Enhanced hardness and plasticity of Cr–CrN coatings with periodic gradient structures

The significant difference of mechanical properties between metal and ceramic often causes plastic deformation incompatibility of metal/ceramic multilayered coatings, leading to premature cracking. To address this issue, we proposed a periodically graded coating architecture for Cr-CrN multilayers that achieve superior hardness and enhanced plasticity compared to conventional multilayers. Using high-power impulse magnetron sputtering (HiPIMS), we precisely controlled cyclic modulation of N₂ gas flow to realize periodic nitrogen content variations from 0 to ~50 at.% and back to 0 within each deposition cycle. The optimized graded coating with a modulation period of ~149 nm exhibits a maximum hardness of 22.54 GPa and a critical cohesive failure load of ~35 N in scratch tests. By analyzing cross-sectional morphologies at different scratch loads in combination with tensile strain–crack density–contact stress evolution, the plastic deformation capacity of the coatings was further determined. The superb mechanical properties are attributed to the gradient nitrogen–Cr layer, in which solid solution of N in Cr increases the interface-affected zone thickness and mitigates excessive stress localization at sharp phase boundaries, while simultaneously promoting grain refinement. The results establish periodic gradient design as a promising strategy to overcome the inherent hardness-plasticity trade-off in conventional multilayer coatings.