Ceramics International 42 (2016) 17174–17178

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Reactive melt infiltration fabrication of C/C-SiC composite: Wettingand infiltration

Yonggang Tong a,b,n, Shuxin Bai b, Xiubing Liang a, Qing H. Qin c, Jiangtao Zhai d

a National Engineering Research Center for Mechanical Product Remanufacturing, Academy of Armored Forces Engineering, Beijing, Chinab College of Aerospace Science and Engineering, National University of Defense Technology, Changsha, Chinac Research School of Engineering, College of Engineering and Computer Science, Australian National University, ACT, Australiad School of Electronics and Information, Jiangsu University of Science and Technology, Zhenjiang, China

a r t i c l e i n f o

Article history:Received 21 July 2016Received in revised form31 July 2016Accepted 1 August 2016Available online 2 August 2016

Keywords:WettingInfiltrationC/C composite preformReactive melt infiltrationLiquid silicon

x.doi.org/10.1016/j.ceramint.2016.08.00742/& 2016 Elsevier Ltd and Techna Group S.r

esponding author.ail address: tygiaarh419@163.com (Y. Tong).

e cite this article as: Y. Tong, et al., Rnational (2016), http://dx.doi.org/10.

a b s t r a c t

Reactive melt infiltration is a fast and economical fabrication process for high performance C/C-SiCcomposite. In order to help understanding reactive melt infiltration production of C/C-SiC composite byliquid silicon, wetting and infiltration of the porous C/C composite preform by liquid silicon were in-vestigated using a sessile drop technique. The contact angle decreased with the increase of time whilethe drop base diameter increased. According to the variation of drop base diameter and contact angle as afunction of time, four different stages corresponding to the interfacial reaction and infiltration of liquidsilicon were identified during wetting of the porous C/C composite preform by the liquid silicon. Theinfiltration height based on wetting curve linearly increased with time, much smaller than that calcu-lated according to Washburn equation, which strongly indicated the reaction control of silicon infiltra-tion.

& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

1. Introduction

C/C-SiC composite is a potential candidate for highly demand-ing engineering applications such as heat shields, structuralcomponents for reentry space vehicles, high performance brakediscs and high temperature heat exchanger tubes due to its lowdensity, high hardness, excellent oxidation resistance, highstrength and thermal shock resistance [1–3]. Reactive melt in-filtration (RMI) has been demonstrated to be an effective techni-que for preparing C/C-SiC composite [4,5]. Its advantages includeshort fabrication period, low cost and near net shape [6]. Duringthe RMI process, liquid silicon infiltrates into a porous C/C preformunder the driving of capillary force. The liquid silicon expands tofill the pores and reacts with the carbon in the porous C/C preform,which ultimately results in a dense carbon and SiC compositematrix. Wetting of the porous C/C preform by liquid silicon is thefoundation of RMI process. In the past years, wetting behavior ofcarbon by liquid silicon has been investigated by several re-searchers. Li et al. [7] used commercial graphite materials withdifferent surface roughness as substrates and tested the contactangles at 1430 °C. Dezellus et al. [8] performed wetting experi-ments with molten silicon on glassy (vitreous) carbon and pseudo-

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eactive melt infiltration fab1016/j.ceramint.2016.08.007

monocrystalline graphite. Israel et al. [9] studied the wetting ofdifferent graphite substrates by liquid silicon. Whalen et al. [10]reported equilibrium contact angles under vacuum at 1426 °Cfrom 40° to 50° for glassy carbon and 5° to 15° for graphite ma-terials. The contact angles on different carbon substrates andmeasured values by different researchers have certain differences.Particularly, the above work focuses on the wetting of glassy car-bon and graphite substrates with very few pores by liquid silicon.No published information is available on the wetting of porous C/Ccomposite preform with large amount of pores by liquid silicon.

