Electrophoretic Deposition of Ti3Si(Al)C2 from Aqueous Suspension

Ying Liang,z,y Ziqi Sun,y,z Jixin Chen,y Xiaoxia Liu,z and Yanchun Zhou*,w,y

zDepartment of Chemistry, Northeastern University, Shenyang 110004, China

yShenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences,Shenyang 110016, China

zGraduate School of Chinese Academy of Science, Beijing 100039, China

Ti3Si(Al)C2 films were electrophoretically deposited at 3 V onindium-tin-oxide (ITO) conductive glass from Ti3Si(Al)C2 aque-ous suspension with 1 vol% solid loading at pH 9 in the absenceof any dispersant. The surface morphology, cross section mi-crostructure, and preferred orientation of the films were inves-tigated by scanning electron microscopy and X-ray diffraction.The as-deposited Ti3Si(Al)C2 films exhibited (00l) preferredorientation and the thickness can be controlled by the deposi-tion–drying–deposition method. These results demonstrate thatelectrophoretic deposition is a simple and feasible method toprepare MAX-phases green films at room temperature.

I. Introduction

Ti3SiC2 is a member of layered ternary ceramics Mn11AXn

(MAX-phases, where M is a transition metal, A is an IIIA orIVA element, X is C or N, and n5 1, 2, or 3) and exhibits salientproperties of low density, high strength and modulus, chemicalstability, damage tolerance at room temperature, good machin-ability, and being resistant to thermal shock and oxidation.1–3

Thin films of this material can be considered as electrical contactlayers and connectors at high temperatures due to its good elec-trical and thermal conductivity.4 In addition, Ti3SiC2 is also acandidate material for high-temperature oxidation-resistant andcorrosion-protective coatings for metallic and other substrates.To improve the high-temperature oxidation resistance, Ti3Si(Al)C2 solid solutions were prepared by substituting a smallamount of Si with Al because of the formation of a protectivea-Al2O3 layer during high-temperature oxidation.5,6 TiC impu-rity in Ti3SiC2 could also be eliminated by forming Ti3Si(Al)C2

solid solutions.5

For the application as protective coatings, a denseTi3Si(Al)C2 film must be deposited on the substrate. To achievesuch a goal, different methods have been used to deposit MAX-phases films including chemical vapor deposition (CVD), mag-netron sputtering (MS), and high-velocity oxy-fuel (HVOF)spraying. Ti3SiC2 coatings could be deposited by CVD fromdifferent gas mixtures, such as TiCl4–SiCl4–CCl4–H2 or TiCl4–SiCl4–CH4–H2.

7–11 CVD allows complex geometrical shape sub-strates to be coated but it is difficult to obtain pure Ti3SiC2 filmfrom the gas phase. Thin-film deposition using MS has beenused for many M3AX2 phases besides Ti3SiC2, e.g., Ti3GeC2

12

and Ti3AlC2,13 as well as M2AX phases such as Ti2GeC,12

Ti2AlC,14,15 Cr2AlC,16,17, and Ti2AlN.18,19 The main challengeof CVD, MS, and HVOF is the temperature, which has to besufficiently high to activate the synthetic reaction. However,higher temperature may damage the substrates, especially forthe deposition of high-temperature oxidation-resistant coatingson steel substrates. Frodelius et al.20 deposited thick coatings ofTi2AlC by applying HVOF spraying from Ti2AlC powder. Thecoatings were dense and exhibited good adhesion to the sub-strates, but contained Ti3AlC2, TiC, and AlxTiy phases formedby diffusion, melting, and solidification processes. To avoid thereaction/decomposition of MAX-phases or the damage of sub-strates, a low-temperature deposition method to fabricate pureMAX-phases films must be used. Electrophoretic deposition(EPD) is an effective technique to prepare a green film of ce-ramics on a substrate from powder suspension at room temper-ature.21 EPD is a process whereby charged particles in aqueousor organic suspension are moved to and deposited on an oppo-sitely charged electrode under the action of an applied electricfield.21,22 The advantages of this method include the possibilityof coating substrates with a complex shape, easy control ofcoating thickness, simplicity of the required equipment, and lowcost.21–24 Very recently, decomposition of MAX phases duringhigh-temperature pressureless sintering was overcome by plac-ing the green body within the powder bed of A-containing com-pounds. And nearly fully dense and phase-pure bulk MAX-phases samples were obtained using pressureless sintering.25 Itis, thus, possible to obtain dense and phase-pure MAX-phasescoating by depositing a green film on a substrate at room tem-perature from aqueous media followed by pressureless sintering.

