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ZrB2–SiC(Al) ceramics with high resistance to oxidation at 1500 °C
Corrosion Science xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
ZrB2–SiC(Al) ceramics with high resistance to oxidation at 1500 °C Yiguang Wang a,⇑, Lei Luo a, Jing Sun a, Linan An b,⇑ a b
Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, PR China Department of Materials Science and Engineering, Advanced Materials Processing and Analysis Centre, University of Central Florida, Orlando, FL 32816, USA
a r t i c l e
i n f o
Article history: Received 7 December 2012 Accepted 23 April 2013 Available online xxxx Keywords: A. Ceramic B. SEM C. Oxidation
a b s t r a c t In this paper, we study the oxidation behaviour of a ZrB2/Al-doped SiC composite at 1500 °C. The composite was prepared by hot-pressing the mixture of ZrB2 and polymer-derived SiC(Al). The oxidation behaviour was studied by measuring the weight change as a function of oxidation time and by observing the structure of the oxide layer. It is shown that the ZrB2–SiC(Al) exhibits different oxidation behaviour and improved oxidation resistance as compared to the conventional ZrB2–SiC without Al-doping. The improvement in oxidation resistance is attributed to that Al-doping could increase the bond strength of the Si–O and suppress the active oxidation of SiC. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Recently, ultrahigh temperature ceramics (UHTCs) have received extensive attentions due to their critical applications in the thermal protection systems of hypersonic aerospace vehicles [1–6]. UHTCs, including ZrB2, ZrC, HfB2, HfC, and TaC, have a set of properties that allow the use of them in harsh environments associated with the atmospheric re-entry and hypersonic flights [1,4], including high melting temperature (>3000 °C), ablation resistance, and high-mechanical strength at high temperatures. Among these UHTCs, ZrB2 is the promising one for the aerospace applications because of its lowest density (6.09 g/cm3) and good resistance to thermal shock [2,7]. Since the oxidation of ZrB2 in service environments of hypersonic flights can deteriorate the properties of the materials and could even damage the vehicles, the oxidation behaviour of ZrB2 and ZrB2-based ceramics at high temperatures is highly concerned [2,8,9]. The oxidation of ZrB2 in air leads to the formation of ZrO2 and B2O3. ZrO2 tends to form a porous structure, thus itself cannot protect the material for further oxidation. On the other hand, B2O3 is a liquid at temperatures higher than 450 °C [10]. Below 1100 °C, B2O3 forms a continuous layer to form an effective barrier to oxygen diffusion. ZrB2 thus exhibits a good oxidation resistance below 1100 °C. However, at temperatures above 1100 °C, the evaporation of B2O3 results in the lost of protection to the underlying ZrB2, leading to the paralinear oxidation kinetics between 1100–1400 °C. At even higher temperatures, the rapid evaporation of B2O3 leads to a fast linear oxidation kinetics [11]. In order to improve the ⇑ Corresponding authors. Tel.: +86 29 88494914 (Y. Wang), tel.: +1 407 823 1009 (L. An). E-mail addresses: [email protected] (Y. Wang), [email protected] (L. An).
oxidation resistance of ZrB2 at higher temperatures, SiC was added into ZrB2 to form ZrB2–SiC composites, which can form a borosilicate layer with lower volatility than B2O3 at temperatures higher than 1200 °C [12,13]. The ZrB2–SiC thus exhibits passive oxidation behaviour over a much larger temperature range than pure ZrB2. The continuous demands for safer and faster aerospace vehicles require their leading edges and nosetips to withstand temperatures over 1500 °C [1–3]. At such high temperatures, the volatility of silica becomes obvious, leading to deterioration of the oxidation resistance of the ZrB2–SiC ceramics. Although attempts have been made to improve the oxidation resistance at temperatures higher than 1500 °C by adding other additives like TaC, MoSi2, or WC [8,14,15], little improvement was observed. Previous studies revealed that aluminium-doped polymer-derived silicon-based ceramics exhibited much better oxidation resistance than SiC ceramics without doping [16–18]. During oxidation, a unique oxide structure in which the aluminium sits in the centre of six-member Si–O rings can be formed, leading to retarding the diffusion of molecular oxygen [16,19]. Such a structure can also reinforce the strength of Si–O bonds, as indicated by a significantly improved resistance to water-vapour corrosion of the aluminium-doped SiCN ceramics [20,21]. It is known that the evaporation of silica could be considered as breaking of Si–O bonds in SiO2 to form volatile SiO. The reinforcement of Si–O bonds can lower the evaporation of silica at high temperatures. In this study, we innovatively introduce polymer-derived aluminium-doped SiC (SiC(Al)) instead of conventional SiC into ZrB2 ceramics in order to further improve the oxidation resistance of ZrB2–SiC at 1500 °C or higher. It is found that Al-doping changes the oxidation behaviour and improves the oxidation resistance of ZrB2–SiC ceramics. The oxidation mechanism of the ZrB2–SiC(Al) will be also elucidated.
