Ceramic Fiber Coating by a Non-Aqueous Sol-Gel Process
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CERAMIC FIBER COATING BY A NON-AQUEOUS SOL-GEL PROCESS
F. C. MONTGOMERY, H. H. STRECKERT, R. 0. HARRINGTON, J. L. KAAE, S. P. PAGUIO, AND 0. R. WALL General Atomics, San Diego, CA
ABSTRACT Thin coatings of Al 0, and Y 03, have been prepared on desized Nicalon yarn. The layers were deposited dy dip coating in homogeneous, alcohol solutions of partially hydrolyzed metal alkoxides. The coating thicknesses could be controlled by multiple dipping. The uniformity of the coatings was affected by the alkoxide concentration. Solutions containing greater than 1 wt % equivalent oxide resulted in oxide bridging the void between the fibers. Transmission Electron Microscopy (TEM) studies on alumina coatings formed by dip coating 300 mesh nickel grids, have shown that either eta or gamma alumina is formed after heating at temperatures as low as 700 °C. The rate of grain growth increases as the temperature increases. In addition, increased grain growth is observed for films produced from solutions with higher water:metal ratios. The strength of coated fibers heat treated in air after coating has been determined. Alumina, and yttria coatings reduce the strength of the fiber after heating at temperatures above 9O0 OC. Scanning Auger analysis suggests that the coating is reacting with the fiber at these high temperatures. INTRODUCTION In the development of strong, tough ceramic fiber/ceramic matrix composites, the chemistry and properties of the fiber-matrix interface play an important role. Fiber coating can provide a means of controlling interfacial bond strength. Thin oxide coatings can be produced by dipping a substrate into a homogeneous solution containing a precursor of the oxide. The precursor solution may be made from three basic types of solutions: hydrolyzable metal salts[l], colloidal sols(2], and polymerized organometallic species(3]. In order to utilize a metal alkoxide, reaction conditions must be found which produce a homogeneous solution. Several parameters affect the homogeneity of the solution as the metal alkoxide hydrolyzes and polymerizes. Some of the important parameters are (1) the hydrolysis and condensation temperature, (2) the concentration of metal alkoxide, (3) the pH of the solution and (4) the ratio of water to metal used during the hydrolysis. These factors can often alter the average molecular weight distribution of the resulting polymer and thus influence the microstructure of the final coating. EXPERIMENTAL Synthesis. Clear solutions of organo-aluminum were prepared by the controlled hydrolysis of dilute alcohol solutions of aluminum sec-butoxide (Alfa Inorganics). Initial experiments used sec-butanol and ethanol as the Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
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solvent, but it was found that the solutions were not stable and, after aging for several hours, formed a precipitate. Thus, the alkoxide was dissolved in 2-ethoxyethanol or 2-(2-ethoxyethoxy)ethanol (J. T. Baker) which had been dried by distilling from barium metal. The solutions were hydrolyzed at 55 OCb
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