electrophoretic co-deposition of diamond/borosilicate glass composite coatings

201 (2007) 7645–7651www.elsevier.com/locate/surfcoat

Surface & Coatings Technology

Electrophoretic co-deposition of diamond/borosilicate glasscomposite coatings

Y.H. Wang a,b, Q.Z. Chen b, J. Cho b, A.R. Boccaccini b,⁎

a State Key Laboratory of Metastable Materials Science & Technology, College of Materials Science & Engineering, Yanshan University,Qinhuangdao, Hebei, 066004, PR China

b Department of Materials, Imperial College London, Prince Consort Road, London SW7 2BP, UK

Received 19 November 2006; accepted in revised form 21 February 2007Available online 2 March 2007

Abstract

Composite coatings made of diamond powders and borosilicate glass have been deposited on stainless steel substrates by electrophoretic co-deposition. Ethanol and acetone suspensions containing diamond powders of particle size 1–2 μm and borosilicate glass powders of size 0.1–0.5 μm were used. Electrophoretic deposition (EPD) parameters were optimized by a trial-and-error-approach. Microstructures of deposited andsintered coatings were investigated by XRD and SEM analysis. The results show that applied voltages up to 10 V led to thin and incompletecoatings. Voltages higher than 50 V resulted in uneven coatings with uncontrolled thickness and poor uniformity. The best results were achievedusing ethanol suspensions. Smooth, uniform and dense coatings with diamond and glass particles distributed uniformly were obtained underapplied voltages in the range of 30–50 Vand a deposition time of 4 min. The concentration ratio of diamond to borosilicate glass in the compositecoatings was in good correlation with the original ratio in suspension, thus control of the coating microstructure and composition is possible.During sintering at 900 °C, the glass particles softened; sintered by viscous flow and spread over the diamond particles surface. Thus a glass layerforms protecting the diamond from oxidization or graphitization and bonding the diamond particles together.© 2007 Elsevier B.V. All rights reserved.

Keywords: Diamond; Borosilicate glass coatings; Electrophoretic deposition

1. Introduction

Electrophoretic deposition (EPD) is an advanced materialprocessing technology, which is based on the motion of chargedparticles in liquid suspensions towards an electrode and thecontrolled deposition of the particles on the electrode substrateunder an applied electric field [1]. EPD can produce homogenouscoatings of controllable thickness involving low processing costand requiring simple equipment [1,2]. EPD is especially suitablefor producing ceramic coatings, ceramic matrix composites,functionally graded materials and thick films [1–5]. Typicalcoatings produced include alumina [6–10], zirconia [6,9,10],silica [11], piezoelectric ceramics [12–14] and TiO2 [15]. EPDalso offers unique advantages for depositing uniform coatings ondifferent substrates with complex shapes or curved surfaces, fromplanar plates to meshes, fibers and porous structures [2,3,14,15].

⁎ Corresponding author. Tel.: +44 20 7594 6731; fax: +44 20 75946757.E-mail address: [email protected] (A.R. Boccaccini).

0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2007.02.037

Diamond is a promising material widely applied in advancedmachining tools, wear-resistant coatings, heat sinks andmicroelectronics due to its excellent mechanical and physicalproperties such as high hardness, high thermal conductivity, andexcellent electron field emission effect [16,17]. Limited workhas been carried out on the EPD of diamond particles [18–21],although some promising applications including electrophoret-ically deposited micro diamond layers as seeds for the growth ofCVD diamond film [22,23] and EPD of diamond nanoparticlesas field emission tips have been considered [24,25]. For mostdiamond based products the diamond particles are required tobe mixed with a metallic or vitreous bond matrix, which shouldsinter to a continuous matrix embedding the diamond phase.This is necessary because single-phase diamond ceramics aredifficult to be densified by pressureless sintering and extremelyhigh temperatures and pressures, involving high-cost equip-ment, are required. Indeed most EPD coatings also require asubsequent heating procedure to increase the bonding betweenthe deposited particles and the substrate as well as to eliminate

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http://dx.doi.org/10.1016/j.surfcoat.2007.02.037

Table 1Compositions of the suspensions used in EPD experiments

Sampleno.

