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 Mo5S3 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 degreesC 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 degreesC 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. (C) 2004 Elsevier Ltd. All rights reserved.