Detection of Fatigue Degradation in Functional Thin Surface Film Layer and Its Inspection System

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Functional surface films have been practically used in the field of High-Tech electronics devices and as protective coating materials of the products which suffer from corrosive environment. Such thin surface films are inevitably exposed to the condition of cyclic strain resulting from various kinds of heat treatment in their production process, and this induce the cracking and/or decohesion of the film. However, no inspection method has been developed in order to evaluate the important mechanical properties of the film such as fatigue strength or bond strength. The aim of the present research is to develop the noble testing method to examine the fatigue properties of the thin surface film.The indentation loading which has been used for hardness measurement was evolved into the cyclic loading system to the specimen surface using a ball indenter. This succeeded in developing intensively cyclically strained micron-scale region beneath the specimen surface. An age-hardened aluminum alloy was cyclically loaded in this testing system, and the formation of the selective fatigued region (Persistent Slip Band) was successfully recognized at the expected position. The position was consistent with the theoretical calculation.A non-linear component, corresponding to the cyclic plastic strain was extract from the experimentally measured stress-strain response using an acoustic wave analysis method. It was demonstrated that the non-linear component relate to the film thickness and the fretting condition between the film and the ball. Constructing databases for various kinds of films is required to improve the reliability of the presented testing method.