- Email: [email protected]
C composites at high temperature
Journal of Alloys and Compounds 662 (2016) 302e307
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Oxidation and ablation resistance of the ZrB2eCrSi2eSi/SiC coating for C/C composites at high temperature Tao Feng a, b, *, Hejun Li a, Manhong Hu a, Hongjiao Lin a, Lu Li a a b
C/C Composites Technology Research Center, Northwestern Polytechnical University, Xi'an, Shaanxi 710072 PR China School of Mechanics, Civil Engineering & Architecture, Northwestern Polytechnical University, Xi'an, Shaanxi 710072 PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 September 2015 Received in revised form 9 November 2015 Accepted 4 December 2015 Available online 11 December 2015
To protect carbon/carbon (C/C) composites from oxidation and ablation at high temperatures, a ZrB2 eCrSi2eSi/SiC coating was prepared on the surface of C/C composites by two-step pack cementation method. The microstructure, oxidation and ablation resistance of the ZrB2eCrSi2eSi/SiC coating were studied. The results show that the coating could effectively protect the C/C composites from oxidation at 1773 K for 310 h and from ablation at 2573 K for 90 s. The oxidation and ablation resistance of the coating was mainly attributed to the good protection of the compound glass. © 2015 Elsevier B.V. All rights reserved.
Keywords: Car/carbon composites Coating Oxidation Ablation
1. Introduction Carbon/carbon (C/C) composites are suitable for aeronautical and aerospace applications because of their excellent properties at high temperatures, such as high strength and modulus, good thermal shock resistance, low thermal expansion coefficient (CTE) and so on [1,2]. However, the applications of C/C are severely limited due to rapid oxidation of C/C above 673 K in hightemperature oxidizing environment [3,4]. Applying coatings is an effective method to prevent oxidation under such conditions [5]. Recently, owing to outstanding oxidation and ablation resistance, the ultra-high temperature ceramic ZrB2 has been introduced in Si-based ceramic coating to protect C/C composites from oxidation and ablation [6e8]. In our previous research, the existence of ZrB2 in the Si-based ceramic coating can help to form a ZreSieO compound glass at high temperatures, which has better oxidation protective ability at higher temperatures than that of the pure SiO2 glass [9e11]. However, Oxygen can diffuses into the coating through the glass layer and continue to react with the coating. Some defects, such as pores and bubbles are formed in the coating due to the excessive depletion with further oxidation. The cracks especially penetrating cracks can preferentially form along
* Corresponding author. E-mail address: [email protected] (T. Feng). http://dx.doi.org/10.1016/j.jallcom.2015.12.011 0925-8388/© 2015 Elsevier B.V. All rights reserved.
the defects, resulting in the failure of the coating [12,13]. Therefore, a stable vitreous oxide layer with low oxygen permeability in oxidation process becomes important. In previous research, CrSi2 has been applied in silicide coating to form a stabile CreSieO compound glass at high temperatures. This CreSieO compound glass has lower oxygen permeability at high temperatures, which is helpful to improve the oxidation protection ability of the coating [14,15]. Therefore, the existence of both ZrB2 and CrSi2 in the Sibased coating is promising to own an outstanding oxidation and ablation resistance. Up to now, little literature is reported that a coating system can effectively provide good oxidation and ablation protection for C/C composites. In this paper, a ZrB2eCrSi2eSi/SiC coating was prepared on the surface of C/C composites by a two-step pack cementation method. The microstructure, oxidation and ablation resistance of the ZrB2eCrSi2eSi/SiC coating were studied. 2. Experimental 2.1. Preparation of coated C/C composites Cubic specimens (15 mm 15 mm 15 mm) for oxidation test and plate specimens (F 30 mm 10 mm) for ablation test used as substrates were cut from a 2D C/C composites bulk with density of 1.72 g/cm3. All specimens were hand-abraded using 340 grit SiC paper, then cleaned with distilled ethanol and dried at 383 K for 4 h.
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303
The ZrB2eCrSi2eSi/SiC coating was prepared on the surface of C/C composites by two-step pack cementation. The precursor powder of the SiC buffer-layer coating for the first step pack cementation was mixed as follows: Si 60e80 wt.%, graphite 15e25 wt.% and Al2O3 5e15 wt.%. Then the C/C specimens and mixtures were put into a graphite crucible and heat-treated at 1873e2073 K for 2 h in argon to produce the buffer-layer SiC coating. The precursor powder for the second step pack cementation was mixed as follows: ZrB2 10e20 wt.%, Si 55e75 wt.%, Cr 5e15 wt.%, graphite 5e10 wt.% and some additives. The as-prepared SiC coated specimens and the second step mixtures were put in a graphite crucible, and then were heat-treated in argon at 2073e2273 K for 2 h.
