K.L. More et al. /Composites: Part A 30(1999)463-470 than the highly ordered graphitic BN(the ideal value of CVI SiC d (002)=3.33 A) when the 'mean d(002) value falls in between that of Bn, and Bng, the hexagonal BN structure is referred to as meso-graphitic BN. Clearly, both bn coat ings(high-O and low-O)had a meso-graphitic structure with individual bn crystallites having varying ranges of atomic order. The statistical variations between the mean d(002)value and range of d(002)values was quite different BN layer for each BN, as shown in by the plot in Fig. 4, the low-O BN had a slightly more graphitic component than the high-O BN. This effect was most pronounced near the surface of the fiber, where the highest oxygen content was found in the Nicalon 0.15 a thin silica layer(<5 nm thick) was identified between Fig. 2. TEM image of a Bn interface coatin the bn and the Nicalon fiber in both composites, as shown Fig. 5. This layer formed during the initial stages of BN deposition at 1100C, at these temperatures and in the ntervals of 1, 10, 100 and 400 h. Each specimen was presence of small amounts of O2(possibly from outgassing prepared as described above for examination in a Hitachi of moisture from walls of furnace), limited decomposition HF 2000 FEG/TEM operated at 200 kV and equipped with a of the Nicalon surface is likely. The oxygen content in the Noran energy-dispersive spectrometer(EDS)and a Gatan BN coatings was measured using both AES and EElS and parallel-detection electron energy-loss spectrometer qualitatively compared by EDS. For the AEs analyses, (PEELS) for compositional analysis of the small composite bars were fractured in situ and depth regions. Analytical TEM was used for the identification of profiles were conducted through several BN coatings the reaction products formed during each exposure and to (primarily in areas where fibers had pulled out and left document structural and chemical changes in the BN and the Bn in the trough). Auger analysis showed that the n/ Nicalon fibers as a function of exposure time end tempera- B< I in the high-O BN coatings and near stoichiometric ture Reaction kinetics associated with the isothermal expo- in the low-O BN coatings. The high-O BN had significant sures were evaluated by measuring the thicknesses of the amounts of oxygen, ranging from 10 to 14 at. % and also reaction products formed within the interfacial region silicon(2-5 at. %) The low-O BN had very low levels of oxygen; the oxygen content was al ways less than 2 at. % and no silicon was found. The oxygen content in the high- 3. Results and discussion O BN was much greater(2x) close to the fiber surface (within -0. 1 um)in the as-processed state The bn coatings in both composites(low-O and high-O A series of TEM images comparing the interface mi BN)were-0. 4 um thick and were structurally similar. A structure observed after oxidation at 950c for the low-o Fig. 2 and representative hign -resolution hg is shown in BN and the high-O BN are shown in Fig. 6(a)-(d).Afte shown in Fig 3. The BN was hexagonal and nano-crystal- he BN-Nicalon interfaces in the high-O BN composite, line in both composites, with a nominal crystallite size of whereas no changes were observed for the low-O BN <5 nm. The degree of order in the hexagonal Bn lattice composites after 100 h(compare Fig. 6(a) and Fig. 6(b)) (graphitization index) can be determined by measuring the More severe degradation occurred at the BN-Nicalon lattice interlayer spacing, d(002)[13]. These measurements interfaces in the high-O BN composite than in the low-O are most often accomplished on bulk BN using X-ray BN composite after oxidation for 400 h, as shown in Fi diffraction(XRD). Since the amount of BN present in the 6(c),(d). A stable 25 nm thick Sio2 glass layer was formed actual composite samples used in this work was extremely after 400 h at the BN-Nicalon interfaces in the low-O small and the crystallite size was also very small, XRD BN. Two glass phases formed during the 400 h oxidation could not be used. Instead, d(002) values were measured at the BN-Nicaloninterface in the high-O BN; the initial m individual crystallites in high-resolution TEM images, SiO2 glass(labeled'S in Fig. 6(d)), the thickness of which such as those labeled in Fig 3. Area ' in Fig 3 represents appeared to remain stable from 100 to 400 h, and a second a crystallite of turbostratic BN (characterized by havin glass which formed between the SiO2 and the bn and was only two-dimensional ordering and a rotation of the layers), found by eELs to be a borosilicate glass( the exact glass BN, where the measured value of d(002)= 3.63 A, and composition is unknown-labeled B in Fig. 6(d)).The Area'B' represents a crystallite of graphitic BN (highly most drastic change at the interface, however, was the ordered in three dimensions), BNg, where d(002 formation of voids within the borosilicate glass phase at a 3.40A. Turbostratic BN will have a larger value of d(002) majority of the interfaces in the high-o BN. These voidsintervals of 1, 10, 100 and 400 h. Each specimen was prepared as described above for examination in a Hitachi HF 2000 FEG/TEM operated at 200 kV and equipped with a Noran energy-dispersive spectrometer (EDS) and a Gatan parallel-detection electron energy-loss spectrometer (PEELS) for compositional analysis of the interfacial regions. Analytical TEM was used for the identification of the reaction products formed during each exposure and to document structural and chemical changes in the BN and the Nicalone fibers as a function of exposure time end tempera￾ture. Reaction kinetics associated with the isothermal expo￾sures were evaluated by measuring the thicknesses of the reaction products formed within the interfacial region. 