High-performance WC–Co coatings via HVOF: Mechanical properties of steel surfaces

1 Introduction

Several thermal spray processes are available for coating metallic parts to improve their surface properties [1,2,3]. Among these processes, the high-velocity oxygen fuel (HVOF) method is widely used to provide protective coatings for various components across numerous industrial sectors [3,4,5,6,7,8]. HVOF is used in various industries, including mould manufacturing, where it is applied in creating hot forging punches, aluminium injection moulds, and graphite moulds. In the automotive sector, the technology is utilised for components such as gas turbines and diesel engines. The petroleum and natural gas industries also employ HVOF for gas turbines that operate on coal-burning natural gas and transportation pipelines [9]. Additionally, it finds applications in the aircraft industry, particularly for aircraft landing gears, and in power plant industries for boiler tubing and coal-fired boilers [10]. HVOF is considered an ideal alternative to hard chrome plating due to its environmental benefits and safety, as well as its ability to produce high-quality results, reduce energy consumption, and lower capital costs [11,12].

The schematic image of the HVOF coating system is given in Figure 1, and the cross-sectional diagram of the coating is presented in Figure 1 [13]. In the HVOF method, the combustion fuel and oxygen are directed to the combustion chamber and the spray powder. The chamber has high heat and pressure, allowing gases to pass through the spray and create supersonic flow [8,9]. The dust particles melt during the flame temperature, passing through the combustion chamber and flowing through the spray. In the HVOF process, particles may be completely or partially melted, depending on the flame temperature and the melting point of the coated material.

Figure 1

HVOF system for coating.

The HVOF thermal spraying process is more suitable for obtaining dense structures with low contamination [5,10]. The high impact velocity of particles in this process causes them to be in a plastic state rather than being melted, creating dense coatings [13]. Several factors come into play when forming HVOF thermal spray coatings [9,13,17]. These include the spray system used, raw material properties like particle size distribution and composition, and process parameters like spray type, fuel, oxygen–fuel ratio, spray distance, and powder feed rate [8,9]. The HVOF process involves melting and accelerating dust particles using thermal and kinetic energies to create the desired coatings [17,18,19,20]. The coating is designed to withstand damage and maintain its structural strength and integrity. Its hardness enables it to resist plastic deformation [16,20,21,22]. These parameters influence the properties of the coating and can be adjusted under controlled conditions [11,12,13,14,15,16].

The HVOF thermal spray process has emerged as a superior technique for depositing tungsten carbide–cobalt (WC–Co)-based coatings, particularly in applications demanding exceptional wear resistance. The high particle velocities and relatively low thermal input characteristic of HVOF facilitate the retention of the WC phase and minimise decarburisation, thereby preserving the intrinsic hardness of the carbide. Simultaneously, the cobalt binder phase ensures adequate toughness and load distribution, mitigating crack propagation under cyclic or abrasive stress [23,24,25].

The properties of HVOF-sprayed WC–Co coatings are significantly influenced by the amount and distribution of WC particles and the amount of the Co-binder phase [14,15]. Many surface coating studies have been carried out using the HVOF method [20,21,22,23,24,25,26]. Coating powders and their contents have affected the surface properties differently.

Zheng et al., Leyland and Matthews, and Abbasi et al. have demonstrated that HVOF-sprayed WC–Co coatings exhibit significantly lower porosity and higher adhesive strength compared to those produced by conventional plasma spray methods, resulting in enhanced tribological performance. The synergistic interaction between the hard WC particles and the ductile Co matrix contributes to a refined microstructure capable of resisting both adhesive and abrasive wear mechanisms, making these coatings ideal for high-performance engineering components subjected to severe contact conditions [18,24,25]. Other authors, whose contents are generally similar to our study, have reached favourable results in terms of mechanical properties due to the new hard phases formed on the surface.

