Indentation Response of Molybdenum Disilicide
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Indentation response of molybdenum disilicide A. Newman, T. Jewett, S. Sampath, C. Berndt, and H. Herman Department of Materials Science and Engineering, Stony Brook, State University of New York, Stony Brook, New York 11794-2275 (Received 3 July 1997; accepted 8 December 1997)
The influence of microstructure on the indentation cracking behavior of molybdenum disilicide (MoSi2 ) has been examined. The indentation response of samples produced by various methods has been measured to examine the elastic/plastic nature, hardness, and fracture toughness. Fracture toughness comparisons were made by measuring indentation crack lengths, observing the elastic/plastic indentation response, and quantifying the differences in the indentation cracking behavior. Further information was gained by monitoring the acoustic activity during indentation for selected specimens. It has been observed that the fine grain size and the dispersion of the silica phase promote microcracking and crack deflection.
I. INTRODUCTION
Composites based on molybdenum disilicide (MoSi2 ) are under consideration for high temperature structural applications due to the material’s high melting point (2030 ±C), excellent oxidation resistance, electrodischarge machinability, and stability with compositing agents.1–3 Additionally, the material is ductile at high temperatures, thus offering potential opportunities for hot forming operations. Several processing approaches are available to fabricate monolithic and composite MoSi2 ; for example, hot pressing,4 hot isostatic pressing,5 self-propagating high temperature synthesis,6 and plasma spray forming.7 Two limitations that seriously restrict consideration of MoSi2 as a potential high temperature structural material are (i) lack of ductility at ambient temperatures, and (ii) poor tensile creep resistance at high temperatures. While MoSi2 is brittle at room temperature, it exhibits dislocation plasticity at high temperatures. Aikin8 performed four-point flexure tests on MoSi2 over the temperature range of 1000 ±C to 1400 ±C and reported the ductile-to-brittle transition temperature (DBTT) of MoSi2 to be between 1300 and 1400 ±C. Hardwick et al.9 also reported the DBTT to be in this range. The high purity of these samples, especially with low silica content, resulted in this high DBTT, which is contrary to the generally believed DBTT of 1000 ±C.8 The fabrication of MoSi2 via a powder processing route yields an intrinsic amount of silica because of the high reactivity of Si with O2 .10 The presence of silica, either as particles or as a thin film at the grain boundaries, influences the mechanical properties of MoSi2 . There are conflicting observations in the literature with regard to the morphology and distribution of the intrinsic silica in polycrystalline MoSi2 . In situ Auger analysis of a 10–20 mm grain size sample, containing silica particles within grains and at grain boundaries and fracturing 2662
http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 9,
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