Tribological Performance of Stellite 6/TiN Composite Coatings on Austenitic Stainless Steel

TL;DR

A new composite coating on stainless steel significantly reduces high-temperature wear, improving valve sealing performance.

Contribution

A Stellite 6/TiN composite coating is developed to achieve superior high-temperature wear resistance.

Findings

The composite coating reduces wear volume by 97.2% at 500 °C compared to uncoated steel.

The coating achieves high hardness (~2110 HV) and excellent adhesion (Lc2 ~74 N).

Oxide-glaze formation stabilizes friction, while oxidation-induced spallation affects wear mechanisms.

Abstract

What are the main findings?
A Stellite 6/TiN composite coating is engineered on F347 stainless steel via plasma transferred arc cladding + PVD, enabling a property-gradient architecture for valve sealing applications.The coating system achieves outstanding high-temperature wear protection, reducing wear volume by 97.2% vs. uncoated F347 at 500 °C.

A Stellite 6/TiN composite coating is engineered on F347 stainless steel via plasma transferred arc cladding + PVD, enabling a property-gradient architecture for valve sealing applications.

The coating system achieves outstanding high-temperature wear protection, reducing wear volume by 97.2% vs. uncoated F347 at 500 °C.

What is the implication of the main finding?
The revealed wear mechanisms indicate that oxide-glaze formation stabilizes Stellite 6 friction, whereas oxidation-induced spallation in Stellite 6/TiN can accelerate combined abrasive/oxidative wear at 500 °C, guiding optimization of TiN design and oxidation resistance.

The revealed wear mechanisms indicate that oxide-glaze formation stabilizes Stellite 6 friction, whereas oxidation-induced spallation in Stellite 6/TiN can accelerate combined abrasive/oxidative wear at 500 °C, guiding optimization of TiN design and oxidation resistance.

To enhance the quality and longevity of valve sealing surfaces, this study fabricated the Stellite 6/TiN composite coatings on F347 austenitic stainless steel. The process involved a plasma transferred arc (PTA) of Stellite 6 coating onto the substrate followed by physical vapor deposition (PVD) of a TiN coating onto the Stellite 6 layer. Mechanical testing revealed that the composite coatings achieved high hardness (~2110 HV), excellent adhesion (critical load Lc2 ~74 N), and superior wear resistance versus uncoated steel and single-layer Stellite 6. At 500 °C, the composite reduced wear volume by 97.2% compared to the uncoated substrate. Wear mechanisms for the Stellite 6 and composite coatings at high temperatures were elucidated, highlighting the role of Stellite 6/TiN composite coatings in enhancing tribological performance.

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Full text

1. Introduction

The coal chemical industry, as a critical sector in energy conversion, requires its core equipment to operate under extreme conditions, including high temperatures (up to 482 °C), high pressure, and environments containing solid particles (e.g., coal–oil slurry and catalysts). Valves, as key control components, face severe challenges such as wear and high-temperature softening during long-term service [1,2]. Traditional valve materials like austenitic stainless steel exhibit moderate corrosion resistance but lack sufficient hardness and wear resistance. Pure cobalt-based alloys, while demonstrating excellent wear resistance, suffer from poor toughness and high costs. To address these limitations, composite coatings are developed through surface modification technologies to enhance the comprehensive performance of valves and meet industrial demands.

