Effect of PVD Nitride Coating Deposition on the High-Temperature Pin–Disc Friction Properties Between WC/Co Carbide and Ti₂AlNb Alloy

Abstract

Suitable nitride coating deposition could improve the wear resistance of WC/Co carbide tools when cutting Ti₂AlNb typical difficult-to-machine alloy. However, there is no clear conclusion on which nitride series coating is suitable for improving the friction characteristics between WC/Co carbide and Ti₂AlNb alloy. In this research, the CrAlN, CrAlN/(CrAlB)N/CrAlN, and TiAlN/ZrN coatings were deposited on WC/Co carbide with the only variable of coating type, which were utilized to conduct the high-temperature pin disc experiments with Ti₂AlNb alloy at 600 °C, respectively. The high-temperature friction characteristics were analyzed by the friction coefficient with time, the alloy wear rate, the surface morphology, and element distribution after wear. The results showed that the three types of coating all improved the high temperature friction and wear characteristics of WC/Co carbide. The Ti₂AlNb alloy also exhibited good surface morphology after wear with TiAlN/ZrN-coated carbide. It is speculated that TiAlN/ZrN coating was the suitable coating deposition on WC/Co carbide tools to improve cutting performance of Ti₂AlNb alloy.

1. Introduction

As a lightweight high-temperature structural material, Ti₂AlNb alloy has seen widespread application in recent years for hot-end components in aeroengines, aerospace vehicle skins, and load-bearing structures. It offers significant advantages including high specific strength, excellent creep resistance, and a low thermal expansion coefficient [1,2,3]. The Ti₂AlNb is taken as the difficult-to-machine alloy due to its superior properties [4]. The WC/Co carbide tools have been widely utilized to cut the Ti₂AlNb alloy, but the rapid wear rate limited the application of WC/Co carbide [5]. The nitride coatings are widely deposited on the carbide cutting tools because of their excellent oxidation resistance and thermal stability [6]. In particular, most of the nitride coatings used on WC/Co carbide tools are deposited by physical vapour deposition (PVD) techniques, owing to their ability to produce dense, adherent films with excellent high-temperature stability [7,8].

The researchers have carried out in-depth research on the performance optimization of nitride coatings [9,10,11]. Numerous studies have demonstrated that PVD coatings significantly enhance the wear resistance and cutting performance of WC/Co carbide tools, especially under elevated-temperature or severe machining conditions [12,13,14]. The CrAlN coating is a nitride coating with excellent performance, which is widely used in the surface strengthening of carbide cutting tools, moulds, and high-temperature wear-resistant parts. The coating is a solid solution-enhanced coating formed by introducing Al element on the basis of CrN, with high hardness, excellent wear resistance, and good oxidation resistance [15]. Lin et al. [16] show that with the increase in the Al content in CrAlN coating, its hardness and elastic modulus increase significantly, and at the same time, its wear rate is lower than that of CrN coating. Bouamerene et al. [17] deposited CrAlN coatings using a DC reactive magnetron sputtering process. It was found that the AlN phase can promote the preferential orientation growth of CrN at high deposition temperatures. The deposition temperature around 300 °C could effectively adjust the microstructure of the CrAlN coatings and improve their mechanical and tribological properties. Yu et al. [18] studied the effect of matrix negative bias on the impact performance of CrAlN coating. The results showed that with the increase in matrix bias, the grain size of the coating gradually decreased and then showed higher hardness and better impact cycle resistance.

The introduction of the B element could effectively inhibit the thermal decomposition process of the CrAlN coating, thus significantly improving its antioxidant properties [19]. Chen et al. [20] systematically studied the chemical composition, phase structure, surface and cross-section microstructure, mechanical properties, and tribological behaviour of the deposited CrAlBN coating. The results revealed the strengthening mechanism of the B element on coating properties. Cai et al. [21] discussed the evolution law of different B contents on the microstructure of the CrAlBN coating. The effects of this evolution on the hardness and toughness of coating were revealed. The tribological behaviour of coating and the formation mechanism of friction oxides were analyzed in detail. Here, “friction oxides” refer to the oxide layers that form dynamically on the contact interface during sliding at elevated temperatures, typically consisting of low-shear compounds such as Al₂O₃, Cr₂O₃, ZrO₂, or TiO₂. These tribo-oxidative films have been reported to reduce interfacial shear and stabilize the friction coefficient under high-temperature conditions [22,23].

