Composites: Part B 45 (2013) 1391–1396

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Composites: Part B

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ZrB2/SiC as a protective coating for C/SiC composites: Effect of hightemperature oxidation on mechanical properties and anti-ablation property

Xiang Yang, Li Wei ⇑, Wang Song, Zhang Bi-feng, Chen Zhao-huiScience and Technology on Advanced Ceramic Fibers and Composites, College of Aerospace and Materials Engineering, National University of Defense Technology,Changsha 410073, People’s Republic of China

a r t i c l e i n f o

Article history:Received 5 May 2012Received in revised form 15 June 2012Accepted 6 July 2012Available online 24 July 2012

Keywords:A. Ceramic–matrix compositesB. CorrosionB. Mechanical propertiesD. Surface analysisE. Chemical vapor deposition

1359-8368/$ - see front matter Crown Copyright � 2http://dx.doi.org/10.1016/j.compositesb.2012.07.007

⇑ Corresponding author. Tel.: +86 731 84576441; faE-mail address: liwei3205@163.com (L. Wei).

a b s t r a c t

C/SiC composites are raising great interest as a thermal shielding for aerospace applications, providedthat they are protected from oxidation by suitable coatings. Conversely, ultra high temperature ceramics,and in particular ZrB2, is among the best oxidation resistant materials as known. A new type of oxidationprotective coating (ZrB2/SiC coating) was produced on C/SiC composites. The composites were producedby precursor infiltration and pyrolysis (PIP), which is a simple and low cost method; the coating was ZrB2

mixed with SiC, which was fabricated by painting slurry on the surface of the composite followed bychemical vapor deposition (CVD) SiC on the top. Mechanical tests were conducted before and after oxi-dation test. Anti-ablation property was tested under oxy-acetylene torch. Oxidation test shows that theuncoated samples are oxidized quickly and the weight loss reaches 29.2% and the reservation of originalflexural strength is only 6.74%, while the weight loss of ZrB2/SiC coated samples is only 5.19%, and thereservation of original flexural strength is 37.4%. ZrB2/SiC coating can provide longtime protection forC/SiC composites at 1973 K. Compared with the uncoated composites, the linear and mass ablation ratesof the coated composites decreased by 62.1% and 46.1%, respectively, after ablation for 30 s. The forma-tion of zirconia and silicon dioxide from the oxidation of ZrB2/SiC improved the ablation resistance of thecomposites, because of the evaporation at elevated temperature, which absorbed heat from the flame andreduced the erosive attack to carbon fibers and SiC matrix.

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1. Introduction

Thermal protection systems represent the key issue for the suc-cessful re-entry of a space vehicle [1]. Substantial improvements inthe operational efficiency and reliability of future reentry vehiclescan be obtained by adopting single-body hot structures (self-sustaining and structurally efficient components), as opposed tocurrently employed tile-covered surfaces, together with sharpaerodynamic profiles (bound to provide maneuverability improve-ments and drag reduction during the ascent phase) instead of bluntedges [2,3]. The fulfillment of the abovementioned needs poses theattention on new materials and/or new manufacturing processesable to provide the required performance increase while decreas-ing manufacturing complexity and costs.

For this application two very different classes of materials havebeen considered: carbon fiber composites (in particular with SiCmatrix) [4,5] and ultra high temperature ceramics (UHTCs)[6–11]. The former possess outstanding thermo mechanical prop-erties coupled with an extreme low density, but need a valid

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oxidation protection; the latter have extremely good oxidationresistance at high temperature but a very high density. It isthought that a suitable coating material for C/C or C/SiC compositescould be found in the UHTCs family.

As one kind of the most popular methods to fabricate C/SiCcomposites, precursor infiltration and pyrolysis (PIP) route hasbeen actively developed. Among the processes available to produceC/SiC composites, PIP process is conversely very cost and timeeffective [12,13].

In the family of UHTCs, the most interesting material for ther-mal shielding applications is zirconium diboride (ZrB2), due to itsextremely high melting temperatures, high hardness, and retainedstrength at high temperatures [14]. In particular its oxidation rateis extremely low so that it could be used as a thermal protectionsystem for space re-entry vehicles. This material was studied in1960s and 1970s, but now, due to recent techniques of preparationand sintering [15–17], a new interest has arisen about its proper-ties. In these researches, a ZrB2–SiC matrix was fabricated by PIPcombined with slurry impregnation (SI) method, and ZrB2–SiCcoatings were used in this process was the same as in the matrixin order to increase the ablation resistance [17]. However, up tonow, these materials had not been reported as a coating on surfaceof PIP-C/SiC composites.

ights reserved.

