K.L. More et al. /Composites: Part 4 30(1999)463-470 oxygen in the Nicalon-BN system is restricted, SiO2 will form before either B2O3 or a borosilicate glass at the interfaces. When a composite is subjected to a mechanical load, microcracking occurs and oxygen will rapidly diffuse to many interior interfaces in the composite. However, this will also result in rapid oxidation of the exposed surfaces causing crack closure, and the amount of oxygen that actu- 3.7 ally reaches the majority of interfaces will still be limited The conditions described by Sheldon et al. are being simu- lated in our experiments by using thin, unstressed samples he amount of oxygen reaching the Nicalon-BN inter- faces is limited to the oxygen that can reach the interfaces either from the environment, which is controlled by the rate of diffusion of oxygen through BN via pipe/grain boundary 3.3 diffusion, or by the diffusion of oxygen already present in High-O BN LOW-O BN the bn to the Nicalon-BN interface. Clearly, the diffu- sion of oxygen that already exists within the Bn lattice will Fig. 4. Statistical variation in d(002)between high-O and low-O BN inter- contribute more significantly to the formation of reaction face coatings. products, particularly when no stresses(causing micro- ks)are present, as the experimen nce given In were elongated in shape and were found around the entire this paper shows. Sheldon et al. also predicted the formation circumference of more than 50% of the fibers within the of borosilicate solid solutions in the later stages of the same single tow examined by TEM. When the oxidation was oxidation process after the formation of a Sio2 layer. A carried out at the intermediate temperature of 600"C for SiO2-B2O3 glass layer does form in the high-O BN after 00 h, interface reaction products were also observed for oxidation exposures at 950C for times exceeding 100 h In the high-O BN composites only, as shown in Fig. 7 fact, a borosilicate glass layer also forms after long exposure (compare to Fig. 6(d)). At 600C, a similar glass phase times at 600C(see Fig. 6(d)and Fig. 7). Voids that form segregation also occurred (labeled'S and 'B in Fig. 7) within the borosilicate glass layer are due to the trapping and the onset of void formation between the silica and boro- of volatile by-products, such as N, BO, etc, resulting from silicate glasses was observed after the 400 h exposure. The the oxidation of Bn to form B2O3 and reaction of the B2O3 voids here were much smaller in size but also more numer with SiO, to form the borosilicate glass. Voids can form in ous. No silica formed in the low-O BN composites at the this layer since the viscosity of the silicate glass is signifi BN-Nicalon interfaces at 600C; the BN-Nicalon cantly reduced due to the presence of boron. The SiO2-B2O interfaces in this sample remained stable at 600C, at least phase diagram [17 shows that, at 950C, even small after 400 h exposures amounts of boron can result in a glass phase with reduced The formation of a silica layer between BN and ceramic- viscosity and lower melting point. At 600C, the amount of grade Nicalon fibers has been observed by other research- boron must be higher to cause the same results. Of course ers after oxidation [6, 14, 15]. Sheldon et al. 16] used ther- the presence of a reduced-viscosity, low-melting point glass modynamic calculations to show that if the amount of at the interfaces will be detrimental to the mechanical of the Oxygen can 'existin hexagonal BN in several different forms; as B2O3, as a borosilicate glass (when silicon is present), and as a BN O, phase, or as a combination of these phases. Brozek and Hubacek [18] suggested that free B2O3 is present in crystalline BN in addition to oxygen in constitutionally bonded water present at the surfaces of eSiO individual crystallites. Evidence for the presence of a BNO, hase in CVD BN was given by Guimon et al. [19] . X-ray electron spectroscopy (XPS) was used to show that signifi cant amounts of oxygen were present in the bn and the b films were nonstoichiometric with n/b< XPs results 10 nm indicated the possibility of a metastable ternary ph BN,OY identified from a boron binding energy intermediate between pure BN and B2O,. It was suggested that this phase Fig. 5. TEM image showing thin silica layer (<5 nm) present at BN- resulted from the substitution of oxygen for nitrogen on Nicalon interface after composite processing. the bn lattice. The results from the work by Guimon arewere elongated in