Journal of the American Ceramic SociefyPeres-Rigueiro et al. Vol. 82, No. 12 by energy-dispersive X-ray microanalysis(EDAX), for the BN revealed that the carbon had disappeared after I h exposure at coating and the BN/fiber interlayer are plotted in Figs. I(B)and 800oC(Fig. 2(A)and that the main constituents were silicon (C), respectively. The first plot shows a significant amount of and oxygen. The thicker interlayer that formed after heat treat arbon and oxygen in the BN coating of the as-received com ment at 1200C allowed a more precise characterization by posite. Other authors 820 also detected oxygen and carbon-in EDAX. In Fig. 2(B), the energy spectrum is compared with that the range 5-10 wt%in BN coatings deposited by CVD. The obtained for the SiO2 standard, prepared as indicated in Section origin of the oxygen was unclear, but oxygen seemed to be Ill. Both spectra are practically identical, indicating that the introduced by contamination during the deposition process terlayer formed during high-temperature exposure was Sio and not any type of silicate The cuation associated with the application of the BN coating No oxidation of the bulk bn was observed. even in the samples treated at 1400C for 1 h. The energy spectrum of the interlayer was carbon rich. As indicated in the Introduction BN coating under this condition is shown in Fig. 3. The only arbon-rich layers appeared spontaneously on the surface of difference from the spectrum obtained in the as-received ma- alon fibers incorporated in glass-ceramic matrice terial is the absence of carbon after high-tem exposure, temperature. 1 The layers also were found when bn coating he peaks of nitrogen and oxygen are unaltered. This finding were applied at high temperature onto Nicalon fibers 8-20 and emphasizes the need to improve our knowledge of the oxida- were attributed to fiber decomposition at the surface, although tion processes in BN. The oxidation resistance of this coating there is no clear consensus on this point improves with crystallinity and purity whereas, on the contrary The effect on the BN/fiber interface of oxidation treatment the presence of water vapor leads to the rapid volatilization of for I h at800°and1200° C is shown in F he borosilicate glasses formed by oxidation. 21 22 ely The composite treated for I h at 1000C presented High-resolution analysis by tem (not shown here)indicated an intermediate appearance and is not plotted here, for the sake that this Bn coating exhibited a turbostratic structure and, as of brevity. The BN/SiC interface remained sharp and free from mentioned above, contained significant amounts of carbon and any reaction layer. The thin BN/fiber interlayer, in contrast oxygen(Fig. I(B). Although the present heat\"preatment atments were grew thicker with temperature, reaching -100 nm after heat performed in a laboratory atmosphere, with a rela treatment at 1200C. The energy spectrum of the interlayer of-50%, the Bn coating was unaffected by heat treatment, BN SiO2 Fiber tandard 吕06 M Energy(kev) Fig. 2. TEM photographs showing BN coating after I h at(A)800 and(B)1200C Energy spectra of the BN/fiber interlayer after I h at 800 and at 1200C are shown below the corresponding micrographsby energy-dispersive X-ray microanalysis (EDAX), for the BN coating and the BN/fiber interlayer are plotted in Figs. 1(B) and 1(C), respectively. The first plot shows a significant amount of carbon and oxygen in the BN coating of the as-received com￾posite. Other authors18,20 also detected oxygen and carbon—in the range 5–10 wt%—in BN coatings deposited by CVD. The origin of the oxygen was unclear, but oxygen seemed to be introduced by contamination during the deposition process. Carbon added to BN reportedly15 prevents fiber-strength degradation associated with the application of the BN coating. The second plot, Fig. 1(C), shows that the present BN/fiber interlayer was carbon rich. As indicated in the Introduction, carbon-rich layers appeared spontaneously on the surface of Nicalon fibers incorporated in glass-ceramic matrices at high temperature.1,2 The layers also were found when BN coatings were applied at high temperature onto Nicalon fibers18–20 and were attributed to fiber decomposition at the surface, although there is no clear consensus on this point. The effect on the BN/fiber interface of oxidation treatment for 1 h at 800° and 1200°C is shown in Figs. 2(A) and (B), respectively. The composite treated for 1 h at 1000°C presented an intermediate appearance and is not plotted here, for the sake of brevity. The BN/SiC interface remained sharp and free from any reaction layer. The thin BN/fiber interlayer, in contrast, grew thicker with temperature, reaching ∼100 nm after heat treatment at 1200°C. The energy spectrum of the interlayer revealed that the carbon had disappeared after 1 h exposure at 800°C (Fig. 2(A)) and that the main constituents were silicon and oxygen. The thicker interlayer that formed after heat treat￾ment at 1200°C allowed a more precise characterization by EDAX. In Fig. 2(B), the energy spectrum is compared with that obtained for the SiO2 standard, prepared as indicated in Section III. Both spectra are practically identical, indicating that the interlayer formed during high-temperature exposure was SiO2 and not any type of silicate. No oxidation of the bulk BN was observed, even in the samples treated at 1400°C for 1 h. The energy spectrum of the BN coating under this condition is shown in Fig. 3. The only difference from the spectrum obtained in the as-received ma￾terial is the absence of carbon after high-temperature exposure; the peaks of nitrogen and oxygen are unaltered. This finding emphasizes the need to improve our knowledge of the oxida￾tion processes in BN. The oxidation resistance of this coating improves with crystallinity and purity whereas, on the contrary, the presence of water vapor leads to the rapid volatilization of the borosilicate glasses formed by oxidation.21,22 High-resolution analysis by TEM (not shown here) indicated that this BN coating exhibited a turbostratic structure and, as mentioned above, contained significant amounts of carbon and oxygen (Fig. 1(B)). Although the present heat treatments were performed in a laboratory atmosphere, with a relative humidity of ∼50%, the BN coating was unaffected by heat treatment, Fig. 2. TEM photographs showing BN coating after 1 h at (A) 800° and (B) 1200°C. Energy spectra of the BN/fiber interlayer after 1 h at 800° and at 1200°C are shown below the corresponding micrographs. 3496 Journal of the American Ceramic Society—Pe´rez-Rigueiro et al. Vol. 82, No. 12