Evaluation of high-Temperature diffusion barriers for the Pt-Mo system
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INTRODUCTION
STUDIES conducted during the 1960's and 1970's focused on the use of a variety of individual coatings as well as on the use of multiple coatings which involved diffusion barrier layers for the protection of refractory metals from oxidation and/or diffusion. II-'l These studies were undertaken with the hope that a system could be developed that would have both high-temperature strength and oxidation resistance. Of particular interest were the refractory metal molybdenum and its alloys. Molybdenum is often considered for use in high temperature systems because it retains usable strength up to approximately 1927 ~ (3500 ~ Its relatively high thermal and electrical conductivities make it ideal for heating applications. Its low vapor pressure allows it to be used in vacuum systems. An alloy of molybdenum, Mo-0.5Ti-0.07Zr-0.01C (TZM), has some of these same properties while having a tensile strength approximately twice that of molybdenum above 1093 ~ (2000 ~ This alloy was originally developed as one of the stronger, weight saving, high-temperature materials needed for the space program. The TZM alloy also has a higher recrystallization temperature and welds more easily than unalloyed molybdenum. The most serious problem with using these and other molybdenum alloys in any oxygen-containing atmosphere is that above 427 ~ (800 ~ molybdenum reacts to form molybdenum trioxide, t12Jwhich vaporizes above 500 ~ (932 ~ At 982 ~ (1800 ~ molybdenum is lost by this reaction at the rate of 1.39 • 10 -3 to 3.47 • 10 -3 m / s (0.02 to 0.05 in/hr), t131 Since alloying of molybdenum does not produce acceptably low oxidation rates, tL31coatings of various metals have been studied. Both nickel and chromium coatings were investigated, t~l and it was shown that chromium failed while nickel protected molybdenum for about 560 hours at 1100 ~ Interdiffusion of the Ni and Mo contributed to failure of the nickel-plated molybdenum. Other types of coatings considered were silicide cementation coatings, t2j chromium-nickel coatings, t31and platinum-group metal coatings. Platinum coatings would prevent the formation of a volatile molybdenum oxide if molybdenum did not so readily diffuse through the RICHARD P. WALTERS and BERNARD S. COVINO, Jr. are Supervisory Research Chemists, Albany Research Center, Bureau of Mines, Albany, OR 97321-2198. Manuscript submitted September 2, 1986. METALLURGICALTRANSACTIONS A
platinum coating and oxidize at the surface. I4'141A l s o , molybdenum, diffusing into the platinum, forms brittle intermetallic phases; this precludes the use of platinum-coated molybdenum for long time periods at elevated temperatures. To determine methods to prevent molybdenum from reaching the platinum surface and forming intermetallic phases, investigations of the use of diffusion barriers between the platinum coatings and the molybdenum substrate were begun.t2] Pure metals, oxides, and other compounds were tried with varying degrees of success. No individual barrier has been found that could prevent the interdiffusion or
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