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Effect of grain size on the tribological behavior of nanocrystalline nickel
Materials Science and Engineering A 373 (2004) 370–373
Short communication
Effect of grain size on the tribological behavior of nanocrystalline nickel R. Mishra, B. Basu, R. Balasubramaniam∗ Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur 208016, India Received 9 September 2003; received in revised form 20 September 2003
Abstract Nanocrystalline nickel (8–28 nm) deposits were produced by direct and pulse current electrodeposition. The microhardness and microstrains in the deposits were estimated. Tribological testing indicated that the coefficient of friction (COF) for nanocrystalline nickel was almost half that of polycrystalline nickel. An important and interesting result is that extremely low COF of 0.16 can be obtained with nanocrystalline Ni coatings. © 2004 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline; Nickel; Tribology; Microhardness; Microstrain
1. Introduction Nanocrystalline materials are of great industrial importance due to their improved properties especially chemical and mechanical properties. These materials are used as bulk as well as coatings to engineering substrates. Electrodeposition technique is an useful method for producing nanocrystalline coatings [1]. Smaller grain size can be obtained by the addition of suitable grain refining agents to the bath and using pulse current deposition. Several studies have indicated that a fine and uniform grain size can be achieved with increase in current density during pulse current electrodeposition [2–4]. Moreover, grain refining agents like saccharin have been used, in different amounts, as additives for further reduction in grain size in the case of Ni electrodeposition [5–8]. These additives are adsorbed on the surface but are not incorporated in the deposit. In the presence of adsorbed additives, the mean free path for lateral diffusion of ions is shortened, which is equivalent to a decrease in the coefficient of diffusion of ions. This decrease in the coefficient of diffusion results in increase in concentration of ions at the steady state and thus an increase in the two-dimensional nucleation rate [9]. As a consequence, two-dimensional growth rates are reduced, leading to fine grain sizes.
∗
Corresponding author. E-mail address: [email protected] (R. Balasubramaniam).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.09.107
In the present study, nanocrystalline nickel on a copper substrate was produced using direct and pulsed current electrodeposition in a Watt’s bath. Saccharine was added to bath to reduce grain size. Although some studies had been undertaken to develop and evaluate the properties of nanocrystalline coatings [1], the research on nanocrystalline nickel coatings, in particular the tribological behavior is not yet reported. In this perspective, the major aim of the present work was to evaluate the tribological potential of the newly developed nanocrystalline nickel coatings. Because of its technological importance, steel has been selected as the counter-body material.
2. Experimental Nanocrystalline nickel was electrodeposited using Watt’s bath (NiSO4 ·6H2 O 240 gm/l, NiCl2 ·6H2 O 30 gm/l, H3 BO3 30 gm/l). Electrodeposition experiments were performed at 45 ◦ C using a current density of 0.3 A/cm2 with both direct and pulsed current. These optimum parameters were arrived at after initial studies in which the effect of temperature and current density on the adherence, uniformity and current efficiency was studied [10]. In the pulsed current electrodeposition, performed using a computer-controlled Perkin Elmer 263A potentiostat, ton was 5 ms and toff was 20 ms. The total time of deposition was 30 min, i.e. cumulative ton . A lead–tin (Pb–7%Sn) alloy was used as anode and cold rolled copper
R. Mishra et al. / Materials Science and Engineering A 373 (2004) 370–373
as cathode material. Saccharine (10 gm/l) was also added to bath in some of the electrodeposition experiments for reducing the grain size. Grain size of the deposits was calculated by the (1 1 1) X-ray diffraction peak broadening. The diffraction patterns were obtained using Cu K␣ radiation at a scan rate of 3◦ /min in a Rich Seifert diffractometer. The full width half maxima (FWHM) of the (1 1 1) Ni diffraction peaks were estimated by pseudo-Voigt curve fitting using a polynomial of second degree. After subtracting the instrumental line broadening, which was estimated using annealed Ni and silica standards, the grain size was estimated by applying the Scherrer equation [11]. The microstrain in the Ni deposits was calculated by method of linear fitting of X-ray data. The slope of the plot of B cos θ versus sin θ provided the microstrain (ε) as per the following relation [12]: B cos θ = 2ε sin θ + 0.9
