Page 31 - Engineered Interfaces in Fiber Reinforced Composites

14 Engineered interjaces in fiber reinforced composites

reinforcements like glass, silica, and alumina, but are less effective with alkaline
surfaces like magnesium, asbestos, and calcium carbonate (Plueddemann, 1974).

2.2.4. Chemical bonding
Chemical bonding is the oldest and best known of all bonding theories. Physical
adsorption mechanisms discussed in Section 2.2.2 depend on van der Waal forces or
the acid-based interaction, while chemical bonding mechanism is based on the
primary bond at the interface. A chemical reaction at the interface is of particular
interest in the study of polymer matrix composites because it offers a major
explanation for the use of silane coupling agents on glass fibers embedded in
thermoset and amorphous thermoplastic matrices. Surface oxidative treatments of
carbon fibers have been known for many years to promote chemical bonding with
many different polymer resins. Recent work (Buxton and Baillie, 1995) has shown
that the adhesion is a two-part process: the first part is the removal of a weak layer
of a graphitic-like structure from the fiber surface particularly at low levels of
treatment; and the second part is chemical bonding at the acidic sites. However,
much further work is still needed to verify this hypothesis.
In this mechanism of adhesion, a bond is formed between a chemical group on the
fiber surface and another compatible chemical group in the matrix, the formation of
which results from usual thermally activated chemical reactions. For example, a
silane group in an aqueous solution of a silane coupling agent reacts with a hydroxyl
group of the glass fiber surface, while a group like vinyl on the other end will react
with the epoxide group in the matrix. The chemical compositions of the bulk fiber
and of the surface for several widely used fiber systems are given in Table 2.2. It is
interesting to note that except for glass fibers, the chemical composition of the
surface does not resemble that of the bulk fiber, and oxygen is common to all fiber
surfaces. Further details regarding the types of surface treatments commonly
applied to a variety of organic and inorganic fibers and their effects on the properties
of the interfaces and bulk composites are given in Chapter 5.

2.2.5. Reaction bonding

Other than in polymer matrix composites, the chemical reaction between elements
of constituents takes place in different ways. Reaction occurs to form a new
compound(s) at the interface region in MMCs, particularly those manufactured by a
molten metal infiltration process. Reaction involves transfer of atoms from one or
both of the constituents to the reaction site near the interface and these transfer
processes are diffusion controlled. Depending on the composite constituents, the
atoms of the fiber surface diffuse through the reaction site, (for example, in the
boron fiber-titanium matrix system, this causes a significant volume contraction due
to void formation in the center of the fiber or at the fiber-compound interface
(Blackburn et al., 1966)), or the matrix atoms diffuse through the reaction product.
Continued reaction to form a new compound at the interface region is generally
harmful to the mechanical properties of composites.