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A study on the mechanism of formation of electrocodeposited Ni–diamond coatings
Surface and Coatings Technology 148 (2001) 234–240
A study on the mechanism of formation of electrocodeposited Ni–diamond coatings E.C. Lee*, J.W. Choi Division of Materials Science and Engineering, Korea University, Seoul 136-701, South Korea Received 24 April 2001; accepted in revised form 26 June 2001
Abstract Co-deposition behavior of diamond particles and nickel onto a steel in nickel sulfamate baths has been investigated using both a rotating disc and wire geometries. Based on the zeta potential measurement and the change of diamond content of the deposits with electrical potential strength, diamond particles attracted to the cathode electrophoretically after adsorption of positive nickel ions. Results of polarization studies showed that since the rate of nickel deposition was controlled by both mass transfer of the nickel ions and electrochemical deposition, diamond content in nickel–diamond deposits decreased with increasing applied potential and stirring speed in the relatively high rotation speed ranges. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Electroco-deposition; Nickel; Diamond; Rotating disc electrode; Electrophoresis
1. Introduction Co-deposited coatings of fine insoluble particles with a metal have been studied for diverse engineering applications such as wear resistance, self-lubrication and dispersion hardening. Diamond containing metal matrix composites (MMCs) have been used for many years for grinding and cutting tools. Electrodeposited Ni–diamond wires have recently been commercialized for precision cutting of semiconducting silicon ingots. These wire saws have mainly been produced by passing fine steel wire into sedimented diamond particles in nickel plating baths, under which conditions diamond powders could be irregularly deposited and the diamond content in Ni–diamond deposits could not be easily controlled. Many works have been reported concerning electrodeposition of a metal with suspended inert particles in the plating solutions w1–6x. Although the mechanism of co-deposition on the influence of electrolysis and hydrodynamics has not * Corresponding author. Tel.: q82-2-3290-3265; fax: q82-2-9283584. E-mail address: [email protected] (E.C. Lee).
been fully predicted, three possible mechanisms of codeposition are generally accepted; namely inert particles can be transported to the cathode by either electrophoretic action, mechanical entrapment by agitation or van der Waals attractive forces, and they are embedded into the cathode during electrolytic deposition of a metal. Gugliemi w1x proposed two-successive adsorption steps for the entrapment of inert particles during electrodeposition. The two-step process thought to be a loose adsorption, which had an essentially physical character and subsequent strong adsorption that was assumed to be electric field assisted. Zahavi and Hazan w2x suggested three major stages of the particle incorporation processes; stage 1 involved transport of suspended particles from the bulk of the plating solution to the cathode surface, stage 2 encompassed particle adsorption to the cathode surface for a critical time period and finally, the engulfment of particles by the growing metal deposit. According to Buelens et al. w3x adsorption of cations such as copper or gold ions on alumina particles occurred when alumina particles immersed in the plating solution, and the adsorbed particles were transported to the electrode and then finally, reduced at the cathode. Foster and Kariapper w4x reported that addition of certain
0257-8972/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 3 5 2 - 4
E.C. Lee, J.W. Choi / Surface and Coatings Technology 148 (2001) 234–240
235
Fig. 1. (a) Experimental apparatus for electrodeposition by rotating disc electrode, and (b) anode and cathode for precision Ni–diamond wire coatings.
