Enhancing toughness of CrN coatings by Ni addition for safety-critical applications

CrN is one of the most important transition metal nitrides, used as protective and anti-wear coating in modern engineering applications. However, CrN coatings are typically brittle and susceptible to catastrophic failure. In this paper, CrNiN coatings with differing Ni contents were deposited on tool steel substrates using a closed field unbalanced magnetron sputtering system. The effects of Ni addition on the microstructure and mechanical characteristics of CrN thin films were studied by combining nanoindentation tests with cross-sectional transmission electron microscopy. A columnar structure was observed in the CrN coating. With an increase in Ni content, the resultant columnar grains exhibited a high aspect ratio. Ni additions to CrN were found to enhance its damage resistance. Notably, the CrN thin film deformed mainly by intercolumnar shear sliding, whereas plastic deformation was favored in the CrNiN thin films exhibiting high aspect ratio columnar grains. More significantly, this change of microstructure with enhanced Ni content led to improved damage tolerance, manifested by a higher load required for crack formation upon a sharp contact (expressed as 1/HE2), and the plastic energy absorbed during nanoindentation.