Combustion chemical vapor deposition (CCVD) of LaPO 4 monazite and beta-alumina on alumina fibers for ceramic matrix composites

Abstract

This research used the low cost, open atmosphere combustion chemical vapor deposition (CCVD SM) method to efficiently deposit protective coatings onto alumina fibers (3M Nextel™ 610) for use in ceramic matrix composites (CMCs). La-monazite (LaPO 4) and beta-alumina were the primary candidate debonding coating materials investigated. The coated fibers provide thermochemical stability, as well as desired debonding/sliding interface characteristics to the CMC. Dense and uniform La-phosphate coatings were obtained at deposition temperatures as low as 900–1000°C with minimal degradation of fibers. However, all of the β-alumina phases required high deposition temperatures and, thus, could not be applied onto the Nextel™ 610 alumina fibers. The fibers appeared to have complete and relatively uniform coatings around individual filaments when 420 and 1260 filament tows were coated via the CCVD process. Fibers up to 3 feet long were fed through the deposition flame in the laboratory of MicroCoating Technologies (MCT). TEM analyses performed at Wright-Patterson AFB on the CCVD coated fibers showed a 10–30 nm thick La-rich layer at the fiber/coating interface, and a layer of columnar monazite 0.1–1 μm thick covered with sooty carbon of <50 nm thick on the outside. A single strength test on CCVD coated fibers performed by 3M showed that the strength value fell in the higher end of data from other CVD coated samples.