Transformation and thermal stability of Li-α-sialon ceramics

Abstract

Two phase α/β and single phase α lithium sialons with different m and n values were produced by hot pressing at 1730–1750°C at 30 MPa for 30–40 min in a graphite resistance furnace. When the two-phase samples were heat-treated at lower (1200–1450°C) temperatures in different packing powders, an increase in the amount of α was observed, due to β-sialon in the as-sintered material reacting with grain boundary liquid to form more α. β→α transformation at low temperatures has not been reported previously in any sialon system and in the present case is believed to occur because the α-sialon phase field in the lithium sialon system shifts slightly towards the β-sialon line at lower temperatures. The thermal stability of lithium α-sialon is good in the centre of the single-phase α region when surrounded by a Li-containing powder bed. However, towards the edges of the single-phase region, compositional changes occur on heat-treatment. Thus, samples with high m, n values decompose into β-sialon plus other Li-containing phases. During heat-treatment of other compositions when surrounded by a BN powder bed, the composition of the α-sialon phase continually readjusts towards the α/β sialon phase region as a function of time and this is followed by decomposition of the α phase. Evaporation of the Li+ stabilising cation is believed to be the main reason for this behaviour. The effects of m and n value, heat treatment parameters and packing powder on the thermal stability of Li α-sialons are discussed.