zrb2–sic(al) ceramics with high resistance to oxidation at 1500°c

Corrosion Science xxx (2013) xxx–xxx

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Corrosion Science

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ZrB2–SiC(Al) ceramics with high resistance to oxidation at 1500 �C

0010-938X/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.corsci.2013.04.037

⇑ Corresponding authors. Tel.: +86 29 88494914 (Y. Wang), tel.: +1 407 823 1009(L. An).

E-mail addresses: wangyiguang@nwpu.edu.cn (Y. Wang), lan@mail.ucf.edu (L.An).

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.d10.1016/j.corsci.2013.04.037

Yiguang Wang a,⇑, Lei Luo a, Jing Sun a, Linan An b,⇑a Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, PR Chinab 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 2012Accepted 23 April 2013Available online xxxx

Keywords:A. CeramicB. SEMC. 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 com-posite was prepared by hot-pressing the mixture of ZrB2 and polymer-derived SiC(Al). The oxidationbehaviour was studied by measuring the weight change as a function of oxidation time and by observingthe structure of the oxide layer. It is shown that the ZrB2–SiC(Al) exhibits different oxidation behaviourand improved oxidation resistance as compared to the conventional ZrB2–SiC without Al-doping. Theimprovement in oxidation resistance is attributed to that Al-doping could increase the bond strengthof the Si–O and suppress the active oxidation of SiC.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Recently, ultrahigh temperature ceramics (UHTCs) have re-ceived extensive attentions due to their critical applications inthe thermal protection systems of hypersonic aerospace vehicles[1–6]. UHTCs, including ZrB2, ZrC, HfB2, HfC, and TaC, have a setof properties that allow the use of them in harsh environmentsassociated with the atmospheric re-entry and hypersonic flights[1,4], including high melting temperature (>3000 �C), ablationresistance, and high-mechanical strength at high temperatures.Among these UHTCs, ZrB2 is the promising one for the aerospaceapplications because of its lowest density (6.09 g/cm3) and goodresistance to thermal shock [2,7]. Since the oxidation of ZrB2 in ser-vice environments of hypersonic flights can deteriorate the proper-ties of the materials and could even damage the vehicles, theoxidation behaviour of ZrB2 and ZrB2-based ceramics at high tem-peratures is highly concerned [2,8,9].

The oxidation of ZrB2 in air leads to the formation of ZrO2 andB2O3. ZrO2 tends to form a porous structure, thus itself cannot pro-tect the material for further oxidation. On the other hand, B2O3 is aliquid at temperatures higher than 450 �C [10]. Below 1100 �C,B2O3 forms a continuous layer to form an effective barrier to oxy-gen diffusion. ZrB2 thus exhibits a good oxidation resistance below1100 �C. However, at temperatures above 1100 �C, the evaporationof B2O3 results in the lost of protection to the underlying ZrB2, lead-ing to the paralinear oxidation kinetics between 1100–1400 �C. Ateven higher temperatures, the rapid evaporation of B2O3 leads to afast linear oxidation kinetics [11]. In order to improve the

oxidation resistance of ZrB2 at higher temperatures, SiC was addedinto ZrB2 to form ZrB2–SiC composites, which can form a borosili-cate layer with lower volatility than B2O3 at temperatures higherthan 1200 �C [12,13]. The ZrB2–SiC thus exhibits passive oxidationbehaviour over a much larger temperature range than pure ZrB2.The continuous demands for safer and faster aerospace vehicles re-quire their leading edges and nosetips to withstand temperaturesover 1500 �C [1–3]. At such high temperatures, the volatility ofsilica becomes obvious, leading to deterioration of the oxidationresistance of the ZrB2–SiC ceramics. Although attempts have beenmade to improve the oxidation resistance at temperatures higherthan 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-de-rived silicon-based ceramics exhibited much better oxidationresistance than SiC ceramics without doping [16–18]. During oxi-dation, a unique oxide structure in which the aluminium sits inthe centre of six-member Si–O rings can be formed, leading toretarding the diffusion of molecular oxygen [16,19]. Such a struc-ture can also reinforce the strength of Si–O bonds, as indicatedby a significantly improved resistance to water-vapour corrosionof the aluminium-doped SiCN ceramics [20,21]. It is known thatthe evaporation of silica could be considered as breaking of Si–Obonds in SiO2 to form volatile SiO. The reinforcement of Si–O bondscan lower the evaporation of silica at high temperatures.

