- Email: [email protected]
carbon composites
Journal of the European Ceramic Society 35 (2015) 3789–3796
Contents lists available at www.sciencedirect.com
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Microstructure and oxidation behavior of sol–gel mullite coating on SiC-coated carbon/carbon composites Wen ZhongLiu, Xiao Peng ∗ , Li Zhuan, Hong Wen, Luo Heng, Yu XiaoYu, Li Yang, Chen WenBo State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, PR China
a r t i c l e
i n f o
Article history: Received 15 May 2015 Received in revised form 25 June 2015 Accepted 27 June 2015 Available online 26 July 2015 Keywords: Sol–gel coating Mullite coating Carbon/carbon composites Oxidation Airbrush spraying
a b s t r a c t A dense, uniform, crack-free and well-bonded sol–gel mullite coating with thickness of about 178 m was successfully fabricated on SiC-coated carbon/carbon (C/C) composites. Mullite coating is produced by airbrush spraying a 3/2-mullite coating precursor solution consisting of fine mullite powder dispersed in a mullite precursor sol and air sintering the as-deposited coating. This method has several benefits such as simplicity of the process, high efficiency, ability to coat complex geometries and cost-effectiveness. The mullite/SiC coating was found to protect C/C composites from oxidation at 1773 K for 100 h with a weight loss rate of only 6.85 × 10−5 g cm−2 h−1 and retained compression strength of 97.44%. The oxidation rate was found to be significantly decreased with oxidation time, possibly due to the densification of mullite coating during the oxidation process. The corresponding high temperature oxidation activation energy of mullite/SiC coated C/C in the temperature range of 1573–1773 K is calculated to be 110.63 kJ/mol. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Carbon/carbon (C/C) composites are considered to be one of the most promising high temperature structural materials due to their low density, high strength-to-weight ratio, good thermal shock resistance and good mechanical properties at elevated temperatures. However, the usefulness of C/C composites is limited in oxidizing environments due to low oxidation start temperature which can be as low as 723 K [1]. Application of coatings, especially multilayer ceramic coatings, to C/C composites is considered to be the most effective way to protect C/C in high temperature oxidizing environments [2–4]. The SiC ceramic coating is usually considered to be one of the best bonding layers for use between the C/C composite and outer ceramic coating layer [5,6]. Among a variety of external coating materials, mullite (3Al2 O3 ·2SiO2 ) ceramic has attracted the most interest because of its excellent heat resistance, good thermal expansion coefficient (CTE) match and chemical compatibility with SiC [7–10]. Various methods have been developed to prepare the mullite coating in the past years [11–14]. However, most of these technologies are either too expensive or are unable to coat complex substrate shapes.
∗ Corresponding author. Fax: +86 73188830131. E-mail address: [email protected] (X. Peng). http://dx.doi.org/10.1016/j.jeurceramsoc.2015.06.033 0955-2219/© 2015 Elsevier Ltd. All rights reserved.
The sol–gel method for producing coatings is a well-known process. It has various advantages such as low processing temperatures, cost-effectiveness, and ability to coat complex substrate geometries. It has also been demonstrated to have a commercial scale-up potential [15,16]. This method offers better control over particle size and morphology of the starting precursor resulting in enhanced sintering ability at low temperatures. This in turn imparts superior mechanical properties to the mullite prepared in this way [17–19]. However, coatings prepared by conventional sol–gel processes suffer limitations due to the presence of cracks, porosity and high internal stresses leading to adhesion failures. Additionally, the thickness of the coatings produced by this method is limited to a few microns. Fortunately, researchers have shown that introduction of fine ceramic powders to form a hybrid coating precursor sol helps to overcome the coating thickness limitations [20,21]. The overall performance of the coating in protecting against oxidation depends mainly on the application process for coating precursor and subsequent sintering procedure. Usually, the sol–gel coatings are applied via dip-coating and spin-coating method. One of the major problems of dip and spincoating methods is that they always introduce non-uniformity in coating thickness near the edges of the substrates [22]. Moreover, in order to obtain reasonable coating thickness, multiple cycles of precursor application-and-sintering are required. Airbrush spraying of ceramic slurries has been successfully used as a low cost processing technique instead of dip or spin method. It helps to
3790
W. ZhongLiu et al. / Journal of the European Ceramic Society 35 (2015) 3789–3796
Fig. 1. Schematic diagram showing the mullite coating preparation process.
form coatings on complex substrate shapes with adequate control over coating thicknesses and provides high deposition rates [23,24]. More importantly, it has a high potential for industrial mass fabrication. However, there is a paucity of studies on its use in the field of sol–gel coating deposition. To the best of the authors’ knowledge, the use of airbrush spraying a mullite precursor sol to obtain a thick mullite coating for improved oxidation resistance has not been documented till date. Therefore, the present article represents a pilot study on the subject. In this article, we present a new sol–gel based mullite coating process, which provides a two-fold advantage: (1) a higher upper-coating thickness, similar to the hybrid precursor sol, and (2) better control over the coating thickness uniformity provided by the airbrush spraying method. For this study, a hybrid mullite precursor coating solution was prepared and applied on a SiC coated C/C substrate by the airbrush spraying process. The phase compositions and microstructures of the as-prepared mullite/SiC coating were then characterized. Oxidation resistance, thermalshock properties, mechanical properties including micro-hardness of the coating, and oxidation kinetics of the dual layer coated C/C composites were investigated. 2. Experimental procedures 2.1. Preparation of the mullite coating precursor solution All the materials used for mullite precursor synthesis were of the AR grade. The mullite coating precursor solution was prepared by mixing a mullite precursor sol with mullite powder, along with other ingredients. The processing steps for synthesizing mullite precursor sol are shown in Fig. 1. Tetraethoxysilane (TEOS) was mixed with aluminum nitrate nonahydrate (ANN) and aluminum acetate (2:5 molar ratio of ANN to aluminum acetate) in pure ethanol medium in the stoichiometric ratio of mullite. Acetic acid was added (0.1–0.2 mol/L) to promote gelation. After 6 h of refluxing at 353 K, a milky, viscous monophasic sol was obtained. The mullite precursor sol and 3/2-mullite powder (70–75 wt.% of the
Fig. 2. Surface XRD spectra of the as-prepared mullite coating on SiC-C/C.
total mixture) were mixed by high energy planetary ball-milling at 300 rpm, with a crack-sealing agent (polyvinylpyrrolidon, PVP, average molecular weight of 1.3 × 106 g/mol, about 0.1–0.25 wt.%) and oxalic acid (3–5 wt.%) as drying control chemical additive (DCCA). Ball milling was performed for 4 h after the addition of all the ingredient and, at the end of the ball-milling process, acetone was added to reach an optimized solution viscosity. The solid concentration in the optimized mullite coating precursor solution was about 20 wt%. The mullite powder was prepared by a sol–gel process and ANN and TEOS were used as the starting materials. In short, the source chemicals were dissolved in absolute ethanol in the molar ratio of 3:1 of ANN: TEOS (3/2-mullite) after which acetic acid was added as a catalyst and chelating agent. Subsequently, the mixture was heated in a water bath under reflux at 353 K for 12 h to form a gel which was then oven dried at 393 K for 12 h and further fired at
W. ZhongLiu et al. / Journal of the European Ceramic Society 35 (2015) 3789–3796
3791
Fig. 3. (a) SEM micrographs of the surface and (b) surface EDS analysis of as-prepared mullite outer coating.
paper and cleaned with ethanol and dried at 373 K for 2 h. The SiC inner layer coating was then deposited on C/C substrates by pack cementation procedure [25]. The mullite coating precursor solution was sprayed onto the SiCC/C substrates using a modified spray gun (R2-F, FRVNK). The spray gun was placed above the heated substrate (393 K on a hot plate) at a distance of 25 cm. After spraying, the coated samples were air sintered for 2.5 h at 1773 K in an electric furnace to form the mullite coating.
