Nucleation and Growth of MoSi 2 Pest during Low Temperature Oxidation
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NUCLEATION AND GROWTH OF MoSi 2 PEST DURING LOW TEMPERATURE OXIDATION
T. C. CHOU* AND T. G. NIEH** *Lockheed Research and Development Division, 0/93-60, B/204, Palo Alto, CA 94304
"**LawrenceLivermore National Laboratory,
L-350, P. 0. Box 808, Livermore, CA 94550
ABSTRACT The phenomenon of "MoSi2 pest" has been studied in detail by low temperature oxidation of various polycrystalline MoSi 2 monoliths and composites. Single crystalline samples were studied to address the nucleation and growth of pest reaction. Pest of MoSi? was observed to occur at temperatures between 375 and 5000C. Samples after pest reactions yielded to powdery reaction products of MoO3 whiskers, SiO 2 clusters, and residual MoSi2 particles. Although all the samples showed complete disintegration after sufficient oxidation durations were rendered, the kinetics of pest disintegration varied among different samples. By performing interrupted oxidation tests, pest reaction was found to consist of nucleation and growth stages, with the former stage acting as the rate-limiting step. Furthermore, the nucleation of pest reaction was found to be closely related to the formation of MO03, which was preferentially formed at such defect sites as intergranular boundaries and cracks. Mechanisms leading to the pest disintegration of MoSi2 are discussed in light of the nucleation and growth of pest reaction. INTRODUCTION Molybdenum disilicide (MoSi 2), possessing a high melting point (20200C), excellent oxidation and corrosion resistant properties at high temperatures [1-6], good thermal conductivity, and reasonable strength and density, is being considered as a promising material for structural applications above 12000C. Although MoSi2 exhibits superior environmental protective capabilities, it suffers from low temperature brittleness and decreased strength at high temperatures. Specifically, the ductile-to-brittle transition temperature (DBTT) of MoSi2 is about 1000oC or higher [7,8], and the yield stress (250-350 MPa) decreases drastically at temperatures above 1200'C. To improve the low temperature toughness and high temperature strength of MoSi2, a composite route has been widely adopted. The most notable reinforcements for MoSi2 being explored are ductile refractory metal Nb [9-11] and SiC ceramic whiskers [7]. Despite these extensive efforts, a fundamental understanding is much needed for the environmental compatibility of MoSi2. The hi h temperature oxidation properties of MoSi 2 have been well-studied during the past 30 years. The excellent oxidation resistance of MoSi2 at high temperatures is attributed to the formation of a self-passivating, glassy silica (Si0 2 ) layer. Low temperature oxidation of MoSi2, on the other hand, has not been adequately addressed; its mechanism is still poorly understood [12]. A well-known, yet unresolved, phenomenon occurring during low temperature oxidation of MoSi2 is pest disintegration, which has been reported to occur at temperatures around 400 - 6000 C [1-2]. (Pest-disintegrated MoSi2 is commonly featured as a l
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