1392 X. Yang et al. / Composites: Part B 45 (2013) 1391–1396

The oxidation mechanism of ZrB2 had been much studied [18–21]. The oxidation began at rather low temperature (between 400and 500 �C), with the formation of ZrO2 and B2O3. When the boronoxide layer on the surface was sufficiently fluid it covered the sur-face and acted as a good diffusion barrier toward oxygen. However,boron oxide partial pressure was not negligible at high tempera-ture [22], so that it began to evaporate. Different strategies to im-prove the oxidation resistance had been tested. SiC addition wasvery beneficial to oxidation resistance [23–31].

In this paper, ZrB2/SiC coating is prepared on the surface of PIP-C/SiC composites by CVD and slurry, microstructure and anti-oxidation property of the as-received composites at 1973 K areinvestigated. In addition, the anti-ablation property is evaluatedunder an oxyacetylene torch flame.

2. Experimental

2.1. Specimens preparation

The substrates (80 mm � 5 mm � 4 mm) were cut from three-dimensional carbon fiber reinforced silicon carbide (C/SiC) com-posites (fabricated by Science and Technology on AdvancedCeramic Fibers and Composites of P.R. China) with a density of2.01 g/cm3 and an average open porosity of 11.4% [12].

Three-dimensional carbon fiber reinforced silicon carbide (C/SiC) composites were fabricated as follow:

Three-dimensional braided carbon fibers (T-300, ex-PAN carbonfiber, Toray) were used as the reinforcement [12]. The fiber volumefraction was �45%. Polycarbosilane (PCS) powders with molecularweight �1742 and soften point �448 K were synthesized in ourlaboratory. Xylene was used as solvent for PCS.

C/SiC composites denoted as raw sample were prepared using9�12 cycles of infiltration of PCS-Xylene solution (mass ratio1:1) and subsequently pyrolysised at 1473 K under N2 (purity:99.99%) atmosphere [12].

After being polished with 500 grit SiC paper, the specimenswere cleaned with ethanol and dried at 373 K for 1 h.

ZrB2/SiC coating was used by slurry in order to increase the oxi-dation resistance. In this process, ZrB2 powders (62.5 lm in diam-eter, crystal, purity: 99%, Dingdong Chemical Engineering InstituteCo., Ltd. P.R. China) with PCS-Xylene solution (mass ratio 4:5:6)were pasted on the composite and subsequently pyrolysised at1473 K under argon (purity: 99.99%) atmosphere.

The technology was different from the results of Li’ report [17]:first, ZrB2 powders were smaller and the ratio of the slurry washigher; secondly, the solution and the pyrolysis process were dif-ferent too. Our technology was cost and time effective.

As for the CVD SiC process, Methyltrichlorosilane (MTS,CH3SiCl3) (a molar ratio of 10 between H2 and MTS) was carriedby bubbling hydrogen in gas phase and argon as the dilute gas toslow down the chemical reaction rate during deposition. The depo-sition temperature was controlled in the 1373 K for 5 h at reducedpressure of 3 kPa.

CVD-SiC coating penetrated in the gaps between ZrB2/SiC pow-ders, then formed a mixture of ZrB2/SiC coatings with a layer ofdense SiC on the top.

2.2. Oxidation tests

To investigate the isothermal oxidation behavior of the as-coated samples, the oxidation resistance test was carried out at1973 K in an electrical furnace. The samples after oxidation testswere weighed by an electronic balance with a sensitivity of±0.001 mg. Three-point bending tests were used to evaluate theflexural strength of C/SiC composites with the span/height ratio

of 15 and a crosshead speed of 0.5 mm/min before and afteroxidation.

The phase and composition of samples before and after oxida-tion were identified by X-ray diffraction (XRD, D8 Advance, CuKa radiation, 5–80�, 2h range, 0.01� wide scanning steps, 1 s/stepacquisition time).

The morphologies and crystalline structures of the sampleswere analyzed by scanning electron microscopy (SEM, JSM-5600LV) and energy dispersive spectroscopy (EDS).

