Low-temperature oxidation of molybdenum disilicide
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INTRODUCTION
M O L Y B D E N U M disilicide (MoSi 2) is a promising high-temperature material because of its moderate density (6.31 g cm-S), high melting temperature (2020 ~ strength retention to 1300 ~ ductility above 900 ~ to 1000 ~ 12-5Jand excellent high-temperature oxidation resistance. MoSi2 can disintegrate catastrophically (pest) during oxidation at T --< 600 ~ Although limited previous research indicates that pest does not occur in dense, crack- and pore-free polycrystalline MoSi2, little is known about the rate of low-temperature oxidation, the nature of the oxidation products, the influences of oxidation atmosphere, sample composition and microstructure, the transition from a nonprotective to a protective oxidation product with increasing temperature, or the effect of hightemperature preoxidation on subsequent low-temperature oxidation kinetics. The objective of this study is to investigate these issues using well-characterized polycrystalline MoSi2 samples and to determine whether problems with low-temperature oxidation resistance are likely to limit the application of MoSiz as a hightemperature material. Fitzer 16.7) discovered the catastrophic disintegration to powder of MoSi2 specimens held in oxygen-bearing atmospheres below 600 ~ and called it "pest." Lowtemperature oxidation and pest of MoSi2 have been studied extensively by Fitzer and c o - w o r k e r s I6-9'11'18A9"2[221 and by others, t1~176 Most investigations have been carried out on powder-metallurgical samples containing -> 10 pet porosity, ingot-metallurgical samples known to contain cracks, or samples whose porosity and crack P.J. MESCHTER, formerly Senior Scientist, McDonnell Douglas Research Laboratories, St. Louis, MO, is Materials Scientist, General Electric Company, Corporate Research and Development, Schenectady, NY 12301. Manuscript submitted January 28, 1991. METALLURGICAL TRANSACTIONS A
density were not characterized. One brief published investigation []91 of low-temperature oxidation of crack- and pore-free, ->98 pet dense polycrystalline MoSi2 showed formation of a protective oxide and no evidence of pest, but oxidation was only qualitatively described and the oxide layer was not analyzed. MoSi2 pest occurs only in oxygen-bearing atmospheres. Seyboit and Westbrook l]31 reported pest in oxygen but not in air, while Rubisch [15] observed pest in air at 400 ~ to 500 ~ Published experimental reports 17'15-171 describe pest as limited to the temperature range of 300 ~ to 600 ~ Pest disintegration is preceded by an incubation period variously reported to be 8 to 700 hours, depending on temperature, oxygen pressure. sample density, and composition. I7"9"~3'~~-t71 The maximum rate of attack occurs between 400 ~ and 500 ~ The time required to initiate catastrophic oxidation at 500 ~ is proportional to (po2) -~ and varies from 180 hours at Po2 = 2.67 • 103 Pa (0.026 atm) to 7 h o u r s a t p o , = 1.01 • 10S P a ( 1 atm). ['6] The time to onset of disintegration is a strong function of sample density, ranging from 10 hours in sint
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