Journal of the American Ceramic SocietyJacobson et al. VoL. 82. No 6 One face has been ground to expose the fibers and BN coatings In addition, oxidation studies in low-water-vapor-containing or oxidation. After oxidation, a borosilicate ring forms around environments were done on SiC/BN/SiC layered structures each fiber These structures consisted of a CVd SiC coupon coated with In a companion paper, oxidation of monolithic BN was -10 um CVD BN, an intermediate layer of CVD SiC,-10 um explored. It was shown that oxidation rates were quite sensi- CVD BN, and finally an overcoat of -100 um CVD SiC. The ive to porosity, oxygen impurity levels, and crystallographic intermediate layer of CVD SiC was of varying thickness- rientation. Two types of chemically-vapor-deposited(CVD) om a -l um layer to -40 um mounds. As is shown, th BN were examined--one deposited at -1900 K and one de intermediate layer was quite useful in elucidating oxidation sited at -1400 K. bn deposited at the higher temperature behavior. For oxidation treatment, one face was gently polished had a higher density and a lower oxygen content and exhibited with nonaqueous solvents to expose the bn and BN/SiC inter- much better oxidation resistance than bn deposited at lower es ior oX idation, as shown in Fig. 1. Oxidation was per formed in the TGA, although weight changes were negligible It was also shown that the oxidation of bn was sensitive to The third system involved minicomposites. These were even small amounts of water vapor in the oxidizing gas stream single tows of -10 Hm unidirectional fibers that were first due to the formation of highly stable HBo,(g)si from the coated with -0. 5 um of Bn and then Sic to simulate a matrix reaction of water vapor and B,Oa(e). In the case of BN depos This is described in more detail in table i and Refs. 10 and 12 ited at-1900 K, the weight change curves exhibited paralinear One face of the minicomposites was ground in order to behavior due to the simultaneous weight gain from bn oxida- the fibers and fiber coatings for oxidation. Oxida tion and weight loss from B,O3(e) volatilization as HBO2 (g ments were done in a horizontal furnace with oxygen h Typical processing temperatures of BN interphases in SiC/ through water in order to create a controlled-water-vapor Sic composites are relatively low (1000C) in order to content stream achieve complete fiber coverage and uniform fiber-coatin thicknesses There are several oxidation studies of these com- (2) Postexposure Examination posites in the literature.9-I One oxidation study shows that a After oxidation exposure, the samples were examined with several techniques, depending on the exposure. The oxidized low-processing-temperature BN on a SiC fiber in an oxide BN/SiC samples were examined with both scanning electron omposite exhibits limited oxidation at 1100C. The SiC fiber xidizes in preference to the bn, which is consistent with cresco and Rutherford backscattering thermochemical models. However the situation changes in the copy(RBS). The RBS was done at SUNY-Albany. Helium presence of water vapor. Morscher et al. o have shown that a atoms with an incident energy of 2 Mev were between the beam and detector path was 14, and the angle low-processing-temperature BN fiber coating volatilizes readily in the presence of water vapor. This is illustrated in Fig between the sample normal and the detector was 7. a ste (b). Less volatilization occurs with BN materials processed at owers database allowed fitting the spectra to 0.01 Mev. the gh temperatures and/or doped with silicon. Brun and Luthra'1 standard"RUMP"analysis code was used 13 Estimated sensi- have recently examined oxidation of the BN phase in SiC/Sic tivity for boron was 0.5 at. % with an uncertainty of+25% nd of their composite and ex- The SiC/BN/SiC layered structures and minicomposites were examined by standard electron optical techniques-SEM pose it to an oxidizing environment containing a high concen- tration of water vapor. They observe BN oxidation to B2O3 and and electron microprobe analysis(EPMA). The Imens volatilization due to water vapor. Borosilicate glasses form, but were mounted and polished perpendicular to the In the case of the layered structure, this is shown in Fig. 2, in the boron appears to be leached out because of the presence of the case of the minicomposites. this