ROLE OF NONDESTRUCTIVE EVALUATION FOR THE PURPOSE OF THE UNDERSTANDING FRACTURE MECHANISM Y. J. Park１*, M. Enoki１, T. Suga２ and T. Kishi３ １Department of Materials Science, School of Engineering, The University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ２Research Center for Advanced Science and Technology, The University of Tokyo 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan ３National Institute for Materialｓ Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ABSTRACT The bonded area is largely affected by the surface morphology and mechanical properties of materials by surface activated bonding (SAB) process, because the room temperature bonding process is carried out under a low temperature and low pressure for short time. Accordingly, interfacial defects cause fatally harmful problems in many cases. For an application of this technique, it is important to know the effect of interfacial defects on fracture behavior. The fracture mechanism and its criterion for the growth of interfacial defects were investigated using the Al/Sapphire joint. It became clear that the growth of interfacial defects is the dominant factor for crack propagation. An estimation of stress intensity factor for the growth of interfacial defects was tried in two ways, stress criterion and energy criterion. A critical stress intensity factor for the growth of interfacial defects was estimated as 0.3-0.5MPam1/2 by the analysis of FEM calculation and experimental observation. KEYWORD room temperature bonding, interfacial fracture, fracture criterion, FEM INTRODUCTION With the traditional method of joining the metals, semiconductors and ceramics, diffusion and reaction driven by heat at high temperature provided bonding with the joint [1-3]. Some harmful effects due to the process involving heating, such as the generation of residual thermal stress [2,4] and the formation of brittle reaction products [3,5] at the interface region, on mechanical and functional property have been noticed as the barrier for the application. In recent years, a novel method of Surface Activated Bonding (SAB) was invented [6-8] and its engineering importance is now being emphasized. The basic concept underlying this technique is that two atomically clean solid surfaces under contact show a strong adhesive force [9]. The surface exposed to ambient atmosphere is covered with oxides and adsorbed layers. By means of sputtering or radical beam treatment in an ultra high vacuum, impurity layers are removed so that a clean surface emerges. Therefore, direct bonding at atomic level is acquired between high-energy state pure surfaces. Because work of adherence between dissimilar solid materials is positive except in extremely rare cases, it is regarded that bonding is achieved only if they are in intimate contact of atomic bonding distance. The SAB process brought about innovative benefits because all the processes are carried out under low temperature and pressure. That is, this technology is free from the interfacial problems pointed out at high temperature and also enables to bond electronic devices that are not allowed to be heated. Until now, research activities have been concentrated on the study of the bonding mechanism and on the investigation into the factors affecting mechanical properties. On the theoretical side, calculations of atomic and electronic structures of interfaces indicating the bonding type have been fruitful [10,11]. There have been the rigorous attempts to understand the phenomena of frictional adherence under high vacuums, like space [6,9,12]. On the other hand, the report was restricted to the results of joint strength as a function of bonding conditions [7,8]. Consequently the situation is that the knowledge on fracture mechanism and its criterion are almost unknown. From the comparison between measured bonded area and finite element method (FEM) simulated one for Al/Sapphire system, authors have already made clear the process of contact deformation in a quantitative manner [13]. Bonded area is largely affected by the surface morphology and mechanical properties of materials, because the SAB process is carried out under a low temperature and pressure for a short time. Accordingly, it can be generally stated that the existence of interfacial defects (unbonded parts) is an unavoidable problem. And moreover, it is already known that the mechanical properties of bonded materials, such as fracture behavior and fracture energy, depend on the bonded area to a great extent [14]. It is commonly agreed that elucidation of the effect of interfacial defects on fracture behavior is the most necessary task for an application of this technology. In this research, fracture mechanism and its criterion of Al/sapphire joint were investigated by a fracture mechanical approach. FABRICATION OF SPECIMEN AND TEARING-OFF TEST
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