Oxidation protection for a variety of transition metals and copper via surface silicides formed with silane containing a
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The reaction of SiH 4 /H 2 mixtures with iron, cobalt, nickel, copper, chromium, molybdenum, and tungsten, at temperatures between 350 and 800 °C and 1 atm of total pressure, was studied. When the amount of water vapor in the gas mixture is carefully controlled, a metal silicide diffusion coating forms at the appropriate treatment temperature. Cu silicide coatings form at temperatures as low as 350 °C. Metal silicide coatings for Fe, Ni, Co, and Cr form at intermediate temperatures (500-700 °C), and higher temperatures (above 700 °C) are required for W and Mo. Composition and structure of the metal silicide coatings were determined by Auger depth profiling and x-ray diffraction. Kinetics of the surface reaction between SiH4 and the metal substrate as well as the behavior of these coatings in oxidizing environments at high temperatures were studied by a microgravimetric technique. The metal silicide coatings provide oxidation protection for Fe, Ni, and Cr in pure O2 up to 1000 °C, for W and Mo in air up to 900 °C, and for Cu exposed to air up to 700 °C.
I. INTRODUCTION
Previously1"3 we reported a gas phase treatment for iron (Fe) and ferrous alloys, using silane hydrogen (SiH4/H2) mixtures, which produces a silicon (Si) diffusion coating. These coatings provide oxidation protection to Fe and mild steels. The Si diffusion coating is composed of a surface silicide layer with an interface of Fe-Si in solid solution. In addition to Fe, several other metals form silicides or solid solution when reacted with Si. Thus, the formation of other surface silicides from reaction with SiH4 appears feasible. In fact, Dubois and Nuzzo4 were able to form surface silicides on a variety of metals when they reacted their thin native oxides with SiH4 in ultra high vacuum (UHV). Caillet and coworkers5"7 have also produced diffusion coatings consisting of solid solutions and metal silicides on Fe, Cu, and Ni under static conditions at elevated temperatures (1100 °C for Fe and Ni). We sought to determine if our gas phase treatment, involving SiH 4 /H 2 at atmospheric pressures and relatively low temperatures, was amenable for modifying the surface of metals other than Fe and ferrous alloys. Attempts to use SiH4 at lower temperatures by other workers8 resulted in Si overlayer coatings. The use of SiH 4 /H 2 gas mixtures in our process serves two purposes: (1) reduce the hazards of handling SiH4 by keepa)
Current address: Pontificia Universidad Catolica de Chile, Facultad de Fisica, Casilla 6177, Santiago 22, Chile. J. Mater. Res., Vol. 6, No. 1, Jan 1991
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ing SiH4 concentration below 2%; and (2) maintain a reducing atmosphere for the metal and the coating by keeping the H 2 to water level ratio high. We have demonstrated1"3 that the control of the level of oxygen contaminants, as defined by the water level, is a crucial factor for obtaining Si diffusion coatings rather than overlay coatings at temperatures between 500 and 700 CC. Since reducing the water level in a mixture below 10
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