ournal Am. Ceran. Soc,81655-60(199 Kinetics of Thermal, Passive Oxidation of Nicalon Fibers Yuntian T Zhu, Seth T. Taylor, Michael G. Stout, Darryl P Butt, and Terry C. Lowe Division of Materials Science and Technology, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 The oxidation of NicalonTM fibers is a concern, because of the outer surface to the interface where oxidation occurs, and its potential as a reinforcement of high-temperature com ii1)reaction to form a new layer of oxide at the interface osites, whose service conditions involve high-temperature, Obviously, one must consider all three oxidation steps if a oxidizing environments. Two limiting types of oxidatio general relationship for oxidation kinetics is to be found. Deal mechanisms are often used to describe the kinetics: chemi- nd grove did an excellent job in deriving the general oxi- cal-reaction-controlled oxidation. at small oxide thick dation kinetics for a flat plate, which can account for all three nesses, and diffusion-controlled oxidation, at large oxide stages. However, their model is inappropriate for describing the thicknesses. Neither mechanism can satisfactorily describe the intermediate region where the oxidation kinetics are area through which the oxygen diffuses and the total interface controlled jointly by both the chemical reaction rate at the area for the oxidation reaction both remain constant. For a erface and the diffusion of oxygen through the oxide cylindrical geometry, as in the case of fibers, the effective area layer. To describe the entire oxidation process with a gen for oxygen diffusion changes along the diffusion path, and the eral relationship, one must consider all stages of the oxida total interface area for the oxidation reaction decreases as the tion process, namely (i) adsorption of oxygen at the outer oxide thickness increases. Filipuzzi and Naslain' attempted to surface of the oxide, (ii) diffusion of oxygen from the outer derive the oxidation kinetics for cylindrical fibers. However surface toward the interface where oxidation occurs, and they considered only diffusion in their work; thus, applicatio (iii)reaction at the interface to form a new layer of oxide. of their equations is limited to thick oxide layers. a general Previously, a very useful general relationship was derived description of the oxidation kinetics for cylindrical fibers, for the oxidation kinetics for a flat plate, which could ac- which can account for all three stages of oxidation, has not count for all three stages of oxidation. However, that equa been reported to date tion is inadequate to describe the oxidation of cylindrical In this paper, we derive the general oxidation kinetics for fibers, because the effective area for oxygen diffusion NicalonTM fibers. All three stages of the oxidation prod hanges along the diffusion path and the oxidation interfa- incorporated into the derivation. Comparison with expe cial area decreases as the oxide thickness increases for cy tal data of Nicalon TM fibers shows good agreement be betwer lindrical fibers. In this paper, we have derived a general heory and the experimental results kinetic relationship for the oxidation of cylindrical fibers which can account for all stages of oxidation. Comparison IL. Theoretical derivation of the theory with experimental data of Nicalon fibers During the oxidation process, oxygen must diffuse inward to shows good agreem the oxidation interface. where the reaction occurs In the case of NicalonTM (SiC)fibers, CO may also form during the oxi- . Introduction dation reaction 1-16 and must diffuse outward to the outer sur- face. For simplicity, we assume that the oxidation process is YLINDRICAL filaments such as NicalonTM(SiC) fibers(Ni controlled by only one diffusing species. Furthermore, pon Carbon Co., Tokyo, Japan) have been increasingl sume that oxidation proceeds via the inward diffusion used as reinforcements for high-temperature composites. As a gen. Nonetheless, the final oxidation kinetics equation nsequence, extensive studies on their oxidation behavior plicable to the case where oxidation is controlled ve been conducted in recent years. - For small oxide thick nesses, the oxidation kinetics are often approximated as being Nicalon TM fibers contain excessive carbon and will undergo further pyrolysis at elevated temperatures, which causes the oxidation occurs, which yields a linear relationship between the fiber diameter to shrink ,,, At the same time, the fiber oxide thickness and time. As the scale grows, the oxidation diameter should increase when the sic is oxidized to form kinetics become governed primarily by the diffusion of oxygen SiO2, because I mol of SiO2 has a larger volume than I mol of fiber nesses both the chemical reaction and diffusion can have vary with diameter, 8 which will affect the pyrolysis during oxidation. Our experimental results have shown that the total The oxidation process typically has three distinct stages: 10 average diameter change of NicalonTM fibers is only 1.4%after () surface adsorption of oxygen, (ii) diffusion of oxygen from oxidation, which is negligibly small. Therefore, we shall as- sume a constant fiber diameter during the oxidation of NicalonTM fibers for simplicity. As a consequence, the follow- ing oxidation model can only be applied where the fiber diam- J. L. Smialek--contributng editor eter does not change significantly during oxidation Shown in Fig. I is a schematic drawing of a fiber with an oxide layer. The fiber diameter is R, which is assumed to re main constant during the oxidation, and the oxide thickness is Manuscript No. 191280 Received January 14, 1997, approved June 17, 1997 x. The radius of the unoxidized core is u. where Alamos National Laboratory (1) B under the auspices of the U.S. Depariment of Energy, under Contract No. As mentioned in the introduction, oxygen usually must go through three steps during the oxidatio n processKinetics of Thermal, Passive Oxidation of Nicalon Fibers Yuntian T. Zhu,* Seth T. Taylor, Michael G. Stout, Darryl P. Butt,* and Terry C. Lowe Division of Materials Science and Technology, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 