and manufacturing ELSEVIER Composites: Part A 30(1999)463 Evaluating the effect of oxygen content in BN interfacial coatings on the stability of SiC/BN/SiC composites K.L. More,, K.S. Ailey,R.A. Lowden, H.T. Lin Metals and Ceramics Division, Oak Ridge National Laboratory, Building 4515, MS 6064. PO Box 2008, Oak Ridge, TN 37931-6064, USA Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695, USA Boron nitride was studied as a fiber-matrix interface coating for Nicalon/SiC composites. The effect of initial O-impurity content within the as-processed bN coatings on the long-term interface stability was investigated at elevated temperatures in flowing oxygen. two types of Nicalon"/SiC composites were used for this study; one composite had a bn coating with 2%oxygen(low-O BN) and another co had BN with an oxygen concentration 11%(high-O BN) in the as-processed state. The high-o BN is actually most representative of BN coatings available commercially. The bn coatings in both the high-o and low-O BN containing composites were structurally similar. The samples used here were thinned to 200 um before oxidation and the final preparation for electron microscopy examination of the interface ion was done after the reactions were completed Thin samples were used to simulate maximum corrosion effects that would occur at the surface of an actual part during service. Ech sample was exposed to flowing oxygen at temperatures as high as 950oC for times up to 400 h After each oxidation experiment, the bN coatings were examined by TEM to quantify the extent of any reaction which occurred at either fiber/BN and BN/SiC matrix interfaces. At 950C for 100 h, there were no interface microstructural changes observed in the low-o bn but there was extensive silica formation at the fiber/BN interfaces in the high-o BN. After 400 h at 950 C, large voids formed at the fiber/BN interface in the high-O BN sample only. Oxygen present within the initial BN coating contributed significantly to the degradation of the interfacial properties of the composite. Several techniques, including transmission electron microscopy (TEM), Auger electron spectroscopy (AES), energy-dispersive spectrometry(EDS), and electron energy-loss spectroscopy(EEls)were used to characterize changes in structure and chemistry of the fiber-matrix interface region and to elucidate and quantify composite degradation mechanisms. c 1999 Elsevier Science ltd. all rights reserved Keywords: A Ceramic matrix composites( CMCs); BN; Oxidation; Characterization 1. Introduction been investigated. bn has received much attention recentl for use in Sic-based composite systems, since it has The fibers and matrix play major roles in determining the improved oxidation resistance compared to carbon [5,6] final properties of a composite, however, it is the fiber- However, there are concerns regarding the long-term stabi matrix interface that has significant influence on the fracture lity of many commercially available Bn coatings. Several behavior and mechanical properties of fiber-reinforced recent studies have addressed the issue of the stability of BN composites. Interface coatings are commonly applied to in oxygen-and water-containing environments [7-9 not only to protect the fibers during matrix processing, but It has also become important to fully evaluate the 'type also to control interfacial forces and provide protection for of bn being used as an interface coating since there are the fibers during use in corrosive environments at elevated many parameters that will influence the bn stability at temperatures [1, 2]. Thus, the development of a stable and different use temperatures. Obviously, all BN coatings are chemically compatible fiber coating is necessary in order for inherently different and understanding the atomic level fiber-reinforced composites to be used in critical applica- structure and chemistry of the bn will provide invaluable tions. Carbon was the most commonly used interlayer in the information for composite life-prediction. The crystal struc- past, but as a result of its poor oxidation resistance at ture and chemical composition of the BN will depend on the elevated temperatures [3, 4], other interface materials have deposition parameters used, i.e., gas precursors and deposi tion temperature will influence such factors as BN stoichio- Corresponding author. Tel. 1-423-5747788: fax: 1-423- metry, degree of crystallinity, and impurity content in the 5744913 BN. All of these factors play a role in the long-term (/99/.see front matter e 1999 Elsevier Science Ltd. All rights reserved 59-835X(98)00135-3Evaluating the effect of oxygen content in BN interfacial coatings on the stability of SiC/BN/SiC composites K.L. Morea,*, K.S. Aileyb , R.A. Lowdena , H.T. Lina a Metals and Ceramics Division, Oak Ridge National Laboratory, Building 4515, MS 6064, PO Box 2008, Oak Ridge, TN 37931-6064, USA b Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695, USA Abstract Boron nitride was studied as a fiber–matrix interface coating for Nicalone/SiC composites. The effect of initial O-impurity content within the as-processed BN coatings on the long-term interface stability was investigated at elevated temperatures in flowing oxygen. Two types of Nicalone/SiC composites were used for this study; one composite had a BN coating with , 2% oxygen (low-O BN) and another composite had BN with an oxygen concentration . 11% (high-O BN) in the as-processed state. The high-O BN is actually most representative of BN coatings available commercially. The BN coatings in both the high-O and low-O BN containing composites were structurally similar. The samples used here were thinned to , 200 mm before oxidation and the final preparation for electron microscopy examination of the interface region was done after the reactions were completed. Thin samples were used to simulate maximum corrosion effects that would occur at the surface of an actual part during service. Ech sample was exposed to flowing oxygen at temperatures as high as 9508C for times up to 400 h. After each oxidation experiment, the BN coatings were examined by TEM to quantify the extent of any reaction which occurred at either the fiber/BN and BN/SiC matrix interfaces. At 9508C for 100 h, there were no interface microstructural changes observed in the low-O BN but there was extensive silica formation at the fiber/BN interfaces in the high-O BN. After 400 h at 9508C, large voids formed at the fiber/BN interface in the high-O BN sample only. Oxygen present within the initial BN coating contributed significantly to the degradation of the interfacial properties of the composite. Several techniques, including transmission electron microscopy (TEM), Auger electron spectroscopy (AES), energy-dispersive spectrometry (EDS), and electron energy-loss spectroscopy (EELS) were used to characterize changes in structure and chemistry of the fiber–matrix interface region and to elucidate and quantify composite degradation mechanisms. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Ceramic matrix composites (CMCs); BN; Oxidation; Characterization 1. Introduction The fibers and matrix play major roles in determining the final properties of a composite, however, it is the fiber– matrix interface that has significant influence on the fracture behavior and mechanical properties of fiber-reinforced composites. Interface coatings are commonly applied to not only to protect the fibers during matrix processing, but also to control interfacial forces and provide protection for the fibers during use in corrosive environments at elevated temperatures [1,2]. Thus, the development of a stable and chemically compatible fiber coating is necessary in order for fiber-reinforced composites to be used in critical applica￾tions. Carbon was the most commonly used interlayer in the past, but as a result of its poor oxidation resistance at elevated temperatures [3,4], other interface materials have been investigated. BN has received much attention recently for use in SiC-based composite systems, since it has improved oxidation resistance compared to carbon [5,6]. However, there are concerns regarding the long-term stabi￾lity of many commercially available BN coatings. Several recent studies have addressed the issue of the stability of BN in oxygen- and water-containing environments [7–9]. It has also become important to fully evaluate the ‘type’ of BN being used as an interface coating since there are many parameters that will influence the BN stability at different use temperatures. Obviously, all BN coatings are inherently different and understanding the atomic level structure and chemistry of the BN will provide invaluable information for composite life-prediction. The crystal struc￾ture and chemical composition of the BN will depend on the deposition parameters used, i.e., gas precursors and deposi￾tion temperature will influence such factors as BN stoichio￾metry, degree of crystallinity, and impurity content in the BN. All of these factors play a role in the long-term Composites: Part A 30 (1999) 463–470 1359-835X/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S1359-835X(98)00135-3 * Corresponding author. Tel.: 1 1-423-5747788; fax: 1 1-423- 5744913