λ D
(1)
where B is the FWHM, θ the diffraction angle, D the grain size and λ (=0.15405 nm) is the wavelength of the radiation used. All the deposition experiments were duplicated and good reproducibility was obtained. The coating thickness was estimated from the weight of Ni electrodeposit, as microstructural studies indicated uniform deposition. The Vickers microhardness of the deposits was measured using 1 N load. In the tribological studies, a commercial (DUCOM, India) fretting wear tester (ball-on-flat contact configuration) was used. All the electrodeposited samples were used as the flat materials. The comparison has been made between the nanocrystalline coatings and the bulk coarse-grained polycrystalline nickel samples, cold rolled and annealed to an average grain size of 61 m. Prior to the tests, both the flat and the steel ball were ultrasonically cleaned. The wear tests were performed at a normal load of 1 N, an oscillation frequency of 8 Hz, tangential displacement amplitude of 100 m and number of fretting cycles was 10,000 for all samples. All the wear tests were performed against steel ball (SAE 52100 grade, 8 mm diameter) without lubrication, at room temperature and in ambient air of approximately 40% relative humidity. The coefficient of friction (COF) was recorded continuously during the wear tests. After the wear tests, the surface structures were studied by optical microscopy.
371
3. Results and discussion The coating thickness, grain sizes, mean diagonal length of the microhardness indentation, microhardness and coefficient of friction of the nickel deposits have been listed in Table 1. Typical XRD patterns of the nanocrystalline Ni coatings, exhibiting characteristic line broadening, have been presented elsewhere [13]. The grain sizes of the nanocrystalline nickel varied between 28 and 8 nm. The coating thickness was of the order of 130 m. It can be noted that the depth of the indentation was lower than 10% of the coating thickness in all the cases and therefore, there was no influence of the substrate on the measured hardness. The hardness generally increased with decreasing the grain size. However, the microhardness for deposit of grain size 8 nm was comparable to that of 10 nm grain size sample. Inverse Hall–Petch relationship has been reported in nanocrystalline nickel in the previous studies [1]. Microhardness of the samples have been listed in Table 1. The hardness increased with decreasing grain size. The high hardness exhibited by nanocrystalline nickel is comparable to the results obtained by Erb [1], Abraham et al. [5] and Ebrahimi et al. [14] have reported hardness in the range of 500–650 kg/mm2 for the grain size ranging from 10 to 25 nm. The hardness value increased with decreasing grain size. This is understandable by Hall–Petch relation, which is valid only up to a certain critical grain size. Below a certain low value of grain size, hardness value decreases. Erb [1] has reported inverse Hall–Petch relation for nanocrystalline nickel. The dislocation pile up mechanism is the basis for Hall–Petch equation. In conventional polycrystalline materials of large grain size, hardness increases with decreasing grain size, because of higher degree of dislocation pile up at grain boundaries. However, in case of nanocrystalline materials, the dislocation pile up mechanism may not be applicable when grain size of the material is less than a certain critical value. Below this critical value of grain size, each grain cannot sustain more than one dislocation and therefore dislocation pile up mechanism cannot be applied. The coefficient of friction was determined as a function of number of cycles. Fig. 1 shows the variation of COF with number of cycles for different systems. In all the cases, it was noticed that the COF increased from a low value to a
Table 1 Estimated coating thickness, grain size, mean diagonal length, microhardness, microstrain and coefficient of friction of the nanocrystalline nickel deposits obtained by electrodeposition using a current density of 0.3 A/cm2 at 45 ◦ C Current
Saccharine addition
Grain size (nm)
Coating thickness (m)
Microstrain
Mean diagonal length (m)
Direct Direct Pulsed Pulsed
No Yes No Yes
28 10 22 8
130.5 126.9 136.0 130.5
0.00175 0.00485 0.00245 0.00885
18.0 16.0 17.2 16.4
61 m
–
–
–
Coarse-grained polycrystalline nickel
± ± ± ±
0.3 0.2 0.4 0.2
Microhardness (kg/mm2 ) 572.34 724.00 626.80 689.45
± ± ± ±
COF
18.6 17.4 28.1 16.5
0.55 0.29 0.31 0.16
269.38 ± 39.6
0.62
372
R. Mishra et al. / Materials Science and Engineering A 373 (2004) 370–373
Fig. 1. Variation of coefficient of friction with number of cycles for Ni of grain sizes 8, 22 and 61 m.
