Tuning the microstructure and mechanical properties in nanocrystalline Cr coatings by sputtering power

A significant sputtering power effect on the microstructure and mechanical properties was investigated in the nanocrystalline Cr coatings that were prepared by employing magnetron sputtering. Experimental results demonstrated that all the Cr coatings exhibited a columnar grain structure with preferred (110) out-of-plane orientation. However, with raising the sputtering power from 50 to 250 W, the surface morphology changed from the faceted pyramid to vermiform-like shape due to the anisotropy of growth rate along different crystallographic plane. Meanwhile, raising sputtering power also increased the grain size and coating thickness, between of which followed a power-law relationship, indicative of an evolutionary overgrowth mechanism. In addition, both the nanoindentation hardness and elastic modulus of Cr coatings exhibited a monotonic increment with reducing the grain size. This size-dependent strengthening behavior was rationally interpreted by the classical Hall-Petch theory, and a grain coalescence model for columnar grain morphology was developed to quantitatively describe the size effect on elastic modulus of nanocrystalline Cr coatings.