ATERIALS GENGE S ENGIEERING ELSEVIER Materials Science and Engineering A262(1999)16-24 Modeling of oxidation behavior of sic-reinforced ceramic matrix composites P. Mogilevsky * A. Zangvil Unicersity of Illinois at Urbana-Champaign, Frederick Seitz Materials Research Laboratory, 104 South Goodwin Avenue, Urbana IL 61801 USA Received 15 June 1998: received in revised form 6 October 1998 Abstract Internal oxidation of Sic reinforcement is a major factor affecting the environmental stability of sic reinforced ceramic matrix omposites(CMCs) for high temperature applications. a simple phenomenological model describing the unidirectional oxidation of Sic reinforced oxide CMCs is presented. The model allows to calculate the thickness of the silica layer formed on a Sic reinforcement as a function of its location(depth beneath the surface)and time, if the oxygen permeabilities of silica and the matrix are known. The oxidation mode can thereby be predicted. Alternatively, the model allows to evaluate the oxygen permeabilities of silica and the matrix from the experimental oxidation data. Moreover, the expected mode of oxidation, I or II can be predicted depending on oxygen permeabilities and volume fraction of the reinforcement phase. Application of the model to the results of the microscopic study of the oxidation of Sic reinforced mullite-zirconia matrix composites allowed to oxygen permeabilities of the matrix and of the growing silica layer on the Sic particles. It was found that while ermeability of the silica layer on the Sic particles may depend significantly on the type of Sic reinforcement, it is rea close to the values obtained from the experiments on direct oxidation of Sic and permeation through vitreous silica permeability of the mullite -ZrO, matrix showed a dependence on the microstructure and composition of the matrix. o 1999 Elsevier Science S.A. All rights reserved. Keywords: Environmental stability: Oxygen permeabilities: Oxidation; Silicon carbide: Ceramic matrix composites 1. Introduction [7-9]. Two oxidation modes of Sic reinforced oxide matrix composite materials have been described, Fig. I Ceramic matrix composite(CMC) materials rein- [8, 9]. Mode I is defined as the case where oxygen reacts forced with SiC particles, whiskers, or platelets have with the whole Sic particle before it diffuses farther received increasing attention due to their potentially into the matrix, resulting in a clear boundary separating high fracture toughness and strength [1-5]. Internal a layer with completely oxidized SiC from the underly oxidation of SiC reinforcement is a major factor affect- ing composite with unoxidized SiC Mode II is the case ng the environmental stability of Sic reinforced CMCs where oxygen can deeply penetrate into the matrix for high temperature applications. Hence, alumina and before Sic particles in the outer region are completely mullite have been selected as matrix materials due to oxidized, leaving a long range of partially oxidized Sic heir excellent high-temperature stability and slow oxy particles behind. Mode I was found in mullite-SiC gen permeation Oxidation of Sic has been studied in detail [6]. The composites, while mode II was observed in mullite- study of oxidation of Sic in alumina, mullite, and ZrO2-SiC composites [10. However, no model conve- mullite-zirconia matrices was performed both micro- niently linking the observed oxidation behavior of Sic topically and via conventional weight gain method reinforced CMCs with the diffusion characteristics of the composite components has been applied to analyze the results. In the present study we propose a simple author.Tel:+1-217-3332367;fax:l-217 phenomenological model for mode II of the oxidation 278: e-mail: mogilevsky@mrlxp2 mrl uiuc. edu of sic reinforced CMcs 0921-5093/99/S- see front matter c 1999 Elsevier Science S.A. All rights reserved. PI:s0921-5093(98)01029-6Materials Science and Engineering A262 (1999) 16–24 Modeling of oxidation behavior of SiC-reinforced ceramic matrix composites P. Mogilevsky *, A. Zangvil Uni6ersity of Illinois at Urbana-Champaign, Frederick Seitz Materials Research Laboratory, 104 South Goodwin A6enue, Urbana, IL 61801, USA Received 15 June 1998; received in revised form 6 October 1998 Abstract Internal oxidation of SiC reinforcement is a major factor affecting the environmental stability of SiC reinforced ceramic matrix composites (CMCs) for high temperature applications. A simple phenomenological model describing the unidirectional oxidation of SiC reinforced oxide CMCs is presented. The model allows to calculate the thickness of the silica layer formed on a SiC reinforcement as a function of its location (depth beneath the surface) and time, if the oxygen permeabilities of silica and the matrix are known. The oxidation mode can thereby be predicted. Alternatively, the model allows to evaluate the oxygen permeabilities of silica and the matrix from the experimental oxidation data. Moreover, the expected mode of oxidation, I or II, can be predicted depending on oxygen permeabilities and volume fraction of the reinforcement phase. Application of the model to the results of the microscopic study of the oxidation of SiC reinforced mullite–zirconia matrix composites allowed to evaluate oxygen permeabilities of the matrix and of the growing silica layer on the SiC particles. It was found that while oxygen permeability of the silica layer on the SiC particles may depend significantly on the type of SiC reinforcement, it is reasonably close to the values obtained from the experiments on direct oxidation of SiC and permeation through vitreous silica. Oxygen permeability of the mullite–ZrO2 matrix showed a dependence on the microstructure and composition of the matrix. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Environmental stability; Oxygen permeabilities; Oxidation; Silicon carbide; Ceramic matrix composites 1. Introduction Ceramic matrix composite (CMC) materials rein￾forced with SiC particles, whiskers, or platelets have received increasing attention due to their potentially high fracture toughness and strength [1–5]. Internal oxidation of SiC reinforcement is a major factor affect￾ing the environmental stability of SiC reinforced CMCs for high temperature applications. Hence, alumina and mullite have been selected as matrix materials due to their excellent high-temperature stability and slow oxy￾gen permeation. Oxidation of SiC has been studied in detail [6]. The study of oxidation of SiC in alumina, mullite, and mullite–zirconia matrices was performed both micro￾scopically and via conventional weight gain method [7–9]. Two oxidation modes of SiC reinforced oxide matrix composite materials have been described, Fig. 1 [8,9]. Mode I is defined as the case where oxygen reacts with the whole SiC particle before it diffuses farther into the matrix, resulting in a clear boundary separating a layer with completely oxidized SiC from the underly￾ing composite with unoxidized SiC. Mode II is the case where oxygen can deeply penetrate into the matrix before SiC particles in the outer region are completely oxidized, leaving a long range of partially oxidized SiC particles behind. Mode I was found in mullite–SiC composites, while mode II was observed in mullite– ZrO2 –SiC composites [10]. However, no model conve￾niently linking the observed oxidation behavior of SiC reinforced CMCs with the diffusion characteristics of the composite components has been applied to analyze the results. In the present study we propose a simple phenomenological model for mode II of the oxidation of SiC reinforced CMCs. * Corresponding author. Tel.: +1-217-3332367; fax: 1-217- 2442278; e-mail: mogilevsky@mrlxp2.mrl.uiuc.edu. 0921-5093/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0921-5093(98)01029-6