was parallel to the fibers r. a series of model materials are examined to Polishing was done with nonaqueous solvents to preserve the study the oxidation of BN interphases in SiC/SiC composites water-soluble phases. In the SEM, energy dispersive sp We consider oxidation in both low-water-vapor environments opy(EDs)was used for elemental where borosilicate formation dominates, and high-water-vapor EPMA. wavelength di dispersive spectroscopy(WDS)was use nvironments where volatilization dominates to allow boron detection in the layered specimens alison.G bon maps were omitted due to embedding of polishing com- Il. Experimental Procedures ound in the soft bn phases. Figure 2 illustrates the locations (marked as regions a, b, and c) for each EPMA image and corresponding elemental maps Model Materials and Oxidation Exposures Three types of model materials were examined. Table I sum- IlL. Results and discussion marizes these model materials and their corresponding expo- sures. The oxidation experiments in low-water-vapor containing environments were done with samples consisting of 1)Oxidation in Low-Water-Vapor Environments--BN/ -2 um thick BN chemically-vapor-deposited on a CVD Sic SiC Model Compounds coupon. These were oxidized in a controlled-atmosphere ther- Consider first the experiments on BN films deposited on mogravimetric apparatus(TGA) CVD SiC. Two types of BN are used, as shown in Table I. The Table L. Model Compounds, Types of BN, and Reaction Temperatures for These Studies bn d Model material mperature°o temperatures(°C Oxidation environment Oxidation-low 2 Hm BN film on CVD <1000 water vapor 19003 Oxidation -low CVD SIC/BN on Cvd -1900° O2/20 ppm H,O Volatilization of Bn- Minicomposite--SIC -10001 O3/1%or10%H2O fibers coated with BI Advanced Ceramics Corp "BN deposited by 3M Cor St Paul, posited by Advanced Ceramics Corp, Lakewood, OH. SiC coupon from Morton Inc "SiC, BN deposited byOne face has been ground to expose the fibers and BN coatings for oxidation. After oxidation, a borosilicate ring forms around each fiber. In a companion paper,8 oxidation of monolithic BN was explored. It was shown that oxidation rates were quite sensi￾tive to porosity, oxygen impurity levels, and crystallographic orientation. Two types of chemically-vapor-deposited (CVD) BN were examined—one deposited at ∼1900 K and one de￾posited at ∼1400 K. BN deposited at the higher temperature had a higher density and a lower oxygen content and exhibited much better oxidation resistance than BN deposited at lower temperatures. It was also shown that the oxidation of BN8 was sensitive to even small amounts of water vapor in the oxidizing gas stream due to the formation of highly stable HBO2(g) species from the reaction of water vapor and B2O3(,). In the case of BN depos￾ited at ∼1900 K, the weight change curves exhibited paralinear behavior due to the simultaneous weight gain from BN oxida￾tion and weight loss from B2O3(,) volatilization as HBO2(g). Typical processing temperatures of BN interphases in SiC/ SiC composites are relatively low (∼1000°C) in order to achieve complete fiber coverage and uniform fiber-coating thicknesses. There are several oxidation studies of these com￾posites in the literature.9–11 One oxidation study9 shows that a low-processing-temperature BN on a SiC fiber in an oxide composite exhibits limited oxidation at 1100°C. The SiC fiber oxidizes in preference to the BN, which is consistent with thermochemical models. However, the situation changes in the presence of water vapor. Morscher et al.10 have shown that a low-processing-temperature BN fiber coating volatilizes readily in the presence of water vapor. This is illustrated in Fig. 1(b). Less volatilization occurs with BN materials processed at high temperatures and/or doped with silicon. Brun and Luthra11 have recently examined oxidation of the BN phase in SiC/SiC composites. They grind off an end of their composite and ex￾pose it to an oxidizing environment containing a high concen￾tration of water vapor. They observe BN oxidation to B2O3 and volatilization due to water vapor. Borosilicate glasses form, but the boron appears to be leached out because of the presence of water vapor. In this paper, a series of model materials are examined to study the oxidation of BN interphases in SiC/SiC composites. We consider oxidation in both