Page 301 - Engineered Interfaces in Fiber Reinforced Composites

282 Engineered interfaces in fiber reinforced composites

of the composites. For polymer matrix composites (PMCs), the fiber coatings should
be able to promote such toughening mechanisms as interfacial debonding, post-
debonding friction, stress redistribution and fiber pull-out, while minimizing
possible reduction of strength and modulus due to the presence of the compliant
coating material. It should be recalled (see Section 5.5 for details) that for ceramic
matrix composites (CMCs), as for some brittle PMCs, the main objective of fiber
surface modification is to make the interface rather weak in order to improve the
fracture toughness, which is the major drawback of CMCs. On the other hand, an
adequate interfacial strength is often needed to offer good strength properties, in
particular in the transverse direction (Chawla, 1993). Therefore, a proper control of
the interface is essential to satisfy these conflicting requirements.
In contrast, for metal matrix composites (MMCs) a strong interfacial bond with
high composite strength and a good resistance to prolonged environmental attack in
service are often desirable because the inherently ductile nature of most metal matrix
materials does not require the composite to be further toughened (Taya and
Arsenault, 1989). The reaction products formed at the interphase region at high
processing temperatures generally increase the chemical bonding, but degrades the
gross mechanical properties. As such, a compromise is required between the desired
mechanical properties. At the same time, for both PMCs, MMCs and CMCs, the
coating materials should provide a means to protect the fibers from chemical
reaction, oxidation, hygrothermal aging and other mechanical degradation (i.e.
reaction barrier coating) during handling, fabrication and in service.
The fiber coating technique, either fully or intermittently along the fiber, has been
proven to be the most effective method for achieving both high strength and high
fracture toughness of fiber composites when appropriate coating materials are
selected, although its application to practical PMCs is still in question. The principal
effect of altering the interfacial properties by fiber coating, including the nature of
interfacial bonding, molecular constituents, morphology and ductility of the
interphase, is to modify the mode of failure and thus the potential energy
absorption capacity which, in turn, determines the fracture toughness of composites.
Because of the simplicity in the application to practical composites compared to
other techniques and the feasibility of direct comparison of fracture behaviors
between composites containing uncoated and coated fibers, the fiber coating
technique has received considerable attention, making significant progress in our
understanding of the underlying physics and failure mechanisms associated with the
presence of the coating layer. The fiber coating techniques are reviewed in the
following sections, which have been developed specifically for brittle PMCs.

7.2.1. Intermittent bonding concept

The intermittent fiber bonding method originates from the early work on failure
processes in single fiber micro-composites (Mullin et al., 1968; Gatti et al., 1969;
Mullin and Mazio, 1972). In these studies, coatings on boron fibers were found to be
effective in isolating fiber fracture by encouraging interface debonding immediately
next to the matrix cracks. The corn-cob shape surface of the boron fiber (see Fig.