Time Dependent Debonding of Aluminum/Alumina Interfaces under Cyclic and Static Loading
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Time Dependent Debonding of Aluminum/Alumina Interfaces under Cyclic and Static Loading J. J. Kruzic, J. M. McNaney, R. M. Cannon, and R. O. Ritchie Materials Sciences Division, Lawrence Berkeley National Laboratory, and Department of Materials Science and Engineering, University of California, Berkeley, CA 94720 ABSTRACT The structural integrity of oxide/metal interfaces is important in many applications. While most attention has focused on the debonding of oxide/metal interfaces by conducting strength and fracture toughness tests, very few investigations have looked at time dependant failure of interfaces under cyclic or static loading. Tests have been conducted on sandwich specimens consisting of 5 - 100 micron thick aluminum layers bonded between either polycrystalline or single crystal Al2O3 to determine cyclic fatigue-crack growth, as well as static loaded moistureassisted crack-growth, properties of Al/Al2O3 interfaces. Under cyclic loading, crack growth was observed to occur predominantly by interfacial debonding, but was also observed to make excursions into the Al2O3. Static loading in a moist environment also caused interfacial cracks to deviate into the Al2O3 or alternatively to arrest. Due to the poor crack growth resistance of the Al2O3, cracks leaving the interface grew at faster rates than those at the interface. Trends in crack trajectories and crack growth rates are explained in terms of the degree of plastic constraint in the aluminum layer, the modulus mismatch, and the effects of environmental mechanisms. INTRODUCTION Oxide/metal interfaces can be found in many engineering applications including microelectronic packaging, multi-layered films, coatings, joints, and composite materials. In order to design reliable engineering systems using such bimaterial interfaces, the structural integrity of the interface must be maintained over the lifetime of the component. While much research has been conducted on the strength and fast fracture behavior of oxide/metal interfaces [1-15], previous work is limited to only a handful of studies that focused on time dependent crack growth at and near interfaces, specifically, fatigue crack growth under cyclic loading [1619] and moisture-assisted crack growth under static loading [20-23]. Additionally, layered material systems are quite common, and in metal layers, plastic deformation is often constrained by the oxide when plasticity extends across the entire layer during failure. Indeed, theoretical investigations have predicted that the plastic zone size, the crack tip opening displacement, and the stress state near a crack tip are affected by the change in plastic constraint of the metal layer due to variations in the layer thickness [14]. There have been, however, very few experimental investigations as to how failure mechanisms are affected by these changes [2,8,15,17]. Trends of increasing strength with decreasing layer thickness in the Al/Al2O3 system have been observed [2], while studies centered on fatigue crack growth properties showed no conclusive trend
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