PHASE STABILITIES AND CORROSION/RECESSION PROPERTIES OF RARE EARTH SILICATES UNDER HIGH SPEED STEAM JET

摘要

In this paper, the phase stabilities and corrosion/recession properties of the rare earth silicates under high speed steam jet environment at high temperatures were reported and discussed. Results of steam jet corrosion study showed that the Lu_2Si_2O_7 phase decomposed into Lu_2SiO_5 phase at 1300°C. However, the formed Lu_2SiO_5 phase at 1300°C decomposed back into Lu_2Si_2O_7 at 1500°C. On the other hand, no phase change occurs in Lu_2SiO_5 phase up to 1500°C under high speed steam jet environment. These experimental results can be explained by hypotheses of competitive decomposition reaction between these two phases and by considering of the formation of liquid phase with silica rich composition on the grain surface. The formed liquid phase removed by high speed steam jet during the exposure test. The phase stabilities of Yb_2Si_2O_7 and Yb_2SiO_5 phases were almost same to that of Lu_2Si_2O_7 and Lu_2SiO_5 phases. In the point of view of phase stability, mono-silicate phase is more stable than di-silicate and Yb_2SiO_5 phase is more stable than Lu_2SiO_5 phase at high temperature under steam jet environment. Nonetheless, on the application of rare earth silicate phases as environment barrier coating (EBC) layer, the oxidation of the silicon nitrate substrate due to gas path through the EBC layer is very important issue that needs to be addressed and overcome.