Page 206 - Engineered Interfaces in Fiber Reinforced Composites

188 Engineered interfaces in jiber reinforced composites

Power supply
1
Matching unit
I

4
~
1 Plasma
f I I

Fiber Fiber
supply take-up

Fig. 5.12. Schematic representation of a continuous cold plasma treatment. After Garbassi and Occhiello
(1993).

been used with varying degrees of success. These liquid-phase treatments are
generally milder than the ones with a gaseous-phase, and do not cause excessive
pitting and hence degradation of the fiber strength. Several factors, such as acid
concentration, exposure time and temperature and mode of treatment, influence the
effectiveness of these oxidative processes. Depending on the type of carbon fibers,
nitric acid treatment in general increases the surface area, surface functionality and
surface oxide contents with increasing treatment time and temperature and acid
concentration (Scolar, 1974). This treatment generally causes an appreciable weight
loss and smoothing of the fiber surface by removing the surface irregularities
(Donnet and Ehrburger, 1977).
Electrolytic or anodic oxidation is fast, uniform and best suited to mass
production. This process is most widely used for treatment of commercial carbon
fibers. The oxidation mechanism of most carbon fibers is characterized by
simultaneous formation of C02 and degradation products that are dissolved in
the electrolyte of alkaline solution or adhere onto the carbon fiber surface in nitric
acid. Only minor changes in the surface topography and the surface area of the fiber
are obtained with a small weight loss, say, normally less than 2%.
Non-oxidative treatments: Several non-oxidative treatment techniques have been
developed for carbon fibers, which include whiskerization and plasma deposition of
organic and polymer coatings. Whiskerization involves a nucleation process and the
growth of very thin and high strength single crystals of the chemical compounds,
such as silicon carbide (Sic), titanium dioxide (TiOz) and silicon nitride (Si3N4), on
the fiber surface perpendicular to the fiber axis (Goan and Prosen, 1969). The
whiskers grow from individual fibers, which usually initiate at the points of defects,
compositional heterogeneities, metallic inclusions or structural irregularities and