Tensile strength silicon carbide fibers

Yajima, S., Hasegawa, Y, Okamura, K., Matsuzawa, T. (1978b). Development of high-tensile strength silicon-carbide fiber using an organosiliconpol3mier precursor. Nature, 273(5663), 525-527. doi 10.1038/273525a0. 

Figure 19.3. Tensile strength of CVD silicon-carbide fiber as a function of temperature.

Yajima. S., Hayashi. J., Omori, M. and Okamura, K. (1976). Development of a silicon carbide fiber with high tensile strength. Nature 261, 683-685. 

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. 

Silicon carbide fibers wifh diameters between 100 and 500 pm and alumina hollow fibers with a wall thickness of 30 pm were successfully produced for microtool applications like heat exchanger tubes, refractory tools, thermocouple protection tubes and electric heating elements. Examples of these two fibers are shown in Fig. 5. The strength of the SiC fibers was shown to be as high as for SiC bulk material 800 MPa (tensile test with a free clamping length of 50 mm), which is sufficient for microtool applications. 

S. Yajima, J. Hayashi, M. Omori, and K. Okamura, Development of Silicon Carbide Fiber with High Tensile Strength, Nature, 1976, 261, 683. 

The other method consists in decomposing halides on an electrically heated metal filament, which then forms the core of the inorganic fiber. Silicon carbide fibers produced in this way (from the decomposition of SiCU with, for example, toluene or acetone) possess tensile strengths of up to 280,000 N/cm. ... 

A stoichiometric CVI silicon carbide fiber made by MER had a diameter of 10 jm. Its tensile strength was 1.1 GPa both at room temperature and at 1400°C, and the fiber was stable to... 

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. 

In spite of the relatively poor tensile strengths of the fibers, carbonized Kevlar possesses characteristics that make it an ideal candidate as a substrate for chemical vapor deposition of silicon carbide including a compatible coefficient of thermal expansion, sufficient tensile strength to survive the coating process, availability from a reliable domestic supplier, and a smooth, even diameter of less than 7 fim (78). 

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). 

Yajima S, Hasegawa Y, Hayashi J, limura M (1978) Synthesis of continuous silicon carbide fiber with high tensile strength and high Young s modulus. Part 1. Synthesis of polycarbosilane as precursor. J Mater Sci 13(12) 2569-2576... 

S. Yajima, K. Okamura, J. Hayashi (1975), Structural analysis in continuous silicon carbide fiber of high tensile strength, Chem. Lett. 1209. 

Fig. I.IB illustrates fibers typical of commercial asbestos, while Fig. l.ll shows Fiberglas and Fig. I.IJ silicon carbide whiskers. Some of the fibers in these examples are bent, occasionally through 180°, indicating considerable flexibility. Whiskers of other compounds can also bend but the tensile strength of these materials is their most remarkable feature. The measured values (Table 1.2) are at least ten times higher than those observed for the same compounds in bulk or in another morphology (Walker and Zoltai, 1979). The numerous investigations into the causes of this unique response have produced several hypotheses.

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