2.3. Ablation tests

Samples (30 mm � 30 mm � 5 mm) for ablation tests were ma-chined from the as-produced composites. The ablation propertiesof the samples were tested with an oxyacetylene torch, and theoxyacetylene flame was parallel to the axial orientation of samples.The pressure and flux of O2 were 0.4 MPa and 1512 L/h, and thoseof C2H2 were 0.095 MPa and 1116 L/h, respectively. The innerdiameter of the nozzle was 8 mm. The distance between the nozzletip and the sample was 10 mm. The sample was fixed in a water-cooled copper concave fixture and exposed to the flame with esti-mated temperature about 2373 K for 30 s. Compared with the tra-ditional ablation process [17], the ablation tests in the paper washarsher. The linear and mass ablation rates of the samples couldbe obtained according to the formulas below:

R1 ¼Ddt

ð1Þ

Rm ¼Dm

tð2Þ

Rl is the linear ablation rate; Dd is the change of the sample’s thick-ness at center region before and after ablation; Rm is the mass abla-tion rate; Dm is the sample’s mass change before and after ablation;t is the ablation time.

The morphologies of the samples were analyzed by scanningelectron microscopy (SEM, JSM-5600LV) combined with energydispersive spectroscopy (EDS).

3. Results and discussion

3.1. Microstructure of the coatings

Fig. 1 shows the SEM micrograph, EDS and X-ray patterns of thecross-section of the as-received coatings obtained by slurry paint-ing and CVD [32]. It reveals that the coatings can be divided twolayers. The first layer is ZrB2/SiC (Fig. 1d) with a porous structure(Fig. 1b), which is infusible phase. However, the porous coatingsprovide a poor oxidation protective ability, due to diffusion of oxy-gen to the C/SiC matrix through the pores in ZrB2/SiC coating. Thesecond layer is SiC coating (Fig. 1e), SiC grains connect each othercompactly. It revealed the formation of dense coating surface andno crack can be found on cross-section of coating (Fig. 1c). FromFig. 1, we can know that the thickness of the first layer is 23 lmand the following SiC layer is 26 lm successively.

3.2. Effect of high temperature oxidation on mechanical properties

In order to verify the effect of the oxidation on mechanicalproperties, the samples were been oxidized at 1973 K in air for2 h. The effect of oxidation on the mechanical behavior of samplesunder investigation was summarized in Table 1.

The mechanical behavior of the composite specimens without asuitable protective coating dramatically got worse with oxidation,the reservation of original flexural strength was only 6.74%.

Fig. 1. SEM micrograph and X-ray patterns of the cross-section of the as-received coating ((a) cross-section of C/SiC; (b) cross-section of ZrB2/SiC coating; (c) cross-section ofSiC coating; (d) X-ray patterns of ZrB2/SiC coating; (e) X-ray patterns of SiC coating).

Table 1Anti-oxidation property of C/SiC composites after oxidation at 1973 K.

Samples Weight loss (%) Retain rate of flexualstrength (%)

C/SiC samples 29.2 6.74ZrB2–SiC coates samples 5.19 37.4

X. Yang et al. / Composites: Part B 45 (2013) 1391–1396 1393

Compared with the uncoated C/SiC, the ZrB2/SiC coating wasable to reduce degradation phenomena in oxidizing environment.The reservation of original flexural strength was 37.4%, ZrB2/SiCcoated C/SiC samples exhibited an obvious increase of oxidationresistance, which indicated that the ZrB2/SiC coating showed a bet-ter high temperature oxidation property.

Morphologies of uncoated samples before and after oxidationare shown in Fig. 2. SEM image (Fig. 2b) shows that no carbon fi-bers in the composites, the matrix of the composites become veryloose and many microcracks emerge (Fig. 2b). The mass loss of un-coated samples reached 29.2%, and the reservation of original flex-ural strength was only 6.74%. While, compared with the uncoatedC/SiC oxidized at 1773 K [32], visible cracks were existed in thesamples. At higher temperature, SiO2 film is a poor barrier to oxy-gen diffusion because of the high evaporation rates of SiO2 and thedeterioration of the oxide film. It was concluded that C/SiC com-posites had been severely damaged after oxidation at 1973 K,which meant that the C/SiC was invalidation after oxidation.

During oxidation tests, because of the open porosity of thecomposites, cracks and pores (Fig. 2b) existing may be the channelsfor the oxidized gas to diffuse into the interior of the composites.The reactions occurring during the oxidation process were asfollows [10]:

2SiCðsÞ þ 3O2ðgÞ ! 2SiO2ðlÞ þ 2COðgÞ ðR1Þ

SiCðsÞ þ 2O2ðgÞ ! SiO2ðlÞ þ CO2ðgÞ ðR2Þ

SiO2ðlÞ ! SiO2ðgÞ ðR3Þ

CðsÞ þ O2ðgÞ ! CO2ðgÞ ðR4Þ

2CðsÞ þ O2ðgÞ ! 2COðgÞ ðR5Þ

According to the reactions between samples and oxygen, theweight of samples would increase after oxidation. Therefore, weconclude the weight loss of the uncoated samples is due to oxida-tion of carbon fiber and evaporation of SiO2, which lead to the fail-ure of C/SiC composites at 1973 K.

