Microstructure and mechanical property evaluation of pulsed DC magnetron sputtered Cr-B and Cr-B-N films

The microstructure of the Cr-B-N thin films changed from a coarse columnar structure to a glassy and featureless morphology as the nitrogen content increased from 15.2 at.% to 54.5 at.%, whereas the corresponding structure developed from an amorphous state to a nanocomposite structure consisting of CrN nanograins and amorphous BN phases. It was found that high hardness, good tribological and brittle properties were obtained for the CrB2 coating. The hardness and elastic modulus of the Cr-B-N thin films decreased with increasing nitrogen content until the nanocomposite structure of nanocrystalline CrN grains and an amorphous BN matrix was formed. However, the hardening effect induced by the nanocomposite structure was limited due to the fact that the small CrN nanograins were surrounded by a thick intergranular soft amorphous BN layer. On the other hand, the fracture toughness and resistance against elastic strain to failure of the Cr-B-N coatings were effectively enhanced by the addition of nitrogen.