3. Results and discussion The BN coatings in both composites (low-O and high-O BN) were ,0.4 mm thick and were structurally similar. A TEM image of a typical BN interface coating is shown in Fig. 2 and representative high-resolution TEM images are shown in Fig. 3. The BN was hexagonal and nano-crystal￾line in both composites, with a nominal crystallite size of ,5 nm. The degree of order in the hexagonal BN lattice (graphitization index) can be determined by measuring the lattice interlayer spacing, d(002) [13]. These measurements are most often accomplished on bulk BN using X-ray diffraction (XRD). Since the amount of BN present in the actual composite samples used in this work was extremely small and the crystallite size was also very small, XRD could not be used. Instead, d(002) values were measured from individual crystallites in high-resolution TEM images, such as those labeled in Fig. 3. Area ‘A’ in Fig. 3 represents a crystallite of turbostratic BN (characterized by having only two-dimensional ordering and a rotation of the layers), BNt, where the measured value of d(002) ˆ 3.63 A˚ , and Area ‘B’ represents a crystallite of graphitic BN (highly ordered in three dimensions), BNg, where d(002) ˆ 3.40 A˚ . Turbostratic BN will have a larger value of d(002) than the highly ordered graphitic BN (the ideal value of d(002) ˆ 3.33 A˚ ) when the ‘mean’ d(002) value falls in between that of BNt and BNg, the hexagonal BN structure is referred to as meso-graphitic BN. Clearly, both BN coat￾ings (high-O and low-O) had a meso-graphitic structure with individual BN crystallites having varying ranges of atomic order. The statistical variations between the mean d(002) value and range of d(002) values was quite different for each BN, as shown in by the plot in Fig. 4; the low-O BN had a slightly more graphitic component than the high-O BN. This effect was most pronounced near the surface of the fiber, where the highest oxygen content was found in the high-O BN. A thin silica layer (,5 nm thick) was identified between the BN and the Nicalone fiber in both composites, as shown in Fig. 5. This layer formed during the initial stages of BN deposition at 11008C; at these temperatures and in the presence of small amounts of O2 (possibly from outgassing of moisture from walls of furnace), limited decomposition of the Nicalon surface is likely. The oxygen content in the BN coatings was measured using both AES and EELS and qualitatively compared by EDS. For the AES analyses, small composite bars were fractured in situ and depth profiles were conducted through several BN coatings (primarily in areas where fibers had pulled out and left BN in the ‘trough’). Auger analysis showed that the N/ B , 1 in the high-O BN coatings and near stoichiometric in the low-O BN coatings. The high-O BN had significant amounts of oxygen, ranging from 10 to 14 at.%, and also silicon (2–5 at.%). The low-O BN had very low levels of oxygen; the oxygen content was always less than 2 at.%, and no silicon was found. The oxygen content in the high￾O BN was much greater (2×) close to the fiber surface (within ,0.1 mm) in the as-processed state. A series of TEM images comparing the interface micro￾structure observed after oxidation at 9508C for the low-O BN and the high-O BN are shown in Fig. 6(a)–(d). After 100 h at 9508C, a 20 nm thick silica layer was identified at the BN–Nicalone interfaces in the high-O BN composite, whereas no changes were observed for the low-O BN composites after 100 h (compare Fig. 6(a) and Fig. 6(b)). More severe degradation occurred at the BN–Nicalone interfaces in the high-O BN composite than in the low-O BN composite after oxidation for 400 h, as shown in Fig. 6(c),(d). A stable 25 nm thick SiO2 glass layer was formed after 400 h at the BN–Nicalone interfaces in the low-O BN. Two glass phases formed during the 400 h oxidation at the BN–Nicalone interface in the high-O BN; the initial SiO2 glass (labeled ‘S’ in Fig. 6(d)), the thickness of which appeared to remain stable from 100 to 400 h, and a second glass which formed between the SiO2 and the BN and was found by EELS to be a borosilicate glass (the exact glass composition is unknown—labeled ‘B’ in Fig. 6(d)). The most drastic change at the interface, however, was the formation of voids within the borosilicate glass phase at a majority of the interfaces in the high-O BN. These voids K.L. More et al. / Composites: Part A 30 (1999) 463–470 465 Fig. 2. TEM image of a BN interface coating in Nicalone–BN–SiC composite