Fabrication of Ceramic Composites Based on a Silicon Nitride Powder with a Precipitated Sintering Additive

The process of spray drying synthesis of the charge compositions based on silicon nitride (Si3N4) with organic compounds of aluminum and yttrium in a molar ratio of 3 : 5 (stoichiometry of yttrium aluminum garnet (YAG)) as a sintering additive is considered. The sintered compositions 91.5 wt % Si3N4 + 8.5 wt % additive (in terms of YAG) were studied using X-ray diffraction analysis, scanning electron microscopy, and thermogravimetric analysis. The charge compositions were annealed in four stages up to a temperature of 1000°C to decompose organic matter and form an oxide phase of the sintering additive. The technology of high-speed (100°C/min) spark plasma sintering (SPS) in vacuum under a uniaxial pressure of 70 MPa was used to fabricate the ceramic samples 10 mm in diameter. The microstructure, mechanical properties, and phase composition of ceramics were studied. The effect of preliminary annealing of charge compositions on the structure, phase composition, and physical and mechanical properties of ceramics were studied. It has been established that preliminary multistage annealing of charge compositions affects the SPS kinetics along with the density and phase composition of the obtained ceramic. It has been established that the kinetics of SPS of the pre-annealed powders has the two-stage type of intense shrinkage. In this case, a denser ceramic microstructure is formed as compared with reaction synthesis of a sintering additive (for not pre-annealed charge composition), but pre-annealing slows down the growth of the elongated β-Si3N4 grains and results in an increase in the volume of a sintering additive phase. It was shown that the sintering of ceramics from unannealed charge compositions results in a lower density but higher hardness of the final material. On the basis of the Young–Cutler model, the activation energy of the SPS process was determined and it was shown that, in both cases, the kinetics of compaction of the “Si3N4–sintering additive” powders is determined by the intensity of the viscous flow of the oxide phase along the grain boundaries of ceramic.