Y Liu et al. Materials Science and Engineering A 466(2007)172-177 4.03 1000°C .05 0·1300°C Oxidation time(h) As-received 00°C Fig 8. Oxidation weight loss of the coated 3D C/SiC composites after oxidized re(C) for 600 min Fig. 9. Residual flexural strength of the coated C/SiC before and after oxidized for 900 min: ('d) Sic/B/SiC coating; sic/Sic/SiC coating and B2O3xSiO2 by XRD results. However, volatilization of the B203 was much more extensive as the temperature increased posite showed a large strength loss(64% retained strength)after [8-10].Then the high partial pressure would create pores in the oxidation at 700C, lager than that of SiC/B/SiC coated com- coating surface. The cracks cannot be completely sealed, and posites after oxidation at the same time. The residual strengths the porous material remained due to B2O3 volatilization of the SiC/SiC/SiC coated composite after oxidation at 1000C Above 1300C, the experiment was terminated because is nearly as same as that of SiC/B/SiC coated composites after of the extensive production and volatilization of B2O3 and oxidation at the same time. After oxidation at 1300oC. the resid B2O3xSiO2, which was verified by the adhesion between the ual strength of the Sic/Sic/SiC coated composite oxidized for samples and the corundum tube. There was not a continuous 10 h is higher than that of SiC/B/SiC coated composites only glassy material on the coating surface. The outer layer CVD SiC oxidized for 2h coating faked off because of the high partial pressure of B20 These results indicate that the Sic/B/Sic coating could and B203 xSiOz. Therefore, the hybrid coating surface seemed provide better oxidation protection for C/SiC composite than loose and flake-like according to SEMresults The compositions Sic/SiC/SiC coating between 700C and 1000C for 600min were SiC, SiO2 and B2O3 xSio by EDS and XRD results due to cracks sealed by B2O3 or B2O3 x Sio2 liquid. But above 1000C, the protection effect of the SiC/B/SiC coating for C/SiC 3.3. Effect of oxidation on the weight change and composite decreases due to the extensive volatilization of B2O3 mechanical properties of the coated composites or B2O3 xSiO2 liquid. The protection effect of the SiC/B/SiC coating for C/SiC composite is worse than that of Sic/sic/SiC Fig8 shows the weight change of the coated C/SiC com- coating at the temperatures greater than 1000C posite oxidized at700°C,1000°cfor600 min and1300°C for 120 min. The weight change of the SiC/SiC/SiC coated 3D- 4. Conclusions C/SiC composites oxidized at the same time and temperature are also shown in Fig 8, in order to compare the oxidation behav- (1)ACVD B coating for C/SiC was synthesized from hydrogen ior of he SiC/B/SiC coated composites. Both the coated C/SiC and boron trichloride. The b coating was found to exhibit a composites showed weight loss. As for SiC/SiC/SiC coated crystal structure. The morphology of CVD SiC coating on composites, the weight loss almost increase linearly with the the CVd B layer is similar to that of CVD SiC coating on xidation time increase from 700C to 1300C. which showed the CVD SiC layer, but the crack width at room temperature that the cracks are not fully healed below 1300 C. However, the seems smaller weight loss is nonlinear with increasing oxidation time, which (2) The coating surface was covered by B2O3 or B2O3xSio can be described as follows: at 700C. the weight loss increases glassy material after oxidation at 700C and 1000C, how for the initial 2 h, then decrease with increasing oxidation time, ever the morphology of coating surface seems iake-like at 1000"C, the weight loss increases dramatically for the initial ter oxidation at 1300C. The coating cracks were fully 2h, then decrease rapidly for 3 h, and finally increases rapidly sealed at700°C, and partially sealed at I000°C. for the last several hours; at 1300C, the weight loss increases (3) The residual strength of the SiC/SiC/SiC coated compos dramatically before the experiment was stopped ite showed a strength loss(76.9% retained strength) after The residual flexural strengths are compared as shown in xidation at 700C. larger than that of SiC/B/SiC coated Fig 9. The residual strength of the SiC/B/SiC coated composite composites after oxidation. The residual strength of the had a small strength loss(87.7% retained strength) after oxida- Sic/SiC/SiC coated composite after oxidation at 1000C tion for 600 min at 700oC and 1000C, and a biggish strength is nearly as same as that of Sic/B/Sic coated composites loss(76.9% retained strength) after oxidation at 1300C for after oxidation at the same time. After oxidation at 1300oC 120 min. The residual strength of the SiC/SiC/Sic coated com- the residual strength of the SiC/Sic/SiC coated composite176 Y. Liu et