Structure and thermal evolution of SiC-based fibers with low oxygen content

Polycarbosilane-derived ceramic grade fibers have been commercialized for a decade under the trade name Nicalon (CGN). However, some problems associated with this fiber have limited its ultimate use as reinforcement of CMCs. Besides SiC (49 mol%), a large amount of free carbon (28 mol%) and an amorphous SiO{sub l.12}C{sub 0.44} phase (23 mol%), are also present in the fiber. At high temperatures (typically above 1200{degrees}C) the silicon oxycarbide phase decomposes, forming gaseous species such as CO and SiO, whose diffusion through the fiber and reaction with the free carbon are believed to create pores and other defects in the fiber. New routes for the fabrication of ceramic fibers have been investigated. The main break-through concerns the curing of PCS filaments by {gamma}-rays or electron beam irradiation (EB). In this process, the cross-linkage between polymeric chains of the precursor is mainly achieved by forming Si-C bonds between polymeric chains, instead of Si-O-Si bridges as in oxygen curing. Compared to fibers cured in air, the strength of fibers cured by electron irradiation remained almost unaffected after heat treatment to 1400{degrees}C. A slight increase in Young`s modulus was even noticed at these temperatures. In this study, the thermal stability in argon of anmore » home-made experimental SiC-based fiber (O : 3 at%), prepared by electron irradiation of PCS filaments, was investigated on the chemical and structural point of view and compared to the Hi Nicalon Si-C based fiber (O : 0.5 at%) known to be obtained by the same process.« less