Effects of phase equilibrium on the oxidation behavior of rare-earth-dopedalpha-sialon ceramics

A series of rare-earth-(RE-)doped alpha-sialons (RExSi12-4.5xAl4.5xO1.5xN16 -1.5x, with x = 0.40 for RE = Nd, Sm, Yb, and x = 0.48 for RE = Y) were pre pared and heat-treated in air at 1350 degrees C for 66-727 h (3-30 days), a nd the variations in composition and structure with time of the formed oxid e scales and matrix materials were investigated. In the oxide scales of the Nd-, Sm-, and Y-containing samples a liquid was formed, apparently in (qua si-)equilibrium with the crystalline phases cristobalite and mullite, while only crystalline Yb2Si2O7, cristobalite, and mullite were observed in the Yb sample. Apparently, the liquid plays an important role in the oxidation process. In the depleted zone, located between the scale and the matrix, th e liquid attacks the matrix phases, and a process takes place in which the originally formed phases dissolve and reprecipitate as more oxygen-rich pha ses. In the Nd- and Sm-doped systems, where the alpha-sialon phase is inher ently metastable at 1350 degrees C, an extensive alpha --> beta-sialon tran sformation takes place, creating still more liquid. As a consequence, the o xidation resistance of alpha-sialons containing Nd and Sm is much lower tha n those containing Y and, in particular, Yb.