heavy monovalent cations such as Thq, Rbq and Csq to acid copper plating solutions would promote codeposition. Celis et al. w5x developed a statistical model to calculate composition of inert particles of co-deposits and it was found to be valid for the electrolytic codeposition of Cu–Al2O3 from acidic sulfate baths and Au–Al2O3 from cyanide baths. Recently, Abi-Akar et al. w6x reported that uniform surface distribution of particles in the metal matrix were obtained for Ni–diamond and Co–chromium carbide systems under a low gravity environment. Celis et al. w7x gave a very good survey on the mathematical models of electrocodeposition developed over the years and they also presented some trends in future developments of composite plating. The previous studies have not been fully described on the deposition process of Ni– diamond and the influence of electrolysis and hydrodynamics. In the present work, we attempted to elucidate the mechanism of electro-deposited Ni–diamond composite coatings-surface charge of diamond particle, transport of diamond particles and nickel ions from the bulk of the bath to the electrode surface, and variation of diamond content in Ni–diamond deposits dependent on the rate of nickel deposition. 2. Experimental Electrodeposition experiments of Ni–diamond composite coatings were carried out in batches in a 200-ml glass vessel, using a 3-cm diameter flat steel rotating
disc mounted on a Teflon holder and 3-cm length 0.15mm diameter steel wire. The set-up is shown schematically in Fig. 1. A standard Calomel electrode (SCE) equipped with a Luggin capillary probe, was positioned close to the rotating disc. A pure platinum electrode was positioned 2 cm below the rotating disc in the plating solution. The rotating speeds of the disc were controlled by an electrode rotator (HR-103A, Hukudo) from 100 to 3000 rpm, which was under laminar flow conditions. For the wire geometry, a low carbon steel wire cathode was positioned vertically at the center of 1.2-, 2.0- or 3.0-cm diameter of cylindrical perforated platinum anode. The chemical compositions of plating baths, steel plate and steel wire were shown in Table 1. The cathodic surfaces were polished by emery papers and fine alumina powders successively, degreased in the alkaline ultrasonic cleaning bath, and pickled in the acid solution. For the electrodeposition test, a rotating disc or steel wire cathode was used in combination with an electrochemical measurement apparatus, such as power supply (HP6642A, Hewlett-Packard), potentiostat (HA-301, Hukudo) and function generator (HG-104, Hukudo). 2– 4, 4–8, 10–15 and 20–30-mm sized artificial diamond particles (GE Co.) were used during the experiments. The 100-ml plating solutions were controlled within 0.28C using a constant temperature bath. During experimentation of precision diamond wire, the electrolyte was stirred by magnetic stirrer; however, no extra agitation was conducted for the rotating disc geometry. The surface morphology and composition of Ni–dia-
236
E.C. Lee, J.W. Choi / Surface and Coatings Technology 148 (2001) 234–240
Table 1 The composition of the: (a) nickel plating bath; (b) steel plate; and (c) steel wire Composition Nickel plating bath Ni(H2NSO3)2Ø4H2O NiCl2Ø6H2O H3BO3
500 gyl 5 gyl 40 gyl
Steel plate C Mn P Al S N O Fe
250 ppm 0.12% 0.012% 0.074% 0.10% 13 ppm 36 ppm Bal.
Steel wire C Mn Si P S Fe
0.82% 0.50% 0.20% 0.005% 0.004% Bal.
Fig. 2. Volume percent of nickel and diamond as a function of applied voltage. Diamond concentrations4 gyl, and rotating speeds500 rpm.
Fig. 3. Surface morphology of nickel matrix with diamond particles using Ni electrolyte containing 4 gyl of 10–15-mm diamond particles at (a) 0.97, (b) 1.20, (c) 1.49 and (d) 1.65 V.
E.C. Lee, J.W. Choi / Surface and Coatings Technology 148 (2001) 234–240
Fig. 4. Volume percent as a function of diamond concentration in the Ni electrolyte. Applied voltages0.97 V (vs. S.C.E.), and rotation speeds500 rpm.
237
Fig. 6. The polarization curves on the effects of diamond concentration in the bath run at 3000 rpm.
Fig. 5. Surface morphology of nickel matrix with diamond particles using Ni electrolyte containing (a) 1, (b) 2, (c) 3 and (d) 4 gyl of diamond particles.
