urna J Am Ceram Soc, 80 [3]609-14(1997) Intermediate-Temperature environmental effects on Boron Nitride-Coated silicon carbide-Fiber-Reinforced Glass-Ceramic Composites Ellen Y. Sun and Hua-Tay Lin Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6068 John brennan United Technologies Research Center, East Hartford, Connecticut 06108 The environmental effects on the mechanical properties of such as load-transfer, crack-deflection, and fiber-pullout pro- fiber-reinforced composites at intermediate temperatures cesses. However, when exposed to an oxidizing environment, were investigated by conducting flexural static-fatigue the carbon interfacial layer can oxidize, resulting in strong experiments in air at 600 and 950%C. The material that fiber-matrix bonding via silica formation at the fiber surface was studied was a silicon carbide/boron nitride(SiC/BN) and/or in fiber-strength degradation because of oxidation or dual-coated Nicalon-fiber-reinforced barium magnesium recrystallization of the fiber. In recent years, research has been aluminosilicate glass-ceramic Comparable time-dependent conducted to control the fiber/matrix interface in the composite failure responses were found at 600 and 950C when the by applying coatings on the fiber surfaces prior to composi maximum tensile stress applied in the bend bar was. 60% processing. The goal is to achieve a fiber coating of relatively of the room-temperature ultimate Flexural strength of low shear strength and good oxidation resistance such that as-received materials. At both temperatures, the materials composites with excellent mechanical properties and useful survived 500 h fatigue tests at lower stress levels. Among engineering lifetimes in oxygen-rich environments can be the samples that survived the 500 h fatigue tests, a 20% obtained -7 Following this approach, barium mag alumi- degradation in the room-temperature flexural strength nosilicate(BMAS)glass-ceramic composites reinforced with was measured in samples tested at 600 C, whereas no deg silicon carbide/boron nitride(SiC/BN) dual-coated Nicalon dation was observed for the samples tested at 950C. Microstructure and chemistry studies revealed interfacial fiber(Nippon Carbon Co., Tokyo, Japan) have been fabricated Previous studies have revealed that this material exhibits better oxidation in the samples that were fatigued at 600C. The mechanical properties and thermal stability at high tempera- growth rate of the Si-C-O fiber oxidation product at 600 C tures(21100 C), compared to composites reinforced with uncoated fibers or carbon-coated fibers, and, hence, is a promis- the interior of the material was oxidized and resulted in a ing candidate for high-temperature structural applications trength degradation and less fibrous fracture. In contrast. the interior of the material remained intact at 950%c More recently, oxidation effects on fiber-reinforced glass- because of crack sealing by rapid silicate formation, and ceramic composites with in-situl-formed carbon interlayers have strength/toughness of the composite was maintained. Also, been found to be more severe at intermediate temperatures at 600C, BN oxidized via volatilization, because no borosil (4000-800oC)than at high temperatures(=1000C). -4These cate was formed studies were conducted without applied stress. At high tempera tures, the carbon interfacial layer can be protected by the oxide scale that formed on the fiber surface at the exposed fiber end L. Introduction The interfacial opening that formed because of carbon removal O XIDATION embrittlement of fiber-reinforced glass and glass an be quickly sealed before oxidation extends into the interior ceramic matrix composites at high temperatures(21100.C of the material. However, at intermediate temperatures, the is well documented. With polymer-derived silicon carbide opening may not be sealed, because of lower rates of silicate (SiC-type fibers, the formation of a thin carbon layer(20- scale formation, resulting in property degradation. On the other 50 nm thick) at the fiber/matrix interface can be obtained during reinforced bMas glass-ceramic composites have indicated that processing at elevated temperatures during composi tion. This weak interfacial layer results in composites with high his composite system shows no strength degradation after strength and toughne ss vIa n multiple toughening mechanisms, annealing at 550C in oxygen for 100 h. Therefore, to examine the environmental effects on this composite system at interme- diate temperatures, the material has been subjected to stresses R.J. Kerans--contributing editor above that which produces microcracking in the matrix and, thus, allows the interior of the composite to be exposed to the environment. In the present study, static-fatigue experiments Manuscript No 191916 Received April 1, 1996: approved September 27, 1996 have been used. Composite materials have been exposed to various applied flexural stresses at 600 and 950C in air. The Research Associates Program admiment of resistance of the composite to stress-induced oxidation has been 960R22464 with Lockheed Martin Energy Research Corp and by an evaluated by observing the static-fatigue behavior at different temperatures and correlating the mechanical properties with Presented in part at the 199 Conference on Composites, Advanced Ceramics, interfacial microstructure and chemistry studies. A parallel or Materials and Structures(Coce Advanced Ceramics and Composites Symposium(Paper No. C-78-96F) come of the study is a better understanding of the oxidation Member, American Ceramic Society. behavior of bn and the sic fibers at theseJ. Am. Ceram. Soc., 80 [3] 609–14 (1997) Intermediate-Temperature Environmental Effects on Boron Nitride-Coated Silicon Carbide-Fiber-Reinforced Glass-Ceramic Composites Ellen Y. Sun* and Hua-Tay Lin* Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831–6068 John J. Brennan* United Technologies Research Center, East Hartford, Connecticut 06108 The environmental effects on the mechanical properties of such as load-transfer, crack-deflection, and fiber-pullout pro- fiber-reinforced composites at intermediate temperatures cesses. However, when exposed to an oxidizing environment, were investigated by conducting flexural static-fatigue the carbon interfacial layer can oxidize, resulting in strong experiments in air at 600