2.2. Oxidation test The oxidation test was carried out in air at 1773 K. The cubic specimens were put directly into isothermal zone at 1773 K, whereafter they were taken out of isothermal zone and cooled to room temperature. Mass of the specimens was measured by an analytical balance (Sartorious CP224S), and then they were put into isothermal zone again for the next oxidation period. Cumulative mass change percentages (DM%) of the specimens were calculated by the following equation
DM% ¼
m1 m0 100% m0
(1)
where m0 and m1 are the mass of the specimens before oxidation and after oxidation, respectively.
Fig. 2. Cross-section backscattering electron image of the coating.
DRm
m0 m1 t
DR1 ¼
d0 d1 t
(2)
(3)
where t is the ablation time; m0 and m1 are the mass of the specimens before and after ablation, respectively; d0 and d1 are the thickness of the specimens before and after ablation, respectively. The thickness of the coated specimens was measured by a micrometre. Mass of the specimens was measured by the analytical balance (Sartorious CP224S).
2.3. Ablation test
2.4. Characterization
The ablation test was carried out in oxyacetylene torch. Plate specimens were installed in water cooled copper concave device and the axial orientation of the plate specimens was parallel to the oxyacetylene torch flame. The inner diameter of the oxyacetylene gun tip was 2 mm, and the distance between the gun tip and the surface of the specimens was 10 mm. The pressure and flux of C2H2 were 0.095 MPa and 0.4 L/S, and those of O2 were 0.4 MPa and 0.4 L/ S, respectively. The heat flux on the surface of the specimens received from the flame was 4.2 MW/m2. The temperature of the oxyacetylene torch flame was above 2573 K [9]. The linear ablation (DRl) and mass ablation rates (DRm) of the coated specimens were calculated by the following equation
The crystalline structure of the double-layer coating was measured with X-ray diffraction (XRD, X'Pert Pro MPD). The morphology and the element distribution of the double-layer coating were analysed by scanning electron microscope (SEM, JSM6460) and energy dispersive spectroscopy (EDS). 3. Results and discussion 3.1. Microstructure of the coating Fig. 1 shows the SEM image and X-ray pattern of the ZrB2eCrSi2eSi/SiC coating surface obtained by the two-step pack
Fig. 1. Backscattering electron image and X-ray pattern of the coating surface: (a) backscattering electron image; (b) X-ray pattern.
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Plentiful interfaces can release the thermal stress and decrease the cracking frequency of the coating [16]. Fig. 2 shows a backscattering electron image along the crosssection of the coating. It can be seen that the mixture including ZrB2 and CrSi2 are distributed among the SiC particles. During the second step pack cementation, the ZrB2 and Cr pack powders can easily penetrate into the SiC coating with the flow of the melting Si. Meanwhile, Cr can react with Si to generate CrSi2. In addition, the average thickness of the double-layer coating is about 200 mm. No obvious gaps or penetrating cracks are found, due to good combination between layer-to-layer and coating-to-substrate. 3.2. Oxidation resistance of the coated C/C composites
Fig. 3. Isothermal oxidation curve of the coated C/C specimens in air at 1773 K.
cementation. Clearly, the coating (Fig. 1(a)) is dense and no visible cracks can be detected. The corresponding XRD pattern (Fig. 1(b)) shows that the double-layer coating is composed of ZrB2, CrSi2, SiC and Si, respectively. Moreover, there are three kinds of particles in white, dark grey and grey colour, respectively. According to EDS and XRD analysis, the white is verified as a mixture including ZrB2 and CrSi2. Dark grey and grey are SiC and Si, respectively. Moreover, these different phases can form plentiful interfaces in the coating.