Stellite 6 is a high-performance cobalt-based alloy widely applied in aerospace, energy, and chemical industries due to its excellent wear resistance, corrosion resistance, and high-temperature strength [3]. Erfanmanesh, M. et al. [4] investigated the effects of laser cladding and laser glazing processes on the microstructure, microhardness, and high-temperature wear resistance of a high-velocity oxy-fuel (HVOF)-sprayed Stellite 6 coating. The results showed that the ST-HVOF coating exhibited a 60% improvement in wear resistance compared to the stainless steel substrate. The ST-Clad and ST-Glazing coatings further enhanced high-temperature wear resistance by 67% and 58%, respectively, relative to the ST-HVOF coating. Researchers have also incorporated ceramic particles into Stellite 6 to enhance its mechanical properties. Yang, J. et al. [5] systematically studied the influence of laser power and WC content on the wear resistance of Stellite 6/WC composite coatings. Their results demonstrated that the Stellite 6/WC composite coating with a cladding power of 550 W and 30 wt.% WC exhibited a wear loss only 1/6th of the substrate material. Similarly, Zhang, H. et al. [6] investigated the impact of WC addition on Stellite 6/WC composite coatings, revealing that the average microhardness of the cladding layer increased proportionally with WC content. The highest microhardness and optimal wear resistance were achieved at a direct WC addition of 12 wt.%.

TiN films are commonly used as hard coatings to enhance the wear resistance of metallic materials due to their high hardness, high melting point and excellent chemical stability [7]. Łępicka, M. et al. [8] examined the PVD of TiN coatings on 316LVM stainless steel and Ti6Al4V alloy. The TiN-coated 316LVM exhibited significantly shallower wear profiles compared to the uncoated steel substrate. However, TiN coatings on Ti6Al4V substrates showed markedly reduced wear resistance, with the average friction distance and total substrate exposure being approximately 9 times lower than those of TiN-coated 316LVM. Gupta, R. et al. [9] prepared oxidation-free titanium nitride (TiN) coatings using conventional plasma spraying, achieving a 15-fold reduction in wear rate compared to the bare Ti-6Al-4V substrate. Ghailane, A. et al. [10] systematically studied the influence of nitrogen flow rate Qv(N_2_) on the nanohardness, adhesion, and structural properties of TiN. Maximum hardness was observed at Qv(N2) = 4 sccm, while Young’s modulus peaked at 5 sccm. The highest H/E ratio, indicative of superior resistance to plastic deformation and wear, occurred at Qv(N_2_) = 3 sccm. For Qv(N2) ≤ 4 sccm, the critical adhesion load (Lc2) of TiN coatings exceeded 30 N, but further increases in nitrogen flow led to a significant decline in adhesion. These findings highlight that Young’s modulus mismatch between the substrate and coating adversely affects adhesion and, consequently, wear resistance. Notably, while hardness has traditionally been regarded as the primary determinant of wear resistance, substantial evidence suggests that elastic modulus also plays a critical role in wear behavior. Leyland and Matthews [11] proposed the H/E ratio (hardness-to-elastic modulus ratio) as a key indicator of coating durability, reflecting the material’s elastic strain tolerance and resilience. This parameter has proven more effective than hardness alone in predicting wear resistance. Additionally, the H^3^/E^2^ ratio has emerged as a potential metric for evaluating fracture resistance in coatings [12].

Duplex coating systems that couple a thick load-bearing layer with a thin PVD hard top layer have been widely studied. A recent HVOF-PVD review summarizes that the sprayed layer is commonly used to provide mechanical support, while the PVD layer offers a dense and hard contact surface, and that such duplex architectures often improve overall protective and tribological performance compared with single-layer coatings [13]. Experimentally, Picas and co-workers [14] evaluated duplex coatings on light alloy substrates, where an HVOF interlayer was combined with PVD top layers such as TiN and TiAlN to enhance tribological behavior. Chen et al. [15] designed an HVOF–PVD duplex coating consisting of an HVOF-sprayed NiCr–Cr_3_C_2_-based layer and an outer PVD nitride film and reported improved wear resistance against a WC counterbody. Beyond thermal spray routes, laser-assisted surface modification followed by PVD TiN has been reported to improve TiN adhesion and wear behavior by creating a more favorable supporting zone beneath the film [16]. Importantly, Co-based HVOF underlayers have also been incorporated into PVD/HVOF duplex systems. Sun et al. [17] compared duplex coatings in which a CrN PVD film was deposited on NiCrBSi and on Stellite 12 HVOF coatings, investigating how the HVOF underlayer influences the tribological behavior of the CrN film. In addition, Li et al. [18] investigated TiN/NiCrBSi and TiN/Stellite 12 duplex coatings from the perspective of corrosion regulation, showing that a PVD TiN film can cover and seal surface defects of HVOF alloy coatings and improve electrochemical performance in chloride environments.

