Growth of silicides and interdiffusion in the Mo-Si system
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I.
INTRODUCTION
MoSi2-based composites are potential candidates for high-temperature applications.[1] In order to understand the creep behavior and the structural stability of the composites, studies of diffusion in MoSi2 and other silicides of Mo are needed, but such studies are limited in the literature. Zmii and co-workers[2,3] explored the silicide formation and interdiffusion in solid-solid diffusion couples assembled with polycrystalline disks of Mo and Si at selected temperatures over 900 7C to 1350 7C and determined empirical energies of activation for the growth of silicide layers in the couples. Bartlett and co-workers[4,5] also studied the growth kinetics of Mo5Si3 and Mo3Si between Mo and MoSi2 over the temperature range from 1200 7C to 1900 7C and observed that the Mo3Si layer formed in the diffusion zone only after the MoSi2 layer was consumed. Mass and Rieck[6] investigated the development of a MoSi2 layer in vapor-solid diffusion couples at temperatures between 1000 7C and 1200 7C and reported fiber textures in the diffusion zone. Basu and Gosh[7] reported the formation of MoSi2 and Mo5Si3 layers between Mo and Si at 1300 7C. This article presents interdiffusion studies carried out with Mo vs Si diffusion couples at selected temperatures between 900 7C and 1350 7C. The diffusion structures developed in these couples are examined for nonstoichiometry and texture development in the MoSi2 layer. Integrated interdiffusion coefficients and energies of activation for interdiffusion are determined for the MoSi2 and Mo5Si3 layers developed in the couples. Average binary interdiffusion coefficients calculated for the MoSi2 phase on the basis of the observed variation in the Si/Mo ratio across the MoSi2 layer
P.C. TORTORICI, formerly Graduate Student, School of Materials Engineering, Purdue University, is Process Engineer, Microelectronics Engineering, Hewlett-Packard Company, Rohnert Park, CA 94928. M.A. DAYANANDA, Professor of Materials Engineering, is with the School of Materials Engineering, Purdue University, West Lafayette, IN 479071289. Manuscript submitted March 27, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
are also reported. Expressions relating the growth-rate contants to integrated interdiffusion coefficients are derived. II.
EXPERIMENTAL PROCEDURE
The 99.95 pct Mo and 99.9999 pct Si samples were obtained from Johnson Matthey Alfa/Aesar.* Disks of *Alfa and Aesar are trademarks of Johnson Matthey, Catalog Co. Inc., Ward Hill, MA.
Mo and Si were cut, metallographically polished from 600 grit SiC paper through 1-mm diamond paste, and employed for the assembly and annealing of Mo vs Si diffusion couples at selected temperatures between 900 7C and 1350 7C. For diffusion annealing at temperatures between 900 7C and 1100 7C, solid-solid couples were assembled with disks of Mo and Si in a jig of Kovar steel. The jig was sealed in a quartz tube, evacuated, flushed with H2, and annealed in a horizontal tube furnace. For annealing at temperatures higher than 1100 7C, sandwich couples assembled
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