Development of a Sintering Process for High-Performance Silicon Nitride.
Abstract : Beta-Si3N4 ceramics with excellent creep and oxidation resistance in the temperature range of 1300 to 1450 C can be produced by the sintering process using small amounts (3-7%) of BeSiN2 as a densification aid. These ceramics are composed of both elongated and equi-axed grains of Beta-Si3N4 solid solution and sometimes contain minor amounts of Si, SiC and BeSiN2, depending on starting composition and sintering conditions. Although samples have been occasionally sintered to 98% relative density, densities near 90% are routinely produced by sintering at 2000-2100 C for 15 min. in 60-80 atm of N2. In addition to optimum amounts of Be and O required for the attainment of high density and consequently, good thermochemical properties of the ceramic, effects of metallic impurities (Ca, Mg and Fe which promote sintering) have been observed. The thermodynamic stability of Si3N4 is outlined and the 'region of sinterability' is established for submicron alpha-Si3N4 powders containing small amounts of BeSiN2. Proposed chemical reactions and mechanism of densification during sintering are discussed. (Author)