Hot corrosion, wear, and oxidation performance of high-velocity oxy-fuel and plasma spray coatings in various environments: A comprehensive review

3.1. Microstructure of HVOF spray coatings and the prepared composite powder

The microstructure of coatings made by HVOF spraying has been thoroughly investigated by many researchers. Understanding the effects of microstructural features on these coatings’ mechanical and tribological properties has been the main goal of these studies. Notably, when it comes to HVOF spray coatings, the feedstock material’s initial microstructure is crucial. This is because it has a significant impact on enhancing multiple coating properties, which are essential for their efficient operation in demanding conditions. The microstructural characteristics of these coatings play a crucial role in enhancing their properties, especially in high-temperature applications. Gurmail et.al. [41] examined the microstructure of composite powders made of NiCrAlY (nickel-chromium-aluminum-yttrium) alloy with a 20% weight fraction of silicon carbide (SiC) and NiCrAlY alloy with a 20% weight fraction of boron carbide (B₄C), as illustrated in Figure 4. To prepare these composite powders, ball milling was used. The findings of the scanning electron microscope (SEM) analysis showed that the metallic and carbide powders were mixed uniformly, with the particle sizes measured at about 38 µm (Figure 4a-b). The rectangular regions in Figure 4(a,b) show the surfaceenergy-dispersive X-Ray spectroscopy (EDS) analysis, confirming the presence of all powder elements, including Ni, C, Al, Cr, B, and Si.

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Figure 4.

(a-d) SEM , (e-f) energy-dispersive spectroscopy (EDS)/and (g-i) EDS mapping analysis, (h-j) EDS line scan results of NiCrAlY–20wt%SiC and NiCrAlY–20wt%B₄C composite powders coatings on T22 steel deposited by the HVOF process. Permission from Elsevier. Copyright © 2021.

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In Figure 4(c,d), the field emission scanning electron microscopy (FE-SEM) images reveal the microstructure of coatings applied onto a steel substrate. Notably, an irregular microstructure is evident in both cases. Within the microstructure, there is a combination of molten, partially molten, and unmelted particles forming the matrix. Upon deposition, these particles interlock and uniformly distribute across the matrix. Additionally, the green circles highlight nodular and spherical morphologies on T22 steel, as indicated by the yellow arrows in Figure 4(c,d). The rectangular regions show EDS analysis confirming the presence of Ni, Cr, Al, Si, and Y. Particularly, the identification of oxygen confirmed the formation of oxides during the thermal spray HVOF process. The cross-sectional microstructure of deposited coatings exhibits characteristic splat-like layered patterns, as depicted in Figure 4(e, f). In the case of NiCrAlY-SiC coatings, a compact and crack-free structure is observed, with the splats densely packed, resulting in an approximate thickness of 108 µm. Similarly, for NiCrAlY-B₄C coatings, a well-packed layered structure is evident, with a thickness of approximately 100 µm. The EDS mapping and line scan illustrated in Figure 4(g, h) for the NiCrAlY-SiC coatings clearly verify the Ni and Cr phases. Si-rich particles are locally agglomerated, particularly near the coating/substrate interface, while Al is present in low concentration and clustered at limited interfacial regions. The uniform distribution of O and C suggests the formation of oxides and carbides throughout the coating. This EDS pattern is consistent with the line scan results presented. Similarly, the outcomes for the NiCrAlY-B₄C coatings are depicted in Figure 4(i,j). The EDS line scan confirmed a Ni–Cr rich coating with minor amounts of Al, Si, C, and O as shown in Figure 4(h). The results, as shown in Figure 4, lead to the conclusion that the two new composite powders were effectively synthesized with a unique microstructure. These powders were subsequently employed as coatings on steel substrates, demonstrating their suitability for applications involving high-temperature corrosion, which is discussed in the subsequent section. Zakeri and his colleagues [42] reported a study wherein they employed a ball milling process to modify MCrAlY powder. They investigated both the microstructure and the high-temperature oxidation behavior of the NiCoCrAlY when subjected to the HVOF process.

In Figure 5(a), the initial NiCoCrAlY powder, as received for the study, displayed spherical particles with a smooth surface. However, after undergoing milling, as depicted in Figure 5(b), the powder exhibited a semi-spherical morphology, forming agglomerated particles with a rough surface morphology. A closer examination of the agglomerated powder in Figure 5(c) revealed that it consisted of nano-sized particles. The red rectangles in Figure 5(a–c) show high-magnification images of conventional and milled NiCoCrAlY nanostructured powder. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) findings further confirmed a substantial reduction in crystal size following the milling process, as illustrated in Figure 5(d) and Figure 5(e), respectively. The particle size measurement indicated that the powder particles fell within the range of 10-30 µm. Tang et al. also reported [43] similar findings, showing that a TEM analysis of the CoNiCrAlY powder revealed an average grain size of 15 nm. The milled nanostructure powder of NiCoCrAlY had an average particle diameter of 33 µm, while the conventional powder had an average size of 37 µm. Powders with spherical shapes and particle sizes between 10 and 45 µm are recommended for HVOF spray coatings [44,45]. Figure 5(f) presents the cumulative particle size distribution curves, with the inset showing SEM images of the powders at different milling times. The surface morphology of the nanostructure coating and the traditional spray coating has been shown in Figure 5(g,h, and i,j). The red inset rectangle shows the high-magnification image of splashed particles in Figure 5(g, i). Furthermore, Figure 5(a′, b′) present the EDS results, confirming the composition of the powder from the region highlighted by the red rectangle in Figure 5(h). When compared to their nanostructured counterparts, conventional coatings show a comparatively higher presence of spherical unmelted particles, according to an analysis of the surface morphology. Results from EDS further examined this observation and verified the elemental mapping of these unmelted particles. Figure 5(k,l) and (m,n) shows the cross-sectional SEM/EDS analysis of the conventional and nanostructured coatings made using the HVOF process. The yellow dotted lines in Figure 5(l) highlight partially melted particles in the coating formed after using the conventional NiCrCoAlY powder. Some of the yellow arrows in Figure 5(n) indicate β precipitates and microcracks in the coating. Figures 5(a1′, b1′) show the EDS results, confirming the composition of the conventional and nano-structured NiCoCrAlY coating powders, respectively. Both types of coatings exhibit a laminated splat morphology, which is a characteristic feature of thermal spray coatings. These kinds of microstructures and particles are especially useful for improving the characteristics of substrate materials, particularly in terms of increasing resistance to oxidation, corrosion, and wear caused by high temperatures.

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Figure 5.

(a-c) SEM results of conventional and as-milled nanostructure NiCoCrAlY powder, respectively. (d) XRD results, (e) TEM, and (f) Particle size distribution curves. (g, h), (i,j) and (k, l), (m, n) surface microstructure and cross-sectional morphologies of the conventional and as-milled nanostructure NiCoCrAlY coatings by HVOF process, respectively. (a’, b’) and (a1’, b1’) the EDS elemental spectrum from (h),and (n) respectively.Permission from Elsevier. Copyright © 2020.

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Figure 6 included a selection of materials, namely NiCrWMoCuCBFe, NiCrAY, and Cr₃C₂-NiCr, both in their initial received state and as milling powders [46]. These materials are employed in HVOF spray coatings, serving to produce a diverse range of coatings on metal substrates with the goal of improving their resistance to high-temperature oxidation, corrosion, and wear.

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Figure 6.

(a–c) SEM, particle size distribution, and inverse pole figure of as-received NiCrWMoCuCBFe powder; (d) XRD patterns of powder and coating; (e–f) SEM surface and cross-section of HVOF-sprayed coating; (g-i) surface and cross-sectional morphology of Cr₃C₂–NiCr coating; (j-m) SEM images of NiCrAlY powders (as-received and milled for 5, 10, and 15 h); (n, o) cross-sections of as-received and 15 h-milled NiCrAlY coatings. Permission from Elsevier. Copyright © 2022.