Our purpose is thus to develop the coating system with densebarriers. The designed ZrB2/SiC coating, from inside to outside, isZrB2/SiC ? SiC. When being oxidized at 1973 K, the coated sampleskept 5.19% mass loss, which suggested that the coatings were thekey point for little change after oxidation. Little fiber was damagedin the composites (Fig. 3b).

The XRD method is employed to study the micro-structure ofthe coatings. In the XRD spectrum shown by Fig. 4, XRD patternsindicate that ZrB2 particles still exist in the composite after oxida-tion, which mean that the coatings show good oxygen resistance athigher temperature (1973 K). After oxidation tests at 1973 K, ZrO2

and zirconium silicate (ZrSiO4) are found, which is infusible phase.Besides the aforementioned reactions for SiC, oxidation reactionconcerning ZrB2 powder took place as follows:

2ZrB2ðsÞ þ 5O2ðgÞ ¼ 2ZrO2ðsÞ þ 2B2O3ðgÞ ðR6Þ

ZrO2ðsÞ þ SiO2ðlÞ ¼ ZrSiO4ðsÞ ðR7Þ

For ZrB2 oxidized at 1973 K, ZrO2 and B2O3 were formed [33].Appreciable volatilization of B2O3 starts at above 1473 K leavingZrO2 on the coating system [34], which is confirmed by XRD pat-

Fig. 2. SEM of C/SiC composites before (a) and after (b) oxidation.

Fig. 3. SEM of ZrB2/SiC coated C/SiC composites after oxidation ((a) surface; (b) cross-section).

Fig. 4. X-ray patterns of C/SiC composites with ZrB2/SiC coating after oxidation.

Table 2Anti-ablation property of C/SiC composites.

Samples Linear ablationrate (mm/s)

Mass ablationrate (g/s)

Uncoated C/SiC composites 0.0622 0.0425Coated C/SiC composites 0.0236 0.0229

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terns of the coated samples after oxidation (Fig. 4). The oxidationchannels could be sealed by the oxides. The SiC matrix togetherwith fiber is less oxidized. Almost no change of the morphologyis observed. The weight loss for the samples was of course thesmaller (5.19%), and the reservation of original flexural strengthwas 37.4%, effects of the coatings on the mechanical propertieswere remarkable.

The coating acts as a diffusion barrier, forming ZrO2, SiO2 andB2O3 already at low temperature; B2O3 and SiO2 then react forminga borosilicate glass. Even if, in principle, due to the evaporation ofB2O3, the layer should be considered as a sacrificial coating, theborosilicate glass seems noneffective in protecting the composite

in the conditions of this work (>1773 K), these materials exhibitprotective oxidation behavior at temperatures above the stabilitylimit of borosilicate glass due to the formation of a protectiveZrO2 layer. The results prove that the ZrB2/SiC coating protectsC/SiC composites effectively at 1973 K.

3.3. Anti-ablation property of coated C/SiC composites

Table 2 shows the ablation property of C/SiC composites, fromthe data of the table, Uncoated C/SiC samples exhibited severeablation under oxy-acetylene torch (Table 2), without protection,composites can be consumed rapidly; the linear and mass ablationrates reached 0.0622 mm/s and 0.0425 g/s. While, the coatingexhibited better anti-ablative property, the coating prevented ma-trix from ablation efficiently, the linear and mass ablation rates ofthe coated samples reach 0.0236 mm/s and 0.0229 g/s and de-crease by 62.1% and 46.1%, respectively.

Compared with the report of C/ZrB2–SiC [17], the linear andmass ablation rates of the coated samples reached 0.071 mm/sand 0.0117 g/s, the C/ZrB2–SiC showed a better mass ablation ratesdue to the better oxidation property of ZrB2–SiC, while, the ZrB2–SiC coated C/SiC showed a better linear ablation rates. It could fullyfulfill the advantages of refractory compounds.

Fig. 5. Ablation morphology of the coated samples (a) center region and (b) transition region.

X. Yang et al. / Composites: Part B 45 (2013) 1391–1396 1395

ZrB2 and SiC were high melted point materials, both the oxidationof coating and melting of oxides consumed large amount of heat;also the evaporation of B2O3 could take away large amounts of heat,which was also beneficial to the protection for C/SiC composites.