For example, Picas et al. [19] examined the potential of three new types of coatings, tungsten carbide–nickel–molybdenum–chromium–iron–cobalt (WC–NiMoCrFeCo), WC–CrC–Ni, and tungsten carbide–iron–chromium–aluminium (WC–FeCrAl), as alternatives to the conventional WC–CoCr coating. They found that the WC–CoCr coating has a slightly higher hardness than the WC–NiMoCrFeCo and WC–FeCrAl coatings. The researchers also discovered that the WC–CrC–Ni coating was significantly the softest and least wear-resistant due to a high percentage of lower-hardness CrC particles. Nahvi and colleagues studied the microstructural and mechanical properties of HVOF-sprayed coatings made of WC–FeCrAl, WC–NiMoCrFeCo, and the traditional WC–Co. They found that while the WC–FeCrAl coating had the highest hardness, the conventional WC–Co coating had the highest fracture toughness amongst the tested coatings [10]. Bolelli and colleagues compared the WC–FeCrAl coating to the WC–CoCr coating. They found that while the sliding wear and ball impact resistance were comparable, the WC–FeCrAl coating showed inferior resistance to dry particle abrasion due to brittleness induced by oxide inclusions [17]. Mahade et al. conducted a study on the effect of laser remelting on a plasma-sprayed coating layer of WC with a 12% Co content. Numerous studies have highlighted the significance of process parameters in achieving the desired outcomes [13,14,16]. Abbasi et al. used HVOF coating (WC–75Cr₃C₂–25NiCr) on stainless steel. After the experimental studies, they also saw that the steel surface properties improved. Numerous studies have shown that the coating process enhances workpiece surface mechanical and tribological properties [24,25,26]. Finding the optimal composition ratio of the hard phase and the best spraying process is essential for enhancing the wear resistance of WC-based coatings. According to various studies, the impact of carbides on the microstructure and performance of these coatings has been thoroughly examined [15,16,17,18].

The aim of this study is to see the effect of pure and mixed or content differences on surface mechanical properties. Since stainless steel is used extensively in the sector, improving the surface properties of this steel will be important and valuable for many sectors. It will be an important output of this study to improve the mechanical properties of this intensively used steel group and to find the ideal content and ideal parameters in this regard. The coating layer’s hardness and microstructure images were examined in detail. The thickness of the coated layer was measured, and the effect of process parameters on hardness and mechanical properties was investigated. The CSM apparatus performed the linear wear test, while surface roughness measurements were done using a 3D (MAHR) surface profilometer. The wear test measured the coated surface’s wear volume, wear rate (WR), and friction coefficient. The surface changes after the wear test were interpreted based on the scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS) images obtained.

2 Experimental procedures

3 Results and discussion

The SEM micrographs of the cross-sectional surfaces of samples (z1 and z7) are shown in Figure 2a and b, respectively. Some porosity was observed on the coating layer. There are two possible reasons for the formation of these porosities. First, the roughness on the metal surface during the spraying of the powder may have obstructed the flow, causing the sudden solidification of the powder [9,10,11,12,13,14]. Second, insufficient wetting on the metal surface may have caused the formation of oxides, oils, and stains on the surface. The grain size of powders can significantly affect the quality of coating formation under the same spraying conditions. Coatings formed with larger grain powders tend to have higher porosity compared to those formed with smaller grain powders [15,16,19]. This is because larger grains create more voids and gaps during the spraying process, leading to increased porosity in the final coating. If coatings made with large-grain powders exhibit higher porosity, then larger-grain powders result in more porous coatings [19,20]. Smaller particles generally establish better interfacial contact with the substrate, which enhances metallurgical or mechanical bonding and improves adhesion. In contrast, larger particles may not fully melt or deform upon impact, resulting in higher porosity, weaker inter-particle cohesion, and less effective bonding with the substrate. Research has demonstrated that optimising the particle size distribution can significantly enhance coating performance by balancing the deposition efficiency, thermal behaviour, and bonding characteristics [21,22].

Figure 2

SEM micrographs of the cross-section of (a) sample z1 and (b) sample z7.

