H. Li et al. /Ceramics International 35(2009)2277-2282 before it evaporates. However, appreciable volatilization of ●sio2 B2O3 starts at above 1200C leaving Zro, on the coating ZrSio system [21]. XRD pattern(Fig. 7c) and element analysis ◆ZrB2 (Fig. 6c) for #3 specimen indicate that ZrB 2 particles still exist (州 in the composite after oxidation. It means that Sic coating protects the interior area of the composite including ZrB powder from further oxidation at higher temperature (1200C). During the oxidation process, the combustion gas with a high flow rate and a larger amount of H2O will accelerate the silica and zirconium oxide formation on the coating and matrix. Different to #l and #2 specimens, the 15253545556575 oxidation channels for #3 specimen could be sealed by the oxides. The PyC interlayer together with fiber is less oxidized. Almost no change of the morphology for #3 specimen was thus D patterns of specimens after oxidation: (a)#I CVD ZrC coating,(b) #2 SiC-ZrC mixture coating and(c)#3 ZrB-Sic mixture coating. observed. The weight loss for this sample is of course the smallest among the three specimens in Fig. 5b. During the oxidation process, the top Zrc will be 4.Conclusions oxidized very fast, similar to #1. As the flame reaches the 2D C/Zrc-Sic composites were fabricated by the CVi mixture layer, the ZrC between SiC powders would be oxidized at the same rate as before. Without ZrC bond. the Sic or its process combined with the PIP method. The results indicate oxide SiOz will be easily blown off owing to mechanical that pure ZrC coatings can be easily oxidized and result in a complete degradation of the composites, whereas ZrB2-CVD erosion of the high gas-flow in combustion atmosphere Sic coating could fully fulfill the advantages of refractory After losing the coating systems, cracks and pores existing in compounds to improve oxidation resistance C/ArC-SiC composites may be the channels for the oxidized gas and stream to diffuse into the interior of the composites Oxidation reactions take place in the whole composite References simultaneously and quickly. The ZrC-SiC matrix cannot withstand oxidation environment with high heat flux and high [1] T.M. Besmann, B W.Sheldon, R.A. Lowden, D.P. Stinton, Vapor phase abrication and properties of continuous filament ceramic comp pressure gas flow any more because the protecting silica layer cIence253(6)(1991)11041109 be easily blown off. The subsequent [2] I. Toshihiro, K Shinji, M. Kenji, H. Toshihiko, K. Yasuhiko, N Toshio, oxidation of fiber and interphase results in complete degrada- Tough, Thermally conductive silicon carbide composite with high strength tion of the composites. Thus, the weight loss for these two up to 1600C in air, Science 282(1998)1295-1297 samples is much higher. The XRD patterns(Fig. 7)and EDS 3] W.H. Glime, J.D. Cawley, Oxidation of carbon fibers and films in ceramic matrix composites: a weak link process, Carbon 33(8)(1995)1053-1060. analysis(Fig. 6)of #I and #2 specimens are almost the same, [4]xG. Luan L.F. Cheng Y D Xu, L.T.Zhang Stressed oxidation behaviors suggesting the identical oxidation behavior of these two of SiC matrix composites in combustion environments, Mater. Lett. 61 composites. Meanwhile, sample #2 shows a close oxidation (2007)4114-4116 morphology and weight loss to #1 with a pure ZrC coating. It is 51 J.R. Strife, J-E Sbeehan, Ceramic coatings for carbon-carbon composites. suggested that, without the protection of the outer coating, the role of SiC powders in sample #2 and the pyrolysized SiC in [6] LFCheng,YDXu,LTZhang,XWYin, Oxidation behavior of three dimensional C/SiC composites in air and combustion gas environments sample #l is the same Carbon38(2000)2103-2108 The ZrB2-SiC mixture