Page 34 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 2. Characterization of interfaces 17

2.3. Physico-chemical characterization of interfaces

2.3.1. Introduction

Composite interfaces exist in a variety of forms of differing materials. A
convenient way to characterize composite interfaces embedded within the bulk
material is to analyze the surfaces of the composite constituents before they are
combined together, or the surfaces created by fracture. Surface layers represent only
a small portion of the total volume of bulk material. The structure and composition
of the local surface often differ from the bulk material, yet they can provide critical
information in predicting the overall properties and performance. The basic
unknown parameters in physico-chemical surface analysis are the chemical
composition, depth, purity and the distribution of specific constituents and their
atomic/microscopic structures, which constitute the interfaces. Many factors such as
process variables, contaminants, surface treatments and exposure to environmental
conditions must be considered in the analysis.
When a solid surface is irradiated with a beam of photons, electrons or ions,
species are generated in various combinations. An analytical method for surface
characterization consists of using a particular type of probe beam and detecting a
particular type of generated species. In spectroscopy, the intensity or efficiency of the
phenomenon of species generation is studied as a function of the energy of the
species generated at a constant probe beam energy, or vice versa. Most spectro-
scopic techniques are capable of analyzing surface composition, and some also allow
an estimation of the chemical state of the atoms. However, it may be difficult to
isolate the contributions of each surface layer of the material being probed to these
properties. Since most surface analysis techniques probe only the top dozen atomic
layers, it is important not to contaminate this region. For this reason and
particularly to reduce gas adsorption, a vacuum always has to be used in
conjunction with these techniques. The emergence of ultrahigh vacuum systems of
less than loT6 Pa (or 7.5 x Torr), due to rapid technological advances in recent
years, has accelerated the development of sophisticated techniques utilizing
electrons, atoms and ions. Amongst the currently available characterization
techniques, the most useful ones for composite interfaces are: infrared (IR) and
Fourier transform infrared (FTIR) spectroscopy, laser Raman spectroscopy, X-ray
photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), secondary
ion mass spectroscopy (SIMS), ion scattering spectroscopy (ISS), solid state nuclear
magnetic resonance (NMR) spectroscopy, wide-angle X-ray scattering (WAXS),
small-angle X-ray scattering (SAXS) and the measurement of the contact angle. A
selected list of these techniques is presented in Table 2.4 along with their atomic
processes and the information they provide. Each technique has its own complexity,
definite applications and limitations. Often the information sought cannot be
provided by a single technique. This has resulted in the design of equipment that
utilizes two or more techniques and obtains different sets of data from the same
surface of the sample (e.g. ISSjSIMS two-in-one and XPS/AES/SIMS three-in-one
equipment). Adamson (1982), Lee (1989), Castle and Watts (1988) and Ishida (1994)