Effect of grain morphology in pressureless-sintered Si 3N 4z.sbnd;Y 2O 3/Al 2O 3 on leaching behavior under hydrothermal conditions

Abstract

The purpose of this work is to study the effect of grain morphology in sintered Si3N4 on the leaching behavior under hydrothermal conditions. The pressureless-sintered Si3N4-Y2O3/Al2O3 specimens were produced using two kinds of commercial grade Si3N4 powders, i.e., prepared by the imide-decomposition method (specimen-A) and the silicon-nitridation method (specimen-B). Differences in microstructures were observed for sintered specimens from these Si3N4 starting powders, i.e., specimen-A resulted in a finer microstructure than specimen-B. Leaching test was carried out using an autoclave under the hydrothermal conditions at 300°C and 8.6MPa for 1-10 days. The two specimens showed the similar trend of weight loss in leaching test, but larger weight loss was measured in specimen-B in the whole leaching time. A parabolic plot of weight loss suggested that the leaching mechanism consisted of two stages. SEM observation revealed that the both leached layers consisted of porous layer at the first stage and flakes on it at the second stage. The porous layer for specimen-A was finer than that for specimen-B and the traces of machining grooves was still visible on both porous layers. The results of this experiment showed that the leaching weight loss of sintered Si3N4 under hydrothermal conditions and the microstructures of the leached layer depended remarkably on the microstructural grain morphology of the sintered body generated by the starting powders and also on its mechanical history by grinding.