The role of plasticity in bimaterial fracture with ductile interlayers
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I. INTRODUCTION ADHESION of thin metallic films is one of the most important reliability issues in microelectronics. A device may fail due to poor adhesion even if the material of the film itself satisfies the design criteria. Numerous mechanisms have been identified as fundamental to the adhesion of thin films and debonding of metal/ceramic interfaces. Interfacial fracture energy, G0, an energy necessary to create free surfaces from a bonded interface, is determined primarily by the true interfacial surface energy, a quantity commonly measured by the contact angle technique.[1] A variety of microstructural factors in the fracture process zone would also affect G0. These include mechanical interlocking, phase intermixing, and/or existing defects along an interface. Interfacial fracture energy may be evaluated by delaminating a thin film from the substrate. However, before a thin metallic film debonds from the substrate, it usually experiences plastic deformation. It is difficult to extract the interfacial fracture energy from the total energy measured. What is measured is the practical work of adhesion. In terms of the critical strain energy release rate, Gcrit, i.e., elastic energy released per unit of fracture area, Gcrit 5 G0 1 Gp 1 other forms of dissipation
[1]
Here, Gp is the crack tip plastic energy dissipation rate. The stronger the interface is (higher G0) the more energy is N.I. TYMIAK and A.A. VOLINSKY, Graduate Students, and W.W. GERBERICH, Professor, are with the Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455. M.D. KRIESE, Research Scientist, is with Osmic Inc., Troy, MI 48084. S.A. DOWNS, Applications Scientist, is with Hysitron Inc., Minneapolis, MN 55439. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
dissipated through plastic deformation. Compared with the other dissipation terms, Gp is prevalent except, possibly, for very thin films and/or high interfacial roughness where frictional losses may be significant.[2] Plastic energy dissipation is suggested as a dominant mechanism involved in the increase of the experimentally measured interfacial fracture toughness with the film thickness increase.[3,4] For a Cu/ SiO2 interface, this trend was observed for adhesion determined with the different methods.[4] However, there is a possibility that intrinsic interfacial strength increases with the increasing film thickness as a consequence of differences in deposition/processing involved in obtaining different film thicknesses. Also, with increasing film thickness, mode mixity may change. The experimentally measured practical work of adhesion generally increases with the crack tip becoming more heavily under mode II conditions.[5] Increasing frictional[2] and/or plasticity[6] losses are suggested
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