Whisker

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 are single crystalline fibers with a diameter of several tenths of a im and lengths of 10 to 200 pm. They are characterized by maximum achievable mechanical properties. Thus, for example, tensile strengths of over 10 GPa have been measured with SiC-whiskers. 

More than 100 different materials have to date been produced as whiskers and characterized. However, only SiC- (50 to 100 t, 1996) and potassium titanate whiskers (ca. 5000 t) have achieved commercial importance. Important properties of whiskers are summarized in Table 5.2-18. 

Whiskers are generally produced from the gas phase at high temperatures 

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. 

Whiskers can be easily mixed with powders and processed as such. Composites are formed with metal (utilization in automobile construction e.g. pi.stons of whisker-reinforced aluminum), ceramic or polymer matrices for frictional applications (e.g. brake and clutch linings). Small quantities command prices of 1500 to 2000 DM/kg. For larger quantities prices of 100 to 150 DM/kg are expected. 

Whiskers grow in the presence of large growth-inhibiting molecules, which usually show a preferential adsorption on certain crystal plains. Incorporation of adsorbed particles in the crystal [86] and internal stress generated during the elec-trodeposition are major causes of whisker 

Under certain conditions of growth, crystals form in a direction in which growth is rapid, developing filamentary crystals that are free from gross imperfection. These filamentary single crystals are called whiskers. An area of interest from a research point of view has been the structure and properties of whiskers of ceramic materials in which extremely high-strength values have been obtained. 

---

These thin wires are supported on a special carrier that can be inserted into the ion source of the mass spectrometer after first growing the whiskers in a separate apparatus. Although the wires are very fragile, they last for some time and are easily renewed. They are often referred to as emitter electrodes (ion emitters). 

By growing thin whiskers along a sharp edge or thin wire, the ends of the whiskers become regions of very small radius of curvature and, consequently, provide very intense electric fields. 

Whey protein Whilhelmy plate techniqi Whipped cream Whipped toppings Wliipping CTeain Whipworm Whisker reinforcements Whiskers... 

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. 

A composite material (1) is a material consisting of two or more physically and/or chemically distinct, suitably arranged or distributed phases, generally having characteristics different from those of any components in isolation. Usually one component acts as a matrix in which the reinforcing phase is distributed. When the continuous phase or matrix is a metal, the composite is a metal-matrix composite (MMC). The reinforcement can be in the form of particles, whiskers, short fibers, or continuous fibers (see Composite materials). 

There are three kinds of metal-matrix composites distinguished by type of reinforcement particle-reinforced MMCs, short fiber- or whisker-reinforced MMCs, and continuous fiber- or sheet-reinforced MMCs. Table 1 provides examples of some important reinforcements used in metal-matrix composites as well as their aspect (length/diameter) ratios and diameters. 

Thermal Stresses and Properties. In general, ceramic reinforcements (fibers, whiskers, or particles) have a coefficient of thermal expansion greater than that of most metallic matrices. This means that when the composite is subjected to a temperature change, thermal stresses are generated in both components. 

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. 

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... 

Reinforcements. The high modulus, high intrinsic strength, and temperature stabiHty make SiC, in the form of whiskers, platelets, and fibers, a promising candidate reinforcement material for metal, polymer, and ceramic matrix composites (qv). 

In all of these processes it is possible to increase the yield of whiskers by a dding metallic impurities, and the sublimation process requires such additions. The vapor—Hquid—sohd (VLS) growth mechanism is often thought to be involved. 

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). 

---