Nicalon fibers properties

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. 

SiC monofilaments produced by the CVD process is generally superior to Nicalon SiC fibers in mechanical properties because of its almost 100% 6-SiC purity while Nicalon is a mixture of SiC, Si02 and free carbon. Representative properties of SiC monofilaments and Nicalon fibers are given in Table 5.15. 

Chemical and phase purity are critical issues that drive precursor design because optimal mechanical properties are achieved only with high purity. For example, ceramics grade Nicalon fibers, with a chemical composition of ca SiCi 45O0 36 and densities 17,9... 

Fareed, A.S., Sonuparlak, B., Lee, C.T., Fortini, A.J., Schiroky, G.H. (1990), Mechanical properties of 2-D Nicalon fiber-reinforced LANXIDE aluminum oxide and aluminum nitride matrix composites , Ceram. Eng. Sci. Proc., 11(7-8), 782-794. 

SiC is an excellent nonoxide ceramic with high-temperature stability and suitable mechanical properties. Since silicon-containing polymers are generally used for preparing nonoxide ceramics, various polymeric precursors with different structures have been designed. Preceramic polycarbosilane (PCS), used for preparing commercial Nicalon fiber,... 

Table 8.1 Flexural (three-pt loading) properties of BMAS matrix/SiC/BN/Nicalon fiber composites24...

Brennen, J.J. 1997. Interfacial design and properties of layered BN(+ C) coated Nicalon fiber-reinforced glass-ceramic matrix composites, in Ceramic Microstructures 96. New York Plenum Press. 

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... 

Table 1 lists some key production and compositional details for a variety of SiC-based fiber types of current interest and availability as CMC reinforcement. The polymer-derived types range from first-generation fibers with very high percentages of oxygen and excess carbon, such as Nicalon and Tyranno Lox M, to the more recent near-stoichiometric (atomic C/Si 1) fibers, such as Tyraimo SA and Sylramic. For the CVD-derived types, such as the SCS family with carbon cores, the only compositional variables in the SiC sheaths are slight excesses of free sihcon or free carbon. Table 2 lists some of the key physical and mechanical properties of the SiC fiber types in their as-produced condition, as well as estimated commercial cost per kilogram, all properties important to fiber application as CMC reinforcement. These SiC fiber properties in Tables 1 and 2 are in most part those published by the indicated commercial vendors. It should be noted that the Sylramic fiber... 

TABLE 3. Mechanical properties of a CVI SiC/Si-B-C composite with a self healing matrix and a multilayer reinforcement of Hi-Nicalon fibers (after [3]). 

TABLE 4. Mechanical properties of 2D CVI enhanced SiC/SiC composite reinforced with 0/90 five harness satin fabrics of Hi-Nicalon fibers (source Power Systems Composites data sheet). 

Unless otherwise noted, all data presented for Prepreg HiPerCompT is for 8-ply laminates, with a balanced [0-90-90-0]s stacking of uniaxial plies, and nominally 22-25% by volume of Hi-Nicalon M fibers. Data for Slurry Cast HiPerComp is for 8-ply laminates made with 0-90,8 harness satin weave cloth and a nominal volume fraction of Hi-Nicalon fiber of 33-38%. Measurement of in-plane properties were generally done in one ofthe primary fiber directions. 

FIGURE 5. Thermal properties ofprepreg and slurry cast HiPerComp composites with Hi-Nicalon fibers. 

K. Nakano, S. Kume, K. Sasaki, and H. Saka, Microstmcture and Mechanical Properties of Hi-Nicalon Fiber Reinforced Si3N4 Matrix Composites, Ceram. Eng. Sci. Proc. 17, [4], pp. 324-332 (1996). 

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. 

Tensile properties of unidirectional BN/SiC coated Hi-Nicalon fiber reinforced celsian matrix composites [36-37] from room temperature to 1200°C in air are shown in Table 6, The value of Young s modulus decreased with increase in test temperature indicating the presence of glassy phase in the matrix. The yield stress decreased from room temperature... 

J. Z. Gyekenyesi and N. P. Bansal, High Temperature Mechanical Properties of Hi-Nicalon Fiber-Reinforced Celsian Composites, in Advances in Ceramic Matrix Composites V (N. P. Bansal, J. P. Singh, and E. Ustundag Eds.), Am. Ceram. Soc., Westerville, OH Ceram. Trans., 1 3,291-306 (2000). 

N. P. Bansal, Effects of Thermal Ageing in air on Microstructure and Mechanical Properties of Hi-Nicalon Fiber-Reinforced Celsian Composites, unpublished work. 

J. J. Brennan, Interfacial Design and Properties ofLayered BN(+C) Coated Nicalon Fiber-Reinforced Glass-Ceramic Matrix Composites, in Ceramic Microstructures Control at the Atomic Level, A. P. Tomsia and A. Glaeser eds.. Plenum Press, New York (1998) 705-712. 

The Nicalon fibers have variable composition which includes a substantial amount of oxygen and free carbon and consist of microcrystalline SiC in Si-C-0 glassy matrix together with thin plates of turbostratic carbon. Their properties vary with the composition.I" They generally have high electrical conductivity. 

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). 

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