Self-lubrication of single-phase high-entropy ceramic enabled by tribo-induced amorphous carbon

Designing single-phase ceramics with favorable self-lubricity is a substantial challenge due to the strong chemical bonds and difficulty to shearing. Most conventional lubricating ceramics only benefit from adding solid lubricants, which largely limits the mechanical properties of the resulting materials. Here, we report a single-phase ceramic inspired by the novel high-entropy design concept and tribo-induced effect, and which can achieve an outstanding self-lubricity (friction coefficient is as low as 0.1) at 200-400 degrees C in a vacuum environment. The self-lubricity is attributed to two factors, namely the decomposition of the (Hf-Mo-Nb-Ta-Ti)C ceramic leading to formation of the amorphous carbon film at the sliding interfaces, and the diffusion of carbon atom under the effect of high-temperature tribo-induced enabling easy shear contact. Based on the experimental and theoretical findings, we formulate a novel design strategy for single-phase self-lubricating ceramics by in-situ forming lubricating phase.