K.L. More et al. /Composites: Part A 30 (1999)463-470 SIoF 10 nm Sio 25 nm fiber 25 nm Fig. 6. TEM images showing the microstructural evolution at Nicalon"-BN interfaces after o at 950C: (a) low-o Bn after 100 h; (b) high-O Bn after 100 h;(c) low-O BN after 400 h;(d) high-O BN after 400 h(silica layer labeled"S and borosilicate glass layer labeledB) &Onsistent with the results found in our work for the high-o BN-Nicalon" interface from the high-O BN after 400h BN coatings: the bn d(002)spacings measured indicated a at 950C in argon is shown in Fig 8. There was no reaction meso-graphitic structure(more turbostratic than the low-o at the BN-nicalon interfaces for the low-O Bn after BN); the coatings had oxygen levels greater than 11 at 400 h in Ar; however, the interface reactions for the high (higher close to the Nicalon surface); the high-O BN coat- O BN were very similar to those observed for the high-O BN ings had a N/B <l. Thus, there is clear evidence for the oxidized for 400 h at 950C (compare Fig 8 and Fig. 6(d)) presence of a similar metastable ternary BN,O, phase in the The role of the oxygen within the BN, therefore, was clearly high-o bn coatings. Auger results also indicated that the silicon found within the high-O BN coating was bonded with oxygen indicating the possibility of a small amount of"free borosilicate glass phase surrounding the bn grains Oxygen is clearly dissolved in the bn structure regardless of the form the oxygen takes. The thermodynamic driving force is for the oxygen to move from within the BN coating to the bn-nicalon interface where reactions with sic will occur. The presence of the dissolved oxygen and oxygen- containing phases in the high-O BN will certainly contribute to the formation of the initial SiO, and subsequent sio B,O3 observed in the later stages of oxidation In order to fully evaluate the role of the oxygen within the bn (as opposed to the contribution of oxygen from the n environment), the composite samples were also annealed in argon under the same time and temperature conditions Fig. 7. TEM image of Nicalon-BN interface in high-o bn after 400 h at used for the oxidation experiments. A TEM image of a 600C (silica layer labeled'S'and borosilicate glass layer labeledB)consistent with the results found in our work for the high-O BN coatings: the BN d(002) spacings measured indicated a meso-graphitic structure (more turbostratic than the low-O BN); the coatings had oxygen levels greater than 11 at.% (higher close to the Nicalon surface); the high-O BN coat￾ings had a N/B , 1. Thus, there is clear evidence for the presence of a similar metastable ternary BNxOy phase in the high-O BN coatings. Auger results also indicated that the silicon found within the high-O BN coating was bonded with oxygen indicating the possibility of a small amount of ‘free’ borosilicate glass phase surrounding the BN grains. Oxygen is clearly dissolved in the BN structure regardless of the form the oxygen takes. The thermodynamic driving force is for the oxygen to move from within the BN coating to the BN–Nicalon interface where reactions with SiC will occur. The presence of the dissolved oxygen and oxygen￾containing phases in the high-O BN will certainly contribute to the formation of the initial SiO2 and subsequent SiO2– B2O3 observed in the later stages of oxidation. In order to fully evaluate the role of the oxygen within the BN (as opposed to the contribution of oxygen from the environment), the composite samples were also annealed in argon under the same time and temperature conditions used for the oxidation experiments. A TEM image of a BN–Nicalone interface from the high-O BN after 400 h at 9508C in argon is shown in Fig. 8. There was no reaction at the BN–Nicalone interfaces for the low-O BN after 400 h in Ar; however, the interface reactions for the high￾O BN were very similar to those observed for the high-O BN oxidized for 400 h at 9508C (compare Fig. 8 and Fig. 6(d)). The role of the oxygen within the BN, therefore, was clearly 468 K.L. More et al. / Composites: Part A 30 (1999) 463–470 Fig. 6. TEM images showing the microstructural evolution at Nicalone–BN interfaces after oxidation at 9508C: (a) low-O BN after 100 h; (b) high-O BN after 100 h; (c) low-O BN after 400 h; (d) high-O BN after 400 h (silica layer labeled ‘S’ and borosilicate glass layer labeled ‘B’). Fig. 7. TEM image of Nicalone–BN interface in high-O BN after 400 h at 6008C (silica layer labeled ‘S’ and borosilicate glass layer labeled ‘B’)