The oxidation curve of the coated C/C composites at 1773 K in air is shown in Fig. 3. It is clear that the ZrB2eCrSi2eSi/SiC coating can provide good oxidation protection for C/C composites. The mass loss of the coated specimens is 0.1% after oxidation at 1773 K for 310 h. The protective time is longer than that of the ZrB2eSiCeSi/Bmodified SiC coating (at 1773 K for 150 h) by Feng [9] and that of the ZrB2eSiC coating (at 1773 K for 216 h) by Li et al. [12]. A smooth and dense glass layer can be found on the coating surface after oxidation at 1773 K for 310 h, as shown in Fig. 4(a). In addition, some microcracks can also be found on the surface of the glass layer. These microcracks are generated in the stage of quick cooling when the specimens are taken out of the isothermal zone for weighing, and can be sealed by the glass when the coating is heated again. According to the XRD pattern of the coating after oxidation (Fig. 4(b)), the glass is composed of SiO2, ZrSiO4 and Cr2O3
Fig. 4. SEM image and X-ray pattern of the surface of the coating after isothermal oxidation at 1773 K: (a) SEM image; (b) X-ray pattern.
Fig. 5. Cross-section electron images of the coating after isothermal oxidation at 1773 K: (a) SEM image; (b) backscattering electron image.
T. Feng et al. / Journal of Alloys and Compounds 662 (2016) 302e307 Table 1 Ablation properties of the coated specimens. Ablation time (s)
Linear ablation rate (mm/s)
Mass ablation rate (mg/s)
30 60 90
0.34 1.23 1.64
0.023 0.35 0.56
obviously. Therefore, the glass is a ZreCreSieO compound glass to provide multiple oxidation protection. The ZrSiO4 dispersants can improve the stability of SiO2 glass at high temperatures and the Cr2O3 dispersants can lower the oxygen diffusion in the glass [11,15], which can improve the oxidation resistance of the coating. In addition, the glass layer covers the whole coating surface. ZrB2 and CrSi2 on the coating surface have been exhausted to form the ZreCreSieO compound glass, resulting in that ZrB2 and CrSi2 phase cannot be measured. During oxidation test, the coating materials will react with oxygen as follows:
305
2ZrB2 ðsÞ þ 5O2 ðgÞ/2ZrO2 ðsÞ þ 2B2 O3 ðgÞ
(4)
4CrSi2 ðsÞ þ 11O2 ðgÞ/2Cr2 O3 ðsÞ þ 8SiO2 ðsÞ
(5)
SiCðsÞ þ 2O2 ðgÞ/SiO2 ðsÞ þ CO2 ðgÞ
(6)
SiðlÞ þ O2 ðgÞ/SiO2 ðsÞ
(7)
According to the above mentioned reactions, the gaseous CO2 and B2O3 are generated. Moreover, Cr2O3 tends to transform volatilisable CrO3 above 1273 K [13,17]. Therefore, the formation of the volatilisable productions such as CO2, B2O3 and CrO3 results in the depletion of the coating materials. As shown in Fig. 5(a), the thickness of the coating has no obvious change compared to that of the virgin specimens (Fig. 2), which can be inferred that the depletion of the coating is slight due to the protection of the compound glass layer. Moreover, no obvious oxidation mark can be found, which can be inferred that the depletion of the coating
Fig. 6. Backscattering images of the coating surface after ablation for different ablative time: (a1, a2) 30 s: (a1) in ablation center region and (a2) in ablation transition region; (b1, b2) 60 s: (b1) in ablation center region and (b2) in ablation transition region; (c1, c2) 90 s: (c1) in ablation center region and (c2) in ablation transition region.
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T. Feng et al. / Journal of Alloys and Compounds 662 (2016) 302e307
Fig. 7. X-ray pattern of the coating surface after ablation in the ablation center region after different ablation time.
