beta-Si3N4 grain growth .1. Effect of metal oxide sintering additives

Si3N4 ceramics with a constant molar fraction of Al2O3 and Y2O3 or Yb2O3 have been microstructurally characterised by scanning electron microscopy in combination with quantitative microscopy, and the three-dimensional beta-Si3N4 grain size distributions were subsequently reconstructed by a stereological method. The Si3N4 ceramics were densified by gas pressure sintering at 1900 degrees C under a pressure of 980 kPa for I, 3, 5 and IO h. The microstructural analysis showed that the Y2O3/Al2O3 ratio in the starting powder mixture affects the beta-Si3N4 grain shape and size distribution. An increased Y2O3/Al2O3 ratio resulted in a higher mean aspect ratio. A replacement of Y2O3 by Yb2O3 increased the mean aspect ratio further and resulted in a more narrow grain size distribution with an increased mean grain volume. The mean grain size increased with densification time, and a grain growth exponent of n = 3.2 implied that the beta-Si3N4 grain growth was rate controlled by diffusion through the liquid phase. The metal oxide composition also determined the formation of secondary crystalline phases. Crack propagation and, hence, fracture toughness was dependent upon beta-Si3N4 grain morphology and the intergranular structure. (C) 1997 Elsevier Science Limited.