KINETICS OF THERMAL, PASSIVE OXIDATION OF NICALON FIBERS

The oxidation of Nicalon(TM) fibers is a concern, because of its poten tial as a reinforcement of high-temperature composites, whose service conditions involve high-temperature, oxidizing environments, Two limit ing types of oxidation mechanisms are often used to describe the kinet ics: chemical-reaction-controlled oxidation, at small oxide thicknesse s, and diffusion-controlled oxidation, at large oxide thicknesses, Nei ther mechanism can satisfactorily describe the intermediate region whe re the oxidation kinetics are controlled jointly by both the chemical reaction rate at the interface and the diffusion of oxygen through the oxide layer. To describe the entire oxidation process with a general relationship, one must consider all stages of the oxidation process, n amely (i) adsorption of oxygen at the outer surface of the oxide, (ii) diffusion of oxygen from the outer surface toward the interface,where oxidation occurs, and (iii) reaction at the interface to form a new l ayer of oxide, Previously, a very useful general relationship was deri ved for the oxidation kinetics for a flat plate, which could account f or all three stages of oxidation, However, that equation is inadequate to describe the oxidation of cylindrical fibers, because the effectiv e area for oxygen diffusion changes along the diffusion path and the o xidation interfacial area decreases as the oxide thickness increases f or cylindrical fibers, In this paper, we have derived a general kineti c relationship for the oxidation of cylindrical fibers, which can acco unt for all stages of oxidation, Comparison of the theory with experim ental data of Nicalon(TM) fibers shows good agreement.