In valve applications, Co-based hardfacing alloys such as Stellite 6 are widely applied to sealing or seat surfaces to resist wear and corrosion, and PTA cladding provides a metallurgically bonded, millimeter-scale supporting layer suitable for service components. Motivated by coal–chemical valve sealing conditions, this study proposes a novel Stellite 6/TiN composite coatings. On one hand, Stellite 6 serves as intermediate coating with hardness and elastic modulus values intermediate between the hard PVD-coated TiN layer and the F347 substrate, which enables a gradual transition in physicochemical properties, thereby facilitating the mitigation of stress concentration. On the other hand, the TiN coating further improves tribological performance and chemical stability. This work evaluates a PTA-deposited Stellite 6 supporting layer on F347 stainless steel coupled with a micrometer-scale TiN film, with emphasis on temperature-dependent friction behavior, volumetric wear quantification, and dominant wear mechanisms.

2. Materials and Methods

2.1. Materials

The substrate material used was ASTM A182 F347 [19]. The substrate was then coated with Stellite 6 by PTA cladding on the surface, followed by TiN deposition by PVD. The chemical compositions of the F347 substrate and the Stellite 6 alloy are listed in Table 1 and Table 2, respectively. Before cladding Stellite 6 onto the F347 substrate, surface impurities such as oxide layers and oil contaminants were removed using an angle grinder and sandpaper to achieve a smooth, metallic-gloss finish. The cladding process was performed to obtain a defect-free Stellite 6 layer, free of visible pores, cracks, or other macroscopic flaws. Subsequently, the cladded surface was ground flat, and a TiN coating was deposited via PVD to produce the final sample. The overall coating architecture and sample preparation procedure are schematically illustrated in Figure 1a.

2.2. Coating Characterization

The microstructure of the Stellite 6 and TiN coatings was characterized using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The sample preparation and characterization workflow are shown in Figure 1b.

To analyze the hardness variation across different regions of the weld overlay, a MH-5 Vickers hardness tester was employed. Figure 1c shows a schematic diagram of the hardness testing process. Tests were conducted under a load of 5 N with a dwell time of 15 s. Measurements were taken starting at 0.4 mm from the fusion line and proceeding perpendicular to the cladding surface at 0.1 mm intervals, totaling 12 points, and the experiment was repeated three times. Macroscopic hardness tests were also performed on the upper surfaces of the Stellite 6 and TiN coatings using a 0.5 N load and 15 s dwell time. Five points were measured on each sample surface, and the results were averaged.

The adhesion strength of the vapor-deposited Stellite 6/TiN coatings were evaluated using an Rtec SMT-5000 nanoindentation scratch tester (Rtec Instruments, Inc., San Jose, CA, USA). The sample preparation and characterization workflow are shown in Figure 1d. The test parameters included a load range of 0.2–100 N, a loading rate of 100 N/min, and a scratch length of 1.5 mm. The experiment was repeated three times. Post-scratch analysis involved SEM observation of scratch morphology and surface topography measurement using a super-depth-of-field microscope. The critical loads Lc1 and Lc2 were defined as follows: Lc1 corresponds to the first sign of adhesive failure (e.g., spalling or cracking), and Lc2 represents the complete exposure of the substrate material [20].

