Ceramic Reinforcing Fibers

Ceramic reinforcing fibers are mainly utilized for the manufacture of composites with  

Both continuously and discontinuously reinforcing fibers are utilized 

Lightweight constructions with improved mechanical properties versus classical metallic materials 

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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. 

Ceramic reinforcing fibers are utilized both in a continuous form (endless fibers) and in a discontinuous form (e.g. whiskers, short fibers). Most of the continuous fibers are utilized in the manufacture of composites with polymer matrices (PMC), where they are in competition with other high performance fibers (boron, carbon fibers), mainly for military or aerospace applications. Discontinuous fibers are generally used for the manufacture of metal matrix (MMC) and ceramic matrix (CMC) composites. 

Chemical Economics Handlrook. 2/1997. Stanford Research Institute, ceramic Reinforcing Fibers, 542.6000K - Z and. 542.6002A - /. and. 542.6002A -C Silica Fibers, 542.6001 P - Z and 542.6002A - U. 

Boltzmann s constant, and T is tempeiatuie in kelvin. In general, the creep resistance of metal is improved by the incorporation of ceramic reinforcements. The steady-state creep rate as a function of appHed stress for silver matrix and tungsten fiber—silver matrix composites at 600°C is an example (Fig. 18) (52). The modeling of creep behavior of MMCs is compHcated because in the temperature regime where the metal matrix may be creeping, the ceramic reinforcement is likely to be deforming elastically. 

Fig. 3. Tensile stress—strain curve for (-) reinforced ceramic and ( " ) fiber-reinforced ceramic composite. A represents the point where the matrix...

Fibrous Composites. These composites consist of fibers in a matrix. The fibers may be short or discontinuous and randomly arranged continuous filaments arranged parallel to each other in the form of woven rovings (coUections of bundles of continuous filaments) or braided (8). In the case of chopped strand mat the random arrangement is planar. In whisker (needle-shaped crystals or filaments of carbon and ceramics) reinforced materials the arrangement is usually three-dimensional and the resulting composites are macroscopically homogeneous. 

Modem machining deals with an increasingly wide range of materials which includes, in addition to the traditional metals, high-chromium and nickel stainless steels, titanium, intermetallics, refractory metals, ceramics, glasses, fiber-reinforced composites, and many others. These materials have widely different properties. They react differently to machining and each presents a special machining problem. 

A rapidly increasing number of publications on polysilanes documents current interest in these polymers (JJ. Polysilanes are potentially applicable in microlithography as high resolution UV-resists (2J, imageable etch barriers ), or contrast enhancement layers (4). They have been successfully used as precursors to Si-C fibers (5J and ceramic reinforcing agents ((L). Polysilanes have also initiated polymerization of vinyl monomers (J ). Doping of polysilanes have increased their conductivity to the level of semiconductors (8). Very recently polysilanes were used as photoconductors (9) and non-linear optical materials (10b... 

Though these materials are thermally very stable and mechanically inert they do only poorly resist mechanical stresses. This drawback can be overcome by reinforcing the monolithic ceramics with fibers. Today SiC-, Si3N4- and siliconcarbonitride fibers are commercially available, which are used in worldwide research activities to develop bulk component parts with composites. 

The micro-residual stresses arise from the differential CTE of the fiber and matrix, and the temperature difference. Table 7.4 gives the linear CTE values for various types of reinforcing fibers and matrix materials used widely for composite fabrication. The CTEs of most fibers and ceramic matrices are relatively lower... 

Figure 5.115 Stress-strain diagrams for lithiumaluminosilicate glass ceramic reinforced with 50% SiC fibers in various orientations. From Ceramic Microstructures, by W. E. Lee and W. M. Rainforth, p. 103. Copyright 1994 by William E. Lee and W. Mark Rainforth, with kind permission of Kluwer Academic Publishers.

Silicon nitride ceramics reinforced with carbon fibers and carbon nanotubes... 

Modulus and creep-resistance of plastics are still inferior to metals, ceramics, and glass, which means there are areas where they cannot compete. We know that molecular rigidity, crystallinity, polarity, and reinforcing fibers can all go a long way toward closing the gap. 

As mentioned previously, one of the major concerns for glass-ceramics reinforced with continuous fibers is the effect of matrix cracks in oxidizing environments. An important parameter in this kind of composite is the... 

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). 

In the reinforcement sector a number of fibers are available, which differ considerably both in price and also in chemical and thermal stability. This has a direct impact on their suitability for the production of fiber composites. Finally there are multiphase materials, in which reinforcing fibers are embedded in the form of short or endless oriented or non-oriented fibers, in a polymeric, metal or ceramic matrix. 

Cylindrically converging shock waves on powders were used to make mixtures of diamonds and hBN. BN fiber reinforced Zr02 was described . A nanostructured composite of magnetic particles of FOj N in a nonmagnetic matrix of BN is made via an inorganic geP. Fabrication of BN-B4 composites was reported . Consolidation of novel sintered composites formed from high pressure crystallization of amorphous ceramics was also described. The literature discusses other ceramics reinforced with BN fibers, as welF . ... 

Ceramics reinforced with such fibers may increase the energy efficiency of engines by allowing them to run as... 

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