GROWTH-RATE CONSTANT AND CHEMICAL DIFFUSIVITY IN SILICIDES MO5SI3 ANDTA5SI3

The growth kinetics and chemical diffusion coefficients in intermetall ic silicides M5Si3 (M = Mo and Ta) were investigated at temperatures b etween 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 parabolica lly. Concentration profiles of Si, Mo and Ta were measured across cros s-sectioned samples using an electron-probe microanalysis and chemical diffusion coefficients were obtained using Wagner's equation on multi layer diffusion systems. Both the parabolic rate constant and chemical diffusion coefficient in the Mo5Si3 phase were approximately one orde r of magnitude larger than those of the Ta5Si3 phase. Activation energ y for the parabolic rate constant was close to that of the chemical di ffusion coefficient for each diffusion couple, showing 297 and 271 kJ/ mol for the Mo5Si3 and Ta5Si3 phases, respectively. Microstructural ob servations indicated that Kirkendall voids formed within the Mo5Si3 la yer close to the MoSi2 phase, whereas there is little formation of voi ds in the Ta5Si3 layer.