Mechanisms of Deformation of Silicon Nitride and Silicon Carbide at High Temperatures
Sheldon M. Wiederhorn, B Hockey, J French
This paper compares the relative merits of liquid-phase sintered Β-Si3N4 with sintered α-SiC for high-temperature applications. These materials represent two extremes of ceramic microstructure: liquid-phase sintered Β-Si3N4 contains grains that are coated by a second phase, whereas sintered α-SiC contains grains that are in direct crystalline contact. As will be shown, the mechanical behavior of the two materials differs substantially. At temperatures up to 1500 degrees C, sintered α-SiC is a creep-resistant solid. At room temperature, however, it is brittle, Klc = (2 to 4) MPa.m1/2, and has a low bending strength, αb = (400 to 500) MPa. By contrast, liquid-phase sintered Β-Si3N4 is not as creep resistant since it contains a residual sintering aid at its grain boundaries that deforms at a lower temperature than the silicon nitride grains. Hence, its temperature capability is less than that of sintered α-SiC. Silicon nitride is, however, tougher, Klc = (6 to 8) MPa.m1/2, and stronger, α-b = (700 to 1000) MPa, than sintered α-SiC. Deformation of liquid-phase sintered Β-Si3N4, and other ceramics with a second phase at the grain boundaries, depends on the refractoriness of that phase, the more refractory the phase, the more resistant the material is to creep. Experimental results on Β-Si3N4 suggest that toughness decreases as creep resistance increases; hence, a trade-off must be made between creep resistance and material toughness to achieve an optimal high temperature microstructure.
, Hockey, B.
and French, J.
Mechanisms of Deformation of Silicon Nitride and Silicon Carbide at High Temperatures, Journal of the European Ceramic Society
(Accessed December 1, 2023)