Ta-10W has great potential in the aerospace and nuclear industries due to its good formability, high melting point, excellent high-temperature strength and corrosion resistance (Byun and Maloy, 2008; Liu et al., 2018; Li et al., 2020; Lv et al., 2020). It has attracted considerable attention for high-temperature structural components in aerospace and the jacket for nuclear waste (Zhou et al., 2015; Novakowski et al., 2018; Wang et al., 2020; Zhang et al., 2020). Most of the workpiece surface failures are mainly caused by wear behaviors. Especially in some elevated-temperature areas, wear occurs frequently and inevitably. Therefore, it is necessary to research the wear mechanisms at elevated temperatures of Ta-10W alloy.

The tribological properties of Ta have been intensively investigated over the past decades. Kommel Lembit investigated the forming of gradient microstructures of pure tantalum under the wear trajectory of a dry sliding wear test (Kommel et al., 2021). The results showed that the coefficient of friction and wear was not dependent on the initial microstructure and hardness of the material, but the depth of the grading layer under the wear trajectory was dependent on the previous treatment history of the material. Many researchers had focused on the gradient nanocrystal structure consisting of recrystallized nanometer-sized grains formed under the surface layer of the sample when Ta was subjected to dry sliding friction (Zhang et al., 2014a; Zhang et al., 2014b). Notably, Ta has different phases, including metastable-tetragonal β-Ta and stable body-centered-cubic α-Ta phase. The β-Ta primarily has a high level of hardness, but it has a high level of brittleness and poor load-bearing ability. It is susceptible to fracture and wear under high load, which causes a high wear rate (Myers et al., 2013). Other studies showed the sample with tantalum gradient transitional layer showed excellent tribological properties, which were in close relation to the high critical load, low surface roughness and the phase transition from metastable β-Ta to stable cubic α-Ta under the action of frictional heat in oxygen-less conditions (Bing et al., 2022). Furthermore, tungsten has a uniform hardness, high melting point and superior oxidation resistance. The tungsten oxide formed in friction can protect the surface from damage (Li et al., 2017). Spark plasma sintering has been used in a few studies to consolidate Ta-W alloys. In a study by Shahram Seyyedin (Seyyedin et al., 2020), the effect of mechanical alloying time on the microstructure, hardness, and tensile properties of Ta-10 (wt%)W alloy sintered at 2000°C for 5 min using spark plasma sintering was investigated. Furthermore, the corrosion resistance of commercial purity titanium (CP-Ti) and Ti-6Al-4V (TC4) alloys coated with Ta-10W via multi-arc ion plating was investigated by Pei Sun, and electrochemical corrosion testing revealed that the Ta-10W coating improved the substrate’s corrosion resistance (Sun et al., 2019). However, there isn’t yet any research on the influence of temperature on the tribological properties of Ta-10W alloy.

In this study, the effect of temperature on the frictional properties of tantalum-tungsten alloys was examined. Like other superalloys, Ta-10W alloy would have a “pest” oxidation phenomenon at about 500°C (Dong et al., 2018), so the tribological tests at 100°C, 200°C and 300°C were carried out. Then, the elevated-temperature tribological samples were characterized by X-Ray diffractometer (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The tribological behaviors and mechanisms of Ta-10W at elevated temperatures were studied.

It is well known that in the process of sliding contact when one surface moved over another surface, the movement of the surface would generate an interchangeable subsurface shear stress (Okonkwo et al., 2012). With the increase of wear time, the plastic strain of the subsurface layer gradually increased until cracks developed and extended to pits or spalling, as seen at 200°C and 300°C in this research. When the surface was lubed under these conditions, surface deformation was taken place, then the surface was elongated. Therefore, the surface showed a fish scale-like structure.

As Gautam G previously pointed out, the temperature rise may lead to the increase of material viscosity at high temperatures. It was reported that due to the temperature rose, the surface softening and excessive oxidation resulted in the continuous increase of friction coefficient (Gautam et al., 2016). Bai Yaping and Guo Yongchun also pointed out this phenomenon. They studied the influence of Al₂O₃ nanoparticles on the tribological behavior of 7,075 Al alloy and found that 7,075 Al alloy was easy to soften at high temperatures, which would lead to serious adhesion between 7,075 Al alloy and the corresponding vermicular cast iron. Thus, the friction coefficient raised with the increase in temperature (Bai et al., 2017).

The structure of tantalum oxide is complex and there are many phase structures, such as α phase, β phase, γ phase, and δ phase. Tantalum oxide films prepared by physical vapor deposition usually have an amorphous structure. When annealed in an air (or oxygen) atmosphere higher than the crystallization temperature (about 873–973 K), the crystal with an orthogonal crystal structure is mainly formed β-Ta₂O₅ (Dimitrova et al., 2001). In addition, Ta₂O₅ also has a body-centered cubic structure α-Ta₂O₅ and hexagonal crystal structure γ-Ta₂O₅. Tantalum oxide formed during friction did not show any features in the XRD pattern, which indicated that the atomic structure remained amorphous.

