High-temperature oxidation behavior of reaction-formed silicon carbide ceramics
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The oxidation behavior of reaction-formed silicon carbide (RFSC) ceramics was investigated in the temperature range of 1100 to 1400 °C. The oxidation weight change was recorded by TGA; the oxidized materials were examined by light and electron microscopy, and the oxidation product by x-ray diffraction analysis (XRD). The materials exhibited initial weight loss, followed by passive weight gain (with enhanced parabolic rates, kp), and ending with a negative (logarithmic) deviation from the parabolic law. The weight loss arose from the oxidation of residual carbon, and the enhanced kp values from internal oxidation and the oxidation of residual silicon, while the logarithmic kinetics is thought to have resulted from crystallization of the oxide. The presence of a small amount of MoSi2 in the RFSC material caused a further increase in the oxidation rate. The only solid oxidation product for all temperatures studied was silica.
I. INTRODUCTION Recently, there has been a surge of interest in the research and development of advanced silicon carbide ceramics intended for a variety of applications in aerospace, energy, nuclear, and automotive industries.1'2 The majority of these applications require components fabricated in net or near-net shapes at low cost. The fabrication of silicon carbide ceramics by reactive melt infiltration of silicon has some important advantages over the more conventional processes of pressureless sintering (PS), hot-pressing (HP), and chemical vapor deposition (CVD). These include full densification without external pressures and at much lower temperatures, and the potential for tailoring the product microstructure and composition by controlling the melt chemistry and engineering the preforms. Early attempts to exploit this technique utilized preforms of silicon carbide and graphitic carbon powders, and after silicon infiltration new silicon carbide grew epitaxially on silicon carbide seed crystals incorporated into the powder mix.3 Those reaction-sintered silicon carbides ("REFEL SiC", and their variants from other sources) often have a somewhat coarse and inhomogeneous microstructure characterized by significant porosity and a residue of unreacted phases. A low cost technique for the processing of reactionformed silicon carbide (RFSC) ceramics has been recently developed, and the properties of a variety of RFSC materials have been reported.2'4 8 In this process, silicon carbide ceramics are made by infiltrating molten silicon or silicon-(Mo, Nb) alloys into microporous carbon preforms (without seed crystals).4"7 The resulting yS-SiC has a fine microstructure and excellent mechanical properties, which are only slightly degraded by exposure to elevated temperatures.8 There is no change of 3232
J. Mater. Res., Vol. 10, No. 12, Dec 1995
http://journals.cambridge.org
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dimension during processing by this reactive infiltration technique. However, complete conversion of the carbon preform into silicon carbide without any residual phase is difficult, requiring a very delicate
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