Structure and stability of medium-entropy M 2AlC ( M = Ti/V/Cr) coatings synthesized by magnetron sputtering

The extension of high-entropy design principles to ceramic MAX phases holds great promise for developing coatings with tailored properties. However, the synthesis and stability of such entropy-stabilized MAX-phase coatings, particularly those with M-site solid solutions, remain underexplored. In this study, medium-entropy (TiVCr)2AlC coatings were synthesized by magnetron sputtering, and the effects of substrate bias and thermal annealing on their microstructure and phase stability were systematically investigated. It was found that higher bias voltages promote coating densification but also lead to Al depletion and the precipitation of a Cr2Al-rich second phase. Even initially, stoichiometric MAX-phase coatings undergo decomposition upon annealing, resulting in Cr2Al formation. This phase instability is strongly correlated with the large atomic-size difference (i.e., δ > 5.5) among M-site elements, which can introduce significant lattice strain. This correlation provides a key perspective for screening stable compositions in multielement MAX-phase design.