EPD materials andparticle size

Compositionof suspensions

Depositionelectrode

1 1–2 μm diamond 3.0 g diamond, 100 ml acetone Anode2 1–2 μm diamond 3.0 g diamond, 100 ml ethanol Anode3 0.1–0.5 μm glass 1.75 g glass, 100 ml acetone Anode4 0.1–0.5 μm glass 1.75 g glass, 100 ml ethanol Anode5 1–2 μm diamond 3.0 g diamond, 100 ml acetone,

50 mg iodineCathode

6 1–2 μm diamond 3.0 g diamond, 100 ml ethanol,50 mg iodine

Cathode

7 0.1–0.5 μm glass 1.75 g glass, 100 ml acetone,50 mg iodine

No coating

8 0.1–0.5μm glass 1.75 g glass, 100 ml ethanol,50 mg iodine

No coating

9 1–2 μm diamond 1.5 g diamond, 1.0 g glass,100 ml acetone

Anode0.1–0.5 μm glass

10 1–2 μm diamond 1.5 g diamond, 1.0 g glass,100 ml ethanol


11 1–2 μm diamond 3.0 g diamond, 1.0 g glass,100 ml ethanol


7646 Y.H. Wang et al. / Surface & Coatings Technology 201 (2007) 7645–7651

porosity. The development of composite coatings containingdiamond particles and a bond matrix is an active field ofresearch in the particular application areas mentioned above. Inthis paper we report results on the fabrication of diamond/borosilicate composite coatings using electrophoretic co-deposition (EPcD). Borosilicate glass was chosen as a vitrifiedbond due to its chemical durability and low thermal expansioncoefficient [26]. It was hypothesized that electrophoretic co-deposition can produce not only a homogenous mixing ofdiamond and bonding phase particles but also a controllablethickness coating. If successful, the EPcD technique shouldprovide a simple method to develop hard coatings for precisionmachining tools or wear-resistant parts.

2. Experimental

Diamond powders with different particle size distributionswere purchased from Funike Superhard Materials Co., China.The borosilicate glass powder used (particle size range 0.1–0.5 μm) was of DURAN® composition: (wt.%): 81 SiO2, 13B2O3, 4 Na2O+K2O, 2 Al2O3 (Schott Glass, Germany), which

Fig. 1. SEM micrographs of the composite coatings deposited under different a

has been used in previous investigations on glass matrixcomposites [26]. Both diamond and glass particles were used asreceived, e.g. without washing or applying any other pre-treatment. Acetone and ethanol suspensions containing boro-silicate glass and diamond powders were prepared. In somesuspensions, a small amount of iodine was added to reverse thecharge on the particles, following previous studies in theliterature [27]. The addition of iodine is based on the proposedreaction of iodine and acetylacetone according to the followingchemical formula:

CH3COCH2COCH3 þ 2I2→IH2CCOCH2COCH2I

þ2Hþ þ 2I−

It is suggested that the protons generated by the reaction areadsorbed on the suspended particles (diamond or glass), makingthem positively charged. However, if the concentration ofiodine is too high, a large amount of H+ ions in the suspensionwill be produced. In this case, the H+ ions become the maincurrent carriers and the speed of the ceramic particles motionwould be reduced. Moreover, a too high concentration of H+

would result in the reduction of the double layer thickness of theparticles and hence in the repulsive force between the particles.This would promote particle agglomeration and hence it wouldgive rise to poorer deposition results.

The compositions of the prepared suspensions are listed inTable 1. The suspensions were ultrasonically treated for 15 minimmediately before electrophoretic deposition. Stainless steel(type 304) plates with dimensions of 10 mm×10 mm×0.2 mmwere used as electrodes. The as-received plates were washedwith the same solvent used in the suspensions prior to EPD.

All EPD experiments were carried out at ambient temper-ature using a two-electrode cell. For each series of experiments,constant voltage in the range of 5–55 V was applied using aThurlby Thandar Instrument EL561 power supply withelectrodes separation of 10 mm and deposition times rangingfrom 0.5 to 10 min. After deposition, the samples were dried for24 h at ambient temperature in a desiccator.