Figure 3

EDS analyses of (a) sample z3 (wt%) and (b) sample z9 (wt%).

The varied process temperature range resulted in minor pores on the substrate surface. The type and shape of the powder material, the coating production method, and the production parameters determine the formation of porosity in the coating.

The structure shows porosity, which is ideal for wear applications [14,15,16]. Hajare and their colleagues discovered that temperature differences could exist between the substrate and coating layers during a particular process. This disparity might be due to quick and unregulated cooling between the two layers, leading to a particle structure divergence. This situation can cause a delay in bonding [20,21]. The even distribution of the material on the powder structure results in more substantial and more consistent interactions between the powders and substrate, leading to a more durable bond [11,12]. This is particularly desirable in many industrial and technical applications. However, some microcracks were observed in specific structures in SEM images of the samples. These are very fine cracks of capillary size in the coating structure, which may have resulted from thermal differences caused by the high process temperature [12,13].

In Figure 3, EDS analyses of sample z3 and sample z9 are performed. The difference between P1 and P2 powder contents is seen from EDS analyses. Upon examining the composition by EDS analyses, it is evident that W is the dominant element in the first region. The second region primarily comprises Co and Ni, while the third region comprises Cr and Al. The tungsten phase is relatively abundant in the first region. In the dark grey area, we can consider oxidation, as discussed in the work of Hajare et al. [20,21,22,25]. Capillary microcracks observed in HVOF coatings typically originate from a combination of three primary mechanisms: thermal mismatch, shrinkage-induced stress, and phase transformation. The difference in thermal expansion coefficients between the coating and the substrate can lead to residual tensile stresses during cooling, which may exceed the material’s fracture strength and initiate microcracks [26]. Additionally, rapid solidification and particle consolidation during spraying can cause volumetric shrinkage; when constrained by the substrate, this shrinkage generates tensile stress and promotes crack formation [27]. Furthermore, certain coating materials may undergo phase transformations during or after spraying, resulting in volume changes and internal stresses that contribute to crack development. A comprehensive understanding of these mechanisms is essential for optimising process parameters and improving coating integrity [19–21]. In a study conducted by Hajare and colleagues, it was noted that dust particles cause less particle melting and dissolution of carbides due to their high velocity [19,21]. In Figure 2, the dark grey area is densely located in the structure, which suggests oxidation, as demonstrated in the work of Hajare et al. [20]. As such, it can be assumed that the metal binder phase melted entirely, while the WC particles remained solid during spraying [20,22,24,25]. In a study conducted by Noblea and his colleagues, it was found that perfect bonding between Co and WC results in a denser structure in the decking of WC–Co compared to WC–10Co–4Cr. This structure has grey and dark spots that indicate the presence of Co and unresolved Cr phases, while the bright spots represent [21,30].

The coating thickness is one of the most significant factors affecting the mechanical and wear properties [31]. The thickness of coatings on the samples was measured by SEM. To begin with, the changes in coating thickness were analysed while keeping the number of passes constant and altering the spray distance, as shown in Figure 4. The coating thickness changes were observed when the number of passes was 10–15–20, and the spray distance was increased from 200 to 300 mm. It is important to note that coating thickness may not always progress symmetrically and can deviate due to spray mouth angle errors during the coating process, depending on the distance [12,24]. In sample z5, the highest coating thickness of 426 µm was achieved at a spray distance of 200 mm with several 20 passes. The coating thickness for the z1 sample was 392 µm, whereas the z2 sample’s coating thickness was 290 µm. The results indicate that experimental parameters like distance and number of passes significantly impact the coating thickness and structure [23,24,31]. In thermal spray coating processes, the spray-to-substrate distance, often referred to as standoff distance, is a critical parameter that significantly influences the hardness of the resulting coating. This distance directly affects the thermal and kinetic energies of the particles upon impact. In the material and spray technique, particles retain sufficient velocity and temperature to flatten effectively on the substrate, forming a dense and well-adhered microstructure with enhanced hardness. If the spray is positioned too close, particles may overheat or oxidise, potentially causing microstructural defects and reducing mechanical integrity. Conversely, excessive spray distance can lead to premature cooling and reduced particle velocity, resulting in poor bonding, increased porosity, and lower hardness values. Therefore, precise control of spray distance is essential to optimise coating properties, as supported by experimental studies, showing peak hardness values at specific spray distances [20,21].