Because of its low friction coefficient, high thermal reflectance, and excellent resistance to adhesive wear, ZrN coating has been widely concerned in the field of high-temperature protection. However, compared to TiAlN materials, ZrN has a relatively low hardness [24], which limits its application in some harsh conditions. In order to make up for this defect, ZrN and TiAlN composite coatings have become a research hotspot in recent years and have shown broad application prospects in the coating field [25,26,27]. In addition, compared with the traditional TiAlN/ZrN double-layer structure, nano-multilayer coatings have attracted wide attention due to their denser microstructure, excellent mechanical properties, and superior thermal stability, which have become an important development direction for the industrial application of high-performance coatings [28,29]. Overall, previous studies have shown that Cr-based nitride coatings tend to form lubricious Cr₂O₃ films at elevated temperatures, while the incorporation of B can further stabilize these oxide layers. Zr-containing coatings, in contrast, often generate low-shear ZrO₂/Al₂O₃ tribo-oxides during sliding, contributing to additional friction reduction. Reported high-temperature friction coefficients for uncoated WC/Co commonly exceed ~0.6, whereas CrAlN-based and Zr-containing coatings typically reduce the value to approximately 0.45–0.55. These findings highlight the diverse tribological pathways through which different nitride coatings may contribute to friction reduction under high-temperature contact conditions.

However, there is no clear conclusion on which nitride series coating is suitable for improving the friction characteristics between WC/Co carbide and Ti₂AlNb alloy. In this research, three representative PVD nitride coatings—CrAlN, CrAlN/(CrAlB)N/CrAlN, and TiAlN/ZrN—were deposited on WC/Co carbide substrates, with the coating type being the only controlled variable. These coated carbide pins were then used to perform pin–disc friction tests against Ti₂AlNb alloy at 25 °C and 600 °C to comparatively evaluate their frictional and wear behaviours. The environment-temperature and high-temperature friction characteristics were analyzed by the friction coefficient with time, the alloy wear rate, the surface morphology, and element distribution after wear. The suitable coating types could be determined according to wear mechanism and experimental results between the various coated WC/Co carbide and Ti₂AlNb alloy to guide for industry production.

2. Experimental Procedure

2.1. Coating Deposition and Charactrization

As shown in Figure 1, the coating deposition was conducted in the RD500-SDU device, which was fabricated by Huasheng company, Guangdong, Dongguan, China. The applied substrate material was Kennametal carbide 2210. The chemical composition of Kennametal carbide 2210 was WC phase and Co phase. The content of WC phase and Co phase were 89.5% and 10.5%, respectively. The metallographic structures of Kennametal carbide 2210 were A02, B00, C00, and E00. The grain size of WC phase was about 0.6–0.8 µm. The hardness of Kennametal carbide 2210 was about 91.5 HRA [28]. Before deposition, all WC–Co pin substrates were mirror-polished to a surface roughness of Ra ≈ 0.02 µm, ultrasonically cleaned for 10 min in both acetone and ethanol, and dried in warm air. An Ar⁺ plasma etching step (−500 V, 5 min) was applied immediately prior to deposition to enhance coating adhesion.

Several customized targets are selected to deposit the three types of coating. The brief deposition process of CrAlN, CrAlN/(CrAlB)N/CrAlN, and TiAlN/ZrN coatings have been listed in Figure 2. The deposition parameters were carefully optimized and are now provided as ranges to ensure process reproducibility while maintaining confidentiality of proprietary data. The ranges provided in Figure 2 reflect the optimized process windows obtained after multiple deposition–evaluation cycles, during which parameters such as bias voltage, nitrogen flow, and target power were adjusted to enhance coating adhesion, hardness, oxidation resistance, and friction stability on our PVD furnace (Guangdong Huasheng, model RD500-SDU). The base pressure before deposition was below 6 × 10⁻³ Pa, the working pressure was in the range of 0.35–0.45 Pa, and the Ar/N₂ flow ratio was maintained between 30:70 and 40:60. The substrate temperature was controlled at 450 ± 20 °C, with a bias voltage of –80 to –100 V, an arc current of 70–90 A, and a deposition duration of 2.0–2.5 h.