coating for #3 specimen is the key [7] T Narushima, T. Goto, Y. Iguchi, T. Hirai, High-temperature oxidation of point for little change before and after oxidation. ZrO2 and chemically vapor-deposited silicon carbide in wet oxygen at 1823 to zirconium silicate(ZrSiO4) are found in #3 specimen after 1923K,J.Am. Ceran.Soc.73(12)(1990)1580-1584. [8 N.S. Jacobson, Corrosion of silicon-based ceramics in combustion envir- oxidation (Figs. 6c and 7c), which reveals that in the onments, J. Am. Ceram. Soc. 76(1)(1993)3-28. ombustion environment, besides the aforementioned reactions [9] C.R. Wang, J M. Yang, W. Hoffman, Thermal stability of refractory for SiC, an oxidation reaction concerning ZrB2 powder takes arbide/boride composites, Mater. Chem. Phys. 74(2002)272-28 [10] J.D. Bull, D.J. Rasky, CC. Karika, Stability characterization of diboride composites under high velocity atmospheric flight conditions, in: Pro- ZrB2+(5/2)02- ZrO2+B2O ceedings of the 24th International SAMPE Technical Conference. Tor- T1092-T1106 Previous studies show that defects are unavoidable in CVd [11] W.G. Fahrenholtz, G.E. Hilmas, A L. Chamberlain, J w. zimme SiC coating [19], which will result in oxygen diffusion inward Processing and characterization of ZrB2-based ultra-high tempe and oxidation of the composites. Thus, oxidation for ZrB monolithic and fibrous monolithic mics, J. Mater. Sci. 39(2004) 5951-5957 powder will be inevitable. For ZrB2 oxidized at elevated [12)A. Rezaie, W.G. Fahrenholtz, G.E. Hilmas, Evolution of structure during temperatures, Zro2 and liquid B2O3 are formed [20]. The B2O3 the oxidation of zirconium diboride-silicon carbide in air up to 1500C, J provides significant oxidation protection at lower temperatures Eur. Ceram.Soc.2702007)2495-2501in Fig. 5b. During the oxidation process, the top ZrC will be oxidized very fast, similar to #1. As the flame reaches the mixture layer, the ZrC between SiC powders would be oxidized at the same rate as before. Without ZrC bond, the SiC or its oxide SiO2 will be easily blown off owing to mechanical erosion of the high gas-flow in combustion atmosphere. After losing the coating systems, cracks and pores existing in C/ZrC–SiC composites may be the channels for the oxidized gas and stream to diffuse into the interior of the composites. Oxidation reactions take place in the whole composite simultaneously and quickly. The ZrC–SiC matrix cannot withstand oxidation environment with high heat flux and high pressure gas flow any more because the protecting silica layer becomes active and can be easily blown off. The subsequent oxidation of fiber and interphase results in complete degrada￾tion of the composites. Thus, the weight loss for these two samples is much higher. The XRD patterns (Fig. 7) and EDS analysis (Fig. 6) of #1 and #2 specimens are almost the same, suggesting the identical oxidation behavior of these two composites. Meanwhile, sample #2 shows a close oxidation morphology and weight loss to #1 with a pure ZrC coating. It is suggested that, without the protection of the outer coating, the role of SiC powders in sample #2 and the pyrolysized SiC in sample #1 is the same. The ZrB2–SiC mixture coating for #3 specimen is the key point for little change before and after oxidation. ZrO2 and zirconium silicate (ZrSiO4) are found in #3 specimen after oxidation (Figs. 6c and 7c), which reveals that in the combustion environment, besides the aforementioned reactions for SiC, an oxidation reaction concerning ZrB2 powder takes place as follows: ZrB2 þ ð5=2ÞO2 ! ZrO2 þ B2O3 (3) Previous studies show that defects are unavoidable in CVD SiC coating [19], which will result in oxygen diffusion inward and oxidation of the composites. Thus, oxidation for ZrB2 powder will be