Transformation and thermal stability of Li-alpha-sialon ceramics

Two phase alpha/beta and single phase alpha lithium sialons with different m and n values were produced by hot pressing at 1730-1750 degrees C at 30 M Pa for 30-40 min in a graphite resistance furnace. When the two-phase sampl es were heat-treated at lower (1200-1450 degrees C) temperatures in differe nt packaging powders, an increase in the amount of alpha was observed, due to beta-sialon in the as-sintend material reacting with grain boundary liqu id to form more alpha. beta-->alpha transformation at low temperatures has not been reported previously in any sialon system and in the present case i s believed to occur because the alpha-sialon phase field in the lithium sia lon system shifts slightly towards the beta-sialon line at lower temperatur es. The thermal stability of lithium alpha-sialon is good in the centre of the single-phase alpha region when surrounded by a Li-containing powder bed . However, towards the edges of the single-phase region, compositional chan ges occur on heat-treatment. Thus, samples with high m, n values decompose into beta-sialon plus other Li-containing phases. During heat-treatment of other compositions when surrounded by a BN powder bed, the composition of t he alpha-sialon phase continually readjusts towards the alpha/beta sialon p hase region as a function of time and this is followed by decomposition of the a phase. Evaporation of the Li+ stabilising cation is believed to be th e main reason for this behaviour The effects of m and n value, heat treatme nt parameters and packing powder on the thermal stability of Li alpha-sialo ns are discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.