Nicalon production process

As can be seen from this figure, the heat-resistance was remarkably improved by the drastic changes in the microstructure from amorphous to polycrystalline structure. Another type of SiC-based fiber, SA fiber (2), has a sintered SiC polycrystalline structure and includes very small amounts of aluminum. This fiber exhibits outstanding high temperature strength, coupled with much improved thermal conductivity and thermal stability compared with the Nicalon and Hi-Nicalon fibers. The fabrication cost of the SA fiber is also reduced to near half of that of the Hi-Nicalon Type S [ 17]. The SA fiber makes SiC/SiC composites even more attractive to the many applications [18]. In the next section, the production process, microstructure and physical properties of the SA fiber are explained in detail. 

Yajima s innovation directly contributed to Si-C-0 fiber (Nicalon NCK, Toyama, Japan) and Si-Ti-C-0 fiber (Tyranno Ube Industries, Tokyo, Japan) production in the early years. The Tyranno fiber production process showed the possibility of introducing various alkoxides in the starting PCS to modify the resulting SiC grain boundaries. 

As noted earlier, CVl is nsed primarily to form ceramic-fiber-reinforced ceramic matrix composites. The most common of these combinations is SiC fiber/SiC matrix composites. One commercially available product has a two-dimensional 0/90 layup of plain weave fabric and fiber volume fraction of about 40%. This same composite can be fabricated with unidirectional fibers and with 45° architectures. The most commonly used SiC fiber for the preforms is Nicalon , the mechanical properties for which were provided earlier in Section 5.4.2.7. A number of other carbide and nitride fibers are also available, including Si3N4, BN, and TiC. Preform geometries can be tailored to the application in order to maximize strength and toughness in the direction of maximnm stresses. The reactions used to form the matrix are similar to those used in CVD processes (cf. Section 7.2.4) and those described previously in Eq. (3.105). 

The discovery by Yajima that polysilanes could be pyrolyzed to silicon carbide was mentioned in the introduction.7 In this process, either (Me2Si) or the cyclic oligomer (Me2Si)6 are synthesized from Me2SiCl2 and are then heated to near 450 °C (Scheme 5.10). This discovery has been commercialized by the Nippon Carbon Co. for the production of NICALON silicon carbide fibers. In this process, methylene groups become inserted into many of the Si-Si bonds to give a polycarbosilane polymer with the idealized 5.14. 

The polymer is heated until molten, forced through an appropriately sized aperture, and solidified as it is drawn into a fibre the commercial process produces multifilament fibres rather than single strands and cross-linking of the filaments adds strength to the final product. The composition of Nicalon fibres is not as simple as this description implies carbon and silica are present in addition to P-SiC. 

This so-called polymer route was introduced by Chantrell and Popper [10], who proposed the use of inorganic polymers as starting materials for the preparation of ceramics, opening up the rather ambitious perspective of easily shaping monolithic green bodies via this route. At the end of the 1960s, Winter et al. [11, 12] pioneered such a process for the production of Si/C/N fibers, and developed it to technical feasibility. Following the route developed by Yajima et al. [13, 14], ceramic fibers (SiC, Nicalon) have been available on... 

The chemistry is similar to that of powder production. The basic precursor is usually polycarbosilane which is a pale-yellow solid with a melting point of 225 C. A typical process flowchart is shown,in Fig. 14.4. The chemical solution is partially thickened by polymerization or by additives and spun directly into fibers. The resulting green fibers are dried and pyrolized.I lt l These fibers are produced commercially by Nippon Carbon Co. and distributed in the US under the trade name of Nicalon by Dow Coming Corp., Midland MI. They are also produced by another Japanese firm, UBE Industries under the tradename of Tyrarmo. 

Figure 14.4 Melt-spinning process for the production of Nicalon fibers.I ...

The resistance of SiC to high-temperature working and oxidation makes it a valuable advanced material. Fibres of p-SiC are produced by CVD using R4 SiCl ( precursors or an alkane and chlorosilane in a reactor similar to that in Fig. 28.26. Fibres marketed under the tradename of Nicalon are produced by a melt-spinning process. This begins with reactions 28.21 and 28.22, the products of which are pyrolysed to give a carbosilane polymer (scheme 28.23). 

Nicalon and Tyranno ceramic fibers, two well-known preceramic derived commercial products, are marketed for structural applications. Nicalon is a SiC based ceramic fiber processed using chemistry and techniques first developed by Yajima and coworkers [6-14]. Tyranno fibers are SiC/TiC based fibers produced via novel modifications to the original Yajima work [15-17]. Elastic moduli and tensile strengths for both fibers are of the order of 250-300 GPa and 2-3 GPa respectively. Textron s CVD SiC fibers (not preceramic) offer tensile strengths of up to 4 GPa [18]. The elastic modulus of sintered, hot pressed SiC is in the range of 400-450 GPa [19]. These compare with tensile strengths of =< 8 GPa and an elastic modulus of= 580 GPa for single crystal, SiC whiskers [18]. 

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