2.3. Wear Test

The tribological properties were evaluated using an Rtec MFT-5000 multi-functional friction and wear tester (Rtec Instruments, Inc., San Jose, CA, USA) in a pin-on-disk configuration. The disk samples included the F347 substrate, Stellite 6 coating on the substrate, and Stellite 6/TiN composite coatings, while the pins were all made of the Stellite 6/TiN composite coatings. Specifically, we state that the pin is a steel substrate coated with the same Stellite 6/TiN composite coatings, and this configuration was chosen to simulate realistic coated–coated contact conditions relevant to valve sealing pairs during alignment. This allows for the evaluation of coating failure after multiple valve cycles. The tests were conducted under an applied load of 5 N in rotational mode at a speed of 100 rpm. All samples underwent dry sliding wear tests at room temperature (RT), 250 °C, and 500 °C, as shown in Figure 1e. The wear trajectory profile was measured using a LEXT OLS5000 3D laser confocal microscope (Olympus Corporation, Hachioji, Tokyo, Japan), and EDS was used to perform elemental qualitative analysis of the wear marks. Briefly, the 3D topography of the wear track was acquired using a laser confocal microscope. The unworn surface adjacent to the wear scar was used to define the reference plane. The wear-loss volume was calculated as the integrated material deficit below the reference plane, whereas the adhesion volume was calculated as the integrated material build-up above the reference plane. To reduce the influence of local heterogeneity, volumetric calculations were performed at three different positions along the wear track and averaged to obtain the reported wear-loss and adhesion volumes. X-ray photoelectron spectroscopy (XPS) technology was used to characterize and analyze samples of Stellite 6/TiN composite coatings after wear at 500 °C, as shown in Figure 1f.

3. Results

3.1. Microstructural Characterization

Figure 2 presents cross-sectional SEM and EDS images of the F347 substrate and Stellite 6 weld overlay. SEM observations revealed a uniform microstructure at the interface between the F347 substrate and Stellite 6 weld overlay, with excellent bonding integrity and a distinct boundary free of cracks. In the central region of the weld overlay, the microstructure predominantly consisted of directionally aligned columnar grains. According to crystal growth theory, face-centered cubic (FCC) crystals preferentially grow along the [001] orientation. As heat dissipation at the bottom of the molten pool was dominated by the substrate, aligned with the [001] growth direction, columnar grains formed at nearly 90° to the interface during solidification. Near the top surface of the weld overlay, the microstructure transitioned to fine equiaxed grains due to multidirectional heat dissipation, rapid cooling rates, and increased undercooling in this region [21]. Figure 2e display EDS line-scanning results across the fusion zone of the Stellite 6 weld overlay, starting from the cladding side. The results indicated that Co and Cr contents gradually decreased from the weld overlay toward the substrate, eventually stabilizing, while Fe content progressively increased. Area-scanning analysis further demonstrated elemental diffusion (Co, Fe, Cr) within the fusion zone during cladding. Specifically, Cr-rich phases segregated in interdendritic regions, whereas Co-rich phases concentrated within dendrite cores [22].

3.2. Mechanical Properties

Figure 3a shows the microhardness profiles of the Stellite 6 weld overlay surface and the Stellite 6/TiN composite coatings surface. The composite coatings significantly enhanced the surface hardness of the Stellite 6 alloy. The Stellite 6 weld overlay exhibited minimal hardness fluctuations, indicating a relatively uniform surface composition, with an average Vickers hardness of 513.84 HV. In contrast, the composite coatings displayed slight hardness variations, suggesting the presence of localized surface defects. Nevertheless, the composite coatings maintained a high average Vickers hardness of 2114.74 HV, representing an approximately fourfold increase over the Stellite 6 coating [23].

To investigate the hardness gradient across the weld overlay cross-section, further hardness measurements were conducted from the substrate to the weld overlay. Figure 3b illustrates the hardness evolution from the F347 substrate to the Stellite 6 weld overlay. The hardness increased progressively from the substrate region (0.4 mm from the fusion line) through the fusion zone to the weld overlay (0.7 mm from the fusion line), eventually stabilizing within the weld overlay. The F347 stainless steel substrate exhibited an average hardness of 240.35 HV0.5, while the Stellite 6 weld overlay achieved an average hardness of 456.83 HV0.5, marking a 1.9-fold improvement over the substrate.

