Availableonlineatwww.sciencedirect.com Science Direct CERAMICS INTERNATIONAL ELSEVIER Ceramics International 35(2009)2277-2282 www.elsevier.com/locate/ceramint Oxidation analysis of 2D C/zrc-sic composites with different coating structures in CHa combustion gas environment Houbu Li", Litong Zhang, Laifei Cheng, Yiguang Wang National Key Laboratory of Themostructure Composite Materials, Nort/westem Polytechnical University, Xi'an 710072, China Received 13 October 2008 28 mber 2008; accepted 29 December 2008 Available online 22 January 2009 Abstract 2D C/Zrc-SiC composites were fabricated by chemical vapor infiltration combined with polymer slurry infiltration and pyrolysis. Liquid highly branched polycarbosilane was used as the pre-ceramic precursor. In order to improve the oxidation resistance, three kinds of coating structures were prepared on C/Zrc-SiC composites: pure zirconium carbide coating, SiC-ZrC coating, and ZrB2-SiC coating. Structural evolutions of the as-produced composites after oxidation in CHa combustion gas atmosphere at about 1800C were investigated and compared Based on a model of the oxidation process, the mixture ZrB2-CVD SiC showed the best oxidation resistance. C 2009 Elsevier Ltd and Techna Group S.r. I. All rights reserved. Keywords: A Precursors: organic: B Composites; B Surfaces: Oxidation mechanism 1. Introduction barrier to oxygen diffusion because of the high evaporation rates of silicon oxide and the deterioration of the oxide film Carbon fiber reinforced silicon carbide composites(C/SiC) Researchers have been looking for new solutions to provide are one of the most promising structural materials for high oxidation/ablation protection for C/Sic composites at ultra temperature applications [1, 2]. C/SiC composites have a high high temperatures [9]. Zirconium and its composites have thermal stability and are usually considered useful up to exceptional properties. Especially, zirconium carbide(ZrC)and 1650C. However, they have a low durability except in inert zirconium boride (zrB2) have attracted much attention. They atmospheres. At higher temperatures(>1700C), the oxidation have melting points over 3000C, relatively low densities, and of the fiber, interphase and matrix cooperatively influence the the abilities to form refractory oxide zirconia scales(melting oxidation behavior of C/SiC composites in oxygen atmosphere point 2770C)[10]. However, these oxide coating are porou 3, 4]. Thus, reducing oxidation at high temperature in oxygen and do not provide oxidation protection. Thus, the poor environment is the challenge to extend application of C/Sic oxidation resistance of Zr B2 and Zrc makes them seldom to be composites. As a rule this problem is usually resolved by used alone. The addition of Sic has been shown to effectively applying oxidation-resistant coatings. improve the oxidation resistance of ZrB2 [11, 12 Because the oxidation of SiC is passive up to 1650C and the n the present paper, 2D C/ZrC-Sic composites were formed Sio2 film has a low oxygen diffusion coefficient, SiC is prepared by chemical vapor infiltration and polymer slurry the fundamental coating material for high temperature oxidation infiltration with a high-branched polycarbosilane(HBPCS)as protection of structural composites [5]. SiC coating prepared by the pre-ceramic precursor. Three kinds of coating structures chemical vapor deposition(CVD)shows different oxidation have been applied on the composites surface, i.e. (i)mixtu behavior in various environments, such as air and combustion Sic with CVd Zrc, (ii) mixture of ZrB, with CVD SiC, and atmosphere [6-8. But at T>1800C, the oxide film is a poor (iii) pure ZrC coating for comparison. Microstructure changes after oxidation in CHa combustion gas environment at about 1800C were investigated and compared. The erosion 86 29 88486068x834: fax: +8629 88494620. mechanism of different coating structures was discussed based nail. com(H. Li) on the oxidation results 2-884234.00 09 Elsevier Ltd and Techna Group S.r.L. All rights reserved 10.1016/j-cera 008.12002
Oxidation analysis of 2D C/ZrC–SiC composites with different coating structures in CH4 combustion gas environment Houbu Li *, Litong Zhang, Laifei Cheng, Yiguang Wang National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi’an 710072, China Received 13 October 2008; received in revised form 28 November 2008; accepted 29 December 2008 Available online 22 January 2009 Abstract 2D C/ZrC–SiC composites were fabricated by chemical vapor infiltration combined with polymer slurry infiltration and pyrolysis. Liquid highly branched polycarbosilane was used as the pre-ceramic precursor. In order to improve the oxidation resistance, three kinds of coating structures were prepared on C/ZrC–SiC composites: pure zirconium carbide coating, SiC–ZrC coating, and ZrB2–SiC coating. Structural evolutions of the