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borosilicate glass composite coatings
Surface & Coatings Technology 201 (2007) 7645 – 7651 www.elsevier.com/locate/surfcoat
Electrophoretic co-deposition of diamond/borosilicate glass composite 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 2007 Available online 2 March 2007
Abstract Composite coatings made of diamond powders and borosilicate glass have been deposited on stainless steel substrates by electrophoretic codeposition. 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 and sintered coatings were investigated by XRD and SEM analysis. The results show that applied voltages up to 10 V led to thin and incomplete coatings. Voltages higher than 50 V resulted in uneven coatings with uncontrolled thickness and poor uniformity. The best results were achieved using ethanol suspensions. Smooth, uniform and dense coatings with diamond and glass particles distributed uniformly were obtained under applied voltages in the range of 30–50 V and a deposition time of 4 min. The concentration ratio of diamond to borosilicate glass in the composite coatings 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 layer forms 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 material processing technology, which is based on the motion of charged particles in liquid suspensions towards an electrode and the controlled deposition of the particles on the electrode substrate under an applied electric field [1]. EPD can produce homogenous coatings of controllable thickness involving low processing cost and requiring simple equipment [1,2]. EPD is especially suitable for producing ceramic coatings, ceramic matrix composites, functionally graded materials and thick films [1–5]. Typical coatings produced include alumina [6–10], zirconia [6,9,10], silica [11], piezoelectric ceramics [12–14] and TiO2 [15]. EPD also offers unique advantages for depositing uniform coatings on different substrates with complex shapes or curved surfaces, from planar 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 advanced machining tools, wear-resistant coatings, heat sinks and microelectronics due to its excellent mechanical and physical properties such as high hardness, high thermal conductivity, and excellent electron field emission effect [16,17]. Limited work has been carried out on the EPD of diamond particles [18–21], although some promising applications including electrophoretically deposited micro diamond layers as seeds for the growth of CVD diamond film [22,23] and EPD of diamond nanoparticles as field emission tips have been considered [24,25]. For most diamond based products the diamond particles are required to be mixed with a metallic or vitreous bond matrix, which should sinter to a continuous matrix embedding the diamond phase. This is necessary because single-phase diamond ceramics are difficult to be densified by pressureless sintering and extremely high temperatures and pressures, involving high-cost equipment, are required. Indeed most EPD coatings also require a subsequent heating procedure to increase the bonding between the deposited particles and the substrate as well as to eliminate
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Table 1 Compositions of the suspensions used in EPD experiments Sample EPD materials and Composition no. particle size of suspensions 1 2 3 4 5
1–2 μm diamond 1–2 μm diamond 0.1–0.5 μm glass 0.1–0.5 μm glass 1–2 μm diamond
6
1–2 μm diamond
7
0.1–0.5 μm glass
8
0.1–0.5μm glass
9
1–2 μm diamond 0.1–0.5 μm glass 1–2 μm diamond 0.1–0.5 μm glass 1–2 μm diamond 0.1–0.5 μm glass
10 11
3.0 g diamond, 100 ml acetone 3.0 g diamond, 100 ml ethanol 1.75 g glass, 100 ml acetone 1.75 g glass, 100 ml ethanol 3.0 g diamond, 100 ml acetone, 50 mg iodine 3.0 g diamond, 100 ml ethanol, 50 mg iodine 1.75 g glass, 100 ml acetone, 50 mg iodine 1.75 g glass, 100 ml ethanol, 50 mg iodine 1.5 g diamond, 1.0 g glass, 100 ml acetone 1.5 g diamond, 1.0 g glass, 100 ml ethanol 3.0 g diamond, 1.0 g glass, 100 ml ethanol
Deposition electrode Anode Anode Anode Anode Cathode Cathode No coating No coating
has been used in previous investigations on glass matrix composites [26]. Both diamond and glass particles were used as received, e.g. without washing or applying any other pretreatment. Acetone and ethanol suspensions containing borosilicate glass and diamond powders were prepared. In some suspensions, a small amount of iodine was added to reverse the charge on the particles, following previous studies in the literature [27]. The addition of iodine is based on the proposed reaction of iodine and acetylacetone according to the following chemical formula: CH3 COCH2 COCH3 þ 2I2 →IH2 CCOCH2 COCH2 I þ2Hþ þ 2I−
Anode Anode Anode
porosity. The development of composite coatings containing diamond particles and a bond matrix is an active field of research in the particular application areas mentioned above. In this paper we report results on the fabrication of diamond/ borosilicate composite coatings using electrophoretic codeposition (EPcD). Borosilicate glass was chosen as a vitrified bond due to its chemical durability and low thermal expansion coefficient [26]. It was hypothesized that electrophoretic codeposition can produce not only a homogenous mixing of diamond and bonding phase particles but also a controllable thickness coating. If successful, the EPcD technique should provide a simple method to develop hard coatings for precision machining tools or wear-resistant parts. 2. Experimental Diamond powders with different particle size distributions were 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, 13 B2O3, 4 Na2O + K2O, 2 Al2O3 (Schott Glass, Germany), which