Sintering of the coatings was carried out in air at 900 °C for1 h, using a heating rate of 20 °C/min. Samples were cooleddown in the furnace.

pplied voltages (suspension 10): (a) 10 V, (b) 40 V; deposition time: 4 min.

Fig. 2. Surface morphologies of coatings for Samples 9 (a) and 10 (b), obtained from acetone and ethanol suspensions, respectively.

7647Y.H. Wang et al. / Surface & Coatings Technology 201 (2007) 7645–7651

The microstructure of the composite coatings before andafter sintering was observed using scanning electron microsco-py (SEM) (JEOL 5610). The samples were coated with goldbefore the examination. X-ray diffraction (XRD) analysis wascarried out using a Philips PW1710 diffractometer to investigatethe phase composition of the EPD coatings before and aftersintering.

3. Results and discussion

3.1. Determination of optimal suspensions and EPD parameters

Two kinds of solvents were used in the present experiments:ethanol and acetone. The suspensions used in Samples 1–4(Table 1) were prepared by adding diamond and borosilicateglass particles separately in the two solvents. All suspensionshad good stability. By a trial-and-error approach it was foundthat the diamond and glass particles were both negativelycharged in these two suspensions and moved towards the anodeunder the applied electric field. This is the necessary conditionto realize the co-deposition for producing composite coatings.

Diamond particles should acquire positive surface charge inorganic solvents containing a small amount of iodine due to the

Fig. 3. Variation of the current intensity with deposition time during EPD ofdiamond and borosilicate glass powders in acetone and ethanol suspensions(Samples 9 and 10, Table 1).

effect of the hydrogen ion [13,15,27]. Samples 5–6 in ourexperiments (Table 1) also confirmed the cathodic deposition ofdiamond particles, which occurred as 50 mg iodine was addedto 100 ml ethanol or acetone suspensions. However, thesuspensions of borosilicate glass powders in the same solventsplus iodine were not stable, borosilicate glass powders settledquickly within minutes during EPD. Therefore co-deposition ofdiamond and borosilicate glass on the cathode was not possibleusing suspensions with added iodine. The particle size wasfound to be a key factor determining the stability of thesuspensions. It was found that suspensions were unstable as theaverage diameter of diamond particles was larger than 5 μm (notshown in Table 1). Suspensions were therefore prepared bydispersing diamond particles of size 1–2 μm. Borosilicate glasspowder of average size 0.1–0.5 μm in ethanol or acetone wasused. Details about the effects of the solvents used on theuniformity of the coatings produced are discussed below.

Fig. 1a and b shows SEMmicrographs of composite coatingsobtained from suspension 10 (see Table 1) using differentvoltages. It was obvious that the applied voltage had a greatinfluence on the thickness and quality of the coatings obtained.Voltages below 10 V led to thin coatings which did notcompletely cover the substrate, as shown in Fig. 1a. Increasing

Fig. 4. Variation of the current intensity with deposition time in pure solvents,e.g. acetone and ethanol, under an applied voltage of 40 V.

Fig. 5. EPD diamond/borosilicate glass composite coating obtained fromacetone suspension (Sample 9) on a scratched substrate (applied voltage: 40 V,deposition time: 4 min), showing preferential growth of the coatings along thescratches.


the applied voltage resulted in higher deposition rate andincreased coating thickness. On the other hand voltages of over50 V led to coatings with uneven morphology and pooruniformity, especially greater deposit thickness was observedon the edges of the substrates. Smooth and uniform coatingscovering the substrates entirely were obtained using voltages inthe range of 30–50 V (Fig. 1b).

Compared with the applied voltage, the deposition time has aless significant influence on the quality of the compositecoatings. In both suspensions the thickness of the compositecoating increased with prolonging the deposition time.However, little increase of the coating thickness was observedwhen the deposition time was increased to 7 or more minutes.The saturation of deposit thickness with increasing depositiontime is usually found in EPD under constant voltage conditions,which is the result of the increasing electric resistance offeredby the growing deposit [3]. A deposition time of 4 min wasfound in the present study to be optimal to obtain high-qualitycoatings.