2.3. Isothermal and thermal shock oxidation tests Fig. 4. Photographic image of the as-prepared mullite/SiC coated C/C samples with different substrate geometries.
1573 K for 2 h. The as-synthesized mullite powder had a narrow particle size distribution and an average particle size of 1.17 m. 2.2. Coating preparation Two kinds of specimens used as substrates were cut from a bulk 2.5D C/C composites with a density of about 1.67 g/cm3 . The specimens for oxidation and thermal shock tests were rectangular with a size of 35 mm × 10 mm × 10 mm, while the cubic specimens for mechanical testing have the size of 10 mm × 10 mm × 10 mm. All the specimens were hand-abraded using 400 and 800 grit SiC
The isothermal and thermal shock oxidation behaviors of the Mullite/SiC coated C/C specimens were studied in static air from 1573 K to 1773 K in an electrical furnace. For the isothermal oxidation tests, the coated specimens were kept at a set oxidation temperature for fixed time after which they were taken out of the furnace. They were then removed from the furnace, cooled to room temperature and weighed in an electronic balance (SHIMADZUAUY220) with a sensitivity of ±0.1 mg. The thermal shock test were carried out by putting the coated samples into an electrical furnace and maintaining at 1773 K for 3 min. Then the samples were taken out of the furnace and cooled to room temperature in 1 min by using compressed air jet cooling. The samples were then put directly into the furnace again for the next thermal shock cycle. The weight change data were collected after every four cycles.
Fig. 5. BSED micrographs of the (a) cross section, (b) high magnification view of substrate/coating interface of as-prepared mullite/SiC coated C/C composites.
3792
W. ZhongLiu et al. / Journal of the European Ceramic Society 35 (2015) 3789–3796
Fig. 6. Line-scanning EDS element analysis of the cross-section of the as-prepared mullite/SiC double layer coating (Ref. Fig. 5b).
Fig. 9. Cross-section micrograph of the mullite/SiC coated sample after undergoing 100 thermal shock cycles (1773 K-R.T) in air.
dation. A scanning electron microscope (SEM, Nova NanoSEM230) equipped with back scatter electron detector (BSED) and energy dispersive spectroscopy (EDS) was used to analyze the morphology and elemental distribution of the coatings. The compressive strength of the specimens was tested in a universal material testing machine (Instron3369, America), perpendicular to the direction of carbon fiber non-woven cloth. The micro-hardness (Hv) of the mullite outer coating was measured randomly from the fine polished cut-section of coated samples using a micro-hardness tester (BUEHLER5104, America). 3. Results and discussion
Fig. 7. Isothermal oxidation curves of the mullite/SiC-C/C composites in air at 1773 K.
Fig. 8. Thermal shock resistance during different cycles at 1773 K-R.T. in air, (cooled by compressed air jet).
2.4. Coating characterization The X-ray diffraction tests were (XRD, Rigaku D/max 2550, Japan) performed at 40 kV and 200 mA using Cu K␣ radiations to analyze the phase composition of the coatings before and after oxi-
The XRD pattern of the as-prepared mullite outer coating is shown in Fig. 2. The main phases in the as-prepared coating are mullite and cristobalite. These form when the monophasic mullite precursors crystallize into an Al2 O3 -rich pseudo-tetragonal mullite at about 1253 K. Then the Al2 O3 -rich mullite gradually transforms into 3:2 orthorhombic mullite by reaction with the silica phase upon being further heated above 1573 K. The latter transformation step is slow because the diffusion rate is limited [26–28]. In addition, no peak of SiC or Si is found, indicating that a dense mullite outer coating with enough thickness has formed. Fig. 3a shows the surface micrograph of the as-prepared mullite outer coating. There are no observable pores or cracks on the surface of mullite coated SiC-C/C samples after sintering for 2.5 h, at 1773 K. This signifies good densification of the mullite outer coating. In addition, the process has effectively coated a wide range of geometries as illustrated in Fig. 4. From the surface EDS analysis (Fig. 3b), it can be seen that the outer coating is composed of Al, Si and O in the stoichiometric ratio approximate corresponding to that of 3/2-mullite. Fig. 5a shows BSED images of the cross-section of the mullite coated samples. The images demonstrate a highly uniform coating thickness of the mullite outer layer throughout the sample surface, including at the sharp corners. The average thickness of the SiC inner coating prepared by pack cementation is about 80.2 m and the mullite outer coating prepared by sol–gel spraying method is about 177.8 m. Even at high magnification (Fig. 5b), no gaps or pores can be observed at the interface of mullite-to-SiC layer, indicating that a good bonding between the two layers has been achieved. In addition, no detectable cracks can be seen in the crosssection of the as-prepared mullite coating, which suggests good thermal expansion match between SiC inner coating and mullite coating.
W. ZhongLiu et al. / Journal of the European Ceramic Society 35 (2015) 3789–3796
3793
Fig. 10. SEM morphologies of the surface ((a)–(e)) and cross-section (f) of mullite coated SiC-C/C composites after different oxidation time at 1773 K. (a) 8 h, (b) 28 h, (c) 60 h, (d) 80 h, (e) 100 h, (f) 100 h. Inset shows the high magnification views of the corresponding mullite coating surface.
Fig. 6 shows the EDS line scanning analysis of the concentration distribution of C, O, Al and Si element in the cross-section of the as-prepared mullite/SiC coatings. The EDS line scan analysis shows that the dual-layer coating can be divided into four parts, designated as a, b, c, and d shown in Fig. 5b. Part a is the C/C composite matrix, part b is the SiC bonding layer, and part c is the mullite (3Al2 O3 ·2SiO2 ) outer coating in accordance with the experimental design. In addition, the distribution pattern of Si and O element suggests the existence of a thin layer of SiO2 glass (Fig. 6, part d) at the interface of mullite-to-SiC layer. This layer is formed by the oxidation of the SiC inner coating during the co-sintering of the mullite outer coating.
