Abstract: Refractory high-entropy/medium-entropy alloys (RHEAs/RMEAs) demonstrate excellent thermal stability and mechanical properties at elevated temperatures. However, under high-temperature non-inert tribological conditions, the differences in oxidation activity among the constituent elements lead to instability at the sliding interface, thereby affecting the wear resistance of the material. In this work, we report a novel strategy to achieve low wear rates in NbTiTa self-lubricating composites by in situ formation of a nanocrystalline-amorphous composite layer and phase boundary transformation of Al₂O₃ nanoparticles. During high-temperature friction, the complex composition (NbTiTa, Al₂O₃, Ag, and CaF₂/BaF₂ eutectics) and the high-density grain boundary of the composites promote the formation of oxide glazes on the sliding surface. An amorphous (NbTiTa-O)-nanocrystalline tribo-layer of approximately 2.1 µm thick is formed at 600 °C, exhibiting a microhardness of approximately 15.8 ± 1.6 GPa and remarkable resistance to plastic deformation. Furthermore, the incompatible deformation between the NbTiTa alloy and Al₂O₃ during friction induces the interphase boundary transition from an incoherent to an amorphous structure. This interfacial transformation effectively absorbs the strain energy of the alloy during friction and inhibits crack nucleation. Consequently, the designed NbTiTa self-lubricating composite maintains an exceptionally low wear rate (10^-7-10^-6 mm³ N^-1 m^-1) at 600 and 800 °C. Therefore, this study provides a universally applicable strategy and valuable insights for the design of high-temperature wear-resistant self-lubricating composites.

Key words: Refractory medium-entropy alloys, High temperature, Self-lubricating composites, Wear mechanisms, Nano-Al₂O₃ reinforcement