inevitable. For ZrB2 oxidized at elevated temperatures, ZrO2 and liquid B2O3 are formed [20]. The B2O3 provides significant oxidation protection at lower temperatures, before it evaporates. However, appreciable volatilization of B2O3 starts at above 1200 8C leaving ZrO2 on the coating system [21]. XRD pattern (Fig. 7c) and element analysis (Fig. 6c) for #3 specimen indicate that ZrB2 particles still exist in the composite after oxidation. It means that SiC coating protects the interior area of the composite including ZrB2 powder from further oxidation at higher temperature (>1200 8C). During the oxidation process, the combustion gas with a high flow rate and a larger amount of H2O will accelerate the silica and zirconium oxide formation on the coating and matrix. Different to #1 and #2 specimens, the oxidation channels for #3 specimen could be sealed by the oxides. The PyC interlayer together with fiber is less oxidized. Almost no change of the morphology for #3 specimen was thus observed. The weight loss for this sample is of course the smallest among the three specimens. 4. Conclusions 2D C/ZrC–SiC composites were fabricated by the CVI process combined with the PIP method. The results indicate that pure ZrC coatings can be easily oxidized and result in a complete degradation of the composites, whereas ZrB2–CVD SiC coating could fully fulfill the advantages of refractory compounds to improve oxidation resistance. References [1] T.M. Besmann, B.W. Sheldon, R.A. Lowden, D.P. Stinton, Vapor phase fabrication and properties of continuous filament ceramic composites, Science 253 (6) (1991) 1104–1109. [2] I. Toshihiro, K. Shinji, M. Kenji, H. Toshihiko, K. Yasuhiko, N. Toshio, A. Tough, Thermally conductive silicon carbide composite with high strength up to 16008C in air, Science 282 (1998) 1295–1297. [3] W.H. Glime, J.D. Cawley, Oxidation of carbon fibers and films in ceramic matrix composites: a weak link process, Carbon 33 (8) (1995) 1053–1060. [4] X.G. Luan, L.F. Cheng, Y.D. Xu, L.T. Zhang, Stressed oxidation behaviors of SiC matrix composites in combustion environments, Mater. Lett. 61 (2007) 4114–4116. [5] J.R. Strife, J.E. Sbeehan, Ceramic coatings for carbon–carbon composites, Am. Ceram. Soc. Bull. 67 (2) (1988) 369–374. [6] L.F. Cheng, Y.D. Xu, L.T. Zhang, X.W. Yin, Oxidation behavior of three dimensional C/SiC composites in air and combustion gas environments, Carbon 38 (2000) 2103–2108. [7] T. Narushima, T. Goto, Y. Iguchi, T. Hirai, High-temperature oxidation of chemically vapor-deposited silicon carbide in wet oxygen at 1823 to 1923 K, J. Am. Ceram. Soc. 73 (12) (1990) 1580–1584. [8] N.S. Jacobson, Corrosion of silicon-based ceramics in combustion envir￾onments, J. Am. Ceram. Soc. 76 (1) (1993) 3–28. [9] C.R. Wang, J.M. Yang, W. Hoffman, Thermal stability of refractory carbide/boride composites, Mater. Chem. Phys. 74 (2002) 272–281. [10] J.D. Bull, D.J. Rasky, C.C. Karika, Stability characterization of diboride composites under high velocity atmospheric flight conditions, in: Pro￾ceedings of the 24th International SAMPE Technical Conference, Tor￾onto, Canada, (1992), pp. T1092–T1106. [11] W.G. Fahrenholtz, G.E. Hilmas, A.L. Chamberlain, J.W. Zimmermann, Processing and characterization of ZrB2-based ultra-high temperature monolithic and fibrous monolithic ceramics, J. Mater. Sci. 39 (2004) 5951–5957. [12] A. Rezaie, W.G. Fahrenholtz, G.E. Hilmas, Evolution of structure during the oxidation of zirconium diboride–silicon carbide in air up to 15008C, J. Eur. Ceram. Soc. 27 (2007) 2495–2501. Fig. 7. XRD patterns of specimens after oxidation: (a) #1 CVD ZrC coating, (b) #2 SiC–ZrC mixture coating and (c) #3 ZrB2–SiC mixture coating. H. Li et al. / Ceramics International 35 (2009) 2277–2282 2281