The isothermal oxidation curves of the mullite/SiC dual layer coated C/C composites at 1773 K in the air are shown in Fig. 7. After oxidation in air for 100 h at 1773 K, the weight loss of the coated sample is only 6.85 × 10−3 g cm−2 with a corresponding weight loss rate of 6.85 × 10−5 g cm−2 h−1 . These values indicate an excellent oxidation resistance of the dual layer coatings. The compression strength values of the samples are shown in Table 1. Compared to the SiC coated C/C samples, the compression strength of the as-prepared mullite coated SiC-C/C slightly decreases from 218.94 MPa to 215.41 MPa. This is mainly due to the oxidation of samples during the sintering process of the mullite coating. After oxidation in air at 1773 K for 100 h, the compres-
3794
W. ZhongLiu et al. / Journal of the European Ceramic Society 35 (2015) 3789–3796
Table 1 The compression strength of the samples. Samples
Compression strength (MPa)
Retaining percentage (%)
SiC coated C/C As-prepared mullite/SiC-C/C Mullite/SiC-C/C after oxidation at 1773 K for 100 h
218.94 ± 22.44 215.41 ± 8.26 209.89 ± 6.32
– 98.39 97.44
sion strength of the mullite/SiC coated samples decreases from 215.41 MPa to 209.89 MPa. This is equivalent to 97.44% strength of the as-prepared SiC-C/C sample. These results confirm that the mullite/SiC dual layer coatings can provide long-term protection from high temperature oxidation on C/C composites. In addition, the relationship between mass change and thermal cycle times is shown in Fig. 8. The weight loss of mullite/SiC coated sample is only 0.375% after 50 cycles, and after 100 thermal cycles, it increases to 3.412%. This suggests that the oxidation rate increases with thermal cycle times. Fig. 9 clearly reflects the presence of penetrating cracks which were found to be one of the main reasons for the failure of mullite/SiC dual layer coatings during the thermal shock cycles. As indicated, these penetrable cracks provide the channels for oxygen to attack the C/C substrate during thermal cycling. This leads to the formation of large horizontal oxidation holes at the interface between the substrate and the coating. In addition, no spallation phenomenon is found at the interface of mullite/SiC coating, confirming excellent interfacial bonding between the two layers. According to the results of curve fitting the weight loss rate curve in Fig. 7, the oxidation behavior of the mullite/SiC-C/C sample can be divided into three processes as A, B and C. Based on the analysis of the oxidation curves, the corresponding oxidation kinetic equations are shown below: WA = 14.31325 − 3.03597t + 0.23781t 2 (0 < t ≤ 8)
(2)
2
(3)
WB = 7.42635 − 0.33217t + 0.00521t (8 ≤ t ≤ 28) WC = 3.47924 − 0.05903t + 3.17485t (28 ≤ t ≤ 100) (×10−2
mg/cm2 )
(1)
2
is the mass loss of the coated sample where W and t (h) is the oxidation time. At the initial oxidation stage A (0–8 h), both the weight loss and weight loss rate of coated samples increases with oxidation time and quickly reaches 4.01 × 10−3 g cm−2 and 5.02 × 10−5 g cm−2 h−1 , respectively. Subsequently, in the stage B (8–28 h), a decrease in the slope is observed, suggesting a decrease in oxidation rate. In the third stage C (28–100 h), weight loss of the coated sample is found to be almost constant while the plot of weight loss rate of sample vs. time follows approximately a parabolic law. This suggests that the oxidation rate of the coated sample is controlled by the diffusion rate of oxygen across the mullite/SiC coating. Fig. 10(a)–(e) are helpful in understanding the microstructural evolution of the mullite outer coatings and corresponding changes that occur during the oxidation of the mullite/SiC coated C/C samples at 1773 K. Fig. 10(a) shows that after oxidation at 1773 K for 8 h, the surface of as-prepared mullite coating evolves into a porous interconnected structure with dense solid regions separated by large pores. Extensive grain growth is also observed here. These pores undergo a coalescence process and provide paths for oxygen diffusion, resulting in accelerated oxidation process (oxidation stage A, Fig. 7). As oxidation proceeds, mullite grains continue to grow by rapid elimination of the smaller grains, In addition, porous regions shrink into dense, solid regions composed of mullite grains which are much larger than the original ones. This is confirmed in Fig. 10(b). Reduction in large pores, together with the increase in dense regions, is consistent with the observed decrease in the
Fig. 11. Micro-hardness of the cross section of the mullite outer coating at different oxidation times (1773 K in air)
oxidation rate of coated samples (oxidation stage B, Fig. 7). Continuation of the oxidation tests at 1773 K leads to dense micro-regions that continue to coalesce as depicted in Fig. 10(c). Finally, the coating becomes fully dense and porosity free (Fig. 10(d)–(e)). This accounts for the parabolic oxidation curve in Fig. 7 stage C. Fig. 10(f) shows backscattered micrograph of mullite/SiC coated C/C after oxidation testing at 1773 K for 100 h. It can be seen that the coating is still intact and no cracks or spallation is present, which signifies the excellent long-term oxidation resistance. Micro-hardness measurements of the mullite coating provide further evidence of the densification of the sol–gel mullite outer coating during oxidation at 1773 K. It is observed in Fig. 11 that micro-hardness of mullite coating is enhanced with increasing oxidation time at 1773 K. This is expected since coating densification is enhanced with increasing sintering time. Fig. 12 shows the surface XRD patterns of mullite/SiC coated C/C samples after oxidation at 1773 K for different times. It shows that glassy phase free stoichiometric 3/2-mullite coating is obtained from monophasic precursors after oxidation at 1773 K for 8 h. As shown previously, the mullite crystals grow with increasing oxidation times. Hence, the mullite peaks are observed to get stronger with oxidation times. After oxidation for 100 h at 1773 K, there was no detectable decomposition of the mullite outer coating, which indicates the excellent stability of the mullite coating. Fig. 13 shows the isothermal oxidation curves of mullite/SiC coated C/C composites held at 1573–1773 K for 28–100 h. The mass loss calculated using the triadic mean method shows increase in oxidation rate with temperature from 1573 K to 1773 K. It is found that the weight loss of the sample follows a parabolic curve with oxidation time at 1573–1773 K, where W2 is a function of oxidation time (Fig. 14). The relationship between weight loss of the samples and oxidation time is parabolic in the temperature range of 1573–1773 K. In addition, an Arrhenius relationship is found between the oxidation weight loss rate and the oxidation temperature of mullite/SiC coated C/C composites in the range 1573–1773 K (Fig. 15).
W. ZhongLiu et al. / Journal of the European Ceramic Society 35 (2015) 3789–3796
3795
Fig. 14. Plots of W2 as a function of oxidation time at the temperature range 1573–1773 K.
Fig. 12. Surface XRD patterns of mullite coating after oxidation at 1773 K for different hours.
Fig. 15. Arrhenius curve of mullite/SiC coated C/C composites at 1573–1773 K.
4. Conclusions
Fig. 13. Isothermal oxidation curves of mullite/SiC coated C/C composites in air at the temperature range of 1573–1773 K.
According to Wu and Li’s research [9–30], the oxidation activation energy is expected to be 112 kJ/mol, when the oxidation process of the coated C/C is controlled by the rate of oxygen diffusion. In the current study, the oxidation activation energy of the mullite/SiC coated C/C composites is calculated to be 110.63 kJ/mol. Therefore, we can conclude that the oxygen diffusion through the coating is the rate-limiting step in the oxidation of the mullite/SiC coated C/C composites in the temperature range of 1573–1773 K. Thus the dual layer coating has excellent oxidation resistance in the 1573–1773 K temperature range.
Sol–gel spraying method was innovatively used in the deposition of mullite coating on SiC coated C/C composites. The mullite coating was deposited by airbrush spraying mullite precursor hybrid sol solution containing measured amounts of mullite fine powder, PVP and DCCA as additives. This is then followed by air sintering the as-deposited coating for 2.5 h at 1773 K. This singlestep spraying-and-sintering technique proved to be suitable for obtaining dense, well-bonded and crack-free mullite outer coating with homogeneous thickness (about 178 m) on SiC coated C/C composites. The process was readily able to coat a range of geometries and complex shapes. The mullite/SiC coatings can effectively protect C/C composites from oxidation in air at 1773 K for up to 100 h with a weight loss of 6.85 × 10−3 g cm−2 and a corresponding weight loss rate of 6.85 × 10−5 g cm−2 h−1 . A theory regarding the oxidation kinetics of the dual layer coated C/C composites in the range 1573–1773 K has also been proposed. The high temperature oxidation activation energy of mullite/SiC dual coating coated C/C composites at 1573–1773 K is calculated to be 110.63 kJ/mol. The oxidation process is found to be predominantly controlled by the diffusion rate of oxygen through the mullite/SiC dual coating in the temperature range of 1573–1773 K.
