Measurement of Plasticity Gradients with Scanning Probe Microscopy
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JAMES D. KIELY AND DAWN A. BONNELL Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104 ABSTRACT
Scanning Tunneling and Atomic Force Microscopy were used to characterize the topography of fractured Au/sapphire interfaces. Variance analysis which quantifies surface morphology was developed and applied to the characterization of the metal fracture surface of the metal/ceramic system. Fracture surface features related to plasticity were quantified and correlated to the fracture energy and energy release rate. INTRODUCTION Plasticity gradients are critical to many aspects of micromechanical behavior including persistent slip bands, crack tip elastic fields, and fracture process zones. Despite their importance, characterization is incomplete since the region over which they exist is often difficult to access experimentally. The advent of scanning probe microscopies allow the topography of plastic zones to be probed on relevant length scales. The topography may be quantified and related to fracture mechanisms through parameters such as roughness and feature length. Previous mathematical calculations of surface roughness are deficient in that they are either length scale independent, e.g. single RMS measurements, or rely on fractal descriptions of the surface. The need exists for a more general approach for quantifying variations in both amplitude and characteristic feature length in scanning probe microscopy images. The objective of this project is to develop the necessary mathematical techniques and to demonstrate their efficacy on a problem of current interest, the fracture of metal/ceramic interfaces. After a description of the sample preparation and the scanning probe microscopy methods, we discuss the strengths and limitations of our technique and the correlations we have found between the topography of fractured gold/sapphire interfaces and their fracture behavior. EXPERIMENTAL
Gold/sapphire was chosen as a model plastic/elastic system since its fracture characteristics have been well characterized [1,2]. The gold/sapphire interface fails in a quasi-brittle manner, i.e. decohesion occurs at the crack tip as it advances along the interface with high velocities while plastic dissipation occurs in the gold [2]. Results are presented from interfaces bonded in two geometries. Failure under well-characterized shear load was obtained from the U.C.S.B. diffusion-bonded 401 Mat. Res. Soc. Symp. Proc. Vol. 318. ©1994 Materials Research Society
laminate configuration, in which a 100gm gold foil was constrained between two sapphire plates. The specimens were indented and then fractured in air and in dry N2. The influence of edge effects was determined by indenting and then loading a 100gim gold foil diffusion bonded between two sapphire beams in four-point bend in ambient air. Prior to failure, slip lines were observed in the gold foil near the tensile edge, indicating that appreciable plastic flow occurred. The interface failed in a quasibrittle manner with no gold remaining
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