238
E.C. Lee, J.W. Choi / Surface and Coatings Technology 148 (2001) 234–240
Fig. 7. Volume percent of nickel and diamond as a function of rotation speed. Applied voltages0.97 V (vs. S.C.E.), and diamond concentrations4 gyl.
mond deposits were analyzed by scanning electron microscope (JSM-5310LV, Jeol), image analyzer and atomic absorption spectrophotometer (Varian Tectron1475). Unless otherwise stated, the experiments were conducted at 0.97 V (vs. S.C.E.), 500 rpm, 508C, 4-gy
Fig. 8. The polarization curves on the effects of rotating speed run at diamond concentration of 0.1 vol.%.
Fig. 9. Zeta potential of diamond as a function of pH.
l diamond concentration and 20-min deposition time for the rotating disc geometry. 3. Results and discussion The effect of applied potential on the diamond composition in Ni–diamond deposits was shown in Fig. 2. Increasing the applied potential decreased the diamond
Fig. 10. Adsorption of nickel ions onto diamond particles as a function of nickel concentration in nickel containing solutions.
E.C. Lee, J.W. Choi / Surface and Coatings Technology 148 (2001) 234–240
239
Fig. 11. The effect of the electric field on the deposition of diamond powder for two different anode–cathode distances. Diamond concentrations4 gyl, and agitations460 rpm.
content in the Ni–diamond composite. The explanation of this phenomenon was that the rate of nickel deposition would be more or less controlled by charge transfer at the cathode; however, diamond particles did not transfer charge even if the particles moved to the cathode electrophoretically. The scanning electron photomicrographs showed the morphology of the nickel matrix with diamond particles dependent on the variation of voltage (Fig. 3). A lot of diamond particles were seen on the surface of the nickel deposit at 0.97 V (vs. S.C.E.). However, at a higher voltage of 1.2 V, nickel covered almost all the diamond particles which were shown as white points and then, diamond particles were completely engulfed into nickel deposits at 1.49 and 1.65 V, respectively. Diamond particles could be seen in the photographs of the deposit cross-section taken at higher voltages. The effects of diamond concentration in the bath on the diamond content of the deposits were shown in Fig. 4. The volume percent of diamond particles in Ni– diamond deposits increased by raising concentration of diamond in the bath. The similar results were reported by Zahavi and Hazan w2x and in several different systems w5,8x. The scanning electron photomicrographs also showed that diamond content in Ni–diamond deposits increased with raising diamond concentration of the bath from 1.0 to 4.0 gyl (Fig. 5). The polarization curves on the effects of diamond concentration were shown in Fig. 6. A rise in diamond concentration of the bath tends to
Fig. 12. The surface morphology of nickel–diamond co-deposits for the precision wire cutter. Size of diamond particles equal to (a) 10– 15, and (b) 20–30 mm, respectively.
increase current density and hence, increase the rate of nickel deposition. The results could be attributed to turbulent flow which occurred near the disc surface. The figure also showed that log (current density) increased non-linearly with applied voltage, and this suggests that the rate of nickel deposition could be controlled both by electrochemical charge and mass transfer of nickel ions from the bulk of the electrolyte to the disc surface. The effect of disc rotation speed on the diamond composition in Ni–diamond deposits was shown in Fig. 7. Increasing rotation speed increased diamond content in the deposits from 25 to 32 vol.% at a rotation speed of 300–500 rpm; however, it decreased by 32 to 15 vol.% in the range of 500–1000 rpm. Similar results were also reported for Ni–diamond w2x, Cu–Al2O3 and Au–Al2O3 w3x and Ni–silicon carbite w9x systems.