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The SEM analysis reveals that the NiCrWMoCuCBFe particles exhibit a predominantly spherical shape, with some of the smaller particles adhering to one another, resulting in the formation of larger particles as shown in Figure 6(a). The size distribution plot (Figure 6(b)), indicates that most of these particles fall within the 20-50 µm range. It has been previously reported that particles ranging from 10-30 µm are appropriate for use in HVOF spray coating technology. This is attributed to the substantial influence of particle size on their melting characteristics during the HVOF process. Figure 6(c) shows the orientation distribution map by inverse pole figure of cross-sectional surface of NiCrWMoCuBFe powder. After applying the NiCrWMoCuCBFe coatings onto the substrate, it was observed that there were some partially melted particles embedded within the coatings (as depicted in Figure 6e). These particles have the potential to enhance the bond strength between the HVOF spray coating and the substrate. However, it’s important to note that cracks and oxide formations tend to develop around these particles simultaneously. A cross-sectional analysis of the coatings revealed that the inner part of the coatings exhibits relatively low levels of porosity. The measured thickness of the coating is approximately 380 µm, as illustrated in Figure 6(f). The XRD results confirm that the grain size was significantly refined following the spray coating process, as depicted in Figure 6(d). Dervis Ozkan reported [47,48] the structural analysis, wear, corrosion, and oxidation performance assessment of hard metal Cr₃C₂-NiCr coating by the HVOF process. The SEM results revealed that the initial particles of the Cr₃C₂-NiCr powder possessed a spherical shape, making them well-suited for HVOF spray coatings (see Figure 6g). Points 1, 2, and 3 indicate the EDS point analyses shown in the inset of Figure 6(g), confirming the presence of Ni, C, and Cr. Further examination of the SEM cross-sectional microstructure demonstrated that the HVOF-sprayed Cr₃C₂-NiCr coating exhibited characteristics such as high density, low porosity, and a low oxide content as shown in Figure 6(h). A magnified image, as depicted in Figure 6(i), shows a low porosity observed in the Cr₃C₂-NiCr coating. Gurmail and Farzin et al. [41,49] conducted a study on the morphological characteristics of NiCrAlY powder both in its as-received state and after undergoing various milling durations ranging from 5 h to 15 h, as depicted in Figure 6(j-m). Additionally, they presented cross-sectional analyses of spray coatings using conventional NiCrAlY powder and the milled powder, as illustrated in Figures 6(n) and (o), respectively. The results show that the powder has a desirable disk-shaped morphology and a notably high surface-to-volume ratio after 15 h of milling. Comparing this specific morphology to traditional spherical NiCrAlY powder, it is advantageous for use in HVOF jet processes.

Wang et al. [50] conducted a study on the HVOF process for applying Tungsten carbide (WC) coatings to the surface of 316L stainless steel, mainly for marine applications. The morphology of the as-received powder is depicted in Figure 7. The spherical particles, which range in size from 15 to 45 µm, are especially suitable for HVOF spray coatings as shown in Figure 7(a,c,e). The magnified images of these powder microstructure are as shown in Figure 7(b,d,f). It’s significant to note that this spherical shape helps the particles distribute heat more evenly, which eventually results in coatings on the substrate that have less oxygen [51–53].

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Figure 7.

(a,d), (c,d) and (e,f) SEM surface morphologies of as-received powder WC-10Co4Cr, WC-10Co, and WC-17Co, respectively. (g,h) Cross-sectional morphologies of WC-10Co4Cr, (i,j) WC-10Co, and (k,l) WC-17Co of the coatings. (m-o) show the XRD results of the as-received powder and coatings. Permission from Elsevier. Copyright © 2023.

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Figure 7(g,i,k) presents the cross-sectional morphologies of three distinct coatings: WC-10Co4Cr (referred to as WCC), WC-10Co (referred to as WC1), and WC-17Co (referred to as WC₂). In these magnified images, it is evident that all three types of coatings exhibit a uniform distribution of WC phases with minimal cracks and pores as shown in Figure 7(h,j,l). The porosity levels in these coatings are as follows: WCC has a porosity level of 0.72% ± 0.16%, WC1 has a porosity level of 0.97% ± 0.22%, and WC₂ has the lowest porosity level at 0.41% ± 0.09%. Understanding the porosity level is crucial when considering the resistance of the coatings to cavitation erosion and corrosion [54]. This is because cracks frequently start and spread from these initial flaws, which eventually affects how well the coatings perform overall in terms of withstanding this kind of damage. The XRD results for the coatings and powder are shown in Figure 7(m,n,o). These results verify that WC and Co phases make up most of the powder, whereas WC, W₂C, and Co₃W₃C phases are present in the coatings. A minor decarburization of WC during the HVOF process leads to the formation of W₂C [32,55]. Tabatabaei et al. [56] did a study where they used the V-shape method to produce a composite material made of NiCrBSi/WC-CoCr and then applied HVOF spray coating. Assessment of these coatings’ tribological characteristics and hardness at 500°C was the main goal of their study. NiCrBSi powder and WC-CoCr powder are the two initial feedstock powders shown in Figure 8(a,b). The researchers used the V-shape mixing process as shown in (Figure 8(c)) obtain the composite powder NiCrBSi/WC-CoCr (Figure 8(d).

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Figure 8.

SEM images of: (a) NiCrBSi powder, (b) WC–CoCr powder (c) V-shape mixing and (d) NiCrBSi + WC-CoCr powder. (e), (f), (g), and (h) SEM images of the coatings with 0, 15, 30, and 45 wt% of WC–CoCr, respectively, with magnified images in the inset. (i) and (j) backscattered SEM images with 0 and 30 wt% of WC–CoCr powder and corresponding EDS mapping and point analysis, (k) shows SEM results of 80Ni-20Cr coating powder, and (l) the cross-section SEM results of as-sprayed coating, (m1, m2, m3) are corresponding mapping analysis of Fe, Cr, and Ni respectively.Permission from Elsevier. Copyright © 2023.

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In the aforementioned study, it was observed that a composite coating consisting of 30 wt% WC–CoCr (hard phase) and 70 wt% NiCrBSi (soft phase) exhibited several advantageous characteristics. This included the development of a well-adhered, uniform, and densely packed microstructure, which, in turn, led to enhancements in the hardness, wear resistance, and friction properties of the coatings. In Figure 8(e-h), cross-sectional SEM images of the coatings are presented for compositions containing 0, 15, 30, and 45 wt% of WC–CoCr, and these images are accompanied by magnified insets displaying thicknesses ranging from 300 µm to 500 µm. These results convincingly demonstrate that the coating exhibits a higher density, uniformity, and reduced porosity. Furthermore, the EDS results, including EDS mapping and point analysis at positions 1 and 2, provide evidence of the even distribution of all elements i.e. Ni, Cr, C, Si, Fe, W and Co within the coating, as illustrated in Figure 8(i). Similarly, Figure 8(j) shows EDS mapping and point EDS analysis for the sample containing 30 wt% WC–CoCr/NiCrBSi coating. These findings reveal the presence of a novel WC phase alongside the predominant Ni solid solution, with intermetallic phases occurring in minor proportions. Binal [57] reported a study focused on investigating the hot corrosion and oxidation behavior of an 80Ni-20Cr coating applied onto a 316L steel substrate using the HVOF process. Figure 8(k,l) shows the as-received particles of 80Ni-20Cr, as well as cross-sectional SEM images of the coating on the stainless-steel substrate. It exhibited a dense and uniform composition, with a relatively low porosity level of approximately 0.7, as determined through analysis using Image J software. Elemental mapping of Fe, Cr and Ni, as illustrated in Figure 8(m1, m2, m3), provided insights into the composition of the coating powder. It indicated that the primary constituents of the coating were Ni and Cr, and these elements were evenly distributed throughout the coating zone. Additionally, traces of Fe were observed within the coating, likely resulting from diffusion from the underlying substrate. Furthermore, the substrate zone displayed the presence of the major elements found in the substrate, namely Fe, Cr, and Ni. This distinctive coating demonstrated a substantial enhancement in both oxidation resistance and corrosion resistance for the stainless-steel substrate, particularly when exposed to high-temperature conditions.

