Oxycarbide fibers Nicalon® fiber

This mechanism accounts for the evolution of a gaseous SiO CO mixture, the growth of the SiC crystals and the deaease in the amounts of free cartxin and silicon oxycarbide. The relative amounts of CO and SiO which are formed and the composition of the final residue (i.e., SiC or SiC + C) depend upon the relative amounts of free caitton and silicon oxycarbide in the fiber. For the Nicalon fiber (NL 200), the main species in the gas phase is CO and the solid residue is SiC. There is therefore enough SiO formed by decomposition of the silicon oxycarbide phase (Equation 9) to consume all the free carbon by Equation 10 [73]. This mechanism also accounts for one of the processes used to produce oxygen-free nearly-stoichiomefric SiC fibers [38] [54]. Since CO and SiO diffuse and escape from the fiber, the decomposition starts near its surface and the decomposition front moves radially towards the fiber axis, yielding a skin/core microstructure [16]. 

The presence of amorphous oxycarbide (Si-O-C) phase was also revealed in the Nicalon fibers at the interface between SiC core and surface SiOa layer (Pumpuck, 1980 Lipowitz, 1987 Laffon, 1989 Porte, 1989), although any thermodynamically stable compounds have not been found in the Si-O-C system. The formation of such an amorphous silicon oxycarbide phase was also suggested during the pyrolysis of organics-substituted polysiloxane gels to form SiC (Zhang, 1990 Babonneau, 1990 Bums, 1992). 

Ceramic fibers of the nonoxide variety such as silioon carbide, silicon oxycarbide such as Nicalon, silicon nitride, boron carbide, etc. have become very important because of their attractive combination of high stiffiiess, high strength and low density. We give brief description of some important nonoxide fibers. 

The highest modulus of a given substrate is obtained with a single crystal structure. Single crystal CVD-SiC whiskers (578 GPa) have a stiffen more highly ordered, structure than polycrystalline CVD-SiC fibers (190-400 GPa), and sapphire whiskers and fibers (415 GPa) are stiffer than slurry spun polycrystalline alumina fibers such as Fiber FP (380 GPa). Superimposed upon this relationship is a compositional factor. Fiber modulus and structural order generally also decrease with increasing compositional complexity, e.g., silicon carbide is intrinsically stiffer than silicon oxycarbide such as Nicalon, and slurry spun alumina fibers are stiffer than sol-gel or melt spun aluminate fibers. 

As the temperature is raised to domain 3, hydrogen corresponding to the residual C-H bonds from the Si-CHrSi backbone is progressively released with a continuous but slow density increase. Above about 1000°C, clusters of free carbon and nanocrystals of p-SiC are formed. At the end of domain 3, ceramic grade Nicalon Si-C-0 fibers consist of a dispersion of free carbon clusters and SiC nanocrystals in an amorphous Si-C-0 matrix [14]. As the temperature is increased to domain 4, growth of p-SiC nanocrystals and decomposition of ternary silicon oxycarbide occurs in the pyrolytic residue, and evolution of CO and SiO is accompanied by further weight loss. 

Thus, the fibers resulting from the pyrolysis at 1200-1300°C of oxygen cured Yajima type PCS precursor fibers in an inert atmosphere are far from consisting of pure SiC since the molar fraction of SiC is about 50%. They also contain significant amounts of partly hydrogenated free carbon and silicon oxycarbide. It Is noteworthy that the value, or perhaps mean value [14] [64], of the term 1-x/2 = 0.447 is dose to 0.5 in Nicalon NL-200 (Figure 7) and that the main tetrahedral units are therefore close to SiOiCz [15]. 

The tensile strength of Si-C-0 fibers decreases after exposure to elevated temperatures. When Nicalon NL 200 fibers are exposed for 1 hour to 1300 C in argon (P = 100 kPa), their mean tensile strength and scale parameter, Co, decrease by 45% while their Weibull modulus remains unchanged [80-83]. Fibers exposed to more severe conditions (e.g., for 5 hours in a vacuum at 1500°C) are so weak that they cannot be tested. Finally, the fact that oxygen-free fibers maintain their tensile strength under similar conditions relates to the absence of silicon oxycarbide and its decomposition process. 

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