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ZrC composite in oxyacetylene torch environment
Composites Science and Technology 71 (2011) 1392–1396
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Ablation behavior and mechanism of 3D C/ZrC composite in oxyacetylene torch environment Dan Zhao, Changrui Zhang, Haifeng Hu ⇑, Yudi Zhang State Key Laboratory of Advanced Ceramic Fibers & Composites, College of Aerospace & Materials Engineering, National University of Defense Technology, Changsha 410073, PR China
a r t i c l e
i n f o
Article history: Received 21 November 2010 Received in revised form 11 May 2011 Accepted 13 May 2011 Available online 17 May 2011 Keywords: A. Carbon fiber A. Zirconium carbide A. Ceramic matrix composites (CMCs) B. Ablation behavior
a b s t r a c t Ablation property of three dimensional carbon fiber reinforced zirconium carbide composite (3D C/ZrC composite) was determined using oxyacetylene torch test with a heat flux of 4187 kW/m2 and flame temperature of over 3000 °C. C/ZrC composite exhibited an excellent configurational stability with a surface temperature of over 2000 °C during 60–300 s period, while 3D C/SiC composite was perforated at 55 s. After ablation for 300 s, the composite showed a mass loss rate of 0.006 g/s and a linear recession rate of 0.004 mm/s. The formation of zirconia melt on the surface of the C/ZrC composite contributed mainly the ablation property improvement. The C/ZrC composite after ablation showed four different layers due to the temperature and pressure gradients: the melting layer, the loose tree-coral-like ZrO2 layer, the undersurface oxidation layer, and the composite layer. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Sharp leading edges, nose caps and rocket engines of hypersonic aerospace vehicles serve in extreme environments with high heat fluxes, high enthalpy, high pressure, and perhaps high velocity ceramic particle erosion. The operation temperatures of these components may increase rapidly from room temperature to over 3000 °C and last from several seconds to several hundreds seconds [1]. Ultra high temperature ceramics (UHTCs), such as carbides and diborides of Zr and Hf, are widely investigated as candidates in environment over 3000 °C because of the high melting points [2]. Further more, Zr-based ceramics are more attractive because of their lower manufacturing costs and densities than Hf-based ceramics, and carbon fiber introduction may improve greatly the fracture toughness and thermal shock resistance of bulk ceramics. Because of the special application environments, it is very important to validate the ablation properties of these materials. Ablation properties can be investigated by ballistic flight experiments and ground-based simulation experiments. Ballistic flight experiments can confirm the ablation properties of materials in real re-entry condition. For example, NASA Ames investigated the HfB2/SiC, ZrB2/SiC ceramics for nose tips (SHARP-B1 project) [3] and reentry strake (SHARP-B2 project) [4] by ballistic flight experiments. However, this method is seldom used because of the considerable cost. More often ground-based simulation tests are used, such as wind tunnel test, plasma art-jet test, and oxyacetylene ⇑ Corresponding author. Tel./fax: +86 731 84576269. E-mail addresses: [email protected] (D. Zhao), [email protected] (C. Zhang), [email protected] (H. Hu), [email protected] (Y. Zhang). 0266-3538/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2011.05.005
torch ablation. Wind tunnel tests can simulate the conditions of high enthalpy and strong gas flows, but it cannot simulate the fully representative flight envelope in terms of Mach and Reynolds number [5]. Plasma art-jet ablation [6,7] can partially simulate the re-entry environment, but the parameters are simple. Again wind tunnel and plasma arc-jet tests are cost. Oxyacetylene torch test is the most convenient method with the lowest cost. It is often used to give a primary evaluation about the ablation properties of materials. A large number of researches have been done using oxyacetylene torch tests to study the ablation properties of UHTCs and ceramic matrix composites. For example, the ablative properties of following materials were reported: ZrB2–SiC composites by Han et al. [8], ZrB2–SiC composites with LaB6 additions by Zhang et al. [9], 2D C/ZrB2–SiC composite by Hu et al. [10] and Wang et al. [11], and pitch-derived ZrC/C by Tong et al. [12]. However, most of the works were focused on the ZrB2-based and ZrC-based bulk ceramics. In this paper, the ablation behavior of C/ZrC composite was investigated by oxyacetylene torch test, and the ablation mechanism was proposed.