3796
W. ZhongLiu et al. / Journal of the European Ceramic Society 35 (2015) 3789–3796
Acknowledgments The project was supported by the National Key Basic Research and Development Program under Grant no. 2011CB605804 and the National Natural Science Foundation of China under Grant no. 51205417. References [1] M.E. Westwood, J.D. Webster, R.J. Day, et al., Oxidation protection for carbon fiber composites, J. Mater. Sci. 31 (6) (1996) 1389–1397. [2] X. Ren, H. Li, K. Li, et al., Oxidation protection of ultra-high temperature ceramic ZrxTa1–xB2–SiC/SiC coating prepared by in-situ reaction method for carbon/carbon composites, J. Eur. Ceram. Soc. 35 (3) (2015) 897–907. [3] E.L. Corral, R.E. Loehman, Ultra-High-temperature ceramic coatings for oxidation protection of carbon–carbon composites, J. Am. Ceram. Soc. 91 (5) (2008) 1495–1502. [4] Q.G. Fu, H.J. Li, Y.J. Wang, et al., B2 O3 modified SiC-MoSi2 oxidation resistant coating for carbon/carbon composites by a two-step pack cementation, Corros. Sci. 51 (10) (2009) 2450–2454. [5] H. Wu, H.J. Li, C. Ma, et al., MoSi2 -based oxidation protective coatings for SiC-coated carbon/carbon composites prepared by supersonic plasma spraying, J. Eur. Ceram. Soc. 30 (15) (2010) 3267–3270. [6] Y.L. Zhang, H.J. Li, X.Y. Yao, et al., Oxidation protection of C/SiC coated carbon/carbon composites with Si–Mo coating at high temperature, Corros. Sci. 53 (6) (2011) 2075–2079. [7] H. Jian-Feng, Z. Xie-Rong, L. He-Jun, et al., Mullite-Al2 O3 -SiC oxidation protective coating for carbon/carbon composites, Carbon 41 (14) (2003) 2825–2829. [8] H. Fritze, J. Jojic, T. Witke, et al., Mullite based oxidation protection for SiC-C/C composites in air at temperatures up to 1900 K, J. Eur. Ceram. Soc. 18 (16) (1998) 2351–2364. [9] J.F. Huang, H.J. Li, X.R. Zeng, et al., Influence of preparation technology on the microstructure and anti-oxidation property of SiC-Al2 O3 -mullite multi-coatings for carbon/carbon composites, Appl. Surf. Sci. 252 (12) (2006) 4244–4249. [10] M. Ferraris, M. Salvo, F. Smeacetto, Cordierite-mullite coating for SiCf /SiC composites, J. Eur. Ceram. Soc. 22 (13) (2002) 2343–2347. [11] K.T. Wang, L.Y. Cao, J.F. Huang, et al., A mullite/SiC oxidation protective coating for carbon/carbon composites, J. Eur. Ceram. Soc. 33 (1) (2013) 191–198. ´ C. Argirusis, G. Borchardt, et al., Oxidation protection of [12] T. Damjanovic, C/C-SiC composites by an electrophoretically deposited mullite precursor, J. Eur. Ceram. Soc. 25 (5) (2005) 577–587. [13] M.L. Auger, V.K. Sarin, The development of CVD mullite coatings for high temperature corrosive applications, Surf. Coat. Technol. 94 (1997) 46–52. [14] C. Ma, H. Li, H. Wu, et al., Mullite oxidation resistant coating for SiC-coated carbon/carbon composites by supersonic plasma spraying, J. Mater. Sci. Technol. 29 (1) (2013) 29–33.
[15] M. Touzin, F. Béclin, Fabrication and characterization of composite sol–gel coatings on porous ceramic substrate, J. Eur. Ceram. Soc. 31 (9) (2011) 1661–1667. [16] D. Wang, G.P. Bierwagen, Sol–gel coatings on metals for corrosion protection, Prog. Org. Coat. 64 (4) (2009) 327–338. [17] O. Ebrahimpour, C. Dubois, J. Chaouki, Fabrication of mullite-bonded porous SiC ceramics via a sol–gel assisted in situ reaction bonding, J. Eur. Ceram. Soc. 34 (2) (2014) 237–247. [18] J. Leivo, V. Meretoja, M. Vippola, et al., Sol–gel derived aluminosilicate coatings on alumina as substrate for osteoblasts, Acta Biomater. 2 (6) (2006) 659–668. [19] M. Touzin, F. Béclin, Fabrication and characterization of composite sol–gel coatings on porous ceramic substrate, J. Eur. Ceram. Soc. 31 (9) (2011) 1661–1667. [20] D.A. Barrow, T.E. Petroff, M. Sayer, Thick ceramic coatings using a sol–gel based ceramic–ceramic 0–3 composite, Surf. Coat. Technol. 76 (1995) 113–118. [21] D.L. Corker, Q. Zhang, R.W. Whatmore, et al., PZT ‘composite’ ferroelectric thick films, J. Eur. Ceram. Soc. 22 (3) (2002) 383–390. [22] S. Ramasamy, S.N. Tewari, K.N. Lee, et al., Slurry based multilayer environmental barrier coatings for silicon carbide and silicon nitride ceramics—I. Processing, Surf. Coat. Technol. 205 (2) (2010) 258–265. [23] F. Baino, C. Vitale-Brovarone, Feasibility of glass-ceramic coatings on alumina prosthetic implants by airbrush spraying method, Ceram. Int. 41 (2) (2015) 2150–2159. [24] A.M. Waetjen, D.A. Polsakiewicz, I. Kuhl, et al., Slurry deposition by airbrush for selective laser sintering of ceramic components, J. Eur. Ceram. Soc. 29 (1) (2009) 1–6. [25] Y. Zhang, H. Li, K.Z. Li, et al., C/SiC/Si-Mo-Cr multilayer coating for carbon/carbon composites for oxidation protection, New Carbon Mater. 27 (2) (2012) 105–109. [26] J. Leivo, M. Lindén, J.M. Rosenholm, et al., Evolution of aluminosilicate structure and mullite crystallization from homogeneous nanoparticulate sol–gel precursor with organic additives, J. Eur. Ceram. Soc. 28 (9) (2008) 1749–1762. [27] E.R. de Sola, F.J. Torres, J. Alarcón, Thermal evolution and structural study of 2:1 mullite from monophasic gels, J. Eur. Ceram. Soc. 26 (12) (2006) 2279–2284. [28] E.R. de Sola, F. Estevan, J. Alarcón, Low-temperature Ti-containing 3:2 and 2:1 mullite nanocrystals from single-phase gels, J. Eur. Ceram. Soc. 27 (7) (2007) 2655–2663. [29] J. Li, R. Luo, Y. Chen, et al., Oxidation behavior and kinetics of SiC/alumina-borosilicate coating for carbon–carbon composites, Appl. Surf. Sci. 255 (5) (2008) 1967–1974. [30] T.M. Wu, Y.R. Wu, Methodology in exploring the oxidation behavior of coated-carbon carbon composites, J. Mater. Sci. 29 (5) (1994) 1260–1264.