It is suggested that the protons generated by the reaction are adsorbed on the suspended particles (diamond or glass), making them positively charged. However, if the concentration of iodine is too high, a large amount of H+ ions in the suspension will be produced. In this case, the H+ ions become the main current carriers and the speed of the ceramic particles motion would be reduced. Moreover, a too high concentration of H+ would result in the reduction of the double layer thickness of the particles and hence in the repulsive force between the particles. This would promote particle agglomeration and hence it would give rise to poorer deposition results. The compositions of the prepared suspensions are listed in Table 1. The suspensions were ultrasonically treated for 15 min immediately before electrophoretic deposition. Stainless steel (type 304) plates with dimensions of 10 mm × 10 mm × 0.2 mm were used as electrodes. The as-received plates were washed with the same solvent used in the suspensions prior to EPD. All EPD experiments were carried out at ambient temperature using a two-electrode cell. For each series of experiments, constant voltage in the range of 5–55 V was applied using a Thurlby Thandar Instrument EL561 power supply with electrodes separation of 10 mm and deposition times ranging from 0.5 to 10 min. After deposition, the samples were dried for 24 h at ambient temperature in a desiccator. Sintering of the coatings was carried out in air at 900 °C for 1 h, using a heating rate of 20 °C/min. Samples were cooled down in the furnace.
Fig. 1. SEM micrographs of the composite coatings deposited under different applied voltages (suspension 10): (a) 10 V, (b) 40 V; deposition time: 4 min.
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Fig. 2. Surface morphologies of coatings for Samples 9 (a) and 10 (b), obtained from acetone and ethanol suspensions, respectively.
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 borosilicate glass particles separately in the two solvents. All suspensions had good stability. By a trial-and-error approach it was found that the diamond and glass particles were both negatively charged in these two suspensions and moved towards the anode under the applied electric field. This is the necessary condition to realize the co-deposition for producing composite coatings. Diamond particles should acquire positive surface charge in organic solvents containing a small amount of iodine due to the
effect of the hydrogen ion [13,15,27]. Samples 5–6 in our experiments (Table 1) also confirmed the cathodic deposition of diamond particles, which occurred as 50 mg iodine was added to 100 ml ethanol or acetone suspensions. However, the suspensions of borosilicate glass powders in the same solvents plus iodine were not stable, borosilicate glass powders settled quickly within minutes during EPD. Therefore co-deposition of diamond and borosilicate glass on the cathode was not possible using suspensions with added iodine. The particle size was found to be a key factor determining the stability of the suspensions. It was found that suspensions were unstable as the average diameter of diamond particles was larger than 5 μm (not shown in Table 1). Suspensions were therefore prepared by dispersing diamond particles of size 1–2 μm. Borosilicate glass powder of average size 0.1–0.5 μm in ethanol or acetone was used. Details about the effects of the solvents used on the uniformity of the coatings produced are discussed below. Fig. 1a and b shows SEM micrographs of composite coatings obtained from suspension 10 (see Table 1) using different voltages. It was obvious that the applied voltage had a great influence on the thickness and quality of the coatings obtained. Voltages below 10 V led to thin coatings which did not completely cover the substrate, as shown in Fig. 1a. Increasing
Fig. 3. Variation of the current intensity with deposition time during EPD of diamond and borosilicate glass powders in acetone and ethanol suspensions (Samples 9 and 10, Table 1).
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.
The microstructure of the composite coatings before and after sintering was observed using scanning electron microscopy (SEM) (JEOL 5610). The samples were coated with gold before the examination. X-ray diffraction (XRD) analysis was carried out using a Philips PW1710 diffractometer to investigate the phase composition of the EPD coatings before and after sintering. 3. Results and discussion 3.1. Determination of optimal suspensions and EPD parameters
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Fig. 5. EPD diamond/borosilicate glass composite coating obtained from acetone suspension (Sample 9) on a scratched substrate (applied voltage: 40 V, deposition time: 4 min), showing preferential growth of the coatings along the scratches.
