Evolution kinetics and microstructure of MoSi2 and Mo5Si3 surface layers on two-phase Mo-9Si-18B alloy during pack-cementation and high-temperature oxidation

Following an investigation of the oxidation behavior of a two-phase Mo-9Si-18B alloy coated with MoSi2, which was prepared by pack-cementation with Si (Intermetallics 2003, 11:763), we have studied the kinetics of the evolution of the MoSi2 layer during cementation and the formation of Mo5Si3 layer during high-temperature oxidation, in order to provide quantitative basis for the surface-coating procedure and the protection performance. The microstructures of the surface layers have also been examined. The growth of MoSi2 on Mo-9Si-18B during pack-cementation at 900 °C obeys the parabolic time law and the rate constant is close to the estimate from the literature data of Mo/Si reaction diffusion. The growth must occur by inward lattice diffusion of Si into MoSi2. The as-prepared MoSi2 coating layer consists of fine grains of the order of about 500 nm, with a [001] texture in the growth direction. The growth constant of Mo5Si3 developed between MoSi2 and the matrix during annealing in air at temperatures between 1300 and 1500 °C is significantly lower than those reported for the binary Mo-Si system, but the integrated interdiffusion coefficient is approximately equal. The activation energy for the growth (about 220 kJ/mol) is lower than in the binary system, while the activation energy of interdiffusion (240 kJ/mol) is not very different. The layer of Mo5Si3 consists of columnar grains with a [001] texture in the growth direction, which might have formed to reduce the thermal stresses arising from its anisotripic thermal expansion during cyclic oxidation.