Chauhan and their colleagues [58] employed a novel blending method, as illustrated in schematic Figure 9(a), to prepare nano/micro CeO₂ powders dispersed with Cr₃C₂-NiCr.

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Figure 9

(a,b) Cr₃C₂-NiCr, (c,d) micro-CeO₂ SEM images, and (e,f) nano-CeO₂ powders TEM results; (A) schematic of the blending and HVOF coating process; (g–i, j–l) cross-sectional and surface SEM of Cr₃C₂-NiCr, micro-, and nano-CeO₂-reinforced coatings; (m–r) high-magnification surface morphologies and EDS analyses; (s–t) elemental mapping confirming CeO₂ dispersion after using HVOF process. Permission from Elsevier. Copyright © 2023.

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The SEM and TEM results for the initial powder are depicted in Figure 9(a-f) for nano-CeO₂ powder. These findings clearly indicate that the morphology of the powder is spherical with pores, and the CeO₂ nanoparticles are agglomerated due to van der Waals forces. Subsequently, these composite powders were applied to a stainless-steel substrate using the HVOF process. The detailed role of micro/nano CeO₂ reinforcement in enhancing mechanical and tribological properties will be discussed in the subsequent section. The cross-sectional results of SEM/EDS mappings (Figure 9g-i) reveal that all coatings have a thickness ranging between 280 and 300 µm, exhibit a lamellar microstructure, and are free of pores and cavities at the coating-substrate interface [58]. The magnified images further confirm the presence of CeO₂, as indicated by the yellow arrows, based on the SEM/EDS results in Figure 9(j–l). The surface microstructure of the as-sprayed coatings is shown in Figure 9(m, n, o), with corresponding magnified images (Figure 9(p, q, r)) for coatings both with and without CeO₂. SEM/EDS mapping further confirms the distribution of Ce, Cr, Ni, C, and O within the coatings, as shown in Figure 9(s, t). EDS analysis also reveals a reduced oxygen content and a more uniform dispersion of CeO₂ in the nanocomposite powder compared with the other coatings. It is worth noting that there are some variations in the concentration of CeO₂ after deposition, with these variations being more pronounced in micro-reinforced coatings in comparison to CeO₂-reinforced nanoparticles Cr₃C₂-NiCr coatings, which aligns with findings in other reported literature [59].

Section 3.1 provided an in-depth survey of recent literature, emphasizing the properties of as-received feedstock materials. This includes a focus on parameters such as particle size, shape, and microstructure. Additionally, the section explores composite powders, particularly NiCrAlY/NiCoCrAlY, prepared through various methods. These composite powders incorporate diverse hard particles, as elaborated upon earlier. The comprehensive analysis of NiCrAlY/NiCoCrAlY composite powders, along with the distribution of hard metal phases, proves to be important. The section clearly demonstrated the requirements, including the microstructure of appropriate composite powder and particle size for the HVOF process. Particle size plays an important role in the HVOF process because it strongly affects melting behavior, splat formation, and the resultant microstructure. The literature shows that powder particle sizes in the range of 10–30 µm are considered optimal for HVOF spraying. Powders within this range achieve more complete melting, enhanced lamellar interlocking, and denser coatings with low porosity, which in turn improve bond strength, hardness, corrosion, and oxidation resistance. For example, nanostructured NiCoCrAlY powders with a ∼15 nm grain size and 30 µm particle diameter produced coatings with finer splat morphology and reduced defects [42]. Similarly, spherical WC–Co powders of 15–45 µm yielded porosity levels below 1%. In contrast, powders with sizes above ∼50 µm are more difficult to fully melt during spraying, often resulting in partially molten or unmelted particles embedded in the coating. These larger particles can locally enhance bonding by mechanical interlocking but typically increase porosity, crack density, and oxide content, which negatively affect mechanical strength, adhesion, and oxidation resistance. Thus, the optimal particle size for HVOF coatings is 10–30 µm, providing improved bonding, adhesion, and resistance to high-temperature corrosion. The next section investigates how these variations significantly impact crucial aspects such as hot corrosion resistance, wear resistance, oxidation, and mechanical properties of coatings produced through HVOF Spraying. Hence, the subsequent section, 3.2, will elaborate on the key findings and insights derived from the recent literature survey.

3.2. Corrosion, wear, and oxidation performance of HVOF spray coatings

This section provides a detailed literature survey aimed at enhancing corrosion, wear, and oxidation resistance through various coatings deposited by the HVOF spray process.

Ozkan reported [47] on the oxidation and wear mechanisms of Cr₃C₂-NiCr hard-metal coatings on a stainless-steel substrate through the HVOF process, exploring different times and temperatures. Additionally, the study investigated the hot corrosion mechanism of Cr₃C₂-NiCr composite coatings in corrosive environments with Na₂SO₄ and V₂O₅. The oxidation results indicate that at 850°C, the thickness of the oxide layer increased as the oxidation time extended from 5 h to 50 h (Figure 10a). Figure 10(a-c) show the cross-sectional morphologies after oxidation at different times, where the yellow labels denote the cross-sectional views and the white arrows indicate mixed oxides, porosity, and inner oxides. Figure 10(b) shows the cross-sectional morphologies after hot corrosion for 1 h, 3 h, and 5 h, exhibiting similar degradation behavior. After 50 h of oxidation, the mixed oxide accelerated due to the increased time and high temperature, thereby enhancing the diffusion of oxygen from the atmosphere to the substrate layer. Elemental mappings confirmed the diffusion of elements and the presence of rich oxides after oxidation (Figure 10(c)). Figure 10(d) presents the elemental mapping after hot corrosion, revealing that vanadium elements penetrate through microcracks toward the coating/substrate interface. The worn surface morphology after the wear test, where oxide layer formation, microcracks, swelling, and fine scratches are observed, as depicted in Figure 10(e). The friction coefficient and volume loss values increase with the applied load, ranging from 4 N to 12 N, as illustrated in Figures 10(f, g). The graph illustrates that the increase in wear traces corresponds to increased volume loss, consequently contributing to elevated wear rates. It has been determined that, under high-temperature conditions, the Cr₃C₂-NiCr composite coatings demonstrate superior resistance to both wear and corrosion in corrosive environments.

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Figure 10.

(a), (b) Cross-sectional results of Cr₃C₂ -NiCr composite coatings after the oxidation at 850 ⁰C (for 5 h, 25 h, and 50 h), and after corrosion (1 h, 3 h, and 5 h), respectively. (c) & (d), respectively, the mapping results for 50 h and 5 h after oxidation and hot corrosion. (e) shows SEM results of wear traces and worn surfaces of the Cr₃C₂ -NiCr coatings under varying loads. (f) and (g) show friction coefficient curves and volume loss/wear rate of the coatings, respectively. The white arrows indicate porosity, mixed oxides,oxide, wear trace, fine scractes, micro-cracks, and oxidation layer. Red arrows indicate the rich phases of Vanadium element using elemental mapping. Permission from Elsevier. Copyright © 2023.

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Wang and colleagues conducted a study [46] on the corrosion and wear performance of NiCrWMoCuCBFe coatings using the HVOF process in various environments, including H2O, HCl, NaOH, and NaCl. The coatings exhibited passivation characteristics in HCl, NaOH, and NaCl solutions (Figure 11a). The results also indicated that the coating had the lowest passivation potential in NaOH and the highest passivation potential in HCl solutions. Based on these passivation characteristics, the corrosion potential and current density of the coating in HCl environments were higher compared to those in NaOH and NaCl environments.

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Figure 11.

(a) Potentiodynamic polarization curve, (b) real-time friction coefficient, (c) average friction coefficient and wear rate, (d) SEM surface results of worn scars with 3D morphology results, (e) FESEM results of worn scars with magnified images in the right side of the NiCrWMoCuCBFe coatings in various environments, i.e., H₂O, HCl, NaOH, and NaCl. Permission from Elsevier. Copyright © 2022.