The oxidation of Nicalon™ fibers is a concern, because of its potential as a reinforcement of high-temperature com￾posites, whose service conditions involve high-temperature, oxidizing environments. Two limiting types of oxidation mechanisms are often used to describe the kinetics: chemi￾cal-reaction-controlled oxidation, at small oxide thick￾nesses, and diffusion-controlled oxidation, at large oxide thicknesses. Neither mechanism can satisfactorily describe the intermediate region where 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 gen￾eral relationship, one must consider all stages of the oxida￾tion process, namely (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 layer of oxide. Previously, a very useful general relationship was derived for the oxidation kinetics for a flat plate, which could ac￾count for all three stages of oxidation. However, that equa￾tion is inadequate to describe the oxidation of cylindrical fibers, because the effective area for oxygen diffusion changes along the diffusion path and the oxidation interfa￾cial area decreases as the oxide thickness increases for cy￾lindrical fibers. In this paper, we have derived a general kinetic relationship for the oxidation of cylindrical fibers, which can account for all stages of oxidation. Comparison of the theory with experimental data of Nicalon™ fibers shows good agreement. I. Introduction CYLINDRICAL filaments such as Nicalon™ (SiC) fibers (Nip￾pon Carbon Co., Tokyo, Japan) have been increasingly used as reinforcements for high-temperature composites. As a consequence, extensive studies on their oxidation behavior have been conducted in recent years.1–9 For small oxide thick￾nesses, the oxidation kinetics are often approximated as being controlled by chemical-reaction kinetics at the interface where oxidation occurs, which yields a linear relationship between the oxide thickness and time. As the scale grows, the oxidation kinetics become governed primarily by the diffusion of oxygen through the oxide layer toward the oxide/core interface, which yields a parabolic relationship. At intermediate oxide thick￾nesses, both the chemical reaction and diffusion can have equally significant roles in the oxidation kinetics. The oxidation process typically has three distinct stages:10 (i) surface adsorption of oxygen, (ii) diffusion of oxygen from the outer surface to the interface where oxidation occurs, and (iii) reaction to form a new layer of oxide at the interface. Obviously, one must consider all three oxidation steps if a general relationship for oxidation kinetics is to be found. Deal and Grove10 did an excellent job in deriving the general oxi￾dation kinetics for a flat plate, which can account for all three stages. However, their model is inappropriate for describing the oxidation kinetics of cylindrical fibers. For a flat plate, the total area through which the oxygen diffuses and the total interface area for the oxidation reaction both remain constant. For a cylindrical geometry, as in the case of fibers, the effective area for oxygen diffusion changes along the diffusion path, and the total interface area for the oxidation reaction decreases as the oxide thickness increases. Filipuzzi and Naslain1 attempted to derive the oxidation kinetics for cylindrical fibers. However, they considered only diffusion in their work; thus, application of their equations is limited to thick oxide layers. A general description of the oxidation kinetics for cylindrical fibers, which can account for all three stages of oxidation, has not been reported to date. In this paper, we derive the general oxidation kinetics for Nicalon™ fibers. All three stages of the oxidation process are incorporated into the derivation. Comparison with experimen￾tal data of Nicalon™ fibers shows good agreement between theory and the experimental results. II. Theoretical Derivation During the oxidation process, oxygen must diffuse inward to the oxidation interface, where the reaction occurs. In the case of Nicalon™ (SiC) fibers, CO may also form during the oxi￾dation reaction11–16 and must diffuse outward to the outer sur￾face. For simplicity, we assume that the oxidation process is controlled by only one diffusing species. Furthermore, we as￾sume that oxidation proceeds via the inward diffusion of oxy￾gen. Nonetheless, the final oxidation kinetics equation is ap￾plicable to the case where oxidation is controlled by the outward diffusion of CO. Nicalon™ fibers contain excessive carbon and will undergo further pyrolysis at elevated temperatures, which causes the fiber diameter to shrink.1,3,11,17 At the same time, the fiber diameter should increase when the SiC is oxidized to form SiO2, because 1 mol of SiO2 has a larger volume than 1 mol of SiC. Furthermore, Nicalon™ fiber has a large diameter varia￾tion from filament to filament, and the free-carbon content may vary with diameter,18 which will affect the pyrolysis during oxidation. Our experimental results have shown that the total average diameter change of Nicalon™ fibers is only 1.4% after oxidation, which is negligibly small. Therefore, we shall as￾sume a constant fiber diameter during the oxidation of Nicalon™ fibers for simplicity. As a consequence, the follow￾ing oxidation model can only be applied where the fiber diam￾eter does not change significantly during oxidation. Shown in Fig. 1 is a schematic drawing of a fiber with an oxide layer. The fiber diameter is R, which is assumed to re￾main constant during the oxidation, and the oxide thickness is x. The radius of the unoxidized core is u, where u=R−x (1) As mentioned in the introduction, oxygen usually must go through three steps during the oxidation process:10 J. L. Smialek—contributing editor Manuscript No. 191280. Received January 14, 1997; approved June 17, 1997. Supported by the Laboratory Directed Research and Development Office of Los Alamos National Laboratory. This work was performed at Los Alamos National Laboratory under the auspices of the U.S. Department of Energy, under Contract No. W-7405-EN-36. *Member, American Ceramic Society. J. Am. Ceram. Soc., 81 [3] 655–60 (1998) Journal 655