Mechanisms of deformation of silicon nitride and silicon carbide at high temperatures

This paper compares the relative merits of liquid-phase sintered beta-Si3N4 With sintered alpha-SiC for high-temperature applications. These materials represent two extremes of ceramic microstructure: liquid-phase sintered be ta-Si3N4 contains grains that are coated by a second phase, whereas sintere d alpha-SiC contains grains that are in direct crystalline contact. As will be shown, the mechanical behavior of the two materials differs substantial ly. At temperatures up to 1500 degrees C, sintered alpha-SiC is a creep-res istant solid At room temperature, however, it is brittle, K-Ic = (2-4) MPa. m(1/2), and has a low bending strength, sigma(b) = (400-500) MPa. By contra st, liquid-phase sintered beta-Si3N4 is not as creep resistant since it con tains a residual sintering aid at its grain boundaries that deforms at a lo wer temperature than the silicon nitride grains. Hence, its temperature cap ability is less than that of sintered alpha-SiC. Silicon nitride is, howeve r, tougher, K-Ic = (6-8) MPa.m(1/2), and stronger, sigma(b) = (700-1000) MP a, than sintered alpha-SiC. Deformation of liquid-phase sintered beta-Si3N4 , and other ceramics with a second phase at the grain boundaries, depends o n the refractoriness of that phase, the more refractory the phase, the more resistant the material is to creep. Experimental results on beta-Si3N4 sug gest that toughness decreases as creep resistance increases; hence, a trade -off must be made between creep resistance and material toughness to achiev e an optimal high temperature microstructure. (C) 1999 Elsevier Science Ltd . All rights reserved.