Y Liu et al/ Corrosion Science 51(2009)820-826 0. 14 wt% at 1200C. The oxidation behaviour of the Sic/a-BCSic Table 2 ating differs from that of the Sicsic/SiC coating. For Sic/Sic) Compare of coatings system on oxidation protection for 3D C/Sic composite O3 glass formation. Only the oxidations trength retained ratio of Tested condition of Sic, carbon fibre and Pyc were referred The oxidation 3D C/SiC composite/6 of the SiCa-BC/SiC coating should be similar as that of SiC/a-BC/SiC 83.0 t%H2O8%O2/78 phitic B-C/SiC, and Sic/B/ Sic coating systems in the same Ar therm 15 times atmosphere. The weights gain of SiC/a-BC/SiC coating can SiC/graphitic 1300°C 02/78% Ar, no uted to the special H20 and O2 atmosphere. The detail protection mechanism would be discussed in next part. Sic/B/SiC [8] 76.9 1300C, 2 h, air, thermal shock 3 The residual flexural strengths of 3D C/SiC composites with three-layers CVD Sic coatings or Sic/a-BC/SiC coatings are shown SiC/SiCsIc 73.0 1300°C15h,22%02/78%Ar,no 7. Apparently the sic thermal shock than that of SiC/SiC/SiC coatings. The strength retained ratios of 3D C/Sic composites coated with SiC/a-BC/SiC coatings oxidized at 00, 1000, and 1200C were 83.2%, 96.4%, and 82.6%, respectively. temperature would decrease when H20 exists [3, 22]. During the On the other hand, those for Sic/Sic/SiC coating were 65.8%, 94.9%, oxidation process, the following reactions would occur. rial, and lead to the strength loss besides oxidation, which proven 2BC5s Oo2(S)+12.302(8)-B2030)+11CO2( 8) from the results of the composites coated with SiC/SiC/SiC coating )+O2(g)→CO2(g) in reference [8] and [21]. Furthermore, strength loss increases with B203 (1)+H2O(8)-2HBO2(g) thermal shock times increasing at previous 20-30 times thermal B203(1)+3H, O(g)-2H3 BO3(g) after 20 times thermal shock. Then, the strength loss ratio would Sic(S)+302(8)-+SiO2(s)+2C0(g) not increase SIC(S)+3H2O(8)-SiO2(s)+CO(g)+3H,(g) (6) The C/Sic composites with several coating systems indicated SiO2(s)+2H20(g) 600-1000°C the low strength retained ratio as listed in Table 2. Though the strength retained ratio of the composite coated with Sic/gra- B203 0)+B203(g) phitic-BCxSiC was also 83.0%, thermal shock has not been applied to the composite. Therefore, the strength loss should mainly be SiO2(8)+B203(1)-0B2O3XSiO2(1) attributed to the thermal shock since the weight increased during B203 XSio2( 21000C B203(g)+Sio2(s) the oxidation process. According to the above results. lude that the Sic/a-BC/SiC coating has a good oxidation protection Reactions (1)-(4) would occur below 700C, leading to the for 3D C/SiC composite up to 1200C for 100 weight loss of the composites. Reactions(5),(6) would occur above700C, leading to the weight gain of the composites Reae 3.3. Protection mechanism of sic/a-BCSiC coatings on 3D C/Sic tion(7)also leads to the weight loss. According to the morphologies weight changes and strength changes, the oxidation process were shown in Fig 8. The main pro- As presented in XPS results(see Table 1). the boron concentra- cess can be described as follows: tion of a-BC was 15 at %. The carbon and oxygen concentration were 82 at. and 3 at.%, respectively. Therefore the a-Bc can be (1)Due to the thermal expansion mismatch between SiC coat- composed of BC55O0.2. A melting point of B2O3 is 450C, and the ing and a-BC coating, were many cracks in as-prepared volatilization point is 900C[22]. Furthermore, the volatilization SiCa-BC/SiC coating as presented in Fig. 8(a). 900 SiC/SiC/Sic 国 SiC/a-BC/SiC 50 100 As-received 700 1200 Temperature/C g. 7. Residual strength for 3D C/SiC composites coated with conventional CVD layer and modified one.0.14 wt.