very high value within the running-in period (500 cycles) and thereafter, COF marginally decreased and attained a steady state value within the next 1000 cycles. The steady state COF for different samples has been tabulated in Table 1. Comparing the steady state COF values, it is clear that extremely low COF (down to 0.16 for 8 nm grain size Ni) is obtainable with electrodeposited Ni coatings. The COF exhibited by 8 nm deposit was lower than 10 nm deposit. Pure nickel of grain size 61 m exhibited a high value of COF of
around 0.6, whereas for nanocrystalline nickel coated surfaces, the COF was significantly lower. The coefficient of friction apparently decreases with decreasing grain size. It has been reported by Alpas [15] for nanocrystalline nickel that COF decreased by a factor of two and hence wear resistance was enhanced. Increase in the hardness by a factor of 5 will cause an associated increase in wear resistance by a factor of 2 [16]. It was observed, prior to tribological testing, that the surfaces of the coatings and bulk Ni were uniform
Fig. 2. Typical topography of the worn surface, as observed in the optical microscope, on the nanocrystalline 10 nm Ni coating. It was fretted against a steel ball of 10 mm diameter at 100 g load for 10,000 cycles with a frequency of 10 Hz at 100 m stroke. Double pointed arrow indicates the fretting direction.
R. Mishra et al. / Materials Science and Engineering A 373 (2004) 370–373
without any undulations and sharp asperities. Fig. 2 shows the characteristic morphological features of the worn coating surface of nanocrystalline Ni. The worn surface is found to be elliptical with larger dimensions along the fretting direction. The most important feature of the work surface is the smooth appearance revealing that polishing coupled with mild wear is the predominant mechanism for obtaining low COF of nanocrystalline Ni. Similar topographical features were also observed for the other nanocrystalline Ni coatings in the present study. In this regard, it is also illuminating to note the microstrains estimated from the XRD data. The results of microstrain calculations, based on Eq. (1), have been listed in Table 1. The microstrain data indicated that electrodeposition synthesis technique resulted in compressive strains in the deposits, which is similar to that reported by Denis and Such [17]. In the present study, the magnitude of compressive stresses was higher in deposits with saccharine addition compared to deposits obtained without saccharine addition. It can be also noticed from Table 1 that the magnitude of microstrain was higher with pulsing as compared to direct current deposits. It is interesting to note that nanocrystalline Ni coating (8 nm grain size) with higher compressive strain than the other coatings, exhibited much lower COF of 0.16. Observing the data presented in Table 1, it is apparent that larger the microstrain, lower was the COF. However, a distinct relationship between microstrain and COF was not clear based on data presented in Table 1.
4. Summary The tribological and microhardness of nanocrystalline nickel synthesized by direct and pulsed current electrodeposition has been addressed. The higher hardness exhibited by these deposit is the prime reason behind for their lower value of COF in fretting experiment. The observation of
373
smooth polished coating surface was indicative of the lower COF.
Acknowledgements RB would like to acknowledge the equipment grant (potentiostat) from Alexander von Humboldt foundation.