Infiltration of the liquid silicon is critical for the RMI fabricationof C/C-SiC composite. It determines the density of the as-producedcomposite and therefore greatly affects the composite's perfor-mance. Liquid silicon infiltration is generally described to be aviscous flow with a parabolic trend as predicted by Washburnequation [11–13]. However, the recent work on infiltration of Si–Nialloy and silicon into graphite material indicated that silicon in-filtration is not limited by the viscous flow but by the reaction atthe infiltration front with a linear variation process [14,15]. Theseinfiltration experiments were performed with graphite materialwith few pores (o15%) and the infiltration was interrupted by theclosure of the small pores in the graphite. The infiltration height isvery small (1–2 mm) due to the closure of the pores, which isdifferent from the silicon infiltration during RMI fabrication of C/C-SiC composite (several millimeters without pore closure). The si-licon infiltration into C/C composite preform with large porosities

rication of C/C-SiC composite: Wetting and infiltration, Ceramicsi

Y. Tong et al. / Ceramics International 42 (2016) 17174–17178 17175

during RMI fabrication of C/C-SiC composite is still not wellunderstood.

The aim of this investigation is to study the wetting and in-filtration of a porous C/C composite preform by liquid silicon. Thewetting behavior and infiltration process of C/C composite pre-forms with large porosity (25%) were investigated, which couldhelp to understand RMI production of C/C-SiC composite.

2. Experimental

Porous C/C composite preforms prepared for RMI production ofC/C-SiC composite were used as the substrate material. Carbonfiber needled felts were used as the reinforcement of the preforms.The carbon fibers were PAN-based (T300, Toray, Japan). The nee-dled felts were prepared by a three-dimensional needling techni-que, starting with repeatedly overlapping the layers of 0° non-woven fiber cloth, short-cut-fiber web, and 90° non-woven fibercloth with needle-punching step by step. Pyrolytic carbon wasdeposited on the surface of carbon fibers by chemical vapor in-filtration and the porous C/C composite preforms were obtained.The density of the preforms was approximately 1.3 g/cm3 and theopen porosity was about 25%. The substrate surfaces were me-chanically ground and polished by diamond paste to a mirrorfinish. Silicon of electronic purity (less than 20 ppb of total im-purities) was cut into small cubic pieces with dimensions of3�3�3 mm for wetting experiments. Before the wetting ex-periment, both C/C composite preforms and silicon were im-mersed in acetone and ultrasonically cleaned and dried at 100 °Cfor 2 h. The wetting experiments were performed in a flowing Ar

Fig. 1. Wetting of liquid silicon on the porous C/C composite preform with the increasmorphology of the wetted sample.

Please cite this article as: Y. Tong, et al., Reactive melt infiltration fabInternational (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.007

(99.999%) atmosphere using a sessile drop method. Detailed de-scription of the experimental procedure was given elsewhere [16].The C/C composite preform substrate was placed horizontally in avacuum chamber while the small cube of silicon was stored in astainless-steel tube with a flexible connector on the top of thedropping device outside the chamber. The stainless-steel tubeconnected an open alumina tube through which the silicon couldbe delivered to the substrate surface when the desired testingtemperature (i.e. 1773 K) was reached, thus enabling the iso-thermal wetting kinetics to be fully monitored and avoiding thepre-interaction of the wetting couples during continuous heating.The drop profiles during spreading were captured using a highresolution charge-coupled device camera. The contact angles to-gether with drop geometric parameters such as drop base dia-meter and height were directly calculated from the drop profilesby using an axisymmetric drop shape analysis program based onthe Laplace equation. After the wetting experiments, the specimenwas embedded in resin, sectioned perpendicular to the interface,and then polished for microstructure observation and phaseidentification. The morphologies of the specimen were observedby Hitachi-S4800 scanning electron microscope (SEM). The che-mical composition was examined by energy dispersive spectro-scopy (EDS). The phases of the specimen were identified by X-raydiffraction (XRD, Rigaku D/Max 2550VB-).

