March 1997 613 Area 1 410 3.510 2.510 210 g1510 c 点11 5000 200 300 400 500 600 Energy Loss (ev Area 2 410 3.510 310 2.510 210 1.510 5000 200 300 400 500600 Energy Loss ( ev) was deposited on the res of regions labeled(A)area I and (B)area 2 in Fig. 4(A)(in both spectra, the carbon peaks are from the carbon coating that Fig. 5. PEELS an diffusion path at600°C ation proceeded along the fibers the simultaneous oxidation of BN and SiC at high temperatures and affected the entire debonded length, resulting in strength (1100C and 950%C)and intermediate temperatures(600oC degradation of the fibers. In the top ply (0), the cracks in the The calculations were initiated with equal amounts of SiC and matrix were likely to propagate when the bridging fibers in the Bn and under a constant pressure of 10 Pa(I atm). The crack wake failed. In the first 90 ply, oxidation extended into results indicated that, in the temperature range of 600-1100C the interior of the composite. These are evidenced by the large the oxygen partial pressure that was necessary to oxidize SiC brittle-failure zone that is observed in the samples that have and form solid-state SiO2(Po. was lower than the oxygen been fatigued at600°C partial pressure that was necessary for solid-/liquid-state B2O, Different oxidation behaviors were observed for the BN formation (Po, ) In other words, solid SiO2 formed before con- oated SiC fibers in a glass-ceramic matrix at intermediate and densed B,O,. At high temperatures, the crack opening was igh temperatures. At 1100C, the BN/fiber interfaces remained quickly closed by the oxidation product of SiC. The availability his intact under static loading conditions and substantial interfacial of oxygen in the sealed crack was limited because solid-state reaction occurred more readily under cyclic loading conditions diffusion of oxygen through the silica sealing is much slower when the maximum applied stress was 103 MPa ,o The inter- than the gas-phase transport prior to crack closure. The oxygen facial reaction at 1 100C exhibited an oxidation sequence such partial pressure in the crack(po, may satisfy such a condition that the BN layer remained unaffected, whereas the SiC fiber <Po, <po, that oxidation of BN is prevented. However, oxidized. On the other hand, the interfacial reaction that was the po, level in the crack at 600"C can be much higher than that nduced at 600C exhibited spontaneous oxidation of BN and at 950C. This is due to two reasons. First, the crack opening is SiC. Thermodynamic calculations were performed to analyze less likely to be sealed by the silicate scale at the externalMarch 1997 Environmental Effects on BN-Coated SiC-Fiber-Reinforced Glass-Ceramic Composites 613 (A) (B) Fig. 5. PEELS analyses of regions labeled (A) area 1 and (B) area 2 in Fig. 4(A) (in both spectra, the carbon peaks are from the carbon coating that was deposited on the TEM foil). diffusion path at 600C. Oxidation proceeded along the fibers the simultaneous oxidation of BN and SiC at high temperatures (1100C19 and affected the entire debonded length, resulting in strength and 950C) and intermediate temperatures (600C). degradation of the fibers. In the top ply (0), the cracks in the The calculations were initiated with equal amounts of SiC and BN and under a constant pressure of
105 matrix were likely to propagate when the bridging fibers in the Pa (1 atm). The crack wake failed. In the first 90 ply, oxidation extended into results indicated that, in the temperature range of 600–1100C, the interior of the composite. These are evidenced by the large the oxygen partial pressure that was necessary to oxidize SiC brittle-failure zone that is observed in the samples that have and form solid-state SiO2 ( pSiC O2 ) was lower than the oxygen been fatigued at 600 partial pressure that was necessary for solid-/liquid-state B2O3 C. Different oxidation behaviors were observed for the BN- formation (pBN O2 ). In other words, solid SiO2 formed before con￾densed B2O3 coated SiC fibers in a glass-ceramic matrix at intermediate and . At high temperatures, the crack opening was high temperatures. At 1100C, the BN/fiber interfaces remained quickly closed by the oxidation product of SiC. The availability intact under static loading conditions and substantial interfacial of oxygen in the sealed crack was limited because solid-state reaction occurred more readily under cyclic loading conditions diffusion of oxygen through the silica sealing is much slower when the maximum applied stress was 103 MPa. than the gas-phase transport prior to crack closure. The oxygen 9,10 The inter￾facial reaction at 1100C exhibited an oxidation sequence such partial pressure in the crack (pO2 ) may satisfy such a condition pSiC O2 pO2 pBN O2 that oxidation of BN is prevented.19 that the BN layer remained unaffected, whereas the SiC fiber However, oxidized.10 On the other hand, the interfacial reaction that was the pO2 level in the crack at 600C can be much higher than that induced at 600C exhibited spontaneous oxidation of BN and at 950C. This is due to two reasons. First, the crack opening is SiC. Thermodynamic calculations were performed to analyze less likely to be sealed by the silicate scale at the external