al. / Materials Science and Engineering A 466 (2007) 172–177 Fig. 8. Oxidation weight loss of the coated 3D C/SiC composites after oxidized for 600 min. and B2O3·xSiO2 by XRD results. However, volatilization of the B2O3 was much more extensive as the temperature increased [8–10]. Then the high partial pressure would create pores in the coating surface. The cracks cannot be completely sealed, and the porous material remained due to B2O3 volatilization. Above 1300 ◦C, the experiment was terminated because of the extensive production and volatilization of B2O3 and B2O3·xSiO2, which was verified by the adhesion between the samples and the corundum tube. There was not a continuous glassy material on the coating surface. The outer layer CVD SiC coating flaked off because of the high partial pressure of B2O3 and B2O3·xSiO2. Therefore, the hybrid coating surface seemed loose and flake-like according to SEM results. The compositions were SiC, SiO2 and B2O3·xSiO2 by EDS and XRD results. 3.3. Effect of oxidation on the weight change and mechanical properties of the coated composites Fig. 8 shows the weight change of the coated C/SiC com￾posite oxidized at 700 ◦C, 1000 ◦C for 600 min and 1300 ◦C for 120 min. The weight change of the SiC/SiC/SiC coated 3D￾C/SiC composites oxidized at the same time and temperature are also shown in Fig. 8, in order to compare the oxidation behav￾ior of he SiC/B/SiC coated composites. Both the coated C/SiC composites showed weight loss. As for SiC/SiC/SiC coated composites, the weight loss almost increase linearly with the oxidation time increase from 700 ◦C to 1300 ◦C, which showed that the cracks are not fully healed below 1300 ◦C. However, the weight loss is nonlinear with increasing oxidation time, which can be described as follows: at 700 ◦C, the weight loss increases for the initial 2 h, then decrease with increasing oxidation time; at 1000 ◦C, the weight loss increases dramatically for the initial 2 h, then decrease rapidly for 3 h, and finally increases rapidly for the last several hours; at 1300 ◦C, the weight loss increases dramatically before the experiment was stopped. The residual flexural strengths are compared as shown in Fig. 9. The residual strength of the SiC/B/SiC coated composite had a small strength loss (87.7% retained strength) after oxida￾tion for 600 min at 700 ◦C and 1000 ◦C, and a biggish strength loss (76.9% retained strength) after oxidation at 1300 ◦C for 120 min. The residual strength of the SiC/SiC/SiC coated com￾Fig. 9. Residual flexural strength of the coated C/SiC before and after oxidized for 900 min: ( ) SiC/B/SiC coating; ( ) SiC/SiC/SiC coating. posite showed a large strength loss (64% retained strength) after oxidation at 700 ◦C, lager than that of SiC/B/SiC coated com￾posites after oxidation at the same time. The residual strengths of the SiC/SiC/SiC coated composite after oxidation at 1000 ◦C is nearly as same as that of SiC/B/SiC coated composites after oxidation at the same time. After oxidation at 1300 ◦C, the resid￾ual strength of the SiC/SiC/SiC coated composite oxidized for 10 h is higher than that of SiC/B/SiC coated composites only oxidized for 2 h. These results indicate that the SiC/B/SiC coating could provide better oxidation protection for C/SiC composite than SiC/SiC/SiC coating between 700 ◦C and 1000 ◦C for 600min due to cracks sealed by B2O3 or B2O3·xSiO2 liquid. But above 1000 ◦C, the protection effect of the SiC/B/SiC coating for C/SiC composite decreases due to the extensive volatilization of B2O3 or B2O3·xSiO2 liquid. The protection effect of the SiC/B/SiC coating for C/SiC composite is worse than that of SiC/SiC/SiC coating at the temperatures greater than 1000 ◦C. 4. Conclusions (1) A CVD B coating for C/SiC was synthesized from hydrogen and boron trichloride. The B coating was found to exhibit a crystal structure. The morphology of CVD SiC coating on the CVD B layer is similar to that of CVD SiC coating on the CVD SiC layer, but the crack width at room temperature seems smaller. (2) The coating surface was covered by B2O3 or B2O3·xSiO2 glassy material after oxidation at 700 ◦C and 1000 ◦C, how￾ever the morphology of coating surface seems flake-like after oxidation at 1300 ◦C. The coating cracks were fully sealed at 700 ◦C, and partially sealed at 1000 ◦C. (3) The residual strength of the SiC/SiC/SiC coated compos￾ite showed a strength loss (76.9% retained strength) after oxidation at 700 ◦C, larger than that of SiC/B/SiC coated composites after oxidation. The residual strength of the SiC/SiC/SiC coated composite after oxidation at 1000 ◦C is nearly as same as that of SiC/B/SiC coated composites after oxidation at the same time. After oxidation at 1300 ◦C, the residual strength of the SiC/SiC/SiC coated composite