the applied voltage resulted in higher deposition rate and increased coating thickness. On the other hand voltages of over 50 V led to coatings with uneven morphology and poor uniformity, especially greater deposit thickness was observed on the edges of the substrates. Smooth and uniform coatings covering the substrates entirely were obtained using voltages in the range of 30–50 V (Fig. 1b). Compared with the applied voltage, the deposition time has a less significant influence on the quality of the composite coatings. In both suspensions the thickness of the composite coating increased with prolonging the deposition time. However, little increase of the coating thickness was observed when the deposition time was increased to 7 or more minutes. The saturation of deposit thickness with increasing deposition time is usually found in EPD under constant voltage conditions, which is the result of the increasing electric resistance offered by the growing deposit [3]. A deposition time of 4 min was found in the present study to be optimal to obtain high-quality coatings. 3.2. EPD composite coating quality analysis Obvious differences in the surface morphologies of the coatings using different solvents were observed. Sample 9 obtained from acetone suspensions exhibited a very rough and uneven surface. Macroscopic protruding was clearly seen as
shown in Fig. 2a. In contrast, ethanol suspensions led to very smooth and uniform coatings, as seen in Fig. 2b (Sample 10). Both suspensions (Table 1) were of the same composition except for the solvents used: the suspensions contained 1.5 g diamond powder and 1 g borosilicate glass powder in 100 ml solvent (ethanol or acetone). EPcD was carried out at 40 V and deposition time was 4 min. The variation of the current intensity with deposition time was recorded and data are shown in Fig. 3. The current intensity generally decreased with time in acetone suspension but the opposite occurred in ethanol suspensions. To investigate the reasons inducing this effect, the variation of the current intensity with time in pure solvents was recorded, as shown in Fig. 4. The same tendency of current intensity decreasing with time was found in acetone but little variation of current intensity in ethanol was measured. This result indicates that stainless steel substrates are readily passivated in acetone during the EPD process. A further investigation was carried out by electrophoretic depositing the composite coatings on stainless steel substrates on which some scratches had been introduced on the surface. As expected, the coating grew preferentially along the scratches which presented fresh surfaces without passivation, as shown in Fig. 5. Compared with acetone suspensions, the ethanol suspension exhibited better conductivity and no passivation of the electrodes was seen to occur during the EPcD process. As a result, high-quality composite coatings of diamond and borosilicate glass particles were obtained. The SEM micrographs in Fig. 6a, b show the surface of the composite coatings obtained from suspensions 10 and 11, respectively, containing diamond and glass in different weight concentration ratios. The coatings covered the substrate entirely and exhibited homogeneous particle packing and a dense structure. Diamond particles (the larger ones) and borosilicate glass particles (the smaller ones) are seen to be distributed uniformly. No large microcracks were observed on the surface after drying. The corresponding XRD results shown in Fig. 7 also confirm the presence of amorphous glass (halo in the XRD trace) and diamond crystals, both present in the deposit. A higher concentration of diamond powder in the coatings was found in those obtained from suspension 10 due to the relatively high diamond particle content in the suspension. It is not possible to identify the concentration ratio of diamond to glass in the coatings by simple XRD analysis, due to the lack of diffraction pattern of
Fig. 6. Composite coatings obtained from Samples 10 (a) and 11 (b) by EPD (applied voltage: 40 V, deposition time: 4 min).
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Fig. 9. SEM micrograph showing the good wetting and bonding between borosilicate glass and diamond particles achieved by sintering at 900 °C for an hour.
3.3. Sintering of composite coatings
Fig. 7. XRD patterns of diamond/borosilicate glass composite coatings corresponding 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 and weighted, then the coating was etched using HF acid to remove the glass phase and the remaining material was weighted again. It was assumed that the remaining coating only contained diamond because of the high chemical stability of diamond in HF. The weight ratios of diamond to glass in the composite coatings made from Samples 10 and 11 were determined in this manner to be 1:0.85 and 1:1.45, respectively.
The glass chosen as the binder or matrix for diamond crystals should exhibit a low softening temperature because high temperature sintering is harmful for diamond which is easily oxidized or graphitized at high temperatures. The borosilicate glass used in the present experiments has a softening temperature of 650 °C [26]. Fig. 8a and b shows micrographs of the composite coating surfaces of Samples 10 and 11 after sintering at 900 °C for 1 h. The borosilicate glass particles softened and sintered by a viscous flow mechanism at the working temperature. The glass thus had a low viscosity at the sintering temperature and easily spread on the diamond particle surfaces infiltrating the pores between them and bonding the particles together. Thus the diamond particles were fully encapsulated in the glass matrix in Sample 10, as shown in Fig. 8a. However, insufficient glass content in Sample 11 resulted in a discontinuous sintering layer (Fig. 8b). The proper ratio of diamond to glass particle concentrations can be controlled by adjusting the composition of the starting suspension not only to ensure that a sufficient amount of glass is present to bind the diamond particles entirely but also to optimize the glass content for achieving the highest possible concentration of diamond. As inferred by the SEM image in Fig. 9, borosilicate glass exhibited a good wetting behaviour of diamond particles at the working temperature (900 °C) and this ensures a strong adhesion between diamond and the vitrified bond.