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Figure 11(b) shows the friction coefficient of the coating under different environmental conditions, indicating that the lowest friction occurs in the HCl solution compared to the other tested media. Notably, the friction coefficient in HCl solutions remains stable, dropping below 0.1 after 500 seconds of friction. In the case of the HCl solution, there is a discernible declining trend in the friction coefficient with an increase in wear distance. This behavior is likely due to the ongoing decrease in friction during the wear-corrosion process, resulting from the formation of corrosion species on the surface in HCl solution. Real-time friction measurements showed that the average friction coefficient was highest in the NaCl solution at 0.32, while the HCl solution exhibited the lowest value at around 0.05. In comparison, the average friction coefficients in water and NaOH solutions were 0.23 and 0.25, respectively (Figure 11c). The NiCrWMoCuCBFe coating exhibited both the lowest friction coefficient and the minimal wear rate when tested in an HCl solution. These findings indicate that the formation of corrosion products played a key role in influencing the coating’s friction and wear performance under acidic conditions. Figure 11(d) presents the SEM and 3D surface analyses of the worn NiCrWMoCuCBFe coating following friction tests in different solutions. The findings indicate that, following friction in H₂O solution (Figure 11d(a₁)), furrows and local exfoliations appeared on the coating surface due to shear frictional action. The 3D image of the coating after friction reveals details about the surface roughness and the width of the wear mark as shown in (Figure 11d(a₂, b₂ and c₂, d₂). In contrast, when subjected to friction in an HCl solution (Figure 11d(b₁)), a significant number of corrosion products emerged on the surface, indicating pitting corrosion. Exposure to the NaOH solution (Figure 11d(c₁)), resulted in the formation of furrows and partial exfoliation on the worn surface. SEM analysis indicated more pronounced corrosion within the wear tracks. Similarly, after friction in the NaCl solution (Figure 11d(d₁)), the wear mark morphology appeared comparable to that seen in the NaOH environment. In summary, the distinct morphological changes observed in the SEM and 3D results emphasize the diverse effects of friction in different solutions on the NiCrWMoCuCBFe coating, ranging from shear-induced furrows and exfoliations in H₂O solution to pitting corrosion (indicating using yellow arrows), in HCl solution and a combination of furrows and corrosion in NaOH and NaCl solutions.

The FESEM results revealed that the worn surface exhibited smoothness, with a few debris particles and micro-cracks indicated using yellow arrows present after friction in water (Figure 11e(a₁,b₁)). The surface comprised dark and bright regions, representing the wear debris (indicated with yellow arrows) and the coating surface, respectively. Additionally, numerous cracks were observed around semi-molten particles, likely originating during the coating preparation process. Upon friction in the HCl solution, numerous corrosion spots emerged on the coating surface, accompanied by cracks and nanoparticles around these corrosion spots indicated with yellow arrows as shown (Figure 11e(a₂,b₂)). Subsequent friction in the NaOH solution resulted in a distinct exfoliation, with nanoparticles evident on the worn spot. This phenomenon is a key factor contributing to its elevated wear rate in this environment (Figure 11e(a₃,b₃)). Similarly, after friction in the NaCl solution, the characteristics of the worn coating surface closely resembled those observed in NaOH (Figure 11e(a₄,b₄)). The above findings suggest that the coating’s performance is significantly influenced by the specific chemical environment, with corrosion, exfoliation, and nanoparticle formation being key factors affecting its wear resistance.

Figure 12a(a₁,a₂,a₃) presents the wear track analysis of the conventional power Cr₃C₂-NiCr (CP), micro CeO₂ (CMR), and nano CeO₂ reinforcement coated samples using FESEM and EDS. The yellow boxes highlight the formation of a tribo-oxide layer in all three samples, as indicated by the yellow arrows. Additionally, there are high-magnification pictures of the wear tracks as shown in Figure 12a (a₄-a₉). Abrasive wear is the predominant mechanism, as evidenced by the distinct grooves that each coating exhibits along the wear path as indicated using yellow arrows. These results are in line with those of coatings applied using the HVOF process that have been previously published. Significant damage was seen to the CP coating, which is consistent with higher wear rates and friction coefficients. Numerous microcracks were also visible (denotes using yellow arrows) in the wear track. Abrasive wear mentioned with yellow arrows occurred on the CMR coating, resulting in grooves resembling those in CP. Despite experiencing abrasive wear as well, CNR outperformed CP and CMR in terms of wear resistance. Furthermore, there were fewer hard carbide particles in the wear debris from CNR compared to CP and CMR. Notably, in line with previous research, no microcracks were found on the CNR wear surface, and the oxide scale adhered better to the coating [58]. The observations revealed that, upon interaction with the corrosive environment (i.e., sodium sulfate vanadate), a layer of corrosion products, specifically Ni₃V₂O₈ and Na₂SO₄, formed on the surface of the coatings for all tested durations as shown in Figure 12b(b1-b3). Cross-sectional results show that after hot corrosion, the coating layer and interface are damaged,minor alumina formation at the interface,while the formation of V₂O₅ +Na₂SO₄, in the coating is indicated by white arrows (Figure 12(b)). The friction coefficients, wear track profiles, and particular wear rates are shown in Figure 12(c,d, and e), respectively. According to the data, CNR’s friction coefficient is lower than CP’s and CMR’s by 44% and 16%, respectively (Figure 12c). The wear track profiles are displayed in Figure 12(d), while Figure 12(e) indicates that CNR has the lowest specific wear rate, followed by CMR and CP. These findings suggest that Cr₃C₂-NiCr coatings enhanced with nano-CeO₂ offer superior tribological and mechanical performance relative to CP and CMR coatings.

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Figure 12.

(a) shows SEM low and magnification results (from white circles) of the wear track and EDS analysis of powder of Cr₃C₂-NiCr (CP), micro-CeO₂ (CMR), and nano-CeO₂ (CNR), respectively, yellow boxes indicates the formation of tribo-oxides layer. (b) shows cross-sectional results of the 80Ni-20Cr coatings after hot corrosion and white arrows indicates formation of minor oxides,coatings, damage interface and the formation of V₂O₅+Na₂SO₄ for 1 h, 3 h, and 5 h, respectively. (c), (d), and (e) depict, respectively, the coefficient of friction (COF), wear track profile, and wear rate of the coating. (f) shows the surface morphology of the coatings following the hot corrosion process and EDS analysis after 1h, 3h, and 5 h, (g) cross-sectional SEM low and high magnifcation results (from white circles) of the 80Ni-20Cr coatings after isothermal oxidation for 5 h, 25 h, 50 h, and 75 h, respectively. Permission from Elsevier. Copyright © 2023.

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Binal conducted a study [57] on 80Ni-20Cr coatings applied to stainless steel using the HVOF process. The research focused on investigating the oxidation and hot corrosion behavior of the coatings over various time periods at high temperatures in a corrosive environment consisting of 55 wt% V₂O₅ and 45 wt% Na₂SO₄ (refer to Figure 12(b,f,g)). In Figure 12f(f₁-f₃), the surface morphology and EDS analysis after hot corrosion for 1, 3, and 5 h have been depicted. The oval (white circles) highlight regions magnified to further confirm the sample morphology after hot corrosion for various times, as shown in Figure 12f(f₄–f₆). The point EDS analysis additionally confirmed that the columnar and rod-like morphology observed after 3 h of hot corrosion corresponds to the Ni₃V₂O₈ phase. Figure 12g(g₁-g₄) presents the cross-sectional results of the coatings after isothermal oxidation for 5, 25, 50, and 75 h. The magnified images further confirm the formation of Al₂O₃ and mixed oxides, as shown in Figure 12g(g₅–g₈). The findings led to the conclusion that all coated samples exhibited high resistance to isothermal oxidation at 750°C without experiencing damage such as peeling or flaking.

