Growth of silicides and interdiffusion in the Mo-Si system

Solid-solid diffusion couples assembled with disks of Mo and Si were annealed at selected temperatures, over the temperature range from 900 °C to 1350 °C, for the development of diffusion structure and determination of interdiffusion coefficients for the silicides of Mo. Layers of MoSi₂ and Mo₅Si₃ were observed to form in the diffusion zone; the MoSi₂ layer was one to two orders of magnitude larger in thickness than the Mo₅Si₃ layer. The MoSo₂ layer developed a columnar microstructure with evidence of texture 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 phase, average values of the interdiffusion coefficients were also estimated. Relations are derived between the growth-rate constant and the integrated interdiffusion coefficient for the MoSi₂ phase. The evaluated activation energies for interdiffusion in the MoSi₂ and Mo₅Si₃ phases are 130±20 and 210±10 kJ/mol, respectively.