2. Experimental procedure 3D fiber reinforcements were prepared by stitching 15 layers of plain weave carbon fiber cloth (3 k, Toray T300) with carbon fiber (3 k Toray) in 3 mm 3 mm space. The mixture of zirconium butoxide (Zr(OC4H9)4, ZTB) and divinylbenzene (DVB) with a molar ratio of ZrO2 (ZTB)/carbon (DVB) = 1/3 was used as ZrC precursor
D. Zhao et al. / Composites Science and Technology 71 (2011) 1392–1396
[13]. 3D C/ZrC composite was obtained by 16 cycles of infiltration with ZrC precursor into the preform, curing, pyrolysis at 1200 °C, and heat treatment at 1600 °C after the eighth and the sixteenth cycle, respectively. 3D C/SiC composite was obtained by 15 cycles of infiltration with polycarbosilane (PCS) into the preform, curing, pyrolysis at 1200 °C [14]. The volume percentages of carbon fiber, matrix and pores of C/ZrC composite were 34.3%, 31.4% and 34.3%, respectively, and those of C/SiC were 36.2%, 47.7% and 16.1%, respectively. Ablation property test was carried out in a flowing oxyacetylene torch environment, with approximately 4187 kW m2 heat flux and 3100 °C flame temperature. The pressures of oxygen and acetylene were 0.4 MPa and 0.095 MPa, respectively; while the gas flow rates were 1512 L/h and 1116 L/h, respectively. The surface temperature of the sample was monitored with an optical pyrometer. The flat-face specimens with a size of 30 mm 30 mm 3.5 mm were mounted in a concave graphite anvil. The distance between the nozzle tip and the surface of the specimen was 10 mm and the inner diameter of the nozzle tip was 2.0 mm. During the test, the ablation gun was ignited firstly. After the flame was steady, the ablation gun was moved vertically to the sample surface. The microstructure of the ablated composite was examined by scanning electron microscopy (SEM, S4800 Hitachi), and the composition was determined by X-ray Diffraction (XRD, Siemens D500). 3. Results and discussion 3.1. Morphology and component The morphologies of the ablated C/ZrC composite and C/SiC composite were shown in Fig. 1. The comparison between Fig. 1a and Fig. 1b showed that there is no significant difference between the morphologies of the C/ZrC composite specimens after ablation for 60 s and 300 s. During ablation, C/ZrC composite exhibited an excellent configurational stability. The color of the ablated surface changed from black into white, and a white, loose ablation layer formed on the surface. XRD analysis indicated that the main component of the white ablation product was monoclinic ZrO2 (mZrO2) (Fig. 2), showing the oxidation of ZrC matrix. The most severe ablation happened in the center of the flame and some ZrO2 melt formed. As a comparison, C/SiC composite was exposed to the oxyacetylene torch in the same condition, and perforated at 55 s (Fig. 1c), gave a linear recession rate of 0.064 mm/s. Concentric circles were observed in C/SiC composite. The results showed that C/ ZrC composite had much better anti-ablation properties than C/SiC composite.
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Fig. 2. XRD pattern of the ablated C/ZrC composite surface.
60–90 s periods. The mass loss rate of C/ZrC composite was 0.012 g/s during first 60 s, then decreased steadily in 60–300 s, and reached the lowest value (0.006 g/s) at 300 s. The linear recession rate of the C/ZrC composite showed a different trend from the mass loss rate. When the C/ZrC composite was ablated for 60 s, its linear recession rate was 0.002 mm/s. The composite showed a negative recession rate because white, loose shell formed on the surface, and apparent measurement found the thickness of the composite increase after ablation test. After 90 s ablation, the linear recession rate increased to 0.014 mm/s because the surface temperature reached near 2300 °C, so ZrO2 melted and was blown all around by the gas flow of oxyacetylene flame, and thus the thickness of the composite decreased. As ZrO2 melt, molten ZrO2 spread on the ablation surface, partially sealed the pores, and acted as an effective barrier against the inward diffusion of oxygen. After 90 s, there was almost no linear ablation, and the ablation front did not progress furthermore, which could be proved by just simple calculation that 90 s 0.014 mm/s 120 s 0.011 mm/s 180 s 0.008 mm/s 300 s 0.004 mm/s. When the ablation time increased to 300 s, the linear recession rate was 0.004 mm/s. Ablation properties of some ultra high temperature materials reported in other works were showed in Table 2. After ablated for 60 s, the 3D C/ZrC composite showed higher mass loss rate and linear recession rate than Cf/UHTC composites fabricated by other processes because it had higher volume percentages of carbon fiber and pores [15–20]. But when ablation time increased, the composite showed a much better anti-ablation property because molten ZrO2 protected the composite. As a comparison, the C/SiC composite showed a mass loss rate of 0.027 g/s and a linear recession rate of 0.064 mm/s, with the linear recession rate one magnitude higher than that of C/ZrC composite because SiC and its ablative product SiO2 had lower boiling temperatures than the flame temperature and sublimated [21].
3.2. Ablation properties 3.3. Microstructure The mass loss rates and linear recession rates of C/ZrC composite and C/SiC composite were shown in Table 1. The mass loss rate of C/ZrC composite decreased with ablation time increase during
The microstructures of the ablation surface were presented in Fig. 3. Three regions may be found on the ablated surface of the
Fig. 1. Morphologies of the ablated composites (a) C/ZrC composite at 60 s, (b) C/ZrC composite at 300 s and (c) C/SiC composite at 55 s.
