen in the layers is uniform. The distributions of aluminum and silicon are non-uniform. An increased content of silicon was observed in areas of the PEO layer near silicon inclusions as well as in areas where silicon inclusions were encountered along the PEO layer synthesis front. Silicon inclusions at the boundary of the initial alloy and oxide ceramic slow down the growth of the PEO layer.
The distribution of elements in the cross-sectional area of the PEO layers obtained with different electrolytes is almost the same. The large cross-sectional area of the samples was analyzed. However, the content of silicon can differ significantly in small areas (
Figure 9). At the same time, the phase composition of PEO layers synthesized in the electrolyte with the addition of H
2O
2 changed significantly. Synthesis in such an electrolyte changes the content of such phases in the PEO layer as γ-Al
2O
3, Al
2O
3·SiO
2, and SiO
2 (compare the data in
Table 1 and
Table 2), which can affect its wear resistance. It depends on the distribution of silicon in the initial state of the alloy. The content of corundum in PEO layers synthesized in an electrolyte with the addition of H
2O
2 remained almost unchanged.
3.4. Structure and Phase Composition of PEO Layer Synthesized in Different Electrolytes on AK12 Alloy
The results of metallographic studies confirmed the negative effect of silicon crystals in the substrate structure (cast alloy AK12) on the advancement of the PEO synthesis front. During the synthesis of PEO layers on the AK12 alloy (as in the synthesis of the AK9 alloy), silicon crystals in the substrate slow down the synthesis process at the interfaces between the silicon crystals and the matrix, and the propagation of their fronts into the depth of the substrate also slows down (
Figure 10). The inhibitory role of crystals is clearly manifested when the crystals are favorably oriented with respect to the synthesis front of the PEO layer. This is the reason for the significant curvilinearity of the front of the synthesis process.
The XRD patterns of the PEO layer synthesized on the AK12 alloy in 3 g/L KOH + 2 g/L Na
2SiO
3 electrolytes are shown in
Figure 11. The refined crystallographic parameters of all phases and their contents are presented in
Table 3. The previous subsection also describes the results of the XRD phase analysis of the PEO layer synthesized on the AK9 alloy in 3 g/L KOH + 2 g/L Na
2SiO
3 electrolyte (
Table 1).
The refined crystallographic parameters and the content of the phases determined by the X-ray diffraction method in the composition of the PEO layer synthesized on the AK12 alloy in the base electrolyte (3 g/L KOH + 2 g/L Na
2SiO
3) are presented in
Table 3. Evidently, the content of the Al
2O
3·SiO
2 phase in the PEO layer on the AK12 alloy is higher than on the AK9 alloy (compare the data presented in
Table 3 and
Table 1). This is a quite predictable result, since initially the silicon content in the composition of the AK12 alloy was higher than that of the AK9 alloy. Further, taking into account the retarding effect of silicon crystals on the advancement of the synthesis front deep into the substrate, one could also expect a thinner PEO layer on the AK12 alloy. Moreover, the smaller amount of б-Al
2O
3 and г-Al
2O
3 in the PEO layer of the AK-12 alloy also agrees with the results for silicon-containing phases. In the PEO layer synthesized on the AK12 alloy in 3 g/L KOH + 2 g/L Na
2SiO
3 electrolyte, the amount of the Al
2O
3·SiO
2 phase is increased (
Figure 11;
Table 3). The refined crystallographic parameters of all phases and their contents are presented in
Table 3.
The addition of hydrogen peroxide to 3 g/L KOH + 2 g/L Na
2SiO
3 electrolytes leads to an increase in the number of microcracks and a decrease in the size of pores in the PEO layer on the AK12 alloy (
Figure 1d). The distribution of elements over the area of the PEO layer on the AK12 alloy, synthesized in the electrolyte, records the accumulation of silicon crystals, which prevent the growth of the PEO layer deep into the aluminum matrix (
Figure 9). Similar to the AK9 alloy, the accumulation of silicon crystals near the synthesis front of the PEO layer in the AK12 alloy also prevents its penetration deep into the aluminum matrix (
Figure 10). And since the silicon content in the AK12 alloy is higher than that in AK9, the effect of silicon crystals on the synthesis process retardation will be stronger, and, consequently, the thickness of the PEO layer in AK12 will be less.
