Heat-exposure effects on the crystalline development and the strength degradation of silicon carbide fibers

Abstract

Strength measurement under tensile stress and analysis of fractured surfaces were conducted for single SiC fibers heat-treated at 1300° C from a tension test. The tensile strength of the heat treated fibers was reduced by 45% compared with as-receive fibers. Pits are formed on the fiber surface during heat-treatment and these pits can be fracture origins under tensile stresses. From the results by TEM, WDS, XRD and SEM analyses, the pits can be identified as a SiC crystal region which grew abnormally with many microvoids. From the SEM observation, it is seen that all the fracture origins are surface flaw-pits. The shape of the fracture origins is a half-penny shaped crack on a surface perpendicular to the tensile axis and the relation between the depth (b) and the width (c) is found to be b = 0.5c. The fracture toughness, KIc, of the heat-treated fibers has been calculated from the relation between the depth of the fracture origin and the tensile strength and it was found that the value of fracture toughn...