Breaking the trade-off of hardness–ductility in (Cr _{1 −x} Mo _x ) ₂AlC MAX phase coatings via a hierarchical structure

The Cr₂AlC MAX phase offers a remarkable combination of excellent electrical conductivity and hot corrosion resistance in extremely harsh environments. However, the strong trade-off between hardness and toughness is rather limited by its nanolaminate structure for desired applications. Taking the solid solution strengthening and gradient hardening synergy, in this work, high-purity Cr₂AlC coatings with various Mo solid solutions were successfully fabricated via a hybrid sputtering technique followed by subsequent annealing. Interestingly, gradually changing the Mo concentration in the (Cr_1−xMo_x)₂AlC (x = 0.05–0.24) coating enabled a hierarchical structure responsible for gradient refinement of the crystal grain size, and the solid solution of Mo atoms at Cr sites and the gradient variation in the Mo content were confirmed via the atomic-resolution transmission electron microscopy (TEM) characterization. Compared with those of the pristine Cr₂AlC coating, the nanoindentation hardness and toughness values of H/E and H³/E² for the hierarchical (Cr_1−xMo_x)₂AlC coating were enhanced by approximately 26%, 12%, and 57%, respectively. On the basis of comprehensive experiments and ab initio simulations, the reasons behind this observation were mainly attributed to the synergistic effect of Mo occupancy with strong bonding at the Cr site and the strengthening of grain refinement induced by the gradient Mo concentration in the (Cr_1−xMo_x)₂AlC coating. These findings not only reveal the underlying mechanism for the Mo solid solution in the Cr₂AlC coating but also offer a new concept for developing ultrahigh-strength ductility materials for the laminar MAX phase.