References [1] U. Erb, Nanostruct. Mater. 6 (1995) 533. [2] M. Cherkaoui, E. Chassaing, K. Vu Quang, Surf. Coat. Technol. 34 (1988) 243. [3] A.F. Zimmerman, D.G. Clark, A.T. Aust, U. Erb, Mater. Lett. 52 (2002) 85. [4] Kh. Saber, C.C. Koch, P.S. Fedkiw, Mater. Sci. Eng. 341 (2003) 174. [5] M. Abraham, P. Holway, M. Thuvander, A. Carezo, G.D.W. Smith, Surf. Eng. 18 (2002) 151. [6] M. Thuvander, M. Abraham, A. Cerezo, G.D.W. Smith, Mater. Sci. Technol. 17 (2001) 961. [7] M. Abraham, P. Holway, A. Cerezo, G.D.W. Smith, Mater. Sci. Forum 386–388 (2002) 397. [8] M.L. Trudeau, Nanostruct. Mater. 12 (1999) 55. [9] M. Paunovic, Fundamentals of Electrochemical Deposition, Wiley, New York, 1998, pp. 83–85. [10] R. Mishra, Studies on nanocrystalline nickel synthesized by electrodeposition, M.Tech. Thesis, Indian Institute of Technology, Kanpur, India, 2003. [11] B.D. Cullity, Elements of X-ray Diffraction, 2nd ed., AddisonWesley, Reading, MA, 1978, pp. 281–285. [12] C. Suryanarayana, M.G. Norton, X-ray Diffraction—A Practical Approach, Plenum Press, New York, 1998, pp. 207–218. [13] R. Mishra, R. Balasubramaniam, Corros. Sci., submitted for publication. [14] F. Ebrahimi, G.R. Bourne, M.S. Kelly, T.E. Matthews, Nanostruct. Mater. 11 (1999) 343. [15] A.T. Alpas, University of Windsor, Windsor, Ontario (quoted in [1]). [16] U. Erb, G. Palumbo, B. Szpunar, K.T. Aust, Nanostruct. Mater. 9 (1997) 261. [17] K. Denis, T.E. Such, Nickel and Chromium Plating, 2nd ed., Butterworths, London, 1986, pp. 37–38.
Short communication
Effect of grain size on the tribological behavior of nanocrystalline nickel R. Mishra, B. Basu, R. Balasubramaniam∗ Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur 208016, India Received 9 September 2003; received in revised form 20 September 2003
Abstract Nanocrystalline nickel (8–28 nm) deposits were produced by direct and pulse current electrodeposition. The microhardness and microstrains in the deposits were estimated. Tribological testing indicated that the coefficient of friction (COF) for nanocrystalline nickel was almost half that of polycrystalline nickel. An important and interesting result is that extremely low COF of 0.16 can be obtained with nanocrystalline Ni coatings. © 2004 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline; Nickel; Tribology; Microhardness; Microstrain
1. Introduction Nanocrystalline materials are of great industrial importance due to their improved properties especially chemical and mechanical properties. These materials are used as bulk as well as coatings to engineering substrates. Electrodeposition technique is an useful method for producing nanocrystalline coatings [1]. Smaller grain size can be obtained by the addition of suitable grain refining agents to the bath and using pulse current deposition. Several studies have indicated that a fine and uniform grain size can be achieved with increase in current density during pulse current electrodeposition [2–4]. Moreover, grain refining agents like saccharin have been used, in different amounts, as additives for further reduction in grain size in the case of Ni electrodeposition [5–8]. These additives are adsorbed on the surface but are not incorporated in the deposit. In the presence of adsorbed additives, the mean free path for lateral diffusion of ions is shortened, which is equivalent to a decrease in the coefficient of diffusion of ions. This decrease in the coefficient of diffusion results in increase in concentration of ions at the steady state and thus an increase in the two-dimensional nucleation rate [9]. As a consequence, two-dimensional growth rates are reduced, leading to fine grain sizes.
