Surface Film Softening Effects in MoSi 2
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SURFACE FILM SOFTENING EFFECTS IN MoSi 2 C. M. CZARNIK, R. GIBALA, M.A. NASTASI*, R. B. SCHWARZ*, S. R. SRINIVASAN*, J. J. PETROVIC* The University of Michigan, Department of Materials Science and Engineering, Ann Arbor, MI 48109-2136 and *Los Alamos National Laboratory, Los Alamos, NM 87545
ABSTRACT Surface film softening, which is associated with the generation of mobile dislocations at a film-substrate interface and their motion into the substrate, has been demonstrated previously for bcc metals and B2 ordered alloy substrates. MoSi 2 exhibits features of limited dislocation mobility and plasticity similar to these materials and would also be expected to exhibit film-enhanced plasticity, but at much higher temperatures. In this investigation, 150 nm thick films of Si02, MoSiý and ZrO 2 were deposited on MoSi 2 substrates by electron beam deposition at -500C in an attempt to observe film softening. In initial experiments, hot hardness testing was utilized primarily to maintain deformation conditions characteristic of dislocation glide. ZrO -coated MoSi 2 demonstrates a large film-induced softening in hardness tests carrie"W out at 0.5 - 1 kgf loads over the temperature range 25 - 13000C.
INTRODUCTION MoSi? has properties of high stiffness, high thermal conductivity, low density, excellent oxidation resistance and potential high strength at elevated temperatures which make it attractive for investigation as a structural material. However, a major drawback to MoSi 2 has been its lack of ductility at temperatures to about 11000C. The brittle-to-ductile transition temperature (BDIT) is generally reported to be in the range of 900-1400 oC [1], and can be attributed to the onset of dislocation climb and/or creep. There have been attempts to reduce the BDTT of this material or to enhance its plasticity, including second-phase toughening, grain size manipulation, and solid solution alloying [2]. In this paper, we examine surface film softening as a possible method to increase plasticity in polycrystalline and single crystal MoSi2 . This phenomenon has been demonstrated in many bcc metals and B2 ordered alloys at relatively low homologous temperatures T< 0.2 Tm, where Tm is the absolute melting temperature. At these temperatures, dislocation glide mechanisms operate exclusively [3]. The interface between the surface film and substrate acts as an operative dislocation source, in effect pumping dislocations into the material under applied stress. Deformation of the film-substrate composite introduces geometrically necessary dislocations in the substrate. If these dislocations are also mobile, enhanced plasticity may result. Enhanced plasticity has been demonstrated for bcc metals and B2 ordered alloys in tension, compression and cyclic deformation experiments [4,5]. In the case of NiA1, for which additional plastic strains on the order of10% have been reported, an A12 03 surface oxide film has the effect of generating dislocations at the interface which then move into the substrate material. In the case of MoSi 2 , the re
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