E.C. Lee, J.W. Choi / Surface and Coatings Technology 148 (2001) 234–240
240
According to Levich w10x, an increase in diamond composition with rotation speed would be due to the increased rate of diamond transport toward the disc surface in the range of lower rotation speed. A further rise in rotation speed would make the diamond particles stay at the deposition surface for too short a period to be adsorbed. The polarization curves with rotation speed were shown in Fig. 8. The current density increased as rotation speed rose from 1000 to 3000 rpm. This meant that the rate of nickel deposition increased with increasing rotation speed and thus, it was partly controlled by mass transfer of nickel ions from the bulk of the electrolyte to the disc surface. Although diamond particles could promote the rate of nickel deposition by turbulent flow near the cathode surface, the deposition rate of diamond itself would not change since diamond particles could not be discharged at the disc surface. The results explained that the diamond content in Ni– diamond deposits decreased with increasing rotation speed in the range of high disc rotation speed. The surface charge of diamond particles was examined by a zetameter (Coulter Delsa 440SX) and the results were shown in Fig. 9. The zeta potential decreased linearly with pH, and the point of zero charge (pzc) pH appeared to be 3.2. Since the pH of the plating bath used was 3.5, diamond particles would be charged negatively without any other positive charged ions in the plating bath except Hq and OHy ions. Hence, diamond particles should adsorb positive Ni2q ions from the bath in order to move into a cathode surface. Fig. 10 showed that the adsorption of Ni2q ions onto diamond particles increased as the concentration of Ni2q ions of a solution increased from 0.4 to 1.0 mmoly l. Similar results were reported on the co-deposition of Cu–Al2O3 system by Buelens et al. w3x, and by Foster and Kariapper w4x. In order to further study the electrophoretic effect of diamond particles, a co-deposition study was conducted by wire geometry. A 0.15-mm diameter steel wire was passed through the center of 12- and 20-mm diameter cylindrical platinum anodes, respectively, in order to co-deposit Ni and diamond, and the results were shown in Fig. 11. In this figure the weight of diamond deposited increased linearly with electric potential strength, which agreed to the electrophoretic equation as follows: us
vŽcmys. EŽVycm.
s
´ß 4ph
where u is mobility (cm2 yV); v, moving velocity of a
particle (cmys); E, electrical potential strength (Vycm); ´, permeability of the fluid (C2 yN cm2); z, zeta potential (volt) and h, viscosity of the fluid (Nycm2 s). A scanning electron photomicrograph of Ni–diamond deposits using a 12-mm diameter cylindrical anode was shown in Fig. 12. It could be seen that 10–15 mm sized diamond particles deposited more abundantly than those of 20–30 mm in size. 4. Conclusions
1. Based on information of the surface charge of diamond particles obtained from zeta potential measurement, diamond particles could adsorb positive nickel ions firstly, in the nickel plating bath and then, were attracted electrophoretically to the cathode. 2. The parameters of the Ni–diamond deposition process, i.e. the applied voltage, stirring speed and concentration of diamond particles in nickel-plating bath, were optimized to obtain suitable diamond content in nickel–diamond deposits. Since the rate of nickel deposition was controlled by both mass transfer of the nickel ions from the bulk of the bath to the electrode surface and electrochemical reaction, diamond content in Ni–diamond deposits decreased with increasing applied potential and stirring speed in the relatively high rotation speed ranges. Acknowledgements The authors would like to thank the Korea Research Foundation for financial support (Project no. 1998-017E00049). References w1x N. Guglielmi, J. Electrochem. Soc. 119 (1972) 1009. w2x J. Zahavi, J. Hazan, Plating Surf. Fin. 70 (2) (1983) 57. w3x C. Buelens, J.P. Celis, J.R. Roos, J. Appl. Electrochem. 13 (1983) 541. w4x J. Foster, A.M.J. Kariapper, Trans. Inst. Met. Fin. 51 (1973) 27. w5x J.P. Celis, J.R. Roos, C. Buelens, J. Electrochem. Soc. 134 (6) (1987) 1402. w6x H. Abi-Akar, C. Riley, G. Maybee, Chem. Mater. 8 (1996) 2601. w7x J.P. Celis, J.R. Roos, C. Buelens, J. Fransaer, Trans. Inst. Metal Fin. 69 (4) (1991) 133. w8x J. Zahavi, H. Kerbel, Plating Surf. Fin. 69 (1) (1982) 76. w9x G. Maurin, A. Lavanant, J. Appl. Electrochem. 25 (1995) 1113. w10x V.G. Levich, Physicochemical Hydrodynamics, Prentice-Hall Inc, 1962, p. 64.