shape and were found around the entire circumference of more than 50% of the fibers within the single tow examined by TEM. When the oxidation was carried out at the intermediate temperature of 6008C for 400 h, interface reaction products were also observed for the high-O BN composites only, as shown in Fig. 7 (compare to Fig. 6(d)). At 6008C, a similar glass phase segregation also occurred (labeled ‘S’ and ‘B’ in Fig. 7) and the onset of void formation between the silica and boro￾silicate glasses was observed after the 400 h exposure. The voids here were much smaller in size but also more numer￾ous. No silica formed in the low-O BN composites at the BN–Nicalone interfaces at 6008C; the BN–Nicalone interfaces in this sample remained stable at 6008C, at least after 400 h exposures. The formation of a silica layer between BN and ceramic￾grade Nicalone fibers has been observed by other research￾ers after oxidation [6,14,15]. Sheldon et al. [16] used ther￾modynamic calculations to show that if the amount of oxygen in the Nicalone–BN system is restricted, SiO2 will form before either B2O3 or a borosilicate glass at the interfaces. When a composite is subjected to a mechanical load, microcracking occurs and oxygen will rapidly diffuse to many interior interfaces in the composite. However, this will also result in rapid oxidation of the exposed surfaces causing crack closure, and the amount of oxygen that actu￾ally reaches the majority of interfaces will still be limited. The conditions described by Sheldon et al. are being simu￾lated in our experiments by using thin, unstressed samples; the amount of oxygen reaching the Nicalone–BN inter￾faces is limited to the oxygen that can reach the interfaces either from the environment, which is controlled by the rate of diffusion of oxygen through BN via pipe/grain boundary diffusion, or by the diffusion of oxygen already present in the BN to the Nicalone–BN interface. Clearly, the diffu￾sion of oxygen that already exists within the BN lattice will contribute more significantly to the formation of reaction products, particularly when no stresses (causing micro￾cracks) are present, as the experimental evidence given in this paper shows. Sheldon et al. also predicted the formation of borosilicate solid solutions in the later stages of the same oxidation process after the formation of a SiO2 layer. A SiO2–B2O3 glass layer does form in the high-O BN after oxidation exposures at 9508C for times exceeding 100 h. In fact, a borosilicate glass layer also forms after long exposure times at 6008C (see Fig. 6(d) and Fig. 7). Voids that form within the borosilicate glass layer are due to the ‘trapping’ of volatile by-products, such as N, BO, etc., resulting from the oxidation of BN to form B2O3 and reaction of the B2O3 with SiO2 to form the borosilicate glass. Voids can form in this layer since the viscosity of the silicate glass is signifi- cantly reduced due to the presence of boron. The SiO2–B2O3 phase diagram [17] shows that, at 9508C, even small amounts of boron can result in a glass phase with reduced viscosity and lower melting point. At 6008C, the amount of boron must be higher to cause the same results. Of course, the presence of a reduced-viscosity, low-melting point glass at the interfaces will be detrimental to the mechanical properties of the composite. Oxygen can ‘exist’ in hexagonal BN in several different forms; as B2O3, as a borosilicate glass (when silicon is present), and as a BNxOy phase, or as a combination of these phases. Brozek and Hubacek [18] suggested that free B2O3 is present in crystalline BN in addition to oxygen in constitutionally bonded water present at the surfaces of individual crystallites. Evidence for the presence of a BNxOy phase in CVD BN was given by Guimon et al. [19]. X-ray electron spectroscopy (XPS) was used to show that signifi- cant amounts of oxygen were present in the BN and the BN films were nonstoichiometric with N/B , 1. XPS results indicated the possibility of a metastable ternary phase, BNxOy, identified from a boron binding energy intermediate between pure BN and B2O3. It was suggested that this phase resulted from the substitution of oxygen for nitrogen on the BN lattice. The results from the work by Guimon are K.L. More et al. / Composites: Part A 30 (1999) 463–470 467 Fig. 4. Statistical variation in d(002) between high-O and low-O BN inter￾face coatings. Fig. 5. TEM image showing thin silica layer (,5 nm) present at BN– Nicalone interface after composite processing