Nicalon fiber tensile strength

Some fibers exhibit fracture mirrors when they fail within a composite (e.g., Nicalon). A semi-empirical calibration has been developed that relates the mirror radius, am, to the in situ fiber tensile strength, S, given by (Fig. 1.20),... 

Figure 6.26 Tensile strength and modulus of Nicalon type fibers as a function of temperature and oxygen content (after Okamura etal, 1993).

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. 

FIGURE 12.14 (a) Plot of the wave number and bandwidth (full width at half height) of the C-C raman peaks for different SiC fibers after various thermal treatment in air or in nonoxidizing atmospheres, (b) Comparison between ultimate tensile strength and sp peak FWHH as a function of thermal treatment for SiC Hi Nicalon fibers. (Adapted from Colomban, P., Raman microscopy and imaging of ceramic fibers in CMCs and MMCs, Ceramic Trans., 103, 517, 2000. With permission.)... 

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. 

UBE Chemicals synthesized amorphous Si-Ti-C-O fibers from the PCS-titanium alkoxide compound polymer. These so called Tyranno fibers show excellent properties and can be spun thinner than the Nicalon fibers (see Table 4). However heating above 1000°C results in a crystallization of the fiber. In the mid 1990s the German company Bayer AG synthesized an amorphous Si-B-N-C fiber, by pyrolysis of a polyborosilazane polymer [56]. This SiBN3C fiber (see Fig. 11) has a tensile strength of 3 GPa and maintains its amorphous character up to 1800°C. The advantage of the production route from liquid to solid to produce SiC has also attracted attention for... 

Tensile testing was carried out in air at a displacement rate of 0.02 /minute. Tensile properties of 0/90° cross-ply Nicalon/BN/SiC/BSAS composites, containing 38-40 volume per cent of fibers, at various temperatures are shown [29-31] in Table 4. Above 1100°C, ultimate strength and the proportional limit fall off fairly rapidly, while modulus decreased by -40% from ambient temperature to 1300°C. Nicalon fiber is known to degrade at 1200°C and degradation rate increases with increase in temperature. Presence of residual glassy phase in the matrix would account for the observed decrease in modulus and increase in failure strain at elevated test temperatures. 

Several other studies on Nicalon-based ceramic fibers have also been conducted in addition to the investigation of oxidation curing of PCS fibers and effect of oxygen in tensile strength of SiC fibers [51]. Similarly, studies dealing with the chemistry, characterization, modification, use, and applications of polysilanes and polycarbosUane are also available [52]. 

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. 

Figure 12. Weibuli plot of tensile strength data for Si-C-0 (Nicalon NL 202) fibers, with a unimodal Weibuli...

The tensile strength of Si-C-0 fibers decreases after exposure to elevated temperatures. When Nicalon NL 200 fibers are exposed for 1 hour to 1300 C in argon (P = 100 kPa), their mean tensile strength and scale parameter, Co, decrease by 45% while their Weibull modulus remains unchanged [80-83]. Fibers exposed to more severe conditions (e.g., for 5 hours in a vacuum at 1500°C) are so weak that they cannot be tested. Finally, the fact that oxygen-free fibers maintain their tensile strength under similar conditions relates to the absence of silicon oxycarbide and its decomposition process. 

Young s modulus and tensile strength of Si-C-0 and Si-C fibers decrease when the test temperature is increased (Figure 13). The room temperature tensile strength of the Si-C fibers (Hi-Nicalon) is higher than that of Si-C-0 fibers (Nicalon NLM 200), but the two fibers have almost the same strength, (1200 MPa), when tested at 1400°C. At any test temperature, the modulus of the Si-C fiber is higher than that of the Si-C-0 fiber [31] [84]... 

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). 

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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