A study on the mechanism of formation of electrocodeposited Ni–diamond coatings E.C. Lee*, J.W. Choi Division of Materials Science and Engineering, Korea University, Seoul 136-701, South Korea Received 24 April 2001; accepted in revised form 26 June 2001
Abstract Co-deposition behavior of diamond particles and nickel onto a steel in nickel sulfamate baths has been investigated using both a rotating disc and wire geometries. Based on the zeta potential measurement and the change of diamond content of the deposits with electrical potential strength, diamond particles attracted to the cathode electrophoretically after adsorption of positive nickel ions. Results of polarization studies showed that since the rate of nickel deposition was controlled by both mass transfer of the nickel ions and electrochemical deposition, diamond content in nickel–diamond deposits decreased with increasing applied potential and stirring speed in the relatively high rotation speed ranges. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Electroco-deposition; Nickel; Diamond; Rotating disc electrode; Electrophoresis
1. Introduction Co-deposited coatings of fine insoluble particles with a metal have been studied for diverse engineering applications such as wear resistance, self-lubrication and dispersion hardening. Diamond containing metal matrix composites (MMCs) have been used for many years for grinding and cutting tools. Electrodeposited Ni–diamond wires have recently been commercialized for precision cutting of semiconducting silicon ingots. These wire saws have mainly been produced by passing fine steel wire into sedimented diamond particles in nickel plating baths, under which conditions diamond powders could be irregularly deposited and the diamond content in Ni–diamond deposits could not be easily controlled. Many works have been reported concerning electrodeposition of a metal with suspended inert particles in the plating solutions w1–6x. Although the mechanism of co-deposition on the influence of electrolysis and hydrodynamics has not * Corresponding author. Tel.: q82-2-3290-3265; fax: q82-2-9283584. E-mail address: [email protected] (E.C. Lee).
been fully predicted, three possible mechanisms of codeposition are generally accepted; namely inert particles can be transported to the cathode by either electrophoretic action, mechanical entrapment by agitation or van der Waals attractive forces, and they are embedded into the cathode during electrolytic deposition of a metal. Gugliemi w1x proposed two-successive adsorption steps for the entrapment of inert particles during electrodeposition. The two-step process thought to be a loose adsorption, which had an essentially physical character and subsequent strong adsorption that was assumed to be electric field assisted. Zahavi and Hazan w2x suggested three major stages of the particle incorporation processes; stage 1 involved transport of suspended particles from the bulk of the plating solution to the cathode surface, stage 2 encompassed particle adsorption to the cathode surface for a critical time period and finally, the engulfment of particles by the growing metal deposit. According to Buelens et al. w3x adsorption of cations such as copper or gold ions on alumina particles occurred when alumina particles immersed in the plating solution, and the adsorbed particles were transported to the electrode and then finally, reduced at the cathode. Foster and Kariapper w4x reported that addition of certain
0257-8972/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 3 5 2 - 4
E.C. Lee, J.W. Choi / Surface and Coatings Technology 148 (2001) 234–240
235
Fig. 1. (a) Experimental apparatus for electrodeposition by rotating disc electrode, and (b) anode and cathode for precision Ni–diamond wire coatings.
