Toughening and high-temperature self-lubricating of high-entropy boride ceramics through h-BN

High-entropy boride (HEB) ceramics demonstrate outstanding high-temperature stability, positioning them as promising candidates for reliable performance in extreme environments. However, their inherent limitations lie in their relatively low fracture toughness, coupled with the unclear elucidation of high-temperature tribological behaviors. To address these challenges, high-entropy (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂ ceramics are utilized as the matrix material in the current investigation, whereas hexagonal boron nitride (h-BN) is introduced as a type of toughening and lubricating phase to develop HEB-hBN composite ceramics. The toughening and high-temperature self-lubrication of the composites are achieved by leveraging the high aspect ratio, lamellar microstructure, and interlayer slip characteristics of h-BN. The results indicate that h-BN enhances the fracture toughness of the composite ceramics by nearly 70%, which is attributed to the optimization of the crack growth path through its lamellar microstructure and facilitating crack deflection and bridging mechanisms due to its high aspect ratio. Moreover, through interlayer slip effects, h-BN combines with B₂O₃ and metal oxides generated by high-temperature oxidation, forming a gradient tribofilm in conjunction with other synergistic lubrication mechanisms. This synergistic interaction results in a nearly 40% reduction in the friction coefficient of the composite ceramics, accompanied by an approximately 60% decrease in the wear rate under high-temperature friction conditions at 1000 °C. Under extreme friction environments ranging from 1000 to 1200 °C, the composite ceramics maintain a friction coefficient consistently below 0.30, with the wear rate stably sustained at an order of magnitude of 10⁻⁵ mm³/(N·m).