Fig. 8. SEM micrographs of Sample 10 (a) and Sample 11 (b) after sintering at 900 °C for 1 h showing different degree of densification due to different relative content of diamond and glass particles.
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and thus continuous grinding is readily established. This effect makes vitrified bond diamond wheels suitable for meeting the requirement of high surface quality in machined hard materials. Compared with the traditional powder metallurgy method in which products are made by a series of processing steps including pressing of powders, sintering, and dressing, our research results about electrophoretic co-deposition of diamond/ borosilicate glass composite coatings provide a simple and costeffective way to develop these materials. 4. Conclusions
Fig. 10. XRD patterns of Sample 10 (a) and Sample 11 (b) after sintering at 900 °C showing formation of cristobalite.
The anodic electrophoretic co-deposition of diamond and borosilicate glass particles was realized using ethanol and acetone suspensions containing diamond particles of size 1– 2 μm and borosilicate glass particles of size 0.1–0.5 μm. The composite coatings obtained from the acetone suspensions were uneven and exhibited uncontrolled microstructure. In contrast, ethanol suspensions led to a smooth and uniform coating covering the substrate entirely. Diamond and glass particles were seen to be homogeneously distributed and no large microcracks were found on the coating surface. The optimum EPD parameters were: applied voltage 30–50 V and deposition time 3–7 min. The ratio of diamond to borosilicate glass in the composite coating can be controlled by adjusting the relative concentration of the constituents in the starting suspensions. The viscous glass at 900 °C wetted the diamond particles, spreading over the diamond surface thus protecting diamond particles from oxidization or graphitization and bonding the diamond particles together during the high temperature treatment. Results have also confirmed that borosilicate glass can be considered a suitable vitrified bond for diamond abrasive products. Acknowledgment
Fig. 10a and b shows the XRD patterns of the composite coatings of Samples 10 and 11, respectively, after sintering. The diffraction peaks of cristobalite phase showed that crystallization of the glass took place during sintering [28]. This is in agreement with previous results on borosilicate glass/carbon nanotube composites [26]. No graphite was found by XRD implying that the glass softening and flow can protect diamond from oxidization and graphitization during sintering. Borosilicate glass (DURAN [26]) thus emerges as an ideal vitrified bond for diamond containing products. The composite of borosilicate glass and diamond particles can find many applications in wear-resistant materials and cutting tools, especially in diamond wheels for grinding and cutting of ceramics, glasses and silicon wafers [29,30]. Vitrified bond grinding wheels based on sintered mixtures of glass and diamond powders have been shown to be suitable for precision ceramic grinding [31]. In comparison, traditional metal and resin bond wheels might exhibit low grinding effect or might lead to poor grinding surface quality. The wear of vitrified diamond wheels can occur through brittle fracture of the bond material, allowing rapid emergence of new abrasive (diamond) particles in a so-called self-sharpening processing
The author WYH expresses his sincere thanks to CSC (China Scholarship Council) for financial support. References [1] A.R. Boccaccini, J.A. Roether, B.J.C. Thomas, et al., J. Ceram. Soc. Jpn. 114 (2006) 1. [2] A.R. Boccaccini, I. Zhitomirsky, Curr. Opin. Solid State Mater. Sci. 6 (2002) 251. [3] P. Sarkar, P.S. Nicholson, J. Am. Ceram. Soc. 79 (1996) 1987. [4] J. Tabellion, R. Clasen, J. Mater. Sci. 39 (2004) 803. [5] A.R. Boccaccini, O. Van der Biest, J.B. Talbot (Eds.), Electrophoretic Deposition: Fundamental and Applications, The Electrochemical Society, Pennington, US, 2002, (Proc. of the 1st International Conference on Electrophoretic Deposition, held in Banff, Canada, 18–22 August 2002). [6] A.M. Popa, J. Vleugels, J. Vermant, et al., J. Eur. Ceram. Soc. 26 (2006) 933. [7] A.R. Boccaccini, C. Kaya, Ceram. Int. 28 (2002) 893. [8] S. Hayashi, T. Aoki, Z.E. Nakagawa, J. Ceram. Soc. Jpn. 113 (2005) 513. [9] K. Maca, H. Hadraba, J. Cihlar, Ceram. Int. 30 (2004) 843. [10] H. Hadraba, K. Maca, J. Cihlar, Ceram. Int. 30 (2004) 853. [11] Y. Castro, B. Ferrari, R. Moreno, et al., Surf. Coat. Technol. 182 (2004) 199.