3. Results and discussion

Fig. 1 shows the spreading process of liquid silicon on theporous C/C composite preform substrate. The time t¼0 refers to

e of time (a) t¼0, (b-d) 0oto43s, (e) 43soto70s, (f) 70sot, and (g) the final

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the complete melting of the solid silicon. Seen from Fig. 1, themelting silicon droplet gets a regular spherical crown shape on theporous C/C composite preform. The contact angle takes relativelyhigh value at the moment of complete melting but decreases ra-pidly with time when to43s and decreases relatively slowly afterabout 43s. After wetting for about 70s, all the liquid silicon spreadsand infiltrates into the porous C/C composite preform, resulting ina uniform and regular spreading circle. This phenomenon is dif-ferent from that of Li et al.'s work [7]. In their study, they found thespreading process of liquid silicon proceeds in a non-uniformmanner and the solid-liquid-vapor three phase line was non-circular and irregular.

Fig. 2(a) shows the drop base diameter and contact angle as afunction of time for silicon spreading over the porous C/C com-posite preform at 1500 °C. The contact angle decreases rapidlywith time while the drop base diameter increases. The initialcontact angle between the porous C/C composite preform and si-licon is 106.7°, a little larger than that between porous graphiteand silicon (90°) [9] and that between glassy carbon and silicon(95°) [7]. As can be seen from Fig. 2(b), the rapid spreading of li-quid silicon occurs at a nearly constant rate equal to about 30 mm/s. This is several orders of magnitude slower than the spreadingrates measured in non-reactive liquid metal/solid systems [17,18]and is about 3.4 times of that of liquid silicon on the porous gra-phite substrates [9]. According to the variation of drop base dia-meter and contact angle as a function of time, four different stagesas shown in Fig. 2(a) can be identified during the wetting of por-ous C/C composite preform by liquid silicon. Stage 1 slight de-crease of contact angle and slight increase of drop base diameterwith time; stage 2 sharp decrease of contact angle and sharp in-crease of drop base diameter with time; stage 3 slow decrease ofcontact angle and constant of drop base diameter with time; stage4 constant of contact angle and drop base diameter.

It is known that carbon reacts with liquid silicon at high tem-peratures and forms a SiC interfacial layer between carbon and

Fig. 2. (a) Drop base diameter and contact angle as a function of time for siliconspreading over the porous C/C composite preform at 1500 °C. t¼0 corresponds tothe complete melting of Si. (b) Spreading rate of the liquid silicon. (c) Logarithmicrepresentation of contact angle and drop base diameter versus time.

Please cite this article as: Y. Tong, et al., Reactive melt infiltration fabInternational (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.007

liquid silicon. The interfacial reaction is believed to influence thewetting process of porous C/C composite preform by liquid silicon.Reaction of carbon with liquid silicon indicates that carbon beginsto dissolve into the liquid silicon once they contact and a con-tinuous SiC layer is then formed between carbon and liquid siliconin a very short time period [7,19,20]. The variation of drop basediameter and contact angle in the above 4 stages indentified inFig. 2(a) closely relates to the interfacial Si–C reaction process andinfiltration of liquid silicon into the porous C/C preform.

At the initial stage of wetting process, silicon is melted andcontacts with the solid carbon. Once the solid carbon contactswith the liquid silicon, it begins to dissolve into the liquid silicon,which leads to the spreading of silicon on the C/C compositepreform substrate. The solubility of carbon in liquid silicon is verysmall [7,19] and the dissolution of carbon becomes saturated in avery short time period. Thus, the dissolution of carbon just makesa slight spreading of silicon on the C/C composite preform andlasts for a very short time period, which results in a slight decreaseof contact angle and slight increase of drop base diameter for avery short time period, corresponding to stage 1 in Fig. 2(a). Due tothe very short lasting time period, stage 1 is hardly seen fromFig. 2(a). However, it can be clearly observed in the logarithmicscale of the contact angle and drop base diameter for time t inFig. 2(c).

