S. Wu et aL. Composites: Part A 37(2006)1396-140 Na,OSIO B-Sic a a-Cristobalite AΔ△△ 1500°C 人A1400°C 1300°C 1200°C 1100°C 人A_10 Fig. 6. XRD patterns of the specimens' surface after corrosion at different temperature for 10 h. accelerated with an increasing temperature and the corro- composite was well protected from oxidation of reactions sion resistance of CVD Sic was poor (1H6) and nearly kept the same strength value as that in room temperature Above 1200C, though the accelerated 4. Discussion oxidation of Sic was strong. the volatilization and subli- mation of Si(oH)4 and Na20. xSiOz on the surfa In the present experimental condition, main reactions creased continuously, which led to weight loss. Along include the following with the experimental processing, the corrosion layer was Na2O(g)+SO: (1) more and more thick, which slow down the gas diffusion into the unreacted region while the weight loss on the sur SiC(s+3H2O(g= SiOz(s)+ 3H2(g)+ cog (2) face layer kept the constant. Moreover, bubbles/corrosion SiC(s)+ 302(g)= 2SiO2(s)+ 2CO(g) (3) pits, as shown in Fig 3(eHg) and 5 were formed in the cor SiOz(s)+ 2H2O()=Si(oH)4g (4) rosion layers. The diameter of them increased with increasing temperature. As a result, the weight gain Na2O(g)+x SiOz(s)= Na2O (5) reached the maximum value at 1300C, while it kept the SiC(s)+3SO3(g)=3SO2(g)+ Sic O C(s)+HO(g)=CO(g)+ hz(g) experimental time increased and decreased with the expe imental time increasing after 5h. Moreover, above C(s)+Orig)=Co (8) 1300C, the H2O, SO3 and O2 gas might diffuse into the composite and reacted with the pyrolytic carbon interlayer O3(g)+C(s)=SO2(g+ COc (9) according to reaction(7)9), though the diffusion was Below 1200C, according to reaction(2)4), accelerated slow. Furthermore, the strength of Hi-Nicalon fibers exhib oxidation of CVD Sic by water resulted in a thin layer ited degradation as temperature increased above 1200C protective SiO2 formed on the coating surface, which led [19-21]. Consequently, the residual flexural strength of to weight gain of the specimen [11-13]. Moreover, a low the composite strongly decreased as temperature increased. melting point Na20. xSio2 [14] was formed by reaction (5), as the surface EDS of specimens after corrosion for 5. Conclusions 10 h at 1100 and 1200C showed in Fig. 4. As a result, the defects in the coating and oxidation film would be The corrosion behavior of the 3D SiC/Sic composite sealed as shown in Fig 3(b)d). This behavior occurred was investigated in environments containing Na2 SO4 until all the available sodium was used up. At or near the vapor, oxygen and water vapor at temperatures from melt/gas interface, SiO2 could reform [15]. Under the coop- 1000 to 1500C. The corrosion behavior of the composite eration of them, the composite exhibited a slow slight was greatly related to temperature. Below 1200C, the ra- weight gain and the corrosion behaviors were similar to pid passive oxidation of CVD Sic coating by oxygen and oxidation in air, which differed from those showed by water vapor led to formation of a protective silica film Smialek et al. and Federer [15-18]. In this case, the and a slight weight gain, and then the residual flexuralaccelerated with an increasing temperature and the corro￾sion resistance of CVD SiC was poor. 4. Discussion In the present experimental condition, main reactions include the following: Na2SO4ðlÞ ¼ Na2OðgÞ þ SO3ðgÞ ð1Þ SiCðsÞ þ 3H2OðgÞ ¼ SiO2ðsÞ þ 3H2ðgÞ þ COðgÞ ð2Þ 2SiCðsÞ þ 3O2ðgÞ ¼ 2SiO2ðsÞ þ 2COðgÞ ð3Þ SiO2ðsÞ þ 2H2OðgÞ ¼ SiðOHÞ4ðgÞ ð4Þ Na2OðgÞ þ xSiO2ðsÞ ¼ Na2O xSiO2ðlÞ ð5Þ SiCðsÞ þ 3SO3ðgÞ ¼ 3SO2ðgÞ þ SiO2ðsÞ þ COðgÞ ð6Þ CðsÞ þ H2OðgÞ ¼ COðgÞ þ H2ðgÞ ð7Þ CðsÞ þ O2ðgÞ ¼ COðgÞ ð8Þ SO3ðgÞ þ CðsÞ ¼ SO2ðgÞ þ COðgÞ ð9Þ Below 1200 C, according to reaction (2)–(4), accelerated oxidation of CVD SiC by water resulted in a thin layer protective SiO2 formed on the coating surface, which led to weight gain of the specimen [11–13]. Moreover, a low melting point Na2O Æ xSiO2 [14] was formed by reaction (5), as the surface EDS of specimens after corrosion for 10 h at 1100 and 1200 C showed in Fig. 4. As a result, the defects in the coating and oxidation film would be sealed as shown in Fig. 3(b)–(d). This behavior occurred until all the available sodium was used up. At or near the melt/gas interface, SiO2 could reform [15]. Under the coop￾eration of them, the composite exhibited a slow slight weight gain and the corrosion behaviors were similar to oxidation in air, which differed from those showed by Smialek et al. and Federer [15–18]. In this case, the composite was well protected from oxidation of reactions (1)–(6) and nearly kept the same strength value as that in room temperature. Above 1200 C, though the accelerated oxidation of SiC was strong, the volatilization and subli￾mation of Si(OH)4 and Na2O Æ xSiO2 on the surface in￾creased continuously, which led to weight loss. Along with the experimental processing, the corrosion layer was more and more thick, which slow down the gas diffusion into the unreacted region while the weight loss on the sur￾face layer kept the constant. Moreover, bubbles/corrosion pits, as shown in Fig. 3(e)–(g) and 5 were formed in the cor￾rosion layers. The maximum diameter of them increased with increasing temperature. As a result, the weight gain reached the maximum value at 1300 C, while it kept the same value for the primordial 2 h, then increased as the experimental time increased and decreased with the exper￾imental time increasing after 5 h. Moreover, above 1300 C, the H2O, SO3 and O2 gas might diffuse into the composite and reacted with the pyrolytic carbon interlayer according to reaction (7)–(9), though the diffusion was slow. Furthermore, the strength of Hi-Nicalon fibers exhib￾ited degradation as temperature increased above 1200 C [19–21]. Consequently, the residual flexural strength of the composite strongly decreased as temperature increased. 5. Conclusions The corrosion behavior of the 3D SiC/SiC composite was investigated in environments containing Na2SO4 vapor, oxygen and water vapor at temperatures from 1000 to 1500 C. The corrosion behavior of the composite was greatly related to temperature. Below 1200 C, the ra￾pid passive oxidation of CVD SiC coating by oxygen and water vapor led to formation of a protective silica film and a slight weight gain, and then the residual flexural Fig. 6. XRD patterns of the specimens surface after corrosion at different temperature for 10 h. 1400 S. Wu et al. / Composites: Part A 37 (2006) 1396–1401