Y.H. Wang et al. / Surface & Coatings Technology 201 (2007) 7645–7651 [12] Y.H. Chen, J. Ma, T. Li, Ceram. Int. 30 (2004) 1807. [13] Y.H. Chen, T. Li, J. Ma, J. Mater. Sci. 38 (2003) 2803. [14] S.Y. Ng, A.R. Boccaccini, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 116 (2005) 208. [15] A.R. Boccaccini, P. Karapappas, J.M. Marijuan, et al., J. Mater. Sci. 39 (2004) 851. [16] C.E. Nebel, Semicond. Sci. Technol. 18 (2003) S1. [17] V. Blank, M. Popov, G. Pivovarov, et al., Diamond Relat. Mater. 8 (1999) 1531. [18] A.N. Zhukov, Y.V. Zviaguilskaya, C. Benndorf, Coll. Surf. A— Physicochem Eng. Asp. 222 (2003) 341. [19] G.L. Bilbro, Diamond Relat. Mater. 11 (2002) 1572. [20] A.M. Affoune, B.L.V. Prasad, H. Sato, et al., Langmuir 17 (2001) 547. [21] I. Zhitomirsky, Mater. Lett. 37 (1998) 72.
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[22] T. Tsubota, S. Ida, N. Okada, et al., Surf. Coat. Technol. 169 (2003) 262. [23] S. Guillaudeu, X. Zhu, D.M. Aslam, Diamond Relat. Mater. 12 (2003) 65. [24] A.A. Rouse, J.B. Bernhard, E.D. Sosa, et al., Appl. Phys. Lett. 75 (1999) 3417. [25] R. Schlesser, M.T. McClure, B.L. McCarson, et al., Diamond Relat. Mater. 7 (1998) 636. [26] A.R. Boccaccini, D.R. Acevedo, G. Brusatin, P. Colombo, J. Eur. Ceram. Soc. 25 (2005) 1515. [27] S. Okamura, T. Tsukamoto, N. Koura, Jpn. J. Appl. Phys. 32 (1993) 4182. [28] M.F. Zawrah, E.M.A. Hamzawy, Ceram. Int. 28 (2002) 123. [29] T. Tanaka, S. Esaki, K. Nishida, et al., Key Eng. Mater. 257 (2004) 251. [30] Y.K. Liu, P.L. Tso, Int. J. Adv. Manuf. Technol. 22 (2003) 396. [31] Y.Y. Zhou, M. Atwood, D. Golini, et al., Wear 219 (1998) 42.
Electrophoretic co-deposition of diamond/borosilicate glass composite 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 2007 Available online 2 March 2007
Abstract Composite coatings made of diamond powders and borosilicate glass have been deposited on stainless steel substrates by electrophoretic codeposition. 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 and sintered coatings were investigated by XRD and SEM analysis. The results show that applied voltages up to 10 V led to thin and incomplete coatings. Voltages higher than 50 V resulted in uneven coatings with uncontrolled thickness and poor uniformity. The best results were achieved using ethanol suspensions. Smooth, uniform and dense coatings with diamond and glass particles distributed uniformly were obtained under applied voltages in the range of 30–50 V and a deposition time of 4 min. The concentration ratio of diamond to borosilicate glass in the composite coatings 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 layer forms 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 material processing technology, which is based on the motion of charged particles in liquid suspensions towards an electrode and the controlled deposition of the particles on the electrode substrate under an applied electric field [1]. EPD can produce homogenous coatings of controllable thickness involving low processing cost and requiring simple equipment [1,2]. EPD is especially suitable for producing ceramic coatings, ceramic matrix composites, functionally graded materials and thick films [1–5]. Typical coatings produced include alumina [6–10], zirconia [6,9,10], silica [11], piezoelectric ceramics [12–14] and TiO2 [15]. EPD also offers unique advantages for depositing uniform coatings on different substrates with complex shapes or curved surfaces, from planar 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 advanced machining tools, wear-resistant coatings, heat sinks and microelectronics due to its excellent mechanical and physical properties such as high hardness, high thermal conductivity, and excellent electron field emission effect [16,17]. Limited work has been carried out on the EPD of diamond particles [18–21], although some promising applications including electrophoretically deposited micro diamond layers as seeds for the growth of CVD diamond film [22,23] and EPD of diamond nanoparticles as field emission tips have been considered [24,25]. For most diamond based products the diamond particles are required to be mixed with a metallic or vitreous bond matrix, which should sinter to a continuous matrix embedding the diamond phase. This is necessary because single-phase diamond ceramics are difficult to be densified by pressureless sintering and extremely high temperatures and pressures, involving high-cost equipment, are required. Indeed most EPD coatings also require a subsequent heating procedure to increase the bonding between the deposited particles and the substrate as well as to eliminate
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Table 1 Compositions of the suspensions used in EPD experiments Sample EPD materials and Composition no. particle size of suspensions 1 2 3 4 5
1–2 μm diamond 1–2 μm diamond 0.1–0.5 μm glass 0.1–0.5 μm glass 1–2 μm diamond
6
1–2 μm diamond
7
0.1–0.5 μm glass
8
0.1–0.5μm glass
9
1–2 μm diamond 0.1–0.5 μm glass 1–2 μm diamond 0.1–0.5 μm glass 1–2 μm diamond 0.1–0.5 μm glass
10 11
3.0 g diamond, 100 ml acetone 3.0 g diamond, 100 ml ethanol 1.75 g glass, 100 ml acetone 1.75 g glass, 100 ml ethanol 3.0 g diamond, 100 ml acetone, 50 mg iodine 3.0 g diamond, 100 ml ethanol, 50 mg iodine 1.75 g glass, 100 ml acetone, 50 mg iodine 1.75 g glass, 100 ml ethanol, 50 mg iodine 1.5 g diamond, 1.0 g glass, 100 ml acetone 1.5 g diamond, 1.0 g glass, 100 ml ethanol 3.0 g diamond, 1.0 g glass, 100 ml ethanol
Deposition electrode Anode Anode Anode Anode Cathode Cathode No coating No coating