materials results in the mass loss of the coated specimens. No debonding or large cracks are detected, which indicates high bonding strength and good thermal compatibility between the coating and C/C substrate. As shown in Fig. 5(b), it is clear that three kinds of components in white, grey and dark grey colour can be observed significantly, which can also indicate that the depletion of the coating materials is slight. In previous research [12,18], as oxygen can diffuse through the ZreSieO (ZrSiO4 and SiO2) compound glass to react with coating materials and generate some volatilisable productions, some pores and bubbles are inevitably generated in the ZrB2-based coating during oxidation. The cracks especially penetrating cracks can form along the pores and bubbles, resulting in the oxidation of C/C substrate. On the contrary, no pores and bubbles can be detected in the ZrB2eCrSi2eSi/SiC coating under the protection of the ZreCreSieO compound glass, as show in Fig. 5(a and b). Therefore, the ZreCreSieO compound glass can effectively inhibit oxygen from diffusing into the coating and protect the coating materials from excessive depletion at high temperatures to avoid the crack nucleation in the coating. 3.3. Ablation resistance of the coated C/C composites Table 1 shows ablation properties of the coated specimens for different ablative time. It is clear that the coating exhibits good ablation resistance at 2573 K. The linear ablation and mass ablation rates of the coated specimens is only 1.64 mm/s and 0.56 mg/s after ablation for 90 s Fig. 6 shows SEM images of the coated specimens in the ablation center region and ablation transition region for 30 s, 60 s and 90 s. This protective time at 2573 K is longer than that of the Fe2O3 modified ZrB2eSiCeSi coating (60 s) by Feng [18], the SiC nanowire toughened ZrB2eSiC coating (60 s) by Zhang [19], the ZrB2eSiC coating (30 s) by Wang [20] and the ZrB2eSiC coating (60 s) by Yao [21]. As shown in Fig. 6(a1, b1 and c1), the crystalline particles become smaller gradually with extending ablative time and the molten ablative production covers the whole coating surface. Meanwhile, the number and dimension of the pores and bubbles become larger significantly as the ablative time progresses, which can be inferred that the depletion of coating materials is more serious than that after oxidation at 1773 K for 310 h. Some micro-cracks are generated in the rapid cooling stage from high temperature to room temperature. According to the corresponding
XRD patterns as shown in Fig. 7, it is clear that the ablative production is composed of SiO2 and ZrO2. In addition, ZrB2 and CrSi2 are consumed gradually with extending ablative time. The pores and bubbles are formed due to the volatilization of the ablation products such as CO2, B2O3 and CrO3. These volatilisible ablation productions can absorb a large amount of heat to reduce the temperature of the coating surface, which can decrease the erosion from the oxyacetylene ablation. In addition, there are two kinds of phases in white and grey colour after ablation. By XRD and EDS analysis, the white phase is ZrO2 and the grey is SiO2. The white phase becomes larger as the ablative time progresses, which can be inferred that the coating materials are consumed gradually. After ablation for 90 s, C/C substrate is not exposed in the ablation center region. Moreover, no pits or grooves can be observed which can be inferred that the coating is dense and can withstand the mechanical denudation from the flame due to high bonding strength and good thermal compatibility between the coating and C/C substrate. Since that the coating suffers lower temperature, pressure and flow rate in the ablation transition region because of the deviation from oxyacetylene flame, the content of the white phase in the ablation transition region (Fig. 6(a2, b2 and c2)) is less significantly, compared with that in ablation center region. The ablation morphology in the ablation transition region is similar to the oxidation morphology (Fig. 4(a)). A glass layer can be found significantly, which can effectively inhibit oxygen from diffusing into the coating. Therefore, the depletion of the coating materials in the ablation transition region are slight, compared with that in the ablation center region. 4. Conclusions A ZrB2eCrSi2eSi/SiC coating has been prepared on the surface of C/C composites by two-step pack cementation method. The coating can effectively protect the C/C composites from oxidation at 1773 K for 310 h and from ablation at 2573 K for 90 s. The oxidation and ablation resistance of the coating is mainly attributed to the good protection of the compound glass. Acknowledgements This work has been supported by the National Natural Science Foundation of China under Grant No. 51402238, the “111” Project under Grant No. B08040, and the Fundamental Research Funds for the Central Universities No. 3102015ZY034. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
