Interface reactions and wetting in carbon-fiber-reinforced glass matrix composites

Abstract

The wetting behavior of Na 2OB 2O 3SiO 2 glasses on pitch- and polyacrylonitrile (PAN)-based carbon fibers was evaluated using a newly developed thin film technique. Thin coatings of various glass compositions were deposited onto the fibers using a sol-gel method, and then microsessile drops were created during their heat treatment in controlled environments. The ternary glasses were non-wetting with contact angles of the order 90°–150°, whereas the binary B 2O 3SiO 2 glasses wet and spread extensively. There appeared to be more extensive etching of the fiber surfaces by the ternary glasses, but in general there was no obvious correlation between wetting and interface reactivity. On the other hand, the addition of transition metal oxides such as Nb 2O 5 and MoO 3 greatly enhanced the interface reactions. More extensive etching and pitting of the fiber surfaces occurred in these systems. The effect of these additions was also evaluated in hot-pressed composites where increases in interfacial shear strength and decreases in composite strength could be related directly to these interface reactions. The presence of carbide interphases was clearly evident in the MoO 3-doped composites, whereas in the ternary borosilicate system the interfaces were clean. The primary differences in the behavior of pitch- and PAN-based fibers were due to their internal microstructure. The PAN fiber surfaces were microporous and only randomly pitted after contact with the glass thin films, and in the composites the interfaces were often smooth. The pitch fiber surfaces were corrugated with pitting occurring preferentially along the grooves; in the composites the interfaces were visibly rough and the fiber pull-outs were splintered. Some selected experiments in pure CO atmospheres verified the important role of CO evolution in the interface reactions—especially in the creation of a weak boundary layer between the fiber and matrix. In general, the chemical effects at the interfaces in these systems are minimal unless multivalent oxides are included in the glass matrix composition.