Y Liu et al. Materials Science and Engineering A 466(2007)172-177 Fig. 5. Morphology of the SiC/B/SiC coating after oxidized at 1000 C for 900 min: (a) surface: (b)crack sealing. 100um 1 Omm Fig. 6. Morphology of the SiC/B/SiC coating after oxidized at 1300C for 120 min: (a)surface: (b)conglutination between B2 O3 glass and crucible 600 min As temperature increased, the surface consisted of SiC, SiO,(g)+ B2 O3()1o0ooB2O3xSiO2()) SiO and b2O3-xSio after oxidized at 1000C for 600 min and 1300°Cfor120min. For the composites with hybrid SiC/B/SiC multilayer coating, B2O3·xSiO2①)→B2O3(g)+xSiO2(s) the oxidation reactions in air from 700oC to 1300C are as 2C(s)+O2(g)→→2CO(g) The cracks in the Cvd SiC coating cannot be sealed at 700oC SiC(s)+302(g)→→2SiO2(s)+2COg) because of the low oxidation rate of cvd Sic if no cvd layer existed. A previous report [14] indicated that the B2O3 4B(s)+302(g)→2B2O3() (2) would flow at 630.C. Therefore, B2O, liquid would be produced at 700oC due to the reaction(2), after the CVd B layer was B2O3() 3) introduced into the hybrid coating. The liquid flows through the cracks in the Cvd sic. which lead to the sealed crack On the other hand, B203 would accelerate the oxidation of CVd Sic by forming a melt, according to B2O3-SiO2 system phase diagram [15]. This melt would be expected to have high oxygen 1300C Oxidized diffusion rates and lead to the observed rapid oxidation rates. The formation of the glassy material coating the surface and 000° C Oxidized cracks was proved by observation of SEM. The compositions were SiC, Sioz and a small amount of B2O3 as the XRD results Avk showed. Because of the low temperature at 700C volatilization 700° C Oxidized of B2O3 is faint as shown in reaction At 1000C, the reactions(1)3) would be faster. So the microcracks in the Sic coating would also result from the D SiC would also prompt the crack sealing. Large amount of B2O3 would be formed with increasing temperature. Then the reactions(4) Fig. 7. XRD spectrums of the Sic/B/SiC coating before and after oxidized at face and crack were also both covered with glassy material as 700°C,1000°Cand1300°C observed by SEM in Fig. 5. The compositions were SiC, SiOzY. Liu et al. / Materials Science and Engineering A 466 (2007) 172–177 175 Fig. 5. Morphology of the SiC/B/SiC coating after oxidized at 1000 ◦C for 900 min: (a) surface; (b) crack sealing. Fig. 6. Morphology of the SiC/B/SiC coating after oxidized at 1300 ◦C for 120 min: (a) surface; (b) conglutination between B2O3 glass and crucible. 600 min. As temperature increased, the surface consisted of SiC, SiO2 and B2O3·xSiO2 after oxidized at 1000 ◦C for 600 min and 1300 ◦C for 120 min. For the composites with hybrid SiC/B/SiC multilayer coating, the oxidation reactions in air from 700 ◦C to 1300 ◦C are as follows: 2SiC(s) + 3O2(g)800 ◦C −→ 2SiO2(s) + 2CO(g) (1) 4B(s) + 3O2(g)500 ◦C −→ 2B2O3(l) (2) B2O3(l)600−1000 ◦C −→ B2O3(g) (3) Fig. 7. XRD spectrums of the SiC/B/SiC coating before and after oxidized at 700 ◦C, 1000 ◦C and 1300 ◦C. SiO2(g) + B2O3(l)1000 ◦C −→ B2O3 · xSiO2(l) (4) B2O3 · xSiO2(l)≥1000 ◦C −→ B2O3(g) + xSiO2(s) (5) 2C(s) + O2(g)400 ◦C −→ 2CO(g) (6) The cracks in the CVD SiC coating cannot be sealed at 700 ◦C because of the low oxidation rate of CVD SiC if no CVD B layer existed. A previous report [14] indicated that the B2O3 would flow at 630 ◦C. Therefore, B2O3 liquid would be produced at 700 ◦C due to the reaction (2), after the CVD B layer was introduced into the hybrid coating. The liquid flows through the cracks in the CVD SiC, which lead to the sealed crack. On the other hand, B2O3 would accelerate the oxidation of CVD SiC by forming a melt, according to B2O3–SiO2 system phase diagram [15]. This melt would be expected to have high oxygen diffusion rates and lead to the observed rapid oxidation rates. The formation of the glassy material coating the surface and cracks was proved by observation of SEM. The compositions were SiC, SiO2 and a small amount of B2O3 as the XRD results showed. Because of the low temperature at 700 ◦C volatilization of B2O3 is faint as shown in reaction (3). At 1000 ◦C, the reactions (1)–(3) would be faster. So the microcracks in the SiC coating would also result from the glassy materials. The thermal expansion of CVD SiC would also prompt the crack sealing. Large amount of B2O3 would be formed with increasing temperature. Then the reactions (4) and (5) would also much more serious. The hybrid coating sur￾face and crack were also both covered with glassy material as observed by SEM in Fig. 5. The compositions were SiC, SiO2