SiC whisker

The SiC whisker-reinforced alumina composite, a model for engineered materials, has opened new vistas for tool material development. Whereas SiC whisker-reinforced alumina is used extensively for the machining of nickel-base superaHoys, SiC whiskers react chemically with steel, causing rapid wear on the rake face. Attempts are underway to replace SiC whiskers with less reactive whiskers such as TiC or TiN. 

SiC whisker-reinforced alumina is a major advance in tool material development, as it provides a means to increase the fracture toughness of the material via the composite material approach. It is entirely possible that in the next century many new whiskers of refractory, hard materials will be made... 

Sihcon carbide whiskers typically have diameters of a few micrometers and lengths up to 5 cm. They may be composed of either P SiC or a-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). 

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

Nonpowder Synthesis. Many ceramic composites (qv) under iavestigation utilize reinforcing ceramic whiskers or fibers to achieve toughening (19). Whiskers (17,19,20) are produced by vapor-synthesis techniques. SiC whiskers can be produced by the rice hull or vapor—soHd (VS) method whereby rice hulls are pyrolyzed to produce a mixture of carbon, C, and Si02, and whiskers are produced by directional growth by vapor... 

Experimentally it has been shown that both frictional bridging and whisker pullout play an important role in toughening industrially manufactured composites. Such investigations confirm that to maximize toughness via both mechanisms requires a high volume fraction of whiskers and a high composite modulus to whisker modulus ratio. For example, consider the effect of 20 vol % SiC whisker E = 500 GPa) reinforcement of various matrices on the toughness as presented in Table 7 (53). 

Whiskers are short fibers, usually a single crystal with an aspect ratio of 10/1 or greater. They have high strength but are difficult to process. Their applications so far has been limited SiC whiskers are reported as random reinforcement for ceramics and in alumina cutting tools. CVD is a common production method.O Beside SiC, the following whiskers have recently been produced by CVD ... 

Whiskers can be incorporated into the metallic matrix using a number of compositeprocessing techniques. Melt infiltration is a common technique used for the production of SiC whisker-aluminum matrix MMCs. In one version of the infiltration technique, the whiskers are blended with binders to form a thick slurry, which is poured into a cavity and vacuum-molded to form a pre-impregnation body, or pre-preg, of the desired shape. The cured slurry is then fired at elevated temperature to remove moisture and binders. After firing, the preform consists of a partially bonded collection of interlocked whiskers that have a very open structure that is ideal for molten metal penetration. The whisker preform is heated to promote easy metal flow, or infiltration, which is usually performed at low pressures. The infiltration process can be done in air, but is usually performed in vacuum. 

The benefits of SiC whisker-reinforced aluminum can be seen in Figure 5.109. The room-temperature stiffness (modulus) of the composite is about 50% higher than that... 

The prediction of the thermal conductivity is not as simple for the composite reinforced with SiC whiskers. In fact, as mentioned earlier, the thermal conductivity of the reinforcement phase plays an important role. However, for SiC whiskers, their chemical composition can vary drastically depending on the manufacturing process, which is not the case for Si3N4 whiskers. In fact, the influence of the manufacturing process is drastic the thermal conductivity of SiC(w) produced by the vapour-solid process is around 20 W/m K as opposed to 100-250 W/m K for whiskers produced by the vapour-liquid-solid process. Using a Si3N4 matrix with a nominal thermal conductivity of... 

W/m K, the thermal conductivity of the Si3N4-SiC(w) composite can increase or decrease depending on the type of SiC whiskers added.23... 

Fracture toughness as a function of SiC whisker content for different composites. 

Rossignol, F., Goursat, P. and Besson, J.L. Microstructure and mechanical behaviour of self-reinforced Si3N4 and Si3N4-SiC whisker composites , J. Eur. Ceram. Soc., 13 (1994) 299-312. 

Jia, D.C., Zhou, Y. and Lei, T.C. Thermal shock resistance of SiC whiskers reinforced Si3N4 ceramic composites , Ceramics International, 22 (1996) 107-112. 

Pezzotti, G., Tanaka, I. and Okamoto, T. Si3N4/SiC-whisker composites without sintering aids III, High temperature behaviour , J. Am. Ceram. Soc., 74[2] (1991) 326-332. 

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