COMBUSTION CHEMICAL-VAPOR-DEPOSITION (CCVD) OF LAPO4 MONAZITE AND BETA-ALUMINA ON ALUMINA FIBERS FOR CERAMIC-MATRIX COMPOSITES

This research used the low cost, open atmosphere combustion chemical v apor deposition (CCVDSM) method to efficiently deposit protective coat ings onto alumina fibers (3M Nextel(TM) 610) for use in ceramic matrix composites (CMCs). La-monazite (LaPO4) and beta-alumina were the prim ary candidate debonding coating materials investigated. The coated fib ers provide thermochemical stability, as well as desired debonding/sli ding interface characteristics to the CMC. Dense and uniform La-phosph ate coatings were obtained at deposition temperatures as low as 900-10 00 degrees C with minimal degradation of fibers. However, all of the b eta-alumina phases required high deposition temperatures and, thus, co uld not be applied onto the Nextel(TM) 610 alumina fibers. The fibers appeared to have complete and relatively uniform coatings around indiv idual filaments when 420 and 1260 filament tows were coated via the CC VD process. Fibers up to 3 feet long were fed through the deposition f lame in the laboratory of MicroCoating Technologies (MCT). TEM analyse s 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 la yer of columnar monazite 0.1-1 mu m thick covered with sooty carbon of < 50 nm thick on the outside. A single strength test on CCVD coated f ibers performed by 3M showed that the strength value fell in the highe r end of data from other CVD coated samples. (C) 1998 Elsevier Science S.A. All rights reserved.