Breaking the trade-off of hardness-ductility in (Cr_1−xMo_x)₂AlC MAX phase coatings via hierarchical structure

Cr₂AlC MAX phase offers a remarkable combination of superior electrical conductivity and hot corrosion resistance in extremely harsh environments. However, a 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, the high-purity Cr₂AlC coatings with various Mo solid solutions were successfully fabricated by a hybrid sputtering technique followed with a subsequent annealing. Interestingly, changing the Mo concentration gradually in (Cr_1-xMo_x)₂AlC (x = 0.05-0.24) coating enabled a hierarchical structure responsible for gradient refinement of crystal grain size, and the solid solution of Mo atoms at Cr-sites and the gradient variation of Mo content were evidenced by the atomic-resolved transmission electron microscopy (TEM) characterization. Comparing to the pristine Cr₂AlC coating, the nanoindentation hardness and toughness relevant values of H/E and H³/E² for hierarchical (Cr_1-xMo_x)₂AlC coating was enhanced approximately 26 %, 12 % and 57 %, respectively. Based on the comprehensive experiments and ab initio simulations, the reasons behind of this observation were mainly attributed to the synergistic effect of Mo occupancy with strong bonding at Cr-site and grain refinement strengthening induced by the gradient Mo concentration in (Cr_1-xMo_x)₂AlC coating. The findings not only provide the underlying mechanism for Mo solid solution in Cr₂AlC coating, but also offer a new concept to develop ultrahigh strength-ductility for required laminar MAX phase materials.