has been used in previous investigations on glass matrix composites [26]. Both diamond and glass particles were used as received, e.g. without washing or applying any other pretreatment. Acetone and ethanol suspensions containing borosilicate glass and diamond powders were prepared. In some suspensions, a small amount of iodine was added to reverse the charge on the particles, following previous studies in the literature [27]. The addition of iodine is based on the proposed reaction of iodine and acetylacetone according to the following chemical formula: CH3 COCH2 COCH3 þ 2I2 →IH2 CCOCH2 COCH2 I þ2Hþ þ 2I−
Anode Anode Anode
porosity. The development of composite coatings containing diamond particles and a bond matrix is an active field of research in the particular application areas mentioned above. In this paper we report results on the fabrication of diamond/ borosilicate composite coatings using electrophoretic codeposition (EPcD). Borosilicate glass was chosen as a vitrified bond due to its chemical durability and low thermal expansion coefficient [26]. It was hypothesized that electrophoretic codeposition can produce not only a homogenous mixing of diamond and bonding phase particles but also a controllable thickness coating. If successful, the EPcD technique should provide a simple method to develop hard coatings for precision machining tools or wear-resistant parts. 2. Experimental Diamond powders with different particle size distributions were 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, 13 B2O3, 4 Na2O + K2O, 2 Al2O3 (Schott Glass, Germany), which
It is suggested that the protons generated by the reaction are adsorbed on the suspended particles (diamond or glass), making them positively charged. However, if the concentration of iodine is too high, a large amount of H+ ions in the suspension will be produced. In this case, the H+ ions become the main current carriers and the speed of the ceramic particles motion would be reduced. Moreover, a too high concentration of H+ would result in the reduction of the double layer thickness of the particles and hence in the repulsive force between the particles. This would promote particle agglomeration and hence it would give rise to poorer deposition results. The compositions of the prepared suspensions are listed in Table 1. The suspensions were ultrasonically treated for 15 min immediately before electrophoretic deposition. Stainless steel (type 304) plates with dimensions of 10 mm × 10 mm × 0.2 mm were used as electrodes. The as-received plates were washed with the same solvent used in the suspensions prior to EPD. All EPD experiments were carried out at ambient temperature using a two-electrode cell. For each series of experiments, constant voltage in the range of 5–55 V was applied using a Thurlby Thandar Instrument EL561 power supply with electrodes separation of 10 mm and deposition times ranging from 0.5 to 10 min. After deposition, the samples were dried for 24 h at ambient temperature in a desiccator. Sintering of the coatings was carried out in air at 900 °C for 1 h, using a heating rate of 20 °C/min. Samples were cooled down in the furnace.
Fig. 1. SEM micrographs of the composite coatings deposited under different applied voltages (suspension 10): (a) 10 V, (b) 40 V; deposition time: 4 min.
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Fig. 2. Surface morphologies of coatings for Samples 9 (a) and 10 (b), obtained from acetone and ethanol suspensions, respectively.
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 borosilicate glass particles separately in the two solvents. All suspensions had good stability. By a trial-and-error approach it was found that the diamond and glass particles were both negatively charged in these two suspensions and moved towards the anode under the applied electric field. This is the necessary condition to realize the co-deposition for producing composite coatings. Diamond particles should acquire positive surface charge in organic solvents containing a small amount of iodine due to the
effect of the hydrogen ion [13,15,27]. Samples 5–6 in our experiments (Table 1) also confirmed the cathodic deposition of diamond particles, which occurred as 50 mg iodine was added to 100 ml ethanol or acetone suspensions. However, the suspensions of borosilicate glass powders in the same solvents plus iodine were not stable, borosilicate glass powders settled quickly within minutes during EPD. Therefore co-deposition of diamond and borosilicate glass on the cathode was not possible using suspensions with added iodine. The particle size was found to be a key factor determining the stability of the suspensions. It was found that suspensions were unstable as the average diameter of diamond particles was larger than 5 μm (not shown in Table 1). Suspensions were therefore prepared by dispersing diamond particles of size 1–2 μm. Borosilicate glass powder of average size 0.1–0.5 μm in ethanol or acetone was used. Details about the effects of the solvents used on the uniformity of the coatings produced are discussed below. Fig. 1a and b shows SEM micrographs of composite coatings obtained from suspension 10 (see Table 1) using different voltages. It was obvious that the applied voltage had a great influence on the thickness and quality of the coatings obtained. Voltages below 10 V led to thin coatings which did not completely cover the substrate, as shown in Fig. 1a. Increasing
Fig. 3. Variation of the current intensity with deposition time during EPD of diamond and borosilicate glass powders in acetone and ethanol suspensions (Samples 9 and 10, Table 1).
