Kinetics of MoSi 2 pest during low-temperature oxidation
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T. G. Nieh Lawrence Livermore National Laboratory, P.O. Box 808, L-350, Livermore, California 94550 (Received 25 September 1992; accepted 8 March 1993)
The kinetics of MoSi 2 pest, caused by oxidation in air, has been studied. Experimental results indicated that pest disintegration occurred at temperatures between 375 and 500 °C. The volumes of test samples increased with oxidation duration. Analysis of change in sample volume versus oxidation duration revealed that the pest disintegration consisted of two stages, namely nucleation (or incubation) and growth. The onset of the growth stage depended on the test temperature. More importantly, changes in sample volume were found to obey a linear relationship with time during the growth stage. Equations were formulated to demonstrate that the growth kinetics of pest disintegration was proportional to the rates of change in sample volume. The rates of volume change during MoSi2 pest were calculated to be 4.9 X 10"5, 2.8 X 10"5, 3.7 X 10~5, and 5.4 X 10"5 cm 3 /s at 375, 400, 425, and 450 °C, respectively; the growth kinetics increased with oxidation temperature. The activation energy for the growth stage of pest disintegration was determined to be 27.6 kcal/mole, which agrees well with the activation energy for the low-temperature oxidation of MoSi 2 .
I. INTRODUCTION Molybdenum disilicide (MoSi2) has been proposed as a promising high-temperature material because of its high melting point (2020 °C), excellent hightemperature oxidation resistance, reasonable strength at temperatures up to 1200-1300 °C, and ductility above 900 "C.1'2 Although monolithic MoSi 2 suffers from lowtemperature brittleness and decreased strength at high temperatures, proper additions of either ductile metals3"6 or hard ceramics7"9 as reinforcements have emerged as a viable technique to render MoSi 2 as a useful hightemperature structural material. While MoSi2 has good oxidation properties at high temperatures, resulting primarily from the formation of a self-passivating SiO 2 layer, it disintegrates catastrophically (also termed "pest") during oxidation at low temperatures (400-600 °C).10 It is worth noting that pest phenomenon has also been observed from various intermetallics (e.g., beryllides, silicides, aluminides, etc.)11 during oxidation in a wide temperature range. Although pest was first discovered in 1955 by Fitzer,10 a fundamental study of its origin was not performed until very recently.2'12"17 By using a variety of MoSi 2 monoliths and composites, it was recently found that pest occurred not only in polycrystalline MoSi 2 , but also in single crystalline
"'Present address: The BDC Group Technical Center, 100 Mountain Avenue, Murray Hill, New Jersey 07974. J. Mater. Res., Vol. 8, No. 7, Jul 1993
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MoSi 2 . Detailed descriptions on the preparation method, density, and microstructure of the various samples are summarized in Table I. Figures 1 and 2 illustrate the morphological characteristics of various MoSi 2 poly crystals (inclu
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