ournal JAm.C.soc,8214-82(1999 High-Temperature Oxidation of Boron Nitride ll, Boron Nitride Layers in Composites Nathan S Jacobson and Gregory N. Morscher NASA Lewis Research Center, Cleveland, Ohio 44135 Darren R. Bryant and Richard E Tressler The Pennsylvania State University, University Park, Pennsylvania 16802 The oxidation of BN composite interphases was examined SiC oxidation can occur at even lower temperatures, because with a series of model materials. Oxidation was examined of the boron interacting with the SiO, scale as it forms. The in both low-water-vapor(-20 ppm H2O/O2) environments SiO, and B2O3 react to form stable borosilicates, and there is at 900C and high-water-vapor(1% and 10%H2O/O2)en- SiO2-B2O3 eutectic at 372.C. Oxygen diffusivities are more vironments at 700 and 800C. The low-water-vapor case rapid in a borosilicate glass, and, hence, SiC oxidation is en was explored with layered BN/SiC materials. This case was hanced.5,6 These reactions lead to degradation of fiber prope dominated by borosilicate glass formation, and the 20 ppm ties as well as fiber/matrix bonding, which then degrades the water vapor gradually removed the boron from the glass, mechanical properties of the composite. Figure 1(a)illustrates er amou nt of SiO2 than would be expected oxidation of a SiC/SiC composite where borosilicate forms from simple SiC oxidation. Layered Sic/BN/SiC materials were also used to study low-water-vapor oxidation effects within the composite. The high-water-vapor case was ex- ed with SiC/BN/SiC minicomposites, and it was dom nated by volatilization of BN as HBO (8), H3 BO3 (8), and H3 BO(8). A model for recession of the BN fiber coating was developed based on the gas-phase diffusion of these species out of the annular region around the SiC fiber and concurrent sealing of this annular region by oxidation . Introduction B N NITRIDE(BN) is a promising interphase material for SiC-fiber-reinforced SiC-matrix composites. It allows the debonding and fiber-sliding requisite for strong and tough com osite behavior. There are several detailed microstructural studies of these as-processed composites. 1, 2 Ideally the micro- structure should consist of the SiC matrix, a BN fiber coating, and the Sic fiber 10 um The critical question is how this structure changes in service ith aggressive gases at elevated temperatures. With no matrix cracks, BN is stable in contact with SiC at high temperatures This has been confirmed with both thermodynamic calculations and experiments. 3 The only reaction noted in a low-oxygen- potential environment is the solubility of any free carbon in BN 3 However, in the case of a matrix crack, bn would be ex posed to high oxygen potentials. bn oxidizes to a liquid oxide B2O3) above-450°Cas 2BN+502(g)=B,O3()+N2(g) (1) At temperatures above -900C, the SiC oxidizes simulta- SiC +=O,(g)+SiO,(s)+co(g) T.A. Parthasarathy--contributing editor Fig. 1. Micrographs of composites and minicomposites with one image of a composite with CVD Bn deposited at <1000C, 100 h in oxygen at 816C(photo courtesy of D. Fox, NASA Lewis)and(b) 191031 Received May 8, 1997; approved September 24, 1998 secondary electron image of a minicomposite with CVD deposited at ican Ceramic Society -1000oC, 100 h in humid laboratory air at 50High-Temperature Oxidation of Boron Nitride: II, Boron Nitride Layers in Composites Nathan S. Jacobson* and Gregory N. Morscher* NASA Lewis Research Center, Cleveland, Ohio 44135 Darren R. Bryant* and Richard E. Tressler* The Pennsylvania State University, University Park, Pennsylvania 16802 The oxidation of BN composite interphases was examined with a series of model materials. Oxidation was examined in both low-water-vapor (∼20 ppm H2O/O2) environments at 900°C and high-water-vapor (1% and 10% H2O/O2) en￾vironments at 700° and 800°C. The low-water-vapor case was explored with layered BN/SiC materials. This case was dominated by borosilicate glass formation, and the 20 ppm water vapor gradually removed the boron from the glass, leaving a larger amount of SiO2 than would be expected from simple SiC oxidation. Layered SiC/BN/SiC materials were also used to study low-water-vapor oxidation effects within the composite. The high-water-vapor case was ex￾plored with SiC/BN/SiC minicomposites, and it was domi￾nated by volatilization of BN as HBO2(g), H3BO3(g), and H3B3O6(g). A model for recession of the BN fiber coating was developed based on the gas-phase diffusion of these species out of the annular region around the SiC fiber and concurrent sealing of this annular region by oxidation. I. Introduction BORON NITRIDE (BN) is a promising interphase material for SiC-fiber-reinforced SiC-matrix composites. It allows the debonding and fiber-sliding requisite for strong and tough com￾posite behavior. There are several detailed microstructural studies of these as-processed composites.1,2 Ideally the micro￾structure should consist of the SiC matrix, a BN fiber coating, and the SiC fiber. The critical question is how this structure changes in service with aggressive gases at elevated temperatures. With no matrix cracks, BN is stable in contact with SiC at high temperatures. This has been confirmed with both thermodynamic calculations and experiments.1,3 The only reaction noted in a low-oxygen￾potential environment is the solubility of any free carbon in BN.3 However, in the case of a matrix crack, BN would be ex￾posed to high oxygen potentials. BN oxidizes to a liquid oxide (B2O3) above ∼450°C as 2BN + 3 2 O2~g! = B2O3~l! + N2~g! (1) At temperatures above ∼900°C, the SiC oxidizes simulta￾neously: SiC + 3 2 O2~g! + SiO2~s! + CO~g! (2) SiC oxidation can occur at even lower temperatures, because of the boron interacting with the SiO2 scale as it forms. The SiO2 and B2O3 react to form stable borosilicates, and there is SiO2–B2O3 eutectic at 372°C.4 Oxygen diffusivities are more rapid in a borosilicate glass, and, hence, SiC oxidation is en￾hanced.5,6 These reactions lead to degradation of fiber proper￾ties as well as fiber/matrix bonding, which then degrades the mechanical properties of the composite.7 Figure 1(a) illustrates oxidation of a SiC/SiC composite where borosilicate forms. T. A. Parthasarathy—contributing editor Manuscript No. 191031. Received May 8, 1997; approved September 24, 1998. *Member, American Ceramic Society. Fig. 1. Micrographs of composites and minicomposites with one edge ground off and exposed to oxidizing gases: (a) secondary electron image of a composite with CVD BN deposited at <1000°C, 100 h in oxygen at 816°C (photo courtesy of D. Fox, NASA Lewis) and (b) secondary electron image of a minicomposite with CVD deposited at ∼1000°C, 100 h in humid laboratory air at 500°C. J. Am. Ceram. Soc., 82 [6] 1473–82 (1999) Journal 1473