Sintering and grain growth behaviour of Si3N4 ceramics

During sintering, $ rm Si sb3N sb4$ transforms through a liquid phase from the metastable $ alpha$ phase to the stable $ beta$ phase. In this work, $ rm Y sb2O sb3, Al sb2O sb3$ and AlN were added as liquid-forming additives, which react with surface SiO$ sb2$ on the $ rm Si sb3N sb4$ particles. The mechanism of pressureless sintering $ rm Si sb3N sb4$ with these additives was a combination of particle rearrangement, solution-precipitation and elimination of closed porosity. The rate of densification and $ alpha$-$ beta$ phase transformation were strongly influenced by the amount and viscosity of liquid phase. The kinetics suggested that the rate of densification increased with $ rm Al sb2O sb3$ content, since the higher viscosity of the liquid provided more capillary pressure. On the other hand, the rate of transformation was improved by a higher $ rm Y sb2O sb3$ content, because of the increased rate of solution and diffusion in the low viscosity liquid environment. In addition, AlN decreased the rates of both densification and transformation, due to its low solubility in the liquid phase.A morphology analysis was carried out on the samples sintered in the range 1600$ sp circ$C to 1900$ sp circ$C and at 1850$ sp circ$C for various soaking times. The coarsening process of the $ beta$ grains occurred with increasing temperature. Moreover, the $ beta$ grain aspect ratio and size distribution also increased with prolonged sintering. The formation of $ beta$ phase indeed improved the mechanical properties of the materials, however, excessive grain growth led to microcracking, which decreased the strength. The best values were obtained at 1850$ sp circ$C for one hour and corresponded with 98% theoretical density and 100% $ beta$ structure. At higher temperature, the strength decreased, due to decomposition of the $ rm Si sb3N sb4.