Figure 4

Variation of coating thickness according to parameters of HVOF-coated samples.

Further analysis showed that the most efficient coating thickness was achieved with a spray distance of 200 mm. Reducing the distance causes a greater amount of dust to come into contact with the surface, which in turn increases the coating thickness. This finding is supported by previous research [17,19,21]. Conversely, increasing the distance can lead to melting issues with the powders, which may affect the homogeneity of the coating before it reaches the surface [11,18,22,25]. Regarding coating application, increasing the number of passes results in a more uniform thickness. As the number of passes increases, more coating powder reaches the surface, leading to a thicker and more consistent coating structure [18,19,24].

The two powders, P1 and P2, have different compositions. P2 contains a higher proportion of Cr and Co phases, making Co an effective binder for coating powders [20,21]. In contrast, P1 has a higher W phase content, which offers high hardness and strong bonding to the surface [15,23,30]. The differences in the number and ratio of phases in P2 may result in thinner coating thicknesses. Additionally, the density of P1 is higher than that of P2, which may lead to higher coating thicknesses with P1. When the size of the powder used in the coating is small, its speed increases. When the powder size changes, the energy on the surface changes. fine powder size reduces the energy. The coatings (P2) deposited parameters were porous and rougher compared to P1, owing to excessive oxidation. This showed that fine-cut size powder with reduced energy parameters was smoother than the sprayed surface finish [30,31,34].

The XRD analysis graphs of samples z1–z6 and samples z7–z12 can be found in Figure 5a and b, respectively. During the XRD analysis, it was discovered that the coated samples with P1 powder contained Cr, W₂C, Cr₂Ni₃, and WC crystalline phases. On the other hand, the samples coated with P2 contained WC, W, and W₂C phases on the surface. These phases are crucial in enhancing resistance to wear and corrosion [24,26,28,29]. The analysis also revealed the presence of crystallisation of amorphous and nanostructured phases [16,18]. It was observed that the WC peak intensities increased at 2θ angles of 34° and 47°, with an increase in spray distance. Additionally, the coating surface had W₂C phases formed. According to Mahade and colleagues’ study, a decrease in WC phases saw the emergence of W₂C phases [13]. They also found the same phases in their studies. During thermal spray processes, it has been observed that Picas et al. found that the WC particles can undergo decarburisation and oxidation, which leads to the formation of oxycarbides, metallic tungsten (W), and tungsten carbide (W₂C). According to their research, the addition of chromium (WC–CoCr) can modify the decomposition of the WC grains in a way that prevents decarburisation and enhances the binding of the coating’s matrix, thereby improving its mechanical properties [19]. Zheng and his colleagues have claimed that during the HVOF process, powders are exposed to oxygen at high temperatures, causing chemical changes. The W and C phases react to form a brittle structure, like the W₂C phase. The source of the W₂C phase, which was also observed in a study conducted by Wei and his team [15], is a type of decarburisation of WC that occurs during the HVOF process. These brittle structures (W₂C) tend to form weak bonds and can reduce the erosion wear resistance of the coating. The presence of cracks in the structure is mainly associated with the size of the powder, the gas flow rate, and the spray temperature. The WC phases, on the other hand, have improved the structure against erosive wear and have advanced mechanical properties in the coating process [15,16,17,18,29].

Figure 5

XRD analysis of samples after HVOF process samples (a) z1–z6 and (b) z7–z12.

