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>

Abstract

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.