Metal silicon carbide whiskers

High stiffness ceramic fibers such as alumina, alumina-silica, silicon carbide, boron, etc. are used as reinforcement fibers for polymeric, metallic, and ceramic matrix composites (Chawla, 1987). Silicon carbide whisker reinforced alumina composites are used as high speed cutting tools (Chawla, 1993). 

Silicon carbide whiskers are generally manufaclured by a metal-catalyzed process, in which carbon and silicon are condensed from the gas phase. Another jtroccss utilizes rice husks decomposed at 500°C. At temperatures of 1800°C the carbon/Si02-mixture formed is converted into a-SiC. Potassium titanate whiskers are synthesized from potassium molybdate, potassium carbonate and litanium dioxide at temperatures above 1200°C. 

The bulk analysis of /3-SiC whiskers shows the least variation in chemistry. In some whiskers, the residual metals content can vary, most likely, as a result of additives that used as catalysts during synthesis. These include iron, cobalt, and chromium. Studies by Karasek et al. [56] have shown that the physical properties of silicon carbide whisker-reinforced composites do not correlate to the bulk properties of the whiskers significantly. This lack of significant correlation is mainly due to the fact that the important phase chemistry of the whisker-matrix interface is controlled by the matrix chemistry and the surface chemistry of the whiskers. There seems to be little impact of the diffusion of materials into or out of the bulk whisker material. 

Two new processes were recently reported [43-44] potentially opening the door to intensive exploration of silicon carbide whiskers in ceramic, metal and polymer matrix composites. One potentially continuous process [43] is an adaptation of the laboratory batch process [5] the other [44] uses highly reactive amorphous forms of Si02 and C to accelerate growth. 

Silica gel, carbon furnace black and cobalt chloride yield silicon carbide whiskers, or Tokawhiskers [30], in a metal catalyzed process at >1450°C. A process variant [9] yields SiC whiskers >1350 C in a fixed bed percolated by a hydrogen flow. The addition of iron above 1450°C affords submicron whiskers ending with a silicon rich droplet. The iron seems to evaporate and condense below 1450°C leaving behind whiskers with silicon rich tip >1450 C. These processes use the same starting materials as the rice hull processes but they also use a metal particle catalyst. As a result, they are believed to proceed by a VLS phase transformation. 

If this carbothermal process is brought to only partial completion (Equation 11a and 11b), a homogeneous mixture of silicon carbide whiskers and silicon nitride powder [10] is obtained which can be fired directly to yield whisker reinforced ceramics. Silicon carbide reinforced alumina composites and silicon carbide whisker reinforced zirconia composites [31] are also products of the "chemical mixing process". The whisker growth rate in the zirconia process can be accelerated by adding metal particle catalysts such as cobalt chloride, thus potentially facilitating a VLS phase transformation. 

Tiny VLS nanowhiskers can be obtained by chemical vapor deposition with less than 1/100th the diameter of ordinary VLS-CVD silicon carbide whiskers by proper selection of the metal catalyst and process conditions. Indeed, these "cobweb" [2] or nanowhiskers have been made with diameters of <20 nm. To the naked eye these whiskers look like a blue cloud created by light scattering similar to that which causes the sky to appear blue. 

Recent advances further enhance their commercial potential in metal matrix composites such as aluminum, nickel, and copper ceramic matrix composites, such as alumina, zirconia and silicon nitride and glass ceramic matrix composites such as lithium aluminosilicate. Silicon carbide whiskers increase strength, reduce crack propagation, and add structural reliability in ceramic matrix composites. Structural applications include cutting tool inserts, wear parts, and heat engine parts. They increase strength and stiffness of a metal, and support the design of metal matrix composites with thinner cross sections than those of the metal parts they replace, but with equal properties in applications such as turbine blades, boilers and reactors. 

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]. 

Researchers in Japan and the United States have systematically studied the preparation of silicon carbide whiskers. The Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS) synthesized SiC whiskers using silicon oxide (Si02) and charcoal under the support of 863 high-technology projects. China Mining University and Shanghai Institute of Ceramics (SIC) of CAS synthesized SiC whiskers by using rice hulls and charcoal + silicon oxide as raw materials, respectively" ... 

The superalloys, as well as alloys of aluminum, magnesium, titanium, and copper, are used as matrix materials. The reinforcement may be in the form of particulates, both continuous and discontinuous fibers, and whiskers concentrations normally range between 10 and 60 vol%. Continuous-fiber materials include carbon, silicon carbide, boron, aluminum oxide, and the refractory metals. However, discontinuous reinforcements consist primarily of silicon carbide whiskers, chopped fibers of aluminum oxide and carbon, or particulates of silicon carbide and aluminum oxide. In a sense, the cermets (Section 16.2) fall within this MMC scheme. Table 16.9 presents the properties of several common metal-matrix, continuous and aligned fiber-reinforced composites. 

Glass LR, Brown RC, Hoskins JA. Health effects of refractory ceramic fibers scientific issues and policy considerations. Occup Environ Med 1995 52 433-440. Sahle W, Laszlo I, Krantz S, Christensson B. Airborne tungsten oxide whiskers in a hard-metal industry preliminary findings. Ann Occup Hyg 1994 38 37-44. Johnson NF, Hoover MD, Thomassen DG, Cheng YS, Dailey A, Brooks AL. In vitro activity of silicon carbide whiskers in comparison to other industrial fibers using four cell culture systems. Am J Ind Med 1992 21 807-823. 

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. 

Cuboid and cycloid niobium monocarbide (NbC) whiskers, 0.1-2.0 pm in diameter and 5-100 pm in length, and having a square-shaped tip, were recently synthesized by heating mixtures of niobium oxide (Nb203) and carbon black at temperatures over 1100°C [38]. Silicon carbide nano-whiskers, 20-50 nm in diameter and 2-5 pm in length, were carbothermally synthesized by reducing ultrafine precipitated silica powders with ultrafine carbon black by microwave heating [29]. These processes proceed without addition of metal particle catalysts, and therefore by a VS phase transformation [14] [29] [38]. 

Metal-Ceramic Composites. Metals such as aluminum, titanium, copper and the intermetallic titanium aluminide, which are reinforced with silicon-carbide fibers or whiskers show an appreciable increase in mechanical properties particularly at elevated temperatures. These composites are being considered for advanced aerospace structures.1 1... 

MMCs are usually reinforced by either monofilaments, discontinuous fibers, whiskers, particulates, or wires. With the exception of wires, which are metals, reinforcements are generally made of advanced ceramics such as boron, carbon, alumina and silicon carbide. The metal wires used are made of tungsten, beryllium, titanium, and molybdenum. Currently, the most important wire reinforcements are tungsten wire in superalloys and superconducting materials incorporating niobium-titanium and niobium-tin in a copper matrix. The most important MMC systems are presented in Table 18.5. 

Short-fibre reinforced metal matrix composites are significantly less expensive than long-fibre reinforced materials and can thus be used in automotive engineering or in sports equipment. For example, short-fibre reinforced aluminium-silicon carbide composites can be used as pistons in diesel engines at elevated temperatures [49]. Golf clubs and bicycle components can also be manufactured from aluminium matrix composites. Frequently, whiskers (see section 6.2.8) are used as short fibres because of their high strength and favourable aspect ratio. 

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