The effect of NbC porosity on reaction-layer microstructure in NbCSi diffusion couples
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The effect of NbC porosity on reaction-layer microstructure in NbC|Si diffusion couples J. Woodford, C-Y. Yang, and Y.A. Chang Department of Materials Science and Engineering, University of Wisconsin—Madison, 1509 University Avenue, Madison, WI 53706 (Received 14 December 1998; accepted 4 October 1999)
Further experimental observations have allowed us to refine and confirm some aspects of our recently proposed mechanism for reactive diffusion between Si single crystal and NbC powder compact, particularly regarding the prediction of Si as the dominant diffusing species and the nature of the dependence of SiC particle morphology on the presence of voids in the NbC end member. In Si|NbC diffusion couples annealed at either 1300 or 1350 °C, a two-phase NbSi2 + SiC reaction layer formed. Although NbSi2 was the matrix in all of the reaction layers, the SiC phase morphology depended upon NbC porosity: when high-porosity NbC was used, SiC was present as discontinuous particles greater than 1-m-across, while when low-porosity or void-free NbC was used, SiC grew cooperatively with NbSi2 in the form of lamellae less than 0.5 m thick. We propose that this difference arises from the effect of voids both as nucleation sites for SiC particles and as channels for unrestricted SiC growth. Marker experiments conclusively show that Si is the dominant diffusing species in the reaction layer.
I. INTRODUCTION
Recently, Henager et al.1,2,3 have demonstrated the possibility of in situ MoSi2/SiC composite synthesis via a solid-state displacement reaction between Mo2C and Si. Chang, Kao, and co-workers4,5,6 have examined the possibility of a similar in situ NbSi2/SiC composite synthesis. They performed bulk diffusion couple experiments at 1300 °C between NbC and single-crystal Si (Fig. 1), which resulted in the formation of a single twophase reaction layer: NbSi2 + –SiC (Fig. 2). The SiC appears in the form of discrete particles, about 1–3 m across, evenly dispersed in an NbSi2 matrix. The NbC used in these diffusion couples was a sintered powder compact of high porosity. Kao et al.7 noted that SiC appeared to grow in the voids just ahead of the reaction layer|NbC interface, and that the SiC particles were similar in size and shape to the voids in the NbC. Based on that, they proposed the following mechanism for the formation of this aggregate-type microstructure. (Note that all Si chemical potentials are given using pure solid Si at 1300 °C as the standard state.) (i) Silicon diffuses rapidly through the reaction layer to the reaction layer|NbC interface. (ii) From there, it diffuses into the NbC by surface diffusion, along the void walls. (iii) As Si diffuses into the NbC, its chemical potential increases therein; when the chemical potential of Si in the NbC void wall reaches about −54 kJ mol−1 (Fig. 3), SiC begins to form via the reaction NbC + nSi → nSiC + NbC1−n . 248
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J. Mater. Res., Vol. 15, No. 1, Jan 2000 Downloaded: 14 Mar 2015
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