Change in properties of polycarbosilane-derived SiC fibers at high temperatures

Abstract

Using two types of polycarbosilane-derived SiC fibers, namely, Hi-Nicalon (1.8 mass% oxygen) and Nicalon NL400 (13 mass% oxygen) changes in their properties and the mechanism of deterioration were investigated at 1873 K. A short period pyrolysis changed rapidly the properties such as gas generation, tensile strength, crystallite size of β-SiC and specific resistivity. The pyrolysis and the resultant crystal growth proceeded from the surface to the core of Nicalon. The fiber strength was proportional to the reciprocal square root of crystallite size. The formation of surface flaw, as well as coarsening of crystals, was an important factor controlling the fiber strength. Nicalon, which was porous after pyrolysis, deteriorated severely. On the other hand, Hi-Nicalon retained high levels of strength because of slow crystal growth and suppressed gas liberation.