Page 209 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 5. Surface treatments of $hers and e8ecr.s on composite properties 191

additional bonding by developing weak adhesion mechanisms at the interface, such
as dispersion force, dipolar interaction and hydrogen bonds.
In the light of the foregoing experimental evidence, the following can be
summarized regarding the effects of carbon fiber surface treatments, depending on
the methods and media employed:
(1) There are substantial changes in fiber surface area with associated variations in
rugosity depending on the oxidative treatment medium.
(2) A weak surface layer may be removed, the removal being more serious in
plasma etching than in wet oxidation.
(3) There is an increase in the polar surface energy.
(4) Chemical modification takes place and carboxyl, hydroxyl and carbonyl groups
are produced on the fiber surface.
The mechanisms of chemical bonding due to the presence of functional groups
have yet to be more thoroughly clarified, which change the energetics of the carbon
fiber surface considerably. Better mechanical anchoring arising from the surface
rugosity and the increased physical surface area involved in adhesion as well as the
beneficial effect of removing the surface weak layers all contribute to an improved
interfacial bond. However, there must be a limit to the improvement in bond
strength by fiber surface treatment only. A decrease in bond strength and other
deteriorating effects are expected to occur if the surface treatment is excessive,
leading to severe damage of the fiber.

5.3.2.3. EJrects of surface treatment on composite properties
The interlaminar shear, flexural and tensile strengths are increased as the principal
effects of carbon fiber surface treatment on composite properties. The enhancement
of these strength properties depend on the fiber elastic modulus, the degree of
surface treatment and the type of resin and curing agent used. The largest
improvement in ILSS is obtained for high modulus fibers. The compressive strength
is also increased slightly (Norita et al., 1986), and the mode I interlaminar fracture
toughness GI, for crack initiation is almost doubled (Ivens et al., 1991) with
increasing degree of treatment. In general, an increase in the interfacial bond
strength, q,, enhances the composite compressive strength by augmenting the load
required to cause the interface to fail in transverse tension due to the fiber Poisson
effect. Delamination is reduced in favor of microbuckling of surface treated fibers
(Drzal and Madhukar, 1993). The improvement in interface bond strength, zb, or
fracture toughness, GI,, due to fiber surface treatments has been confirmed using
microcomposite tests, e.g. the fragmentation tests (Drzal et al., 1983a, b) and the
fiber pull-out tests (Baillie and Bader, 1991). However, all these beneficial effects of
improved strength properties are inevitably accompanied by a loss in the impact
fracture toughness of unidirectional laminates or notched tensile strength of angle-
ply laminates, as shown in Fig. 5.15 (Goan et al., 1973; Dauksys, 1973). Therefore, a
careful balance has to be sought to ensure both adequate strength and toughness
properties.
More recently Drzal and coworkers (Madhukar and Drzal, 1991a, b, 1992a, b;
Drzal and Madhukar, 1993) have spent significant research efforts to establish the