Contents lists available at www.sciencedirect.com
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Microstructure and oxidation behavior of sol–gel mullite coating on SiC-coated carbon/carbon composites Wen ZhongLiu, Xiao Peng ∗ , Li Zhuan, Hong Wen, Luo Heng, Yu XiaoYu, Li Yang, Chen WenBo State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, PR China
a r t i c l e
i n f o
Article history: Received 15 May 2015 Received in revised form 25 June 2015 Accepted 27 June 2015 Available online 26 July 2015 Keywords: Sol–gel coating Mullite coating Carbon/carbon composites Oxidation Airbrush spraying
a b s t r a c t A dense, uniform, crack-free and well-bonded sol–gel mullite coating with thickness of about 178 m was successfully fabricated on SiC-coated carbon/carbon (C/C) composites. Mullite coating is produced by airbrush spraying a 3/2-mullite coating precursor solution consisting of fine mullite powder dispersed in a mullite precursor sol and air sintering the as-deposited coating. This method has several benefits such as simplicity of the process, high efficiency, ability to coat complex geometries and cost-effectiveness. The mullite/SiC coating was found to protect C/C composites from oxidation at 1773 K for 100 h with a weight loss rate of only 6.85 × 10−5 g cm−2 h−1 and retained compression strength of 97.44%. The oxidation rate was found to be significantly decreased with oxidation time, possibly due to the densification of mullite coating during the oxidation process. The corresponding high temperature oxidation activation energy of mullite/SiC coated C/C in the temperature range of 1573–1773 K is calculated to be 110.63 kJ/mol. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Carbon/carbon (C/C) composites are considered to be one of the most promising high temperature structural materials due to their low density, high strength-to-weight ratio, good thermal shock resistance and good mechanical properties at elevated temperatures. However, the usefulness of C/C composites is limited in oxidizing environments due to low oxidation start temperature which can be as low as 723 K [1]. Application of coatings, especially multilayer ceramic coatings, to C/C composites is considered to be the most effective way to protect C/C in high temperature oxidizing environments [2–4]. The SiC ceramic coating is usually considered to be one of the best bonding layers for use between the C/C composite and outer ceramic coating layer [5,6]. Among a variety of external coating materials, mullite (3Al2 O3 ·2SiO2 ) ceramic has attracted the most interest because of its excellent heat resistance, good thermal expansion coefficient (CTE) match and chemical compatibility with SiC [7–10]. Various methods have been developed to prepare the mullite coating in the past years [11–14]. However, most of these technologies are either too expensive or are unable to coat complex substrate shapes.
∗ Corresponding author. Fax: +86 73188830131. E-mail address: [email protected] (X. Peng). http://dx.doi.org/10.1016/j.jeurceramsoc.2015.06.033 0955-2219/© 2015 Elsevier Ltd. All rights reserved.
The sol–gel method for producing coatings is a well-known process. It has various advantages such as low processing temperatures, cost-effectiveness, and ability to coat complex substrate geometries. It has also been demonstrated to have a commercial scale-up potential [15,16]. This method offers better control over particle size and morphology of the starting precursor resulting in enhanced sintering ability at low temperatures. This in turn imparts superior mechanical properties to the mullite prepared in this way [17–19]. However, coatings prepared by conventional sol–gel processes suffer limitations due to the presence of cracks, porosity and high internal stresses leading to adhesion failures. Additionally, the thickness of the coatings produced by this method is limited to a few microns. Fortunately, researchers have shown that introduction of fine ceramic powders to form a hybrid coating precursor sol helps to overcome the coating thickness limitations [20,21]. The overall performance of the coating in protecting against oxidation depends mainly on the application process for coating precursor and subsequent sintering procedure. Usually, the sol–gel coatings are applied via dip-coating and spin-coating method. One of the major problems of dip and spincoating methods is that they always introduce non-uniformity in coating thickness near the edges of the substrates [22]. Moreover, in order to obtain reasonable coating thickness, multiple cycles of precursor application-and-sintering are required. Airbrush spraying of ceramic slurries has been successfully used as a low cost processing technique instead of dip or spin method. It helps to
3790
W. ZhongLiu et al. / Journal of the European Ceramic Society 35 (2015) 3789–3796
Fig. 1. Schematic diagram showing the mullite coating preparation process.
form coatings on complex substrate shapes with adequate control over coating thicknesses and provides high deposition rates [23,24]. More importantly, it has a high potential for industrial mass fabrication. However, there is a paucity of studies on its use in the field of sol–gel coating deposition. To the best of the authors’ knowledge, the use of airbrush spraying a mullite precursor sol to obtain a thick mullite coating for improved oxidation resistance has not been documented till date. Therefore, the present article represents a pilot study on the subject. In this article, we present a new sol–gel based mullite coating process, which provides a two-fold advantage: (1) a higher upper-coating thickness, similar to the hybrid precursor sol, and (2) better control over the coating thickness uniformity provided by the airbrush spraying method. For this study, a hybrid mullite precursor coating solution was prepared and applied on a SiC coated C/C substrate by the airbrush spraying process. The phase compositions and microstructures of the as-prepared mullite/SiC coating were then characterized. Oxidation resistance, thermalshock properties, mechanical properties including micro-hardness of the coating, and oxidation kinetics of the dual layer coated C/C composites were investigated. 2. Experimental procedures 2.1. Preparation of the mullite coating precursor solution All the materials used for mullite precursor synthesis were of the AR grade. The mullite coating precursor solution was prepared by mixing a mullite precursor sol with mullite powder, along with other ingredients. The processing steps for synthesizing mullite precursor sol are shown in Fig. 1. Tetraethoxysilane (TEOS) was mixed with aluminum nitrate nonahydrate (ANN) and aluminum acetate (2:5 molar ratio of ANN to aluminum acetate) in pure ethanol medium in the stoichiometric ratio of mullite. Acetic acid was added (0.1–0.2 mol/L) to promote gelation. After 6 h of refluxing at 353 K, a milky, viscous monophasic sol was obtained. The mullite precursor sol and 3/2-mullite powder (70–75 wt.% of the
Fig. 2. Surface XRD spectra of the as-prepared mullite coating on SiC-C/C.
total mixture) were mixed by high energy planetary ball-milling at 300 rpm, with a crack-sealing agent (polyvinylpyrrolidon, PVP, average molecular weight of 1.3 × 106 g/mol, about 0.1–0.25 wt.%) and oxalic acid (3–5 wt.%) as drying control chemical additive (DCCA). Ball milling was performed for 4 h after the addition of all the ingredient and, at the end of the ball-milling process, acetone was added to reach an optimized solution viscosity. The solid concentration in the optimized mullite coating precursor solution was about 20 wt%. The mullite powder was prepared by a sol–gel process and ANN and TEOS were used as the starting materials. In short, the source chemicals were dissolved in absolute ethanol in the molar ratio of 3:1 of ANN: TEOS (3/2-mullite) after which acetic acid was added as a catalyst and chelating agent. Subsequently, the mixture was heated in a water bath under reflux at 353 K for 12 h to form a gel which was then oven dried at 393 K for 12 h and further fired at
W. ZhongLiu et al. / Journal of the European Ceramic Society 35 (2015) 3789–3796
3791
Fig. 3. (a) SEM micrographs of the surface and (b) surface EDS analysis of as-prepared mullite outer coating.
paper and cleaned with ethanol and dried at 373 K for 2 h. The SiC inner layer coating was then deposited on C/C substrates by pack cementation procedure [25]. The mullite coating precursor solution was sprayed onto the SiCC/C substrates using a modified spray gun (R2-F, FRVNK). The spray gun was placed above the heated substrate (393 K on a hot plate) at a distance of 25 cm. After spraying, the coated samples were air sintered for 2.5 h at 1773 K in an electric furnace to form the mullite coating.
