Page 30 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 2. Characterization of interfaces 13

1
0 0.5 1.0 1.5
Distance from fiber (pm)
Fig. 2.5. Modulus data as a function of distance from the fiber surface of a carbon fiber-epoxy matrix
composite which are measured from nanoindentation experiments. After Williams et al. (1990).
the average modulus of the interphase of a thickness of approximately 500 nm
formed between a single carbon fiber and epoxy matrix is about one-quarter of that
in the bulk matrix. However, the presence of a stiff fiber mitigates the effect of a soft
interphase, increasing the effective modulus of the interphase beyond that of the
bulk matrix in close vicinity of the fiber (Garton and Daly, 1985; Thomason 1990;
Tsai et al., 1990; Williams et al., 1990). Fig. 2.5 shows typical Young’s modulus data
obtained from nanoindentation experiments on a carbon fiber-epoxy system where
the Young’s modulus of the bulk matrix material is 3.8 GPa.
In MMCs, interdiffusion also plays an important role in promoting reaction
between elements of each constituent at the interface region. The special type of
interdiffusion that takes place in conjunction with chemical reaction in MMCs is
called an exchange reaction, which is described in Section 2.2.5. However, interdif-
fusion in MMCs may not be always beneficial because undesirable compounds are
often formed, particularly when the oxide films present on the fibers are completely
disrupted under extremely high temperature and pressure in a solid state process
(Metcalfe, 1974). To prevent or at least reduce the interaction, it is necessary to apply
an effective diffusion barrier in the form of a coating on the fiber, or alloying elements
in the matrix, to be discussed in Chapter 5. The selection of an appropriate diffusion
barrier relies on a detailed knowledge of the nature of the interaction taking place at
the interface region, which is specific to each fiber-matrix system.

2.2.3. Electrostatic attraction

A difference in electrostatic charge between constituents at the interface may
contribute to the force of attraction bonding. The strength of the interface will
depend on the charge density. Although this attraction is unlikely to make a major
contribution to the final bond strength of the interface, it could be important when
the fiber surface is treated with some coupling agent. This type of bonding will
explain why silane finishes are especially effective for certain acidic or neutral