Microstructure and mechanical properties of a multilayered CoCrNi/Ti coating with varying crystal structure

Medium entropy alloys (MEAs), such as CoCrNi, have been demonstrated to combine high hardness and excellent ductility, thereby outperforming many high entropy alloys reported to date. In this study, a multilayered CoCrNi/Ti coating was deposited onto a M2 steel substrate using a DC magnetron sputtering system. Columnar grains can be observed in both the CoCrNi and Ti layers. A high density of periodic twin boundaries, aligned in a direction normal to the growth direction, was also observed within the columnar CoCrNi grains. Moreover, different crystal structures were identified for different CoCrNi layers. The outermost CoCrNi layer exhibited a FCC structure, whilst in contrast, both the middle and bottom CoCrNi layers exhibited a BCC structure. It was assumed that Shockley partial dislocations were responsible for the FCC to BCC transition occurring in both the bottom and middle CoCrNi layers. A high hardness of 7.6 GPa and elastic modulus of 233 GPa were determined for this multilayered coating by nanoindentation testing. Further, extraordinary damage tolerance was found in the multilayered CoCrNi/Ti coating under indentation loading. The steady shear banding behaviour during deformation may benefit energy dissipation and promote structural plasticity.