Abstract
Micro-Vickers indentation tests in conjunction with acoustic emission (AE) monitoring were performed for TiN films deposited on three kinds of substrates, soft and hard steel by physical vapor deposition (PVD) method. We aimed to evaluate the intrinsic strength of thin films. In this study, three types of substrate with different hardness were employed. One was a WC-Co cermet, denoted as hard substrate, while the others were mild steel (SS400) and metastable austenitic stainless steel (SUS304), denoted as soft substrate. Equidistance step-wise terrace, or rippled cracks were observed and detected AEs only for soft substrates materials. Detected AE waveforms were analyzed using the polarity pattern in order to classify the cracking behavior. Most AEs detected were produced by Mode-I cracking with opening vector in the direction parallel to the film surface. Internal stress in the films was analyzed by the finite element method (FEM) to study the influence of substrates on the fracture mechanism. It was revealed that bending stress was developed by indenter penetration and it induced rippled crack initiation for the soft substrate materials. It was also found that the rippled cracks initiated at a certain distance of spacing. Using the FEM and the value of cracking space (Lc), we proposed the method to determine the bending strength of the thin films.
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