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Table 1 Ablation properties of the C/ZrC composite and C/SiC composites. Samples
C/ZrC
C/SiC
Ablated time (s)
Surface temperature (°C)
Mass loss rate (g/s)
Linear recession rate (mm/s)
60 90 120 180 300 55
2028 2285 2291 2293 2297 1876
0.012 0.010 0.008 0.007 0.006 0.027
0.002 0.014 0.011 0.008 0.004 0.064
C/ZrC composite: melting region (marked as A), porous region (marked as B) and marginal region (marked as C). Region A in the center of the sample formed from the viscous ZrO2 melt at
3000 °C or above, higher than the melting point of ZrO2 (2800 °C). After the ablation, the sample cooled fast and cracks appeared due to the CTE mismatch of the ablation layer and the composite. The porous region B was around the melting region (Fig. 3a). This region was so small that it could not be clearly observed by the naked eye. In this region, ZrO2 melted, but in small amount, and only partially sealed the pores, so porous layer, instead of continuous melting layer, was developed (Fig. 3b). The marginal region C was looser than the others. In region C carbon fibers were oxidized and disappeared (Fig. 3c), leaving confusing morphology of matrix like carbon fiber bundles. Enlarged parts of the square and circular areas of Fig. 3c showed long slots (Fig. 3d) and ZrC foam (Fig. 3e) due to the burn-out of carbon fiber. The above observation and analysis gave that ZrO2 melt had rather high viscosity and strong adhesion to the composite, and was dif-
Table 2 Ablation properties of ultra high temperature materials reported in other works. Materials
Carbon fiber (%, vol)
ZrC (%)
Oxyacetylene flame test condition
C/C–ZrC
10
As the same as our work
C/SiC–ZrC
34.0
2.50 (wt) 4.14 (wt) 12.1 (wt) 23.5 (vol) 33.3 (vol) –
20.3 ZrC coated C/C
–
As the same as our work
The flow rates of oxygen and acetylene were 1.960 L/s and 0.696 L/s, respectively
Ablation time (s)
Mass loss rate (g/ s)
Linear recession rate (mm/ s)
References
60 60 60 60
0.0011 0.00062 0.00028 0.008
0.0012 0.00051 0.00042 0.008
[15–17]
60
0.006
240
[18]
0.004 2
0.0001 (g/cm )
Fig. 3. Ablated surface morphologies of C/ZrC composite (a–e) and C/SiC (f).
0.0003
[19]
D. Zhao et al. / Composites Science and Technology 71 (2011) 1392–1396
ficult to be blown away by the gas flow, thus prevented the composite from further oxidation, so the composite had excellent anti-ablative properties. The surface morphology of the C/ZrC composite after ablation test was completely different from that of C/SiC composite in that the former was composed mainly of ZrO2 matrix, while the latter was needle-like carbon fiber after ablation (Fig. 3f) [21]. This is because SiC matrix and its oxidation product had low boiling points than the flame temperature. They sublimated under the oxyacetylene flame, and left behind carbon fibers. Instead, ZrO2 melt from ZrC matrix oxidation had a high melting point (2800 °C) and high viscosity at 2000 °C, was difficult to be blown away and left on the surface, while carbon fiber was removed for the oxidation environment. This is the basic reason for the ablation property improvement of C/ZrC composite in oxyacetylene torch test. A loose, tree-coral-like ZrO2 layer without any carbon fiber was observed under the surface ablation layer (see Fig. 4). The rela-
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tively large holes with size of U 10 lm formed because of the burn-out of carbon fiber. Tree-coral-like ZrO2 formed because of the lower temperature and pressure. The surface ablation layer hindered the heat transfer and gas flow, so ZrO2 only melted in small amount, recrystallized, and ejected the micro-pores from the matrix. ZrO2 presented as a loose layer instead of a continuous layer. After the loose ZrO2 layer was peeled from the composite, the undersurface layer was the oxidized composite and integral (Fig. 5). White matrix existed mostly between the fiber tows (Fig. 5a). Carbon fiber paralleled the flow of the flame was needle-like with white matrix adhered on the needlepoint (Fig. 5b), which prevented the oxidative gases from contact with the carbon fiber directly and protected the carbon fiber to a certain extent, thus the lateral surfaces of the fibers were attacked first. The morphology of the fiber bundles that were vertical to the flow of the flame was shown in Fig. 5c. Needle-like carbon fibers were observed with matrix between the fibers, unlike the morphology of the ablated C/SiC composite, because ZrO2 has a high melting point and a high viscosity and is difficult to be blown away. There is also tree-coral-like ZrO2 adhered on the lateral surfaces of the carbon fiber (Fig. 5d). The oxidation of carbon fiber and ZrC matrix might lead to a lot of active points. ZrO2 formed here, grew, and branched. Then branch-like matrix formed on the surface of carbon fibers. 3.4. Ablation mechanism Ablation is a combination of chemical reaction (oxidation mainly), thermal–physical process and mechanical erosion. During the oxyacetylene torch ablation, following reactions occurred.