0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.04.037
Please cite this article in press as: Y. Wang et al., ZrB2–SiC(Al) ceramics with high resistance to oxidation at 1500 °C, Corros. Sci. (2013), http://dx.doi.org/ 10.1016/j.corsci.2013.04.037
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Y. Wang et al. / Corrosion Science xxx (2013) xxx–xxx
2. Experimental procedure Liquid polycarbosilane (LPCS, Laboratory of Special Advanced Materials, Xiamen University, Xiamen, China) and aluminium acetylacetonate powder (purity P97%, Acros Organics, Geel, Belgium) were used as the starting materials for synthesis of SiC(Al). Previous studies [16,22] indicated that the concentration of doping aluminium should be no more than 8 atm% in order to get an optimal oxidation resistance. Accordingly, we chose the mass ration of LPCS to aluminium acetylacetonate to be 9:1. The mixture was stirred for 10 h, followed by heat-treatment at 250 °C for 2 h under the protection of flowing argon to allow the complete reaction of the two. Flourier transformation infrared (FTIR) analysis revealed that the treatment made aluminium chemically bond to the polycarbosilane to form polyaluminocarbosilane (PACS) [23]. The resultant PACS was cured at 400 °C for 4 h, followed by pyrolysis at 1000 °C for 4 h to convert to SiC(Al) ceramics. The composition of the as-synthesized SiC(Al) was measured to be SiAl0.040C1.20O0.276 [23]. For comparison, SiC powder without Al-doping was also synthesized by directly curing and pyrolyzing the LPCS using the same procedure. The obtained SiC powders had a composition of SiC1.24O0.227 [23]. For making the ceramic composites, the ZrB2 powder (0.5 lm, 99% purity, Beijing Mountain Technical Development Centre, Beijing, China) and SiC(Al) (or SiC) powder were mixed together by ball-milling for 5 h in a nylon vial with ethanol as the medium. The milling media were zirconia balls. The volume ratio of ZrB2 to SiC(Al) (or SiC) was 80 to 20. After ball-milling, the mixture was dried in a vacuum oven. The resultant mixed powders were then put into a graphite die, followed by hot pressing at 1800 °C for 1 h under a uniaxial pressure of 28 MPa in vacuum. The samples obtained by using SiC(Al) and SiC are denoted as ZrB2–SiC(Al) and ZrB2–SiC, respectively. For further comparison, the sample with aluminium concentration as the same as the ZrB2–SiC(Al) was also prepared by the same sintering process using ZrB2, polymer-derived SiC powder and AlN (1 lm, 99% purity, YiNuo High-tech Material Co., Ltd., Qinhuangdao, China) (molar ratio of 1:0.596:0.011) as the starting materials. The obtained ceramic is denoted as ZrB2–SiC–AlN. The specimens of 10 mm 5 mm 4 mm were cut from the hot-pressed materials for oxidation studies. One surface of the sample was polished to 0.5 finish using diamond paste. Isothermal oxidation study was carried out at 1500 °C in an alumina tube furnace (GSL-1600X, Hefei Kejing Materials Technology Co., Ltd., Hefei, China). Specimens were placed on zirconia plates with the polished surface upwards, and then heated to the setting temperature in the furnace with the protection of high-purity argon, followed by exposure to air flowing at 10 ml/min. After oxidation, the samples were cooled in flowing high purity argon. The weight gain as a function of oxidation time was recorded by a balance with the accuracy of 0.1 mg (Mettler Tolendo AG135, Greifensee, Switzerland). The experiments were repeated at least three times for each materials and oxidation-time intervals. The oxidized samples were characterized by X-ray diffraction (XRD, Rigaku D/max-2400, Tokyo, Japan) with Cu Ka radiation, scanning electron microscopy (SEM, JEOL 6700F, Tokyo, Japan), and energy dispersive spectroscopy (EDS).
