SiC fibers

Sihcon carbide is also a prime candidate material for high temperature fibers (qv). These fibers are produced by three main approaches polymer pyrolysis, chemical vapor deposition (CVD), and sintering. Whereas fiber from the former two approaches are already available as commercial products, the sintered SiC fiber is still under development. Because of its relatively simple process, the sintered a-SiC fiber approach offers the potential of high performance and extreme temperature stabiUty at a relatively low cost. A comparison of the manufacturing methods and properties of various SiC fibers is presented in Table 4 (121,122). 

Fig. 3. Load—deflection curve for a SiC—C—SiC composite in four-point bending. Note the extreme change in behavior fora composite fabricated with a 0.17-p.m carbon layer between the SiC fiber and the SiC matrix as compared with a composite with no interfacial layer (28).

Fig. 10. Crack pinning by a SiC fiber in a glass matrix, photographed using an optical microscope and Nomarski contrast. Fiber ties perpendicular to plane of micrograph lines represent crack position at fixed intervals of time, crack mnning left to right.

T. Ishikawa, T. Nagaoki (Eds.), in Recent Carbon Technology Including Carbon SiC Fibers, JEC Press, Cleveland, OH, 1983. 

Properties. CVD boron fibers have high strength, high modulus, and low density. Their properties are summarized and compared with SiC fibers and other inorganic fibers in Table 19.2 (data supplied by the manufacturers). 

Properties. Properties of SiC fibers are shown in Table 19.2. They are similar to those of CVD boron fibers except that SiC is more refractory and less reactive than boron. CVD-SiC fibers retain much of their mechanical properties when exposed to high temperature in air up to 800°C for as long as one hour as shown in Fig. 19.3. [ 1 SiC reacts with some metals such as titanium in which case a diffusion barrier is applied to the fiber (see Sec. 2.5 below). 

Applications. Most applications of CVD SiC fibers are still in the development stage. They include the followingP k b... 

The cyclotrisilazane (R = Me) produced in reaction (14) is recycled at 650°C [by reaction with MeNHo) the reverse of reaction (14)] to increase the yield of processible polymer. Physicochemical characterization of this material shows it to have a softening point at 190°C and a C Si ratio of 1 1.18. Filaments 5-18 pm in diameter can be spun at 315°C. The precursor fiber is then rendered infusible by exposure to air and transformed into a ceramic fiber by heating to 1200°C under N2- The ceramic yield is on the order of 54% although, the composition of the resulting amorphous product is not reported. The approach used by Verbeek is quite similar to that employed by Yajima et al. (13) in the pyrolytic preparation of polycarbosilane and its transformation into SiC fibers. 

The incorporation of decaborane into polycarbosilane-based polymers has been used in the densification of the polymer-derived SiC fibers. Improved densifi-cation resulted in the formation of fibers with density as high as 2840 kg/m3 corresponding to an increase of 89%.154... 

These high demands are not yet fulfilled by any available fiber coating. Only a C-coated SiC-fiber (NL 607) from Nippon Carbon is commercially available. To meet the above demands, a multilayer fiber coating is necessary. In a joint effort with ABB Heidelberg and TU Chemnitz, a CVD C-coating on C- and SiC-fibers and a C/SiC double CVD coating on C-fibers was developed and tested (Fig. 3). 

The oxidation-resistant SiC fiber was prepared from polycarbosilane containing Zr(OC4H9)4 by the same process as that used for the aforementioned tita-nia/silica fiber, except that the calcination was performed in Ar atmosphere at 1400 °C. In this case, the polycarbosilane and Zr(OC4H9)4 were effectively converted into SiC-based bulk ceramic and zirconium oxide (cubic zirconia). Before the conversion, bleed-out of the zirconium compound proceeded effectively. AES depth analysis of the fiber surface showed an increase in the concentration of zirconium towards the surface. This construction was confirmed by the TEM image of the cross-section near the fiber surface. This indicates the direct production of a SiC-based fiber covered with a Zr02 surface layer, which... 

Fig. 39 Alkali resistance of the Zr02/SiC fiber with comparative results... 

Ferber, M.K., Wereszczak, A.A., Hansen, D.H. and Homeay, J. (1993). Evaluation of interfacial mechanical properties in SiC fiber-reinforced macro-defect-free cement composites. Composites Sci. Technol. 49, 23-33. 

Roman, I. and Aharonov, R. (1992). Mechanical interogation of interfaces in monofilament model composites of continuous SiC fiber-aluminium matrix. Acta Metall. Mater. 40, 477-485. 

Weihs, T.P. and Nix, W. (1991), Experimental examination of the push-down technique for measuring the sliding resistance of SiC fibers in a ceramic matrix. J. Am. Ceram. Soc. 74, 524-534. 

Fig. 4.24. Plot of partial debond stress, oJJ, as a function of debond length, f, for untreated SiC fiber-glass matrix composite. After Kim et al. (1991).

Numerical treatment of Eq. (4.104) gives Zmax values for the different composite systems as shown in Table 4.3. It is worth emphasizing that for the SiC fiber-glass matrix composites, z, ax values are very small relative to values, irrespective of the fiber surface treatments and when compared to other epoxy matrix based composites. 

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