N.S. Jacobson, D.M. Curry, Carbon 44 (2006) 1142e1150. F.J. Buchanan, J.A. Little, Corros. Sci. 35 (1993) 1243e1250. A.V.K. Westwood, B. Rand, S. Lu, Carbon 42 (2004) 3071e3080. X.M. Hou, K.C. Chou, Corros. Sci. 52 (2010) 1093e1097. E. Corral, R. Loehman, J. Am. Ceram. Soc. 91 (2008) 1495e1502. H.J. Zhou, L. Gao, Z. Wang, S.M. Dong, J. Am. Ceram. Soc. 93 (2010) 915e919. X.H. Zhang, P. Hu, J.C. Han, S.H. Meng, Comp. Sci. Technol. 68 (2008) 1718e1726. Y.D. Blum, J. Marschall, D. Hui, S. Yound, J. Am. Ceram. Soc. 91 (2008) 1453e1460. T. Feng, H.J. Li, X.H. Shi, X. Yang, S.L. Wang, Corros. Sci. 67 (2013) 292e297. X.Y. Yao, H.J. Li, Y.L. Zhang, H. Wu, X.F. Qiang, Ceram. Int. 38 (2012) 2095e2100. X.R. Ren, H.J. Li, Y.H. Chu, Q.F. Fu, K.Z. Li, Surf. Coat. Technol. 247 (2014) 61e67. L. Li, H.J. Li, X.M. Yin, Y.H. Chu, X. Chen, Q.G. Fu, J. Alloy. Compd. 645 (2015) 164e170. T. Feng, H.J. Li, Q.G. Fu, X. Yang, H. Wu, Corros. Sci. 53 (2011) 4102e4108. T. Feng, H.J. Li, Q.G. Fu, X. Yang, H. Wu, Carbon 50 (2012) 2171e2178. T. Feng, H.J. Li, Q.G. Fu, X.H. Shi, M.H. Hu, L. Liu, J. Alloys Compd. 622 (2015) 1049e1054. H.J. Li, H. Xue, Y.J. Wang, Q.G. Fu, D.J. Yao, Surf. Coat. Technol. 201 (2007) 9444e9447. A. Tomasi, R. Ceccato, M. Nazmy, S. Gialanella, Mater. Sci. Eng. A 239e240
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[20] D.N. Wang, Y. Zeng, X. Xiong, G.D. Li, Z.K. Chen, W. Sun, Y.L. Wang, Ceram. Int. 41 (2015) 7677e7686. [21] X.Y. Yao, H.J. Li, Y.L. Zhang, K.Z. Li, Q.G. Fu, H. Peng, J. Therm. Spray. Technol. 22 (2013) 531e537.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Oxidation and ablation resistance of the ZrB2eCrSi2eSi/SiC coating for C/C composites at high temperature Tao Feng a, b, *, Hejun Li a, Manhong Hu a, Hongjiao Lin a, Lu Li a a b
C/C Composites Technology Research Center, Northwestern Polytechnical University, Xi'an, Shaanxi 710072 PR China School of Mechanics, Civil Engineering & Architecture, Northwestern Polytechnical University, Xi'an, Shaanxi 710072 PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 September 2015 Received in revised form 9 November 2015 Accepted 4 December 2015 Available online 11 December 2015
To protect carbon/carbon (C/C) composites from oxidation and ablation at high temperatures, a ZrB2 eCrSi2eSi/SiC coating was prepared on the surface of C/C composites by two-step pack cementation method. The microstructure, oxidation and ablation resistance of the ZrB2eCrSi2eSi/SiC coating were studied. The results show that the coating could effectively protect the C/C composites from oxidation at 1773 K for 310 h and from ablation at 2573 K for 90 s. The oxidation and ablation resistance of the coating was mainly attributed to the good protection of the compound glass. © 2015 Elsevier B.V. All rights reserved.
Keywords: Car/carbon composites Coating Oxidation Ablation
1. Introduction Carbon/carbon (C/C) composites are suitable for aeronautical and aerospace applications because of their excellent properties at high temperatures, such as high strength and modulus, good thermal shock resistance, low thermal expansion coefficient (CTE) and so on [1,2]. However, the applications of C/C are severely limited due to rapid oxidation of C/C above 673 K in hightemperature oxidizing environment [3,4]. Applying coatings is an effective method to prevent oxidation under such conditions [5]. Recently, owing to outstanding oxidation and ablation resistance, the ultra-high temperature ceramic ZrB2 has been introduced in Si-based ceramic coating to protect C/C composites from oxidation and ablation [6e8]. In our previous research, the existence of ZrB2 in the Si-based ceramic coating can help to form a ZreSieO compound glass at high temperatures, which has better oxidation protective ability at higher temperatures than that of the pure SiO2 glass [9e11]. However, Oxygen can diffuses into the coating through the glass layer and continue to react with the coating. Some defects, such as pores and bubbles are formed in the coating due to the excessive depletion with further oxidation. The cracks especially penetrating cracks can preferentially form along
* Corresponding author. E-mail address: [email protected] (T. Feng). http://dx.doi.org/10.1016/j.jallcom.2015.12.011 0925-8388/© 2015 Elsevier B.V. All rights reserved.
