Page 228 - Engineered Interfaces in Fiber Reinforced Composites

210 Engineered interfaces in fiber reinforced composites

used mainly to promote the wetting of fibers by molten metals, without due
consideration of the structure and integrity of the coating layer formed. The
efficiency of the plating process can be enhanced by producing an ion in a suitable
electrolyte. Thermal spraying is a process in that a coating precursor is heated
rapidly in a hot gaseous medium and simultaneously projected at a high velocity
onto the substrate fiber surface. The sol-gel method involves dipping or spinning of
fibers in a colloidal dispersion of particles in a carrier liquid, which is followed by
aggregation, gelation and final drying to form a thin layer of coating in the form of
very fine particles.

5.5.3. Carbon fibers

Various fabrication processes and the properties of carbon fibers are discussed in
Section 5.3.1. In general, carbon fibers are unique in that they do not react with
corrosive environment, except oxygen, at moderately high temperatures. They
exhibit even a slight increase in strength at temperatures up to 2200-2775OC,
depending on the precursor material and the heat treatment temperature used
during manufacture. Once processed, the carbon fibers display no changes in grain
size until they exceed their initial processing temperature. Therefore, many CMCs
containing carbon fibers can be used at very high operating temperatures without
much property degradation.
Carbon fiber reinforced aluminum matrix composites are very attractive for
structural applications due to their high specific strength, high modulus and near
zero CTE. However, there are major problems associated with the fabrication
processes of these composites; namely chemical reactivity of aluminum and poor
wetting of fiber by the matrix at its melting point. The interface shear strength of the
composite normally increases with increasing amount of reaction product, which in
turn leads to a decrease in the composite longitudinal strength (Yoon and Okura,
1990). High resolution electron microscopy and XPS reveal that the carbide, e.g.,
A&, form and grow by nucleation preferentially at the edge planes of the carbon
fiber surface (Diwanji and Hall, 1992). The amount of the interfacial reaction
product, A14C3, in carbon fiber-aluminum matrix composites depends on the
surface structure and treatment of carbon fibers. The interfacial shear strength is
enhanced significantly after oxidative treatment of the fiber surface, which is
attributed to the increased number of exposed crystalline edges where the carbides
arc preferentially nucleated.
One common remedy to the problem of excessive interfacial reaction is the
application of an appropriate fiber coating. The structure and morphology of nickel
coated carbon fibers for aluminum matrix have been studied by Abraham et al.
(1989, 1990). NiA13 intermetallics are found near the coated carbon fiber surface
without evidence of interaction at the coated fiber-matrix interface region. It is
believed that the interaction between the nickel coating and the carbon fiber took
place during the coating process, and the intermetallics acted as a nucleating site for
NiA13 precipitate during the composite synthesis. A Cu coating on carbon fiber is
also shown to be effective to reduce the chemical reaction (Abraham et al., 1992).