The coating topographies and chemical element composition were obtained with SEM (JSM-6510, JEOL, Tokyo, Japan) with the coupled energy-dispersive spectrometer (EDS) with various magnification rates. The XRD spectrum of deposited coatings was obtained with the XRD instrument coupling film accessory (incidence angle 0.5°, scanning rate 1 deg/min). X-ray diffraction (XRD) measurements were performed using Cu Kα radiation (λ = 1.5406 Å) produced by a Cu anode, and scans were collected in the 2θ range of 30–70°. The coating nano-hardness was detected with the agilent nano-indenter G200 (Agilent Technologies, Santa Clara, CA, USA). The main setting parameters included the maximum load 5 mN, the experimental loading rate 10.00 mN/min, and duration time 5 s. The coating–substrate adhesion strength was measured using an Anton Paar scratch tester (Anton Paar, Graz, Austria) (scratch speed 10 mm/min, load range 1–150 N).

2.2. Design of Pin–Disc Friction Experiment

The WC/Co carbide was designed with the size of ϕ4.8 mm × 12.7 mm, which was assumed as the pin part. Several carbide specimens were deposited with PVD CrAlN, CrAlN/(CrAlB)N/CrAlN, and TiAlN/ZrN coating as depicted in Section 2.1, respectively. The Ti₂AlNb specimen was designed with the size of 27 mm × 15 mm × 3 mm, which was assumed as the disc part. The main metallographic microstructure, main chemical composition, and mechanical properties of the utilized Ti₂AlNb alloy have been listed in the published research [28]. The WC–Co pin after polishing showed Ra ≈ 0.02 µm, and the Ti₂AlNb disc exhibited Ra ≈ 0.05 µm. These values were used as the baseline for evaluating surface evolution after wear.

Figure 3 depicted the MDW-02 high-speed reciprocating friction testing machine. The experiment uses the pin–disc reciprocating friction and obtains the change in friction coefficient with time through a weighing sensor. The applied load was set as 40 N, the reciprocating friction speed of the pin was 40 mm/s, the reciprocating distance was 10 mm, and the tests were performed at frequencies of 2 Hz under both 25 °C and 600 °C conditions. The duration time was about 10 min.

The friction coefficient (μ) is defined as the ratio of the normal force applied by the two contact surfaces to the friction force that hinders the movement of these surfaces. Changes in wear parameters and friction pair materials will affect the friction coefficient value. The weighing sensor was utilized to determine the time-varied friction coefficient between Ti₂AlNb specimens and different flathead pins at various wear environments. The friction coefficient could be determined with Equation (1).

$$\mu ={\displaystyle \frac{F_f}{F_n}}$$

(1)

where F_f and F_n were the friction force detected using the weighing sensor and the normal force of the worn sample, respectively.

The wear amount was measured with a precision balance with an accuracy of 0.1 mg. Each specimen was measured more than three times to decrease the error. The wear rate could be calculated using Equation (2). After the tribological tests, the wear morphologies of the Ti₂AlNb alloy surface were analyzed with laser confocal microscopy.

$$I_{\omega }={\displaystyle \frac{\mathrm{\Delta }m}{F\times S}}$$

(2)

where Δm was the mass loss of the Ti₂AlNb specimen, F was the applied load, and S was the sliding distance.

The worn surface topography and its chemical element composition of Ti₂AlNb alloy were measured with SEM (JSM-6510) using the coupled energy–dispersive spectrometer (EDS). The surface roughness and surface topography of Ti₂AlNb alloy, uncoated carbide sample, and coated carbide sample before and after the friction tests were obtained with the laser confocal microscope VK-X250K (Keenshi company, Osaka, Japan). More than three positions were selected and averaged as the measurement results of line surface roughness.