In this study, we innovatively introduce polymer-derived alu-minium-doped SiC (SiC(Al)) instead of conventional SiC into ZrB2

ceramics in order to further improve the oxidation resistance ofZrB2–SiC at 1500 �C or higher. It is found that Al-doping changesthe oxidation behaviour and improves the oxidation resistance ofZrB2–SiC ceramics. The oxidation mechanism of the ZrB2–SiC(Al)will be also elucidated.

oi.org/

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2. Experimental procedure

Liquid polycarbosilane (LPCS, Laboratory of Special AdvancedMaterials, Xiamen University, Xiamen, China) and aluminium ace-tylacetonate powder (purity P97%, Acros Organics, Geel, Belgium)were used as the starting materials for synthesis of SiC(Al). Previ-ous studies [16,22] indicated that the concentration of doping alu-minium should be no more than 8 atm% in order to get an optimaloxidation resistance. Accordingly, we chose the mass ration of LPCSto aluminium acetylacetonate to be 9:1. The mixture was stirredfor 10 h, followed by heat-treatment at 250 �C for 2 h under theprotection of flowing argon to allow the complete reaction of thetwo. Flourier transformation infrared (FTIR) analysis revealed thatthe treatment made aluminium chemically bond to the polycar-bosilane to form polyaluminocarbosilane (PACS) [23]. The resul-tant PACS was cured at 400 �C for 4 h, followed by pyrolysis at1000 �C for 4 h to convert to SiC(Al) ceramics. The composition ofthe as-synthesized SiC(Al) was measured to be SiAl0.040C1.20O0.276

[23]. For comparison, SiC powder without Al-doping was alsosynthesized by directly curing and pyrolyzing the LPCS using thesame procedure. The obtained SiC powders had a composition ofSiC1.24O0.227 [23].

For making the ceramic composites, the ZrB2 powder (0.5 lm,99% purity, Beijing Mountain Technical Development Centre, Bei-jing, China) and SiC(Al) (or SiC) powder were mixed together byball-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 mixturewas dried in a vacuum oven. The resultant mixed powders werethen put into a graphite die, followed by hot pressing at 1800 �Cfor 1 h under a uniaxial pressure of 28 MPa in vacuum. The sam-ples obtained by using SiC(Al) and SiC are denoted as ZrB2–SiC(Al)and ZrB2–SiC, respectively.

For further comparison, the sample with aluminium concentra-tion as the same as the ZrB2–SiC(Al) was also prepared by the samesintering process using ZrB2, polymer-derived SiC powder and AlN(1 lm, 99% purity, YiNuo High-tech Material Co., Ltd., Qinhuang-dao, 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 thehot-pressed materials for oxidation studies. One surface of thesample was polished to 0.5 finish using diamond paste. Isothermaloxidation study was carried out at 1500 �C in an alumina tube fur-nace (GSL-1600X, Hefei Kejing Materials Technology Co., Ltd., He-fei, China). Specimens were placed on zirconia plates with thepolished surface upwards, and then heated to the setting temper-ature in the furnace with the protection of high-purity argon, fol-lowed by exposure to air flowing at 10 ml/min. After oxidation,the samples were cooled in flowing high purity argon. The weightgain as a function of oxidation time was recorded by a balance withthe accuracy of 0.1 mg (Mettler Tolendo AG135, Greifensee, Swit-zerland). The experiments were repeated at least three times foreach materials and oxidation-time intervals. The oxidized sampleswere 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 spectros-copy (EDS).

3. Results and discussion

The densities of the obtained ceramics were measured by Archi-medes 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 theo-retical densities which were calculated by using 6.09 g/cm3 forZrB2, 3.26 g/cm3 for AlN, and 2.6 g/cm3 for SiC(Al) [23]. The SEM

Please cite this article in press as: Y. Wang et al., ZrB2–SiC(Al) ceramics with hi10.1016/j.corsci.2013.04.037

images of the polished surfaces of these ceramics are shown inFig. 1. The dark grains in Fig. 1 are SiC and the grey ones areZrB2. All the ceramics have the similar morphology. Fig. 2 is theXRD patterns of the ceramics. Only ZrB2 and SiC are observed inthe XRD patterns; no oxides and other phases were detected fromthe XRD.