ZrCðsÞ þ 3=2O2 ðgÞ ¼ ZrO2 ðsÞ þ COðgÞ
ð1Þ
CðsÞ þ 1=2O2 ðgÞ ¼ COðgÞ
ð2Þ
ZrO2 ðsÞ ¼ ZrO2 ðlÞ
ð3Þ
Fig. 4. Morphologies of the tree-coral-like ZrO2 layer.
Fig. 5. Microstructures of the undersurface oxidation layer.
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D. Zhao et al. / Composites Science and Technology 71 (2011) 1392–1396
of the composite, which greatly improved the ablation property of C/ZrC composite. After the ablation, the composite was divided into four layers as the melting layer, the loose tree-coral-like ZrO2 layer, the undersurface oxidation layer and the composite layer. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (Nos. 90816020 and 90916019). Fig. 6. Schematic diagram of the ablated C/ZrC composite.
References The reactions (1) and (2) occurred in all ablation regions, and reaction (3) only happened in high temperature areas, especially in the surface ablation layer of the composite. The ablation process may be divided into two steps. Firstly, oxidation of carbon fiber and ZrC matrix began at relative lower temperature to form a porous structure, and the oxygen diffusion tunnel was the open pores and cracks in the composite. In this step the ablation rate was controlled by the oxidation rate of carbon and ZrC. Secondly, ZrO2 melted while the temperature was raised over 2800 °C. In the center of the sample, high speed gas flow led to ZrO2 melt spread on the ablated surface. In other regions, some of the ZrO2 melt flowed and filled some pores. The ZrO2 melt formation prevented the further oxidation of carbon fibers and ZrC matrix, showing an excellent anti-ablative behavior. Fig. 6 shows the structure of the ablated C/ZrC composite. The ablated material could be divided into four layers due to the temperature and pressure gradients: the surface melting layer, the loose tree-coral-like ZrO2 layer, the undersurface oxidation layer and the composite layer. On the surface of the composite, the temperature was the highest. ZrO2 melted, spread on the ablated surface, partially plugged the pores of the matrix, and protected the composite. Under the melting layer, the temperature and pressure decreased, and carbon fiber was burnt-out. ZrO2 melted with small amount, recrystallized and ejected the pores from the matrix, and present as tree-coral-like layer. In the oxidation layer, the temperature and pressure decreased more significantly, ZrC was oxidized into loose ZrO2. Needle-like carbon fiber formed because the lateral surface of fibers was eroded first. 4. Conclusions Ablation behavior of the C/ZrC composite was investigated by oxyacetylene torch test. The C/ZrC composite showed a mass loss rate of 0.012 g/s and a negative recession rate of 0.002 mm/s when it was ablated for 60 s. As the ablation time increased, the mass loss rate of the C/ZrC composite decreased gradually, while the linear recession rate increased first and decreased later. The mass loss rate and linear recession rate of ablation for 300 s was 0.006 g/s and 0.004 mm/s, respectively. C/ZrC composite showed much better ablation resistance than C/SiC composite because ZrO2 melt on the surface sealed the pores
[1] Kontinos DA, Gee K, Prabhu DK. Temperature constraints at the sharp leading edge of a crew transfer vehicle. AIAA 2001-2886; 2001. [2] Savino R, Fumo MDS, Paterna D, Serpico M. Aerothermodynamic study of UHTC-based thermal protection systems. Aerosol Sci Technol 2005;9:151–60. [3] Kolodziej P, Salute J, Keese DL. First flight demonstration of a sharp ultra-high temperature ceramic nosetip. NASA TM-112215; 1997. [4] Loomis M, Plamer G. Pre-flight CFD analysis of art jet and flight environments for the SHARP-B2 flight experiment. AIAA 2001-0982; 2001. [5] Monteverde F, Savino R, Fumo MDS, Maso AD. Plasma wind tunnel testing of ultra-high temperature ZrB2–SiC composites under hypersonic re-entry conditions. J Eur Ceram Soc 2010;30:2313–21. [6] Zhang X, Hu P, Han J, Meng S. Ablation behavior of ZrB2–SiC ultra high temperature ceramics under simulated atmospheric re-entry conditions. Compos Sci Technol 2008;68:1718–26. [7] Yin J, Zhang H, Xiong X, Huang B, Zuo J. Ablation properties of carbon/carbon composites with tungsten carbide. Appl Surf Sci 2009;255:5036–40. [8] Han J, Hu P, et al. Oxidation-resistant ZrB2–SiC composites at 2200 °C. Compos Sci Technol 2008;68:799–806. [9] Zhang XH, Hu P, Han JC, Xu L, Meng SH. The addition of lanthanum hexaboride to zirconium diboride for improved oxidation resistance. Scripta Mater 2007;57:1036–9. [10] Hu HF, Wang QK, Chen ZH, Zhang CR, Zhang YD, Wang J. Preparation and characterization of C/SiC–ZrB2 composites by precursor infiltration and pyrolysis process. Ceram Int 2010;36:1011–6. [11] Wang Y, Liu W, Cheng L, Zhang L. Preparation and properties of 2D C/ZrB2–SiC ultra high temperature ceramic composites. Mater Sci Eng 2009;524:129–33. [12] Tong Q, Shi J, Song Y, Guo Q, Liu L. Resistance to ablation of pitch-derived ZrC/C composites. Carbon 2004;42:2495–500. [13] Zhao D, Zhang C, Hu H, Zhang Y, Wang J. A simple way to prepare precursors for zirconium carbide. J Mater Sci 2010;45:6401–5. [14] Hu H, Chen Z, Xiao J, Zheng W. Synthesis and characterization of polysilazane precursor. J Mater Sci Lett 1999;18:1271–2. [15] Shen XT, Li KZ, Li HJ, Fu QG, et al. The effect of zirconium carbide on ablation of carbon/carbon composites under an oxyacetylene flame. Corros Sci 2011;53:105–12. [16] Shen XT, Li KZ, Li HJ, Lan FT, et al. Effect of ZrO2 from the oxidation of ZrC on