The XRD patterns of the PEO layer synthesized on the AK12 alloy in a (3 g/L KOH + 2 g/L Na
2SiO
3 + 3g/L H
2O
2) electrolyte are shown in
Figure 12. The refined crystallographic parameters of all phases and their contents are shown in
Table 4.
The addition of 3g/L H
2O
2 to the electrolyte significantly reduces the quantitative content of the Al
2O
3·SiO
2 phase (sillimanite), and a small percentage of 3Al
2O
3·2SiO
2 phase (3.2 wt%) is revealed in the structure of the PEO layer on the AK12 alloy (
Table 4). At the same time, the amount of г-Al
2O
3 and silicon oxide SiO
2 increases.
The number of pores in the structure of the synthesized PEO layer on the AK12 alloy increased with the addition of hydrogen peroxide to the base electrolyte (3 g/L KOH + 2 g/L Na
2SiO
3). This was visible on the surface of the PEO layer (
Figure 1d) and confirmed in its cross section (
Figure 13). However, in both cases (both on the surface and in the cross section), the pores in the structure were smaller than in the case of synthesis in the basic electrolyte. The PEO layer in the AK12 alloy (in the addition of H
2O
2 to the electrolyte composition) propagates into the depth of the substrate nonuniformly (the same as in the case of the synthesis of the PEO layer in the base electrolyte), since silicon crystals slow down its propagation (
Figure 13b). Consequently, silicon crystals in cast aluminum alloys limit the synthesis of PEO layers, and the addition of H
2O
2 to the base electrolyte has little effect on eliminating this disadvantage.
The high content of silicon in silumins contributes to the formation of high-temperature phases in PEO coatings, in particular α-Al2O3, mullite 3Al2O3·2SiO2 and especially sillimanite-Al2O3·SiO2. The amount of sillimanite increases due to the inclusion of the volatile compounds SiO and SiH in the oxide ceramic coating.
The dense arrangement of silicon crystals near the oxidation front exhausts the adjacent matrix for the content of aluminum in it, and the synthesis of aluminum-silicon-containing oxides becomes more complicated. Therefore, the content of pure silicon oxide in the PEO layer on the AK12 alloy increases.
A spectral analysis of the distribution of elements in the PEO layers on the AK12 alloy is shown in
Figure 14. It has been established that the distribution of oxygen and aluminum in the layers is more or less uniform. The distribution of silicon is non-uniform. The silicon content increased in the areas of the oxide ceramic PEO layer near the silicon inclusions as well as in the areas where there were silicon inclusions during the growth of the oxide ceramic layer along the PEO layer synthesis front (
Figure 14). Silicon inclusions at the interface of the original alloy substrate and oxide ceramics slow down the growth of the oxide ceramic PEO layer. The amount of silicon around the silicon inclusions is greater than in the PEO layers on the AK9 alloy.
The addition of hydrogen peroxide to the electrolyte does not lead to significant differences in the distribution of elements in the oxide ceramic layer on the AK12 alloy.
Despite a rather noticeable change in the phase composition of the PEO layer synthesized in electrolyte with the addition of hydrogen peroxide, the distribution of elements over the area of the PEO layer on the AK12 alloy did not undergo significant changes.
3.5. Microhardness of PEO Layers, Synthesized on Silumins
The microhardness of PEO layers significantly depends on the alloying of aluminum alloys. PEO layers synthesized on silumins have significantly lower microhardness than PEO layers synthesized on Al–Cu and Al–Cu–Mg alloys. Thus, the PEO layer on the D16 alloy (analog of AA2024 ANSI USA), synthesized in the basic alkaline electrolyte-(3 g/L KOH + 2 g/L Na
2SiO
3), has a microhardness of 1600–1900 HV [
28]. The PEO layer on the AMg6 alloy (analog of AA 5056 ANSI USA) synthesized in the basic alkaline electrolyte (3 g/L KOH + 2 g/L Na
2SiO
3) has the microhardness of 1200–1600 HV [
30]. Whereas the PEO layers on AK9 and AK12 silumins, synthesized in the same electrolyte, have significantly lower microhardness. Their values do not exceed 850–960 HV (
Figure 15).
