Metal ceramic interface toughness I: Plasticity on multiple length scales
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Metal ceramic interface toughness I: Plasticity on multiple length scales J. D. Kielya) and D. A. Bonnell The University of Pennsylvania, Philadelphia, Pennsylvania 19104 (Received 29 September 1997; accepted 13 March 1998)
The fracture toughness of Ni-sapphire interfaces was measured as a function of interfacial embrittlement. Embrittlement was controlled by segregating sulfur to the interface, by limiting the presence of moist air in the test environment, and by altering the distribution of interfacial particulates. Fracture energies scaled with the degree of embrittlement and ranged from 8.5 to 34.2 Jym2 . Scanning probe microscopy revealed four distinct plasticity features, the heights of which ranged from 1 mm to 0.5 nm. Plasticity generation processes are determined based on the variation of feature height and position with fracture energy, allowing features associated with the interface decohesion process to be identified.
I. INTRODUCTION AND BACKGROUND
Metal-ceramic interfaces are found in many advanced material applications ranging from structural composites to microelectronics and are of significant technological importance. Demands on the performance of metal-ceramic systems are application-specific, but a universal requirement is mechanical integrity. Strong metal-ceramic interfaces are generally desirable, but strength is affected by a number of extrinsic factors, including geometry, the existence of flaws, and the presence of interfacial reaction products. One goal of interfacial fracture mechanics is to eliminate extrinsic factors affecting strength in order to identify the intrinsic toughness of the interface. Understanding of the intrinsic resistance to fracture of metal-ceramic interfaces has advanced considerably in recent years, and the effects of many extrinsic factors have been identified. One of the strongest determinants of the fracture resistance is the dimension of the metal phase. The smaller the metal phase, the smaller the plastic deformation, and the smaller the fracture resistance.1–3 However, along with the reduction in fracture toughness comes an increase in the failure stress since hydrostatic stresses dominate deviatoric.4,5 A balance between fracture stress and fracture toughness must be found for engineering applications, focusing efforts on small volumes of metals embedded in ceramic matrices. A thin (10 –250 mm) layer of metal is additionally desirable for exploring fundamental relationships because small-scale yielding often applies, so fracture can be analyzed using two-dimensional linear elastic fracture mechanics. a)
Currently at Sandia National Laboratories, Albuquerque, New Mexico 87185-1413. J. Mater. Res., Vol. 13, No. 10, Oct 1998
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In addition to extrinsic factors, intrinsic properties of the interface, especially the work of adhesion, have great influence on behavior. Several researchers have demonstrated the high sensitivity of the fracture resistance on the work
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