3. Results and discussion The densities of the obtained ceramics were measured by Archimedes method to be 5.31, 5.30 and 5.32 g/cm3 for the ZrB2–SiC, ZrB2–SiC(Al) and ZrB2–SiC–AlN, respectively, closing to their theoretical densities which were calculated by using 6.09 g/cm3 for ZrB2, 3.26 g/cm3 for AlN, and 2.6 g/cm3 for SiC(Al) [23]. The SEM
images of the polished surfaces of these ceramics are shown in Fig. 1. The dark grains in Fig. 1 are SiC and the grey ones are ZrB2. All the ceramics have the similar morphology. Fig. 2 is the XRD patterns of the ceramics. Only ZrB2 and SiC are observed in the XRD patterns; no oxides and other phases were detected from the XRD. Fig. 3 shows the plots of the specific weight change as a function of oxidation time for the three samples at 1500 °C. The plots are in the format of weight change per area vs. the square root of oxidation time, thereby the slope of the tangent of the curves is the parabolic oxidation rate constant (kp) of the materials. It is seen that the ZrB2–SiC exhibits an accelerate oxidation behaviour where the oxidation rate continually increases with time. The insert in Fig. 3 reveals that the oxidation of the ZrB2–SiC ceramic obeys linear behaviour, indicating that the oxide layer formed on ZrB2–SiC ceramic cannot provide sufficient protection to the substrate in the present case. In contrast, the ZrB2–SiC(Al) follows a typical parabolic oxidation behaviour, suggesting that the dense oxide layer was formed on the ZrB2–SiC(Al) ceramic, which can effectively protect the substrate from further oxidation. The parabolic oxidation rate constant is calculated to be 2.76 mg/cm2h1/2. The figure shows that the ZrB2–SiC–AlN also exhibits parabolic oxidation behaviour, but with the oxidation rate constant of 5.89 mg/cm2 h1/2, which is twice higher than that for the ZrB2–SiC(Al). The above results clearly demonstrate that Al-doping has profound effect on the oxidation behaviour of ZrB2–SiC based materials, including (i) Al-doping changed oxidation behaviour of the materials from linear oxidation to parabolic behaviour; and (ii) the ZrB2–SiC(Al) exhibits improved oxidation resistance than the ZrB2–SiC. Another interesting thing is that the ZrB2–SiC(Al) exhibits a lower oxidation rate than the ZrB2–SiC–AlN even though they contain the same amount of Al-doping. The high-temperature oxidation of ZrB2–SiC is a process combining weight gain due to the oxidation of ZrB2 and SiC, and weight loss due to the evaporation of B2O3 and SiO [12,24]. Therefore, direct observation of the oxide layer is intuitive to better understand its oxidation behaviour. Fig. 4 shows the cross-sections of the oxidized samples at 1500 °C for 10 h. The average thickness of the oxide layers is 390 lm, 400 lm and 60 lm for ZrB2–SiC, ZrB2– SiC–AlN and ZrB2–SiC(Al), respectively. The thickness of the oxide layer on the ZrB2–SiC(Al) is about 7 times thinner than those on the ZrB2–SiC and ZrB2–SiC–AlN, further confirming the significantly improved oxidation resistance of the ZrB2–SiC(Al). Closer examination reveals that there are three layers on the top of the ZrB2–SiC substrate (Fig. 4a and d): a very thin discontinuous porous SiO2 outer layer, a dense layer containing ZrO2–SiO2– ZrSiO4, and a SiC-depleted porous ZrB2 layer adjacent to the unaffected ZrB2–SiC substrate, similar to those reported previously [7,8,12,24]. The ZrB2–SiC–AlN has the oxide structure similar to that on the ZrB2–SiC (Fig. 4b). In contrast, the oxide layer on the top of ZrB2–SiC(Al) is drastically different (Fig. 4c). First, the oxide contains only two layers instead of three layers: a SiO2 top layer and a ZrO2–SiO2–ZrSiO4 inner layer. Second, there is no obvious SiC depleted porous ZrB2 layer. And third, the top SiO2 layer is dense and continuous. EDS analysis near to the interface between ZrB2–SiC(Al) and the inner oxide layer (Fig. 4f, Point A and Point B) confirms that there is no obvious SiC-depleted layer in the oxide. No aluminium can be detected by EDS for bother ZrB2–SiC–AlN (Fig. 4e) and ZrB2–SiC(Al). It is likely because the concentration of aluminium in the samples is lower than the limitation of EDS. The oxidation mechanism for ZrB2–SiC ceramic has been intensively studied in the past years [1,7–13,24,25]. It is well accepted that the oxidation of ZrB2–SiC at high temperatures results in three-layer structure: silica layer on the top, a middle layer of ZrSiO4 or a mixture of ZrSiO4–ZrO2–SiO2, and the third SiC-depleted layer containing ZrB2 and sometimes a little ZrO2. The third
Please cite this article in press as: Y. Wang et al., ZrB2–SiC(Al) ceramics with high resistance to oxidation at 1500 °C, Corros. Sci. (2013), http://dx.doi.org/ 10.1016/j.corsci.2013.04.037
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Fig. 1. SEM images showing the surface morphologies of the polished ceramics (a) ZrB2–SiC; (b) ZrB2–SiC(Al); and (c) ZrB2–SiC–AlN.
