Evaluation of the fracture toughness of Ti1-xZrxN hard coatings: Effect of compositions

Abstract

The objectives of this study were to evaluate the fracture toughness of Ti1-xZrxN hard coatings using the internal energy induced cracking (IEIC) method and to investigate the compositional effect on the fracture toughness, from which the optimum composition for fracture toughness could be attained. Ti1-xZrxN was selected to be the model system, because Ti1-xZrxN remained single phase structure in the entire compositional range at deposition temperature below 500 °C. Three compositions of Ti1-xZrxN coatings, x = 0.25, 0.55 and 0.85, were deposited by unbalanced magnetron sputtering. The parameters of IEIC method included the residual stress determined by the laser curvature method, Young's modulus obtained from nanoindentation and the film thickness measured from SEM cross-sectional image. The residual stress and film thickness before specimen fracture were used to calculate the elastic stored energy (Gs), from which the fracture toughness (Gc) could be derived. The resultant Gc of the Ti1-xZrxN coatings varied with Zr fraction, ranging from 26.0 to 48.7 J/m2, and reaching a maximum at a composition of Ti0.15Zr0.85N. The results showed that adding Zr atoms into TiN could effectively increase the fracture toughness. The maximum increase of fracture toughness lay in the intermediate range of composition (Zr = 0.55–0.85), suggesting that different properties of Ti and Zr atoms may play an important role on the fracture toughness of single phase Ti1-xZrxN thin films. The increase of Gc with Zr composition may be correlated with the change of configurational entropy of the Ti-Zr-N system. The atomic size difference of Zr and Ti may be crucial on increasing fracture toughness. The increase of fracture toughness for Zr-dominant Ti0.15Zr0.85N coating was higher than that for Ti-dominant Ti0.75Zr0.25N coating. This asymmetrical behavior could be attributed to the difference in lattice constants between Ti-rich and Zr-rich compounds, where the capability of increasing Gs may be higher for a smaller Ti atom incorporated into a larger Zr site in ZrN lattice.