Compressive creep of hot pressed silicon carbide

Abstract

Silicon carbide has a good match of chemical, mechanical and thermal properties and therefore is considered an excellent structural ceramic for high temperature applications. The aim of the present work is compressive creep evaluation of liquid phase sintered silicon carbide with aluminum and rare earth oxide as sintering aids. Rare earth oxides are possible additives considering their highly refractory remnant grain-boundary phase and lower synthesis costs compared to high purity rare earth. Samples were prepared with silicon carbide powder (90wt%) and aluminum oxide (5wt%) plus rare earth oxide (5wt%) additions. Powders were mixed, milled and hot pressed at 1800°C in argon atmosphere. Compressive creep tests were carried out under stress from 150 to 300MPa and temperatures from 1300 to 1400°C. At lower creep test temperatures, the obtained stress exponent values were correlated to mechanisms based on diffusion. At intermediate temperatures, grain-boundary sliding becomes operative, accommodated by diffusion. At higher temperatures cavities are discernible. Oxidation reactions and ionic diffusion result on surface oxidized layer, grain-boundary amorphous and intergranular crystalline Al6Si2O13, δ-Y2Si2O7 and YAG phases. In this case cavitation and amorphous phases redistribution enhance grain-boundary sliding, not accommodated by diffusion. Coalescence occurs at triple point and multigrain-junctions, with subsequent strain rate acceleration and cavitational creep.