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.

Current tribocorrosion research of metallic materials and their surface protective coatings mainly focuses on their short-term properties, with test time of 0.5‒2.0 h and a sliding distance 50‒500 m, which may significantly deviate from the practical long-term service condition and thus cause a catastrophe of marine equipments. In this study, three carbon-based multilayer coatings (Ti/DLC, TiC_x/DLC, and Ti‒TiC_x/DLC) were deposited on S32750 substrates, and both short-term and long-term tribocorrosion behaviors were investigated. The experimental results indicate that the coatings substantially improve the tribocorrosion resistance of the S32750 stainless steel. During the short-term tribocorrosion test, TiC_x/DLC exhibited the best tribocorrosion resistance owing to its high hardness. During the long-term tribocorrosion test, however, Ti‒TiC_x/DLC coating indicated the best anti-tribocorrosion performance owing to its excellent fracture toughness together with high hardness. Moreover, under 5 N, Ti‒TiC_x/DLC can withstand a long-term test of more than 24 h. Additionally, under a higher load of 20 N, the Ti‒TiC_x/DLC with a corresponding sliding distance of approximately 1,728 m maintained a low friction coefficient of approximately 0.06. However, the coating was completely worn out; this is attributable to the formation of tribocorrosion products consisting of graphitized carbon and nanocrystalline Fe_xO_y.