During the wear process, the scar surface was covered with a large number of stable oxide films, resulting in a reduction in the wear rate (Ahmadi et al., 2018). Figure 11 showed a schematic description of the friction film. In this work, the influence of oxide type and particle diameter on the transition behavior of light wear was studied (Kato and Komai, 2007). Wear had been reduced from severe wear to light wear. The transition to light wear was caused by the formation of wear protective film on the friction surface. It was observed that these friction films were generated by friction sintering of the provided oxide particles (Kato, 2008).

Particles are joined together during the sintering process by atomic transport events at temperatures below the melting point (German, 1984). Sintering is fueled by a decrease in the system’s free energy, which is manifested as the elimination of surface area. Usually, sintering is controlled by atomic diffusion rates, which are controlled by surfaces, grain boundaries, and lattices. Generally, the following formula may be used to express the sintering rate for particles, S: S = a D m d n (3) where D is the diffusion constant of part within the sintering material, d is the particle diameter, and a, m, and n are constants. Brown and Ashby (Brown and Ashby, 1980) had given the gas diffusion constant within the chemical compound, D as, D = D 0 ⁡ exp Q T m R T (4)

Where D₀ is the diffusion frequency coefficient of about 5.3 × 10⁻⁴ m²/s for oxides, Q is the activation energy of about 194.6 kJ/mol, R is the universal gas constant of about 8.3 J/mol K, T_m is the melting point of the oxide, and T is that the absolute temperature. This equation means that oxides at a low melting point or high temperature have relatively high diffusivity. Table 3 showed the melting points of some oxides. A study by Kato H showed that the provision of Fe₂O₃ and CuO nanoparticles caused a mild wear change under the conditions used, while the provision of Al₂O₃, ZnO or TiO₂ particles did not (Kato and Komai, 2007). Zhou et al. also reported that the addition of iron oxide particles during the friction process led to the formation of a friction layer (Zhou et al., 2018). Adachi Koshi proved the possibility of friction sintering of wear particles in the process of friction between alumina and itself under high operating pressure and high temperature through experiments (Adachi and Kato, 2000). The study of Zhou showed that for ultra-fine powder compacts, the sintering began as soon as the temperature increased from room temperature (Zhou et al., 1989). Several recent studies had indicated that the metal matrix properties could be augmented with particles such as Al₂O₃, Al₃Fe, or solid lubricants (graphite) (Yang et al., 2010; Agarwal et al., 2014; Li et al., 2015). When the tribological tests were carried out at 200°C and 300°C, tantalum oxide had a high diffusivity, thus forming a friction film, which changed the wear from heavy to light, so the wear rate decreased.

In addition, there were α-Ta nanoparticles assembled into nanoribbons on the reverse side, further reducing friction and wear (Bing et al., 2022). When the strength of the bonding point was lower than that of the two materials of the friction pair, shear occurred at the interface. At this point, the wear was very small, and the material transfer was not significant, although the friction coefficient increased. This slight adhesive wear usually occurred when there was an oxide film, vulcanization film or another coating on the metal surface (Sun, 2007). So at 200°C and 300°C, the friction coefficients increased, but the wear rates were small.

The purpose of this study was to experimentally research the influence of temperature on the frictional wear of Ta-10W. Elevated-temperature tribological tests for Ta-10W with a normal load of 4 N and a time of 30 min were conducted. Using SEM and EDS, topography and morphology of the scars were performed.

The primary wear mechanism of the material at 100 °C was abrasive wear, and adhesive wear and slight oxidative wear occurred at 200°C and 300°C. The friction coefficients were graphed for different temperatures. The average friction coefficients under the three temperatures fluctuated close to the average level, with an average of 0.57, 0.58, and 0.66, respectively. It was observed that the friction coefficients increased with the temperature which was due to the increase of metal viscosity at elevated temperatures.

We measured the mass difference of the sample before and after the friction and wear test with an analytical balance and calculated the wear rates. The results indicated that the wear rate at 100°C was 1.19 × 10⁻¹⁰ mm³N⁻¹m⁻¹, and the wear rates at 200°C and 300°C were greatly reduced, which were 0.36 × 10⁻¹⁰ mm³N⁻¹m⁻¹ and 0.38 × 10⁻¹⁰ mm³N⁻¹m⁻¹, respectively. It was observed that the wear rate decreased as the temperature increased, resulting in a transition from severe wear to light wear. At 200°C and 300°C, a large amount of stable oxide films covered the scar surface, resulting in a lower wear rate.