The kinetics of multilayered titanium-silicide coatings grown by the pack cementation method
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INTRODUCTION
THE high-temperature oxidation of titanium results in the formation of a fast growing, nonprotective TiO2 scale, tj~ Additionally, titanium has a large solubility for oxygen,~2~ and the mechanical properties of titanium alloys are degraded by the inward penetration of oxygen, t3,4[ A protective coating is required to take advantage of the high strength and low density of titanium at temperatures above 550 ~ in an oxidizing environment. An ideal coating for titanium should form a slow growing A1203 or SiO2 scale that prevents the inward penetration of oxygen and other contaminants. Thermodynamic calculations predict that SiO2 is stable on a binary Ti-Si alloy only if it contains a silicon content of at least 37.5 at. pct, and A1203 is stable on binary Ti-A1 alloys only with an aluminum content exceeding 50 at. pct. t5,6l Then a SiO2 scale is stable in contact with the phases TiSi2, TiSi, TisSi4, and TisSi3, but TiO2 is stable on Ti3Si. Abba e t al. tTl used TisSi3 coatings to protect titanium; slow oxidation kinetics were observed with the formation of a mixed SiO2/TiO2 scale, in agreement with the thermodynamic stability calculations for SIO2. Becker e t al. tSJ oxidized various TiAl-base alloys, in which the aluminum content exceeded 50 at. pct, and observed a mixed TiO2/A1203 scale without the formation of continuous AleO3. In this case, the growth kinetics of oxide phases undermined the optimistic expectations of thermodynamic stability calculations. Based on thermodynamics, all of the silicide phases except Ti3Si should form a protective SiO2 scale, and limited interdiffusion between a silicide coating and the titanium substrate should not degrade the ability of the protective silicide layer to stabilize SiO2. Unlike A1203, SiO2 usually forms as an amorphous scale, which may BRIAN V. COCKERAM, formerly Graduate Student with The Ohio State University, is Postdoctoral Researcher, Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210. ROBERT A. RAPP, Mars G. Fontana Professor, is with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210. Manuscript submitted June 27, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
flow and heal cracks at high temperature. Therefore, the use of silicide coatings is a viable approach for protecting titanium against oxidation scaling and matrix contamination at high temperature. Due to the importance of titanium silicides in electronic device applications, 191 their oxidation kinetics tl~ and diffusional growth [~3,ta,15J have been studied. However, data for the diffusion-controlled growth of silicides are limited to TiSi2 and TiSi. Optimized use and understanding of silicide coatings require diffusion data for the individual layers and global growth kinetics for the overall coating. The objective of this study is to measure and interpret the growth rates for multilayered silicide coatings that are formed on titanium by the pack cementation method at 950 ~ 1050 ~ and 1150 ~
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