The aim of this work was to investigate the feasibility of fab-ricating Ti3Si(Al)C2 films from aqueous media by EPD. It isknown that EPD in an aqueous medium limits the voltage usedfor film deposition due to the hydrolysis of water.26 Neverthe-less, the economical and environmentally benign benefits of us-ing water have encouraged researchers to consider aqueousmedium for EPD. In order to obtain high-quality films, a sta-ble aqueous suspension containing Ti3Si(Al)C2 particles wasprepared by adjusting the pH value of the suspension. Homo-geneous Ti3Si(Al)C2 green films were obtained through the EPDprocess at room temperature in this suspension.

II. Experimental Procedure

(1) Materials

Ti3(Si0.95Al0.05)C2 powders were used as raw materials for elect-ropheretic deposition. These powders were synthesized via asolid–liquid reaction method, and then ball milled for 24 h tobreak up the agglomerates. The detailed preparation procedurecan be found in our previous work.5 The average particle sizeand specific surface area of Ti3Si(Al)C2 powder were 3.3 mm and5.37 m2/g, respectively, determined by using an acoustic andelectroacoustic spectrometer (DT-1200, Dispersion TechnologyInc., Bedford Hills, NY) and a Brunauer–Emmitt–Teller accel-

R. Koc—contributing editor

*Member, The American Ceramic Society.This work was financially supported by the National Outstanding Young Scientist

Foundation for Y. C. Zhou, Natural Sciences Foundation of China under Grant Nos.50232040, 50302011, 90403027, 50772114, and 50832008.

wAuthor to whom correspondence should be addressed. e-mail: yczhou@imr.ac.cn

Manuscript No. 26735. Received August 24, 2009; approved January 20, 2010.

Journal

J. Am. Ceram. Soc., 93 [7] 1916–1921 (2010)

DOI: 10.1111/j.1551-2916.2010.03682.x

r 2010 The American Ceramic Society

1916

erated surface area and porosimetry system (ASAP-2010,Micromeritics, Norcross, GA).

(2) Suspension Preparation

The suspensions used for all measurements were prepared byadding 1 vol% Ti3Si(Al)C2 powders to deionized water. A den-sity of 4.5 g/cm3 for Ti3Si(Al)C2 was used in the calculations.The suspension was stirred for 10 min and then was ultrason-ically deflocculated for 10 min. The pH was adjusted by a titra-tion of sodium hydroxide (NaOH) and hydrochloric acid (HCl).z-potential measurement was performed for 1 vol% Ti3Si(Al)C2

suspension using an acoustic and electroacoustic spectrometer(DT-1200, Dispersion Technology Inc.). The z-potential was re-corded as a function of the pH of the suspension.

(3) EPD of Ti3Si(Al)C2

EPD experiments were carried out using a PARSTATs

2273advanced electrochemical system with Powersuite software(Princeton Applied Research, Oak Ridge, TN). A platinummesh was used as the cathode and an ITO glass substrate hav-ing a sheet resistance of 30 O/cm2 was used as the anode. TheITO glass was precleaned with ethanol and acetone. The twoelectrodes, placed parallel to each other with a separation of1 cm, were dipped into the Ti3Si(Al)C2 suspension. Film depo-sition was carried out onto the ITO glass substrate (1.5 cm� 2cm) at 3 V for 1–10 min. The deposited mass on the substrate wasobtained by weighing the ITO glass substrate before and after thedeposition (after drying for 24 h in air at room temperature).

(4) Characterization

The phase identification of Ti3Si(Al)C2 powders and as-depos-ited films was performed by X-ray diffraction (XRD, Rigaku D/max-2400, Tokyo, Japan) with CuKa radiation. The surface andcross-section microstructures of the films were observed by scan-ning electron microscopy (SEM, LEO SUPRA 35, Oberkochen,Germany).

III. Results and Discussion

(1) Ti3Si(Al)C2 Powder Dispersion in Aqueous Suspension

For EPD, a stable suspension is the basic requirement. Thez-potential of particles is an indicator of the stability of the sus-pension and also is a key factor in the EPD process.27,28 It playsroles in: (1) the stabilization of the suspension by determiningthe intensity of repulsive interaction between particles, (2) de-termining the direction and migration velocity of particle duringEPD, and (3) determining the green density of the deposit.28

Figure 1 displays the z-potential curve as a function of pH for

the Ti3Si(Al)C2 suspension. Ti3Si(Al)C2 particles are found tohave negative surface charge in a wide pH range and the z-po-tential reaches a higher negative value in the pH range of 8–10.A high absolute z-potential represents a well-dispersed and sta-ble suspension. The suspension without any dispersant at pH 9can retain excellent stability after sedimentation for 2 h or even alonger time, although its absolute z-potential is not high enough.

