Indentation Creep of Molybdenum: Comparison Between Thin Film and Bulk Material
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K. B. Yoder*, D. S. Stone*, J. C. Lin** and R1 A. Hoffinan** *Department of Materials Science and Engineering, University of Wisconsin-Madison "**SurfaceTechnology Division, ALCOA Research Center, Alcoa Park, PA. ABSTRACT Indentation creep, load relaxation, and rate-change experiments probe room temperature and 80°C creep properties of a 1.3 Am-thick molybdenum film on silicon. The film, with 0.51 GPa compressive stress, 8 GPa hardness and estimated 40 um grain size, was deposited using steered-arc evaporation at -17V bias. Despite its small grain size and high hardness, the thin film behaves like bulk molybdenum does: the rate sensitivity of the hardness is only weaklydependent on measurement path (as with bulk material), and activation volumes calculated based on strain rate sensitivity are consistent with those of bulk molybdenum. We suspect deformation mechanisms are similar to those in bulk molybdenum under similar conditions. INTRODUCTION Hard coatings often have grain sizes below 10 um and defect concentrations exceeding equilibrium concentrations by many orders of magnitude. It is doubtful that the laws governing hardness and flow of bulk materials will straightforwardly extrapolate to such microstructures. By carefully studying hard coatings, materials scientists have the opportunity to examine new deformation phenomena. Recently, it has been proposed that materials scientists can use the rate- and temperaturedependence of the hardness to probe kinetics of low-temperature deformation in hard coatings, thereby gaining insight into deformation mechanisms. As a model system for other highmelting-point materials, bulk molybdenum has been studied to identify how phenomena familiar under uniaxial loading manifest themselves under indentation [1]. (A model system is required for the initial studies; otherwise, the difficulty of interpreting unfamiliar phenomena becomes compounded by uncertainties in the technique). It has been found, in this model system, that experimental results based on indentation display close similarities to data based on uniaxial loading. For instance, the hardness can be separated into components: H= H, +H*
(1)
where H, and H * are analogous to the athermal and frictional components of the flow stress, respectively [2]. This work is now being extended to explore the behaviors of thin molybdenum coatings. At this time the focus is not on novel microstructures. Instead, present studies focus on whether coatings, when deliberately deposited to produce bulk-like microstructures, have bulk-like properties. In this article, we investigate the creep properties of one such molybdenum film. We examine two aspects of the deformation: namely, whether the
651 Mat. Res. Soc. Symp. Proc. Vol. 356 01995 Materials Research Society
measured strain rate sensitivity is path-dependent, as it is in the bulk material, and what the magnitude of the strain rate sensitivity is, in comparison to that of a bulk material. EXPERIMENTAL WORK A specimen was first deposited under conditions producing metallurgical-
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