| Thermodynamic and kinetics of pack siliconizing on pure molybdenum were investigated in the range of 1273K-1473K. Siliconizing was carried out in a hydrogen atmosphere using 40wt.%Si-5wt.%NaF-55wt.%Al₂O₃ packs. The growth rate of MoSi₂ coating was observed to be proportional to the square root of time. Apparent activation energy for growth of the MoSi₂ coating was obtained between that for gas diffusion through Si depleted zone and that for solid diffusion of Si in MoSi₂. The rate limiting step for pack siliconizing process was subject to theoretical considerations. Three models such as a gas diffusion model from the pack to substrate surface, a solid diffusion model in MoSi₂, and an equilibrium model of gas diffusion and solid diffusion were evaluated, Theoretically predicted results based on the quasi steady state parabolic kinetic theory induced from an equilibrium model and experimentally obtained results for pack siliconizing on pure molybdenum were found to be good argeement. The Si halide vapors contributed to growth of the MoSi₂ wating were SiF₂, SiF₃, SiF₄, SiH₃F, SiH₂F₂, SiH₃F and the deposition of Si from Si halide vapors on the molybdenum substrate may occur by reactions involving hydrogen reduction, disproportionation, or a Na vapor species. |
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