As is well-known, the stability of a colloidal system is gov-erned by the interparticle energy, which depends on the inter-particle interaction, involving van derWaals (vdW), electrostatic,steric, and depletion forces, in the absence and presence of pro-cessing additives29–33

Vtotal ¼ VvdW þ Velect þ Vsteric þ Vstructural (1)

where VvdW is the attractive potential energy due to long-termvan der Waals interactions between particles, Velect the repulsivepotential energy resulting from electrostatic interactions be-tween like-charged particle surfaces, Vsteric the repulsive poten-tial energy resulting from steric interactions between particleswhose surfaces are coated with adsorbed polymeric species, andVstructual the potential energy resulting from the presence ofnonadsorbed species in the solution, which may either increaseor decrease suspension stability. The first two terms of Eq. (1)constitute the well-known Derjaguin, Landau, Verwey, andOverbeek (DLVO) theory, which predict the stability of colloi-dal particles suspended in polar liquids.32,33

For Ti3Si(Al)C2, the surface hydroxyl groups Si–OH and Ti–OH are formed when the particles are exposed to a humid at-mosphere. In an aqueous solvent, hydrolysis reaction will occurat the interface of Ti3Si(Al)C2 surface and water. Based on theion distribution diagram by Yasrebi et al.,34 at pH47.7, themain Si-containing species on the particle surface are HSiO3

�

and SiO32� due to the dissociation of Si–OH groups. Similarly, at

pH44.7, the main Ti-containing specie on the particle surface isTi–O�.35 Because the pH of our suspension is 9, the surfaces ofparticles are covered with HSiO3

�, SiO32�, and Ti–O� anions,

which results in particles repulsing due to electrostatic effect anddispersing stably in the suspension. Because the EPD processonly needs a short time, the aforementioned stable suspensionshould be sufficient for our experiment. Hence in the followingdeposition, the pH of the suspension was controlled to be 9.

(2) Deposition and Characterization of Ti3Si(Al)C2 Films

As Ti3Si(Al)C2 particles have a negative charge at pH 9, theymove toward the anode under a constant voltage field. Figure 2is the diagram of the deposition current as a function of depo-sition time. It shows that the current density decreases contin-uously during EPD process. Initially, current decreasesdrastically in a short time after the voltage is applied, and

Fig. 1. Variation of z-potential with pH. Fig. 2. Variation of current density with deposition time at 3 V.

July 2010 EPD of Ti3Si(Al)C2 from Aqueous Suspension 1917

then decreases gradually as the deposition continues. This dras-tic drop in a short time is due to the sudden increase of theconcentration of charged Ti3Si(Al)C2 particles near the anodewhen the dc electric field is applied at the beginning. Under thiscircumstance, a concentration potential with the direction op-posite to that of the applied electric field would be generated,resulting in the slowdown of particles movement and the

reduction of the current. After the initial surge of particles de-posited on the anode, the concentration near the anode becomesstable.36,37

The gradual and continued decrease of deposition current isdue to the anisotropic character of electrical conductivity forTi3Si(Al)C2. Zhou and Sun38 calculated the band structure ofTi3SiC2 and concluded that the electrical conductivity parallel to

Fig. 3. Scanning electron micrographs of Ti3Si(Al)C2 films by electrophoretic deposition for (a) 1 min, (b) 3 min, (c) 5 min, (d) 7 min, and (e) 10 min.

1918 Journal of the American Ceramic Society—Liang et al. Vol. 93, No. 7

the basal plane is higher than that along the c-axis. Therefore, inour experiment, if the particles deposited on the substrate have apreferred orientation with the c-axis being perpendicular to theITO substrate, the electrical resistance of the working electrodewill increase resulting in the decrease of deposition current.

SEMmicrographs of Ti3Si(Al)C2 films fabricated by the EPDprocess are shown in Fig. 3, which illustrate the effect of depo-sition time on the film quality. At the beginning of EPD (seeFigs. 3(a)–(c)), less Ti3Si(Al)C2 particles deposit on the ITOsubstrate and the density of the Ti3Si(Al)C2 films increases withthe increasing deposition time. After 7 and 10 min of deposition(in Figs. 3(d) and (e)), the substrates are fully covered byTi3Si(Al)C2 and the films show the highest density. To quanti-tatively describe the mass change during deposition, the mass ofthe Ti3Si(Al)C2 deposit at 3 V versus time is shown in Fig. 4. Asexpected, the deposit weight increases with the deposition time.