3. Results and Discussion

3.1. Coating Topography and Properties

Figure 4 depicted the surface topography and cross-sectional topography of the CrAlN, CrAlN/(CrAlB)N/CrAlN, and TiAlN/ZrN coatings. The detected droplets were the typical drawback due to the high stacking power accumulation when the high-energy ion bombed with the target materials. The pits were the void defects due to the foreign particles in PVD coatings which could not be firmly bound to the coating and spontaneously failed under high stress effects. A similar phenomenon was also found in the research [30]. The average coating thicknesses were detected as 3.6 ± 0.1 µm (CrAlN), 3.0 ± 0.1 µm (CrAlBN multilayer), and 1.4 ± 0.1 µm (TiAlN/ZrN). The surface roughness of the as-deposited coatings was further quantified by 3D optical profilometry, and the Sa values of CrAlN, CrAlN/(CrAlB)N/CrAlN, and TiAlN/ZrN were measured to be 152.5 nm, 187.1 nm, and 120.0 nm, respectively (Figure 4). These values are consistent with the typical roughness level of arc-deposited nitride coatings and correspond well to the macroparticle distribution observed in the SEM surface images.

Figure 5 shows the X-ray diffraction (XRD) patterns of the CrAlN, CrAlN/(CrAlB)N/CrAlN, and TiAlN/ZrN coatings deposited on WC–Co substrates. All three coatings exhibit a typical cubic B1-NaCl-type structure, consistent with the characteristic phases of CrN- and ZrN-based nitrides. For the CrAlN and CrAlN/(CrAlB)N/CrAlN coatings, the main diffraction peaks are indexed to the (111), (200), and (220) planes of c-Cr(Al)N, confirming that Al and B atoms were successfully incorporated into the CrN lattice. A slight shift in the (200) peak toward higher 2θ angles is observed, which can be attributed to lattice contraction caused by the substitution of smaller Al atoms and the solid-solution effect of B. The TiAlN/ZrN coating exhibits diffraction peaks corresponding to c-ZrN and c-Ti(Al)N phases. The overlapping of these peaks indicates that ZrN and TiAlN layers are well combined, forming a dense nanocomposite coating structure. These results verify that all three coatings maintain stable cubic phases with good crystallinity and uniform structure.

Figure 6 depicted the mechanical properties of deposited CrAlN, CrAlN/(CrAlB)N/CrAlN, and TiAlN/ZrN-coated carbides. The sequence of measured nano-hardness HIT for the deposited coatings is CrAlN/(CrAlB)N/CrAlN < CrAlN < TiAlN/ZrN. The sequence of measured nano-hardness E* for the deposited coatings is TiAlN/ZrN < CrAlN/(CrAlB)N/CrAlN < CrAlN. The sequence of critical Lc₁ for the deposited coatings is CrAlN/(CrAlB)N/CrAlN < CrAlN < TiAlN/ZrN. It was found that the TiAlN/ZrN coating exhibited better hardness and coating–substrate coherence.

