Nicalon 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. 

Non-whisker SiC fibers are also of imporfance in MMCs, and they are currently available in two commercial forms Tyranno and Nicalon . As with the whisker form, the primary advantages of SiC fibers is their oxidation resistance and high-temperature mechanical property retention relative to other fibers. The high-temperature strength of three commercially available Nicalon SiC fibers is shown in Figure 5.111. 

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). 

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... 

In addition to not offering properties expected for phase pure SiC, the original Yajima process suffers from other drawbacks that include a multistep precursor synthesis and the inability to self-cure. Finally, the presence of oxygen limits the upper use temperature for both Nicalon and Tyranno fibers to ca 1200 °C because above this temperature CO and SiO gases evolve, generating defects (large crystallites, pores and voids) that contribute to substantial decreases in mechanical properties. 

The Nicalon fibre (10-20 pm) available commercially consists of a mixture of P-SiC free carbon and Si02 [28], The properties of Nicalon start to degrade above about 600°C because of the thermodynamic instability of the composition and microstructure. Ceramic-grade Nicalon fibres, designated the NL series, having low oxygen content are also available. 

Blissett et al. (1998) reported that thermal shock effects on the residual flexural properties of the Nicalon /CAS were more evident at intermediate temperature differentials, i.e. AT= 450-600°C, and this was attributed to the observed matrix cracking. 

The properties of the Nicalon /SiC (PIP) system followed a similar pattern (A/c(ou) = 400°C, A rc(omc) = 500°C), though this system failed through an interlaminar shear failure process (delamination) and the property reduction saturated at A T= 600°C. The Nicalon /SiC (CVI) system failed by fracture through fibre planes but its properties (ou, omc, WOF) had the same critical temperature difference, A Tc = 700°C. The pre- and post-quench stress-displacement curves for this material can be seen in Fig. 15.9. However, measurement of the Young s modulus of this system before and after quenching by means of a dynamic mechanical resonance technique showed the onset of decrease at ATC(E) = 400°C, i.e. significantly lower than the A 7C of the other properties. 

Blissett, M.J., Smith, P. A., Yeomans, J.A. (1997), Thermal shock behaviour of unidirectional silicon carbide fibre reinforced calcium aluminosilicate , J. Mater. Sci., 32, 317-325. Blissett, M.J., Smith, P.A., Yeomans, J.A. (1998), Flexural mechanical properties of thermally treated unidirectional and cross-ply Nicalon-reinforced calcium aluminosilicate composites , J. Mater. Sci., 33, 4181 —4190. 

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...

FIGURE 12.12 Examples of FGM CMCs (left) the combination of NLM Nicalon SiC fiber reinforced zirconia (in white) and mullite (in black) matrices (right) a Nasicon matrix reinforced with mullite (Nextel) and SiC (NLMTM) fibers (see the sketch). (Reprinted from Colomban, R, Process for fabricating a ceramic matrix composite incorporating woven fibers and materials with different compositions and properties in the same composite, Mater. TechnoL, 10, 89, 1995. With permission.)... 

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