Formation of in situ reinforced microstructures in α-sialon ceramics: Part III. Static and dynamic ripening

Abstract

Dual cation (Yb + Y)-stabilized α-sialon ceramics with either stoichiometric composition or nonstoichiometric composition that yield less than 3 vol% of an additional intergranular liquid/glass phase were consolidated by spark plasma sintering (SPS). This process allows very fast heating and cooling, thus providing a unique possibility to monitor and manipulate the kinetics of phase transformation and grain growth during sintering. Below a temperature threshold, full densification and complete α-sialon formation are accompanied by very limited grain growth. The grain growth kinetics were investigated both by post heat-treatment of SPS pre-consolidated monophasic α-sialon bodies consisting of sub-micron sized equiaxed grains in a conventional graphite furnace using extended holding times (hours) and directly rapid annealing in the SPS apparatus above the temperature threshold (within minutes). Post heat treatment in the graphite furnace yielded in situ reinforced microstructures consisting of interlocking elongated grains only in the presence of an additional intergranular liquid/glass phase. Direct annealing by SPS process yielded in situ reinforced microstructures whether or not an additional liquid/glass was involved. The former microstructures are formed via the static Ostwald ripening mechanism whereas the latter ones are generated via a dynamic ripening mechanism. This demonstrates that the dynamic ripening provides an efficient means of developing in situ reinforced microstructure in α-sialon ceramics with improved mechanical properties.