as-produced composites after oxidation in CH4 combustion gas atmosphere at about 1800 8C were investigated and compared. Based on a model of the oxidation process, the mixture ZrB2–CVD SiC showed the best oxidation resistance. # 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Precursors: organic; B. Composites; B. Surfaces; Oxidation mechanism 1. Introduction Carbon fiber reinforced silicon carbide composites (C/SiC) are one of the most promising structural materials for high temperature applications [1,2]. C/SiC composites have a high thermal stability and are usually considered useful up to 1650 8C. However, they have a low durability except in inert atmospheres. At higher temperatures (>1700 8C), the oxidation of the fiber, interphase and matrix cooperatively influence the oxidation behavior of C/SiC composites in oxygen atmosphere [3,4]. Thus, reducing oxidation at high temperature in oxygen environment is the challenge to extend application of C/SiC composites. As a rule this problem is usually resolved by applying oxidation-resistant coatings. Because the oxidation of SiC is passive up to 1650 8C and the formed SiO2 film has a low oxygen diffusion coefficient, SiC is the fundamental coating material for high temperature oxidation protection of structural composites [5]. SiC coating prepared by chemical vapor deposition (CVD) shows different oxidation behavior in various environments, such as air and combustion atmosphere [6–8]. But at T > 1800 8C, the oxide film is a poor barrier to oxygen diffusion because of the high evaporation rates of silicon oxide and the deterioration of the oxide film. Researchers have been looking for new solutions to provide oxidation/ablation protection for C/SiC composites at ultrahigh temperatures [9]. Zirconium and its composites have exceptional properties. Especially, zirconium carbide (ZrC) and zirconium boride (ZrB2) have attracted much attention. They have melting points over 3000 8C, relatively low densities, and the abilities to form refractory oxide zirconia scales (melting point 2770 8C) [10]. However, these oxide coating are porous and do not provide oxidation protection. Thus, the poor oxidation resistance of ZrB2 and ZrC makes them seldom to be used alone. The addition of SiC has been shown to effectively improve the oxidation resistance of ZrB2 [11,12]. In the present paper, 2D C/ZrC–SiC composites were prepared by chemical vapor infiltration and polymer slurry infiltration with a high-branched polycarbosilane (HBPCS) as the pre-ceramic precursor. Three kinds of coating structures have been applied on the composites surface, i.e. (i) mixture of SiC with CVD ZrC, (ii) mixture of ZrB2 with CVD SiC, and (iii) pure ZrC coating for comparison. Microstructure changes after oxidation in CH4 combustion gas environment at about 1800 8C were investigated and compared. The erosion mechanism of different coating structures was discussed based on the oxidation results. www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 35 (2009) 2277–2282 * Corresponding author. Tel.: +86 29 88486068x834; fax: +86 29 88494620. E-mail address: houbuli@gmail.com (H. Li). 0272-8842/$34.00 # 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2008.12.002
H. Li et al. /Ceramics International 35 (2009) 2277-2282 2. Experimental procedure CH4, and H2 were used as precursors. The deposition temperature was controlled in the 1300-1400C range [15]. 2.1. Fabrication of 2D C/LrC-SiC composites For the preparation of the other two kinds of coating structures, the same heat-treatment on the composites at 1400C The whole process for sample preparation is shown in Fig. 1. for 2 h was firstly conducted. A SiC powder(1I um, Beijing T-300 carbon fiber (Toray, Japan) was employed to fabricate Mountain Technical Development Center, China) layer was then fiber preforms. The preform with fiber content of 40 vol% was pasted on the composite, followed by depositing a ZrC coating shaped by lamination of 2D-carbon cloth. A thin pyrolytic (#2). In this process, CVD ZrC will penetrate in the gaps between carbon layer was deposited on surface of the carbon fiber as the Sic powders and form a mixture of Sic-ZrC coating with a layer interfacial layer by chemical vapor infiltration(CVD) with C3H6 of dense ZrC on the top. Likewise, ZrB2 powder(1 um, Beijing at 960C. The sample was then pretreated for graphitization at Mountain Technical Development Center, China)was pasted on 1800C for 2 h. The C/Sic composite was prepared by the composite followed by depositing a Sic layer on its top(#3) chemical vapor infiltration of SiC at 1000C of the prepared reform Methyltrichlorosilane (MTS, CH SiCl3)was used for 2.3. Oxidation tests the Sic precursor. After CVI, the composites were machined and polished to obtain 40 mm x 20 mm x 3.5 mm substrates The oxidation tests were carried out in a Ch, combustion wind The liquid highly branched polycarbosilane with allyl tunnel(Fig. 2). The wind tunnel possesses a controlled groups was synthesized in the Advanced Materials Laboratory environment chamber [16] providing various kinds and con- at Xiamen University [13, 14]. 