VISCOSITY OF AMORPHOUS OXIDE SCALES ON SISIC AT ELEVATED-TEMPERATURES

The high-temperature corrosion resistance of non-oxide Si-containing c eramics is governed by the properties of the protecting silica scales, which develop in oxidizing atmospheres. Bilayered scales of about 8 m u m total thickness with crystalline and amorphous layers were grown o n reaction-bonded SIC (SiSiC) to study viscosity aspects of the scale. As the amorphous layer formed at the free surface, specimens were sca le bonded by means of high-temperature compression and thereafter test ed in symmetric shear relaxation experiments. With this novel techniqu e the scale viscosity was determined in the range between 920 degrees and 1020 degrees C. The tests revealed a strong temperature dependence with viscosity values between those of pure silica and commercial alu minosilicate glass. The results are discussed in terms of impurities d eliberately introduced into the oxidation atmosphere during the format ion of the silica scale on SiSiC.