Figure 4 presents the 3D surface topography, frictional behavior, scratch depth, and load profiles of the scratch track on the Stellite 6/TiN coatings surface. Within the scratch distance range of 0–0.48 mm (prior to reaching the critical load Lc1 = 31.53 N), the contact area between the indenter and the coating surface gradually expanded, engaging more surface asperities, which resulted in significant fluctuations in the friction coefficient. As the normal force progressively increased, the friction coefficient stabilized and rose steadily, accompanied by TiN coating fracturing and peripheral spalling at the scratch edges [24]. When the normal force reached the critical load Lc2 = 73.78 N, coating failure became evident through surface morphological analysis, exposing the underlying Stellite 6 cladding layer with a metallic luster [25]. Subsequently, the friction coefficient sharply increased and began to fluctuate, indicating direct interaction between the indenter and the Stellite 6 cladding layer [26].

3.3. Friction Properties

Figure 5a illustrates the evolution of friction coefficients over time for the F347 substrate-Stellite 6/TiN tribological pair at different test temperatures. During the initial wear stage, the friction coefficients fluctuate between 0.1 and 1.2 across all temperatures. After 40 min, the fluctuation of the friction coefficient curves at HT250 °C and HT500 °C became significantly smaller and stabilized around 0.5. Figure 5b compares the friction coefficient evolution of Stellite 6-Stellite 6/TiN tribological pair at varying temperatures. The friction coefficient at RT is higher than that at 250 °C and 500 °C. This trend is associated with temperature-dependent changes in interfacial shear, including thermal softening and the increasing contribution of oxide/third-body layers at elevated temperatures. As the test progressed, the gap between HT250 °C and HT500 °C friction coefficients gradually narrowed, converging to approximately 0.4.

Figure 5c depicts the friction coefficient curves for the Stellite 6/TiN-Stellite 6/TiN tribological pair during a 60 min test at different temperatures. Temperature exerted a pronounced influence on the TiN coating: at RT, the friction coefficient remained stable at about 0.1, whereas at elevated temperatures, it increases significantly with marked fluctuations. This behavior likely stems from thermal softening of the TiN coating and degradation of wear resistance at high temperatures. Notably, the RT friction coefficient remained nearly constant during the initial wear stage. In contrast, the HT250 °C sample exhibited a gradual rise in friction coefficient, while the HT500 °C sample showed a rapid increase [27,28]. This divergence suggests that high-temperature conditions weaken the adhesion of the TiN coating and promote spalling. Furthermore, TiN undergoes oxidation at elevated temperatures, forming brittle titanium oxides. The spalled oxide debris enters the friction interface, exacerbating abrasive wear and driving the observed friction coefficient increase.

The wear tracks of the F347 substrate and Stellite 6 coating at different temperatures were measured via 2D and 3D profilometry, as shown in Figure 6. Based on optical microscopy (OM) observations of the wear morphology, the friction mechanism between the F347 substrate and Stellite 6/TiN coatings were identified as adhesive wear. This is consistent with the worn-track morphology and the pronounced material pile-up features. Analysis of 2D wear profiles revealed that the F347 substrate, with its inherently low hardness at room temperature, experienced further thermal softening at elevated temperatures. As the temperature increased, the wear volume of F347 progressively rose, accompanied by an increase in the number and height of salient peaks, indicating intensified adhesive wear under high-temperature conditions. At HT250 °C and HT500 °C, the friction fluctuations become smaller after ~40 min and the friction coefficient curves stable. This stabilization is attributed to the establishment of a more continuous transfer layer on the soft F347 surface: once such a layer forms, sliding tends to occur within or on top of the third-body layer rather than by repeated metal–metal junction formation, thereby reducing stick–slip severity and smoothing the friction trace.

