Abstract
The objectives of this research were to measure the fracture toughness (Gc) of Zr(N,O) hard coatings using internal energy induced cracking (IEIC) method, and investigate the effect of oxygen on the fracture toughness of the Zr(N,O) coatings. Zr(N,O) coatings were deposited by unbalanced magnetron sputtering with different oxygen flow rates while maintaining constant nitrogen and argon flow rates. The results showed that Gc was texture dependent; for ZrN coatings with (111) texture was 26.7±2.1 J/m2 while that for coatings with (200) texture was 9.0±0.9 J/m2. The fracture toughness of Zr(N,O) coatings with small amount of zirconium oxynitride was between 8.7±0.9 and 8.9±0.6 J/m2. The oxynitride could decrease the residual stress which was the driving force for crack propagation, and thereby allowing the critical thickness to increase. Therefore, the fracture toughness remained nearly the same as that of ZrN with (200) texture. Both oxygen flow and oxygen contents were found to affect the structure and the accompanying properties of the Zr(N,O) coatings. Oxygen flow mainly disturbed the migration of Zr adatoms and the subsequent film formation; consequently, the texture of Zr(N,O) films switched from (111) to (200), and the electrical resistivity increased because of the decrease of crystallinity due to the effect of oxygen flow. The oxygen contents of the Zr(N,O) films increased, but not substantially, with increasing oxygen flow rate. Since lower energy processing parameters were used in this study, the formation of oxide or oxynitride phases was not as distinct as that in our previous studies. The fractography showed that the main crack propagated stepwise instead of by surface spallation, which could be due to the stress distribution in the specimen where the edge stress was much larger than the shear stress induced by the in-depth stress gradient.