Exceptional mechanical properties of (Ti _{1− x} Zr _x ) ₂ AlC MAX phase coatings with amorphous-nanocrystalline oxidation layers

The brittleness of hard ceramic materials poses a significant challenge to their practical applications due to the trade-off between hardness and toughness. Here, we propose a hierarchical structure strategy that utilizes alloying and vacuum oxidation to fabricate MAX phase coatings with an amorphous-nanocrystalline oxidation layer on the surface. Hierarchical (Ti_1-xZr_x)₂AlC coatings (x = 0.05~0.18) were prepared via magnetron sputtering followed by vacuum annealing and exhibited enhanced hardness and fracture toughness with increasing Zr content. A maximum hardness of 19.4 GPa and fracture toughness of 4.1 MPa·m^1/2 were achieved in the (Ti_0.82Zr_0.18)₂AlC coating, significantly surpassing previously reported MAX phase coatings. The enhanced hardness primarily originated from the formation of solid solutions of Zr at the M-site and second phase Zr₃Al₂, while the exceptional toughness was attributed to the amorphous-nanocrystalline structure in the surface oxidation layer and the gradient characterization of the coating. These findings provide a pioneered approach based on alloying and oxidation for developing hard yet tough MAX phase coatings and other ceramic materials.