3.2. Friction Coefficient and Wear Rate

Figure 7a illustrates the evolution of the friction coefficient during the high-temperature pin–disc tests between Ti₂AlNb alloy and WC/Co pins with different PVD nitride coatings at 600 °C. For all pin–disc pairs, the friction coefficient increases sharply in the initial stage due to the establishment of real contact area and the rapid formation of early tribo-films. After this running-in stage, the friction coefficient gradually enters a fluctuating steady regime. These fluctuations are observed throughout the entire test duration and indicate the continuous occurrence of stick–slip behaviour at the sliding interface, which is typical for Ti₂AlNb due to its strong adhesion tendency at elevated temperatures. Despite this fluctuation, the coated carbide pins show reduced steady-state friction levels compared with the uncoated WC/Co. Specifically, the CrAlN, CrAlN/(CrAlB)N/CrAlN, and TiAlN/ZrN coatings yield reductions of 2.83%, 5.3%, and 11.32%, respectively, with TiAlN/ZrN showing the most pronounced friction-reducing effect owing to its ability to form low-shear oxide films during sliding. Figure 7b presents the specific wear rates of the Ti₂AlNb discs corresponding to the four test conditions. Unlike the friction coefficient trends, the wear behaviour of Ti₂AlNb shows a more distinct differentiation among the coating systems. The CrAlN/(CrAlB)N/CrAlN coating produces a lower wear rate than that observed for the uncoated WC/Co pin, indicating a reduced degree of friction-induced damage on the alloy surface. In contrast, the CrAlN and TiAlN/ZrN coatings lead to substantial increases in the alloy wear rate, reaching 211% and 700% higher than the uncoated condition, respectively. This indicates that although TiAlN/ZrN exhibits the lowest friction coefficient, its interaction with Ti₂AlNb under 600 °C sliding conditions results in more pronounced material removal on the alloy side. These contrasting outcomes highlight that friction reduction does not necessarily correspond to reduced counter-body wear and that different coatings influence the tribological response of Ti₂AlNb through distinct wear mechanisms, including adhesion, oxide-film formation, ploughing, and tribo-oxidation dynamics. It should be noted that the wear loss of the Ti₂AlNb disc at 25 °C was below the detection limit of the weighing system, resulting in non-reproducible values. Therefore, only the wear-rate results at 600 °C, where measurable and repeatable mass loss occurred, are presented.

3.3. Surface Wear Morphology

This section compares the worn-surface morphologies of the Ti₂AlNb counter-face after sliding against uncoated WC/Co pins and three coated pins—CrAlN, CrAlN/(CrAlB)N/CrAlN, and TiAlN/ZrN—at 25 °C and 600 °C. Emphasis is placed on correlating groove formation, adhesive transfer, peeling pits, and ploughing features with temperature and coating architecture, and on linking these observations to the roughness evolution and to the tribological trends reported in Section 3.2.

At 25 °C, the Ti₂AlNb surface after sliding against uncoated WC/Co shows a coexistence of adhesive and abrasive wear (Figure 8). Parallel micro-grooves along the sliding direction indicate ploughing, while scattered peeling pits are present along the track. Local bright, featureless regions suggest material transfer and plastic flow stemming from severe adhesive junctions; sporadic white speckles correspond to loosely attached third-body debris. The overall track is relatively rough, with a combination of furrows and shallow delamination, reflecting the hardness mismatch and the strong tendency of Ti₂AlNb to adhere at low temperature. At 600 °C, the worn surface appears flatter and more uniform, and the adhesive regions are reduced. This smoothing is consistent with the formation of a thin oxide film on Ti₂AlNb (further supported by elemental evidence in Section 3.4), which lowers interfacial shear and mitigates severe junction growth. Nevertheless, faint grooves remain visible, implying that abrasive contributions persist even under oxidative conditions. This baseline evolution (rough → smoother) serves as a reference for assessing how each coating modifies the counter-face topography.

For the CrAlN/(CrAlB)N/CrAlN multilayer, the Ti₂AlNb worn surface at 25 °C exhibits typical plough-like zones interspersed with shallow adhesive patches (Figure 9). The ploughed areas are less deep and more finely textured than in the uncoated case, indicating that the multilayer coating reduces the effective penetration of asperities and suppresses large-scale adhesion. The presence of B in the middle layer promotes grain-boundary stabilization and high-temperature oxidation resistance; even at room temperature this refined microstructure is associated with reduced chip accumulation in the track bottom and mitigated shallow spalling. At 600 °C, the wear track becomes smoother and more uniform. The multilayer promotes the development of dense oxide films on both the coating and the Ti₂AlNb surface, producing a “two-film” interface that damps shear fluctuations. The observed morphology—shallower grooves, fewer peeling pits, and less debris congestion—indicates a transition toward mild oxidative wear. The reduced tendency for severe adhesion and the relatively even topography are consistent with the lower alloy-side wear rate reported for this pair in Figure 7 further confirming the transition toward mild oxidative wear at elevated temperature.

