EXAMPLES OF EVOLUTION OF MICROSTRUCTURE IN CERAMICS AND COMPOSITES

In this article we describe a number of studies involving the direct o bservation of microstructural evolution. In general these investigatio ns were carried out to establish the mechanistic paths involved. The m aterials studied range from fibers being evaluated for use in high-tem perature ceramic composites to energetic materials used as propellants . In particular we discuss the room temperature imaging of materials d ifficult to image by conventional means and the use of the chamber atm osphere to influence microstructural evolution. Imaging of hydroxyapat ite formed by chemical means is briefly described as an example of a d ifficult microstructure. Microstructural evolution during calcium alum inate cement hydration relies on the chamber atmosphere to control moi sture loss from the hydrating specimens. In some instances microstruct ural evolution with heating occurred independently of the chamber atmo sphere. Grain growth in PZT films formed by sol-gel processes depends strongly on temperature but does not appear to depend on the chamber a tmosphere. This is also the case for the combustion of nitroamine prop ellants in that their combustion does not depend on access to an exter nal source of oxygen. In other studies, the chamber atmosphere played an indirect role in determining microstructure. However, the mechanist ic path driving microstructural evolution in copper-based inks used as conductive paths on electronic substrates is atmosphere dependent. Th ese inks are formulated from copper powder, glass, and an organic bind er, and the interaction of the binder with an oxidizing atmosphere all ows it to be burned out before significant interaction occurs between the copper powder and the glass. Finally, the microstructural variatio ns during the oxidation of structural composites at high temperature w ere used to allow assessments of their likely failure mechanisms. (C) 1993 Wiley-Liss, Inc.