3.2. EPD composite coating quality analysis

Obvious differences in the surface morphologies of thecoatings using different solvents were observed. Sample 9obtained from acetone suspensions exhibited a very rough anduneven surface. Macroscopic protruding was clearly seen as

Fig. 6. Composite coatings obtained from Samples 10 (a) and 11

shown in Fig. 2a. In contrast, ethanol suspensions led to verysmooth and uniform coatings, as seen in Fig. 2b (Sample 10).Both suspensions (Table 1) were of the same compositionexcept for the solvents used: the suspensions contained 1.5 gdiamond powder and 1 g borosilicate glass powder in 100 mlsolvent (ethanol or acetone). EPcD was carried out at 40 V anddeposition time was 4 min. The variation of the current intensitywith deposition time was recorded and data are shown in Fig. 3.The current intensity generally decreased with time in acetonesuspension but the opposite occurred in ethanol suspensions.

To investigate the reasons inducing this effect, the variationof the current intensity with time in pure solvents was recorded,as shown in Fig. 4. The same tendency of current intensitydecreasing with time was found in acetone but little variation ofcurrent intensity in ethanol was measured. This result indicatesthat stainless steel substrates are readily passivated in acetoneduring the EPD process. A further investigation was carried outby electrophoretic depositing the composite coatings onstainless steel substrates on which some scratches had beenintroduced on the surface. As expected, the coating grewpreferentially along the scratches which presented fresh surfaceswithout passivation, as shown in Fig. 5.

Compared with acetone suspensions, the ethanol suspensionexhibited better conductivity and no passivation of theelectrodes was seen to occur during the EPcD process. As aresult, high-quality composite coatings of diamond andborosilicate glass particles were obtained. The SEM micro-graphs in Fig. 6a, b show the surface of the composite coatingsobtained from suspensions 10 and 11, respectively, containingdiamond and glass in different weight concentration ratios. Thecoatings covered the substrate entirely and exhibited homoge-neous particle packing and a dense structure. Diamond particles(the larger ones) and borosilicate glass particles (the smallerones) are seen to be distributed uniformly. No large microcrackswere observed on the surface after drying. The correspondingXRD results shown in Fig. 7 also confirm the presence ofamorphous glass (halo in the XRD trace) and diamond crystals,both present in the deposit. A higher concentration of diamondpowder in the coatings was found in those obtained fromsuspension 10 due to the relatively high diamond particlecontent in the suspension. It is not possible to identify theconcentration ratio of diamond to glass in the coatings bysimple XRD analysis, due to the lack of diffraction pattern of

(b) by EPD (applied voltage: 40 V, deposition time: 4 min).

Fig. 9. SEM micrograph showing the good wetting and bonding betweenborosilicate glass and diamond particles achieved by sintering at 900 °C for anhour.

Fig. 7. XRD patterns of diamond/borosilicate glass composite coatingscorresponding to Samples 10 (a) and 11 (b).


the amorphous glass. To quantify the ratio of diamond to glass,the deposited coating was detached from the substrate andweighted, then the coating was etched using HF acid to removethe glass phase and the remaining material was weighted again.It was assumed that the remaining coating only containeddiamond because of the high chemical stability of diamond inHF. The weight ratios of diamond to glass in the compositecoatings made from Samples 10 and 11 were determined in thismanner to be 1:0.85 and 1:1.45, respectively.

Fig. 8. SEMmicrographs of Sample 10 (a) and Sample 11 (b) after sintering at 900 °Cof diamond and glass particles.