% at 1200 C. The oxidation behaviour of the SiC/a-BC/SiC coating differs from that of the SiC/SiC/SiC coating. For SiC/SiC/ SiC coating, there was no B2O3 glass formation. Only the oxidations of SiC, carbon fibre and PyC were referred. The oxidation behaviour of the SiC/a-BC/SiC coating should be similar as that of SiC/gra￾phitic B–C/SiC, and SiC/B/SiC coating systems in the same oxidation atmosphere. The weights gain of SiC/a-BC/SiC coating can be attrib￾uted to the special H2O and O2 atmosphere. The detail protection mechanism would be discussed in next part. The residual flexural strengths of 3D C/SiC composites with three-layers CVD SiC coatings or SiC/a-BC/SiC coatings are shown in Fig. 7. Apparently the SiC/a-BC/SiC coatings have higher strength than that of SiC/SiC/SiC coatings. The strength retained ratios of 3D C/SiC composites coated with SiC/a-BC/SiC coatings oxidized at 700, 1000, and 1200 C were 83.2%, 96.4%, and 82.6%, respectively. On the other hand, those for SiC/SiC/SiC coating were 65.8%, 94.9%, and 88.6%, respectively. Thermal shock would also damage mate￾rial, and lead to the strength loss besides oxidation, which proven from the results of the composites coated with SiC/SiC/SiC coating in reference [8] and [21]. Furthermore, strength loss increases with thermal shock times increasing at previous 20–30 times thermal shock. According to reference [21], the strength loss ratio is 10% after 20 times thermal shock. Then, the strength loss ratio would not increase. The C/SiC composites with several coating systems indicated the low strength retained ratio as listed in Table 2. Though the strength retained ratio of the composite coated with SiC/gra￾phitic-BCx/SiC was also 83.0%, thermal shock has not been applied to the composite. Therefore, the strength loss should mainly be attributed to the thermal shock since the weight increased during the oxidation process. According to the above results, we can con￾clude that the SiC/a-BC/SiC coating has a good oxidation protection for 3D C/SiC composite up to 1200 C for 100 h. 3.3. Protection mechanism of SiC/a-BC/SiC coatings on 3D C/SiC composites As presented in XPS results (see Table 1), the boron concentra￾tion of a-BC was 15 at.%. The carbon and oxygen concentration were 82 at.% and 3 at.%, respectively. Therefore the a-BC can be composed of BC5.5O0.2. A melting point of B2O3 is 450 C, and the volatilization point is 900 C [22]. Furthermore, the volatilization temperature would decrease when H2O exists [3,22]. During the oxidation process, the following reactions would occur. 2BC5:5O0:2ðsÞ þ 12:3O2ðgÞ ! B2O3ðlÞ þ 11CO2ðgÞ ð1Þ CðsÞ þ O2ðgÞ ! CO2ðgÞ ð2Þ B2O3ðlÞ þ H2OðgÞ ! 2HBO2ðgÞ ð3Þ B2O3ðlÞ þ 3H2OðgÞ ! 2H3BO3ðgÞ ð4Þ SiCðsÞ þ 3O2ðgÞ ! SiO2ðsÞ þ 2COðgÞ ð5Þ SiCðsÞ þ 3H2OðgÞ ! SiO2ðsÞ þ COðgÞ þ 3H2ðgÞ ð6Þ SiO2ðsÞ þ 2H2OðgÞ ! 6001000 C SiðOHÞ4ðgÞ ð7Þ B2O3ðlÞ ! 6001000 C B2O3ðgÞ ð8Þ SiO2ðgÞ þ B2O3ðlÞ ! 1000 C B2O3  xSiO2ðlÞ ð9Þ B2O3  xSiO2ðlÞ ! 1000 C B2O3ðgÞ þ SiO2ðsÞ ð10Þ Reactions (1)–(4) would occur below 700 C, leading to the weight loss of the composites. Reactions (5), (6) would occur above700 C, leading to the weight gain of the composites. Reac￾tion (7) also leads to the weight loss. According to the morphologies weight changes and strength changes, the oxidation process were shown in Fig. 8. The main pro￾cess can be described as follows: (1) Due to the thermal expansion mismatch between SiC coat￾ing and a-BC coating, there were many cracks in as-prepared SiC/a-BC/SiC coating as presented in Fig. 8(a). 0 100 200 300 400 500 600 700 800 900 007 1000 0021 Residual Strength /MPa Temperature / o C As-received SiC/SiC/SiC SiC/a-BC/SiC Fig. 7. Residual strength for 3D C/SiC composites coated with conventional CVD layer and modified one. Table 2 Compare of coatings system on oxidation protection for 3D C/SiC composites. Coating system Strength retained ratio of 3D C/SiC composite/% Tested condition SiC/a-BC/SiC 83.0 1200 C, 100 h, 14% H2O/8% O2/78% Ar, thermal shock 15 times SiC/graphitic B–C/SiC [9] 83.0 1300 C, 15 h, 22% O2/78% Ar, no thermal shock SiC/B/SiC [8] 76.9 1300 C, 2 h, air, thermal shock 3 times SiC/SiC/SiC [9] 73.0 1300 C, 15 h, 22% O2/78% Ar, no thermal shock 824 Y. Liu et al. / Corrosion Science 51 (2009) 820–826