Whisker production

Whiskers are short fibers, usually single crystals, with an aspect ratio of 10/1 or greater. They have high strength and are used as a random reinforcement for ceramics or metals, especially SiC whiskers in alumina cutting tools 

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Recently Schwetz [169] gave an overview of the different mechanisms forming SiC whiskers. One of the processes technically used for SiC whisker production has been mentioned before in Sect. 4.1.2, the pyrolysis of rice husks. This process counts among the type VS (Vapor-Solid mechanism) since the dominating reaction responsible for the whisker growth is the second step of the reaction sequence ... 

S.3.2.2.3 Vapor-Solid Reaction At present the principal commercial method for SiC whisker production is the carbothermic reduction of low cost silica sources at temperatures of 1500-1700°C. The reaction for the formation of VS-cubic (3SiC whiskers occurs in two steps ... 

Figure 13 Sharpened, diamond-coated silicon whisker. Product bulletin, Containerless Research Inc., Evanston, IL., with permission from Dr. P. C. Nordine, President.

Silicon carbide whiskers, usually produced from silica and carbon, are widely used for ceramic matrix composites [38]. Tbe vapor-liquid-solid (VLS) process is accepted as the mechanism for SiC whisker production (Fig. 15.1.4). This process requires formation of a liquid phase of a transition metal at the reaction sites. Elemental sources of Si and C are usually supplied in vapor phase form as SiO and CH4, respectively [39]. The vapors are deposited on the surface of the liquid metal and dissolved into the liquid metal droplets. Since the whiskers are precipitated from the dissolved components, metal droplets are often observed at the tip of the whiskers [39]. 

SiC whiskers can be fabricated by the reaction of silicon and carbon to form a gaseous species that can be transported and reacted in the vapor phase. This type of formation is referred to as a vapor-sohd reaction and occurs by the following multistep process 1) SiOj + C-SiO + CO and 2) SiO -I-2C-SiC -I- CO. Although the raw materials used in these carbothermal processes vary widely, nearly aU of the commercial and high-volume SiC whisker products are formed by this method. These reactions occur at temperatures greater than 1400°C and in an inert or nonoxidizing atmosphere. In addition, a catalyst is added to assure the formation of whiskers rather than particulate during the reaction. 

Metal-Matrix Composites. A metal-matrix composite (MMC) is comprised of a metal ahoy, less than 50% by volume that is reinforced by one or more constituents with a significantly higher elastic modulus. Reinforcement materials include carbides, oxides, graphite, borides, intermetahics or even polymeric products. These materials can be used in the form of whiskers, continuous or discontinuous fibers, or particles. Matrices can be made from metal ahoys of Mg, Al, Ti, Cu, Ni or Fe. In addition, intermetahic compounds such as titanium and nickel aluminides, Ti Al and Ni Al, respectively, are also used as a matrix material (58,59). P/M MMC can be formed by a variety of full-density hot consolidation processes, including hot pressing, hot isostatic pressing, extmsion, or forging. 

Fibrous materials may be naturally occurring or synthetically manufactured by thermal or chemical processes (Fig. 1) (see Fibers, survey). Refractory fibers are generally used in industrial appHcations at temperatures between 1000°C and 2800°C. These fibers may be oxides or nonoxides, vitreous or polycrystalline, and may be produced as whiskers, continuous filaments, or loose wool products. 

Directed Oxidation of a Molten Metal. Directed oxidation of a molten metal or the Lanxide process (45,68,91) involves the reaction of a molten metal with a gaseous oxidant, eg, A1 with O2 in air, to form a porous three-dimensional oxide that grows outward from the metal/ceramic surface. The process proceeds via capillary action as the molten metal wicks into open pore channels in the oxide scale growth. Reinforced ceramic matrix composites can be formed by positioning inert filler materials, eg, fibers, whiskers, and/or particulates, in the path of the oxide scale growth. The resultant composite is comprised of both interconnected metal and ceramic. Typically 5—30 vol % metal remains after processing. The composite product maintains many of the desirable properties of a ceramic however, the presence of the metal serves to increase the fracture toughness of the composite. 

The term s plastic, polymer, resin, elastomer, and reinforced plastic (RP) are some-what synonymous. However, polymer and resin usually denote the basic material. Whereas plastic pertains to polymers or resins containing additives, fillers, and/or reinforcements. Recognize that practically all materials worldwide contain some type of additive or ingredient. An elastomer is a rubberlike material (natural or synthetic). Reinforced plastics (also called composites although to be more accurate called plastic composites) are plastics with reinforcing additives, such as fibers and whiskers, added principally to increase the product s mechanical properties. 

The reactants were ultrasonically dispersed in ethylendiamine, then transferred to a Teflon-lined autoclave and gradually heated up to 160°C. The products were filtered out, washed with absolute alcohol, normal HC1 solution and water and finally dried in vacuum. In the product nano-fibres and near single-crystal whiskers were obtained. 

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