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.
The microstructure of the composite coatings before and after sintering was observed using scanning electron microscopy (SEM) (JEOL 5610). The samples were coated with gold before the examination. X-ray diffraction (XRD) analysis was carried out using a Philips PW1710 diffractometer to investigate the phase composition of the EPD coatings before and after sintering. 3. Results and discussion 3.1. Determination of optimal suspensions and EPD parameters
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Fig. 5. EPD diamond/borosilicate glass composite coating obtained from acetone suspension (Sample 9) on a scratched substrate (applied voltage: 40 V, deposition time: 4 min), showing preferential growth of the coatings along the scratches.
the applied voltage resulted in higher deposition rate and increased coating thickness. On the other hand voltages of over 50 V led to coatings with uneven morphology and poor uniformity, especially greater deposit thickness was observed on the edges of the substrates. Smooth and uniform coatings covering the substrates entirely were obtained using voltages in the range of 30–50 V (Fig. 1b). Compared with the applied voltage, the deposition time has a less significant influence on the quality of the composite coatings. In both suspensions the thickness of the composite coating increased with prolonging the deposition time. However, little increase of the coating thickness was observed when the deposition time was increased to 7 or more minutes. The saturation of deposit thickness with increasing deposition time is usually found in EPD under constant voltage conditions, which is the result of the increasing electric resistance offered by the growing deposit [3]. A deposition time of 4 min was found in the present study to be optimal to obtain high-quality coatings. 3.2. EPD composite coating quality analysis Obvious differences in the surface morphologies of the coatings using different solvents were observed. Sample 9 obtained from acetone suspensions exhibited a very rough and uneven surface. Macroscopic protruding was clearly seen as
shown in Fig. 2a. In contrast, ethanol suspensions led to very smooth and uniform coatings, as seen in Fig. 2b (Sample 10). Both suspensions (Table 1) were of the same composition except for the solvents used: the suspensions contained 1.5 g diamond powder and 1 g borosilicate glass powder in 100 ml solvent (ethanol or acetone). EPcD was carried out at 40 V and deposition time was 4 min. The variation of the current intensity with deposition time was recorded and data are shown in Fig. 3. The current intensity generally decreased with time in acetone suspension but the opposite occurred in ethanol suspensions. To investigate the reasons inducing this effect, the variation of the current intensity with time in pure solvents was recorded, as shown in Fig. 4. The same tendency of current intensity decreasing with time was found in acetone but little variation of current intensity in ethanol was measured. This result indicates that stainless steel substrates are readily passivated in acetone during the EPD process. A further investigation was carried out by electrophoretic depositing the composite coatings on stainless steel substrates on which some scratches had been introduced on the surface. As expected, the coating grew preferentially along the scratches which presented fresh surfaces without passivation, as shown in Fig. 5. Compared with acetone suspensions, the ethanol suspension exhibited better conductivity and no passivation of the electrodes was seen to occur during the EPcD process. As a result, high-quality composite coatings of diamond and borosilicate glass particles were obtained. The SEM micrographs in Fig. 6a, b show the surface of the composite coatings obtained from suspensions 10 and 11, respectively, containing diamond and glass in different weight concentration ratios. The coatings covered the substrate entirely and exhibited homogeneous particle packing and a dense structure. Diamond particles (the larger ones) and borosilicate glass particles (the smaller ones) are seen to be distributed uniformly. No large microcracks were observed on the surface after drying. The corresponding XRD results shown in Fig. 7 also confirm the presence of amorphous glass (halo in the XRD trace) and diamond crystals, both present in the deposit. A higher concentration of diamond powder in the coatings was found in those obtained from suspension 10 due to the relatively high diamond particle content in the suspension. It is not possible to identify the concentration ratio of diamond to glass in the coatings by simple XRD analysis, due to the lack of diffraction pattern of
Fig. 6. Composite coatings obtained from Samples 10 (a) and 11 (b) by EPD (applied voltage: 40 V, deposition time: 4 min).