Against CrAlN, the Ti₂AlNb surface at 25 °C shows more pronounced furrowing than with the multilayer, together with adhesive transfer layers and shallow spalling islands (Figure 10). This points to a mixed mode of abrasive + adhesive wear, in line with the hardness and adhesion ranking where CrAlN sits between the multilayer and TiAlN/ZrN. The early-stage debris is not fully evacuated and tends to be trapped at the groove bottoms, which can seed secondary ploughing events. At 600 °C, the wear track becomes more uniform and flatter. The Cr- and Al-rich coating promotes the formation of a continuous mixed oxide film (Cr₂O₃/Al₂O₃), which reduces interfacial shear and stabilizes the friction. While micro-grooves are still present, their depth and continuity are attenuated relative to 25 °C, and the density of peeling pits declines. The resulting oxidative wear regime is milder, and the morphological signature coheres with the moderate friction reduction reported in Figure 7, further supporting the temperature-induced transition toward a more stable tribological state.

For TiAlN/ZrN, the Ti₂AlNb worn surface at 25 °C displays distinct furrows and uneven adhesive patches, indicating that abrasive micro-cutting is active and adhesive junctions intermittently grow and rupture (Figure 11). The morphology is rougher than with the multilayer at the same temperature, reflecting the stronger asperity interactions before protective oxides have substantially formed. At 600 °C, the track becomes visually smoother, and overt furrows are less prominent. This is consistent with the development of lubricious oxides (Al₂O₃/ZrO₂ on the coating side and oxides on Ti₂AlNb), which reduce stick–slip severity and promote a stable oxidative wear regime. However, localized lamellar spallation of the multilayer architecture can occur under high thermal load and cyclic shear, generating third-body fragments that intermittently participate in abrasion. This dual effect—global smoothing by oxides but local roughening by spalled fragments—explains why the steady-state friction is low whereas the alloy-side wear rate can increase (Figure 7). Morphologically, the track thus appears largely uniform with sporadic peeling sites and fine debris clusters, which is consistent with the low steady-state friction and the alloy-side wear trend reported in Figure 7.

The above observations align with the roughness evolution summarized in Figure 12. On the Ti₂AlNb counter-face (Figure 12a), the surface worn against coated pins is generally smoother than against the uncoated pin at 600 °C, reflecting the lubricating role of oxide films. Among the coatings, the multilayer CrAlN/(CrAlB)N/CrAlN yields particularly even tracks with fewer peeling sites, consistent with its lower alloy-side wear rate. The TiAlN/ZrN case shows an apparently smooth average track but with localized irregularities associated with spallation-derived debris, compatible with the higher alloy-side wear measured despite low friction. On the pin/coating side (Figure 12b), increasing temperature tends to aggravate uncoated WC/Co wear (roughening), whereas the coatings exhibit distinct thermal responses: the multilayer benefits from dense oxides and interface stability (smoother), CrAlN achieves a moderate roughness reduction via Cr₂O₃/Al₂O₃ formation, and TiAlN/ZrN retains smooth coating regions yet can display local roughening where spallation/third-body effects arise. These morphology–roughness correlations rationalize the tribological metrics in Section 3.2 (Figure 7) and set up the compositional/oxidative analysis in Section 3.4, wherein element-resolved maps (O, Cr, Al, Zr, etc.) substantiate the oxide-film mediation of shear and the third-body contribution to mixed wear.

Across all pairs, room-temperature wear is dominated by abrasive + adhesive interactions, manifested by furrows, adhesive transfer layers, and shallow spalling. Upon heating to 600 °C, the system transitions toward oxidative wear, with overall track smoothing and reduced adhesive junctions due to protective films on both counterparts. The CrAlN/(CrAlB)N/CrAlN multilayer most effectively suppresses counter-face material removal (smoother Ti₂AlNb with fewer peeling pits), while TiAlN/ZrN achieves the lowest friction but may generate localized debris via lamellar peeling that heightens alloy-side wear. CrAlN exhibits intermediate behaviour consistent with its microstructural and oxidative attributes. These morphological signatures are fully consistent with the friction/wear trends in Section 3.2 and prefigure the elemental distributions analyzed in Section 3.4.

