Fracture toughness measurement on TiN hard coatings using internal energy induced cracking

The purpose of this research was to develop a new energy-based method to measure the fracture toughness of hard coatings using internal energy induced cracking (IEIC). TiN coating was selected to be the model material due to its well-established mechanical properties and nearly elastic isotropy. TiN film was continuously deposited until reaching the thickness at which the film was fractured due to stored energy accumulation. The residual stress before crack initiation was used to evaluate the stored energy (Gs), from which fracture toughness (Gc) can be derived. The residual stress of the TiN coating was measured by laser curvature method and the Young's modulus of TiN coatings was determined by nanoindentation. The resultant fracture toughness of random-textured TiN coatings was 16.5 J/m2, which was comparable to the data from previous literatures. A relationship between critical thickness and residual stress was derived from the fracture toughness Gc, which can be used to control the coating thickness by adjusting the residual stress. The advantages of this method are without externally applying stress and special sample preparation, and the substrate effect can be avoided. However, there are some disadvantages in applying the IEIC method. To insure that the fracture process is within the film, good adhesion and high residual stress are required. In addition, this method can only be applied to hard coatings due to the requirement of small plastic zone size.