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Fig. 9. SEM micrograph showing the good wetting and bonding between borosilicate glass and diamond particles achieved by sintering at 900 °C for an hour.
3.3. Sintering of composite coatings
Fig. 7. XRD patterns of diamond/borosilicate glass composite coatings corresponding 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 and weighted, then the coating was etched using HF acid to remove the glass phase and the remaining material was weighted again. It was assumed that the remaining coating only contained diamond because of the high chemical stability of diamond in HF. The weight ratios of diamond to glass in the composite coatings made from Samples 10 and 11 were determined in this manner to be 1:0.85 and 1:1.45, respectively.
The glass chosen as the binder or matrix for diamond crystals should exhibit a low softening temperature because high temperature sintering is harmful for diamond which is easily oxidized or graphitized at high temperatures. The borosilicate glass used in the present experiments has a softening temperature of 650 °C [26]. Fig. 8a and b shows micrographs of the composite coating surfaces of Samples 10 and 11 after sintering at 900 °C for 1 h. The borosilicate glass particles softened and sintered by a viscous flow mechanism at the working temperature. The glass thus had a low viscosity at the sintering temperature and easily spread on the diamond particle surfaces infiltrating the pores between them and bonding the particles together. Thus the diamond particles were fully encapsulated in the glass matrix in Sample 10, as shown in Fig. 8a. However, insufficient glass content in Sample 11 resulted in a discontinuous sintering layer (Fig. 8b). The proper ratio of diamond to glass particle concentrations can be controlled by adjusting the composition of the starting suspension not only to ensure that a sufficient amount of glass is present to bind the diamond particles entirely but also to optimize the glass content for achieving the highest possible concentration of diamond. As inferred by the SEM image in Fig. 9, borosilicate glass exhibited a good wetting behaviour of diamond particles at the working temperature (900 °C) and this ensures a strong adhesion between diamond and the vitrified bond.
Fig. 8. SEM micrographs of Sample 10 (a) and Sample 11 (b) after sintering at 900 °C for 1 h showing different degree of densification due to different relative content of diamond and glass particles.
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and thus continuous grinding is readily established. This effect makes vitrified bond diamond wheels suitable for meeting the requirement of high surface quality in machined hard materials. Compared with the traditional powder metallurgy method in which products are made by a series of processing steps including pressing of powders, sintering, and dressing, our research results about electrophoretic co-deposition of diamond/ borosilicate glass composite coatings provide a simple and costeffective way to develop these materials. 4. Conclusions
Fig. 10. XRD patterns of Sample 10 (a) and Sample 11 (b) after sintering at 900 °C showing formation of cristobalite.
The anodic electrophoretic co-deposition of diamond and borosilicate glass particles was realized using ethanol and acetone suspensions containing diamond particles of size 1– 2 μm and borosilicate glass particles of size 0.1–0.5 μm. The composite coatings obtained from the acetone suspensions were uneven and exhibited uncontrolled microstructure. In contrast, ethanol suspensions led to a smooth and uniform coating covering the substrate entirely. Diamond and glass particles were seen to be homogeneously distributed and no large microcracks were found on the coating surface. The optimum EPD parameters were: applied voltage 30–50 V and deposition time 3–7 min. The ratio of diamond to borosilicate glass in the composite coating can be controlled by adjusting the relative concentration of the constituents in the starting suspensions. The viscous glass at 900 °C wetted the diamond particles, spreading over the diamond surface thus protecting diamond particles from oxidization or graphitization and bonding the diamond particles together during the high temperature treatment. Results have also confirmed that borosilicate glass can be considered a suitable vitrified bond for diamond abrasive products. Acknowledgment
Fig. 10a and b shows the XRD patterns of the composite coatings of Samples 10 and 11, respectively, after sintering. The diffraction peaks of cristobalite phase showed that crystallization of the glass took place during sintering [28]. This is in agreement with previous results on borosilicate glass/carbon nanotube composites [26]. No graphite was found by XRD implying that the glass softening and flow can protect diamond from oxidization and graphitization during sintering. Borosilicate glass (DURAN [26]) thus emerges as an ideal vitrified bond for diamond containing products. The composite of borosilicate glass and diamond particles can find many applications in wear-resistant materials and cutting tools, especially in diamond wheels for grinding and cutting of ceramics, glasses and silicon wafers [29,30]. Vitrified bond grinding wheels based on sintered mixtures of glass and diamond powders have been shown to be suitable for precision ceramic grinding [31]. In comparison, traditional metal and resin bond wheels might exhibit low grinding effect or might lead to poor grinding surface quality. The wear of vitrified diamond wheels can occur through brittle fracture of the bond material, allowing rapid emergence of new abrasive (diamond) particles in a so-called self-sharpening processing
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