ablation of ZrC modified carbon/carbon composites. J Inorg Mater 2009;24(5):943–7. in Chinese. [17] Shen XT, Li KZ, Li HJ, Du HY, et al. Microstructure and ablation properties of zirconium carbide doped carbon/carbon composites. Carbon 2010;48(2):344–51. [18] Wang QK, Hu HF, Chen ZH. Effect of ZrC content on the microstructure and properties of 2D C/SiC-ZrC composites via precursor infiltration and pyrolysis. J Aeronaut Mater 2009;29(3):72–6 [in Chinese]. [19] Sun W, Xiong X, Huang BY, Li GD, et al. ZrC ablation protective coating for carbon/carbon composites. Carbon 2009;47(14):3368–71. [20] Wang YG, Zhu XJ, Zhang LT, Cheng LF. Reaction kinetics and ablation properties of C/C–ZrC composites fabricated by reactive melt infiltration. Ceram Int 2011;37(4):1277–83. [21] Fang D, Chen Z, Song Y, Sun Z. Morphology and microstructure of 2.5 dimension C/SiC composites ablated by oxyacetylene torch. Ceram Int 2009;35:1249–53.
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Ablation behavior and mechanism of 3D C/ZrC composite in oxyacetylene torch environment Dan Zhao, Changrui Zhang, Haifeng Hu ⇑, Yudi Zhang State Key Laboratory of Advanced Ceramic Fibers & Composites, College of Aerospace & Materials Engineering, National University of Defense Technology, Changsha 410073, PR China
a r t i c l e
i n f o
Article history: Received 21 November 2010 Received in revised form 11 May 2011 Accepted 13 May 2011 Available online 17 May 2011 Keywords: A. Carbon fiber A. Zirconium carbide A. Ceramic matrix composites (CMCs) B. Ablation behavior
a b s t r a c t Ablation property of three dimensional carbon fiber reinforced zirconium carbide composite (3D C/ZrC composite) was determined using oxyacetylene torch test with a heat flux of 4187 kW/m2 and flame temperature of over 3000 °C. C/ZrC composite exhibited an excellent configurational stability with a surface temperature of over 2000 °C during 60–300 s period, while 3D C/SiC composite was perforated at 55 s. After ablation for 300 s, the composite showed a mass loss rate of 0.006 g/s and a linear recession rate of 0.004 mm/s. The formation of zirconia melt on the surface of the C/ZrC composite contributed mainly the ablation property improvement. The C/ZrC composite after ablation showed four different layers due to the temperature and pressure gradients: the melting layer, the loose tree-coral-like ZrO2 layer, the undersurface oxidation layer, and the composite layer. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Sharp leading edges, nose caps and rocket engines of hypersonic aerospace vehicles serve in extreme environments with high heat fluxes, high enthalpy, high pressure, and perhaps high velocity ceramic particle erosion. The operation temperatures of these components may increase rapidly from room temperature to over 3000 °C and last from several seconds to several hundreds seconds [1]. Ultra high temperature ceramics (UHTCs), such as carbides and diborides of Zr and Hf, are widely investigated as candidates in environment over 3000 °C because of the high melting points [2]. Further more, Zr-based ceramics are more attractive because of their lower manufacturing costs and densities than Hf-based ceramics, and carbon fiber introduction may improve greatly the fracture toughness and thermal shock resistance of bulk ceramics. Because of the special application environments, it is very important to validate the ablation properties of these materials. Ablation properties can be investigated by ballistic flight experiments and ground-based simulation experiments. Ballistic flight experiments can confirm the ablation properties of materials in real re-entry condition. For example, NASA Ames investigated the HfB2/SiC, ZrB2/SiC ceramics for nose tips (SHARP-B1 project) [3] and reentry strake (SHARP-B2 project) [4] by ballistic flight experiments. However, this method is seldom used because of the considerable cost. More often ground-based simulation tests are used, such as wind tunnel test, plasma art-jet test, and oxyacetylene ⇑ Corresponding author. Tel./fax: +86 731 84576269. E-mail addresses: [email protected] (D. Zhao), [email protected] (C. Zhang), [email protected] (H. Hu), [email protected] (Y. Zhang). 0266-3538/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2011.05.005
torch ablation. Wind tunnel tests can simulate the conditions of high enthalpy and strong gas flows, but it cannot simulate the fully representative flight envelope in terms of Mach and Reynolds number [5]. Plasma art-jet ablation [6,7] can partially simulate the re-entry environment, but the parameters are simple. Again wind tunnel and plasma arc-jet tests are cost. Oxyacetylene torch test is the most convenient method with the lowest cost. It is often used to give a primary evaluation about the ablation properties of materials. A large number of researches have been done using oxyacetylene torch tests to study the ablation properties of UHTCs and ceramic matrix composites. For example, the ablative properties of following materials were reported: ZrB2–SiC composites by Han et al. [8], ZrB2–SiC composites with LaB6 additions by Zhang et al. [9], 2D C/ZrB2–SiC composite by Hu et al. [10] and Wang et al. [11], and pitch-derived ZrC/C by Tong et al. [12]. However, most of the works were focused on the ZrB2-based and ZrC-based bulk ceramics. In this paper, the ablation behavior of C/ZrC composite was investigated by oxyacetylene torch test, and the ablation mechanism was proposed.
