A MECHANISM FOR REACTIVE DIFFUSION BETWEEN SI SINGLE-CRYSTAL AND NBC POWDER COMPACT

Based on our recent experimental observations, a growth mechanism for the reactive diffusion between Si single crystal and NbC powder compac t is proposed. In Si-NbC diffusion couples annealed at 1300 degrees C, a two-phase NbSi2 + SiC reaction layer formed with NbSi2 as the matri x and SiC as discontinuous particles. The NbSi2 grain sizes and SiC pa rticle sizes are both in the mu m range. We propose that the SiC parti cles nucleated at the void surfaces in the NbC powder compact. This pr oposed nucleation mechanism offers a potential way of controlling the SiC particle size by changing the void size and void density of the Nb C powder compact. It is also pointed out that this microstructure requ ires Si to be the dominant diffusing species. Si must diffuse through the reaction layer, while C only has to undergo local rearrangement, a nd Nb need not diffuse at all.