the defects, resulting in the failure of the coating [12,13]. Therefore, a stable vitreous oxide layer with low oxygen permeability in oxidation process becomes important. In previous research, CrSi2 has been applied in silicide coating to form a stabile CreSieO compound glass at high temperatures. This CreSieO compound glass has lower oxygen permeability at high temperatures, which is helpful to improve the oxidation protection ability of the coating [14,15]. Therefore, the existence of both ZrB2 and CrSi2 in the Sibased coating is promising to own an outstanding oxidation and ablation resistance. Up to now, little literature is reported that a coating system can effectively provide good oxidation and ablation protection for C/C composites. In this paper, a ZrB2eCrSi2eSi/SiC coating was prepared on the surface of C/C composites by a two-step pack cementation method. The microstructure, oxidation and ablation resistance of the ZrB2eCrSi2eSi/SiC coating were studied. 2. Experimental 2.1. Preparation of coated C/C composites Cubic specimens (15 mm 15 mm 15 mm) for oxidation test and plate specimens (F 30 mm 10 mm) for ablation test used as substrates were cut from a 2D C/C composites bulk with density of 1.72 g/cm3. All specimens were hand-abraded using 340 grit SiC paper, then cleaned with distilled ethanol and dried at 383 K for 4 h.
T. Feng et al. / Journal of Alloys and Compounds 662 (2016) 302e307
303
The ZrB2eCrSi2eSi/SiC coating was prepared on the surface of C/C composites by two-step pack cementation. The precursor powder of the SiC buffer-layer coating for the first step pack cementation was mixed as follows: Si 60e80 wt.%, graphite 15e25 wt.% and Al2O3 5e15 wt.%. Then the C/C specimens and mixtures were put into a graphite crucible and heat-treated at 1873e2073 K for 2 h in argon to produce the buffer-layer SiC coating. The precursor powder for the second step pack cementation was mixed as follows: ZrB2 10e20 wt.%, Si 55e75 wt.%, Cr 5e15 wt.%, graphite 5e10 wt.% and some additives. The as-prepared SiC coated specimens and the second step mixtures were put in a graphite crucible, and then were heat-treated in argon at 2073e2273 K for 2 h.
2.2. Oxidation test The oxidation test was carried out in air at 1773 K. The cubic specimens were put directly into isothermal zone at 1773 K, whereafter they were taken out of isothermal zone and cooled to room temperature. Mass of the specimens was measured by an analytical balance (Sartorious CP224S), and then they were put into isothermal zone again for the next oxidation period. Cumulative mass change percentages (DM%) of the specimens were calculated by the following equation
DM% ¼
m1 m0 100% m0
(1)
where m0 and m1 are the mass of the specimens before oxidation and after oxidation, respectively.
Fig. 2. Cross-section backscattering electron image of the coating.
DRm
m0 m1 t
DR1 ¼
d0 d1 t
(2)
(3)
where t is the ablation time; m0 and m1 are the mass of the specimens before and after ablation, respectively; d0 and d1 are the thickness of the specimens before and after ablation, respectively. The thickness of the coated specimens was measured by a micrometre. Mass of the specimens was measured by the analytical balance (Sartorious CP224S).
2.3. Ablation test
2.4. Characterization
The ablation test was carried out in oxyacetylene torch. Plate specimens were installed in water cooled copper concave device and the axial orientation of the plate specimens was parallel to the oxyacetylene torch flame. The inner diameter of the oxyacetylene gun tip was 2 mm, and the distance between the gun tip and the surface of the specimens was 10 mm. The pressure and flux of C2H2 were 0.095 MPa and 0.4 L/S, and those of O2 were 0.4 MPa and 0.4 L/ S, respectively. The heat flux on the surface of the specimens received from the flame was 4.2 MW/m2. The temperature of the oxyacetylene torch flame was above 2573 K [9]. The linear ablation (DRl) and mass ablation rates (DRm) of the coated specimens were calculated by the following equation
The crystalline structure of the double-layer coating was measured with X-ray diffraction (XRD, X'Pert Pro MPD). The morphology and the element distribution of the double-layer coating were analysed by scanning electron microscope (SEM, JSM6460) and energy dispersive spectroscopy (EDS). 3. Results and discussion 3.1. Microstructure of the coating Fig. 1 shows the SEM image and X-ray pattern of the ZrB2eCrSi2eSi/SiC coating surface obtained by the two-step pack
Fig. 1. Backscattering electron image and X-ray pattern of the coating surface: (a) backscattering electron image; (b) X-ray pattern.