In order to increase the microhardness of PEO layers on silumins, a strong oxidizing agent (hydrogen peroxide) was added to the electrolyte for the synthesis of PEO layers [
28]. The formation of aluminum oxide in plasma discharges is described by the following formulas:
Based on the law of the active masses, it is possible to increase the yield of aluminum oxide according to the reactions (1)–(4) by increasing concentrations of reagents, in particular O, O
2, OH, and OH
− [
31].
Active electrolysis of water and hydrogen peroxide occurs in the plasma-discharge channels, and, as a result, during PEO, they easily decompound with the release of oxygen.
As a result of the thermal decomposition of hydrogen peroxide in the plasma discharge channels, a larger amount of oxygen is formed. This effect leads to an increase in the coating thickness. The thickness of PEO layers on AK9 and AK12 alloys in electrolytes with different hydrogen peroxide contents is shown in
Figure 16. The addition of hydrogen peroxide to the electrolyte leads to an increase in the thickness of the PEO layers, both on the AK9 alloy and on the AK12 alloy. The maximum thickness of 240–300 μm is observed on the PEO layers formed on the AK9 alloy at concentrations of 3 and 5 g/L H
2O
2 (
Figure 16, white bars). A further increase in the concentration of peroxide leads to a decrease in the thickness of the coating. The thickness of the PEO layers formed on the AK12 alloy is somewhat smaller. This alloy has a larger amount of silicon, which, due to its properties, slows down the growth of the coating (
Figure 16, dark bars). An increase in the concentration of hydrogen peroxide in the electrolyte to 7 g/L leads to some reduction in the coating thickness. This is explained by a significant increase in the pH of the electrolyte and the predominance of aluminum oxide dissolution processes over its synthesis processes.
The increase in thickness leads to the synthesis of more corundum in the PEO layer. The addition of 3 g/L H
2O
2 to the base electrolyte (3 g/L KOH + 2 g/L Na
2SiO
3) significantly increases the microhardness of the PEO layers on silumins. Thus, the microhardness of the PEO layer on the AK9 alloy increased from 960 to 1050 HV, and the microhardness of the PEO layer on the AK12 alloy increased from 850 to 1000 HV. It should be noted that separate measurements of the microhardness of PEO layers on the AK9 alloy synthesized in electrolyte with the addition of 3 g/L H
2O
2 have a value of 2000 HV. This corresponds to the microhardness of corundum. PEO layers on the AK12 alloy do not have such microhardness. Although the phase analysis of the PEO layers on the AK12 alloy indicates the presence of corundum in the layer. This may be due to the influence of SiO and SiH on the formation of a large number of small micropores in the PEO layers, which was clearly manifested on the surface of the PEO layer on the AK12 alloy (
Figure 2d). The rather high microhardness of the synthesized PEO layers on both aluminum alloys makes it possible to predict their rather high abrasive wear resistance.
3.6. Abrasive Wear Resistance of PEO Layers, Synthesized on Silumins
The abrasive wear resistance in tests with a fixed abrasive is one of the most common test methods used to study the properties of materials. The abrasive wear resistance of PEO layers synthesized on AK9 and AK12 alloys was studied at their contact with a SM-2 electrocorundum disk on a 7K15 ceramic bond with a hardness of 1800–1900 HV.
The obtained results indicate that the oxide ceramic layers synthesized on AK9 alloy in the basic electrolyte with the addition of 3 g/L of hydrogen peroxide have the highest wear resistance (
Figure 17). The oxide ceramic layers synthesized on AK12 alloy in electrolytes with different H
2O
2 contents have lower abrasive wear resistance. The lowest abrasive wear resistance for PEO layers synthesized on AK12 alloy was observed in 3 g/L KOH + 2 g/L Na
2SiO
3 electrolytes. An increase in the amount of hydrogen peroxide in the electrolyte leads to an increase in the wear resistance the oxide ceramic layers. In contrast to the AK9 alloy, the layers synthesized in a 3 g/L KOH + 2 g/L Na
2SiO
3 + 5 g/L H
2O
2 electrolyte have the highest wear resistance. A further increase in hydrogen peroxide concentration in the electrolyte leads to a decrease in wear resistance. PEO layers synthesized in an electrolyte with 7 g/L H
2O
2 have a lower growth rate and lower corundum content [
31]. This happens due to the increase in pH of the electrolyte and the dissolution of the oxide layer.
