Thermal expansion of high-modulus fibers

Abstract

A resistance-heating technique was used to measure the axial thermal expansion of high-modulus boron and silicon carbide (SiC) fibers from room temperature to 700 and 1500°C, respectively. Both types of fibers investigated in this study were manufactured by a chemical vapor deposition (CVD) process. The boron fibers examined here are composed of boron and a tungsten boride core arising from reaction of deposited boron with a tungsten wire substrate. The composition of the SiC fibers consists of a SiC sheath with a carbon-rich outer coating surrounding an unreacted pyrolytic graphite coated carbon core. The thermal expansion of boron fibers was found to increase parabolically with temperature up to 700°C. Above this temperature the fiber contracted due to void migration and subsequent residual stress relaxation. For SiC fibers, a relatively small initial expansion from room temperature to 450°C was observed. Above 450°C the expansion was found to increase linearly with temperature up to 1300°C, where a hysteresis effect was observed involving a 50% reduction in expansion. Possible explanations for this hysteresis effect were considered and different theories presented. Volume percentage of carbon core was varied and found to have negligible effect on expansion. The conclusion was reached that expansion of these SiC fibers is controlled by the SiC sheath.