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Plentiful interfaces can release the thermal stress and decrease the cracking frequency of the coating [16]. Fig. 2 shows a backscattering electron image along the crosssection of the coating. It can be seen that the mixture including ZrB2 and CrSi2 are distributed among the SiC particles. During the second step pack cementation, the ZrB2 and Cr pack powders can easily penetrate into the SiC coating with the flow of the melting Si. Meanwhile, Cr can react with Si to generate CrSi2. In addition, the average thickness of the double-layer coating is about 200 mm. No obvious gaps or penetrating cracks are found, due to good combination between layer-to-layer and coating-to-substrate. 3.2. Oxidation resistance of the coated C/C composites
Fig. 3. Isothermal oxidation curve of the coated C/C specimens in air at 1773 K.
cementation. Clearly, the coating (Fig. 1(a)) is dense and no visible cracks can be detected. The corresponding XRD pattern (Fig. 1(b)) shows that the double-layer coating is composed of ZrB2, CrSi2, SiC and Si, respectively. Moreover, there are three kinds of particles in white, dark grey and grey colour, respectively. According to EDS and XRD analysis, the white is verified as a mixture including ZrB2 and CrSi2. Dark grey and grey are SiC and Si, respectively. Moreover, these different phases can form plentiful interfaces in the coating.
The oxidation curve of the coated C/C composites at 1773 K in air is shown in Fig. 3. It is clear that the ZrB2eCrSi2eSi/SiC coating can provide good oxidation protection for C/C composites. The mass loss of the coated specimens is 0.1% after oxidation at 1773 K for 310 h. The protective time is longer than that of the ZrB2eSiCeSi/Bmodified SiC coating (at 1773 K for 150 h) by Feng [9] and that of the ZrB2eSiC coating (at 1773 K for 216 h) by Li et al. [12]. A smooth and dense glass layer can be found on the coating surface after oxidation at 1773 K for 310 h, as shown in Fig. 4(a). In addition, some microcracks can also be found on the surface of the glass layer. These microcracks are generated in the stage of quick cooling when the specimens are taken out of the isothermal zone for weighing, and can be sealed by the glass when the coating is heated again. According to the XRD pattern of the coating after oxidation (Fig. 4(b)), the glass is composed of SiO2, ZrSiO4 and Cr2O3
Fig. 4. SEM image and X-ray pattern of the surface of the coating after isothermal oxidation at 1773 K: (a) SEM image; (b) X-ray pattern.
Fig. 5. Cross-section electron images of the coating after isothermal oxidation at 1773 K: (a) SEM image; (b) backscattering electron image.
T. Feng et al. / Journal of Alloys and Compounds 662 (2016) 302e307 Table 1 Ablation properties of the coated specimens. Ablation time (s)
Linear ablation rate (mm/s)
Mass ablation rate (mg/s)
30 60 90
0.34 1.23 1.64
0.023 0.35 0.56
obviously. Therefore, the glass is a ZreCreSieO compound glass to provide multiple oxidation protection. The ZrSiO4 dispersants can improve the stability of SiO2 glass at high temperatures and the Cr2O3 dispersants can lower the oxygen diffusion in the glass [11,15], which can improve the oxidation resistance of the coating. In addition, the glass layer covers the whole coating surface. ZrB2 and CrSi2 on the coating surface have been exhausted to form the ZreCreSieO compound glass, resulting in that ZrB2 and CrSi2 phase cannot be measured. During oxidation test, the coating materials will react with oxygen as follows:
305
2ZrB2 ðsÞ þ 5O2 ðgÞ/2ZrO2 ðsÞ þ 2B2 O3 ðgÞ
(4)
4CrSi2 ðsÞ þ 11O2 ðgÞ/2Cr2 O3 ðsÞ þ 8SiO2 ðsÞ
(5)
SiCðsÞ þ 2O2 ðgÞ/SiO2 ðsÞ þ CO2 ðgÞ
(6)
SiðlÞ þ O2 ðgÞ/SiO2 ðsÞ
(7)
According to the above mentioned reactions, the gaseous CO2 and B2O3 are generated. Moreover, Cr2O3 tends to transform volatilisable CrO3 above 1273 K [13,17]. Therefore, the formation of the volatilisable productions such as CO2, B2O3 and CrO3 results in the depletion of the coating materials. As shown in Fig. 5(a), the thickness of the coating has no obvious change compared to that of the virgin specimens (Fig. 2), which can be inferred that the depletion of the coating is slight due to the protection of the compound glass layer. Moreover, no obvious oxidation mark can be found, which can be inferred that the depletion of the coating
Fig. 6. Backscattering images of the coating surface after ablation for different ablative time: (a1, a2) 30 s: (a1) in ablation center region and (a2) in ablation transition region; (b1, b2) 60 s: (b1) in ablation center region and (b2) in ablation transition region; (c1, c2) 90 s: (c1) in ablation center region and (c2) in ablation transition region.