heavy monovalent cations such as Thq, Rbq and Csq to acid copper plating solutions would promote codeposition. Celis et al. w5x developed a statistical model to calculate composition of inert particles of co-deposits and it was found to be valid for the electrolytic codeposition of Cu–Al2O3 from acidic sulfate baths and Au–Al2O3 from cyanide baths. Recently, Abi-Akar et al. w6x reported that uniform surface distribution of particles in the metal matrix were obtained for Ni–diamond and Co–chromium carbide systems under a low gravity environment. Celis et al. w7x gave a very good survey on the mathematical models of electrocodeposition developed over the years and they also presented some trends in future developments of composite plating. The previous studies have not been fully described on the deposition process of Ni– diamond and the influence of electrolysis and hydrodynamics. In the present work, we attempted to elucidate the mechanism of electro-deposited Ni–diamond composite coatings-surface charge of diamond particle, transport of diamond particles and nickel ions from the bulk of the bath to the electrode surface, and variation of diamond content in Ni–diamond deposits dependent on the rate of nickel deposition. 2. Experimental Electrodeposition experiments of Ni–diamond composite coatings were carried out in batches in a 200-ml glass vessel, using a 3-cm diameter flat steel rotating
disc mounted on a Teflon holder and 3-cm length 0.15mm diameter steel wire. The set-up is shown schematically in Fig. 1. A standard Calomel electrode (SCE) equipped with a Luggin capillary probe, was positioned close to the rotating disc. A pure platinum electrode was positioned 2 cm below the rotating disc in the plating solution. The rotating speeds of the disc were controlled by an electrode rotator (HR-103A, Hukudo) from 100 to 3000 rpm, which was under laminar flow conditions. For the wire geometry, a low carbon steel wire cathode was positioned vertically at the center of 1.2-, 2.0- or 3.0-cm diameter of cylindrical perforated platinum anode. The chemical compositions of plating baths, steel plate and steel wire were shown in Table 1. The cathodic surfaces were polished by emery papers and fine alumina powders successively, degreased in the alkaline ultrasonic cleaning bath, and pickled in the acid solution. For the electrodeposition test, a rotating disc or steel wire cathode was used in combination with an electrochemical measurement apparatus, such as power supply (HP6642A, Hewlett-Packard), potentiostat (HA-301, Hukudo) and function generator (HG-104, Hukudo). 2– 4, 4–8, 10–15 and 20–30-mm sized artificial diamond particles (GE Co.) were used during the experiments. The 100-ml plating solutions were controlled within 0.28C using a constant temperature bath. During experimentation of precision diamond wire, the electrolyte was stirred by magnetic stirrer; however, no extra agitation was conducted for the rotating disc geometry. The surface morphology and composition of Ni–dia-
236
E.C. Lee, J.W. Choi / Surface and Coatings Technology 148 (2001) 234–240
Table 1 The composition of the: (a) nickel plating bath; (b) steel plate; and (c) steel wire Composition Nickel plating bath Ni(H2NSO3)2Ø4H2O NiCl2Ø6H2O H3BO3
500 gyl 5 gyl 40 gyl
Steel plate C Mn P Al S N O Fe
250 ppm 0.12% 0.012% 0.074% 0.10% 13 ppm 36 ppm Bal.
Steel wire C Mn Si P S Fe
0.82% 0.50% 0.20% 0.005% 0.004% Bal.
Fig. 2. Volume percent of nickel and diamond as a function of applied voltage. Diamond concentrations4 gyl, and rotating speeds500 rpm.
Fig. 3. Surface morphology of nickel matrix with diamond particles using Ni electrolyte containing 4 gyl of 10–15-mm diamond particles at (a) 0.97, (b) 1.20, (c) 1.49 and (d) 1.65 V.
E.C. Lee, J.W. Choi / Surface and Coatings Technology 148 (2001) 234–240
Fig. 4. Volume percent as a function of diamond concentration in the Ni electrolyte. Applied voltages0.97 V (vs. S.C.E.), and rotation speeds500 rpm.
237
Fig. 6. The polarization curves on the effects of diamond concentration in the bath run at 3000 rpm.
Fig. 5. Surface morphology of nickel matrix with diamond particles using Ni electrolyte containing (a) 1, (b) 2, (c) 3 and (d) 4 gyl of diamond particles.
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E.C. Lee, J.W. Choi / Surface and Coatings Technology 148 (2001) 234–240
Fig. 7. Volume percent of nickel and diamond as a function of rotation speed. Applied voltages0.97 V (vs. S.C.E.), and diamond concentrations4 gyl.
mond deposits were analyzed by scanning electron microscope (JSM-5310LV, Jeol), image analyzer and atomic absorption spectrophotometer (Varian Tectron1475). Unless otherwise stated, the experiments were conducted at 0.97 V (vs. S.C.E.), 500 rpm, 508C, 4-gy
Fig. 8. The polarization curves on the effects of rotating speed run at diamond concentration of 0.1 vol.%.
