A study of pest oxidation in polycrystalline MoSi 2
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MoSi2 is a promising high-temperature material with low density (6.3 g/cm 3 ), high melting point (2020 °C), and good oxidation resistance at temperatures to about 1900 °C. However, in the intermediate temperature range between 400 and 600 °C, it is susceptible to a "pest" reaction which causes catastrophic disintegration by a combination of oxidation and fracture. In this study, we have used polycrystalline MoSi2, produced by arc-casting of the pure elements and by cold and hot pressing of alloy powders, to characterize the pest reaction and to determine the roles of composition, grain or phase boundaries, and physical defects on the oxidation and fracture of specimens exposed to air at 500 °C. It was found that pest disintegration occurs through transport of oxygen into the interior of the specimen along pre-existing cracks and/or pores, where it reacts to form MoO 3 and SiO 2 . The internal stress produced during the formation of MoO 3 results in disintegration to powder. Near the stoichiometric ratio, the susceptibility to pest disintegration increases with increasing molybdenum content and with decreasing density. Silicon-rich alloys were able to form protective SiO2 and showed no indication of disintegration, even at densities as low as 60%.
I. INTRODUCTION Molybdenum disilicide (MoSi2), with a density of 6.3 g/cm 3 , a melting temperature of approximately 2020 °C, and excellent high-temperature oxidation resistance, is being considered for structural uses at temperatures as high as 1250 °C.1^ At these temperatures, the strength and oxidation resistance of MoSi2 is better than that of most metals and ceramic-based composites. However, as with many intermetallics, MoSi2 suffers from several mechanical property deficiencies, including low ductility below 1000 °C and poor strength and creep resistance at temperatures above 1250 °C. Research efforts are underway in attempts to correct these deficiencies.2-3'5"10 As these alloying efforts progress, a better understanding of the oxidation behavior of this system at all temperatures will be required, especially with respect to the effect of alloying additions on oxidation resistance. Some of these studies have been done or are currently underway.3'11"13 The excellent high-temperature oxidation resistance of MoSi2 is derived from a protective silica (SiO2) layer which forms in oxidizing atmospheres according to the 14 reaction 5MoSi 2
7O 2 —• Mo 5 Si 3 + 7SiO 2
(1)
Once formed at temperatures above 900 °C, SiO2 protects the underlying MoSi2 at lower temperatures and during thermal cycling.11"13'15 However, oxidation at temperatures between about 400 and 600 °C often results in mechanical disintegration of the specimen. This reaction has been termed "pesting" by Fitzer, who first
described the phenomenon in 1955.16 At these temperatures (below about 800 °C), the formation of MoO 3 competes with the formation of SiO2 according to the following reaction14:
(In some studies,17-18 small amounts of MoO 2 in the pesting oxide have also been identified.) The exact mech
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