20 wt% ZrC powder with an centrations of oxidizing gas. For each kind of coating structure, average grain size of I um(Beijing Mountain Technical the oxidation test was repeated at least three times. During the test, Development Center, China)was mixed into HBPCS by the specimen was vertically exposed to the flame for 30 min A magnetic stirring. The slurry was vacuum infiltrated into the mixture of O2 and CHa was charged into the combustion chamber substrates and pyrolysized at 900C for two cycles to realize and spark ignited. The gas-flow velocity was about 20 m/s. The further densification flame temperature at the center was 1800+ 30C, which was stabilized by controlling the flux of each component gas(mass 2. 2. Coating systems preparation flow controller was 58501 series of BROOKS, Japan). The calculated combustion gas composition is listed in Table 1 ZrB,/ZrC/SiC coatings were used in order to increase the oxidation resistance. The first kind of coating structure(#1)was 2.4. Measurement and characterization prepared as follows: two more cycles of polymer slurry infiltration and pyrolysis(Plp)process were conducted on the composite. Microstructures of the specimens were observed by subsequently, aheat-treatmentin flowing argon at 1400Cfor 2 h scanning electron microscope(SEM, JSM-6700F)and the a ZrC layer was deposited on the surface of this specimen by spectroscopy (EDS). X-ray diffraction (XRD) investigation chemical vapor deposition. As for the CVD ZrC process, ZrCL, were carried out by using a Rigaku D/max-2400 diffractompe. n (Tokyo, Japan) with copper Ko radiation Data were digitally recorded in a continuous scan in the range of angle(20) from Carbon fiber preform PyC 15°to75° with a scanning rate of0.087s CVI+PIP 3. Results and discussion C/ZrC-SiC composite 3.1. Microstructure of the 2D C/LrC-SiC composite Fig 3 shows the cross-sections of as-produced C/ZrC-Sic Machined and treated omposite. Between fibers and the Zrc-Sic matrix, SiC matrix Coating preparation CVD Zrc SiC particle ZrB2 particle +CVD Zrc +CVD SiC #1 I. holder 2. norzle 3 chamber 4. oil pip 5. entrance 6. spary 7. igniter 8, substrates different coating structures. Fig. 2. Schematic of the combustion wind tunnel
2. Experimental procedure 2.1. Fabrication of 2D C/ZrC–SiC composites The whole process for sample preparation is shown in Fig. 1. T-300TM carbon fiber (Toray, Japan) was employed to fabricate fiber preforms. The preform with fiber content of 40 vol% was shaped by lamination of 2D-carbon cloth. A thin pyrolytic carbon layer was deposited on surface of the carbon fiber as the interfacial layer by chemical vapor infiltration (CVI) with C3H6 at 960 8C. The sample was then pretreated for graphitization at 1800 8C for 2 h. The C/SiC composite was prepared by chemical vapor infiltration of SiC at 1000 8C of the prepared preform. Methyltrichlorosilane (MTS, CH3SiCl3) was used for the SiC precursor. After CVI, the composites were machined and polished to obtain 40 mm 20 mm 3.5 mm substrates. The liquid highly branched polycarbosilane with allyl groups was synthesized in the Advanced Materials Laboratory at Xiamen University [13,14]. 20 wt% ZrC powder with an average grain size of 1 mm (Beijing Mountain Technical Development Center, China) was mixed into HBPCS by magnetic stirring. The slurry was vacuum infiltrated into the substrates and pyrolysized at 900 8C for two cycles to realize further densification. 2.2. Coating systems preparation ZrB2/ZrC/SiC coatings were used in order to increase the oxidation resistance. The first kind of coating structure (#1) was preparedasfollows: two morecycles ofpolymer slurryinfiltration and pyrolysis (PIP) process were conducted on the composite. Subsequently, a heat-treatmentinflowing argon at 1400 8C for 2 h was carried out in order to stabilize the composite matrix. Finally, a ZrC layer was deposited on the surface of this specimen by chemical vapor deposition. As for the CVD ZrC process, ZrCl4, CH4, and H2 were used as precursors. The deposition temperature was controlled in the 1300–1400 8C range [15]. For the preparation of the other two kinds of coating structures, the same heat-treatment on the composites at 1400 8C for 2 h was firstly conducted. A SiC powder (1 mm, Beijing Mountain Technical Development Center, China) layer was then pasted on the composite, followed by depositing a ZrC coating (#2). In this process, CVD ZrC will penetrate in the gaps between SiC powders and form a mixture of SiC–ZrC coating with a layer of dense ZrC on the top. Likewise, ZrB2 powder (1 mm, Beijing Mountain Technical Development Center, China) was pasted on the composite followed by depositing a SiC layer on its top (#3). 