∗
Corresponding author. E-mail address: [email protected] (R. Balasubramaniam).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.09.107
In the present study, nanocrystalline nickel on a copper substrate was produced using direct and pulsed current electrodeposition in a Watt’s bath. Saccharine was added to bath to reduce grain size. Although some studies had been undertaken to develop and evaluate the properties of nanocrystalline coatings [1], the research on nanocrystalline nickel coatings, in particular the tribological behavior is not yet reported. In this perspective, the major aim of the present work was to evaluate the tribological potential of the newly developed nanocrystalline nickel coatings. Because of its technological importance, steel has been selected as the counter-body material.
2. Experimental Nanocrystalline nickel was electrodeposited using Watt’s bath (NiSO4 ·6H2 O 240 gm/l, NiCl2 ·6H2 O 30 gm/l, H3 BO3 30 gm/l). Electrodeposition experiments were performed at 45 ◦ C using a current density of 0.3 A/cm2 with both direct and pulsed current. These optimum parameters were arrived at after initial studies in which the effect of temperature and current density on the adherence, uniformity and current efficiency was studied [10]. In the pulsed current electrodeposition, performed using a computer-controlled Perkin Elmer 263A potentiostat, ton was 5 ms and toff was 20 ms. The total time of deposition was 30 min, i.e. cumulative ton . A lead–tin (Pb–7%Sn) alloy was used as anode and cold rolled copper
R. Mishra et al. / Materials Science and Engineering A 373 (2004) 370–373
as cathode material. Saccharine (10 gm/l) was also added to bath in some of the electrodeposition experiments for reducing the grain size. Grain size of the deposits was calculated by the (1 1 1) X-ray diffraction peak broadening. The diffraction patterns were obtained using Cu K␣ radiation at a scan rate of 3◦ /min in a Rich Seifert diffractometer. The full width half maxima (FWHM) of the (1 1 1) Ni diffraction peaks were estimated by pseudo-Voigt curve fitting using a polynomial of second degree. After subtracting the instrumental line broadening, which was estimated using annealed Ni and silica standards, the grain size was estimated by applying the Scherrer equation [11]. The microstrain in the Ni deposits was calculated by method of linear fitting of X-ray data. The slope of the plot of B cos θ versus sin θ provided the microstrain (ε) as per the following relation [12]: B cos θ = 2ε sin θ + 0.9
λ D
(1)
where B is the FWHM, θ the diffraction angle, D the grain size and λ (=0.15405 nm) is the wavelength of the radiation used. All the deposition experiments were duplicated and good reproducibility was obtained. The coating thickness was estimated from the weight of Ni electrodeposit, as microstructural studies indicated uniform deposition. The Vickers microhardness of the deposits was measured using 1 N load. In the tribological studies, a commercial (DUCOM, India) fretting wear tester (ball-on-flat contact configuration) was used. All the electrodeposited samples were used as the flat materials. The comparison has been made between the nanocrystalline coatings and the bulk coarse-grained polycrystalline nickel samples, cold rolled and annealed to an average grain size of 61 m. Prior to the tests, both the flat and the steel ball were ultrasonically cleaned. The wear tests were performed at a normal load of 1 N, an oscillation frequency of 8 Hz, tangential displacement amplitude of 100 m and number of fretting cycles was 10,000 for all samples. All the wear tests were performed against steel ball (SAE 52100 grade, 8 mm diameter) without lubrication, at room temperature and in ambient air of approximately 40% relative humidity. The coefficient of friction (COF) was recorded continuously during the wear tests. After the wear tests, the surface structures were studied by optical microscopy.
