H. Li et al. /Ceramics International 35 (2009)2277-2282 额 C/Zrc-Sic atics of coating structures for (a)#I CVD Zrc coating, (b )#2 SiC-ZrC mixture coating and (c)#3 ZrB-Sic mixture coating observed on the top of #l(Fig 4a) and #2(Fig 4b)specimens. morphologies of these oxidized specimens are shown in Fig. 6 As we know, before the deposition of ZrC, cracks will be Compared with the un-oxidized samples(Fig. 4d and e), severe formed on composites surface due to the intrinsic shrinkage damage of #I and #2 specimens is observed(Fig 6a and b) during the polymer-ceramic conversion process(Fig. 4a). while the surface of #3 specimen shows almost no change During the CVD process, ZrC will firstly fill such cracks.(Figs. 4f and 6c). The weight loss of #l and #2 specimens are Another ZrC layer will be subsequently deposited on the top of 7.08% and 6.50%, i.e. much higher than that of 1.06% for #3 the composite(Fig 4a) specimen. XRD patterns(Fig. 7)and element analysis of #l and As shown in Fig. 4c, the ZrB 2 particles are embedded in the #2 specimens(Fig. 6a and b) after oxidation reveal that the CVD SiC to form a mixture layer of Zr B2 and SiC. Another Sic oxidation products include SiOz and ZrO2(white phase)or their layer is on its top, which is formed during the deposition compounds. The reactions occurring during the oxidation process of SiC filling the gaps between ZrB2 particles. process at about 1800C are as follows [17] Similarly, a mixture layer of SiC and Zrc is formed on the surface of the #2 specimen with a CVD ZrC layer on its to SC+(3/2)O2→SiO2+CO (1) Fig. 4b). Meanwhile, a small amount of micro-cracks are found on the surface of#I and #2 specimens(Fig. 4d and e). ZrC +(3/2)02-ZrO2+CO This may be due to the mismatch of thermal expansion Sample #I has been severely damaged after exposure in CH4 coefficient between CVD ZrC and SiC. The schematic pictures combustion gas environment although a ZrC coating had been of the three different coatings are shown in Fig. 5 deposited on its surface. When oxidized, ZrC forms porous ZrO2 and CO. These materials show rapid oxidation with linear 3.3. Oxidation behavior of 2D C/ZrC-SiC composites with (non-protective)reaction kinetics, which allows entire speci- different coating structures mens to be oxidized in relatively short periods of time Moreover, the water vapor formed by the reactions in the The three specimens were exposed simultaneously in a CHa combustion atmosphere will also accelerate the oxidation of the combustion wind tunnel for 30 min at about 1800C. The composites [18]. The coating schematic picture of #2 is shown .00mm15 K 14.4mm x40 SE Fig. 6. Surface morphologies of the specimens after oxidation:(a)#I CVD ZrC coating, (b)#2 SiC-ZrC mixture coating and (c)#3 ZrB2-SiC mixture coatingobserved on the top of #1 (Fig. 4a) and #2 (Fig. 4b) specimens. As we know, before the deposition of ZrC, cracks will be formed on composites surface due to the intrinsic shrinkage during the polymer–ceramic conversion process (Fig. 4a). During the CVD process, ZrC will firstly fill such cracks. Another ZrC layer will be subsequently deposited on the top of the composite (Fig. 4a). As shown in Fig. 4c, the ZrB2 particles are embedded in the CVD SiC to form a mixture layer of ZrB2 and SiC. Another SiC layer is on its top, which is formed during the deposition process of SiC filling the gaps between ZrB2 particles. Similarly, a mixture layer of SiC and ZrC is formed on the surface of the #2 specimen with a CVD ZrC layer on its top (Fig. 4b). Meanwhile, a small amount of micro-cracks are found on the surface of #1 and #2 specimens (Fig. 4d and e). This may be due to the mismatch of thermal expansion coefficient between CVD ZrC and SiC. The schematic pictures of the three different coatings are shown in Fig. 5. 3.3. Oxidation behavior of 2D C/ZrC–SiC composites with different coating structures The three specimens were exposed simultaneously in a CH4 combustion wind tunnel for 30 min at about 1800 8C. The morphologies of these oxidized specimens are shown in Fig. 6. Compared with the un-oxidized samples (Fig. 4d and e), severe damage of #1 and #2 specimens is observed (Fig. 6a and b), while the surface of #3 specimen shows almost no change (Figs. 4f and 6c). The weight loss of #1 and #2 specimens are 7.08% and 6.50%, i.e. much higher than that of 1.06% for #3 specimen. XRD patterns (Fig. 7) and element analysis of #1 and #2 specimens (Fig. 6a and b) after oxidation reveal that the oxidation products include SiO2 and ZrO2 (white phase) or their compounds. The reactions occurring during the oxidation process at about 1800 8C are as follows [17]: SiC þ ð3=2ÞO2 ! SiO2 þ CO (1) ZrC þ ð3=2ÞO2 ! ZrO2 þ CO (2) Sample #1 has been severely damaged after exposure in CH4 combustion gas environment although a ZrC coating had been deposited on its surface. When oxidized, ZrC forms porous ZrO2 and CO. These materials show rapid oxidation with linear (non-protective) reaction kinetics, which allows entire speci￾mens to be oxidized in relatively short periods of time. Moreover, the water vapor formed by the reactions in the combustion atmosphere will also accelerate the oxidation of the composites [18]. The coating schematic picture of #2 is shown Fig. 5. The schematics of coating structures for (a) #1 CVD ZrC coating, (b) #2 SiC–ZrC mixture coating and (c) #3 ZrB2–SiC mixture coating. Fig. 6. Surface morphologies of the specimens after oxidation: (a) #1 CVD ZrC coating, (b) #2 SiC–ZrC mixture coating and (c) #3 ZrB2–SiC mixture coating. 2280 H. Li et al. / Ceramics International 35 (2009) 2277–2282