EFFECTS OF HEAT-TREATMENT IN A WET HYDROGEN ATMOSPHERE ON THE RELIABILITY OF SINTERED ALPHA-SILICON CARBIDE

Sintered alpha-SiC was exposed, for times up to 2 h, to a flowing wet H-2 atmosphere (P-H2O = 1 x 10(-4) MPa) at temperatures of 1300 degree s, 1400 degrees, and 1500 degrees C. The effect of such conditions on the reliability of the ceramic was estimated by comparing the Weibull modulus of the groups of specimens, tested in four-point flexure, befo re and after exposure. The Weibull modulus of as-polished specimens wa s 6.7, indicating a wide variation in room-temperature flexural streng th. The Weibull modulus was increased to 14.2 by the heat treatment fo r 2 h in wet H-2 at 1400 degrees C. The average strength was also impr oved from 347 to 446 MPa by such exposure. Heat treatment at 1300 degr ees and 1500 degrees C also improved the reliability of the material, as indicated by increases in the Weibull modulus, but to less a degree than did exposure at 1400 degrees C. The increases in reliability and average strength were attributed to the blunting of surface flaws by the formation of a thin SiO2 layer on the sample surface.