Figure 6 illustrates the change in hardness with parameters. The measurements were obtained from three different regions of z1–z12 samples after coating. The hardness values varied with the difference in powder contents. Specifically, the coatings coated with P1 powder exhibited higher hardness values. This can be attributed to the higher density of P1 powder and the presence of hard phases (WC) in its content, as indicated in Somasundaram et al., Picas et al., and Hajare and Gogte [16,19,20]. Changes in process parameters have been found to affect the hardness values of coatings [29,30]. Specifically, an increase in the number of passes in the coating process led to an increase in the amount of hard powders on the surface, increasing the coating hardness [20,23]. With each additional pass, the coating thickness increases, leading to a denser and harder coating. This is because multiple passes ensure that more material is deposited, filling in any voids and creating a more uniform and compact layer. Also, more passes mean more particles impacting the surface, which can lead to work hardening. This process increases the hardness of the coating as the particles deform the surface and create a denser structure. Additional passes help in reducing porosity by filling in gaps and voids in the coating. A lower porosity level generally correlates with higher hardness, as the material is more compact and less prone to deformation [22,30,33].

Figure 6

Hardness values of coated samples for different parameters.

On the other hand, changes in spray distance were also found to impact hardness values. An increase in spray distance decreased hardness values because it reduced the density of coating powders [20,25,26]. The highest average hardness values were measured for samples z5 and z11, which were 960 and 900 HV, respectively. These values were obtained at a spray distance of 200 mm and after 20 passes. For example, a decrease in the spray distance from 300 to 200 m in the z6 sample increased the hardness from 820 HV to 960 HV [25]. Some researchers claim that the difference is due to issues with the focus between the sample and the spray [5,15]. It has been observed that changes in the chemical composition can affect the hardness values [17,18,19,20,25]. The hardness value of a coating layer increases due to the formation of new, complex and brittle phases. However, the presence of oxidised and carbide phases on the surface improves the mechanical properties of the coating layer [10,12]. According to the literature, a homogeneous and dense coating that is well bonded to the substrate contributes to high hardness. The WC hard phase has H and E values of approximately 24 and 680 GPa, respectively. Additionally, the coating layer has fewer internal defects and lower porosity values, which are crucial in achieving high hardness values [29,30,31].

Figure 7a and b illustrates the results of the samples’ friction coefficient and WR calculations. The key finding from these results is that varying surface hardness values contribute to differences in surface wear resistance and friction coefficients. The friction coefficient values range from 0.3 to 0.59. The lowest coefficient of friction was observed in sample Z5, while the highest values were found in samples Z2 (P1 powder) and Z8 (P2 powder). In the wear tests, the increase in load had an impact on the friction coefficient values. As the WR increased, the coefficient also increased, reaching a maximum value of 0.57. Initially, WC coating showed an increasing trend followed by a steady-state rate. While this coating increases initially, a decreasing trend occurs, and subsequently it becomes stable. An increment in the friction coefficient can be associated with higher shear stresses at the initial stage due to a smaller contact area between the two mating surfaces. As the sliding time increases, the actual contact area increases, resulting in a stable coefficient of friction [21,26,33]. It is not always the case that increases in the wear load and WR will lead to higher friction coefficient values. The opposite can happen, where the worn products get plastered on the surface, causing a decrease in wear values [27,34].

Previous studies have indicated that oxide wear products often adhere to the surface during testing, which lowers the friction coefficient values [25,26,27,28,29,30,31,32,33,34]. While experimental conditions may show a slight increase in the friction coefficient with higher loads and speeds, this increase is generally not significant. In some cases, research findings indicate a decreasing trend instead [15,19,21,29]. Recent studies have demonstrated that the rate at which materials wear down largely depends on the roughness of their surfaces [27,30,31]. As wear increases, surface roughness typically decreases, which in turn affects the coefficient of friction. When conducting wear tests, it is observed that the coefficient of friction tends to rise with an increase in load. However, this trend does not always hold at higher wear speeds. At elevated speeds, there is often reduced contact with the surface, which can lead to a slight decrease or stabilisation of the coefficient of friction. Additionally, the adhesion of wear products to the surface can complicate the establishment of contact [35,36].