Figure 5 displays the XRD patterns of the films deposited onITO glass substrates. The upper curve in Fig. 5(a) reveals acrystalline Ti3SiC2 phase with some ITO (tin-doped indium ox-ide) substrate peaks such as In2O3 and In2Sn2O7�x and theamorphous phase corresponds to the ITO substrate (see inset inFig. 5(a)). With the increase in deposition time, the ITO sub-strate is gradually covered by Ti3Si(Al)C2 particles so that ITOsubstrate peaks and the amorphous phase also become weakaccordingly in Fig. 5(b). For the films prepared by EPD, (00l)(l5 2, 4, 6, and 8) reflections are very strong so that the peakintensity of (008) is stronger than that of (104) in Fig. 5(a). Incontrast, the peak intensity of (104) is stronger than that of (008)for the raw powders. This indicates that Ti3Si(Al)C2 films ob-tained by EPD exhibit preferred orientation. Figure 5(b) shows

the XRD patterns for the films deposited at 3 V from 1 to 10min. The (00l) peaks of all the films are stronger than those ofthe raw powders. Zhang and Wang39 systematically studied thesurface properties of Ti3SiC2 using the density functional theory.The calculated cleavage energy for each possible cleavage siteshows that Ti–Si is the weakest layer. Upon mechanical milling,the material would cleave along the weakly bonded (00l) planes.When the Ti2–Si bond is cleaved, there will be two kinds ofsurfaces. One is the Si(Ti2) (00l) and the other is the Ti2(C) (00l)termination, where Si or Ti is the surface atom. As mentionedabove, as the pH of the suspension used in our experiment is 9,the surfaces of particles are covered with HSiO3

� and SiO32� an-

ions or Ti–O1 anions due to the dissociation of Si–OH or Ti–OHgroups. In other words, the surface of (00l) planes tends to havemore negative charges. We conclude that the (00l) surface of theparticle may be more responsive to the positive electric field, sothat films with (00l) preferred orientation have been depositedand the peak intensity of (00l) plane is stronger.40

In order to quantitatively describe the preferred orientationof the films, the Lotgering orientation factor is used to evaluatethe degree of orientation41,42

F ¼ p� p0

1� p0(2)

where p or p05SI(00l)/SI(hkl). The value of p was calculated fromthe XRD peak intensities for the films by EPD and the value ofp0 was calculated from the XRD patterns collected on the rawpowders.

Fig. 4. Dependence of the Ti3Si(Al)C2 deposit mass on the depositiontime at 3 V. The solid line is guided for the eye.

Fig. 5. X-ray diffraction patterns of the as-deposied films and raw powders (a) comparison between film by electrophoretic deposition (EPD) for 5 minand raw powders (b) Ti3Si(Al)C2 films for different deposition time.

Fig. 6. Lotgering orientation factor for films by electrophoretic depo-sition (EPD) versus different deposition time at 3 V. The solid line isguided for the eye.

July 2010 EPD of Ti3Si(Al)C2 from Aqueous Suspension 1919

According to Eq. (2), it is known that the larger the F is, thestronger the preferred orientation appears. Fig. 6 presents theLotgering orientation factor as a function of deposition time.Obviously, the degree of orientation is affected by the depositiontime. F decreases with the deposition time as the coverage rate ofthe substrate increases with an increasing deposition time(Fig. 3). In other words, if the coverage rate of the substrateincreases, the orientation of the film decreases. Compared withthe ITO substrate, the distribution of particles deposited on thesubstrate is more irregular. Therefore, the alignment of the fol-lowing deposition stacked on the substrate, which has beencovered with particles, may be disturbed by the irregular distri-bution of particles so that the orientation of (00l) decreases.

In order to prepare highly dense, uniform, and much thickerfilm, the EPD process carried out three cycles of deposition dry-ing of the deposit. Moreover, the film is a three-layer deposit andeach layer was dried in the air for 10 min before the next cycle.The aim of using the deposition–drying–deposition method is toeliminate the oxygen and also enhance the quality of the depos-ited film. It is shown in Fig. 7 that a uniform Ti3Si(Al)C2 filmcan be made in this way and the thickness is 10 mm.

IV. Conclusion

A well-dispersed Ti3Si(Al)C2 aqueous suspension with 1 vol%solid loading was prepared without any dispersant at pH 9.Uniform Ti3Si(Al)C2 films with a thickness up to 10 mm can beformed from a stable Ti3Si(Al)C2 aqueous suspension usingEPD process under a low applied dc voltage of 3 V. Ti3Si(Al)C2

films obtained under the applied electric field exhibit the(00l) preferred orientation. The preferred orientation declinedwith the deposition time. These results show that EPD from anaqueous suspension is a feasible low-cost and environmentallybenign method to fabricate Ti3Si(Al)C2 green films at roomtemperature.

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