Oxidation behavior of Si-C-O fibers (Nicalon) in alumina

Porous Nicalon-alumina compacts with a bulk density of 2.32 Mg/m(3) were ox idized in Ar-25%O-2 gas mixture at 1373-1773 K, and subsequently they were exposed to argon at 1773 K. The mass change of compacts was thermogravimetr ically determined during oxidation and exposure. The oxidized and exposed f ibers were characterized through AES and XRD analysis. As the oxidation tem perature increased, the oxide film was successively transformed into amorph ous silica, cristobalite and mullite. The fiber core was only slightly deco mposed during oxidation above 1473 K. The oxidation rate could be described by the two-dimensional disc-contracting formula: reaction control at earli est stage and diffusion control at later stage. The oxidation of Nicalon wa s greatly accelerated by the presence of alumina. Excess thickening in oxid e film caused the thermal decomposition of the fiber core during exposure i n argon: >0.8 mum for cristobalite film and >1.5 mum for mullite film. (C) 2001 Kluwer Academic Publishers.