In terms of the influence of load and speed on wear, it has been found that load has a more significant effect on the WR than speed [24,25,26]. In our study, while the coefficient of friction increased with higher loads, it either remained the same or decreased with increasing speed.

Hardness (H) is the primary mechanical property that determines the wear resistance of materials, while the modulus of elasticity (E) also plays a role in this resistance. A high H/E ratio indicates that a material has good wear resistance. An appropriate combination of hardness and modulus can help minimise interfacial stress discontinuities in a layer/substrate system. This allows the layer to flex in unison with the substrate when applying a normal load. A greater H/E ratio increases the potential for elastic deformation, which can enhance the tribological performance [30,34].

After examining the WR values of several samples, it was found that the z5 and z11 samples had the lowest WR values. This was because the z5 sample was coated under ideal coating parameters. The tests were conducted under specific conditions involving a sliding distance of 400 m, a 2 N load, and a velocity of 0.1 m·s⁻¹. The WR increased when the test conditions were modified [15,19]. Additionally, there was a noticeable change in the wear resistance with increased speed. The z2 (P1) and z11 (P2) samples had the highest WR when tested at a speed of 0.2 m·s⁻¹ under a 3N. After examining the WR values of several samples, it was determined that the z5 and z11 samples exhibited the lowest WRs. The z5 sample achieved this due to being coated under optimal coating conditions. The tests were conducted under controlled conditions, which included a specific sliding distance of 400 m. Under the applied load, samples z5 (P1) and z11 (P2) exhibited the lowest wear rates. After examining the WR values of several samples, it was found that the z5 and z11 samples exhibited the lowest WRs. This was attributed to the z5 sample being coated under ideal coating parameters. The tests were conducted under specific conditions, which included a sliding distance of 400 m, a load of 2 N, and a velocity of 0.1 m·s⁻¹. When the test conditions were modified, the WR increased [15,19]. Additionally, a noticeable change in wear resistance was observed with increased speed. The z2 (P1) and z11 (P2) samples showed the highest WRs when tested at a speed of 0.2 m·s⁻¹ under a 3 N load, whereas the z5 (P1) and z11 (P2) samples maintained the lowest WRs.

During the wear tests, it was observed that an increase in speed resulted in greater surface deformation, which in turn led to elevated wear values. The hard and brittle nature of the surfaces also contributed slightly to the higher wear values. Previous studies have demonstrated that increasing the load results in a higher WR [29,31]. As the load was increased during the experiment, particles from the oxide coating broke away from the surface and became embedded, causing a slight decrease in wear values [20,27]. However, the oxide particles embedded on the surface and the increasing wear products tended to reduce wear over time. This phenomenon is illustrated in Figure 7 and is supported by previous studies [7,8,12,18,22,35].

Figure 7

(a) Results of the friction coefficient-sliding distance of samples after the wear test (distance, rate, and load) and (b) WR of coated samples.