Fig. 9. Zeta potential of diamond as a function of pH.
l diamond concentration and 20-min deposition time for the rotating disc geometry. 3. Results and discussion The effect of applied potential on the diamond composition in Ni–diamond deposits was shown in Fig. 2. Increasing the applied potential decreased the diamond
Fig. 10. Adsorption of nickel ions onto diamond particles as a function of nickel concentration in nickel containing solutions.
E.C. Lee, J.W. Choi / Surface and Coatings Technology 148 (2001) 234–240
239
Fig. 11. The effect of the electric field on the deposition of diamond powder for two different anode–cathode distances. Diamond concentrations4 gyl, and agitations460 rpm.
content in the Ni–diamond composite. The explanation of this phenomenon was that the rate of nickel deposition would be more or less controlled by charge transfer at the cathode; however, diamond particles did not transfer charge even if the particles moved to the cathode electrophoretically. The scanning electron photomicrographs showed the morphology of the nickel matrix with diamond particles dependent on the variation of voltage (Fig. 3). A lot of diamond particles were seen on the surface of the nickel deposit at 0.97 V (vs. S.C.E.). However, at a higher voltage of 1.2 V, nickel covered almost all the diamond particles which were shown as white points and then, diamond particles were completely engulfed into nickel deposits at 1.49 and 1.65 V, respectively. Diamond particles could be seen in the photographs of the deposit cross-section taken at higher voltages. The effects of diamond concentration in the bath on the diamond content of the deposits were shown in Fig. 4. The volume percent of diamond particles in Ni– diamond deposits increased by raising concentration of diamond in the bath. The similar results were reported by Zahavi and Hazan w2x and in several different systems w5,8x. The scanning electron photomicrographs also showed that diamond content in Ni–diamond deposits increased with raising diamond concentration of the bath from 1.0 to 4.0 gyl (Fig. 5). The polarization curves on the effects of diamond concentration were shown in Fig. 6. A rise in diamond concentration of the bath tends to
Fig. 12. The surface morphology of nickel–diamond co-deposits for the precision wire cutter. Size of diamond particles equal to (a) 10– 15, and (b) 20–30 mm, respectively.
increase current density and hence, increase the rate of nickel deposition. The results could be attributed to turbulent flow which occurred near the disc surface. The figure also showed that log (current density) increased non-linearly with applied voltage, and this suggests that the rate of nickel deposition could be controlled both by electrochemical charge and mass transfer of nickel ions from the bulk of the electrolyte to the disc surface. The effect of disc rotation speed on the diamond composition in Ni–diamond deposits was shown in Fig. 7. Increasing rotation speed increased diamond content in the deposits from 25 to 32 vol.% at a rotation speed of 300–500 rpm; however, it decreased by 32 to 15 vol.% in the range of 500–1000 rpm. Similar results were also reported for Ni–diamond w2x, Cu–Al2O3 and Au–Al2O3 w3x and Ni–silicon carbite w9x systems.
