A Theory for Multi Damage Evaluation of TiN Thin Film
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A Theory for Multi Damage Evaluation of TiN Thin Film Kazunori Misawa1, Tomonaga Okabe2, Masaaki Yanaka3, Masao Shimizu1, Nobuo Takeda2 1
Department of Mechanical Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 230, Japan 2 Department of Advanced Energy, Graduate School of Frontier Sciences, The University of Tokyo, c/o Komaba Open Laboratory (KOL), Takeda Lab., 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan 3 Toppan Technical Research Institute, Sugito-machi, Kitakatsushika-gun, Saitama 345, Japan
ABSTRACT The present paper experimentally studies the cracking phenomena of a TiN thin film on a titanium alloy, and presents a new approach to predict the number of cracks under tensile load. An elastoplastic shear-lag model is developed to obtain the stress distribution caused by the film cracks, which is found to agree well with that calculated with FEM. The number of thin film cracks is predicted using a Monte Carlo simulation using the present approach, and favourably compared with the experimental results. INTRODUCTION Engineering titanium alloys have attractive mechanical properties and have been used in the aerospace field. However, its poor wear characteristics prevent titanium alloys from being widely used as an engineering material. Then, the surface treatment has been required to overcome this problem. One of such techniques is the thin film coating. However, many studies[1, 2] have already reported that the multiple film cracks are generated by the residual thermal stress and/or the applied load, and can be the cause of the decrease in wear resistance. The main aim of the present study is to experimentally investigate the crack phenomena of a TiN thin film on the titanium alloy substrate and to propose an appropriate crack evolution model. In the present paper, we propose an elastoplastic shear-lag model for a bi-material (Ti-alloy/TiN) plate. This model can easily and accurately calculate the stress distribution in the film on the elastoplastic substrate. Utilizing the present model, a Monte Carlo simulation is conducted for predicting the number of film cracks based on the strength distribution of the observed initial cracks. EXPERIMENTAL PROCEDURE A TiN film was deposited on Ti-alloy flat plates (SP-700 (NKK Corporation)) with dimensions of 100 mm x 10.0 mm x 2.0 mm by the arc ion plating method. The
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thickness of TiN film was 1.7 µm. A uniaxial tensile test was performed at a constant crosshead speed of 0.4 mm/min. EXPERIMENTAL RESULTS Figure 1 shows crack density versus the applied strain εc for film thickness 1.7 µm. The crack density was defined as the number of cracks per unit length (1mm). An initial crack was observed at about εc = 0.2 %. The crack density kept increasing as the applied strain increased, and was saturated at about
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