G. Singh and colleagues [9] studied the hot corrosion resistance of a carbide-based composite coating applied to boiler steel using the HVOF technique, exposing it to a Na₂SO₄–60% V₂O₅ environment at 900°C. The study revealed that the uncoated bare steel samples experienced severe corrosion, resulting in the formation of non-protective oxides Figure 13a(a₁). In contrast, the NiCrAlY-SiC and 20B₄C composite coatings demonstrated improved corrosion resistance Figure 13a(a₂, a₃). The SEM surface micrograph confirmed that surface spallation was more pronounced in the bare samples compared to the composite-coated samples, as shown in Figure 13a(a₁-a₃) after 50 cycles. Figure 13b(b₁-b₅) illustrates the surface morphology of both the bare sample and the sample after coating, as analyzed by SEM. EDS analysis from the yellow points marked in pink rectangular box confirmed the presence of various oxides on the sample surfaces following corrosion. The results indicate the formation of poorly developed oxides on the corroded bare steel surface, as depicted in Figure 13. The EDS results further show the rich contents of Fe and O on the corroded bare steel surface, suggesting the likely formation of Fe₂O₃. The scale formed on NiCrAlY-SIC coatings exhibits a homogeneous structure. EDS analysis confirms the presence of Ni, Al, Cr, O, and small amounts of Fe and C in the oxide scale. In the case of NiCrAlY-B₄C coatings, the corrosion-induced scale appears more compact, dominated by Ni, Cr, and Al, with minor traces of other particles. It is concluded from the results that SEM and EDS analyses provided insights into the surface characteristics and elemental compositions, revealing distinctive features of oxide scales formed on bare steel, NiCrAlY-SIC, and NiCrAlY-B₄C coatings after corrosion.

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Figure 13.

(a): Surface morphology after hot corrosion: (a₁) uncoated T22 steel, (a₂) NiCrAlY–20SiC coated, and (a₃) NiCrAlY–20B₄C coated samples. (b): SEM/EDS analysis (b₁–b₅); orange dots within the purple rectangle indicate the locations selected for EDS analysis. (c): Surface morphology after hot corrosion: (c₁-c₂) bare 316L stainless steel and (c₃-c₄) WC–10CoCr coating; red arrows indicate fractures, pits, and cracks. (d): Cross-sectional SEM images of (d₁) bare steel and (d₂) WC–10Co–4Cr coating; in (d₁), yellow circles and red arrows indicate spallation pits, while red lines and yellow arrows denote corrosion pits; in (d₂), red lines show splat separation, and yellow circles with red arrows indicate micro-holes. Permission from Elsevier. Copyright © 2021.

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Kumar et al. [60] reported on the microstructure, oxidation, and hot corrosion behavior of a WC-10Co-4Cr coating on 316LSS using the HVOF process in Na₂SO₄-88%Fe₂(SO₄)₃ environments at 800°C. Figure 13(c)(c₁-c₄) presents the SEM analysis of both the bare and coated samples of 316LSS after exposure to the Na₂SO₄-88%Fe₂(SO₄)₃ environment. The morphology of the bare sample appears uneven, with the magnified image revealing fractures, pits, and cracks (see red arrows) on the surface following SEM results. In contrast, the coated sample processed through HVOF shows the presence of an unmelted powder marked using red arrows, as depicted in Figure 13c(c₃-c₄). Cross-sectional results of both the bare and coated corroded samples are presented in Figure 13d(d₁,d₂). The outcomes clearly illustrate spalling pits (see yellow circles and red arrows) and corrosion pits indicated using red lines and yellow arrows on the uncoated sample surface. In comparison, the coated sample exhibits micro holes (yellow circles and red arrows) and surface separation indicated using red dotted lines attributed to the high-speed melting and semi-fused particles during the process. The result indicates that the WC-10Co-4Cr coating applied on 316LSS using the HVOF process sample exhibits improved oxidation resistance and enhanced resistance to hot corrosion when compared to the uncoated 316LSS sample.

In a separate study, Kumar et al. [61] fabricated WC-12Co and WC-10Co-4Cr coatings on a steel substrate using the HVOF process. Subsequently, they investigated hot corrosion in a corrosive environment containing Na₂SO₄-82%Fe₂(SO₄)₃ at 650°C. The micrographs presented in Figure 14(a-c) illustrate the outcomes of 50 cycles of molten salt corrosion at 650°C on a bare sample, WC-10CoCr-coated, and WC-12Co-coated AISI316L substrate. The SEM morphologies of the uncoated sample and coated samples up to 50 cycles at 650°C in salty environments are depicted in Figure 14(d-f) at various magnifications. The results indicate that the uncoated sample exhibits cracks, corrosion pits, and spalling pits as indicated using red, black circles and yellow red arrows respectively (Figure 14(d, d1)). After post-corrosion testing, the WC-10CoCr-coated surface shows a globular structure, and cracking occurs due to constant oxidation on the sample surface caused by uneven heating and cooling (Figure 14e). Additionally, Figure 14(e1) reveals localized corrosion in Co-rich regions. The SEM morphology of the WC-12Co-coated surfaces after the corrosion test is shown in Figure 14(f, f1). Both the WC and Co matrix are visible in WC-Co coatings (Figure 14f), and the magnified image highlights voids and micro-cracks as mentioned using yellow arrows (Figure 14f1). The graph in Figure 14(g) compares the mass gain versus the number of cycles at 650°C after hot corrosion for stainless steel with WC-12Co and WC-10CoCr coatings compared to the bare sample. The results demonstrate that the coated samples gained less mass compared to the uncoated sample after different cycles of hot corrosion. Furthermore, the Kp value confirms that the HVOF-sprayed coated samples exhibit higher corrosion resistance than the bare steel sample in the Fe₂(SO₄)₃-12%Na₂SO₄ environments. XRD results confirm the formation of various oxide types on the coating surface subjected to hot corrosion environments at 650°C (Figure 14h,h1). The graph illustrates the cumulative weight gain of WC-10CoCr, which had the lowest weight gain compared to WC-12Co and the bare steel sample (Figure 14i). These findings collectively highlight the potential of WC-10CoCr coatings in mitigating the deleterious effects of hot corrosion on stainless steel, making them a promising candidate for applications in harsh environments. These observations highlight that the apparent differences in reported performance of WC-Co, WC-CoCr, and WC-NiCr coatings are largely influenced by substrate coating compatibility, differences in HVOF process parameters such as flame temperature and particle velocity, and the evolution of oxides and its morphologies or intermetallics during exposure. For example, the addition of Cr promotes protective formation of Cr₂O₃ and CoCr₂O₄ scales, improving hot corrosion resistance compared to WC-Co, whereas variations in porosity and WC decarburization into W₂C phases can accelerate degradation of coatings. Thus, the reported differences across WC-based systems are not contradictory but arise from these microstructural and processing-dependent factors.

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Figure 14.

Micrographs of (a) WC-10CoCr coated, (b) uncoated AISI316L, and (c) WC-12Co coated AISI316L, respectively, after hot corrosion. SEM results of the (d,d1) uncoated sample, (e,e1) WC-10CoCr coated, and (f,f1) WC-12Co coated AISI316L in the Na₂SO₄-82%Fe₂(SO₄)₃ environment under hot corrosion. g) shows plot of hot corrosion cycle vs square of weight gain/area, (h,h1) shows XRD pattern for coating WC-10CoCr and WC-12Co respectively after hot corrosion. (i) shows the graphs between corrosion cycles Vs. accumulative weight gain for all corresponding three samples. Permission from Elsevier. Copyright © 2023.

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Hao and their colleagues [62] prepared NiCrAlY, NiCoCrAlYHfSi, and NiCoCrAlYTa coatings using the HVOF process and investigated their tribological properties at 800 C.

