EFFECT OF GRAIN-BOUNDARY CRYSTALLIZATION ON THE MECHANICAL AND ALKALINE CORROSION CHARACTERISTICS OF SINTERED SILICON-NITRIDE

To evaluate an effect of grain boundary crystallization on corrosion characteristics of sintered silicon nitride in alkaline, weight-loss was measured during corrosion test in boiling 30 mass% NaOH aqueous solution up to 200h for as-sintered and heat-treated Si3N4 (1100 degrees C, 3h + 1400 degrees C, 6h). As-sintered and heat-treated Si3N4 showed 0.8 mass% and O.1 mass% weight decrease after 200h, respectively, The grain boundary crystallization was effective to improve the corrosion resistance of Si3N4, This could be attributed that quarternary oxynitrides Y8Si4N4O14 and Y20Si12N4O18, called as J-phase and H-phase, respectively, crystallized in intergranular liquid by the heat treatment, resulting in a decrease of residual secondary glass phase in grain boundaries. Mechanical characteristics including crushing load of Si3N4 ball, fracture toughness and hardness were evaluated, and the grain boundary crystallization improved these properties slightly.