Microstructure and tribological properties of electrodeposited Ni Co alloy deposits

Transcription

1 Applied Surface Science 242 (2005) Microstructure and tribological properties of electrodeposited Ni Co alloy deposits Liping Wang a,b, Yan Gao a,b, Qunji Xue a, Huiwen Liu a, Tao Xu a, * a State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou , PR China b Graduate School of the Chinese Academy of Sciences, Beijng , PR China Received in revised form 24 August 2004; accepted 30 August 2004 Available online 20 October 2004 Abstract Ni Co alloys with different compositions and microstructures were produced by electrodeposition. The effects of Co content on the composition, surface morphology, phase structure, hardness and tribological properties of Ni Co alloys were investigated systemically. Results showed that the morphology and grain size of alloys are mainly influenced by the Co content and the phase structure of Ni Co alloys gradually changed from fcc into hcp structure with the increase of Co content. The hardness of Ni Co alloys with a maximum around 49 wt.% Co followed the Hall Petch effect. It was found that the improvement of wear resistance of Ni-rich alloys with hardness increase fits Archard s law. In addition, the Co-rich alloys exhibited much lower friction coefficient and higher wear resistance when compared with Ni-rich alloys. It has been concluded that hcp crystal structure in Corich alloys contributed to the remarkable friction reduction effect and better anti-wear performance under the dry sliding wear conditions. # 2004 Elsevier B.V. All rights reserved. Keywords: Ni Co alloy; Electrodeposition; Structure; Tribological properties 1. Introduction Ni Co alloys have been investigated as important engineering materials for several decades because of their unique properties, such as high-strength, good wear resistance, heat-conductive, electrocatalytic * Corresponding author. Tel.: ; fax: address: lpwang@lsl.ac.cn (T. Xu). activity [1 5]. Additionally, the use of Ni Co alloys has been extended to the production of threedimensional, complex-shaped finished components by the electroforming technique [6,7]. The investigations on the electrodeposited Ni Co alloys have shown that their microstructure and properties were found to depend strongly on the Co content, which can be controlled by the experimental parameters, such as bath composition, temperature, ph value, and current density, etc. [1,3,8]. The effects of plating parameters /$ see front matter # 2004 Elsevier B.V. All rights reserved. doi: /j.apsusc