3.3. Sintering of composite coatings

The glass chosen as the binder or matrix for diamond crystalsshould exhibit a low softening temperature because hightemperature sintering is harmful for diamond which is easilyoxidized or graphitized at high temperatures. The borosilicateglass used in the present experiments has a softening temperatureof 650 °C [26]. Fig. 8a and b showsmicrographs of the compositecoating surfaces of Samples 10 and 11 after sintering at 900 °C for1 h. The borosilicate glass particles softened and sintered by aviscous flow mechanism at the working temperature. The glassthus had a low viscosity at the sintering temperature and easilyspread on the diamond particle surfaces infiltrating the poresbetween them and bonding the particles together. Thus thediamond particles were fully encapsulated in the glass matrix inSample 10, as shown in Fig. 8a. However, insufficient glasscontent in Sample 11 resulted in a discontinuous sintering layer(Fig. 8b). The proper ratio of diamond to glass particleconcentrations can be controlled by adjusting the compositionof the starting suspension not only to ensure that a sufficientamount of glass is present to bind the diamond particles entirelybut also to optimize the glass content for achieving the highestpossible concentration of diamond.As inferred by the SEM imagein Fig. 9, borosilicate glass exhibited a good wetting behaviour ofdiamond particles at the working temperature (900 °C) and thisensures a strong adhesion between diamond and the vitrifiedbond.

for 1 h showing different degree of densification due to different relative content

Fig. 10. XRD patterns of Sample 10 (a) and Sample 11 (b) after sintering at900 °C showing formation of cristobalite.


Fig. 10a and b shows the XRD patterns of the compositecoatings of Samples 10 and 11, respectively, after sintering. Thediffraction peaks of cristobalite phase showed that crystalliza-tion of the glass took place during sintering [28]. This is inagreement with previous results on borosilicate glass/carbonnanotube composites [26]. No graphite was found by XRDimplying that the glass softening and flow can protect diamondfrom oxidization and graphitization during sintering. Borosil-icate glass (DURAN [26]) thus emerges as an ideal vitrifiedbond for diamond containing products. The composite ofborosilicate glass and diamond particles can find manyapplications in wear-resistant materials and cutting tools,especially in diamond wheels for grinding and cutting ofceramics, glasses and silicon wafers [29,30].

Vitrified bond grinding wheels based on sintered mixtures ofglass and diamond powders have been shown to be suitable forprecision ceramic grinding [31]. In comparison, traditionalmetal and resin bond wheels might exhibit low grinding effector might lead to poor grinding surface quality. The wear ofvitrified diamond wheels can occur through brittle fracture ofthe bond material, allowing rapid emergence of new abrasive(diamond) particles in a so-called self-sharpening processing

and thus continuous grinding is readily established. This effectmakes vitrified bond diamond wheels suitable for meeting therequirement of high surface quality in machined hard materials.Compared with the traditional powder metallurgy method inwhich products are made by a series of processing stepsincluding pressing of powders, sintering, and dressing, ourresearch results about electrophoretic co-deposition of diamond/borosilicate glass composite coatings provide a simple and cost-effective way to develop these materials.

4. Conclusions

The anodic electrophoretic co-deposition of diamond andborosilicate glass particles was realized using ethanol andacetone suspensions containing diamond particles of size 1–2 μm and borosilicate glass particles of size 0.1–0.5 μm. Thecomposite coatings obtained from the acetone suspensions wereuneven and exhibited uncontrolled microstructure. In contrast,ethanol suspensions led to a smooth and uniform coatingcovering the substrate entirely. Diamond and glass particleswere seen to be homogeneously distributed and no largemicrocracks were found on the coating surface. The optimumEPD parameters were: applied voltage 30–50 V and depositiontime 3–7 min. The ratio of diamond to borosilicate glass in thecomposite coating can be controlled by adjusting the relativeconcentration of the constituents in the starting suspensions.The viscous glass at 900 °C wetted the diamond particles,spreading over the diamond surface thus protecting diamondparticles from oxidization or graphitization and bonding thediamond particles together during the high temperaturetreatment. Results have also confirmed that borosilicate glasscan be considered a suitable vitrified bond for diamond abrasiveproducts.

Acknowledgment

The author WYH expresses his sincere thanks to CSC (ChinaScholarship Council) for financial support.

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electrophoretic co-deposition of diamond/borosilicate glass composite coatings

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