Fig. 2. XRD patterns of the as-received ceramics.
layer is believed to be formed due to the active oxidation of SiC at the interface of oxide and ZrB2–SiC matrix [12,25]. According to the thermodynamic study [25,26], the oxygen partial pressure at the interface is in the range of 1011–1014 Pa at 1500 °C, low enough to allow the active oxidation of SiC [25,26]. Thus, the oxidation process of the present ZrB2–SiC can be schematically illustrated in Fig. 5. At the beginning, ZrB2 is oxidized to ZrO2 and B2O3. The B2O3 easily evaporates out at 1500 °C, leaving porous ZrO2. At the interface between the oxide and the unaffected substrate, the SiO is formed due to the active oxidation of SiC. The SiO diffuses out to the surface along porous ZrO2. With the increase in oxygen pressure at the top of the sample, SiO should be further oxidized to form SiO2 or reacted with ZrO2 to form ZrSiO4. While previous studies reported that the protective oxide layer can be formed for the ZrB2–SiC ceramic and the material exhibited parabolic oxidation behaviour at 1500 °C, discontinuous silica layer and linear oxidation behaviour are observed in the present study. It could be due to that the current oxidation experiment was performed
Fig. 3. Specific weight change as a function of square root of oxidation time for ZrB2–SiC, ZrB2–SiC(Al), and ZrB2–SiC–AlN. The insert figure shows the specific weight change as a function of oxidation time for ZrB2–SiC.
under a flowing gas condition. The transition between SiO2 and SiO can be expressed by following equation,
SiO2 ðsÞ ¼ SiOðgÞ þ 1=2O2 ðgÞ
ð1Þ
The flowing gas can bring SiO with it and make the Eq. (1) move towards right-hand direction, favouring the formation of SiO. As for the ZrB2–SiC(Al), the passive parabolic oxidation was observed. As aforementioned, the only difference between the ZrB2– SiC and ZrB2–SiC(Al) is that a small amount of aluminium existed in SiC for the ZrB2–SiC(Al) ceramics. It is thereby reasonable to deduce that aluminium doping in SiC could suppress the active oxidation of SiC. This can be understood as follow. Previous studies [20,21] demonstrated that the activity of SiO2 can be decreased by a suitable amount of Al-doping. Accordingly, Eq. (1) will move towards left-hand direction, meaning SiO2 is favoured to form over
Please cite this article in press as: Y. Wang et al., ZrB2–SiC(Al) ceramics with high resistance to oxidation at 1500 °C, Corros. Sci. (2013), http://dx.doi.org/ 10.1016/j.corsci.2013.04.037
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Fig. 4. Morphologies ((a) ZrB2–SiC; (b) ZrB2–SiC–AlN; (c) ZrB2–SiC(Al)) and EDS analysis ((d) ZrB2–SiC; (e) ZrB2–SiC–AlN; (f) ZrB2–SiC(Al)) of the cross-sections of the oxidized ceramics.
SiO. Thereby, the dense protective SiO2 layer was formed for the ZrB2–SiC(Al). It is a puzzle that while the ZrB2–SiC–AlN also exhibited parabolic oxidation behaviour, its oxidation rate is higher than the ZrB2–SiC(Al) even though they contain the same amount of Al. This is likely due to that the AlN usually locates at the grain boundaries
as a sintering aid [27], instead of uniformly distributes within SiC as in polymer-derived SiC(Al). During the oxidation, the unique aluminium-doped SiO2 structure cannot be formed. In this case, aluminium serving as an impurity, has no strengthening effect on the silica structure. The oxidation rate of ZrB2–SiC–AlN ceramics is thus close to, or even worse than that of ZrB2–SiC.
Please cite this article in press as: Y. Wang et al., ZrB2–SiC(Al) ceramics with high resistance to oxidation at 1500 °C, Corros. Sci. (2013), http://dx.doi.org/ 10.1016/j.corsci.2013.04.037
Y. Wang et al. / Corrosion Science xxx (2013) xxx–xxx
Fig. 5. Schematic oxide structure of ZrB2–SiC ceramics.
4. Conclusions The oxidation behaviour of the ZrB2–SiC(Al), as well as the ZrB2–SiC and ZrB2–SiC–AlN, was studied at 1500 °C for up to 10 h. The measurement on weight gain as a function of oxidation time shows that both ZrB2–SiC(Al) and ZrB2–SiC–AlN exhibit parabolic oxidation behaviour, while the ZrB2–SiC exhibits linear oxidation behaviour. The ZrB2–SiC(Al) exhibits lowest oxidation rate, Observations on the oxide layers reveal that a protective layer is formed on the ZrB2–SiC(Al), but not on the ZrB2–SiC and ZrB2– SiC–AlN. These results clearly demonstrate that the ZrB2–SiC(Al) has much improved oxidation resistance as compared to the other two. The improved oxidation resistance is ascribed to that the Si–O bonds is strengthened by Al-doping, thus the active oxidation of SiC is suppressed.
Acknowledgement This work is financially supported by the Chinese Natural Science Foundation (Grant # 51172181 and # 90916030), ‘‘111’’ Project (B08040), and Program for New Century Excellent Talents in University.
