THERMAL-EXPANSION OF SILICON-CARBIDE MONOFILAMENTS AND SILICON-CARBIDE BOROSILICATE COMPOSITES

The coefficient of thermal expansion (CTE) of a 140-mum-diameter SiC monofilament was determined to be 6.5 +/- 0.5 x 10(-6)-degrees-C-1 in the temperature range 25-degrees-900-degrees-C. Heat treatment of the fibers at 1400-degrees-C for 90 min resulted in both an increase in the intensities of the SiC peaks and a reduction of their width, indicating some grain growth. Furthermore, heat treatment of the fibers for 2 h at 1600-degrees-C in vacuum reduced their CTE to 5.7 x 10(-6)-degrees-C-1. The anomalously high CTE and its decrease upon heat treatment were attributed to the presence of a large fraction of the atoms in the boundaries between the nanometer beta-SiC grains. The thermal expansion coefficients of a series of SiC/borosilicate composites were measured as a function of fiber volume fraction in the temperature range 25-degrees-500-degrees-C and found to follow the rule of mixtures.