and 950
C. The material that fiber–matrix bonding via silica formation at the fiber surface was studied was a silicon carbide/boron nitride (SiC/BN) and/or in fiber-strength degradation because of oxidation or dual-coated Nicalon-fiber-reinforced barium magnesium recrystallization of the fiber. In recent years, research has been aluminosilicate glass-ceramic. Comparable time-dependent conducted to control the fiber/matrix interface in the composite failure responses were found at 600
and 950
C when the by applying coatings on the fiber surfaces prior to composite maximum tensile stress applied in the bend bar was .60% processing. The goal is to achieve a fiber coating of relatively of the room-temperature ultimate flexural strength of low shear strength and good oxidation resistance such that as-received materials. At both temperatures, the materials composites with excellent mechanical properties and useful survived 500 h fatigue tests at lower stress levels. Among engineering lifetimes in oxygen-rich environments can be the samples that survived the 500 h fatigue tests, a 20% obtained.5–7 Following this approach, barium magnesium alumi- degradation in the room-temperature flexural strength nosilicate (BMAS) glass-ceramic composites reinforced with was measured in samples tested at 600
C, whereas no degra- silicon carbide/boron nitride (SiC/BN) dual-coated Nicalon
dation was observed for the samples tested at 950
C. fiber (Nippon Carbon Co., Tokyo, Japan) have been fabricated. Microstructure and chemistry studies revealed interfacial Previous studies have revealed that this material exhibits better oxidation in the samples that were fatigued at 600
C. The mechanical properties and thermal stability at high tempera- growth rate of the Si-C-O fiber oxidation product at 600
C tures (
1100C), compared to composites reinforced with was not sufficient to seal the stress-induced cracks, so that uncoated fibers or carbon-coated fibers, and, hence, is a promis- the interior of the material was oxidized and resulted in a ing candidate for high-temperature structural applications.8–10 strength degradation and less fibrous fracture. In contrast, More recently, oxidation effects on fiber-reinforced glass- the interior of the material remained intact at 950
C ceramic composites with in-situ-formed carbon interlayers have because of crack sealing by rapid silicate formation, and been found to be more severe at intermediate temperatures strength/toughness of the composite was maintained. Also, (400–800C) than at high temperatures (
1000C).11–14 These at 600
C, BN oxidized via volatilization, because no borosili- studies were conducted without applied stress. At high tempera- cate was formed. tures, the carbon interfacial layer can be protected by the oxide scale that formed on the fiber surface at the exposed fiber ends. I. Introduction The interfacial opening that formed because of carbon removal can be quickly sealed before oxidation extends into the interior OXIDATION embrittlement of fiber-reinforced glass and glass- of the material. However, at intermediate temperatures, the ceramic matrix composites at high temperatures (
1100C) opening may not be sealed, because of lower rates of silicate is well documented.1–4 With polymer-derived silicon carbide scale formation, resulting in property degradation. On the other (SiC)-type fibers, the formation of a thin carbon layer (20– hand, preliminary studies on the SiC/BN dual-coated fiber- 50 nm thick) at the fiber/matrix interface can be obtained during reinforced BMAS glass-ceramic composites have indicated that processing at elevated temperatures during composite fabrica- this composite system shows no strength degradation after tion. This weak interfacial layer results in composites with high annealing at 550C in oxygen for 100 h.15 Therefore, to examine strength and toughness via multiple toughening mechanisms, the environmental effects on this composite system at interme￾diate temperatures, the material has been subjected to stresses above that which produces microcracking in the matrix and, R. J. Kerans—contributing editor thus, allows the interior of the composite to be exposed to the environment. In the present study, static-fatigue experiments Manuscript No. 191916. Received April 1, 1996; approved September 27, 1996. have been used. Composite materials have been exposed to Research at ORNL sponsored by the U.S. Department of Energy, Division of various applied flexural stresses at 600 and 950C in air. The Materials Sciences, Office of Basic Energy Sciences, under Contract No. DE-AC05- resistance of the composite to stress-induced oxidation has been 96OR22464 with Lockheed Martin Energy Research Corp. and by an appointment of author EYS to the ORNL Postdoctoral Research Associates Program administered evaluated by observing the static-fatigue behavior at different jointly by ORISE and ORNL. Author JJB was supported at UTRC by the Air Force temperatures and correlating the mechanical properties with Office of Scientific Research. Presented in part at the 1996 Conference on Composites, Advanced Ceramics, interfacial microstructure and chemistry studies. A parallel out- Materials and Structures (Cocoa Beach, FL, Jan. 1996), Environmental Effects on come of the study is a better understanding of the oxidation Advanced Ceramics and Composites Symposium (Paper No. C-78-96F). * Member, American Ceramic Society. behavior of BN and the SiC fibers at these temperatures. 609