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Fig. 7. X-ray pattern of the coating surface after ablation in the ablation center region after different ablation time.
materials results in the mass loss of the coated specimens. No debonding or large cracks are detected, which indicates high bonding strength and good thermal compatibility between the coating and C/C substrate. As shown in Fig. 5(b), it is clear that three kinds of components in white, grey and dark grey colour can be observed significantly, which can also indicate that the depletion of the coating materials is slight. In previous research [12,18], as oxygen can diffuse through the ZreSieO (ZrSiO4 and SiO2) compound glass to react with coating materials and generate some volatilisable productions, some pores and bubbles are inevitably generated in the ZrB2-based coating during oxidation. The cracks especially penetrating cracks can form along the pores and bubbles, resulting in the oxidation of C/C substrate. On the contrary, no pores and bubbles can be detected in the ZrB2eCrSi2eSi/SiC coating under the protection of the ZreCreSieO compound glass, as show in Fig. 5(a and b). Therefore, the ZreCreSieO compound glass can effectively inhibit oxygen from diffusing into the coating and protect the coating materials from excessive depletion at high temperatures to avoid the crack nucleation in the coating. 3.3. Ablation resistance of the coated C/C composites Table 1 shows ablation properties of the coated specimens for different ablative time. It is clear that the coating exhibits good ablation resistance at 2573 K. The linear ablation and mass ablation rates of the coated specimens is only 1.64 mm/s and 0.56 mg/s after ablation for 90 s Fig. 6 shows SEM images of the coated specimens in the ablation center region and ablation transition region for 30 s, 60 s and 90 s. This protective time at 2573 K is longer than that of the Fe2O3 modified ZrB2eSiCeSi coating (60 s) by Feng [18], the SiC nanowire toughened ZrB2eSiC coating (60 s) by Zhang [19], the ZrB2eSiC coating (30 s) by Wang [20] and the ZrB2eSiC coating (60 s) by Yao [21]. As shown in Fig. 6(a1, b1 and c1), the crystalline particles become smaller gradually with extending ablative time and the molten ablative production covers the whole coating surface. Meanwhile, the number and dimension of the pores and bubbles become larger significantly as the ablative time progresses, which can be inferred that the depletion of coating materials is more serious than that after oxidation at 1773 K for 310 h. Some micro-cracks are generated in the rapid cooling stage from high temperature to room temperature. According to the corresponding
XRD patterns as shown in Fig. 7, it is clear that the ablative production is composed of SiO2 and ZrO2. In addition, ZrB2 and CrSi2 are consumed gradually with extending ablative time. The pores and bubbles are formed due to the volatilization of the ablation products such as CO2, B2O3 and CrO3. These volatilisible ablation productions can absorb a large amount of heat to reduce the temperature of the coating surface, which can decrease the erosion from the oxyacetylene ablation. In addition, there are two kinds of phases in white and grey colour after ablation. By XRD and EDS analysis, the white phase is ZrO2 and the grey is SiO2. The white phase becomes larger as the ablative time progresses, which can be inferred that the coating materials are consumed gradually. After ablation for 90 s, C/C substrate is not exposed in the ablation center region. Moreover, no pits or grooves can be observed which can be inferred that the coating is dense and can withstand the mechanical denudation from the flame due to high bonding strength and good thermal compatibility between the coating and C/C substrate. Since that the coating suffers lower temperature, pressure and flow rate in the ablation transition region because of the deviation from oxyacetylene flame, the content of the white phase in the ablation transition region (Fig. 6(a2, b2 and c2)) is less significantly, compared with that in ablation center region. The ablation morphology in the ablation transition region is similar to the oxidation morphology (Fig. 4(a)). A glass layer can be found significantly, which can effectively inhibit oxygen from diffusing into the coating. Therefore, the depletion of the coating materials in the ablation transition region are slight, compared with that in the ablation center region. 4. Conclusions A ZrB2eCrSi2eSi/SiC coating has been prepared on the surface of C/C composites by two-step pack cementation method. The coating can effectively protect the C/C composites from oxidation at 1773 K for 310 h and from ablation at 2573 K for 90 s. The oxidation and ablation resistance of the coating is mainly attributed to the good protection of the compound glass. Acknowledgements This work has been supported by the National Natural Science Foundation of China under Grant No. 51402238, the “111” Project under Grant No. B08040, and the Fundamental Research Funds for the Central Universities No. 3102015ZY034. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
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