Abstract
The objective of this study was to measure the fracture toughness of ZrN hard coatings and to investigate the effect of texture on the fracture toughness. Following Griffith's criterion and cohesive zone model, the stored elastic strain energy (Gs) inside the film derived from the elastic mismatch strain and the stress difference before and after cracking outset was used to evaluate the fracture toughness of hard coatings. For the proposed energy-based method, elastic constant was measured by nanoindentation, film thickness was determined from SEM cross-sectional image, and residual stress was obtained from the laser curvature method or XRD cos2αsin2ψ method. The results showed that for a ZrN coating with (111) texture coefficient of 0.71, the fracture toughness was estimated to be ranged from 23.9 to 42.4 J/m2. The hardness of the ZrN coatings was not changed with increasing film thickness. Stress gradient may play an important role in the fracture mode. Fracture of the ZrN coating may be initiated at a position of local maximum stress gradient accompanying with defects. The stress gradient in the ZrN coating may be originated from the competitive stress generation mechanisms. As the film thickness was above 4.2 µm, the (200) orientation increased in the strongly (111) textured ZrN coatings, which may be related to the stress relief. When the stored energy in the ZrN coating was higher than 31.6 J/m2, it was partly released accompanying with the stress relief. The release of stored energy was considered to be the driving force of texture inversion.