2.3. Isothermal and thermal shock oxidation tests Fig. 4. Photographic image of the as-prepared mullite/SiC coated C/C samples with different substrate geometries.
1573 K for 2 h. The as-synthesized mullite powder had a narrow particle size distribution and an average particle size of 1.17 m. 2.2. Coating preparation Two kinds of specimens used as substrates were cut from a bulk 2.5D C/C composites with a density of about 1.67 g/cm3 . The specimens for oxidation and thermal shock tests were rectangular with a size of 35 mm × 10 mm × 10 mm, while the cubic specimens for mechanical testing have the size of 10 mm × 10 mm × 10 mm. All the specimens were hand-abraded using 400 and 800 grit SiC
The isothermal and thermal shock oxidation behaviors of the Mullite/SiC coated C/C specimens were studied in static air from 1573 K to 1773 K in an electrical furnace. For the isothermal oxidation tests, the coated specimens were kept at a set oxidation temperature for fixed time after which they were taken out of the furnace. They were then removed from the furnace, cooled to room temperature and weighed in an electronic balance (SHIMADZUAUY220) with a sensitivity of ±0.1 mg. The thermal shock test were carried out by putting the coated samples into an electrical furnace and maintaining at 1773 K for 3 min. Then the samples were taken out of the furnace and cooled to room temperature in 1 min by using compressed air jet cooling. The samples were then put directly into the furnace again for the next thermal shock cycle. The weight change data were collected after every four cycles.
Fig. 5. BSED micrographs of the (a) cross section, (b) high magnification view of substrate/coating interface of as-prepared mullite/SiC coated C/C composites.
3792
W. ZhongLiu et al. / Journal of the European Ceramic Society 35 (2015) 3789–3796
Fig. 6. Line-scanning EDS element analysis of the cross-section of the as-prepared mullite/SiC double layer coating (Ref. Fig. 5b).
Fig. 9. Cross-section micrograph of the mullite/SiC coated sample after undergoing 100 thermal shock cycles (1773 K-R.T) in air.
dation. A scanning electron microscope (SEM, Nova NanoSEM230) equipped with back scatter electron detector (BSED) and energy dispersive spectroscopy (EDS) was used to analyze the morphology and elemental distribution of the coatings. The compressive strength of the specimens was tested in a universal material testing machine (Instron3369, America), perpendicular to the direction of carbon fiber non-woven cloth. The micro-hardness (Hv) of the mullite outer coating was measured randomly from the fine polished cut-section of coated samples using a micro-hardness tester (BUEHLER5104, America). 3. Results and discussion
Fig. 7. Isothermal oxidation curves of the mullite/SiC-C/C composites in air at 1773 K.
Fig. 8. Thermal shock resistance during different cycles at 1773 K-R.T. in air, (cooled by compressed air jet).
2.4. Coating characterization The X-ray diffraction tests were (XRD, Rigaku D/max 2550, Japan) performed at 40 kV and 200 mA using Cu K␣ radiations to analyze the phase composition of the coatings before and after oxi-
The XRD pattern of the as-prepared mullite outer coating is shown in Fig. 2. The main phases in the as-prepared coating are mullite and cristobalite. These form when the monophasic mullite precursors crystallize into an Al2 O3 -rich pseudo-tetragonal mullite at about 1253 K. Then the Al2 O3 -rich mullite gradually transforms into 3:2 orthorhombic mullite by reaction with the silica phase upon being further heated above 1573 K. The latter transformation step is slow because the diffusion rate is limited [26–28]. In addition, no peak of SiC or Si is found, indicating that a dense mullite outer coating with enough thickness has formed. Fig. 3a shows the surface micrograph of the as-prepared mullite outer coating. There are no observable pores or cracks on the surface of mullite coated SiC-C/C samples after sintering for 2.5 h, at 1773 K. This signifies good densification of the mullite outer coating. In addition, the process has effectively coated a wide range of geometries as illustrated in Fig. 4. From the surface EDS analysis (Fig. 3b), it can be seen that the outer coating is composed of Al, Si and O in the stoichiometric ratio approximate corresponding to that of 3/2-mullite. Fig. 5a shows BSED images of the cross-section of the mullite coated samples. The images demonstrate a highly uniform coating thickness of the mullite outer layer throughout the sample surface, including at the sharp corners. The average thickness of the SiC inner coating prepared by pack cementation is about 80.2 m and the mullite outer coating prepared by sol–gel spraying method is about 177.8 m. Even at high magnification (Fig. 5b), no gaps or pores can be observed at the interface of mullite-to-SiC layer, indicating that a good bonding between the two layers has been achieved. In addition, no detectable cracks can be seen in the crosssection of the as-prepared mullite coating, which suggests good thermal expansion match between SiC inner coating and mullite coating.
W. ZhongLiu et al. / Journal of the European Ceramic Society 35 (2015) 3789–3796
3793
Fig. 10. SEM morphologies of the surface ((a)–(e)) and cross-section (f) of mullite coated SiC-C/C composites after different oxidation time at 1773 K. (a) 8 h, (b) 28 h, (c) 60 h, (d) 80 h, (e) 100 h, (f) 100 h. Inset shows the high magnification views of the corresponding mullite coating surface.
Fig. 6 shows the EDS line scanning analysis of the concentration distribution of C, O, Al and Si element in the cross-section of the as-prepared mullite/SiC coatings. The EDS line scan analysis shows that the dual-layer coating can be divided into four parts, designated as a, b, c, and d shown in Fig. 5b. Part a is the C/C composite matrix, part b is the SiC bonding layer, and part c is the mullite (3Al2 O3 ·2SiO2 ) outer coating in accordance with the experimental design. In addition, the distribution pattern of Si and O element suggests the existence of a thin layer of SiO2 glass (Fig. 6, part d) at the interface of mullite-to-SiC layer. This layer is formed by the oxidation of the SiC inner coating during the co-sintering of the mullite outer coating.
