Growth Rate Constant and Chemical Diffusivity in Silicides Mo5Si3 and Ta5Si3

Christian Francis, Sohma Hideaki, Tanaka Toshiaki, Tanaka Hiroyuki, Ohsasa Ken-ichi, Narita Toshio

doi:10.2320/matertrans1989.39.286

The growth kinetics and chemical diffusion coefficients in intermetallic silicides M₅Si₃ (M=Mo and Ta ) were investigated at temperatures between 1273 and 1673 K in a vacuum capsule using Mo/MoSi₂ and Ta/TaSi₂ couples, where both disilicides were coated on Mo and Ta substrate by a CVD method. It was found that both the M₅Si₃ layers grew parabolically. Concentration profiles of Si, Mo and Ta were measured across cross-sectioned samples using an electron-probe microanalysis and chemical diffusion coefficients were obtained using Wagner’s equation on multilayer diffusion systems. Both the parabolic rate constant and chemical diffusion coefficient in the Mo₅Si₃ phase were approximately one order of magnitude larger than those of the Ta₅Si₃ phase. Activation energy for the parabolic rate constant was close to that of the chemical diffusion coefficient for each diffusion couple, showing 297 and 271 kJ/mol for the Mo₅Si₃ and Ta₅Si₃ phases, respectively. Microstructural observations indicated that Kirkendall voids formed within the Mo₅Si₃ layer close to the MoSi₂ phase, whereas there is little formation of voids in the Ta₅Si₃ layer.

Growth Rate Constant and Chemical Diffusivity in Silicides Mo<SUB>5</SUB>Si<SUB>3</SUB> and Ta<SUB>5</SUB>Si<SUB>3</SUB>

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