Mechanisms of Deformation of Silicon Nitride and Silicon Carbide at High Temperatures

Published

October 16, 2008

Author(s)

Sheldon M. Wiederhorn, B Hockey, J French

Abstract

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, K_lc = (2 to 4) MPa.m^1/2, and has a low bending strength, α_b = (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, K_lc = (6 to 8) MPa.m^1/2, 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.

Citation

Journal of the European Ceramic Society

Pub Type

Journals

Keywords

ceramics, creep, deformation, silicon carbide, silicon nitride

Citation

Wiederhorn, S. , Hockey, B. and French, J. (2008), Mechanisms of Deformation of Silicon Nitride and Silicon Carbide at High Temperatures, Journal of the European Ceramic Society (Accessed April 28, 2024)