MELILITE FORMATION IN A SAMARIUM-STABILIZED ALPHA-SIALON CERAMIC DURING POSTSINTERING HEAT-TREATMENTS

The formation of the melilite solid solution phase (M'), Sm2Si3-xAlxO3 +xN4-x, in an alpha-sialon sample of overall composition Sm0.6Si9.28Al 2.69O1.36N14.76, was studied as a function of time in the temperature interval 1375-1525 degrees C. The alpha-sialon ceramic contained only minor amounts of the 21R sialon polytype and some residual grain-bound ary glass before heat treatment, In situ studies by high-temperature X -ray diffraction were combined with postsintering heat treatment follo wed by quenching, The M'-phase was found to be formed by two different mechanisms: either crystallization of the residual grain-boundary liq uid or a direct decomposition of the alpha-sialon phase, The liquid cr ystallized during the first 10-15 min of heat treatment, yielding a ra pid M'-phase formation, and further formation of M'-phase continued at a much slower rate, related to the decomposition of alpha-sialon.