The weight loss of samples from AK9 and AK12 alloys in the initial state and with PEO layers synthesized in electrolytes 3 g/L KOH + 2 g/L Na
2SiO
3 and 3 g/L KOH + 2 g/L Na
2SiO
3 + 3 g/L H
2O
2 are compared after abrasive wear resistance tests (
Figure 18). The wear resistance of AK9 alloy in its initial state is higher than that of AK12 alloy. This is explained by the higher content and larger sizes of solid silicon crystals in the AK12 alloy. During the tests, they break away from the alloy and act as an abrasive. This leads to increased weight loss of the sample with the PEO layer during the test. The oxide ceramic layers synthesized on both alloys in a 3 g/L KOH + 2 g/L Na
2SiO
3 electrolyte significantly increase their wear resistance. The abrasive wear resistance of the PEO layer synthesized on the AK9 alloy is almost 80 times higher than that of the alloy in the initial state. The abrasive wear resistance of the PEO layer on the AK12 alloy is about 20 times higher than that of the alloy in its initial state. Consequently, the PEO layers on the AK9 alloy have an order of magnitude higher wear resistance than the AK12 alloy. This effect can be explained by the smaller number of silicon inclusions and their smaller sizes, and the more uniform structure of the synthesized oxide ceramic layers. The network of microcracks observed on the surface of the PEO layers on both alloys did not negatively affect their abrasive wear resistance. Perhaps this is due to the relaxation of tensile stresses in PEO layers due to the formation of microcracks in them. The addition of hydrogen peroxide to the electrolyte leads to an increase in the corundum content of the PEO layers [
31] and an increase in their wear resistance. The abrasive wear resistance of the PEO layer on the AK9 alloy increased by 30% and on the AK12 alloy by 70% after adding hydrogen peroxide to the base electrolyte. It is possible that this effect is due to smaller pores on the surface of the PEO layer on the AK12 alloy synthesized in an electrolyte with the addition of H
2O
2. In addition, an increase in the abrasive wear resistance can occur as a result of the grinding of the wheel oxide phases γ-Al
2O
3 and SiO
2 with low microhardness (as a consequence of contact with corundum abrasive) and the formation of a finely dispersed powder that acts as a solid lubricant.
During the synthesis of coatings, the optimal current densities and the time of synthesis of PEO layers were determined. All coatings were synthesized at a current density of 15 A/dm
2 for 2 h. This made it possible to obtain a larger amount of corundum [
32]. An increase in current density and time leads to a significant increase in the power of plasma channels. The SiO and SiH compounds are formed in the plasma discharge channels and exit the coating. This leads to the formation of layers with a large number of pores and cracks. The structure of such coatings is often given in the literature [
33,
34,
35,
36,
37]. On the basis of the research carried out, simple electrolytes have been developed that can significantly increase the microhardness and wear resistance of AK9 and AK12 alloys. The results presented in the article are compared with those known in the literature. A particularly interesting method is the synthesis of PEO layers with TiC nanoparticles inside them [
36]. The successful synthesis of PEO layers with nanosized TiC inclusions was reported in a number of works [
36,
37,
38]. Although the microhardness of such coatings is higher, namely 1300–1600 HV, their wear resistance is lower than that obtained in this study. This can be explained by the effect of solid TiC nanoparticles embedded in the oxide matrix. These small particles act as an abrasive and further reduce the wear resistance of the synthesized layers. At the same time, in our case, the hardest phase in the synthesized PEO layers was α-Al
2O
3 corundum. The rest phases contained in the PEO layers can act as a solid lubricant and prevent rapid wear of the PEO layer. Despite the noted disadvantages of the abrasive action of nano-inclusions, the idea of introducing nanoparticles into electrolytes for the synthesis of PEO layers is interesting and promising for further development.