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Please cite this article in press as: Y. Wang et al., ZrB2–SiC(Al) ceramics with high resistance to oxidation at 1500 °C, Corros. Sci. (2013), http://dx.doi.org/ 10.1016/j.corsci.2013.04.037
Contents lists available at SciVerse ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
ZrB2–SiC(Al) ceramics with high resistance to oxidation at 1500 °C Yiguang Wang a,⇑, Lei Luo a, Jing Sun a, Linan An b,⇑ a b
Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, PR China Department of Materials Science and Engineering, Advanced Materials Processing and Analysis Centre, University of Central Florida, Orlando, FL 32816, USA
a r t i c l e
i n f o
Article history: Received 7 December 2012 Accepted 23 April 2013 Available online xxxx Keywords: A. Ceramic B. SEM C. Oxidation
a b s t r a c t In this paper, we study the oxidation behaviour of a ZrB2/Al-doped SiC composite at 1500 °C. The composite was prepared by hot-pressing the mixture of ZrB2 and polymer-derived SiC(Al). The oxidation behaviour was studied by measuring the weight change as a function of oxidation time and by observing the structure of the oxide layer. It is shown that the ZrB2–SiC(Al) exhibits different oxidation behaviour and improved oxidation resistance as compared to the conventional ZrB2–SiC without Al-doping. The improvement in oxidation resistance is attributed to that Al-doping could increase the bond strength of the Si–O and suppress the active oxidation of SiC. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Recently, ultrahigh temperature ceramics (UHTCs) have received extensive attentions due to their critical applications in the thermal protection systems of hypersonic aerospace vehicles [1–6]. UHTCs, including ZrB2, ZrC, HfB2, HfC, and TaC, have a set of properties that allow the use of them in harsh environments associated with the atmospheric re-entry and hypersonic flights [1,4], including high melting temperature (>3000 °C), ablation resistance, and high-mechanical strength at high temperatures. Among these UHTCs, ZrB2 is the promising one for the aerospace applications because of its lowest density (6.09 g/cm3) and good resistance to thermal shock [2,7]. Since the oxidation of ZrB2 in service environments of hypersonic flights can deteriorate the properties of the materials and could even damage the vehicles, the oxidation behaviour of ZrB2 and ZrB2-based ceramics at high temperatures is highly concerned [2,8,9]. The oxidation of ZrB2 in air leads to the formation of ZrO2 and B2O3. ZrO2 tends to form a porous structure, thus itself cannot protect the material for further oxidation. On the other hand, B2O3 is a liquid at temperatures higher than 450 °C [10]. Below 1100 °C, B2O3 forms a continuous layer to form an effective barrier to oxygen diffusion. ZrB2 thus exhibits a good oxidation resistance below 1100 °C. However, at temperatures above 1100 °C, the evaporation of B2O3 results in the lost of protection to the underlying ZrB2, leading to the paralinear oxidation kinetics between 1100–1400 °C. At even higher temperatures, the rapid evaporation of B2O3 leads to a fast linear oxidation kinetics [11]. In order to improve the ⇑ Corresponding authors. Tel.: +86 29 88494914 (Y. Wang), tel.: +1 407 823 1009 (L. An). E-mail addresses: [email protected] (Y. Wang), [email protected] (L. An).
oxidation resistance of ZrB2 at higher temperatures, SiC was added into ZrB2 to form ZrB2–SiC composites, which can form a borosilicate layer with lower volatility than B2O3 at temperatures higher than 1200 °C [12,13]. The ZrB2–SiC thus exhibits passive oxidation behaviour over a much larger temperature range than pure ZrB2. The continuous demands for safer and faster aerospace vehicles require their leading edges and nosetips to withstand temperatures over 1500 °C [1–3]. At such high temperatures, the volatility of silica becomes obvious, leading to deterioration of the oxidation resistance of the ZrB2–SiC ceramics. Although attempts have been made to improve the oxidation resistance at temperatures higher than 1500 °C by adding other additives like TaC, MoSi2, or WC [8,14,15], little improvement was observed. Previous studies revealed that aluminium-doped polymer-derived silicon-based ceramics exhibited much better oxidation resistance than SiC ceramics without doping [16–18]. During oxidation, a unique oxide structure in which the aluminium sits in the centre of six-member Si–O rings can be formed, leading to retarding the diffusion of molecular oxygen [16,19]. Such a structure can also reinforce the strength of Si–O bonds, as indicated by a significantly improved resistance to water-vapour corrosion of the aluminium-doped SiCN ceramics [20,21]. It is known that the evaporation of silica could be considered as breaking of Si–O bonds in SiO2 to form volatile SiO. The reinforcement of Si–O bonds can lower the evaporation of silica at high temperatures. In this study, we innovatively introduce polymer-derived aluminium-doped SiC (SiC(Al)) instead of conventional SiC into ZrB2 ceramics in order to further improve the oxidation resistance of ZrB2–SiC at 1500 °C or higher. It is found that Al-doping changes the oxidation behaviour and improves the oxidation resistance of ZrB2–SiC ceramics. The oxidation mechanism of the ZrB2–SiC(Al) will be also elucidated.
