Y Liu et al/ Materials Science and Engineering A 498(2008)430-436 1000°C Multilayer matrices Oxidation temperature (C) Fig 3. Micro-crack deflexion model in the SiC-BCx multilayer matrices of the mod- ted for 10h ngth of the modified composites and C/SiC before and 2B2Cs)+(172)0(g)03B2O3()+4CO2(g) 10 B2O(1) (10) siO2()+B2O2()∞B2O3,x5i02() (11) 1000 B2O3· aSiO(1) B203(g)+xSiO2(s) “002 2C(s)+O2(g)302co(g) The reactions( 8)and (9) led to weight gain. The other reactions led 006 to weight loss At 700 C, B2O3 liquid would be produced at 700C due to the reaction(8), which led to weight gain. At the same time reactions(10)and(13) would occur which led to weight loss. A previous report 2] indicated that the B2O3 would flow at 630.C Oxidation time /h The liquid flowed through the cracks in the composites, and led to Fig 4. Weight change of modified composites after oxidized for 10h the partially sealing of the crack since the formed speed of B203 from the reaction(9)and flowing speed of B2O3 liquid were not enough faster. The weight change was collective results of reac could be prolonged, and the energy of the micro-crack tions(9)and(13). Therefore, the larger weight loss than that after ogress would be consumed. Therefore the strain to fracture of xidation at 1000 and 1300C and the weight loss occurred at the whole oxidation process based on the same cause. At 1000C, the reactions(8)-(11)and(13)would be faster. Large 3.3. Oxidation resistance of the composites in static( amount of B203 would be formed at this temperature. At the same ime, Sioz would be formed. B203 would accelerate the oxida tion of CVD Sic by forming a melt, according to B2O3-SiOz system Fig 4 shows the weight change of the modified composite after phase diagram [ 21]. The thermal expansion of CVD Sic would also oxidation at 700, 1000 and 1300C. There are similar trend of prompt the crack sealing On the other hand, B2O3 volatilization weight change for the modified composite at different oxidation would also occur at the same time. The weig an co formao s was also temperature, which can be described as: the weight change n-collective results of B2O3 formation, Sioz formation, CO near with oxidation time prolonging, and the weight loss increases and B2O3 volatilization. Based on the above causes, the weight for the initial 2h, then decrease for the next 8 h. The weight changes was less than that of composites after oxidation at 1000C, which after oxidation for 2, 5, and 10 h, respectively, were included in showed that the sealing of the micro-cracks are better than that of Table 2. The maximum weight losses all occurred after oxidation for composites after oxidation at 700C. 2h. At the same time, the weight gain ratio increased with increas- At 1300C, the weight change curve was similar as that at ing temperature. According to Ref [20], the BCx can be written as 1000 C. But the weight losses were less than that at 1000 C dur B3C2. For the modified composites, the oxidation reactions in air ing the initial 2 h. And the weight gains were larger than that at 1000C during the last 4h. This phenomenon can be contributed to the faster formation of B2O3 and B2O3 SiO2 liquid glass. Due to 2SiC(s)+302(g)→2SiO2(1)+2C0(g) 3) the small weight changes at all oxidation temperatures, we can con- clude that the C/(Sic-BCx)n composite have low weight loss after Table 2 oxidized from 700 to 1300.C The maximum, minimum and final weight changes of modified composites after The residual flexural strength of C/SiC and the modified compos oxidized at different temperature in air during 10h ites were compared as shown in Fig. 5. The residual strength of the temperature°c) 1000 modified composite stayed nearly constant after oxidation for 10h change after 2h 0.015 0.053 at 700C, then a little strength increase(102.7% retained strength) hange after 5 h oxidation(%) -0.016 after oxidation at 1000 and 1300.C The residual strength of the change after 10 h oxidation(%)-0021 0.015 C/SiC composites showed a strength loss(73% retained strength)Y. Liu et al. / Materials Science and Engineering A 498 (2008) 430–436 433 Fig. 3. Micro-crack deflexion model in the SiC–BCx multilayer matrices of the mod￾ified composites. Fig. 4. Weight change of modified composites after oxidized for 10 h. progress could be prolonged, and the energy of the micro-crack progress would be consumed. Therefore, the strain to fracture of the composites was improved. 