3.4. Analysis of Worn Surface Elements

To further clarify the elemental evolution and oxidation behaviour during frictional wear, the worn surfaces of Ti₂AlNb alloys against uncoated and coated WC/Co pins were analyzed by scanning electron microscopy (SEM) combined with energy-dispersive spectroscopy (EDS). The elemental maps reveal the compositional changes and oxidation extent at both 25 °C and 600 °C, providing direct evidence for the transition from abrasive/adhesive wear to oxidative wear as discussed in Section 3.3.

As shown in Figure 13, for the uncoated WC/Co–Ti₂AlNb pair, the EDS mappings at 25 °C and 600 °C illustrate the evolution of the surface chemistry. At 25 °C, a slight increase in O and C elements is detected near the grooves (Figure 13b), indicating that minor adhesion and ploughing abrasion occur simultaneously. The main detected elements are Ni > Nb > Cr (Figure 13c), confirming that WC/Co debris adheres to the Ti₂AlNb surface due to localized high stress. When the temperature increases to 600 °C, the oxygen signal in Figure 13d increases significantly, while Ti and Al signals decrease, suggesting the formation of continuous TiO₂ and Al₂O₃ oxide films. These oxides act as a lubricating layer, explaining the smoother surface and lower friction observed at high temperature. The uncoated pair thus transitions from abrasive–adhesive wear to mild oxidative wear with increasing temperature.

As illustrated in Figure 14, in the CrAlN/(CrAlB)N/CrAlN–Ti₂AlNb pair, the surface chemical composition of Ti₂AlNb after friction at 25 °C shows enrichment of C, Cr, and Ni, indicating material transfer from the coating and partial adhesive interaction at the interface. The absence of significant changes in Ti or Al content implies that substrate exposure is limited and that the multilayer structure effectively confines the wear to the surface. The combined presence of Cr and B in the coating favours grain-boundary strengthening and high-temperature stability, preventing extensive delamination. After heating to 600 °C, the EDS maps show a pronounced increase in O content across the wear track, while the proportions of Cr and Al remain stable. This demonstrates the formation of a dense Cr₂O₃/Al₂O₃ composite oxide film on the contact surface. The B element, distributed near the oxide–substrate interface, contributes to refined oxide grain boundaries and enhanced film adhesion, thereby improving the protective efficiency of the oxide scale. Consequently, the CrAlBN multilayer coating induces balanced oxidation behaviour, maintaining a moderate friction coefficient and minimal counter-face damage.

According to Figure 15, for the CrAlN–Ti₂AlNb pair, the elemental maps exhibit similar trends but with less uniform oxidation. At 25 °C, the wear surface of Ti₂AlNb contains elevated Cr and C contents due to adhesive transfer of coating fragments, and small O enrichment areas suggest initial oxide nucleation. The wear mechanism at this stage remains a mix of abrasive and adhesive modes. At 600 °C, the O content increases markedly, and continuous Cr₂O₃ and Al₂O₃ oxides are detected. These oxides act as a solid-lubricating barrier, impeding further oxygen diffusion and improving wear stability. Compared to the multilayer coating, the CrAlN single layer shows weaker oxidation uniformity but still exhibits substantial improvement over the uncoated pair. This observation supports the tribological results in Figure 7, where CrAlN demonstrates intermediate friction and wear behaviour, consistent with its moderate oxidation resistance and mechanical robustness.