2 L. Wang et al. / Applied Surface Science 242 (2005) on the composition and morphology of Ni Co deposits were compared in many literatures [4,5,9]. Golodnitsky et al. recently studies the effects of Co content on the tensile strength, internal stress and high-temperature oxidation of Ni Co alloys [3]. Their activities for the oxygen evolution reaction and hydrogen evolution reaction were also studied on electrodeposited Ni Co ultramicroelectrodes [10,11]. Moreover, much interest is focused on the magnetic properties of Ni Co alloys due to the application of these alloys in various magnetic devices, especially in microsystem technology for manufacture of sensors, actuators and inductors [12,13]. It is reported that the magnetic properties of Ni Co alloy are greatly influenced by the composition and phase structure of Ni Co alloy [14]. Unfortunately, there are very limited studies focused on the friction and wear properties of Ni Co alloys as a function of their microstructure and composition. In the present paper, Ni Co alloys with different Co content were electrodeposited on AISI-1045 steel substrates. The composition, microstructure, mechanical, and tribological properties of Ni Co alloys were compared systemically in order to specifically correlate the structure and tribological properties of Ni Co alloys. 2. Experimental Ni Co alloys were electrodeposited from a typical Watts-type electrolyte, containing Nickel sulfate (200 g/l), sodium chloride (20 g/l), boric-acid (30 g/ l), sodium lauryl sulfate (0.1 g/l) and cobalt sulfate (0 80 g/l). In addition, pure Ni was also produced for comparison purpose. The Ni Co alloys were deposited on AISI-1045 steel substrates by choosing a current density of 3 A/dm 2 at a bath temperature of 45 8C. The anode was a pure Ni plate. The ph of the bath was kept at 4.0 adjusted by ammonia water or dilute sulfuric-acid. Before deposition, the substrates were mechanically polished to a mm surface finish, the substrate was then degreased in acetone with ultrasonic cleaning for 5 min, rinsed in the running water to remove contamination on the substrate surface. After than, the steel substrates were activated for 20 s in the 20 vol.% HCl solution, and finally rinsed with distilled water. The surface morphology and microstructure of the alloy deposits were investigated using a JSM-5600Lv scanning electron microscopy (SEM). The compositions of Ni Co alloys were determined with energy dispersive X-ray spectroscopy (EDS) analysis tool attached to SEM. The crystal structure and phase composition of alloy deposits were studied by X-ray diffraction (XRD). Microhardness of the deposits was determined using a Vicker s microhardness indenter with a load of 50 g for 10 s, indentations were made on the 50 mm thick deposits. The final value quoted for the hardness of a deposit was the average of 10 measurements. The tribological behavior was tested on a reciprocating ball-on-disk UMT-2MT tribometer (Center for tribology, Inc., California, USA) at room temperature with a relative humidity of 45 55% under dry sliding conditions. AISI stainless steel ball (diameter 4 mm with hardness of RC 62) was used as the counter body; all tests were performed under a load of 3 N with a sliding speed of 55 mm s 1. The friction coefficient and sliding time were recorded automatically during the test. The wear volume loss was measured using a surface profilometer, wear rates of all the alloy deposits were calculated using the equation of K ¼ V=SF, where V is the wear volume loss in mm 3, S the total sliding distance in m and F the normal load in N. 3. Results and discussion 3.1. Composition of Ni Co alloys The dependence of the composition of Ni Co alloys on the concentration of Co 2+ ions in the electrolyte at a fixed concentration of Ni 2+ ions is presented in Fig. 1.It is clearly observed that the Co content in alloy deposits increased gradually with the increase of Co 2+ concentration in the electrolyte. Note that the percentage of Co in the alloys was always higher than in the electrolyte in agreement with [3,5], which is confirmed by the anomalous codeposition of Ni Co alloy. Namely, the less noble metal (Co) is preferentially deposited. A generally accepted explanation for these anomalous phenomena was the change of the near-electrode ph, the formation of metal hydroxyl and their competitive adsorption [15,16].

3 328 L. Wang et al. / Applied Surface Science 242 (2005) Fig. 1. The alloy compositions as a function of Co 2+ concentrations in the baths Morphology and phase structure of Ni Co alloys Typical surface morphologies of Ni Co alloys with different Co content are shown in Fig. 2b f, respectively. Fig. 2a shows a typical morphology of a Watt Ni deposit, which has relatively large grain size (3 10 mm) and showed polyhedral crystallites. Sequentially increasing Co content from 7 to 49 wt.% (Fig. 2b d) results in a gradual decrease in the grain size of the Ni Co alloy down to a sub-micron grain size. When the Co content reached the 49 wt.%, close observation of SEM morphology at highmagnification (Fig. 2g) revealed that the Ni Co alloys have spherical cluster surface piled with a large number of equally sized grains with spherical-shape. At above 49 wt.% Co, the grain size of Ni Co deposits, however, increased with the increase of Co content in alloys. When increasing Co content up to 81 wt.%, the morphology of the Ni Co alloys changes dramatically, and with less compact structure, the Ni Co alloy showed a rather regularly branched structure with extended acicular 3 6 mm length crystallites (Fig. 2f). The phase composition and structure of pure Ni and Ni Co alloys with different Co contents were investigated using XRD shown in Fig. 3. As can be seen from Fig. 3a, the pure Ni deposit exhibits facecentered cubic (fcc) lattice with remarkable (2 0 0) growth orientation, which can be attributed to the Fig. 2. SEM morphologies of Ni Co alloy deposits with their Co contents of (a) 0 wt.%, (b) 7 wt.%, (c) 27 wt.%, (d) 49 wt.%, (e) 66 wt.%, (f) 81 wt.%, (g) high-magnification of Ni 49 wt.% Co alloy. largest grain size of pure Ni. With the codeposition of Co, the Ni Co solid solution was formed. As can be seen in Fig. 3b f, both the crystal structure and phase composition are mainly dependent on the Co contents in alloys. For the Ni-rich alloys with Co content lower than 49 wt.%, the Ni Co alloys show complete fcc phase structure, which is in agreement with previously reported results [2,3]. Furthermore, the (1 1 1) growth orientation gradually increased with the increase of Co content, and the FWHM of the Bragg line for the (1 1 1) peak also increased correspondingly, which is in accordance with the gradual reduction of grain size when increasing the Co content from 0 to 49 wt.% in Ni Co alloys as shown in Fig. 2b d. Moreover, when