The inclusion of hard phases like WC in coating powders enhances performance. Furthermore, phases high in Cr and Co have been shown to improve the wear resistance. For WC–Co-based powders, tribo-films made up of tungsten trioxide (WO₃) were noted on the surface. The oxidised tribo-film creates a layer over the wear tracks, effectively filling any delaminated areas and pores within the coating. The presence of oxygen suggests that oxidation could play a role in forming wear debris. This tribo-film helps reduce friction and wear by offering low shear strength during sliding movements [36]. Compounds such as WO₃ exhibit excellent lubricating properties. Furthermore, the wear products that detach from the surface often appear as oxides, which are thought to enhance the wear resistance. The single-phase oxides such as Cr₂O₃, WO₃, and CoO, along with binary-phase oxides like CoWO₄, Cr₂WO₆, CrWO₄, and NiWO₄, are frequently observed in WC-based coatings [37,38]. The single-phase oxides, such as Cr₂O₃, WO₃, and CoO, along with binary-phase oxides like CoWO₄, Cr₂WO₆, CrWO₄, and NiWO₄, are commonly found in WC-based coatings. With reduced friction, a smaller force is applied to the surface of the sample, resulting in less energy being released as heat. Excessive heating due to friction can raise component surface temperatures to dangerous levels, resulting in softening and deformation. When the load or speed is increased during a wear experiment, the temperature rises due to the sliding motion. This increase in temperature causes the material in the matrix to soften. As the temperature continues to rise, an oxide layer forms on the surface, composed of various metals such as W, Co, and Cr [39]. This oxide layer smooths the surface and reduces the friction coefficient, as noted in studies of Wei et al., Picas et al., Wang et al., Russel et al., and Song et al. [15,19,23,27,30]. When we look at the wear results of the z9 and z11 samples, there is not a slight difference. z11 WR is much lower than that of z9. According to the Archard equation (V = {K W L}/{H}), the coefficient of friction is not the only factor influencing the amount of wear. Hardness and the wear products that detach from the surface also play significant roles in determining the wear quantity. Consequently, excessive fragmentation of coating powders on the surface during wear can sometimes lead to discrepancies in the wear amounts predicted by formulas [37,40].

During wear tests, several observations were made regarding the wear trace, as illustrated in Figure 8a–f. It was noted that tiny shivers and transverse microcracks appeared on the wear surface. Additionally, some plastic deformation was observed on the worn area. The tests also resulted in the formation of long, thin grooves, and images revealed the presence of generated wear debris (Figure 8a–c). Furthermore, it was noted that a small amount of adhesion wear occurred due to the rapid removal of carbide particles, as depicted in Figure 8e and f. The worn surface of the coated steel exhibits flakes, oxides, and minor scars in certain areas, which can be attributed to adhesion and tribo-oxidation interactions. Studies have shown that hard surfaces treated with coatings can develop surface cracks and ruptures due to their brittle nature [38,39,40]. Even a few cracks can lead to delamination layers, resulting in the creation of fine wear debris. In contrast, relatively coarse and coaxial debris can also be observed on the surface (Figure 8c and d). [10,12,29]. Hard phases within the coating layer are essential for reducing wear, as highlighted in Tribhuwan and Mishra, Wang et al., and Leyland and Mathews [22,23,24]. Worn surface images (Figure 8e and f) reveal flattened asperities and areas that have undergone plastic deformation. Additionally, ploughing and abrasion features are evident in Figure 8g. This can be explained by the occurrence of debris particles that become trapped between the pin and the groove. These particles can lead to increased volume loss due to their rolling and rubbing actions [29,30,31,32]. Hanief et al. observed that carbide oxidation occurred during sliding tests [35,36,37,38]. This oxidation may lead to the formation of a thin protective film on the coating. Furthermore, increased heating at the interface promotes oxidation at higher loads. Consequently, developing a protective oxide layer under high loads can result in a reduced friction coefficient [36,39,40].

Figure 8

SEM images of the worn surface of samples (a) z1, (b) z3, (c) z4, (d) z7, (e) z8, (f) z9, and (g) z10.

Tribo-oxidation mechanisms contribute to surface protection during sliding by forming oxide layers that act as solid lubricants, thermal barriers, and wear-resistant films. Under sliding contact, frictional heating promotes the formation of oxides such as Cr₂O₃, depending on the material system. These oxides can reduce direct metal-to-metal contact, thereby lowering friction and WRs [41,42]. Additionally, some tribo-oxidised surfaces exhibit self-healing behaviour, where the oxide layer fills microcracks or defects, maintaining surface integrity during prolonged sliding. The formation of quasi-viscous transfer layers and partially ductile subsurface oxides further enhances the material’s ability to adapt to harsh tribological conditions, offering both mechanical protection and chemical stability [42,43].

