Whisker silicon carbide

Whipple s rules, 20 138 Whisker reinforcement, 5 554, 555, 654 performance in ceramic—matrix composites, 5 572-575 physical properties, 5 557t synthesis, 5 642-643 and toughening, 5 622 Whiskers, silicon carbide, 22 533-534 White... 

Among existing synthetic whiskers, silicon carbide (SiC) whiskers have the highest hardness, modulus, tensile strength, and heat resistance temperature and are divided into a-SiC and... 

Other ceramic cutting-tool materials include alumina, Si-Al-0-N, alumina-carbide composites and, more recently, a composite of silicon nitride reinforced with silicon carbide whiskers. This last material can be produced by chemical-vapor infiltration (CVI) and has high strength and toughness as shown in Table 18.3.Cl... 

FIGURE 5.6 This is a fractured sample of a ceramic composite (alumina with 30 volume-percent silicon carbide whiskers). The lighter regions of circular or cylindrical shape are randomly oriented whiskers protruding from the fractured surface. The rod-like depressions in the surface mark places where whiskers nearly parallel with the fracture were pulled out. Courtesy, Roy W. Rice, W. R. Grace and Company. 

More recently, Stanicioiu, Chinta Hartner (1959) attempted to reinforce the cement with glass fibres, but this was not successful. The most serious study on the reinforcement of dental silicate cement was made by J. Aveston (in Wilson et al., 1972). Silicon carbide whiskers, carbon fibres and alumina powder were introduced into the cement mix. Unfortunately, the glass powder/liquid ratio had to be reduced, and the strength gained by reinforcement was thereby lost. It is clear that dental silicate cement cannot be strengthened by fibre or particulate reinforcement. 

Soluble polysilane polymers can also be used as precursors to silicon carbide. The first such application, using (PhMeSi)n-(Me2Si)m copolymers ("Polysilastyrene"), was to strengthen silicon nitride ceramics. The Si3N4 ceramic body was soaked in polysilane and refired, leading to the formation of silicon carbide whiskers in the pore spaces and a consequent increase in strength. (U)... 

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.

Lapin CA, Craig DK, Valerio MG, et al A subchronic inhalation toxicity study in rats exposed to silicon carbide whiskers. Fundam Appl Toxicol 16 128-146, 1991... 

Vaughan GL, Trently SA The toxicity of silicon carbide whiskers, a review. J Environ Sci Health A Environ Sci Eng Toxic Hazard Subst Control 31(8) 2033-54, 1996... 

Whiskers (nonoxides) (aluminum nitride, boron carbide, silicon carbide, silicon nitride, tungsten carbide, beryllium carbide)... 

Silicon carbide whiskers typically have diameters of a few micrometers and lengths up to 5 cm. They may be composed of either P-SiC or CC-SiC, the latter in one or more polytypes, and occur mosdy as hair- or ribbonlike crystals. Despite many attempts to produce SiC whiskers on a large scale at low cost, they have never acquired a wide importance. SiC whiskers have been reviewed (111—120). 

Silicon carbide has been described as a mild inhalation irritant (143). The threshold limit value for silicon carbide in the atmosphere is 5 mg/m3. Because of increased interest in SiC whiskers as a reinforcement for composites, the ASTM has established Subcommittee E34.70 on Single-Crystal Ceramic Whiskers to write standards for handling this form of SiC (144). 

These comments on whiskers apply roughly equally to alumina, silicon carbide, and silicon nitride, the three types on which the most work has been done. 

Metals and ceramics (claylike materials) are also used as matrices in advanced composites. In most cases, metal matrix composites consist of aluminum, magnesium, copper, or titanium alloys of these metals or intermetallic compounds, such as TiAl and NiAl. The reinforcement is usually a ceramic material such as boron carbide (B4C), silicon carbide (SiC), aluminum oxide (A1203), aluminum nitride (AlN), or boron nitride (BN). Metals have also been used as reinforcements in metal matrices. For example, the physical characteristics of some types of steel have been improved by the addition of aluminum fibers. The reinforcement is usually added in the form of particles, whiskers, plates, or fibers. 

Campbell, G.H., Rtihle, M., Dagleish, B.J. and Evans, A.G. Whiskers toughening a comparison between aluminium oxide and silicon nitride toughened with silicon carbide , J. Am. Ceram. Soc., 73[3] (1990) 521-530. 

Russell, L.M., Donaldson, K.Y., Hasselman, D.P.H., Corbin, N.D. Petrovic, J.J. and Rhodes, J.F. Effect of vapor-liquid-solid and vapor-solid silicon carbide whiskers on the effective thermal diffusivity/conductivity of silicon nitride matrix composites , J. Am. Ceram. Soc., 74[4] (1991) 874-877. 

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