The surface morphology and EDS analysis of the wear marks on NiCrAlY have been shown in Figure 15(a). The results reveal the severe plastic deformation marks on the wear surface. Micro-cracks during friction lead to the exfoliation of splats, causing the COF of this coating to fluctuate more rapidly compared to the other coatings. Similarly, wear marks on the NiCoCrAlYHfSi coating resemble those on NiCrAlY, but adhesion and abrasion are milder (Figure 15b). Results also show that there are no obvious abrasive marks on the worn surface for the NiCoCrAlYTa coating, and the coating exhibited greater adhesion (Figure 15c). Furthermore, EDS results confirmed (Figure 15(a₁,b₁,c₁)) the oxygen content after high-temperature friction analysis. The NiCoCrAlYTa coating exhibits high-performance tribological properties, which is why it was selected to investigate the formation mechanism of the glazed layer and its effect on wear and friction performance (Figure 15d). Raman results on the worn and non-worn surfaces confirm similar chemical reactions during friction, with various oxides such as Al₂O₃, Cr₂O₃, and NiO appearing (Figure 15e,f). Figure 15(g,h) shows TEM results of the cross-sectional, Figure 15(i-k) High resolution transmission electron microscopy (HRTEM) results of region 1 indicated using yellow rectangle, and Figure 15l is the diffraction pattern of region 2 of the NiCoCrAlYTa coating is the diffraction pattern of region 2 of the NiCoCrAlYTa coating. TEM results confirm the presence of amorphous and crystalline phases, as well as the formation of various oxides and the Ni₃Al phase. Figure 15(m,n) shows the mechanical properties, hardness, and load-displacement curve after nano-indentation, respectively. All three types of coatings significantly enhance micro-hardness compared to stainless steel. The micro-hardness of 316L steel is 176.88 HV and gradually increases for coatings 1 to 3, 554.81 HV, 569.61 HV, and 599.61 HV, respectively (Figure 15m). Figure 15(o,p) depicts real-time COF value graphs, wear, and average COF values of the three types of coatings, respectively. During friction, the COF of NiCrAlY first decreases slowly and then gradually levels, with its value ranging from 0.28 to 0.40 in the stable period. The COF value for NiCoCrAlYHfSi coatings is about 0.40, and for the NiCoCrAlYTa coating, it is very stable, fluctuating slightly around 0.33. The average coefficient values gradually decrease in the sequence 0.45 ± 0.017, 0.42 ± 0.012, and 0.34 ± 0.016, respectively, for NiCrAlY, NiCoCrAlYHfSi, and NiCoCrAlYTa coatings. The wear rate of the coatings exhibits a similar trend to that of COF for all three types of coatings. The wear and friction mechanisms [62] of MCrAlY coatings are presented in Figure 16. In conclusion, the comprehensive analysis of Raman spectroscopy, TEM results, mechanical properties, and friction characteristics reveals that NiCoCrAlYTa coating demonstrates superior wear resistance and stable friction behavior compared to NiCrAlY and NiCoCrAlYHfSi coatings, supported by the presence of various oxides, crystalline phases, and enhanced micro-hardness.

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Figure 15.

(a–c) SEM/EDS results of the worn surfaces of NiCrAlY, NiCoCrAlYHfSi, and NiCoCrAlYTa coatings on 316L stainless steel. (d) SEM/EDS mapping of the NiCoCrAlYTa wear mark. (e,f) Raman spectra of the wear track and coatings. (g,h) TEM cross-sections of the NiCoCrAlYTa coating; (i–k) HRTEM images of region 1; (l) diffraction pattern of region 2. (m,n) microhardness and nanoindentation results. High-temperature tribological performance: (o) Coefficient of friction-distance curves, and (p) the average coefficient of friction and wear rate values. Permission from Elsevier. Copyright © 2023.

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Figure 16.

Shows the schematic for the wear and fraction mechanism of the NiCrAlY, NiCoCrAlYHfSi, and NiCoCrAlYTa coatings for the work presented in Figure 15 of this article . Permission from Elsevier. Copyright © 2023.

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The effectiveness of HVOF coatings on steel substrates to improve their tribological characteristics has also been studied by other researchers. Kaur et al. [63] reported on the use of the HVOF process to coat boiler steel with Cr₃C₂-NiCr, showing enhanced resistance to wear, erosion, and hot corrosion in boiler steel applications. For both coated and uncoated samples, the researchers used a molten salt mixture of Ni and Fe under particular circumstances. In the molten salt environments of Ni and Fe, the coated samples demonstrated excellent resistance to wear and corrosion. A study on the HVOF spray method for applying Cr₃C₂-NiCr, Ni-Cr, and WC-Co coatings on a steel substrate was carried out by Sidhu and his associates [64]. At a temperature of roughly 900°C, the study examined the hot corrosion resistance of these deposited coatings in the presence of Na₂SO₄-V₂O₅ environments.

The results showed that the surface was protected from hot corrosion attacks by the WC coating. On the other hand, the bare sample showed signs of severe corrosion, including peeling, scale cracking, and spallation. In order to apply tungsten carbide coatings in piston, valve, and aircraft applications, Bonora et al. [65] reported the HVOF process. When compared to traditional electroplating coatings, the results show that HVOF coatings have better mechanical and tribological properties on the preparation of novel WC-Cr₃C₂-CoNi coatings that are applied to a stainless-steel substrate by means of the HVOF TSC technique. The performance of these coatings with different WC-Cr₃C₂ contents is specifically examined in the study in relation to traditional WC coatings. The results indicate that the WC- Cr₃C₂-CoNi coating with a 20 wt% concentration of WC-Cr₃C₂ significantly improves the wear and corrosion resistance of the coatings. Furthermore, the homogeneous distribution of Ni and Cr in the WC- Cr₃C₂-CoNi coating results in a more stable oxide film during the corrosion process, thereby enhancing the corrosion resistance of the coatings.

4.1. Studies on the microstructures of as-received powder and as-prepared coatings, as well as the corrosion, wear, and oxidation performance of plasma spray coatings

This section focuses on the microstructure of as-received feedstock materials and the unique features of coatings produced by the PS process. It also examines how the microstructure influences the mechanical, wear, corrosion, and oxidation resistance of these coatings.

Reddy and colleagues [68] used the PS coatings process in an aggressive Na₂SO₄+60 percent V₂O₅ salt environment at 700°C to study the hot corrosion and oxidation behavior of NiCrAlY-TiO₂ and NiCrAlY-Cr₂O₃-YSZ coatings on alloy steel. Figure 19(a,b) illustrates the morphology of NiCrAlY-TiO₂ and NiCrAlY-Cr₂O₃-YSZ powders. The results reveal that the particles exhibit a spherical shape with sizes ranging from -45 to +15 μm, produced through the gas atomized technique. The SEM micrographs and EDS analysis of the corroded surfaces of the samples, with and without coatings, are presented in Figure 19(c-e). These findings confirm the microstructure, the formation of oxides, and the elemental composition of the oxides, as mentioned using yellow arrows. Figure 19(f) illustrates the elemental mapping of the corroded surface of NiCrAlY-TiO₂, indicating the predominant presence of Al, Cr, O, Ni, S, Ti, and Na in the formed scale. The distribution of these elements is clearly depicted in the mapping results, as shown in Figure 19(f). Additionally, a layer of iron is evident in the substrate portion. In Figure 19(g), the top layer of the NiCrAlY-Cr₂O₃-YSZ-coated scale predominantly comprises O, forming a thick layer in the measured scale. Al and Cr are observed at the boundaries of Ni-rich PS splats. The weight gain versus the number of cycles for both coated and uncoated samples is presented in Figure 19(h). The results indicate mass gains of 111.13, 38.97, and 29.94 mg/cm² for the steel substrate, NiCrAlY+TiO₂, and NiCrAlY+Cr₂O₃+YSZ coatings, respectively. The lower weight gain suggests that cermet coatings protect against corrosion in aggressive environments. The parabolic plots demonstrate that the NiCrAlY-Cr₂O₃-YSZ coating, with lower Kp values (Figure 19(i)), provides more effective protection in salt environments compared to the uncoated samples. In conclusion, the study found that coated samples exhibited less mass gain, lower Kp values, lower porosity, higher hardness values, higher bond strength, and better corrosion resistance at high temperatures compared to the uncoated samples.