Once the dissolution of carbon in the liquid silicon becomessaturated, an interfacial SiC layer is thought to be formed on thesolid carbon by the reaction between carbon and silicon, whichcan be demonstrated by the cross-section morphology of C/Ccomposite preform reacting with the liquid silicon for 35 s (Fig. 3).Both EDS and XRD analysis indicate the formation of SiC layer onthe solid carbon at the substrate and liquid silicon interface. Thereaction between silicon and carbon greatly improves the wettingbetween carbon and silicon and thus, spreading of silicon proceedsat a rather high speed rate as shown in Fig. 2(b). The contact anglesharply decreases and drop base diameter sharply increases, i.e.the stage 2 in Fig. 2(a). Stage 3 has constant drop base diameter,which means the wetting spreading of silicon has stopped. How-ever, the contact angle decreases with time, which should becaused by the infiltration of silicon into the porous C/C compositepreform. After infiltrating of silicon for a certain time, the liquidsilicon completely infiltrates into the porous C/C composite pre-form. The contact angle becomes 0° and the wetting and infiltra-tion of silicon completes, corresponding to the stage 4 in Fig. 2(a) with constant of contact angle and drop base diameter.

Fig. 4 shows the infiltration height as a function of time duringthe wetting of C/C composite preform by the liquid silicon. Theinfiltration height was calculated according to the method in re-ference [14] based on the wetting curve in Fig. 2(a). As seen, theinfiltration height linearly increases with infiltration time, stronglysuggesting that infiltration is governed by the interfacial-reactionformation of SiC on the pore walls at the infiltration front, ratherthan the viscous flow. This is consistent with the results from si-licon infiltration of graphite material with few pores [14,15]. Inorder to further confirm the silicon infiltration kinetics, the in-filtration height as a function of time based on viscous flow of li-quid silicon was calculated according to Washburn equation [12],

σ θμ

=( )

hr

tcos2 1

2

where s and μ are the surface tension and viscosity of the liquidsilicon, θ the equilibrium contact angle of the liquid silicon on thesolid carbon and r the effective pore radius characteristic of thepreform.

Pore-size distribution of the C/C composite preform substratewas determined using a mercury porosimetry. The pore size

rication of C/C-SiC composite: Wetting and infiltration, Ceramicsi

Fig. 3. (a) Cross-section morphology of the specimen after silicon wetting for 35 s (b) EDS analysis patterns (c) XRD patterns.

Fig. 4. Infiltration height of liquid silicon into porous C/C composite preform as afunction of time.

Fig. 5. The pore size distribution of the C/C composite perform.

Y. Tong et al. / Ceramics International 42 (2016) 17174–17178 17177

distribution of the C/C preform is shown in Fig. 5. As can be seen,the pore diameter mainly ranges from 8 mm to 100 mm. Themedian pore diameter on the basis of volume is 19 mm. Thus, the

Please cite this article as: Y. Tong, et al., Reactive melt infiltration fabInternational (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.007

effective pore radius, r, is 9.5 mm based on the median pore dia-meter. Other parameters [7,13,21,22] used in the Eq. (1) are listedin Table 1. Substituting these parameters into Eq. (1), the in-filtration height for different time based on viscous flow of liquidsilicon can be calculated. The calculated results are also shown inFig. 4. As can be seen, the infiltration height at different time basedon viscous flow of liquid silicon is much larger than that in presentwork. The calculated infiltration height for 10 s can reach about240 mm, which is far from the facts. All the above information

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Table 1Physical properties of liquid silicon.

Density (Kg/cm3)

Viscosity (Kg/ms)

Contact angle oncarbon (°)

Surface tension(N/m)

Silicon 2330 6.4�10�4 20 0.82

Y. Tong et al. / Ceramics International 42 (2016) 17174–1717817178

confirms the governing of silicon infiltration by the interfacialreaction formation of SiC on the pore walls at the infiltration front.

4. Conclusions

Wetting and infiltration of the porous C/C composite preformby liquid silicon was investigated using a sessile drop technique.The contact angle decreased with time while the drop base dia-meter increased. The initial contact angle between the porous C/Ccomposite preform and liquid silicon was 106.7° and finallyreached 0° due to the complete infiltration of silicon into theporous preform. The infiltration height linearly increased withtime, which strongly indicated the reaction control of siliconinfiltration.

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