0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.04.037
Please cite this article in press as: Y. Wang et al., ZrB2–SiC(Al) ceramics with high resistance to oxidation at 1500 °C, Corros. Sci. (2013), http://dx.doi.org/ 10.1016/j.corsci.2013.04.037
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2. Experimental procedure Liquid polycarbosilane (LPCS, Laboratory of Special Advanced Materials, Xiamen University, Xiamen, China) and aluminium acetylacetonate powder (purity P97%, Acros Organics, Geel, Belgium) were used as the starting materials for synthesis of SiC(Al). Previous studies [16,22] indicated that the concentration of doping aluminium should be no more than 8 atm% in order to get an optimal oxidation resistance. Accordingly, we chose the mass ration of LPCS to aluminium acetylacetonate to be 9:1. The mixture was stirred for 10 h, followed by heat-treatment at 250 °C for 2 h under the protection of flowing argon to allow the complete reaction of the two. Flourier transformation infrared (FTIR) analysis revealed that the treatment made aluminium chemically bond to the polycarbosilane to form polyaluminocarbosilane (PACS) [23]. The resultant PACS was cured at 400 °C for 4 h, followed by pyrolysis at 1000 °C for 4 h to convert to SiC(Al) ceramics. The composition of the as-synthesized SiC(Al) was measured to be SiAl0.040C1.20O0.276 [23]. For comparison, SiC powder without Al-doping was also synthesized by directly curing and pyrolyzing the LPCS using the same procedure. The obtained SiC powders had a composition of SiC1.24O0.227 [23]. For making the ceramic composites, the ZrB2 powder (0.5 lm, 99% purity, Beijing Mountain Technical Development Centre, Beijing, China) and SiC(Al) (or SiC) powder were mixed together by ball-milling for 5 h in a nylon vial with ethanol as the medium. The milling media were zirconia balls. The volume ratio of ZrB2 to SiC(Al) (or SiC) was 80 to 20. After ball-milling, the mixture was dried in a vacuum oven. The resultant mixed powders were then put into a graphite die, followed by hot pressing at 1800 °C for 1 h under a uniaxial pressure of 28 MPa in vacuum. The samples obtained by using SiC(Al) and SiC are denoted as ZrB2–SiC(Al) and ZrB2–SiC, respectively. For further comparison, the sample with aluminium concentration as the same as the ZrB2–SiC(Al) was also prepared by the same sintering process using ZrB2, polymer-derived SiC powder and AlN (1 lm, 99% purity, YiNuo High-tech Material Co., Ltd., Qinhuangdao, China) (molar ratio of 1:0.596:0.011) as the starting materials. The obtained ceramic is denoted as ZrB2–SiC–AlN. The specimens of 10 mm 5 mm 4 mm were cut from the hot-pressed materials for oxidation studies. One surface of the sample was polished to 0.5 finish using diamond paste. Isothermal oxidation study was carried out at 1500 °C in an alumina tube furnace (GSL-1600X, Hefei Kejing Materials Technology Co., Ltd., Hefei, China). Specimens were placed on zirconia plates with the polished surface upwards, and then heated to the setting temperature in the furnace with the protection of high-purity argon, followed by exposure to air flowing at 10 ml/min. After oxidation, the samples were cooled in flowing high purity argon. The weight gain as a function of oxidation time was recorded by a balance with the accuracy of 0.1 mg (Mettler Tolendo AG135, Greifensee, Switzerland). The experiments were repeated at least three times for each materials and oxidation-time intervals. The oxidized samples were characterized by X-ray diffraction (XRD, Rigaku D/max-2400, Tokyo, Japan) with Cu Ka radiation, scanning electron microscopy (SEM, JEOL 6700F, Tokyo, Japan), and energy dispersive spectroscopy (EDS).
