Nicalon carbide, fiber

Manufacture of P-Silicon Carbide. A commercially utilized appHcation of polysdanes is the conversion of some homopolymers and copolymers to siHcon carbide (130). For example, polydimethyl silane is converted to the ceramic in a series of thermal processing steps. SiHcon carbide fibers is commercialized by the Nippon Carbon Co. under the trade name Nicalon (see Refractory fibers). 

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 multifilament fiber (10-20 xm diameter) as commercially produced consists of a mixture of /3-SiC, free carbon and SiOj. The properties of this fiber are summarized in Table 6.5. The properties of Nicalon start to degrade at temperatures above about 600°C because of the thermodynamic instability of composition and microstructure. A ceramic grade of Nicalon, called Hi Nicalon, having low oxygen content is also available Yet another version of a multifilament silicon carbide fiber is Tyranno, produced by Ube Industries, Japan. This is made by pyrolysis of poly (titano carbosilanes) and contains between 1.5 and 4wt% titanium. 

This process is carried out in Japan by Nippon Carbon Co. to make NICALON silicon carbide fibers, with high tensile strength and excellent temperature and oxidation resistance. It can also be used to generate coatings and solid objects. Modifications of the basic process include the addition of borosiloxanes as catalysts, and the incorporation of titanium, in the form of titanium alkoxides, to produce fibers containing titanium and oxygen as well as silicon and carbon. 

A continuous CVD fiber coating process is being explored for the preparation of a low cost, high strength, thermally stable silicon carbide fiber tow. By depositing a 5 pm thick layer of stoichiometric SiC onto each filament of a carbon fiber tow it is possible to prepare fibers that are 89 vol.% SiC which have twice the tensile strength of the commonly employed Nicalon fiber. In addition, the CVD fiber has superior resistance to creep. An economic analysis indicates that the fibers could be produced for 50/lb compared to 300/lb for Nicalon. 

Takeda, M. 1996. Mechanical and structural analysis of silicon carbide fiber Hi-Nicalon types. Ceramic Engineering and Science Proceedings 17(4-5) 35 2. 

Hi-Nicalon/Celsian composites are stable up to use temperature of 1100°C in oxidizing environments and degrade at higher temperatures due to the instability of polymer-derived fibers. The stability of Celsian matrix composites may be extended to higher temperatures by more uniform and stable interface coating(s) and by reinforcement with more advanced silicon carbide fiber (Sylramic) for applications as hot components (combustion liner, air foil, nozzle, etc.) in turbine engines. 

C. A 90% converted CVI silicon carbide fiber had about the same diameter. Its tensile strength was higher than that of the 100% CVI SiC fiber (1.7 GPa) at room temperature and at 1300°C, but the fiber lost strength above 1300 C. By way of comparison, a CVD-derived sheath/core SiC/C fiber made by Textron had a diameter of 140 pirn. Its strength was 4.0 GPa at room temperature and 1.0 GPa at 1400°C. It was weaker than the 100% CVI fiber at 1500°C. Nicalon, the fourth SiC fiber shown in Figure 14, was stable to about 1250°C, but lost strength above 1300 C. 

Simon, G. and Bunsell, A.R. (1984) Mechanical and structural characterization of the nicalon silicon-carbide fiber. J. Mater. Sci., 19 3649-3657. 

Nicalon, Silicon carbide fiber, Dow Coming Corp. 

Yajima et al. have reported the synthesis of continuous silicon carbide fibers from an organic polymer [30]. SiC fibers have been commercially produced by Nippon Carbon Co. under the trade name Nicalon , and similar SiC fibers containing Ti are produced by UBE Industries under the trade name Tyranno fiber [31]. 

The commercially produced continuous and multifilament Nicalon (Hercules) fiber is produced from polydimethylsilane however other organosilicon polymers have been used for the production of silicon carbide fiber. Polydimethylsilane is first distilled to remove the low molecular weight components, and polymer of average molecular weight 1500 is melt spun at 280°C and cured in air at 200°C. The fiber is then heat treated between 800 and 1500°C in nitrogen or vacuum. Optimum mechanical properties are achieved at ca 1250°C. Listed properties of the Nicalon fiber are modulus 200 GPa and tensile strength 2.8 GPa (1). 

Silicon carbide fibers are Nicalon (Nippon Carbon Co.). 

This well known work formed the basis for Nippon Carbon "Nicalon " silicon carbide fibers, currently distributed in the United States by the Dow Corning Corporation. 

Table 5.9 presents room-temperature properties of one of these materials, enhanced silicon carbide fiber-reinforced silicon carbide, which has a SiC matrix containing proprietary additives that improve oxidation resistance. The composite is reinforced with a plain weave fabric woven from CG Nicalon silicon carbide fibers. 

Other organosilicon polymer precursors for ceramics have either been prepared or improved by means of transition metal complex-catalyzed chemistry. For instance, the Nicalon silicon carbide-based ceramic fibers are fabricated from a polycarbosilane that is produced by thermal rearrangement of poly(dimethylsilylene) [18]. The CH3(H)SiCH2 group is the major constituent of this polycarbosilane. 

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). 

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. 

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