Mechanical and fracture toughness studies of amorphous SiC–N hard coatings using nanoindentation

Abstract

a: SiC x – N y films were deposited on Si(111) and sapphire substrates at ambient temperatures by reactive magnetron sputtering of SiC in a mixed Ar/N2 discharge. Nanoindentation was used to characterize the mechanical (hardness and elastic modulus) and fracture (toughness) properties. A conventional sharp Berkovich diamond indenter (three-sided pyramid) was used to assess the mechanical properties. The hardness and elastic modulus slightly increased from 23 and 220 GPa to 27 and 270 GPa with incorporation of N2 (up to 4.4%). The fracture toughness (Kc) was characterized by replacing the aforementioned indenter with a cube corner indenter, which also is trigonal but has the geometry of a cube corner with a smaller angle between the axis of symmetry and a face. The purpose of the switch was to reduce the much higher cracking thresholds associated with the Berkovich and Vickers indenters. The smaller volume of thin films requires this reduction for reliability. The nucleation and propagation of median-radial indentation cracks was made feasible by the cube corner geometrically displacing more volume than the Berkovich under similar loads. A substrate fracture toughness effect was observed dependent upon indenter penetration depth. Fracture toughness was relatively constant (≈3.1 MPa√m) from 19 (threshold load) to 40 mN; however, loads ⩾50 mN resulted in a decrease to ≈2.4 MPa√m since the greater penetration depths approaching the sapphire substrate played a role. The structure, composition, and bonding of the films were qualitatively and quantitatively assessed by x-ray diffraction, energy dispersive x-ray spectroscopy, and micro-Raman spectroscopy, respectively, and subsequently correlated to the overall film properties.