The brittleness of hard ceramic materials poses a significant challenge to their practical application because of 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 the hardness and fracture toughness increased simultaneously 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, whereas the exceptional toughness was attributed to the amorphous-nanocrystalline structure in the surface oxidation layer and the gradient structure of the coatings. These findings provide a pioneering approach based on alloying and oxidation for developing hard yet tough MAX phase coatings and other ceramic materials.

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.

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.