BETA-SI3N4 GRAIN-GROWTH .1. EFFECT OF METAL-OXIDE SINTERING ADDITIVES

Si3N4 ceramics with a constant molar fraction of Al2O3 and Y2O3 or Yb2 O3 have been microstructurally characterised by scanning electron micr oscopy in combination with quantitative microscopy, and the three-dime nsional beta-Si3N4 grain size distributions were subsequently reconstr ucted by a stereological method. The Si3N4 ceramics were densified by gas pressure sintering at 1900 degrees C under a pressure of 980 kPa f or I, 3, 5 and IO h. The microstructural analysis showed that the Y2O3 /Al2O3 ratio in the starting powder mixture affects the beta-Si3N4 gra in shape and size distribution. An increased Y2O3/Al2O3 ratio resulted in a higher mean aspect ratio. A replacement of Y2O3 by Yb2O3 increas ed the mean aspect ratio further and resulted in a more narrow grain s ize distribution with an increased mean grain volume. The mean grain s ize increased with densification time, and a grain growth exponent of n = 3.2 implied that the beta-Si3N4 grain growth was rate controlled b y diffusion through the liquid phase. The metal oxide composition also determined the formation of secondary crystalline phases. Crack propa gation and, hence, fracture toughness was dependent upon beta-Si3N4 gr ain morphology and the intergranular structure. (C) 1997 Elsevier Scie nce Limited.