low-water-vapor environments, where borosilicate formation dominates, and high-water-vapor environments, where volatilization dominates. II. Experimental Procedures (1) Model Materials and Oxidation Exposures Three types of model materials were examined. Table I sum￾marizes these model materials and their corresponding expo￾sures. The oxidation experiments in low-water-vapor￾containing environments were done with samples consisting of ∼2 mm thick BN chemically-vapor-deposited on a CVD SiC coupon. These were oxidized in a controlled-atmosphere ther￾mogravimetric apparatus (TGA). In addition, oxidation studies in low-water-vapor-containing environments were done on SiC/BN/SiC layered structures. These structures consisted of a CVD SiC coupon coated with ∼10 mm CVD BN, an intermediate layer of CVD SiC, ∼10 mm CVD BN, and finally an overcoat of ∼100 mm CVD SiC. The intermediate layer of CVD SiC was of varying thickness— from a ∼1 mm layer to ∼40 mm mounds. As is shown, this intermediate layer was quite useful in elucidating oxidation behavior. For oxidation treatment, one face was gently polished with nonaqueous solvents to expose the BN and BN/SiC inter￾faces for oxidation, as shown in Fig. 1. Oxidation was per￾formed in the TGA, although weight changes were negligible. The third system involved minicomposites. These were single tows of ∼10 mm unidirectional fibers that were first coated with ∼0.5 mm of BN and then SiC to simulate a matrix. This is described in more detail in Table I and Refs. 10 and 12. One face of the minicomposites was ground in order to expose the fibers and fiber coatings for oxidation. Oxidation treat￾ments were done in a horizontal furnace with oxygen bubbled through water in order to create a controlled-water-vapor￾content stream. (2) Postexposure Examination After oxidation exposure, the samples were examined with several techniques, depending on the exposure. The oxidized BN/SiC samples were examined with both scanning electron microscopy (SEM) and Rutherford backscattering spectros￾copy (RBS). The RBS was done at SUNY-Albany. Helium atoms with an incident energy of 2 MeV were used; the angle between the beam and detector path was 14°, and the angle between the sample normal and the detector was 7°. A stopping powers database allowed fitting the spectra to 0.01 MeV. The standard “RUMP” analysis code was used.13 Estimated sensi￾tivity for boron was 0.5 at.% with an uncertainty of ±25%. The SiC/BN/SiC layered structures and minicomposites were examined by standard electron optical techniques—SEM and electron microprobe analysis (EPMA). The specimens were mounted and polished perpendicular to the exposed face. In the case of the layered structure, this is shown in Fig. 2; in the case of the minicomposites, this was parallel to the fibers. Polishing was done with nonaqueous solvents to preserve the water-soluble phases. In the SEM, energy dispersive spectros￾copy (EDS) was used for elemental identification. In the EPMA, wavelength dispersive spectroscopy (WDS) was used to allow boron detection in the layered specimens. Elemental maps were done for boron, nitrogen, oxygen, and silicon. Car￾bon maps were omitted due to embedding of polishing com￾pound in the soft BN phases. Figure 2 illustrates the locations (marked as regions a, b, and c) for each EPMA image and corresponding elemental maps. III. Results and Discussion (1) Oxidation in Low-Water-Vapor Environments—BN/ SiC Model Compounds Consider first the experiments on BN films deposited on CVD SiC. Two types of BN are used, as shown in Table I. The Table I. Model Compounds, Types of BN, and Reaction Temperatures for These Studies Study Model material BN deposition temperature (°C) Oxidation temperatures (°C) Oxidation environment Oxidation—low 2 mm BN film on CVD <1000† 900 O2/20 ppm H2O water vapor SiC ∼1900‡ Oxidation—low CVD SiC/BN on CVD ∼1900¶ 900 O2/20 ppm H2O water vapor SiC coupon§ 1100 Volatilization of BN— Minicomposite—SiC ∼1000†† 700 O2/1% or 10% H2O high water vapor fibers coated with BN 800 in SiC matrix † BN deposited by General Electric Co., Schnectady, NY. ‡ BN deposited by Advanced Ceramics Corp., Lakewood, OH. § SiC coupon from Morton Inc. ¶ SiC, BN deposited by Advanced Ceramics Corp. ††BN deposited by 3M Corp., St. Paul, MN. 1474 Journal of the American Ceramic Society—Jacobson et al. Vol. 82, No. 6