In Situ Synthesis and Wide-Temperature Tribological Properties of Biphasic Structure High-Entropy (Mg_1/6Ni_1/6Co_1/6Cu_1/6Zn_1/6Al_1/6)₃O₄ Ceramics

High-entropy oxide ceramics (HEOCs) have shown great potential in the field of wear-resistant lubricant materials in a wide temperature range due to their high-temperature phase stability and oxidation resistance brought by their multimajor elemental properties. However, the mechanical properties and tribological behavior of single-phase HEOCs are difficult to optimize simultaneously, which constitutes a major bottleneck limiting their practical applications. In this study, a synergistic enhancement strategy based on a rock-salt/spinel biphasic structure was proposed, successfully achieving a co-optimization of mechanical and tribological properties for (Mg_1/6Ni_1/6Co_1/6Cu_1/6Zn_1/6Al_1/6)₃O₄ HEOCs. Utilizing the in situ thermal decomposition of Al(OH)₃ to generate alumina as the aluminum source, rock-salt/spinel biphasic synergistic high-entropy ceramics with uniformly dense components and free of heterogeneous phases were successfully synthesized at relatively low sintering temperatures. The resulting biphasic ceramics exhibited outstanding mechanical properties, with a Vickers hardness reaching 802.1 Hv─significantly higher than that of classic rock-salt HEOCs. Tribological studies over a wide temperature range demonstrated that the ceramics achieved an ultralow coefficient of friction (COF = 0.07) and extremely low wear rate (1.5 × 10^-6 mm³/N·m) at 300 °C, with the mechanism revealing a unique "microplasticity-coordinated oxidative lubrication" mechanism induced by the biphasic structure. At 600 °C, complex adhesion, abrasive, and fatigue wear mechanisms cause dramatic increases in both the friction coefficient and wear rate. Notably, at 900 °C, the COF dropped significantly to 0.44, which was attributed to Cu enrichment and the formation of a Cu-based oxide molten lubricating layer. This work not only confirms the ability of biphasic structures to synergistically enhance the service performance of HEOCs across wide temperature ranges but also provides a novel paradigm for designing next-generation high-performance wear-resistant lubricating ceramic materials based on biphasic high-entropy strategies.