Figure 7(a1,b1,c1) demonstrate that the wear tracks on Stellite 6 exhibited grooving features, characteristic of abrasive wear as the dominant mechanism. Compared with F347, the Stellite 6 disk shows a higher load-bearing capability and less severe adhesive transfer. However, at high temperatures, material adhesion occurred, suggesting a hybrid wear mechanism combining abrasive and adhesive wear [29]. Overall, the wear resistance of Stellite 6 at elevated temperatures was inferior to its room-temperature performance. Notably, between HT250 °C and HT500 °C, the wear volumes showed minimal difference, with maximum wear depths remaining around −6 μm, indicating that Stellite 6 maintains robust friction resistance in environments below 500 °C [30]. Importantly, the Stellite 6 weld overlay demonstrated significantly enhanced tribological properties compared to the bare substrate. A detailed mechanistic explanation of this improvement will be discussed in Section 4.

Figure 8 displays the OM and 3D profilometry images of the Stellite 6/TiN coatings after 60 min of friction-wear testing at different temperatures. At room temperature, even after 60 min of testing, no significant wear tracks were observed, and the friction coefficient remained stable. In contrast, at HT250 °C, though the coating did not fail completely, clearer grooves and increased track roughness are observed, and the 2D profile exhibits deeper local valleys, consistent with progressive local spalling of TiN. Such spalling introduces hard debris into the interface, promoting third-body abrasion and leading to the gradual rise and enhanced fluctuations of friction. However, at HT500 °C, the TiN coating exhibited severe spalling, with partial regions fully delaminated, exposing the underlying substrate with a metallic luster. The friction mechanism between Stellite 6/TiN and Stellite 6/TiN coating pairs is dominated by abrasive wear, where spalled TiN debris likely intensify interfacial abrasive wear [31,32]. The resulting mixed contact state and abundant hard debris intensify abrasive interactions and cause the markedly higher and more unstable friction at elevated temperature.

4. Discussion

Figure 9a illustrates the wear volumes of the bare substrate, laser-clad Stellite 6 coating, and Stellite 6/TiN composite coating under friction testing at different temperatures. Due to the extremely low wear volume of the Stellite 6/TiN composite coatings at room temperature, which lacked statistical significance because surface roughness could not be fully eliminated, this data point was excluded from the figure. The results demonstrate that the Stellite 6/TiN composite coatings exhibited significantly lower wear volumes compared to the other two materials, particularly under high-temperature conditions. For instance, at 500 °C, the wear volume of the Stellite 6 overlay decreased by 73.1% relative to the bare substrate. The Stellite 6/TiN coating further reduced the wear volume by 89.7% compared with Stellite 6 and by 97.2% compared with the substrate. Figure 9b presents the adhesion volumes (i.e., material accumulation above the substrate surface) for the bare substrate and laser-clad Stellite 6 coating. Adhesion volume arises from the abrasive disk material transfer, spalling of pin material, and oxide formation, reflecting the severity of adhesive wear at the friction interface. The results reveal that the F347 substrate suffered the most severe adhesive wear. This is attributed to its low hardness, where surface asperities undergo plastic deformation under load, and part of them generate debris (wear particles), while the other part is transferred to the pin specimen. This transfer increases the real contact area between friction pairs, facilitating atomic bonding and adhesive interactions. Within the Stellite 6 overlay, the coexistence of Co-rich dendrite cores and Cr-enriched interdendritic regions indicates a “tough matrix + hard constituent” synergy: the Co-rich matrix accommodates contact stresses, while the Cr-enriched interdendritic regions hinder abrasive grooving and plastic plowing. This is consistent with the measured hardness increase from the F347 substrate to the Stellite 6 overlay, which explains the markedly reduced wear-loss volume and the significantly lower adhesion volume of Stellite 6 compared with F347. At RT, HT250 °C, and HT500 °C, the adhesion volumes of the Stellite 6 weld overlay were reduced by 90.3%, 88.2%, and 88.5%, respectively, compared to the bare substrate.