2.3. Oxidation tests The oxidation tests were carried out in a CH4 combustion wind tunnel (Fig. 2). The wind tunnel possesses a controlled environment chamber [16] providing various kinds and concentrations of oxidizing gas. For each kind of coating structure, the oxidation test was repeated at least three times. During the test, the specimen was vertically exposed to the flame for 30 min. A mixture of O2 and CH4 was charged into the combustion chamber and spark ignited. The gas-flow velocity was about 20 m/s. The flame temperature at the center was 1800 30 8C, which was stabilized by controlling the flux of each component gas (mass flow controller was 5850i series of BROOKS, Japan). The calculated combustion gas composition is listed in Table 1. 2.4. Measurement and characterization Microstructures of the specimens were observed by scanning electron microscope (SEM, JSM-6700F) and the elemental analysis was conducted by energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) investigations were carried out by using a Rigaku D/max-2400 diffractometer (Tokyo, Japan) with copper Ka radiation. Data were digitally recorded in a continuous scan in the range of angle (2u) from 158 to 758 with a scanning rate of 0.088/s. 3. Results and discussion 3.1. Microstructure of the 2D C/ZrC–SiC composite Fig. 3 shows the cross-sections of as-produced C/ZrC–SiC composite. Between fibers and the ZrC–SiC matrix, SiC matrix Fig. 1. Experimental procedure for preparation of C/ZrC–SiC composites with different coating structures. Fig. 2. Schematic of the combustion wind tunnel. 2278 H. Li et al. / Ceramics International 35 (2009) 2277–2282
H. Li et al. /Ceramics International 35(2009)2277-2282 Table 1 particles in the composites(Fig. 3b). ZrC slurry prefers to fill Combustion gas composition large pores between the fiber bundles. Hence, the channels from he surface to the inner of composite can be easily blocked by Zrc O Other and pyrolysized SiC. Further infiltration of the slurry thus becomes much difficult after several PlP cycles. The density 16.1 55.3 27.6 .0 measured by the Archimedes method for the final composite about 2.34 g/cm with an open porosity of about 12% which was formed by CVd process is obviously observed (Fig. 3b). Furthermore, it can be seen that there are still few pores 3.2. Microstructure of coatings in the composite(Fig 3a). During the PIP process, the liquid PCS will fill surrounding both the Cvd Sic matrix and the Zrc The cross-sections of different coating structures are show particles. After pyrolysis, the derived Sic integrates ZrC in Fig. 4. ZrC coating with a thickness of about 8 um is p200u Fig 3. Cross-sections of as-received C/ZrC-SiC composites CF: carbon fiber, (a)a low magnification and(b)a high magnification. Pyrolysized Sic CVD SIC 经态100 Fig. 4. Cross-sections of different coating structures (a)#I CVD ZrC coating, (b)#2 Sic-ZrC mixture coating, (c)#3 ZrBx-SiC mixture coating and surface morphologies for(d)#l (e)#2, and (f)#3 specimens
which was formed by CVD process is obviously observed (Fig. 3b). Furthermore, it can be seen that there are still few pores in the composite (Fig. 3a). During the PIP process, the liquid PCS will fill surrounding both the CVD SiC matrix and the ZrC particles. After pyrolysis, the derived SiC integrates ZrC particles in the composites (Fig. 3b). ZrC slurry prefers to fill large pores between the fiber bundles. Hence, the channels from the surface to the inner of composite can be easily blocked by ZrC and pyrolysized SiC. Further infiltration of the slurry thus becomes much difficult after several PIP cycles. The density measured by the Archimedes method for the final composite is about 2.34 g/cm3 with an open porosity of about 12%. 3.2. Microstructure of coatings The cross-sections of different coating structures are shown in Fig. 4. ZrC coating with a thickness of about 8 mm is Table 1 Combustion gas composition. Components O2 H2O CO2 Other mol.% 16.1 55.3 27.6 1.0 Fig. 3. Cross-sections of as-received C/ZrC–SiC composites. CF: carbon fiber, (a) a low magnification and (b) a high magnification. Fig. 4. Cross-sections of different coating structures (a) #1 CVD ZrC coating, (b) #2 SiC–ZrC mixture coating, (c) #3 ZrB2–SiC mixture coating and surface morphologies for (d) #1 (e) #2, and (f) #3 specimens. H. Li et al. / Ceramics International 35 (2009) 2277–2282 2279
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 coating
observed 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 specimens 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
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)2495-2501
in 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 degradation 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. 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