Abstract
In last two decades, extensive studies have been dedicated in establishing a standard method on toughness measurement for thin film materials. However, there has no standard methodology or test procedure up to now. Stress based or energy based methods have been proposed on this subject, where the externally applying stress is usually required, and therefore special substrate geometry is designed, or micro-scaled strain gauge or atomically sharp cracks need to be fabricated. In addition, the stress distribution in the specimen should be monitored during the measurement. These requirements may introduce the complicated substrate effect and thereby increasing the difficulty of fracture toughness measurement. This research was in an attempt to develop a new method without applying external stress for measuring fracture toughness of hard coatings. TiN was selected as a model material, owing to its well-established mechanical properties and nearly elastic isotropy. The proposed method involved residual stress measurements by XRD and laser curvature methods, and Young’s modulus obtained from nanoindentation. The difference of stress before and after crack initiation was used to evaluate the average storage energy (Gs), from which fracture toughness was derived. The results showed that the fracture toughness of random-textured TiN coatings was 16.7J/m2, which is comparable to previous reported data. The integrated stored energy (GIS) was assessed from Gs by considering the stress gradient measured from cos2αsin2ψ XRD method accompanying with layer-by-layer analysis. GIS can be regarded as the local energy distribution and is a guidance of local fracture location. GIS distribution was found to be consistent with the fracture morphologies of the coatings. Furthermore, the fracture toughness can be used to determine the critical thickness which is useful in thickness control. A new elastic constant named AEXEC (average effective X-ray elastic constant) was suggested, which can reduce the statistical fluctuation in stress measurement. The AEXEC also provides a simple and nondestructive way to acquire reliable XECs that are comparable to those determined by nanoindentation.