The isothermal oxidation curves of the mullite/SiC dual layer coated C/C composites at 1773 K in the air are shown in Fig. 7. After oxidation in air for 100 h at 1773 K, the weight loss of the coated sample is only 6.85 × 10−3 g cm−2 with a corresponding weight loss rate of 6.85 × 10−5 g cm−2 h−1 . These values indicate an excellent oxidation resistance of the dual layer coatings. The compression strength values of the samples are shown in Table 1. Compared to the SiC coated C/C samples, the compression strength of the as-prepared mullite coated SiC-C/C slightly decreases from 218.94 MPa to 215.41 MPa. This is mainly due to the oxidation of samples during the sintering process of the mullite coating. After oxidation in air at 1773 K for 100 h, the compres-
3794
W. ZhongLiu et al. / Journal of the European Ceramic Society 35 (2015) 3789–3796
Table 1 The compression strength of the samples. Samples
Compression strength (MPa)
Retaining percentage (%)
SiC coated C/C As-prepared mullite/SiC-C/C Mullite/SiC-C/C after oxidation at 1773 K for 100 h
218.94 ± 22.44 215.41 ± 8.26 209.89 ± 6.32
– 98.39 97.44
sion strength of the mullite/SiC coated samples decreases from 215.41 MPa to 209.89 MPa. This is equivalent to 97.44% strength of the as-prepared SiC-C/C sample. These results confirm that the mullite/SiC dual layer coatings can provide long-term protection from high temperature oxidation on C/C composites. In addition, the relationship between mass change and thermal cycle times is shown in Fig. 8. The weight loss of mullite/SiC coated sample is only 0.375% after 50 cycles, and after 100 thermal cycles, it increases to 3.412%. This suggests that the oxidation rate increases with thermal cycle times. Fig. 9 clearly reflects the presence of penetrating cracks which were found to be one of the main reasons for the failure of mullite/SiC dual layer coatings during the thermal shock cycles. As indicated, these penetrable cracks provide the channels for oxygen to attack the C/C substrate during thermal cycling. This leads to the formation of large horizontal oxidation holes at the interface between the substrate and the coating. In addition, no spallation phenomenon is found at the interface of mullite/SiC coating, confirming excellent interfacial bonding between the two layers. According to the results of curve fitting the weight loss rate curve in Fig. 7, the oxidation behavior of the mullite/SiC-C/C sample can be divided into three processes as A, B and C. Based on the analysis of the oxidation curves, the corresponding oxidation kinetic equations are shown below: WA = 14.31325 − 3.03597t + 0.23781t 2 (0 < t ≤ 8)
(2)
2
(3)
WB = 7.42635 − 0.33217t + 0.00521t (8 ≤ t ≤ 28) WC = 3.47924 − 0.05903t + 3.17485t (28 ≤ t ≤ 100) (×10−2
mg/cm2 )
(1)
2
is the mass loss of the coated sample where W and t (h) is the oxidation time. At the initial oxidation stage A (0–8 h), both the weight loss and weight loss rate of coated samples increases with oxidation time and quickly reaches 4.01 × 10−3 g cm−2 and 5.02 × 10−5 g cm−2 h−1 , respectively. Subsequently, in the stage B (8–28 h), a decrease in the slope is observed, suggesting a decrease in oxidation rate. In the third stage C (28–100 h), weight loss of the coated sample is found to be almost constant while the plot of weight loss rate of sample vs. time follows approximately a parabolic law. This suggests that the oxidation rate of the coated sample is controlled by the diffusion rate of oxygen across the mullite/SiC coating. Fig. 10(a)–(e) are helpful in understanding the microstructural evolution of the mullite outer coatings and corresponding changes that occur during the oxidation of the mullite/SiC coated C/C samples at 1773 K. Fig. 10(a) shows that after oxidation at 1773 K for 8 h, the surface of as-prepared mullite coating evolves into a porous interconnected structure with dense solid regions separated by large pores. Extensive grain growth is also observed here. These pores undergo a coalescence process and provide paths for oxygen diffusion, resulting in accelerated oxidation process (oxidation stage A, Fig. 7). As oxidation proceeds, mullite grains continue to grow by rapid elimination of the smaller grains, In addition, porous regions shrink into dense, solid regions composed of mullite grains which are much larger than the original ones. This is confirmed in Fig. 10(b). Reduction in large pores, together with the increase in dense regions, is consistent with the observed decrease in the
Fig. 11. Micro-hardness of the cross section of the mullite outer coating at different oxidation times (1773 K in air)
oxidation rate of coated samples (oxidation stage B, Fig. 7). Continuation of the oxidation tests at 1773 K leads to dense micro-regions that continue to coalesce as depicted in Fig. 10(c). Finally, the coating becomes fully dense and porosity free (Fig. 10(d)–(e)). This accounts for the parabolic oxidation curve in Fig. 7 stage C. Fig. 10(f) shows backscattered micrograph of mullite/SiC coated C/C after oxidation testing at 1773 K for 100 h. It can be seen that the coating is still intact and no cracks or spallation is present, which signifies the excellent long-term oxidation resistance. Micro-hardness measurements of the mullite coating provide further evidence of the densification of the sol–gel mullite outer coating during oxidation at 1773 K. It is observed in Fig. 11 that micro-hardness of mullite coating is enhanced with increasing oxidation time at 1773 K. This is expected since coating densification is enhanced with increasing sintering time. Fig. 12 shows the surface XRD patterns of mullite/SiC coated C/C samples after oxidation at 1773 K for different times. It shows that glassy phase free stoichiometric 3/2-mullite coating is obtained from monophasic precursors after oxidation at 1773 K for 8 h. As shown previously, the mullite crystals grow with increasing oxidation times. Hence, the mullite peaks are observed to get stronger with oxidation times. After oxidation for 100 h at 1773 K, there was no detectable decomposition of the mullite outer coating, which indicates the excellent stability of the mullite coating. Fig. 13 shows the isothermal oxidation curves of mullite/SiC coated C/C composites held at 1573–1773 K for 28–100 h. The mass loss calculated using the triadic mean method shows increase in oxidation rate with temperature from 1573 K to 1773 K. It is found that the weight loss of the sample follows a parabolic curve with oxidation time at 1573–1773 K, where W2 is a function of oxidation time (Fig. 14). The relationship between weight loss of the samples and oxidation time is parabolic in the temperature range of 1573–1773 K. In addition, an Arrhenius relationship is found between the oxidation weight loss rate and the oxidation temperature of mullite/SiC coated C/C composites in the range 1573–1773 K (Fig. 15).
W. ZhongLiu et al. / Journal of the European Ceramic Society 35 (2015) 3789–3796
3795
Fig. 14. Plots of W2 as a function of oxidation time at the temperature range 1573–1773 K.
Fig. 12. Surface XRD patterns of mullite coating after oxidation at 1773 K for different hours.
Fig. 15. Arrhenius curve of mullite/SiC coated C/C composites at 1573–1773 K.
4. Conclusions
Fig. 13. Isothermal oxidation curves of mullite/SiC coated C/C composites in air at the temperature range of 1573–1773 K.
According to Wu and Li’s research [9–30], the oxidation activation energy is expected to be 112 kJ/mol, when the oxidation process of the coated C/C is controlled by the rate of oxygen diffusion. In the current study, the oxidation activation energy of the mullite/SiC coated C/C composites is calculated to be 110.63 kJ/mol. Therefore, we can conclude that the oxygen diffusion through the coating is the rate-limiting step in the oxidation of the mullite/SiC coated C/C composites in the temperature range of 1573–1773 K. Thus the dual layer coating has excellent oxidation resistance in the 1573–1773 K temperature range.
Sol–gel spraying method was innovatively used in the deposition of mullite coating on SiC coated C/C composites. The mullite coating was deposited by airbrush spraying mullite precursor hybrid sol solution containing measured amounts of mullite fine powder, PVP and DCCA as additives. This is then followed by air sintering the as-deposited coating for 2.5 h at 1773 K. This singlestep spraying-and-sintering technique proved to be suitable for obtaining dense, well-bonded and crack-free mullite outer coating with homogeneous thickness (about 178 m) on SiC coated C/C composites. The process was readily able to coat a range of geometries and complex shapes. The mullite/SiC coatings can effectively protect C/C composites from oxidation in air at 1773 K for up to 100 h with a weight loss of 6.85 × 10−3 g cm−2 and a corresponding weight loss rate of 6.85 × 10−5 g cm−2 h−1 . A theory regarding the oxidation kinetics of the dual layer coated C/C composites in the range 1573–1773 K has also been proposed. The high temperature oxidation activation energy of mullite/SiC dual coating coated C/C composites at 1573–1773 K is calculated to be 110.63 kJ/mol. The oxidation process is found to be predominantly controlled by the diffusion rate of oxygen through the mullite/SiC dual coating in the temperature range of 1573–1773 K.
