Effect of Preformed Silica Protective Film on Oxidation Behavior of SiC Ceramics during Soaking in Oxygen-Deficient Atmosphere

Effect of preformed silica protective film on the oxidation behavior of porous SiC ceramics at 1700K in Ar-O2 atmosphere with PO2 in the active oxidation range was investigated and following results were obtained. (1) when the inner pore surface of the ceramics was incompletely covered by the silica layer, CO evolution rate exhibited a peak indicative of the occurrence of the active oxidation soon after the dropping operation for PO2 from 2 or 10kPa to 0.02kPa. This active oxidation accompanied the silica reduction. (2) The sample oxidized for 10h at PO2=10kPa showed no peaking in the CO evolution rate after the PO2 dropping operation. Thickness of the silica layer of this oxidized sample was about 0.2μm. This sample showed no weight change during the successive soaking for 10h at PO2=0.02kPa. (3) As the scheme without the weight change during the soaking in the oxygen-deficient atmosphere, the following reaction was assumed, SiC+4/3O2->2/3SiO2+1/3SiO(g)+CO This reaction forcing the volume increase in the reacting zone, was assumed to contribute to the preservation of the silica protective film. (4) It was pointed out that, in the case of the well-passivated SiC ceramics, at least minor correction should be done in Gulbransen's theory which is known to be valid for the fresh ceramics.