Fig. 3 shows the plots of the specific weight change as a functionof oxidation time for the three samples at 1500 �C. The plots are inthe format of weight change per area vs. the square root of oxida-tion time, thereby the slope of the tangent of the curves is the par-abolic oxidation rate constant (kp) of the materials. It is seen thatthe ZrB2–SiC exhibits an accelerate oxidation behaviour wherethe oxidation rate continually increases with time. The insert inFig. 3 reveals that the oxidation of the ZrB2–SiC ceramic obeys lin-ear behaviour, indicating that the oxide layer formed on ZrB2–SiCceramic cannot provide sufficient protection to the substrate inthe present case. In contrast, the ZrB2–SiC(Al) follows a typical par-abolic oxidation behaviour, suggesting that the dense oxide layerwas formed on the ZrB2–SiC(Al) ceramic, which can effectively pro-tect the substrate from further oxidation. The parabolic oxidationrate constant is calculated to be 2.76 mg/cm2h1/2. The figure showsthat the ZrB2–SiC–AlN also exhibits parabolic oxidation behaviour,but with the oxidation rate constant of 5.89 mg/cm2 h1/2, which istwice higher than that for the ZrB2–SiC(Al). The above resultsclearly demonstrate that Al-doping has profound effect on the oxi-dation behaviour of ZrB2–SiC based materials, including (i) Al-dop-ing changed oxidation behaviour of the materials from linearoxidation to parabolic behaviour; and (ii) the ZrB2–SiC(Al) exhibitsimproved oxidation resistance than the ZrB2–SiC. Another interest-ing thing is that the ZrB2–SiC(Al) exhibits a lower oxidation ratethan the ZrB2–SiC–AlN even though they contain the same amountof Al-doping.

The high-temperature oxidation of ZrB2–SiC is a process com-bining weight gain due to the oxidation of ZrB2 and SiC, and weightloss due to the evaporation of B2O3 and SiO [12,24]. Therefore, di-rect observation of the oxide layer is intuitive to better understandits oxidation behaviour. Fig. 4 shows the cross-sections of the oxi-dized samples at 1500 �C for 10 h. The average thickness of theoxide layers is 390 lm, 400 lm and 60 lm for ZrB2–SiC, ZrB2–SiC–AlN and ZrB2–SiC(Al), respectively. The thickness of the oxidelayer on the ZrB2–SiC(Al) is about 7 times thinner than those onthe ZrB2–SiC and ZrB2–SiC–AlN, further confirming the signifi-cantly improved oxidation resistance of the ZrB2–SiC(Al).

Closer examination reveals that there are three layers on the topof the ZrB2–SiC substrate (Fig. 4a and d): a very thin discontinuousporous SiO2 outer layer, a dense layer containing ZrO2–SiO2–ZrSiO4, and a SiC-depleted porous ZrB2 layer adjacent to the unaf-fected ZrB2–SiC substrate, similar to those reported previously[7,8,12,24]. The ZrB2–SiC–AlN has the oxide structure similar tothat on the ZrB2–SiC (Fig. 4b). In contrast, the oxide layer on thetop of ZrB2–SiC(Al) is drastically different (Fig. 4c). First, the oxidecontains only two layers instead of three layers: a SiO2 top layerand a ZrO2–SiO2–ZrSiO4 inner layer. Second, there is no obviousSiC depleted porous ZrB2 layer. And third, the top SiO2 layer isdense and continuous. EDS analysis near to the interface betweenZrB2–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 concentrationof aluminium in the samples is lower than the limitation of EDS.

The oxidation mechanism for ZrB2–SiC ceramic has been inten-sively studied in the past years [1,7–13,24,25]. It is well acceptedthat the oxidation of ZrB2–SiC at high temperatures results inthree-layer structure: silica layer on the top, a middle layer ofZrSiO4 or a mixture of ZrSiO4–ZrO2–SiO2, and the third SiC-de-pleted layer containing ZrB2 and sometimes a little ZrO2. The third

gh resistance to oxidation at 1500 �C, Corros. Sci. (2013), http://dx.doi.org/

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.Fig. 3. Specific weight change as a function of square root of oxidation time forZrB2–SiC, ZrB2–SiC(Al), and ZrB2–SiC–AlN. The insert figure shows the specificweight change as a function of oxidation time for ZrB2–SiC.