As shown in Figure 16, for the TiAlN/ZrN–Ti₂AlNb pair, the EDS mappings reveal distinctive element migration behaviours. At 25 °C, localized enrichment of Ti, Al, and Zr is observed on the alloy surface, indicating coating material transfer and intermittent adhesion. The oxygen content remains low, showing that oxidation is minimal at this stage. As the temperature rises to 600 °C, strong O enrichment occurs, accompanied by the detection of Al₂O₃ and ZrO₂ oxides, confirming the formation of lubricious mixed oxide layers that effectively reduce interfacial shear stress. However, EDS also identifies localized regions with depleted coating elements and increased W and Co signals, signifying coating spallation and exposure of the carbide substrate. The detached TiAlN/ZrN fragments and newly formed oxides behave as third-body abrasives, which locally increase the wear rate of Ti₂AlNb despite the overall friction reduction. This duality—oxidation-induced lubrication versus debris-induced abrasion—accounts for the low friction but relatively high alloy-side wear observed for this coating at 600 °C.

The comparative EDS analysis across all friction pairs reveals a consistent pattern: as temperature increases, oxygen incorporation intensifies, and oxide species evolve from discontinuous patches into continuous protective films. For uncoated contacts, these oxides are sparse and easily disrupted; for coated pairs, particularly CrAlN and CrAlBN systems, dense and adherent oxide scales form on both counterparts, stabilizing friction and suppressing adhesion. The TiAlN/ZrN system, in contrast, promotes low-shear mixed oxides but experiences localized structural failure under cyclic thermal stress. Overall, the elemental evidence corroborates the morphological observations in Section 3.3 and supports the conclusion that the wear mechanism shifts from abrasive/adhesive wear at 25 °C to oxidative wear at 600 °C, with coating composition and multilayer design dictating the oxidation pathway and tribological outcome.

4. Conclusions

In this study, three PVD nitride coatings—CrAlN, CrAlN/(CrAlB)N/CrAlN, and TiAlN/ZrN—were deposited on WC/Co carbide substrates to investigate their effects on the high-temperature tribological behaviour against Ti₂AlNb alloy. Through systematic analysis of microstructure, mechanical properties, frictional behaviour, and worn surface characteristics, the friction-reduction and wear-resistance mechanisms of these coatings were elucidated. The results provide a reference for optimizing coating design in the machining of Ti₂AlNb and other high-temperature intermetallic alloys.

• All three coatings exhibit a dense cubic B1-NaCl structure and maintain good phase stability under high-temperature conditions. The CrAlN coating showed a uniform microstructure and stable (200) texture, while B incorporation in CrAlN/(CrAlB)N/CrAlN refined grains and enhanced oxidation resistance through solid-solution and grain-boundary strengthening. The TiAlN/ZrN multilayer coating presented a well-defined nanoscale lamellar interface, achieving the highest hardness and adhesion strength due to synergistic effects between Ti(Al)N and ZrN layers. These structural and mechanical differences laid the foundation for distinct tribological responses during high-temperature sliding.
• Under 600 °C pin–disc tests, all coatings reduced the friction coefficient compared with uncoated WC/Co carbide, indicating effective suppression of adhesive wear. The Ti₂AlNb counter-face exhibited smoother worn surfaces at elevated temperature, attributed to the formation of lubricious oxide films (Al₂O₃, ZrO₂, and Cr₂O₃). Among the tested coatings, TiAlN/ZrN demonstrated the lowest steady-state friction coefficient, while CrAlN/(CrAlB)N/CrAlN maintained superior thermal stability with minimal oxidation-induced degradation. These results confirm that friction behaviour is governed by both coating microstructure and dynamic oxide film evolution at elevated temperature.
• Wear mechanisms transitioned from abrasive and adhesive wear at room temperature to oxidative wear at 600 °C. TiAlN/ZrN exhibited the highest resistance to coating delamination and retained smooth surface morphology after testing, indicating superior load-bearing capacity and interfacial cohesion. In contrast, CrAlN/(CrAlB)N/CrAlN minimized material removal from the Ti₂AlNb counter-face by forming protective oxide layers on both surfaces. Therefore, TiAlN/ZrN is the most promising coating for enhancing tool-side durability and friction reduction, while CrAlN/(CrAlB)N/CrAlN offers advantages in reducing counterpart wear. Future work should couple high-temperature friction results with cutting tests to establish quantitative correlations between tribological performance and machining service life.