Figure 10 shows the SEM and EDS analysis images of the wear marks of Stellite 6 overlay coating worn at HT500 °C for 60 min. The image shows the existence of two regions of dark and bright contrast. The chemical composition of the bright region (B) is mainly cobalt, chromium and tungsten. These elements correspond to the primary components of Stellite 6, which underwent a limited oxidation process. In contrast, the elemental composition of the dark region (A) indicates a significant presence of oxygen, accompanied by notable enrichments of iron and titanium. This discrepancy is attributed to the accelerated oxidation of iron and titanium in this region at elevated temperatures, leading to the formation of iron and titanium oxides [33]. The titanium element comes from the TiN coating of the pin specimen, which forms an oxide film and partially flakes off during high temperature wear. The dark areas (A) show severe plastic deformation and material transfer from the pin, which is typical evidence of adhesive wear [34]. The mechanism of high temperature wear of the Stellite 6 coating can be explained as follows: at elevated temperatures, oxidation becomes more pronounced because the specimen surface is thermally activated during sliding, promoting oxide formation across the wear track. As a result of the applied compressive force, the oxide layer formed on the brittle coating surface breaks up and part of the coating separates from the wear surface. Another portion of the oxidized and fractured particles reintegrate and form dispersed oxides on the wear surface, which may solidify into a glaze layer or become embedded in the wear surface. The glaze layer, in turn, adheres firmly to the substrate and reduces the actual surface contact area between the pin and the wear surface, which can reduce the coefficient of friction and increase wear resistance [4,35]. This is consistent with the coefficient of friction curve shown in Figure 5b in Section 3.3: the highest coefficient of friction is found at room temperature (up to about 0.7), while the coefficient of friction decreases at high temperatures (about 0.4).

Figure 11 presents SEM micrographs and EDS analysis of wear tracks on the Stellite 6/TiN composite coatings after 60 min of high-temperature wear testing at 500 °C. As shown in Figure 11a, two distinct regions (dark and bright) were observed. The dark region (C) exhibited milder wear characteristics, with EDS elemental analysis revealing minimal oxygen content, suggesting the TiN coating remained relatively intact without significant fragmentation. In contrast, the bright region (D) displayed severe wear accompanied by oxygen enrichment and nitrogen depletion, indicative of TiN oxidation to form TiO_2_ or Ti_2_O_3_ [36,37].

Furthermore, elemental diffusion from the Stellite 6 layer occurred at elevated temperatures. Co demonstrated homogeneous diffusion across the wear track, while iron Fe accumulated preferentially in the severely worn region (D), coinciding with oxygen enrichment. This implies the coexistence of titanium oxides and trace iron oxides in region (D). Notably, mismatched thermal expansion coefficients (TECs) among titanium nitride, iron, and titanium oxides induced localized thermal stresses, leading to partial coating/oxide spallation. This phenomenon intensified with increasing temperature, and this is evident in the accelerated increase in the friction coefficient at 500 °C compared to 250 °C, as shown in Figure 5c.

Table 3 summarizes the dominant wear mechanisms of the three disk conditions from RT to 500 °C and links each mechanism to the most direct experimental evidence in this work. By mapping temperature, disk material, and the corresponding morphology and chemistry signatures, the table highlights the mechanism transition from transfer-dominated wear on F347, to groove-controlled wear with oxide-assisted stabilization on Stellite 6, and finally to spalling-driven, debris-mediated wear on the TiN-topped system at elevated temperature.

