Polysilazane fibers

Polysilazane fibers are rendered infusible by humidity or in the absence of oxygen by ammonia. The final step of producing ceramic fibers is the pyrolysis. The cured fibers are heated at 1200 -1300°C in argon, nitrogen, or in vacuo, and SiC- or SiC/SijN fibers with a diameter of around 15 /xm are obtained. Heating up silicon-polymers, whether polysilanes or polysilazanes, results in the evolution of CH4 and H2. 

In the area of preceramic polysilazanes, sufficient progress has been made to produce precursors for silicon nitride fibers, coatings and as binders for silicon nitride powder. However, particular problems still remain to be solved particularly with regard to reducing impurity levels and improving densification during pyrolysis. 

Summary A brief review of the preparation of silicon containing preceramic polymers to prepare silicon carbide and silicon carbonitride fibers is given. Methylchlorodisilanes are converted to polysilanes and polysilazanes which yield ceramic fibers after meltspinning, curing, and pyrolysis. 

The polysilazanes were also melt spun, cured, and pyrolyzed to give silicon carbonitride fibers (Eq. 7). The carbon content of these fibers depends on the molecular composition of the polysilazane and the pyrolysis gas. When ammonia is used as reactive gas pure silicon nitride fibers will be obtained (Eq. 8) [14]. 

Figure 4. shows the route from the high boiling residue of the direct synthesis to silicon carbo-nitride fibers. Methylchlorodisilanes and trichlorosilanes as additives are mixed in a specific ratio and react with methylamine and a small amount of ammonia to form an aminodisilane/oligosilazane. The subsequent polycondensation reaction of this mixture by heating to 250 °C yields a soluble and melt spinnable polysilazane. In comparision with the polysilane the properties of the polysilazane depend on the ratios of the disilanes/silanes and methylamine/ammonia and also on the reaction conditions. 

Development has been conducted to fabricate SiC-based matrix/Si-C-based fiber composites by polymer infiltration. Nicalon fibers are used, and various polymers such as polysilazanes, polysiloxanes, and polycarbosilanes are used to yield matrices of SiCO, SiNC, and SiC. As with CVI of SiC/SiC composites, carbon acts as an interface layer but does not result in stability at high temperatures. 

Access to phase pure silicon nitride materials via processable precursors is limited to just three approaches. The first, shown in reaction 6, provides one of the first oligomers exploited as a preceramic polymer24,253. This simple polysilazane, containing only Si, N and H, is known to be relatively unstable and will crosslink on its own to give intractable gels. Furthermore, it does not offer the 3Si I4N stoichiometry required for Si3N4. Nonetheless, it is useful as a binder and for fiber-reinforced ceramic matrix composites (CMCs)31. 

Thus the chemistry leading to the desired ceramic product is quite satisfactory most of the requirements mentioned earlier are met. Initial evaluation of the polysilazane shows that it has promise in three of the main potential applications of preceramic polymers in the preparation of ceramic fibers and ceramic coatings and as a binder for ceramic powders. 

Gaul and co-workers (13,14) prepared a variety of polysilazanes by using chlorosilanes and hexamethyldisilazane to control polymer molecular weight, rheology, and spinnability (equation 9). Fibers were prepared from the poly(methyldisilylazane) (MPDZ) system (equation 10). 

A novel process for polysilazane preparation has been developed via transaminationy which does not require the use of chlorosilane intermediates. Silicon nitride, especially as fibers or coatings,... 

The drawing of these precursor polysilazane polymers to form fibers and their subsequent pyrolysis to silicon nitride fibers is a complex process that will be reported separately. 

Besides the continuous fibers, application of metallorganic polymers to heat-resistant coatings, dense ceramic moldings, porous bodies, and SiC matrix sources in advanced ceramics via polymer infiltration pyrolysis (PIP) have been developed. Novel precursor polymers have been synthesized and investigated for ceramics in addition to PCS (Table 19.1). For SiC ceramics, various Si-C backbone polymers have been synthesized. Their polymer nature (e.g., viscosity, stability, cross-linking mechanism, and ceramic yield) are, however, fairly different from PCS. On the other hand, polysilazane, perhydropolysilazane, polyb-orazine, aluminum nitride polymers, and their copolymers have been investigated... 

Routes via polysilazanes, in contrast, lead to soluble or fusible intermediates that are prerequisites for the formation of coatings and fibers. Although a route enabling the formation of polysilazanes was already reported by Stock in 1921 [41], numerous routes, all leading to polysilazane intermediates, have been discussed in the literature only recently [42-49]. Dichlorosilanes and ammonia or alkyl amines are used as starting compounds. 

A soluble polysilazane was obtained by the ammonolysis of an adduct between SiH2Cl2 and pyridine [25]. A formula for the polymer is (SiH2NH)o.46(SiH2)o.36(SiH3)o.l8, and a structural model is shown in Fig. 3. This polymer is produced as the precursor for the Si3N4 fiber and Si-N coating by Tonen Corporation. 

D. Synthesis of Silicon Nitride Fibers from Polysilazanes... 

Si-C-N-0 and Si-C-N fibers are made by spinning, curing and pyrolyzing polysilazane (PSZ) or polycarbosilazane (PCSZ) precursors. PSZs have a Si-N backbone, carbon being present in pendent groups, whereas, in PCSZs, carbon is contained in a Si-C-N backbone. 

Slip infiltrated, HlPed carbon fiber reinforced material, slip infiltrated SiC fiber reinforced nitrided SisN4, and polysilazane solution infiltrated pyrolyzed composites with SiC fibers have been fabricated [159]. All materials exhibited non-brittle fracture. 

---