Growth of silicides and interdiffusion in the Mo-Si system

Solid-solid diffusion couples assembled with disks of Mo and Si were anneal ed at selected temperatures, over the temperature range from 900 degrees C to 1350 degrees C, for the development of diffusion structure and determina tion of interdiffusion coefficients for the silicides of Mo. Layers of MoSi 2 and Mo5Si3 were observed to form in the diffusion zone; the MoSi2 layer w as one to two orders of magnitude larger in thickness than the Mo5Si3 layer . The MoSi2 layer developed a columnar microstructure with evidence of text ure and preferential growth of grains in the direction of diffusion. The Si -to-Mo ratio, determined by microprobe analysis across the thickness of the MoSi2 layer, varied within the approximate range from 1.9 to 2.0. From the concentration profiles, integrated interdiffusion coefficients as well as energies of activation for interdiffusion were determined for the silicide layers. On the basis of the observed stoichiometric range for the MoSi2 pha se, average values of the interdiffusion coefficients were also estimated. Relations are derived between the growth-rate constant and the integrated i nterdiffusion coefficient for the MoSi2 phase. The evaluated activation ene rgies for interdiffusion in the MoSi2 and Mo5Si3 phases are 130 +/- 20 and 210 +/- 10 kJ/mol, respectively.