3.3. Oxidation resistance of the composites in static air environment Fig. 4 shows the weight change of the modified composite after oxidation at 700, 1000 and 1300 ◦C. There are similar trend of weight change for the modified composite at different oxidation temperature, which can be described as: the weight change is non￾linear with oxidation time prolonging, and the weight loss increases for the initial 2 h, then decrease for the next 8 h. The weight changes after oxidation for 2, 5, and 10 h, respectively, were included in Table 2. The maximum weight losses all occurred after oxidation for 2 h. At the same time, the weight gain ratio increased with increas￾ing temperature. According to Ref. [20], the BCx can be written as B3C2. For the modified composites, the oxidation reactions in air from 700 to 1300 ◦C are as follows: 2SiC(s) + 3O2(g)800 ◦C −→ 2SiO2(l) + 2CO(g) (8) Table 2 The maximum, minimum and final weight changes of modified composites after oxidized at different temperature in air during 10 h Oxidation temperature (◦C) 700 1000 1300 The weight change after 2 h oxidation (%) −0.074 −0.015 −0.053 The weight change after 5 h oxidation (%) −0.011 0 −0.016 The weight change after 10 h oxidation (%) −0.021 0.015 0.042 Fig. 5. Residual flexural strength of the modified composites and C/SiC before and after oxidized for 10 h. 2B3C2(s) + (17/2)O2(g)600 ◦C −→ 3B2O3(l) + 4CO2(g) (9) B2O3(l)600−1000 ◦C −→ B2O3(g) (10) SiO2(l) + B2O3(l)1000 ◦C −→ B2O3 · xSiO2(l) (11) B2O3 · xSiO2(l)≥1000 ◦C −→ B2O3(g) + xSiO2(s) (12) 2C(s) + O2(g)400 ◦C −→ 2CO(g) (13) The reactions (8) and (9) led to weight gain. The other reactions led to weight loss. At 700 ◦C, B2O3 liquid would be produced at 700 ◦C due to the reaction (8), which led to weight gain. At the same time, reactions (10) and (13) would occur which led to weight loss. A previous report [2] indicated that the B2O3 would flow at 630 ◦C. The liquid flowed through the cracks in the composites, and led to the partially sealing of the crack since the formed speed of B2O3 from the reaction (9) and flowing speed of B2O3 liquid were not enough faster. The weight change was collective results of reac￾tions (9) and (13). Therefore, the larger weight loss than that after oxidation at 1000 and 1300 ◦C and the weight loss occurred at the whole oxidation process based on the same cause. At 1000 ◦C, the reactions (8)–(11) and (13) would be faster. Large amount of B2O3 would be formed at this temperature. At the same time, SiO2 would be formed. B2O3 would accelerate the oxida￾tion of CVD SiC by forming a melt, according to B2O3–SiO2 system phase diagram [21]. The thermal expansion of CVD SiC would also prompt the crack sealing. On the other hand, B2O3 volatilization would also occur at the same time. The weight change was also collective results of B2O3 formation, SiO2 formation, CO formation and B2O3 volatilization. Based on the above causes, the weight loss was less than that of composites after oxidation at 1000 ◦C, which showed that the sealing of the micro-cracks are better than that of composites after oxidation at 700 ◦C. At 1300 ◦C, the weight change curve was similar as that at 1000 ◦C. But the weight losses were less than that at 1000 ◦C dur￾ing the initial 2 h. And the weight gains were larger than that at 1000 ◦C during the last 4 h. This phenomenon can be contributed to the faster formation of B2O3 and B2O3·SiO2 liquid glass. Due to the small weight changes at all oxidation temperatures, we can con￾clude that the C/(SiC–BCx)n composite have low weight loss after oxidized from 700 to 1300 ◦C. The residual flexural strength of C/SiC and the modified compos￾ites were compared as shown in Fig. 5. The residual strength of the modified composite stayed nearly constant after oxidation for 10 h at 700 ◦C, then a little strength increase (102.7% retained strength) after oxidation at 1000 and 1300 ◦C. The residual strength of the C/SiC composites showed a strength loss (73% retained strength)