Abstract
Fracture behavior of Si3N4 and SiC ceramics was investigated in corrosive environments at high temperature. Corrosion tests were made under various stress conditions using specimens coated with molten salts at l000°C in air. Flexural strengths of corroded specimens were investigated at room temperature. Stress-time-dependent and stress-cycle-dependent fracture tests were also performed in air at l000°C using specimens coated with a moltan salt. The following results were obtained by analyzing the chemical reaction and fracture process. ( 1 ) Molten salts of sodium corroded Si3N4 at l000°C in air. The major mode of corrosion attack was the dissolution of the grain boundary phase. The formation of large pits resulted in strength degradation. Strength degradation of SiC was only 5%. ( 2 ) Applied stress affected the morphology of bubbles in layers corroded by molten salts. A model is proposed for the defect formation under tensile and compressive stress. ( 3 ) Stress-cycle-dependent strength of Si3N4 was lower than stress-time-dependent strength under a sodium salt condition at 1000°C.
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