3. Results and discussion The densities of the obtained ceramics were measured by Archimedes method to be 5.31, 5.30 and 5.32 g/cm3 for the ZrB2–SiC, ZrB2–SiC(Al) and ZrB2–SiC–AlN, respectively, closing to their theoretical densities which were calculated by using 6.09 g/cm3 for ZrB2, 3.26 g/cm3 for AlN, and 2.6 g/cm3 for SiC(Al) [23]. The SEM
images of the polished surfaces of these ceramics are shown in Fig. 1. The dark grains in Fig. 1 are SiC and the grey ones are ZrB2. All the ceramics have the similar morphology. Fig. 2 is the XRD patterns of the ceramics. Only ZrB2 and SiC are observed in the XRD patterns; no oxides and other phases were detected from the XRD. Fig. 3 shows the plots of the specific weight change as a function of oxidation time for the three samples at 1500 °C. The plots are in the format of weight change per area vs. the square root of oxidation time, thereby the slope of the tangent of the curves is the parabolic oxidation rate constant (kp) of the materials. It is seen that the ZrB2–SiC exhibits an accelerate oxidation behaviour where the oxidation rate continually increases with time. The insert in Fig. 3 reveals that the oxidation of the ZrB2–SiC ceramic obeys linear behaviour, indicating that the oxide layer formed on ZrB2–SiC ceramic cannot provide sufficient protection to the substrate in the present case. In contrast, the ZrB2–SiC(Al) follows a typical parabolic oxidation behaviour, suggesting that the dense oxide layer was formed on the ZrB2–SiC(Al) ceramic, which can effectively protect the substrate from further oxidation. The parabolic oxidation rate constant is calculated to be 2.76 mg/cm2h1/2. The figure shows that the ZrB2–SiC–AlN also exhibits parabolic oxidation behaviour, but with the oxidation rate constant of 5.89 mg/cm2 h1/2, which is twice higher than that for the ZrB2–SiC(Al). The above results clearly demonstrate that Al-doping has profound effect on the oxidation behaviour of ZrB2–SiC based materials, including (i) Al-doping changed oxidation behaviour of the materials from linear oxidation to parabolic behaviour; and (ii) the ZrB2–SiC(Al) exhibits improved oxidation resistance than the ZrB2–SiC. Another interesting thing is that the ZrB2–SiC(Al) exhibits a lower oxidation rate than the ZrB2–SiC–AlN even though they contain the same amount of Al-doping. The high-temperature oxidation of ZrB2–SiC is a process combining weight gain due to the oxidation of ZrB2 and SiC, and weight loss due to the evaporation of B2O3 and SiO [12,24]. Therefore, direct observation of the oxide layer is intuitive to better understand its oxidation behaviour. Fig. 4 shows the cross-sections of the oxidized samples at 1500 °C for 10 h. The average thickness of the oxide layers is 390 lm, 400 lm and 60 lm for ZrB2–SiC, ZrB2– SiC–AlN and ZrB2–SiC(Al), respectively. The thickness of the oxide layer on the ZrB2–SiC(Al) is about 7 times thinner than those on the ZrB2–SiC and ZrB2–SiC–AlN, further confirming the significantly improved oxidation resistance of the ZrB2–SiC(Al). Closer examination reveals that there are three layers on the top of the ZrB2–SiC substrate (Fig. 4a and d): a very thin discontinuous porous SiO2 outer layer, a dense layer containing ZrO2–SiO2– ZrSiO4, and a SiC-depleted porous ZrB2 layer adjacent to the unaffected ZrB2–SiC substrate, similar to those reported previously [7,8,12,24]. The ZrB2–SiC–AlN has the oxide structure similar to that on the ZrB2–SiC (Fig. 4b). In contrast, the oxide layer on the top of ZrB2–SiC(Al) is drastically different (Fig. 4c). First, the oxide contains only two layers instead of three layers: a SiO2 top layer and a ZrO2–SiO2–ZrSiO4 inner layer. Second, there is no obvious SiC depleted porous ZrB2 layer. And third, the top SiO2 layer is dense and continuous. EDS analysis near to the interface between ZrB2–SiC(Al) and the inner oxide layer (Fig. 4f, Point A and Point B) confirms that there is no obvious SiC-depleted layer in the oxide. No aluminium can be detected by EDS for bother ZrB2–SiC–AlN (Fig. 4e) and ZrB2–SiC(Al). It is likely because the concentration of aluminium in the samples is lower than the limitation of EDS. The oxidation mechanism for ZrB2–SiC ceramic has been intensively studied in the past years [1,7–13,24,25]. It is well accepted that the oxidation of ZrB2–SiC at high temperatures results in three-layer structure: silica layer on the top, a middle layer of ZrSiO4 or a mixture of ZrSiO4–ZrO2–SiO2, and the third SiC-depleted layer containing ZrB2 and sometimes a little ZrO2. The third
Please cite this article in press as: Y. Wang et al., ZrB2–SiC(Al) ceramics with high resistance to oxidation at 1500 °C, Corros. Sci. (2013), http://dx.doi.org/ 10.1016/j.corsci.2013.04.037
Y. Wang et al. / Corrosion Science xxx (2013) xxx–xxx
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Fig. 1. SEM images showing the surface morphologies of the polished ceramics (a) ZrB2–SiC; (b) ZrB2–SiC(Al); and (c) ZrB2–SiC–AlN.
