Synthesis of nanocomposite thin films based on the Mo-Si-C ternary system and compositional tailoring through controlled
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Mo-Si-C system was first investigated by Nowotny in 1954,[1] resulting in the publication of an isothermal section at 1600 7C. The only ternary phase identified in the system was Mo≤5Si3C≤1, which belongs to a group of phases with the general formula TI3TII,2M3X,1 (and a collective name: the ‘‘Nowotny’’ phase), where T refers to a transition metal, M represents Si or Ge, and Xx refers to a nonmetal such as C, O, B, or N.[2] The characteristic feature of the Nowotny phase is the presence of carbon-centered tetrahedra. In the Mo-Si-C system, the Nowotny phase is considered to form by minor additions of carbon to Mo5Si3, resulting in destabilization of the tetragonal structure and formation of the carbon-stabilized Nowotny phase.[1–5] Subsequent work on the Mo-Si-C system resulted in the publication of isothermal sections at 1727 7C by Brewer and Krikorian in 1956[3] and at 1200 7C by van Loo et al. in 1982.[4] A summary of the phases present in the Mo-Si-C system is given in Table I.[6,7] The presence of three-phase regions on either side of the MoSi2-SiC tie-line (i.e., MoSi2-SiC-Nowotny and MoSi2SiC-Si) points to the necessity of carefully controlling the composition of the starting materials during synthesis of MoSi2-SiC composites. If the initial starting composition is silicon deficient, the possibility of incorporating free silicon in the composite would severely limit the maximum tem-
S. GOVINDARAJAN, Process Engineer, is with White Oaks Semiconductor Corporation, Richmond, VA 23150. J.J. MOORE, Professor and Head, Department of Metallurgical and Materials Engineering, and Director, Advanced Coatings and Surface Engineering Laboratory, is with the Colorado School of Mines, Golden, CO 80401. J. DISAM, General Manager, Metals, Refractories, and Ceramics, is with Schott Glaswerke, Mainz, D-6500 Germany. Manuscript submitted December 1, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
perature at which the material could be used, since silicon melts at ;1420 7C. Steinmetz et al.[8] conducted an interesting study of the Mo-Si-C system, with the objective of finding a suitable diffusion barrier between a molybdenum disilicide coating and a molybdenum substrate. The approach examined was to introduce a layer of Mo2C on a molybdenum substrate (by reaction of molybdenum with a mixture of methane (1 pct) and hydrogen (99 pct), at 1250 7C to 1450 7C) followed by silicidation of the surface through a pack cementation technique (viz., the carburized substrate was placed in a pack containing silicon powder, and SiCl4 was introduced into the pack at a pressure of 80 torr (1.065 3 104 Pa) at 25 7C). The silicided component was then annealed at various temperatures in a vacuum furnace (base pressure of 1026 torr (1.33 3 1024 Pa)). The conclusion arrived at was that the best choice of a barrier layer (based on a comparison between the rate of formation of the less-oxidationresistant silicide Mo5Si3 in a Mo-MoSi2 couple and a Mobarrier layer-MoSi2 couple), consisted of a 45-mm-thick layer of Mo2C. The synthesis of multilayer str
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