Page 22 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 2

CHARACTERIZATION OF INTERFACES

2.1. Introduction

The physico-chemical aspect of composite interfaces is a difficult subject and our
understanding of this feature is still far from complete. Two important topics will be
reviewed in this chapter. They are the theory of bonding at the fiber-matrix interface
and the analytical techniques to characterize the interface. The nature or origin of
the bonding between the fiber and matrix is discussed in terms of the theories of
adhesion with associated mechanisms of bonding. Examples of specific fiber-matrix
systems are provided along with their corresponding mechanisms of adhesion.
Various physico-chemical analytical techniques, which have been devised to
characterize the surface properties of fibers and composite interfaces, are also
extensively reviewed with corresponding analytical models for evaluation of the
experimental data. Advantages and limitations of each method are also presented.
Proper characterization of composite interfaces, whether it is for chemical,
physical or mechanical properties, is extremely difficult because most interfaces with
which we are concerned are buried inside the material. Furthermore, the
microscopic and often nanoscopic nature of interfaces in most useful advanced
fiber composites requires the characterization and measurement techniques to be of
ultrahigh magnification and resolution for sensible and accurate solutions. In
addition, cxperiments have to be carried out in a well-controlled environment using
sophisticated testing conditions (e.g. in a high vacuum chamber). There are many
difficulties often encountered in the physico-chemical analyses of surfaces.

2.2. Theories of adhesion and types of bonding

The nature of bonding is not only dependent on the atomic arrangement,
molecular conformation and chemical constitution of the fiber and matrix, but also
on the morphological properties of the fiber and the diffusivity of elements in each
constituent. It follows therefore that the interface is specific to each fiber-matrix
system (Kim and Mai, 1991). Adhesion in general can be attributed to mechanisms
including, but not restricted to, adsorption and wetting, electrostatic attraction,

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