We can see the surface deformation patterns after examining the 3D profilometer images of the wear marks shown in Figure 9a and b. The x-axis typically denotes the length, while the y-axis denotes the width. Together, they form a grid that maps the surface area. The z-axis represents the vertical dimension, indicating the height or depth of the surface features. The z-axis measures the deviations from a reference plane, showing the peaks and valleys of the surface.

Figure 9

3D profilometer of the worn surface of (a) sample z1 (2 N load in wear test) and (b) sample z7.

It was noted that the depth of the wear marks increased with higher intensity [34,35,36,41,42,43]. The wear heights of the coatings, observed in the x-, y-, and z-planes, varied, with the WR being greatest in the z-direction. In contrast, wear was less pronounced in other directions, particularly in the y direction, where the least amount of wear occurred. The wear depths on the affected surfaces are illustrated using different colours, accompanied by a numeric scale bar on the side [32,34,37,38,43,44].

Upon examining the coating layer, it was found that there were no significant losses, and excessive wear pits could not form. Notably, the scarcity of deep red areas in the top picture is striking. The relationship between wear depths and the amount of wear resulted in significant findings regarding the creation of wear-resistant coatings. It has been observed that coatings made with P1 powder exhibit better wear resistance than those made with P2 powder. The wear on the surface of the sample made with P2 powder was more severe, and the surface losses were more significant (Figure 9b). Different wear patterns were observed depending on the direction of the wear. Overall, it has been noted that there was no severe wear under the experimental conditions. Additionally, the structure of the powder and the differences in the hard phases play a crucial role in determining the characteristics of wear observed on the surface [36,45].

4 Conclusions

The study aimed to enhance the surface properties of stainless steel materials, which are extensively utilised in various industries, yielding valuable results. This improvement will help extend the lifespan of stainless steel surfaces across multiple sectors. Achieving the right powder content and coating parameters will significantly benefit the study’s users. Two powder mixtures were used as coating powder, one containing 50 wt% Sulzer Metco 5810 (WC and 12 wt% Co powder) and 50 wt% Amdry 960 (Ni, Cr, and Al), and the other containing 50 wt% Woka 3653 and 50 wt% Amdry 960 (P1 and P2). Coatings made with 50 wt% Sulzer Metco 5810 (WC and 12 wt% Co powder) and 50 wt% Amdry 960 (P1) powder were denser and less porous compared to those created with 50 wt% Woka 3653 and 50 wt% Amdry 960 (P2) powders. It is believed that the powders’ particle size distributions and densities influence the porosity, affecting the coating layer’s morphology. The P1 powders achieved a coating thickness of 416 μm when using a spray distance of 200 mm and applying 20 passes. As the number of passes increases and the spray distance decreases, more coating powder reaches the surface, thereby altering both the thickness and structure of the coating.

The coated surface revealed the presence of hard phases, including WC, W, W₂C, Cr₂Ni₃, Co₃C, and Cr. These hard phases significantly contributed to the mechanical properties of the coating. The z5 sample, created using P1 powder at a spray distance of 200 mm over 20 passes, recorded the highest average hardness value of 960 HV. In contrast, the coated layer produced by HVOF exhibited the lowest hardness value at 631 HV. When comparing the 50 wt% Sulzer Metco 5810 (WC and 12 wt% Co powder) and 50 wt% Amdry 960 (Ni, Cr, and Al) and the other containing 50 wt% Woka 3653 and 50 wt% Amdry 960 (P1 and P2) powders, it was noted that coatings made with P1 powder generally exhibited slightly higher hardness values.

Overall, both coatings enhanced the wear resistance of the surfaces. Variations in parameters influenced wear and hardness values. As the spray distance decreased and the number of passes increased, there were positive improvements in surface mechanical values. Notably, the wear resistance of coatings using P1 powder appears to be somewhat better than that of those made with P2. The overall amount of wear increased with the WR and load for both powders. Additionally, the friction coefficient values displayed a trend that was proportional to the amount of wear. The WRs and friction coefficient values decreased due to the oxidation that occurs on the surface during wear and the adhesion of wear products to the surface.