Fig. 2. XRD patterns of the as-received ceramics.
layer is believed to be formed due to the active oxidation of SiC at the interface of oxide and ZrB2–SiC matrix [12,25]. According to the thermodynamic study [25,26], the oxygen partial pressure at the interface is in the range of 1011–1014 Pa at 1500 °C, low enough to allow the active oxidation of SiC [25,26]. Thus, the oxidation process of the present ZrB2–SiC can be schematically illustrated in Fig. 5. At the beginning, ZrB2 is oxidized to ZrO2 and B2O3. The B2O3 easily evaporates out at 1500 °C, leaving porous ZrO2. At the interface between the oxide and the unaffected substrate, the SiO is formed due to the active oxidation of SiC. The SiO diffuses out to the surface along porous ZrO2. With the increase in oxygen pressure at the top of the sample, SiO should be further oxidized to form SiO2 or reacted with ZrO2 to form ZrSiO4. While previous studies reported that the protective oxide layer can be formed for the ZrB2–SiC ceramic and the material exhibited parabolic oxidation behaviour at 1500 °C, discontinuous silica layer and linear oxidation behaviour are observed in the present study. It could be due to that the current oxidation experiment was performed
Fig. 3. Specific weight change as a function of square root of oxidation time for ZrB2–SiC, ZrB2–SiC(Al), and ZrB2–SiC–AlN. The insert figure shows the specific weight change as a function of oxidation time for ZrB2–SiC.
under a flowing gas condition. The transition between SiO2 and SiO can be expressed by following equation,
SiO2 ðsÞ ¼ SiOðgÞ þ 1=2O2 ðgÞ
ð1Þ
The flowing gas can bring SiO with it and make the Eq. (1) move towards right-hand direction, favouring the formation of SiO. As for the ZrB2–SiC(Al), the passive parabolic oxidation was observed. As aforementioned, the only difference between the ZrB2– SiC and ZrB2–SiC(Al) is that a small amount of aluminium existed in SiC for the ZrB2–SiC(Al) ceramics. It is thereby reasonable to deduce that aluminium doping in SiC could suppress the active oxidation of SiC. This can be understood as follow. Previous studies [20,21] demonstrated that the activity of SiO2 can be decreased by a suitable amount of Al-doping. Accordingly, Eq. (1) will move towards left-hand direction, meaning SiO2 is favoured to form over
Please cite this article in press as: Y. Wang et al., ZrB2–SiC(Al) ceramics with high resistance to oxidation at 1500 °C, Corros. Sci. (2013), http://dx.doi.org/ 10.1016/j.corsci.2013.04.037
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Y. Wang et al. / Corrosion Science xxx (2013) xxx–xxx
Fig. 4. Morphologies ((a) ZrB2–SiC; (b) ZrB2–SiC–AlN; (c) ZrB2–SiC(Al)) and EDS analysis ((d) ZrB2–SiC; (e) ZrB2–SiC–AlN; (f) ZrB2–SiC(Al)) of the cross-sections of the oxidized ceramics.
SiO. Thereby, the dense protective SiO2 layer was formed for the ZrB2–SiC(Al). It is a puzzle that while the ZrB2–SiC–AlN also exhibited parabolic oxidation behaviour, its oxidation rate is higher than the ZrB2–SiC(Al) even though they contain the same amount of Al. This is likely due to that the AlN usually locates at the grain boundaries
as a sintering aid [27], instead of uniformly distributes within SiC as in polymer-derived SiC(Al). During the oxidation, the unique aluminium-doped SiO2 structure cannot be formed. In this case, aluminium serving as an impurity, has no strengthening effect on the silica structure. The oxidation rate of ZrB2–SiC–AlN ceramics is thus close to, or even worse than that of ZrB2–SiC.
Please cite this article in press as: Y. Wang et al., ZrB2–SiC(Al) ceramics with high resistance to oxidation at 1500 °C, Corros. Sci. (2013), http://dx.doi.org/ 10.1016/j.corsci.2013.04.037
Y. Wang et al. / Corrosion Science xxx (2013) xxx–xxx
Fig. 5. Schematic oxide structure of ZrB2–SiC ceramics.
4. Conclusions The oxidation behaviour of the ZrB2–SiC(Al), as well as the ZrB2–SiC and ZrB2–SiC–AlN, was studied at 1500 °C for up to 10 h. The measurement on weight gain as a function of oxidation time shows that both ZrB2–SiC(Al) and ZrB2–SiC–AlN exhibit parabolic oxidation behaviour, while the ZrB2–SiC exhibits linear oxidation behaviour. The ZrB2–SiC(Al) exhibits lowest oxidation rate, Observations on the oxide layers reveal that a protective layer is formed on the ZrB2–SiC(Al), but not on the ZrB2–SiC and ZrB2– SiC–AlN. These results clearly demonstrate that the ZrB2–SiC(Al) has much improved oxidation resistance as compared to the other two. The improved oxidation resistance is ascribed to that the Si–O bonds is strengthened by Al-doping, thus the active oxidation of SiC is suppressed.
Acknowledgement This work is financially supported by the Chinese Natural Science Foundation (Grant # 51172181 and # 90916030), ‘‘111’’ Project (B08040), and Program for New Century Excellent Talents in University.
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Please cite this article in press as: Y. Wang et al., ZrB2–SiC(Al) ceramics with high resistance to oxidation at 1500 °C, Corros. Sci. (2013), http://dx.doi.org/ 10.1016/j.corsci.2013.04.037