Exploring the oxidation behaviors of the Ti-V-Cr-Mo high-entropy MAX at 800°C for its self-lubricity

Given the unique nanolaminate structure, the MAX phase ceramics which are composed of alternatively stacked two-dimensional (2D) metal (M) carbides (X) and main group (A) atomic layers have shown promising properties at elevated temperatures. Recently, an excellent self-lubricating performance of a high-entropy (HE) (TiVCrMo)3AlC2 MAX is demonstrated at 800°C in the air. However, the formation mechanism of a self-lubricative tribofilm is still waiting for clarified. In this work, the HE (TiVCrMo)3AlC2 MAX is synthesized and its oxidation behaviors are systematically evaluated at 800°C in the air. A two-stage parabolic oxidation behavior is detected, which can be mainly ascribed to the volatilization of MoO3 species coupled with the thickening of the compound oxide scale. Meanwhile, the HE composition results in chemically complex phases including Rutile and vacancy-ordered TiO species in the oxide scale, which contribute to a higher ion diffusion rate for mass transportation. Consequently, the continued volatilization of MoO3 is beneficial for the enhanced lubricity and anti-wear performance of the (TiVCrMo)3AlC2 MAX at 800°C.