Y. Wang et al. / Corrosion Science xxx (2013) xxx–xxx 3

layer is believed to be formed due to the active oxidation of SiC atthe interface of oxide and ZrB2–SiC matrix [12,25]. According to thethermodynamic study [25,26], the oxygen partial pressure at theinterface is in the range of 10�11–10�14 Pa at 1500 �C, low enoughto allow the active oxidation of SiC [25,26]. Thus, the oxidationprocess of the present ZrB2–SiC can be schematically illustratedin Fig. 5. At the beginning, ZrB2 is oxidized to ZrO2 and B2O3. TheB2O3 easily evaporates out at 1500 �C, leaving porous ZrO2. At theinterface between the oxide and the unaffected substrate, the SiOis formed due to the active oxidation of SiC. The SiO diffuses outto the surface along porous ZrO2. With the increase in oxygen pres-sure at the top of the sample, SiO should be further oxidized toform SiO2 or reacted with ZrO2 to form ZrSiO4. While previousstudies reported that the protective oxide layer can be formedfor the ZrB2–SiC ceramic and the material exhibited parabolic oxi-dation behaviour at 1500 �C, discontinuous silica layer and linearoxidation behaviour are observed in the present study. It couldbe due to that the current oxidation experiment was performed

Please cite this article in press as: Y. Wang et al., ZrB2–SiC(Al) ceramics with hi10.1016/j.corsci.2013.04.037

under a flowing gas condition. The transition between SiO2 andSiO can be expressed by following equation,

SiO2ðsÞ ¼ SiOðgÞ þ 1=2O2ðgÞ ð1Þ

The flowing gas can bring SiO with it and make the Eq. (1) move to-wards right-hand direction, favouring the formation of SiO.

As for the ZrB2–SiC(Al), the passive parabolic oxidation was ob-served. As aforementioned, the only difference between the ZrB2–SiC and ZrB2–SiC(Al) is that a small amount of aluminium existedin SiC for the ZrB2–SiC(Al) ceramics. It is thereby reasonable to de-duce that aluminium doping in SiC could suppress the active oxi-dation of SiC. This can be understood as follow. Previous studies[20,21] demonstrated that the activity of SiO2 can be decreasedby a suitable amount of Al-doping. Accordingly, Eq. (1) will movetowards left-hand direction, meaning SiO2 is favoured to form over

gh resistance to oxidation at 1500 �C, Corros. Sci. (2013), http://dx.doi.org/

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 oxidizedceramics.

4 Y. Wang et al. / Corrosion Science xxx (2013) xxx–xxx

SiO. Thereby, the dense protective SiO2 layer was formed for theZrB2–SiC(Al).

It is a puzzle that while the ZrB2–SiC–AlN also exhibited para-bolic oxidation behaviour, its oxidation rate is higher than theZrB2–SiC(Al) even though they contain the same amount of Al. Thisis likely due to that the AlN usually locates at the grain boundaries

Please cite this article in press as: Y. Wang et al., ZrB2–SiC(Al) ceramics with hi10.1016/j.corsci.2013.04.037

as a sintering aid [27], instead of uniformly distributes within SiCas in polymer-derived SiC(Al). During the oxidation, the uniquealuminium-doped SiO2 structure cannot be formed. In this case,aluminium serving as an impurity, has no strengthening effect onthe silica structure. The oxidation rate of ZrB2–SiC–AlN ceramicsis thus close to, or even worse than that of ZrB2–SiC.

gh resistance to oxidation at 1500 �C, Corros. Sci. (2013), http://dx.doi.org/

Fig. 5. Schematic oxide structure of ZrB2–SiC ceramics.

Y. Wang et al. / Corrosion Science xxx (2013) xxx–xxx 5

4. Conclusions

The oxidation behaviour of the ZrB2–SiC(Al), as well as theZrB2–SiC and ZrB2–SiC–AlN, was studied at 1500 �C for up to10 h. The measurement on weight gain as a function of oxidationtime shows that both ZrB2–SiC(Al) and ZrB2–SiC–AlN exhibit para-bolic oxidation behaviour, while the ZrB2–SiC exhibits linear oxida-tion behaviour. The ZrB2–SiC(Al) exhibits lowest oxidation rate,Observations on the oxide layers reveal that a protective layer isformed 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 othertwo. The improved oxidation resistance is ascribed to that the Si–Obonds is strengthened by Al-doping, thus the active oxidation ofSiC is suppressed.

Acknowledgement

This work is financially supported by the Chinese Natural Sci-ence Foundation (Grant # 51172181 and # 90916030), ‘‘111’’ Pro-ject (B08040), and Program for New Century Excellent Talents inUniversity.

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