Figure 12 shows the XPS image of the Stellite 6/TiN composite coatings after undergoing friction wear at 500 °C. As illustrated in Figure 12a, the XPS broad spectrum reveals the presence of Ti and O on the surface of the Stellite 6/TiN composite coatings after Ar+ ion etching, accompanied by traces of N and C. Figure 12b presents the image obtained through peak fitting of the Ti 2p high-resolution XPS spectrum. This analysis was performed using MultiPak software (Version 9.9.0.8, ULVAC-PHI, Inc., Chigasaki, Japan), with the energy difference between the 2p1/2 and 2p3/2 peaks constrained and the peak intensity ratio set to 0.5 to ascertain the valence state of Ti [38]. In the XPS spectrum, peaks at 458.86 eV and 464.74 eV were identified as TiO_2_, and peaks at 457 eV and 463.21 eV were identified as Ti_2_O_3_. In combination with the high-resolution N 1s spectrum, the main peak with a binding energy of 396.76 eV was identified as the N-Ti bond. Consequently, the peaks at 455.02 eV and 461.16 eV in the Ti 2p spectrum are identified as TiN. Given TiO_2_’s propensity to undergo oxidation into higher-valent oxides, such as Ti_2_O_3_ and TiO_2_, the presence of TiO_2_ in the surface layer of the film was undetectable.

Figure 13 displays the process of high temperature oxidation of Stellite 6/TiN composite coatings. The oxidation process of TiN can be considered as the substitution of N elements by ambient O elements and the simultaneous release of N atoms as follows:

$$[eqn]$$ $$[eqn]$$

During the initial stage of high-temperature wear, the TiN coating begins to slowly oxidize upon contact with oxygen in the air, forming an extremely unstable TiO oxide layer on its surface. This layer quickly transforms into a Ti_2_O_3_ oxide layer. The absence of Ti^2+^ in the Ti 2p high-resolution XPS analysis further corroborates this finding. When subjected to standard loading conditions, the oxide layer exhibits a propensity for fracture and delamination, thereby initiating the oxidation process of the underlying TiN coating. As high-temperature friction wear progresses, the Ti_2_O_3_ oxide layer on the surface of the TiN coating transforms into a TiO_2_ layer. Notably, the thermal expansion coefficient (TEC) mismatch between the surface TiO_2_ layer (8.3 × 10^−6^/K) and the underlying TiN (9.35 × 10^−6^/K) generates interfacial thermal stresses, potentially initiating microcracks or spallation. These defects act as oxygen diffusion channels, accelerating internal oxidation [24,39]. Nitrogen gas released during this process diffuses outward through the TiO_2_ layer. This nitrogen depletion may progressively reduce the nitrogen content in the subsurface TiN, leading to its transformation into low-nitrogen titanium nitride phases (e.g., Ti_2_N) [40]. Consequently, the oxygen-deficient subsurface region with reduced nitrogen content promotes the partial decomposition of original TiN into Ti_2_N under elevated temperatures [41]. A similar nitrogen release and nitride decomposition process has been observed during high-temperature oxidation of arc ion-plated Ti/TiN multilayer coatings [42].

5. Conclusions

In this study, Stellite 6 and Stellite 6/TiN composite coatings were deposited on ASTM A182 F347 austenitic stainless steel. The microstructural characteristics, mechanical properties, and tribological performance of the coatings were systematically evaluated, and their high-temperature oxidation mechanisms were elucidated. The key conclusions are as follows:

1. A PTA-cladded Stellite 6 load-bearing layer combined with a PVD TiN top layer forms a property-gradient coating system on F347 steel, targeting sealing surfaces of valves in coal chemical industry. The architecture provides a clear hardness hierarchy, which underpins the improved wear resistance and reduced material transfer compared with the bare substrate.
2. The main industrial implication is improved sealing-surface integrity. The Stellite 6 layer provides load support and resists deformation. The TiN top layer suppresses metallic junction formation at the sliding interface, thereby mitigating adhesive transfer.
3. Wear was quantified from 3D surface profiles and reported as two components, wear-loss volume and adhesion volume. This is more informative for sealing applications because it captures both removal and transfer-related damage.
4. Future work should focus on thermal-cycling durability, optimization of TiN thickness and residual stress to mitigate spalling, and long-term testing under service-representative media and contact conditions for coal chemical valves.