New developments in alpha-SiAlON ceramics

Abstract

Two different alpha-sialon compositions have been densified by either pressureless sintering or capsul-free sinter HIPing using either single cation namely strontium, lanthanum, cerium, neodymium or their equimolar mixtures with calcium/ytterbium as a principal cation. The resulting materials have been heat-treated at 1450 degrees C for a maximum of 720 h to observe the alpha --> beta sialon transformation. Effect of starting compositions on the phase stabilities, microstructure, room temperature mechanical properties and optical transparent nature of sintered materials have been investigated. SEM observations for oxygen rich starting compositions revealed that alpha-sialon grains exist in needle like morphology in all the systems studied after both pressureless sintering and HIPing. Surprisingly, the nitrogen rich starting compositions give rise to coloured, optically transparent alpha-sialon ceramics after capsul-free HIPing. Clear evidence of alpha-sialon formation with incorporation of large cations namely, La3+ Ce3+, Nd3+ and Sr2+, has been established by XRD, SEM and EDX analysis. The observed phenomenon could possibly extend the usefulness of alpha-sialons ceramics to both structural and functional applications