MAX phase forming mechanism of M–Al–C (M = Ti, V, Cr) coatings: In-situ X-ray diffraction and first-principle calculations

The interesting hybrid properties of ceramics and metals induced by unique nano-laminated structures make the Mn+1AXn (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.