High-entropy diboride: A novel high-temperature selflubricating 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 significant degradation of mechanical properties, severely limiting their engineering applications. Benefiting from the excellent high-temperature mechanical and lubrication potential of novel high-entropy borides, the present work innovatively proposed a strategy of using high-entropy ceramic components to provide lubrication functions and successfully designed and prepared a novel high-entropy (Ti0.2V0.2Nb0.2Ta0.2Mo0.2)B2 system based on the tribological element design principles. The material achieves synergistic enhancement of both mechanical and tribological performance. Its microhardness, fracture toughness, and flexural strength are 23.8±0.9 GPa, 5.4±0.3 MPa·m1/2, and 415±17 MPa, respectively. Furthermore, compared with conventional single-phase ceramics, high-entropy (Ti0.2V0.2Nb0.2Ta0.2Mo0.2)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 °C, while the wear rate maintains at a low level ((8.8±0.7)×10-5 mm3/(N·m)). This outstanding high-temperature tribological performance is attributed primarily 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 highentropy (Ti0.2V0.2Nb0.2Ta0.2Mo0.2)B2 substrate.