3796
W. ZhongLiu et al. / Journal of the European Ceramic Society 35 (2015) 3789–3796
Acknowledgments The project was supported by the National Key Basic Research and Development Program under Grant no. 2011CB605804 and the National Natural Science Foundation of China under Grant no. 51205417. References [1] M.E. Westwood, J.D. Webster, R.J. Day, et al., Oxidation protection for carbon fiber composites, J. Mater. Sci. 31 (6) (1996) 1389–1397. [2] X. Ren, H. Li, K. Li, et al., Oxidation protection of ultra-high temperature ceramic ZrxTa1–xB2–SiC/SiC coating prepared by in-situ reaction method for carbon/carbon composites, J. Eur. Ceram. Soc. 35 (3) (2015) 897–907. [3] E.L. Corral, R.E. Loehman, Ultra-High-temperature ceramic coatings for oxidation protection of carbon–carbon composites, J. Am. Ceram. Soc. 91 (5) (2008) 1495–1502. [4] Q.G. Fu, H.J. Li, Y.J. Wang, et al., B2 O3 modified SiC-MoSi2 oxidation resistant coating for carbon/carbon composites by a two-step pack cementation, Corros. Sci. 51 (10) (2009) 2450–2454. [5] H. Wu, H.J. Li, C. Ma, et al., MoSi2 -based oxidation protective coatings for SiC-coated carbon/carbon composites prepared by supersonic plasma spraying, J. Eur. Ceram. Soc. 30 (15) (2010) 3267–3270. [6] Y.L. Zhang, H.J. Li, X.Y. Yao, et al., Oxidation protection of C/SiC coated carbon/carbon composites with Si–Mo coating at high temperature, Corros. Sci. 53 (6) (2011) 2075–2079. [7] H. Jian-Feng, Z. Xie-Rong, L. He-Jun, et al., Mullite-Al2 O3 -SiC oxidation protective coating for carbon/carbon composites, Carbon 41 (14) (2003) 2825–2829. [8] H. Fritze, J. Jojic, T. Witke, et al., Mullite based oxidation protection for SiC-C/C composites in air at temperatures up to 1900 K, J. Eur. Ceram. Soc. 18 (16) (1998) 2351–2364. [9] J.F. Huang, H.J. Li, X.R. Zeng, et al., Influence of preparation technology on the microstructure and anti-oxidation property of SiC-Al2 O3 -mullite multi-coatings for carbon/carbon composites, Appl. Surf. Sci. 252 (12) (2006) 4244–4249. [10] M. Ferraris, M. Salvo, F. Smeacetto, Cordierite-mullite coating for SiCf /SiC composites, J. Eur. Ceram. Soc. 22 (13) (2002) 2343–2347. [11] K.T. Wang, L.Y. Cao, J.F. Huang, et al., A mullite/SiC oxidation protective coating for carbon/carbon composites, J. Eur. Ceram. Soc. 33 (1) (2013) 191–198. ´ C. Argirusis, G. Borchardt, et al., Oxidation protection of [12] T. Damjanovic, C/C-SiC composites by an electrophoretically deposited mullite precursor, J. Eur. Ceram. Soc. 25 (5) (2005) 577–587. [13] M.L. Auger, V.K. Sarin, The development of CVD mullite coatings for high temperature corrosive applications, Surf. Coat. Technol. 94 (1997) 46–52. [14] C. Ma, H. Li, H. Wu, et al., Mullite oxidation resistant coating for SiC-coated carbon/carbon composites by supersonic plasma spraying, J. Mater. Sci. Technol. 29 (1) (2013) 29–33.
[15] M. Touzin, F. Béclin, Fabrication and characterization of composite sol–gel coatings on porous ceramic substrate, J. Eur. Ceram. Soc. 31 (9) (2011) 1661–1667. [16] D. Wang, G.P. Bierwagen, Sol–gel coatings on metals for corrosion protection, Prog. Org. Coat. 64 (4) (2009) 327–338. [17] O. Ebrahimpour, C. Dubois, J. Chaouki, Fabrication of mullite-bonded porous SiC ceramics via a sol–gel assisted in situ reaction bonding, J. Eur. Ceram. Soc. 34 (2) (2014) 237–247. [18] J. Leivo, V. Meretoja, M. Vippola, et al., Sol–gel derived aluminosilicate coatings on alumina as substrate for osteoblasts, Acta Biomater. 2 (6) (2006) 659–668. [19] M. Touzin, F. Béclin, Fabrication and characterization of composite sol–gel coatings on porous ceramic substrate, J. Eur. Ceram. Soc. 31 (9) (2011) 1661–1667. [20] D.A. Barrow, T.E. Petroff, M. Sayer, Thick ceramic coatings using a sol–gel based ceramic–ceramic 0–3 composite, Surf. Coat. Technol. 76 (1995) 113–118. [21] D.L. Corker, Q. Zhang, R.W. Whatmore, et al., PZT ‘composite’ ferroelectric thick films, J. Eur. Ceram. Soc. 22 (3) (2002) 383–390. [22] S. Ramasamy, S.N. Tewari, K.N. Lee, et al., Slurry based multilayer environmental barrier coatings for silicon carbide and silicon nitride ceramics—I. Processing, Surf. Coat. Technol. 205 (2) (2010) 258–265. [23] F. Baino, C. Vitale-Brovarone, Feasibility of glass-ceramic coatings on alumina prosthetic implants by airbrush spraying method, Ceram. Int. 41 (2) (2015) 2150–2159. [24] A.M. Waetjen, D.A. Polsakiewicz, I. Kuhl, et al., Slurry deposition by airbrush for selective laser sintering of ceramic components, J. Eur. Ceram. Soc. 29 (1) (2009) 1–6. [25] Y. Zhang, H. Li, K.Z. Li, et al., C/SiC/Si-Mo-Cr multilayer coating for carbon/carbon composites for oxidation protection, New Carbon Mater. 27 (2) (2012) 105–109. [26] J. Leivo, M. Lindén, J.M. Rosenholm, et al., Evolution of aluminosilicate structure and mullite crystallization from homogeneous nanoparticulate sol–gel precursor with organic additives, J. Eur. Ceram. Soc. 28 (9) (2008) 1749–1762. [27] E.R. de Sola, F.J. Torres, J. Alarcón, Thermal evolution and structural study of 2:1 mullite from monophasic gels, J. Eur. Ceram. Soc. 26 (12) (2006) 2279–2284. [28] E.R. de Sola, F. Estevan, J. Alarcón, Low-temperature Ti-containing 3:2 and 2:1 mullite nanocrystals from single-phase gels, J. Eur. Ceram. Soc. 27 (7) (2007) 2655–2663. [29] J. Li, R. Luo, Y. Chen, et al., Oxidation behavior and kinetics of SiC/alumina-borosilicate coating for carbon–carbon composites, Appl. Surf. Sci. 255 (5) (2008) 1967–1974. [30] T.M. Wu, Y.R. Wu, Methodology in exploring the oxidation behavior of coated-carbon carbon composites, J. Mater. Sci. 29 (5) (1994) 1260–1264.