22 Y Liu et al /Corrosion Science 51 (2009)820-826 CVD a-BC surface CVD SiC CVD a-BC 100m CVD SIC 500um c CVDSiC surface Fig. 2. Surface and cross section of CVD SiC/a-BCSiC coatings (a)CVD a-BC surface(b)Cross-section morphology of hybrid coatings (c)CVD SiC surface. 700C only Sic phase was identified. The cristobalite occurs after oxidation at 1000C. the obvious cristobalite exists after oxidation at 1200C which was corresponding wi morphologies as shown in Fig. 4(f). Fig 6 shows the weight change curves of the coated C/SiC com posite after oxidation at 700, 1000, and 1200C up to 100 h in 14 voL% H20/8 voL% O2/78 voL% Ar atmosphere. Clearly showing the weight changes are small at all the test temperatures. The max- Sic imum of weight loss is only 0.49 wt% after 100 h-oxidation at 700C. The weight change curves of difference each temperature have great differences, which can be described detailed as Fig 3. Interfacial morphology between CVD a-BC and CVD SiC coating. (1)At 700C, the weight of the composites lost continuously. During the first 10h, the weight loss is very obvious with 0.0072 wt %/h rate. It is small with 0.0032 wt %/h rate from (2)At 1000C, the surface morphologies have changed obvi- 10 to 68 h. At the last 32 h, it is obvious again with ously compared with that of coatings before oxidation. 0.0074 wt %h rate ccording to surface morphologies in the Fig 4(c)and(d 2)At 1000C, the weight of the composites increased continu luch glass occur on the coatings surface. The crack in coat- ously at the first 41 h, then the weight decreased with ings is sealed by glass. The positions among CVD SiC coa increasing oxidation time. The weight gain has a maximum lations are also filled by glass. Some new cracks occur in the of 0.11 wt% after oxidation for 41 h. Finally, the weight loss glass surface layer after oxidation which due to the mis is 0011 wt% after oxidation for 100 h match of thermal expansion coefficient between coatings (3)At 1200C. the weight of the composites increased continu and glass layer, though no spallation of the glass occurred ously after oxidation for 100 h. The maximum of weight gain at this temperature. is 0. 155 wt%, and the final weight gain is 0. 14 wt. %. The sus- (3)At 1200C, the surface was fully covered with tained weight gain indicates that the coating is healed an shown in Fig 4(e)and(f). There are many pores an no oxygen and water intrude into the interface and fibre. n the glass layer. The cracks resulted from the misr layer. The pores in the glass layer could be formed due to coated with Sic/Sic/Sic, sic B/SiC or Sic/graphitic B-C/SiC had its the volatilization of B203 glass. The crystallization phenom- weight loss at 700-1300C for 10 or 15 h in air atmosphere. At ena can be found in the glass layer as shown in Fig 4(f). The 700C oxidation, the weights of Sic/Sic/SiC, Sic/graphitic B-C/ glass crystallized owing to the effects of high temperature SiC, and Sic/B/SiC lost above 0.71 wt% for 15 h, 0. 19 wt% for (1200C)and long time(100 h), which was proven by XRD 15 h, and 0.003 wt% for 10 h, respectively. At 1100C oxidation, easurement Fig. 5 shows the XRD patterns measured on the weights of Sic/Sic/Sic and Sic/graphitic B-C/Sic lost above the test specimens oxidized at different temperatures. At 0.16 wt% for 15 h, 0.51 wt% for 15 h, respectively. At 1000C oxi-(2) At 1000 C, the surface morphologies have changed obvi￾ously compared with that of coatings before oxidation. According to surface morphologies in the Fig. 4(c) and (d), much glass occur on the coatings surface. The crack in coat￾ings is sealed by glass. The positions among CVD SiC coagu￾lations are also filled by glass. Some new cracks occur in the glass surface layer after oxidation which due to the mis￾match of thermal expansion coefficient between coatings and glass layer, though no spallation of the glass occurred at this temperature. (3) At 1200 C, the surface was fully covered with glass as shown in Fig. 4(e) and (f). There are many pores and cracks in the glass layer. The cracks resulted from the mismatch of thermal expansion coefficient between coatings and glass layer. The pores in the glass layer could be formed due to the volatilization of B2O3 glass. The crystallization phenom￾ena can be found in the glass layer as shown in Fig. 4(f). The glass crystallized owing to the effects of high temperature (1200 C) and long time (100 h), which was proven by XRD measurement. Fig. 5 shows the XRD patterns measured on the test specimens oxidized at different temperatures. At 700 C only SiC phase was identified. The cristobalite occurs after oxidation at 1000 C. The obvious cristobalite exists after oxidation at 1200 C which was corresponding with morphologies as shown in Fig. 4(f). Fig. 6 shows the weight change curves of the coated C/SiC com￾posite after oxidation at 700, 1000, and 1200 C up to 100 h in 14 vol.% H2O/8 vol.% O2/78 vol.% Ar atmosphere. Clearly showing the weight changes are small at all the test temperatures. The max￾imum of weight loss is only 0.49 wt.% after 100 h-oxidation at 700 C. The weight change curves of difference each temperature have great differences, which can be described detailed as follows: (1) At 700 C, the weight of the composites lost continuously. During the first 10 h, the weight loss is very obvious with 0.0072 wt.%/h rate. It is small with 0.0032 wt.%/h rate from 10 to 68 h. At the last 32 h, it is obvious again with 0.0074 wt.%/h rate. (2) At 1000 C, the weight of the composites increased continu￾ously at the first 41 h, then the weight decreased with increasing oxidation time. The weight gain has a maximum of 0.11 wt.% after oxidation for 41 h. Finally, the weight loss is 0.011 wt.% after oxidation for 100 h. (3) At 1200 C, the weight of the composites increased continu￾ously after oxidation for 100 h. The maximum of weight gain is 0.155 wt.%, and the final weight gain is 0.14 wt.%. The sus￾tained weight gain indicates that the coating is healed and no oxygen and water intrude into the interface and fibre. According to the previous results [8,9], the C/SiC composites coated with SiC/SiC/SiC, SiC/B/SiC or SiC/graphitic B–C/SiC had its weight loss at 700–1300 C for 10 or 15 h in air atmosphere. At 700 C oxidation, the weights of SiC/SiC/SiC, SiC/graphitic B–C/ SiC, and SiC/B/SiC lost above 0.71 wt.% for 15 h, 0.19 wt.% for 15 h, and 0.003 wt.% for 10 h, respectively. At 1100 C oxidation, the weights of SiC/SiC/SiC and SiC/graphitic B–C/SiC lost above 0.16 wt.% for 15 h, 0.51 wt.% for 15 h, respectively. At 1000 C oxi￾Fig. 2. Surface and cross section of CVD SiC/a-BC/SiC coatings. (a) CVD a-BC surface. (b) Cross-section morphology of hybrid coatings. (c) CVD SiC surface. Fig. 3. Interfacial morphology between CVD a-BC and CVD SiC coating. 822 Y. Liu et al. / Corrosion Science 51 (2009) 820–826