of Processing Types
Right at the outset, the very latest glass-matrix composites, such as those suggested by ASI (1), have already been compared to more traditional (if still young) ones, like silicon- carbon composites (SiC-SiC type), and found to have limitations. In a discussion following his state-of-the-art review, Naslain (13:495) answered a question precisely on this point, revealing that the glass (or glass-ceramic) matrix composites had been designed for only medium temperature (600-1,000 (C), if long-duration, exposures in oxidizing atmospheres, but they are still limited by creep of the matrix and oxidation of the interphase (which is principally carbon). Still, the glass-ceramic composites do have advantages, among which are the speed with which they can be fabricated, the variety of compositions of matrix that can be used, and the absence of significant residual porosity (13:495). Considering that the majority of research work has been devoted to matrix, rather than fiber materials, it is interesting to note from Naslain as well that the major portion of the cost of CMCs is related to the fiber, rather than the matrix (13:496).
Earlier, and perhaps the very earliest, developments were carbon-on-carbon composites that were designed to withstand short exposures at extremely high temperatures (e.g., in rocket engines or for re-entry thermal protection), but part of the material was expected to be (and proved to be) consumed or destroyed in each successive application (13:485). These composites had the major disadvantage of suffering from oxidation even at very low temperatures, as low as 500(C (13:485).
Chemical Vapor Deposition (CVD) CMCs. Sun, et al. have attempted to overcome "embrittlement and loss of strength and toughness" among glass-ceramic matrix composites
by applying CVD coatings, such as boron nitride (BN) and SiC-on-BN, onto fibers of Nicalon and an "Si-N-C based HPZ fiber" (hydridopolysilazane, f...