Unveiling the wide-temperature tribological mechanisms of (MgCoNiCuZn)O high-entropy oxide: The critical role of dynamic CuO precipitation and re-dissolution

The rock-salt structured (MgCoNiCuZn)O high-entropy oxide (HEO) ceramics exhibit great potential for high-temperature applications, yet its tribological behavior across a wide temperature range remains to be fully deciphered. This work systematically investigated the wide-temperature tribological mechanisms of (MgCoNiCuZn)O from room temperature (RT) to 800 ℃. The results indicate that the HEO's hardness and wear resistance displayed a non-monotonic dependence on temperature, while simultaneously demonstrating a reversible entropy-driven phase transition. At 400 ℃, thermodynamic instability triggered CuO precipitation, which disrupts the high-entropy structure and leads to a significant degradation in hardness and increased the wear rate. However, at the elevated temperature of 800 ℃, an entropy-driven re-dissolution of CuO partially restores the high-entropy structure, leading to a recovery in hardness. Concurrently, a continuous, lubricious oxide film formed on the worn surface. Density functional theory (DFT) calculations reveal that the low binding energy and high electron activity of CuO underpin its preferential precipitation and its key role in forming the Cu-rich oxide lubricating film. The synergy between phase re-stabilization and surface film formation results in excellent high-temperature wear resistance, with a minimum wear rate of approximately 0.53×10^-5 mm³·(N·m)^-1 at 800 ℃. This work elucidates the phase evolution and tribological mechanisms of HEO across a wide temperature range, providing valuable guidance for designing advanced wear-resistant materials for extreme environments.