E.C. Lee, J.W. Choi / Surface and Coatings Technology 148 (2001) 234–240
240
According to Levich w10x, an increase in diamond composition with rotation speed would be due to the increased rate of diamond transport toward the disc surface in the range of lower rotation speed. A further rise in rotation speed would make the diamond particles stay at the deposition surface for too short a period to be adsorbed. The polarization curves with rotation speed were shown in Fig. 8. The current density increased as rotation speed rose from 1000 to 3000 rpm. This meant that the rate of nickel deposition increased with increasing rotation speed and thus, it was partly controlled by mass transfer of nickel ions from the bulk of the electrolyte to the disc surface. Although diamond particles could promote the rate of nickel deposition by turbulent flow near the cathode surface, the deposition rate of diamond itself would not change since diamond particles could not be discharged at the disc surface. The results explained that the diamond content in Ni– diamond deposits decreased with increasing rotation speed in the range of high disc rotation speed. The surface charge of diamond particles was examined by a zetameter (Coulter Delsa 440SX) and the results were shown in Fig. 9. The zeta potential decreased linearly with pH, and the point of zero charge (pzc) pH appeared to be 3.2. Since the pH of the plating bath used was 3.5, diamond particles would be charged negatively without any other positive charged ions in the plating bath except Hq and OHy ions. Hence, diamond particles should adsorb positive Ni2q ions from the bath in order to move into a cathode surface. Fig. 10 showed that the adsorption of Ni2q ions onto diamond particles increased as the concentration of Ni2q ions of a solution increased from 0.4 to 1.0 mmoly l. Similar results were reported on the co-deposition of Cu–Al2O3 system by Buelens et al. w3x, and by Foster and Kariapper w4x. In order to further study the electrophoretic effect of diamond particles, a co-deposition study was conducted by wire geometry. A 0.15-mm diameter steel wire was passed through the center of 12- and 20-mm diameter cylindrical platinum anodes, respectively, in order to co-deposit Ni and diamond, and the results were shown in Fig. 11. In this figure the weight of diamond deposited increased linearly with electric potential strength, which agreed to the electrophoretic equation as follows: us
vŽcmys. EŽVycm.
s
´ß 4ph
where u is mobility (cm2 yV); v, moving velocity of a
particle (cmys); E, electrical potential strength (Vycm); ´, permeability of the fluid (C2 yN cm2); z, zeta potential (volt) and h, viscosity of the fluid (Nycm2 s). A scanning electron photomicrograph of Ni–diamond deposits using a 12-mm diameter cylindrical anode was shown in Fig. 12. It could be seen that 10–15 mm sized diamond particles deposited more abundantly than those of 20–30 mm in size. 4. Conclusions
1. Based on information of the surface charge of diamond particles obtained from zeta potential measurement, diamond particles could adsorb positive nickel ions firstly, in the nickel plating bath and then, were attracted electrophoretically to the cathode. 2. The parameters of the Ni–diamond deposition process, i.e. the applied voltage, stirring speed and concentration of diamond particles in nickel-plating bath, were optimized to obtain suitable diamond content in nickel–diamond deposits. Since the rate of nickel deposition was controlled by both mass transfer of the nickel ions from the bulk of the bath to the electrode surface and electrochemical reaction, diamond content in Ni–diamond deposits decreased with increasing applied potential and stirring speed in the relatively high rotation speed ranges. Acknowledgements The authors would like to thank the Korea Research Foundation for financial support (Project no. 1998-017E00049). References w1x N. Guglielmi, J. Electrochem. Soc. 119 (1972) 1009. w2x J. Zahavi, J. Hazan, Plating Surf. Fin. 70 (2) (1983) 57. w3x C. Buelens, J.P. Celis, J.R. Roos, J. Appl. Electrochem. 13 (1983) 541. w4x J. Foster, A.M.J. Kariapper, Trans. Inst. Met. Fin. 51 (1973) 27. w5x J.P. Celis, J.R. Roos, C. Buelens, J. Electrochem. Soc. 134 (6) (1987) 1402. w6x H. Abi-Akar, C. Riley, G. Maybee, Chem. Mater. 8 (1996) 2601. w7x J.P. Celis, J.R. Roos, C. Buelens, J. Fransaer, Trans. Inst. Metal Fin. 69 (4) (1991) 133. w8x J. Zahavi, H. Kerbel, Plating Surf. Fin. 69 (1) (1982) 76. w9x G. Maurin, A. Lavanant, J. Appl. Electrochem. 25 (1995) 1113. w10x V.G. Levich, Physicochemical Hydrodynamics, Prentice-Hall Inc, 1962, p. 64.