form the desired ceramic. Three basic differences from carbon fiber processing, however, dominate the Si ceramic fiber technology. They are:

• The insensitivity of final fiber properties to orientation steps during processing

• Detrimental effects of oxidative crosslinking on ceramic fiber properties. In carbon fiber production, oxygen containing moieties less thermodynamically stable than the desired carbon structure are expelled from the structure during pyrolysis. In Si-based ceramics SiO₂ is the thermodynamically most stable form and the compound least desirable in the final ceramic structure.

• Carbon fiber is normally produced from high molecular weight, mechanically sound, environmentally stable precursor fibers amenable to fiber-handling technology. The precursors for silicon-based nonoxide ceramic fibers are brittle and environmentally unstable, leading to major difficulties in handling and storage. This results in fibers of relatively short lengths and a high degree of variability.

Over the past few years it has been shown that the strength of SiC and Si₃N₄ fibers is a very strong function of the flaw content of the final ceramic and that this flaw content is, to a very large extent, a function of impurities and lack of homogeneity in the starting polymer. It was further shown that strength loss at elevated temperatures and in various chemical environments is, to a significant degree, caused by exacerbation of existing flaws rather than by creation of new ones. It was also established that all Si-based ceramic fibers derived from polymeric precursors are classically brittle materials and that they all fit a universal curve of strength (at room or elevated temperature) versus reciprocal root of flaw size, as shown in Figure 3.6.

While linear, high molecular weight, Si-containing polymers of excellent spinability are known, these are not useful as precursors for ceramic fibers because, under the condition needed to cure the fibers, thermodynamically stable Si ring systems are formed and fiber integrity is lost. To overcome the ring-formation tendency, precursor polymers of low molecular weight and high degree of branching are used. Some of the more commonly used precursor structures are shown in Figure 3.5. All of these polymers contain non-stochiometric amounts of Si, C, nitrogen, and oxygen (based on the desired final ceramic), and are difficult to characterize. All of these factors make continuous spinning of infinite-length fibers and yarns both difficult and expensive.

As already stated, to maximize ceramic fiber properties, the cure step must minimize introduction of oxygen and, because of fiber instability and brittleness, be kinetically compatible with spinning speeds as much as possible.