PROCESSING, MICROSTRUCTURE, AND WEAR BEHAVIOR OF SILICON-NITRIDE HOT-PRESSED WITH ALUMINA AND YTTRIA

Commercial silicon nitride powder with Al2O3 and Y2O3 additives was ho t-pressed to complete density. The resulting microstructure contained elongated grains with no trace of remaining alpha-Si3N4. The aspect ra tio of the elongated grains increased with increasing soak time at a f ixed hot-pressing temperature. X-ray diffraction analysis showed that the crystalline phase in the hot-pressed samples was beta-sialon (Si6- zAlzOzN8-z) with z values that increased with soak time. The fracture strength and fracture toughness of the samples increased as the aspect ratio of the grains increased. The Vickers hardness decreased slightl y as the soak time was increased, which was attributed to a grain size effect. Wear tests of silicon nitride against silicon nitride were co nducted on a reciprocating pin-on-disk apparatus with paraffin oil as a lubricant. Correlation studies of wear with microstructure and mecha nical properties were performed. The wear rate increased rapidly with increasing soak time in spite of the increased strength and toughness. This was attributed to increased third-body wear caused by pullout of pieces from the wear surface. The pullout mechanism was not conclusiv ely identified. However, TEM examination showed clear evidence of disl ocation motion under the wear scar. Grain boundary microstresses cause d by the anisotropic thermal expansion and elastic properties of the e longated grains may have contributed to the observed pullout.