The surface morphologies of C/SiC composites with ZrB2/SiCcoating after ablation for 30 s are shown in Fig. 5, a great changein morphology occurred on the surface of the composites. The cen-tral region was the core of oxyacetylene flame and its ablation wasthe more severe of the two regions. The coating became loosen andporous after being oxidized, with the ablation going on, the coatingwas consumed out by the shearing action of the oxyacetyleneflame, then C/SiC composites was oxidized. SiC matrix was oxi-dized and its oxides sublimated completely, leaving naked nee-dle-shaped carbon fibers in the central region. In the transitionzone, the mechanical denudation became weakened, the coatingwas not consumed out, the oxides of the coating still remained atthe surface, and the oxides covered the coating and preventedthe C/SiC from ablation further.

To reveal the ablation behavior further, the EDS of the center re-gion and the transition region are listed in Fig. 6. There are largeamounts of white phase generated in the center region. EDS resultsshows that there are C, O and Si, which indicated that the whitephase is composed of carbides and oxides of Si, the white and grayphases could be distinguished as SiC and SiO2, respectively. So thecoatings were almost consumed out in the center region.

In the transition region, the oxides of the coating still remainedat the surface, EDS results shows that there are Zr, B, C, O and Si,which indicated that the phase was composed of carbides and oxi-des of Si, B and Zr.

During ablation, the center region suffers the highest tempera-ture and pressure, in the transition erosion region, the temperature

Fig. 6. EDS and center region SEM images of the coated sampl

and pressure decrease significantly. The reactions about SiC coat-ing ((Eqs. (R1), (R2), (R3)) happen firstly, and then the reactions(R6) and (R7). In fact, B2O3 has an unusually low melting point(773 K) and a high vapor pressure [34]. Therefore, at high temper-ature, B2O3 vaporizes quickly. The rapid volatilization of B2O3 leadto the formation of large voids and cracks in the layer areas(Fig. 4b), then results in increased oxidation of ZrB2 since ZrO2 isnot a highly protective oxide. The introduction of SiC remarkablyimproves the oxidation resistance of the ZrB2 material due to theformation of silica glass (Eq. (R7)) which is more viscous, havinga higher melting temperature and a lower vapor pressure, andmore resistant to oxygen diffusion[35].

It can be found that: Eq. (R3) is a melting process and consumeslots of heat during ablation; there are large amounts of gas gener-ated, which can also take heat away from the coatings surface. Thetemperature of the central region can be lowered according to thereactions, which is the reason of well anti-ablative property ofZrB2/SiC coating.

Mono SiC cannot protect from ablation effectively and samplesurface is ablated seriously [36], mass ablation rate and linear abla-tion rate are both greater than those of coated C/SiC composites(Table 2). The melting points of ZrB2 and ZrO2 are 3245 �C and2700 �C, respectively, so ZrB2 will not reach its melting point inthe ablation center of composites, the mechanical corrosion resis-tance and anti-erosion property of the coating can be enhanced.Furthermore, with a low saturated vapor pressure, ZrO2 can endurehigh-temperature ablation with little evaporation and loss greatly.Meanwhile, ZrO2 can endure high-temperature ablation with littleevaporation and loss greatly. Meanwhile ZrO2 can absorb a largeamount of heat during melting to reduce the surface temperatureof the sample and weaken oxyacetylene ablation.

es after ablation ((a) center region; (b) transition region).

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4. Conclusions

In conclusion, ZrB2/SiC as a protective coating was obtained onthe surface of C/SiC composites by slurry painting and CVD, theneffect of high temperature oxidation on mechanical propertiesand anti-ablation property were compared and investigated.

(1) ZrB2/SiC coating showed better oxygen resistance at highertemperature (1973 K). Without coating, the weight loss ofC/SiC composites reached 29.2% and retained only 6.74% oforiginal flexural strength; while, C/SiC composites withZrB2/SiC coating retained 37.4% of original flexural strengthand the weight loss reached 5.19%. ZrB2/SiC coating couldprovide longtime protection for C/SiC composites at highertemperature (1973 K).

(2) The linear and mass ablation rates of C/SiC changed mark-edly under oxyacetylene torch for 30 s with coatings ornot. The linear and mass ablation rates of the coated com-posites decreased by 62.1% and 46.1%, respectively. The gas-ses released during ablation could take away a lot of heat,which was also helpful to the protection for the composites.The results indicated that it could fully fulfill the advantagesof refractory compounds.

Acknowledgments

The authors are grateful to National Natural Science Foundationof China (90916002) for financial support. In addition the authorswould like to thank Processor Q.S. Ma for help with valuablediscussions.

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