Viscosity of amorphous oxide scales on SiSiC at elevated temperatures

The high-temperature corrosion resistance of non-oxide Si-containing ceramics is governed by the properties of the protecting silica scales, which develop in oxidizing atmospheres. Bilayered scales of about 8 mu m total thickness with crystalline and amorphous layers were grown on reaction-bonded SIC (SiSiC) to study viscosity aspects of the scale. As the amorphous layer formed at the free surface, specimens were scale bonded by means of high-temperature compression and thereafter tested in symmetric shear relaxation experiments. With this novel technique 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 aluminosilicate glass. The results are discussed in terms of impurities deliberately introduced into the oxidation atmosphere during the formation of the silica scale on SiSiC.