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

In this work the oxidation kinetics of a porous (21.5% porosity) nitride bo nded silicon carbide containing 20% SiC was investigated in the temperature range of 1100-1400 degreesC. Two oxidation stages were found which corresp onded to (i) a rapid parabolic oxidation rate during short term oxidation e xposure (0-10 hours), and (ii) a parabolic scale growth after some exposure time. During short term oxidation a continuous oxide film developed, but i t was unable to block internal oxidation at the pore channels. An experimen tal activation energy of 55 kJ/mol was obtained which was attributed to int ially rapid external and internal oxidation through the open pore network. After 1-10 hours depending on the actual temperature, closure of the surfac e pores through oxidation lead to a transition where a continuous SiO2 scal e grew on the outer sample surface through diffusion. An activation energy of 132 kJ/mol was associated with this parabolic growth suggesting that inw ard oxygen diffusion through the SiO2 scale was rate limiting. Metallograph ic observations indicated severe cracking of the scale developed. This was attributed to the relatively large shrinkage (approximate to1%) associated with the beta-alpha cristobalite transformation occurring at temperatures b elow 250 degreesC. Moreover, X-ray diffraction indicated the presence of cr istobalite and tridymite, but it was unable to identify a discontinuous pha se developed beneath the SiO2 scale during oxidation. (C) 2000 Kluwer Acade mic Publishers.