2. Experimental procedure 3D fiber reinforcements were prepared by stitching 15 layers of plain weave carbon fiber cloth (3 k, Toray T300) with carbon fiber (3 k Toray) in 3 mm 3 mm space. The mixture of zirconium butoxide (Zr(OC4H9)4, ZTB) and divinylbenzene (DVB) with a molar ratio of ZrO2 (ZTB)/carbon (DVB) = 1/3 was used as ZrC precursor
D. Zhao et al. / Composites Science and Technology 71 (2011) 1392–1396
[13]. 3D C/ZrC composite was obtained by 16 cycles of infiltration with ZrC precursor into the preform, curing, pyrolysis at 1200 °C, and heat treatment at 1600 °C after the eighth and the sixteenth cycle, respectively. 3D C/SiC composite was obtained by 15 cycles of infiltration with polycarbosilane (PCS) into the preform, curing, pyrolysis at 1200 °C [14]. The volume percentages of carbon fiber, matrix and pores of C/ZrC composite were 34.3%, 31.4% and 34.3%, respectively, and those of C/SiC were 36.2%, 47.7% and 16.1%, respectively. Ablation property test was carried out in a flowing oxyacetylene torch environment, with approximately 4187 kW m2 heat flux and 3100 °C flame temperature. The pressures of oxygen and acetylene were 0.4 MPa and 0.095 MPa, respectively; while the gas flow rates were 1512 L/h and 1116 L/h, respectively. The surface temperature of the sample was monitored with an optical pyrometer. The flat-face specimens with a size of 30 mm 30 mm 3.5 mm were mounted in a concave graphite anvil. The distance between the nozzle tip and the surface of the specimen was 10 mm and the inner diameter of the nozzle tip was 2.0 mm. During the test, the ablation gun was ignited firstly. After the flame was steady, the ablation gun was moved vertically to the sample surface. The microstructure of the ablated composite was examined by scanning electron microscopy (SEM, S4800 Hitachi), and the composition was determined by X-ray Diffraction (XRD, Siemens D500). 3. Results and discussion 3.1. Morphology and component The morphologies of the ablated C/ZrC composite and C/SiC composite were shown in Fig. 1. The comparison between Fig. 1a and Fig. 1b showed that there is no significant difference between the morphologies of the C/ZrC composite specimens after ablation for 60 s and 300 s. During ablation, C/ZrC composite exhibited an excellent configurational stability. The color of the ablated surface changed from black into white, and a white, loose ablation layer formed on the surface. XRD analysis indicated that the main component of the white ablation product was monoclinic ZrO2 (mZrO2) (Fig. 2), showing the oxidation of ZrC matrix. The most severe ablation happened in the center of the flame and some ZrO2 melt formed. As a comparison, C/SiC composite was exposed to the oxyacetylene torch in the same condition, and perforated at 55 s (Fig. 1c), gave a linear recession rate of 0.064 mm/s. Concentric circles were observed in C/SiC composite. The results showed that C/ ZrC composite had much better anti-ablation properties than C/SiC composite.
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Fig. 2. XRD pattern of the ablated C/ZrC composite surface.
