High strength and fracture toughness sintered silicon nitride body mfr. - by wet milling silicon nitride powder with additives, drying and sintering, esp. for machine and tool cutting and wear parts

摘要

In mfg. a sintered silicon nitride body, silicon nitride powder is wet-milled with additives and the mixt. is dried and sintered. The silicon nitride powder used comprises 0-85% alpha-silicon nitride and 15-100 wt.% beta-silicon nitride, with a mean crystallite size less than or equal to 0.2 microns and a max. crystallite size of 0.45 microns. The crystallites may have an elongated shape (length/thickness greater than 1). USE/ADVANTAGE - For machine and machine tool parts (especially wear parts) and for cutting tools. The material has a strength and fracture toughness which are higher than those of conventional silicon nitride ceramics. In an example, a silicon nitride powder with a beta/(alpha+beta) ratio of 1.0, a mean crystallite size of 0.15 microns, a max. crystallite size of less than or equal to 0.4 microns (determined by x-ray diffraction) and a free specific surface of 10 m2/g was ball-milled with 10 wt.% yttria, 2 wt.% alumina, 1 wt.% stearic acid and sintering aids in ethanol, followed by drying. The mixture was sintered at 1900 deg.C under a max. nitrogen pressure of 5 MPa. The fracture toughness and strength of the product were 10.5 MPa m1/2 and 850 MPA respectively. Corresp. fracture toughness and strength values obtd. for a powder comprising 96 wt.% alpha-silicon nitride and 4 wt.% beta-silicon nitride, which was mixed, pressed and sintered under the same conditions, were 8.0 MPam power (1/2) and 750 MPa respectively.