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Figure 19.

Morphology of the (a) powder NiCrAlY-TiO₂ and (b) NiCrAl-Cr₂O₃-YSZ, respectively. SEM/EDS results of (c) alloy steel, (d) NiCrAlY-TiO_2, and (e) NiCrAl-Cr₂O₃-YSZ coatings, respectively, after being exposed to high-temperature corrosion environment of Na₂SO₄+60%V₂O₅ salt for 50 cycles at 700°C. cross-sectional elemental mapping analysis of (f) NiCrAlY-TiO₂ and (g) NiCrAl-Cr₂O₃-YSZ coatings, respectively, after corrosion for 50 cycles at 700°C. (h) and (i) show the mass gain curve and corresponding parabolic plots of alloy steel, NiCrAlY-TiO₂, and NiCrAl-Cr₂O₃-YSZ samples, respectively, in a corrosive environment at 700°C.Permission from Elsevier. Copyright © 2021.

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Jonca et al. [69] conducted a study on the oxidation behavior of a sprayed CoNiCrAlY/h-BN following exposure to temperatures of 750°C and 900°C. Additionally, they investigated the formation of protective alumina at high temperatures using various characterization techniques. The morphologies of the as-prepared CoNiCrAlY/h-BN coatings have been illustrated in Figure 20(a-d). The results revealed that the coating comprised unmelted particles, pores, and a lamellar structure. EDS analysis confirmed the formation of various oxides and mixed oxides during the deposition process. The oxidation behavior of the thermally sprayed CoNiCrAlY/h-BN coating was investigated after exposure to temperatures of 750°C and 900°C. The study also delved into the formation of protective alumina at high temperatures, employing various characterization techniques. The morphologies of the CoNiCrAlY/h-BN coating have been illustrated in Figure 20(a-d). The results revealed that the coating comprised unmelted particles, pores, and a lamellar structure. EDS analysis confirmed the formation of various oxides and mixed oxides during the deposition process.

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Figure 20.

(a-d) Surface morphologies of the CoNiCrAlY/h-BN coating. (e) XRD patterns after 500 h oxidation at 750°C and 900°C. (f) Mass change vs. time for 718 + NiAl + CoNiCrAlY/h-BN, CoNiCrAlY/h-BN, 718 alloy, and NiAl coatings at 750°C and 900°C. (g,h) Raman spectra after 500 h oxidation. (i–l) Surface morphologies and (m–p) cross-sectional views after 100 h and 500 h oxidation at 750°C and 900°C . Permission from Elsevier. Copyright © 2019.

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Figure 20(e) presents the XRD results of the CoNiCrAlY/h-BN at 750 and 900°C after 500 h of oxidation. The coating surface exhibited the presence of γ and γ/ phases, Cr₂O₃, as well as mixed oxides of CoNiO₂ and CoCr₂O₄. The XRD peak intensities were notably higher at 900°C compared to 750°C. The mass change curves illustrating the oxidation performance of various samples over a 500-h period at 750°C and 900°C are displayed in Figure 20(f). The CoNiCrAlY/h-BN sample showed the most stable oxidation behavior among the tested coatings, with oxidation rate constants (Kp) of 1.12 × 10⁻¹¹ mg² cm⁻⁴ s⁻¹ at 750°C and 2.29 × 10⁻¹⁰ mg² cm⁻⁴ s⁻¹ at 900°C. The Raman spectra of the CoNiCrAlY/h-BN coating after 500 h of oxidation exposure at the corresponding temperatures are displayed in Figure 20(g) and (h). The obtained results verify that the coatings contain various oxides, including spinel oxides, Cr₂O₃, and alumina (α-Al₂O₃, a very stable and protective oxide) [70-72]. The continuous formation of an α-Al₂O₃ scale protects the coating from further oxidation, thereby extending its lifespan at high temperatures [73-78]. The surface morphology of the coating at 750°C for 100 h and 900°C for 500 h is depicted in Figure 20(i, j) and (k, l), respectively. The results show that at lower temperatures (750°C), the morphology exhibits needle or crystal-type structures, indicating the characteristics of metastable or θ-Al₂O₃. However, at higher temperatures and longer oxidation times, θ-Al₂O₃ transforms into α-Al₂O₃, leading to a significant reduction in the needle-type morphology content. Figure 20(m, o, and n, p) displays the cross-sectional morphology of the CoNiCrAlY/h-BN coatings. The results reveal the formation of various oxides, as confirmed by SEM/EDS analysis. Additionally, the presence of unmelted particles suggests a larger source of aluminum than the surrounding material, explaining the varying oxidation behavior across different parts of the coating and the heterogeneous distribution of oxides.

Alam and Das [5] investigated the hot corrosion resistance of a WC-CoCr coating applied to a steel substrate using the PS method. In their study, both coated and uncoated specimens were subjected to a corrosive mixture of Na₂SO₄ and 25% NaCl at 700°C over 50 thermal cycles. Figure 21(a) displays the cross-sectional analysis of the coated samples, confirming the uniform thickness of the coating on the steel substrate. Figure 21(b,c) depicts the morphology of oxidizing species at various magnifications. The magnified SEM results confirm the opening of some pores (mentioned using orange arrows) at the surface due to the penetration of oxidizing species. Once oxidation has occurred on all interior surfaces accessible (orange arrows), resulting in the formation of oxides at splat boundaries and within porosity, the coating undergoes a densification process. This densification leads to a slow-down in oxide diffusion, with further oxide development primarily restricted to the surface of the specimen.

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Figure 21.

SEM analysis of WC-CoCr-coated steel: (a) cross-section , (b) surface morphology of oxidation products and (c) high magnification SEM image of oxidation products; (d, e) XRD patterns before and after hot corrosion; (f, g) SEM/EDS with elemental mapping of uncoated steel post-corrosion; (h–j) oxide formation, splat morphology, and SEM/EDS with mapping of coated samples after corrosion (k-n) SEM/EDS spectra and elemental distributions; (o) mass gain, (p) cumulative weight gain, and (q) mass gain squared vs. time comparison between uncoated and coated samples. Permission from Springer nature. Copyright © 2023.

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Figure 21(d) and (e) show the XRD patterns of WC-CoCr-coated samples prior to and following hot corrosion. Before corrosion, distinct peaks corresponding to WC and WC₂ phases are clearly observed. The XRD results confirm the sharp peaks of WC and WC₂ before hot corrosion. After hot corrosion testing, the main phases observed are WO₃, CoW₃, C, and CoWO₄. Additionally, chromium (Cr) develops the Cr₂O₃ phase, and its spinel oxide CoCr₂O₄ acts as an intermediate phase. The formation of this intermediate phase contributes to enhanced corrosion resistance of the coating in a Na₂SO₄-25% NaCl salt environment. SEM/EDS analysis of the uncoated sample, illustrated in Figure 21(f) and (g) along with elemental mapping, reveals significant activity of iron (Fe) and chlorine (Cl) in the corrosion process. Upon oxidation, Fe promotes the formation of the unprotective oxide Fe₂O₃. Further interaction with the steel substrate at high temperatures causes molten salt diffusion, leading to slits and cracks. These slits and cracks are the initial sites for oxide formation and oxidation. Figure 21(h) illustrates the splats and oxides developed on the coatings ,as indicated using orange arrows. SEM/EDS results show the coated sample after hot corrosion in Figure 21(i,j) with the corresponding elemental mapping. Figure 21(k–n) presents the SEM/EDS elemental spectra, and the corresponding elemental percentage distribution of the coatings. The presence of oxides and their distribution indicate the response of the coating to hot corrosion conditions, indicating its protective capabilities in an aggressive environment. The presence of tungsten and chromium oxides, formed as scales on the surface and boundaries, enhances the resistance of WC-CoCr coatings to hot corrosion. These oxides serve as effective barriers, preventing the diffusion of corrosive elements and inhibiting their penetration through the coatings. Figure 21(o) illustrates the mass gain per unit curve versus the number of hot corrosion cycles for both the coated and uncoated samples in a molten salt environment at 700°C. The findings reveal a 74.45% reduction in weight gain for the WC-CoCr-coated sample compared to the uncoated sample in the corrosive environment. Figure 21(p) further demonstrates that the coated samples experienced lower weight gain when compared to the uncoated sample in corrosive environments. These graphs were generated following hot corrosion in an air environment, specifically in a corrosive atmosphere containing Na₂SO₄ + 25% NaCl at a temperature of 700°C. The graphs are intended to illustrate the corrosion behavior of both uncoated and WC-CoCr-coated samples under the test conditions. As shown in Figure 21(q), the data follow a parabolic trend, which is typical for hot corrosion kinetics. The parabolic rate constant (Kp) for the coated sample at 700°C is lower than that of the uncoated one, indicating that the WC-CoCr coating significantly enhances resistance to hot corrosion at this temperature.