60–90 s periods. The mass loss rate of C/ZrC composite was 0.012 g/s during first 60 s, then decreased steadily in 60–300 s, and reached the lowest value (0.006 g/s) at 300 s. The linear recession rate of the C/ZrC composite showed a different trend from the mass loss rate. When the C/ZrC composite was ablated for 60 s, its linear recession rate was 0.002 mm/s. The composite showed a negative recession rate because white, loose shell formed on the surface, and apparent measurement found the thickness of the composite increase after ablation test. After 90 s ablation, the linear recession rate increased to 0.014 mm/s because the surface temperature reached near 2300 °C, so ZrO2 melted and was blown all around by the gas flow of oxyacetylene flame, and thus the thickness of the composite decreased. As ZrO2 melt, molten ZrO2 spread on the ablation surface, partially sealed the pores, and acted as an effective barrier against the inward diffusion of oxygen. After 90 s, there was almost no linear ablation, and the ablation front did not progress furthermore, which could be proved by just simple calculation that 90 s 0.014 mm/s 120 s 0.011 mm/s 180 s 0.008 mm/s 300 s 0.004 mm/s. When the ablation time increased to 300 s, the linear recession rate was 0.004 mm/s. Ablation properties of some ultra high temperature materials reported in other works were showed in Table 2. After ablated for 60 s, the 3D C/ZrC composite showed higher mass loss rate and linear recession rate than Cf/UHTC composites fabricated by other processes because it had higher volume percentages of carbon fiber and pores [15–20]. But when ablation time increased, the composite showed a much better anti-ablation property because molten ZrO2 protected the composite. As a comparison, the C/SiC composite showed a mass loss rate of 0.027 g/s and a linear recession rate of 0.064 mm/s, with the linear recession rate one magnitude higher than that of C/ZrC composite because SiC and its ablative product SiO2 had lower boiling temperatures than the flame temperature and sublimated [21].
3.2. Ablation properties 3.3. Microstructure The mass loss rates and linear recession rates of C/ZrC composite and C/SiC composite were shown in Table 1. The mass loss rate of C/ZrC composite decreased with ablation time increase during
The microstructures of the ablation surface were presented in Fig. 3. Three regions may be found on the ablated surface of the
Fig. 1. Morphologies of the ablated composites (a) C/ZrC composite at 60 s, (b) C/ZrC composite at 300 s and (c) C/SiC composite at 55 s.
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Table 1 Ablation properties of the C/ZrC composite and C/SiC composites. Samples
C/ZrC
C/SiC
Ablated time (s)
Surface temperature (°C)
Mass loss rate (g/s)
Linear recession rate (mm/s)
60 90 120 180 300 55
2028 2285 2291 2293 2297 1876
0.012 0.010 0.008 0.007 0.006 0.027
0.002 0.014 0.011 0.008 0.004 0.064
C/ZrC composite: melting region (marked as A), porous region (marked as B) and marginal region (marked as C). Region A in the center of the sample formed from the viscous ZrO2 melt at
3000 °C or above, higher than the melting point of ZrO2 (2800 °C). After the ablation, the sample cooled fast and cracks appeared due to the CTE mismatch of the ablation layer and the composite. The porous region B was around the melting region (Fig. 3a). This region was so small that it could not be clearly observed by the naked eye. In this region, ZrO2 melted, but in small amount, and only partially sealed the pores, so porous layer, instead of continuous melting layer, was developed (Fig. 3b). The marginal region C was looser than the others. In region C carbon fibers were oxidized and disappeared (Fig. 3c), leaving confusing morphology of matrix like carbon fiber bundles. Enlarged parts of the square and circular areas of Fig. 3c showed long slots (Fig. 3d) and ZrC foam (Fig. 3e) due to the burn-out of carbon fiber. The above observation and analysis gave that ZrO2 melt had rather high viscosity and strong adhesion to the composite, and was dif-
Table 2 Ablation properties of ultra high temperature materials reported in other works. Materials
Carbon fiber (%, vol)
ZrC (%)
Oxyacetylene flame test condition
C/C–ZrC
10
As the same as our work
C/SiC–ZrC
34.0
2.50 (wt) 4.14 (wt) 12.1 (wt) 23.5 (vol) 33.3 (vol) –
20.3 ZrC coated C/C
–
As the same as our work
The flow rates of oxygen and acetylene were 1.960 L/s and 0.696 L/s, respectively
Ablation time (s)
Mass loss rate (g/ s)
Linear recession rate (mm/ s)
References
60 60 60 60
0.0011 0.00062 0.00028 0.008
0.0012 0.00051 0.00042 0.008
[15–17]
60
0.006
240
[18]
0.004 2
0.0001 (g/cm )
Fig. 3. Ablated surface morphologies of C/ZrC composite (a–e) and C/SiC (f).
