Fibers in Metal Matrices

The books by Chung [1], Peebles Jr. [2] and Matthews [3] give an introduction to the study of metal matrix composites (MMC). The effective reinforcement of metal matrices with carbon fibers will confer the following properties  

Increase the strength and modulus at room and elevated temperatures. 

Reduce the density and hence improve the specific strength and modulus. 

A high modulus fiber will reduce the coefficient of thermal expansion and increase the thermal conductivity (Table 16.1). 

It is not surprising that with all these benefits, considerable effort has been expended on achieving reinforcement of metals. 

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At the time that the author first became involved with carbon fibers, the immediate emphasis on research was to attempt to incorporate carbon fiber in metal matrices (e.g. Figures 3.52 3.54). It was soon realized that this route was beset with many technical problems and attention was switched to polymer matrices, which appeared to be a better way to further the initial sales of carbon fibers. Over the years, much work has now been undertaken with metal matrices but, unfortunately, some of this work is not available for open publication, or is restricted to U.S. citizens only. 

CARBON FIBERS IN METAL MATRICES 23.12.1 Electromagnetic interference (EMi) and heat dissipation [244]... 

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. 

Ceramic coatings on fibers and powders have a variety of uses. For example, porous ceramic coatings on nanoscale metallic or ceramic particles can improve the catalytic properties of a powder. Also, the carbon fibers used as reinforcement in metallic matrices can be coated with a thin ceramic film (such as SiC or TiN) to reduce the rate of interdiffusion that may occur between the matrix materials and the fibers, and enhance the wetting of the fiber surface by metals. ... 

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. 

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. 

There are four types of SCS fibers depending on the thickness of the final SiC coating designed for different metal matrices. They are the standard SCS, SCS-2, SCS-6 and SCS-8. Fig. 5.30 illustrates schematically the cross sections of two commercially produced SiC fibers, the standard SCS and SCS-6 fibers, according to DiCarlo (1988). Both types of fibers consist of a carbon core of 37 pm in diameter, a SiC sheath of varying thickness and a carbon-rich surface coating of 0-4 pm in... 

Boron fibers exhibit good chemical compatibility with polymer matrices, hence their utili/.ation in composites therewith. At the production temperature of 577 - 677°C, boron reacts with the surface of metal matrices to form... 

Fibers in RPs are primarily used to reinforce a resin by transferring the stress under an applied load from the weaker resin matrix to the much stronger fiber. Plastics provide valuable and versatile materials for use as matrices, but other materials, such as metals, ceramics, and cements, are... 

Polyacrylonitrile, PAN, CHj-CHCC N)-, leads on pyrolysis over an intermediate stage to complete carbonization, as depicted in Fig. 3.51. The final product is used to make strong carbon fibers. The polyacrylonitrile fibers turn yellow to red on the formation of the ladder stmcture shown in the figure. On final carbonization the fiber turns black and consists of graphite-like structures. The carbon fibers have found many applications due to their high tensile modulus in the fiber direction and their, compared to metals, low weight. They are used as fiber reinforcement in epoxy matrices. Typical applications range from aircraft and aerospace to industrial, transportation, and recreational equipment. 

The most common matrices are the low-density metals, such as aluminum and aluminum alloys, and magnesium and its alloys. Some work has been carried out on lead alloys, mainly for bearing applications, and there is interest in the reinforcement, for example, of titanium-, nickel- and iron-base alloys for higher-temperature performance. However, the problems encountered in achieving the thermodynamic stability of fibers in intimate contact with metals become more severe as the potential service temperature is raised, and the bulk of development work at present rests with the light alloys. 

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