Page 190 - Engineered Interfaces in Fiber Reinforced Composites

112 Engineered interfaces in Jiber reinforced composites

and some ceramic fibers, such as boron (B/W), Sic and A1203 fibers. The methods
of surface treatment, the choice of reaction barrier coatings and the resulting
mechanisms for improving the mechanical performance of a given fiber are different
for different types of matrix material as for the thermodynamic and chemical
compatibilities required. To fully understand the mechanisms of bonding or failure
at the interface region and thus to apply the many different surface treatment
techniques, it is also necessary to have an adequate understanding of the microstruc-
ture/properties of the fibers concerned. Proper characterization of the interfaces
modified by surface treatments or fiber coatings, and evaluation of the mechanical
performance of the composites made therefrom are as important as the development
of novel techniques of surface modification. Extensive and in-depth discussions on
surface analytical techniques and mechanical testing methods are already given in
Chapters 2 and 3, respectively.

5.2. Glass fibers and silane coupling agents

5.2.1. Structure und properties of gluss$bers

A variety of chemical compositions of mineral glasses have been used to produce
fibers. The most commonly used are based on silica (SOz) with additions of oxides
of calcium, aluminum, iron, sodium, and magnesium. The polyhedron network
structure of sodium silicate glass is schematically illustrated in Fig. 5.1, where each
polyhedron is a combination of oxygen atoms around a silicon atom bonded
together by covalent bonds. The sodium ions are not linked to the network, but only
form ionic bonds with oxygen atoms. As a result of the three-dimensional network
structure of glass, the properties of glass fibers are isotropic, as opposed to most

Silicon atom
0 Oxygen atom
0 Sodium ion

Fig. 5.1. Two dimensional illustration of the polyhedron network structure of sodium silicate glass. After
Hull (1981).