0.0003
[19]
D. Zhao et al. / Composites Science and Technology 71 (2011) 1392–1396
ficult to be blown away by the gas flow, thus prevented the composite from further oxidation, so the composite had excellent anti-ablative properties. The surface morphology of the C/ZrC composite after ablation test was completely different from that of C/SiC composite in that the former was composed mainly of ZrO2 matrix, while the latter was needle-like carbon fiber after ablation (Fig. 3f) [21]. This is because SiC matrix and its oxidation product had low boiling points than the flame temperature. They sublimated under the oxyacetylene flame, and left behind carbon fibers. Instead, ZrO2 melt from ZrC matrix oxidation had a high melting point (2800 °C) and high viscosity at 2000 °C, was difficult to be blown away and left on the surface, while carbon fiber was removed for the oxidation environment. This is the basic reason for the ablation property improvement of C/ZrC composite in oxyacetylene torch test. A loose, tree-coral-like ZrO2 layer without any carbon fiber was observed under the surface ablation layer (see Fig. 4). The rela-
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tively large holes with size of U 10 lm formed because of the burn-out of carbon fiber. Tree-coral-like ZrO2 formed because of the lower temperature and pressure. The surface ablation layer hindered the heat transfer and gas flow, so ZrO2 only melted in small amount, recrystallized, and ejected the micro-pores from the matrix. ZrO2 presented as a loose layer instead of a continuous layer. After the loose ZrO2 layer was peeled from the composite, the undersurface layer was the oxidized composite and integral (Fig. 5). White matrix existed mostly between the fiber tows (Fig. 5a). Carbon fiber paralleled the flow of the flame was needle-like with white matrix adhered on the needlepoint (Fig. 5b), which prevented the oxidative gases from contact with the carbon fiber directly and protected the carbon fiber to a certain extent, thus the lateral surfaces of the fibers were attacked first. The morphology of the fiber bundles that were vertical to the flow of the flame was shown in Fig. 5c. Needle-like carbon fibers were observed with matrix between the fibers, unlike the morphology of the ablated C/SiC composite, because ZrO2 has a high melting point and a high viscosity and is difficult to be blown away. There is also tree-coral-like ZrO2 adhered on the lateral surfaces of the carbon fiber (Fig. 5d). The oxidation of carbon fiber and ZrC matrix might lead to a lot of active points. ZrO2 formed here, grew, and branched. Then branch-like matrix formed on the surface of carbon fibers. 3.4. Ablation mechanism Ablation is a combination of chemical reaction (oxidation mainly), thermal–physical process and mechanical erosion. During the oxyacetylene torch ablation, following reactions occurred.
ZrCðsÞ þ 3=2O2 ðgÞ ¼ ZrO2 ðsÞ þ COðgÞ
ð1Þ
CðsÞ þ 1=2O2 ðgÞ ¼ COðgÞ
ð2Þ
ZrO2 ðsÞ ¼ ZrO2 ðlÞ
ð3Þ
Fig. 4. Morphologies of the tree-coral-like ZrO2 layer.
Fig. 5. Microstructures of the undersurface oxidation layer.
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of the composite, which greatly improved the ablation property of C/ZrC composite. After the ablation, the composite was divided into four layers as the melting layer, the loose tree-coral-like ZrO2 layer, the undersurface oxidation layer and the composite layer. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (Nos. 90816020 and 90916019). Fig. 6. Schematic diagram of the ablated C/ZrC composite.
References The reactions (1) and (2) occurred in all ablation regions, and reaction (3) only happened in high temperature areas, especially in the surface ablation layer of the composite. The ablation process may be divided into two steps. Firstly, oxidation of carbon fiber and ZrC matrix began at relative lower temperature to form a porous structure, and the oxygen diffusion tunnel was the open pores and cracks in the composite. In this step the ablation rate was controlled by the oxidation rate of carbon and ZrC. Secondly, ZrO2 melted while the temperature was raised over 2800 °C. In the center of the sample, high speed gas flow led to ZrO2 melt spread on the ablated surface. In other regions, some of the ZrO2 melt flowed and filled some pores. The ZrO2 melt formation prevented the further oxidation of carbon fibers and ZrC matrix, showing an excellent anti-ablative behavior. Fig. 6 shows the structure of the ablated C/ZrC composite. The ablated material could be divided into four layers due to the temperature and pressure gradients: the surface melting layer, the loose tree-coral-like ZrO2 layer, the undersurface oxidation layer and the composite layer. On the surface of the composite, the temperature was the highest. ZrO2 melted, spread on the ablated surface, partially plugged the pores of the matrix, and protected the composite. Under the melting layer, the temperature and pressure decreased, and carbon fiber was burnt-out. ZrO2 melted with small amount, recrystallized and ejected the pores from the matrix, and present as tree-coral-like layer. In the oxidation layer, the temperature and pressure decreased more significantly, ZrC was oxidized into loose ZrO2. Needle-like carbon fiber formed because the lateral surface of fibers was eroded first. 4. Conclusions Ablation behavior of the C/ZrC composite was investigated by oxyacetylene torch test. The C/ZrC composite showed a mass loss rate of 0.012 g/s and a negative recession rate of 0.002 mm/s when it was ablated for 60 s. As the ablation time increased, the mass loss rate of the C/ZrC composite decreased gradually, while the linear recession rate increased first and decreased later. The mass loss rate and linear recession rate of ablation for 300 s was 0.006 g/s and 0.004 mm/s, respectively. C/ZrC composite showed much better ablation resistance than C/SiC composite because ZrO2 melt on the surface sealed the pores
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