371
3. Results and discussion The coating thickness, grain sizes, mean diagonal length of the microhardness indentation, microhardness and coefficient of friction of the nickel deposits have been listed in Table 1. Typical XRD patterns of the nanocrystalline Ni coatings, exhibiting characteristic line broadening, have been presented elsewhere [13]. The grain sizes of the nanocrystalline nickel varied between 28 and 8 nm. The coating thickness was of the order of 130 m. It can be noted that the depth of the indentation was lower than 10% of the coating thickness in all the cases and therefore, there was no influence of the substrate on the measured hardness. The hardness generally increased with decreasing the grain size. However, the microhardness for deposit of grain size 8 nm was comparable to that of 10 nm grain size sample. Inverse Hall–Petch relationship has been reported in nanocrystalline nickel in the previous studies [1]. Microhardness of the samples have been listed in Table 1. The hardness increased with decreasing grain size. The high hardness exhibited by nanocrystalline nickel is comparable to the results obtained by Erb [1], Abraham et al. [5] and Ebrahimi et al. [14] have reported hardness in the range of 500–650 kg/mm2 for the grain size ranging from 10 to 25 nm. The hardness value increased with decreasing grain size. This is understandable by Hall–Petch relation, which is valid only up to a certain critical grain size. Below a certain low value of grain size, hardness value decreases. Erb [1] has reported inverse Hall–Petch relation for nanocrystalline nickel. The dislocation pile up mechanism is the basis for Hall–Petch equation. In conventional polycrystalline materials of large grain size, hardness increases with decreasing grain size, because of higher degree of dislocation pile up at grain boundaries. However, in case of nanocrystalline materials, the dislocation pile up mechanism may not be applicable when grain size of the material is less than a certain critical value. Below this critical value of grain size, each grain cannot sustain more than one dislocation and therefore dislocation pile up mechanism cannot be applied. The coefficient of friction was determined as a function of number of cycles. Fig. 1 shows the variation of COF with number of cycles for different systems. In all the cases, it was noticed that the COF increased from a low value to a
Table 1 Estimated coating thickness, grain size, mean diagonal length, microhardness, microstrain and coefficient of friction of the nanocrystalline nickel deposits obtained by electrodeposition using a current density of 0.3 A/cm2 at 45 ◦ C Current
Saccharine addition
Grain size (nm)
Coating thickness (m)
Microstrain
Mean diagonal length (m)
Direct Direct Pulsed Pulsed
No Yes No Yes
28 10 22 8
130.5 126.9 136.0 130.5
0.00175 0.00485 0.00245 0.00885
18.0 16.0 17.2 16.4
61 m
–
–
–
Coarse-grained polycrystalline nickel
± ± ± ±
0.3 0.2 0.4 0.2
Microhardness (kg/mm2 ) 572.34 724.00 626.80 689.45
± ± ± ±
COF
18.6 17.4 28.1 16.5
0.55 0.29 0.31 0.16
269.38 ± 39.6
0.62
372
R. Mishra et al. / Materials Science and Engineering A 373 (2004) 370–373
Fig. 1. Variation of coefficient of friction with number of cycles for Ni of grain sizes 8, 22 and 61 m.
very high value within the running-in period (500 cycles) and thereafter, COF marginally decreased and attained a steady state value within the next 1000 cycles. The steady state COF for different samples has been tabulated in Table 1. Comparing the steady state COF values, it is clear that extremely low COF (down to 0.16 for 8 nm grain size Ni) is obtainable with electrodeposited Ni coatings. The COF exhibited by 8 nm deposit was lower than 10 nm deposit. Pure nickel of grain size 61 m exhibited a high value of COF of
around 0.6, whereas for nanocrystalline nickel coated surfaces, the COF was significantly lower. The coefficient of friction apparently decreases with decreasing grain size. It has been reported by Alpas [15] for nanocrystalline nickel that COF decreased by a factor of two and hence wear resistance was enhanced. Increase in the hardness by a factor of 5 will cause an associated increase in wear resistance by a factor of 2 [16]. It was observed, prior to tribological testing, that the surfaces of the coatings and bulk Ni were uniform
Fig. 2. Typical topography of the worn surface, as observed in the optical microscope, on the nanocrystalline 10 nm Ni coating. It was fretted against a steel ball of 10 mm diameter at 100 g load for 10,000 cycles with a frequency of 10 Hz at 100 m stroke. Double pointed arrow indicates the fretting direction.
