High-entropy diboride: A novel high-temperature self-lubricating ceramic with enhanced mechanical and tribological properties

Ceramic-based lubrication materials are among the best choices for solving high-temperature load-bearing and lubrication problems. However, traditional ceramics achieve self-lubrication by adding lubricating phases, which often leads to a significant degradation of mechanical properties, severely limiting their engineering applications. Benefiting from the excellent high-temperature mechanical and lubrication potential of the novel high-entropy borides, the present work innovatively proposed the strategy of using high-entropy ceramics' own components to provide lubrication functions and successfully designed and prepared a novel high-entropy (Ti_0.2V_0.2Nb_0.2Ta_0.2Mo_0.2)B₂ based on the tribological element design principles. The material achieves synergistic enhancement of both mechanical and tribological performances. Its microhardness, fracture toughness, and flexural strength are up to 23.8±0.9 GPa, 5.4±0.3 MPa·m^1/2, and 415±17 MPa, respectively. Furthermore, compared with the conventional single-phase ceramics, the high-entropy (Ti_0.2V_0.2Nb_0.2Ta_0.2Mo_0.2)B₂ with Al₂O₃ balls as the mating partner demonstrates exceptional overall tribological properties across a wide temperature range. Notably, the friction coefficient is as low as 0.12±0.01 at 1000 ℃, while the wear rate maintains at a low level ((8.8±0.7)×10^-5 mm³/Nm). This outstanding high-temperature tribological performance is primarily attributed to the novel high-temperature solid-liquid synergistic lubrication mechanism generated by liquid-phase B₂O₃ and solid-phase layered MoO₃ and V₂O₅, as well as the excellent high-temperature support provided by the high-entropy (Ti_0.2V_0.2Nb_0.2Ta_0.2Mo_0.2)B₂ substrate.