Indentation behavior of a Brittle Film/Brittle Substrate Composite
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INDENTATION BEHAVIOR OF A BRITTLE FILM/BRITTLE SUBSTRATE COMPOSITE C. M. CZARNIK, R. GIBALA, 0. BARON, M. NASTASI* AND T. R. JERVIS* The University of Michigan, Department of Materials Science and Engineering, Ann Arbor, MI 48109-2136 *Los Alamos National Laboratory, Los Alamos, NM 87545 ABSTRACT Surface-film induced plasticity caused by the generation of dislocations at the filmsubstrate interface has been demonstrated previously for bcc metals and B2 ordered alloys deformed at low homologous temperatures. More recently, we have observed similar effects at elevated temperatures for ZrO 2-coated MoSi 2, which also demonstrates a film-induced reduction in hardness for microhardness tests over 250 C - 1300 0C. In this investigation, 120 nm - 480 nm thick ZrO2 films were deposited on (001) single crystal MoSi 2 by electron beam deposition. These composites were used to characterize the film-induced softening of MoSi 2 at room temperature, where both MoSi 2 and ZrO 2 are brittle. Indents were made through the film into the substrate at 0.5 kgf loads using Vickers and Knoop indenters. Atomic Force Microscopy was employed to measure the geometry of Knoop indents after load removal, and to compare the results to the loaded indentation shapes. Increasing film thickness reduces the length of radial cracks in the substrate and lowers the composite hardness. The film also changes the relative amounts of deformation associated with material pileup at the surface near the edges of the indent, suggesting a change in deformation mechanism from pileup around the indenter edges to deformation by dislocation motion in the bulk. INTRODUCTION MoSi 2 exhibits properties of excellent oxidation resistance, low density, high thermal conductivity, high stiffness, and potential high strength at elevated temperatures. However, limited plasticity at low and intermediate temperatures (1300'C) have been major obstacles to its widespread use. Compression testing of [001] MoSi 2 single crystals at 1250 0 C indicates very small (
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