A mechanism for reactive diffusion between Si single crystal and NbC powder compact
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A mechanism for reactive diffusion between Si single crystal and NbC powder compact C. R. Kao Department of Chemical Engineering, National Central University, Chung Li, Taiwan
J. Woodford and Y. A. Chang Department of Materials Science and Engineering, University of Wisconsin– Madison, Madison, Wisconsin 53706-1595 (Received 23 August 1995; accepted 27 December 1995)
Based on our recent experimental observations, a growth mechanism for the reactive diffusion between Si single crystal and NbC powder compact is proposed. In Si–NbC diffusion couples annealed at 1300 ±C, a two-phase NbSi2 1 SiC reaction layer formed with NbSi2 as the matrix and SiC as discontinuous particles. The NbSi2 grain sizes and SiC particle sizes are both in the mm range. We propose that the SiC particles nucleated at the void surfaces in the NbC powder compact. This proposed nucleation mechanism offers a potential way of controlling the SiC particle size by changing the void size and void density of the NbC powder compact. It is also pointed out that this microstructure requires Si to be the dominant diffusing species. Si must diffuse through the reaction layer, while C only has to undergo local rearrangement, and Nb need not diffuse at all.
I. INTRODUCTION
Refractory metal silicides and their composites are being explored as potential high temperature materials in the aerospace industry and elsewhere due to their high melting points, good high-temperature strength, and high-temperature oxidation resistance. Henager et al.1–4 synthesized a MoSi2 –SiC composite in situ using a solid-state reaction between Mo2 C and Si. This approach may offer a potential route for making high-temperature structural composites economically. The thermodynamic and kinetic principles governing this type of in situ composite synthesis via solid-state reactions have been given elsewhere.5,6 However, in order to optimize further the mechanical properties of such composites, it is desirable that aspects of the final microstructure, such as the size of the SiC particles, be engineered by changing the various processing variables such as temperature, reaction time, particle size of starting materials, etc. Such an optimization process demands an understanding of the diffusion and reaction mechanisms for this class of in situ synthesis reactions. Recently, data concerning the reaction kinetics and microstructural evolution for the reactive diffusion between Si and NbC became available.7 The objective of the present paper is to present a probable mechanism for the reactive diffusion between NbC and Si based on experimental observations.7 II. SUMMARY OF EXPERIMENTAL OBSERVATIONS
x 19st 1/2 2 3.51/2 d .
Since the experimental details were given in earlier papers,7,8 only a brief summary will be presented here. 850
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The materials used were Si single crystal and 99.9% NbC sintered powder compact of near-stoichiometric composition. The NbC was originally intended for use as a sputter target. Deta
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