Temperature-dependent tribological behavior and interface evolution of plasma sprayed molybdenum coatings

The present study investigates the dry sliding tribological characteristics and interfacial architecture evolution of atmospheric plasma sprayed molybdenum (Mo) coating deposited on Inconel 718 alloy, over a temperature range of 25°C (room temperature, RT) ~ 500°C. Comprehensive studies reveal that plasma sprayed Mo coating suffers increasing oxidation and strain transition when temperature exceeds 300°C. In-situ mechanical measurement confirms the Mo coating maintains a desirable thermomechanical stability at the temperature range. The Mo/Al2O3 tribopair exhibits a parabolic trend in friction and wear rate. Specifically, the Mo coating attains a low wear rate (~10-5 mm³/N·m) at both RT and 500°C. However, severe wear happens at 300°C. The worn Mo surface develops a tribolayer primarily composed of MoO2 and MoO3. Despite their presence, these two molybdenum oxides do not effectively reduce friction and mitigate wear when interacting with the Al2O3 surface. Electron Backscatter Diffraction (EBSD) and Molecular Dynamics (MD) simulations demonstrate that the near-surface structure of the Mo coating transforms predominantly due to the compression-induced wear rather than dislocation evolution. Under combined friction and temperature effects, the Mo coating at RT, 300°C, and 500°C exhibits variations in crystallographic texture, dislocation density, and dislocation length. Compared to the subsurface characteristics at 300°C, the near-surface architecture at RT and 500°C shows higher different orientations under thermal shear stress: (104) at RT, (100) at 300°C, (213) at 500°C. It is precisely these dislocation characteristics and texture orientations that contribute to the enhancement of wear resistance at both RT and 500°C. In conclusion, the coating exhibits temperature-dependent tribological behavior where optimal performance with low friction coefficient (0.39) and wear rate (8.8 × 10⁻⁵ mm³/N·m) is achieved at 500°C due to protective MoO₃-dominated tribolayer formation, while severe wear occurs at 300°C caused by brittle MoO₂. This study enhances our understanding of high-temperature tribology in Mo-coated mechanical components and the corresponding microstructural changes at tribo-surfaces and interfaces under unlubricated conditions.