Influence of porosity on the high temperature oxidation of a SiC-reinforced Si3N4 ceramic composite

ISSN： 0022-2461

Source： Journal of Materials Science, Vol.35, Iss.23, 2000-12, pp. : 5995-6004

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Abstract

In this work the oxidation kinetics of a porous (21.5% porosity) nitride bonded silicon carbide containing 20% SiC was investigated in the temperature range of 1100–1400 °C. Two oxidation stages were found which corresponded to (i) a rapid parabolic oxidation rate during short term oxidation exposure (0–10 hours), and (ii) a parabolic scale growth after some exposure time. During short term oxidation a continuous oxide film developed, but it was unable to block internal oxidation at the pore channels. An experimental activation energy of 55 kJ/mol was obtained which was attributed to intially rapid external and internal oxidation through the open pore network. After 1–10 hours depending on the actual temperature, closure of the surface pores through oxidation lead to a transition where a continuous SiO_2 scale grew on the outer sample surface through diffusion. An activation energy of 132 kJ/mol was associated with this parabolic growth suggesting that inward oxygen diffusion through the SiO_2 scale was rate limiting. Metallographic observations indicated severe cracking of the scale developed. This was attributed to the relatively large shrinkage (≈1%) associated with the β–α cristobalite transformation occurring at temperatures below 250 °C. Moreover, X-ray diffraction indicated the presence of cristobalite and tridymite, but it was unable to identify a discontinuous phase developed beneath the SiO_2 scale during oxidation.