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).

ZrC ceramics have good prospects for application in high-temperature and high-pressure environments, but their low toughness limits their practical applications. Zr-Al-C ternary layered compounds have good mechanical properties, so in this paper, single-phase dense Zr₃Al₃C₅ ceramics were prepared by spark plasma sintering using Zr, Al and C powder as raw materials, and the reaction process and mechanical properties of Zr₃Al₃C₅ compounds were investigated. The results showed that at 700℃, a large amount of Zr-Al intermetallic compounds were generated. At 900 ℃, the ZrAl₃ compounds were transformed into Zr₂Al₃ and ZrAl₂, and Al₄C₃ and ZrC compounds were formed simultaneously. When the temperature reached 1300 ℃, the Zr₃Al₃C₅ compounds appeared, but during the sintering process, Zr₂Al₃C₄ compounds were generated due to the local inhomogeneity caused by the extremely easy melting of Al. When the temperature reached 1700 ℃, single-phase dense Zr₃Al₃C₅ ceramics were prepared. The hardness and Young's modulus of Zr₃Al₃C₅ ceramics reached 10.9 GPa and 310 GPa, respectively, and the fracture toughness was 3.8 MPa·m^1/2 ± 0.16 MPa·m^1/2. The lamellar structure and elongated grain structure allowed the energy dissipation during fracture and improved the fracture toughness.

ZrB₂-based composite ceramics have great potential for applications at ultra-high temperatures, but the low toughness and poor oxidation resistance of them limit their practical applications. In this work, the Zr, B₄C, Al, C and Si raw powders were used to prepare ZrB₂–SiC–Zr₂Al₄C₅ composite ceramics by reactive spark plasma sintering. The reaction processes and mechanism were studied. The effects of Zr₂Al₄C₅ content and oxidation temperature on high-temperature oxidation behavior of multiphase ceramics were investigated for revealing the anti-oxidation mechanism. The results indicated that the B₄C reacted with Zr to form ZrB₂ at 900℃, and then the SiC formed at 1100℃. At 1400℃, the Zr₂Al₄C₅ formed from the reaction of Zr₃Al₃C₅, ZrC and Al₄C₃ which formed at the early initial reaction stage. And the Zr₂Al₄C₅ transformed into Zr₃Al₄C₆ with further increasing temperature. The main phases of the composite ceramics after oxidation are ZrO₂, ZrSiO₄, aluminosilicate and SiO₂ glass. With increasing Zr₂Al₄C₅ content, the surface of the oxide layer of the composite ceramics became uneven, loose and porous, while obvious long strip grains and inhomogeneous distribution of glassy phases were also observed. This study not only provides a new experimental reference for in-depth understanding of the high-temperature oxidation behavior of ZrB₂–SiC based multiphase ceramics, but also provides new experimental and theoretical support for developing high-performance ZrB₂–SiC based high-temperature composite ceramics.