Microstructure and nanomechanical properties of cubic boron nitride films prepared by bias sputter deposition

Abstract

Nanoindentations, using a triangular based cube-corner shaped diamond tip, were tried to study the mechanical properties of cubic boron nitride ( cBN) films. Since the cBN film has a layered structure consisting of an initial sp 2-bonded BN (amorphous BN and turbostratic BN) layer and a sp 3-bonded BN ( cBN) layer, which was previously studied by cross-sectional transmission electron microscopy (TEM), cBN films in different growth stages were prepared for the measurements. Hardness and elastic modulus were evaluated as a function of the penetration depth from the obtained loading-unloading curves using a method described by Oliver et al. The maximum hardness and elastic modulus of a 110-nm thick cBN film (including an approximately 50-nm thick initial layer) were approximately 10 and 3 times, respectively, larger than the values evaluated for a silicon wafer substrate. The dependence of the evaluated hardness and the elastic modulus on film thickness and penetration depth can be explained in terms of the layered structure of the cBN film, i.e. soft initial sp 2-bonded BN layer with low elastic modulus, and hard cBN layer with high elastic modulus.