R. Mishra et al. / Materials Science and Engineering A 373 (2004) 370–373
without any undulations and sharp asperities. Fig. 2 shows the characteristic morphological features of the worn coating surface of nanocrystalline Ni. The worn surface is found to be elliptical with larger dimensions along the fretting direction. The most important feature of the work surface is the smooth appearance revealing that polishing coupled with mild wear is the predominant mechanism for obtaining low COF of nanocrystalline Ni. Similar topographical features were also observed for the other nanocrystalline Ni coatings in the present study. In this regard, it is also illuminating to note the microstrains estimated from the XRD data. The results of microstrain calculations, based on Eq. (1), have been listed in Table 1. The microstrain data indicated that electrodeposition synthesis technique resulted in compressive strains in the deposits, which is similar to that reported by Denis and Such [17]. In the present study, the magnitude of compressive stresses was higher in deposits with saccharine addition compared to deposits obtained without saccharine addition. It can be also noticed from Table 1 that the magnitude of microstrain was higher with pulsing as compared to direct current deposits. It is interesting to note that nanocrystalline Ni coating (8 nm grain size) with higher compressive strain than the other coatings, exhibited much lower COF of 0.16. Observing the data presented in Table 1, it is apparent that larger the microstrain, lower was the COF. However, a distinct relationship between microstrain and COF was not clear based on data presented in Table 1.
4. Summary The tribological and microhardness of nanocrystalline nickel synthesized by direct and pulsed current electrodeposition has been addressed. The higher hardness exhibited by these deposit is the prime reason behind for their lower value of COF in fretting experiment. The observation of
373
smooth polished coating surface was indicative of the lower COF.
Acknowledgements RB would like to acknowledge the equipment grant (potentiostat) from Alexander von Humboldt foundation.
References [1] U. Erb, Nanostruct. Mater. 6 (1995) 533. [2] M. Cherkaoui, E. Chassaing, K. Vu Quang, Surf. Coat. Technol. 34 (1988) 243. [3] A.F. Zimmerman, D.G. Clark, A.T. Aust, U. Erb, Mater. Lett. 52 (2002) 85. [4] Kh. Saber, C.C. Koch, P.S. Fedkiw, Mater. Sci. Eng. 341 (2003) 174. [5] M. Abraham, P. Holway, M. Thuvander, A. Carezo, G.D.W. Smith, Surf. Eng. 18 (2002) 151. [6] M. Thuvander, M. Abraham, A. Cerezo, G.D.W. Smith, Mater. Sci. Technol. 17 (2001) 961. [7] M. Abraham, P. Holway, A. Cerezo, G.D.W. Smith, Mater. Sci. Forum 386–388 (2002) 397. [8] M.L. Trudeau, Nanostruct. Mater. 12 (1999) 55. [9] M. Paunovic, Fundamentals of Electrochemical Deposition, Wiley, New York, 1998, pp. 83–85. [10] R. Mishra, Studies on nanocrystalline nickel synthesized by electrodeposition, M.Tech. Thesis, Indian Institute of Technology, Kanpur, India, 2003. [11] B.D. Cullity, Elements of X-ray Diffraction, 2nd ed., AddisonWesley, Reading, MA, 1978, pp. 281–285. [12] C. Suryanarayana, M.G. Norton, X-ray Diffraction—A Practical Approach, Plenum Press, New York, 1998, pp. 207–218. [13] R. Mishra, R. Balasubramaniam, Corros. Sci., submitted for publication. [14] F. Ebrahimi, G.R. Bourne, M.S. Kelly, T.E. Matthews, Nanostruct. Mater. 11 (1999) 343. [15] A.T. Alpas, University of Windsor, Windsor, Ontario (quoted in [1]). [16] U. Erb, G. Palumbo, B. Szpunar, K.T. Aust, Nanostruct. Mater. 9 (1997) 261. [17] K. Denis, T.E. Such, Nickel and Chromium Plating, 2nd ed., Butterworths, London, 1986, pp. 37–38.