Abstract: The interesting hybrid properties of ceramics and metals induced by unique nano-laminated structures make the M_n+1AX_n (MAX) phase attractive as a potential protective coating for vital structural components in harsh systems. However, an extremely narrow phase-forming region makes it difficult to prepare MAX phase coatings with high purity, which is required to obtain coatings with high-temperature anti-oxidation capabilities. This work describes the dependence of the phase evolution in deposited M-Al-C (M = Ti, V, Cr) coatings as a function on temperature using in-situ X-ray diffraction analysis. Compared to V₂AlC and Cr₂AlC MAX phase coatings, the Ti₂AlC coating displayed a higher phase-forming temperature accompanied by a lack of any intermediate phases before the appearance of the Ti₂AlC MAX phase. The results of the first-principle calculations correlated with the experience in which Ti₂AlC exhibited the largest formation energy and density of states. The effect of the phase compositions of these three MAX phase coatings on mechanical properties were also investigated using ex-situ Vickers and nano-indenter tests, demonstrating the improved mechanical properties with good stability at high temperatures. These findings provide a deeper understanding of the phase-forming mechanism of MAX phase coatings to guide the preparation of high-purity MAX phase coatings and the optimization of MAX phase coatings with expected intermediate phases such as Cr₂C, V₂C etc., as well as their application as protective coatings in temperature-related harsh environments.

Key words: MAX phase coating, Phase-formation mechanism, In-situ X-ray diffraction, First-principle calculations, Mechanical properties