Bolelli and colleagues [79] employed an innovative PS technique involving a hybrid injection of dry and liquid feedstock to fabricate a composite coating comprising a NiCrAlY metal matrix containing Al₂O₃ and h-BN sub-micron particles. The findings revealed a significant increase in hardness and sliding wear resistance as the content of Al₂O₃ and h-BN sub-micron particles was increased. This improvement was especially prominent at room temperature, with beneficial effects also observed at elevated temperatures of 400°C and 700°C. The presence of Al₂O₃ and h-BN contributed to the development and stabilization of a protective oxide-based tribo-film, which acted as a barrier to minimize direct interaction between the coating and the counter surface. Figure 22(a-d) depicts cross-sectional micrographs of different coatings, with their thickness approximately ranging from 300 to 400 µm. The microstructure reveals the presence of pores, clusters of Al₂O₃ and h-BN particles, as indicated by black arrows and circles, and a cone-shaped defect marked by a black dashed line. SEM results confirm that at room temperature, the worn surface shows the development of a tribo-film composed of oxide clusters, as indicated by black arrows in Figure 22(e, f), corresponding to samples Ny10Al10BN0 and Ny10Al10BN5. The wear reducing effect is less pronounced at 400°C (Figure 22(g, h)) and 700 °C (Figure 22(i, j)) compared to room temperature. The oxidized tribo-film clusters reduce abrasive grooving and adhesive tearing by mitigating direct contact between surfaces. This is evident in sample Ny10Al10BN0 (1 wt% Al₂O_3, no h-BN; Figure 22k) compared to Ny10Al10BN5 (10 wt% Al₂O_3, 9 wt% h-BN; Figure 22l). Wear scars at 400°C and 700°C (Figure 22(m, n)) show surfaces covered by various tribo-films as indicated by oval circles. These clusters consist of smeared debris patches, including larger metallic lamellar fragments (Figure 22(o)) within a matrix of oxidized submicron particles (Figure 22(p)). TEM images of loose debris confirm the presence of partially crystalline submicron and nanoparticles (Figure 22(q)). The high-resolution detail in Figure 22(r) reveals lattice planes, and the corresponding selected-area electron diffraction (SAED) pattern (from position marked as a white circle) displays diffraction spots and rings indicative of a glassy phase.

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Figure 22.

SEM cross-sectional images of coatings: (a) Ny10Al10BN0, (b) Ny10Al10BN1.25, (c) Ny10Al10BN2.5, and (d) Ny20Al10BN2.5, showing internal porosity and the presence of particles using black arrows and circles respectively. SEM of wear scars for Ny10Al10BN0 and Ny10Al10BN5 at: (e, f) room temperature, (g, h) 400°C, and (i, j) 700°C. Wear features on Ny10Al10BN0: (k) at room temperature, (m) at 400°C with white oval circles indicate tribo-film, and Ny10Al10BN5: (l) at room temperature, (n) at 700°C. Wear debris from Ny10Al10BN5 at room temperature: (o) SEM of smeared debris indicated using white arrows, (p) detailed SEM view, (q) TEM of loose particles, (r) HRTEM with SAED pattern. Permission from Elsevier. Copyright © 2017.

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Numerous researchers have reported the enhancement of tribological properties through PS coating on steel substrates. Doddamani et al. [80] studied three coating systems- NiCrAlY/WC-Co/Chemosphere/MoS₂/CaF₂, NiCrAlY/WC-Co/Chemosphere/MoS₂/CaSO₄, and NiCrAlY/WC-Co/Chemosphere-applied to steel substrates via plasma spraying. Their research focused on assessing the tribological performance of these coatings and the steel substrate over temperatures ranging from room temperature to 600°C under dry lubrication. Results showed that all coatings exhibited lower wear rates than the 321steel substrate throughout the testing conditions. Among them, the NiCrAlY/WC-Co/Chemosphere coatings containing MoS₂ combined with CaF₂ or CaSO₄ demonstrated reduced friction coefficients compared to the NiCrAlY/WC-Co/Chemosphere coating without these additives and the bare substrate. Analysis of the worn surfaces showed that MoS₂ functioned as an efficient lubricant at 200°C, while tribo-chemical formation of compounds like CaMoO₄ and MoO₃ maintained lubrication at 600°C. According to the study’s findings, the coatings under investigation—NiCrAlY/WC-Co/Chemosphere/MoS₂/CaF₂ and NiCrAlY/WC-Co/Chemosphere/MoS₂/CaSO₄ in particular-show improved tribological performance over a broad temperature range, indicating their potential use in situations requiring dry, high-temperature lubrication conditions. In a study conducted by Indupuri and Kumar [81], the microstructural, mechanical, tribological, and corrosion performance of plasma-sprayed aluminum nitride (AlN) coatings in a harsh environment were examined. Ball milling was used to create the feedstock powder, which was composed of Al and AlN in a 70:30 weight percent ratio. Both the c-AlN and h-AlN phases were present in the coating, according to the microstructural analysis. Hardness and elastic modulus were estimated to be around 7.2 GPa and 280 GPa, respectively. A low wear rate of roughly 0.18 × 10⁻³ mm³ N^-1 m^-1 and a COF of 0.45 ± 0.02 were found by tribological performance evaluation. Additionally, in a 3.5 w% NaCl solution, the AlN coating’s corrosion resistance behavior showed a low corrosion rate of 2.95 × 10⁻⁹ mm/s, suggesting the possibility of an extended lifetime for the coated components. These results support the recommendation of the plasma-sprayed AlN coating as a high-performance wear resistance solution. Sudana and Madhu et al. [82], examined the application of cermet coating by plasma spraying to alloy steel for use in aerospace. In this study, alloy steel was successfully coated with three different coatings using atmospheric PS technology: 35% (WC-Co)/65% (Cr₃C₂NiCr), 70% CrAlY + 30% TiO₂, and 70% NiCrAlY + 25% Cr₂O₃ + 5% YSZ. The porosity levels of the coatings were 2.7–3.5%. At 700°C, a ductile erosive mechanism exhibiting significant plastic deformation was demonstrated by all coatings. In high-temperature erosion environments, the 70% NiCrAlY + 30% TiO₂ coating outperformed the others in terms of protection. The coating’s superior performance compared to other coatings can be attributed to its enhanced ductility, uniform microstructure, and reduced porosity. Increased wear resistance was another benefit of adding chromium. The coated samples exhibited greater material loss than the uncoated steel, which is probably because the alumina particles embedded in the coated surface provided protection. The NiCrAlY-Ag-Mo coating using the plasma-sprayed coatings technique, and its tribological characteristics were examined in non-lubricated conditions between 20°C and 800°C. The findings show that, when compared to the pure NiCrAlY coating, adding molybdate improves the composite coating’s microhardness while also lowering porosity. According to the results, the NiCrAlY–Ag–Mo composite coating continuously showed low friction coefficients of roughly 0.3 over the whole temperature range. Furthermore, the wear rates for this coating were consistently on the order of 10⁻⁵ mm³ N^-1 m^-1 at all tested temperatures.