Page 225 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 5. Surjace treatments ofjibers and effects on composite properties 207

preserved at service temperatures. The effects of interface reaction depend on the
nature and reactivity of fiber and matrix combinations; and the morphology and
mechanical properties of the reaction products are expected to be responsible for the
diverse fracture behavior at the interface region. Both in CMCs and MMCs, a
suitable coating should be chosen such that the bonding mechanism at the
interphase region becomes primarily mechanical in nature. In MMCs, the coating
should also allow the fibers to be properly wetted by the molten matrix material
because the wettability plays an important role in improving the interface bonding.
For example, metal oxide fibers, such as A1203, are not readily wetted by many
metals unless they are very reactive or the fibers are coated with appropriate
materials (Ward-Close and Partridge 1990).
On the other hand, because of the accelerated reactivity of the surfaces of many
fibers when in contact with metal and ceramic matrices at elevated temperatures,
considerable precautions need to be exercised to ensure the fiber-matrix compat-
ibility and avoid fiber degradation, The composite constituents and the reaction
barrier coatings must also be chemically stable at the processing and service
temperatures. Any chemical interactions occurring between the fibers, coating and
matrix during the manufacturing processes would influence the interface bond
strength. Once fabricated, the service temperature must not exceed some maximum
level, otherwise an interdiffusion-controlled reaction may occur between the
elements of fiber, coating and matrix to form compound layers of substantial
thicknesses which are often detrimental to the mechanical performance of the
composites. A balance is thus always required between the reaction necessary for
efficient interfacial bonding and fiber degradation caused by excessive reaction. A
coating is also required to protect the fiber from mechanical degradation during
handling.
The choice of a coating for a given combination of fiber and matrix materials
depends on the processing and service requirements. The criteria for thermodynamic
stability in the temperature range encountered during the fabrication process and in
service are clearly of most importance. In this regard, highly stable oxides such as,
Y203, MgO, Zr03, Hf02, SiOz, A1203, SnOz and other non-reactive refractory
species such as C, W, Mo, BN, Sic, are considered to be strong candidates among
many coating materials hitherto developed. Any coating material certainly has an
inherent upper temperature limit, although this limit can be enhanced by modifying
the elements of the coating material or by introducing multi-layer coatings. For
example, a BN coating tends to oxidize in air at about 6OO0C, the behavior of which
is considered to be slightly better than that with a carbon coating. A Sic coating
tends to form a reaction product Si02 at temperatures above 1200°C. Some porous
oxide coatings that eliminate the problem of an upper temperature limit appear to
provide an opportunity, but further research is needed regarding their influence on
fiber strength and degradation along with their macro structural stability, before
they can be accepted for wider applications.
Another important mechanical property of a coating layer is the coefficient of
thermal expansion (CTE). Residual stresses generated due to the differential thermal
contraction between the composite constituents are extremely detrimental to the