4 L. Wang et al. / Applied Surface Science 242 (2005) wt.%, as shown in Fig. 3f that a very strong hcp (0 0 2) texture with pronounced (1 0 0) and (1 1 0) peaks were observed, which is commonly observed in both conventionally electrodeposited Co and nanocrystalline Co [17,18]. Therefore, it can be concluded that the phase structure of Ni Co alloys gradually changed from fcc into hcp with the increase of Co content as shown in Fig Microhardness of Ni Co alloy deposits Fig. 3. XRD patterns of Ni Co alloy deposits with their Co contents of (a) 0 wt.%, (b) 7 wt.%, (c) 27 wt.%, (d) 49 wt.%, (e) 66 wt.%, (f) 81 wt.%. the Co content was increased to 66 wt.%, the presence of (1 0 0) peak demonstrated the initial formation of a hexagonal close packed (hcp) lattice, indicating that the crystal structure of the Ni Co alloy changed from complete fcc lattice into a mixed (majority of fcc) + (minority of hcp) phase as shown in Fig. 3e. At above Fig. 4a presented the microhardness of Ni Co alloys as a function of Co content in alloys. It is clearly that microhardness of Ni Co alloys increased initially with Co content varying from 0 to approximately 49 wt.%, and then gradually decreased as Co content increased further above 49 wt.%. The explanation to this gradual reduction of microhardness is the gradual increase of grain size with the increase of Co content in Co-rich alloys as shown in Fig. 2e f. Note that the microcrystalline Ni Co alloys show the maximum hardness at approximately 49 wt.% Co, which can be associated with the smallest grain size as mentioned in microstructure analysis sector. Normally, strengthening of polycrystalline materials by grain size refinement is technologically attractive because it generally does not adversely affect ductility and toughness [19], which can be represented by the classical Hall Petch effect: H ¼ H 0 þ kd 0:5 (1) where H 0 is hardness constant, k constant, and d diameter of grain. The hardness change with average grain size (d) of the Ni Co deposits is shown in Fig. 4b Fig. 4. Microhardness as function of Co content (a) and d 0.5 (b) of the Ni Co deposits.

5 330 L. Wang et al. / Applied Surface Science 242 (2005) in the form of a Hall Petch plot. It is obvious that the Ni Co alloy deposits exhibit a nearly constant Hall Petch gradient; such a relationship has also been observed on pure Ni, pure Co and pure Zn from other studies [20,21] Friction and wear properties The effect of Co content on friction coefficient of Ni Co alloys were shown in Fig. 5. It is observed that the friction coefficient of pure Ni and Ni Co alloys with Co content lower than 49 wt.% (Ni-rich alloys) were quite close. With the further increase of Co content, the Corich alloys showed excellent friction reduction behavior. The Co-rich alloy deposit with Co content higher than 81 wt.% exhibited the smallest friction coefficient (more than two times lower than Ni and Ni-rich alloys), followed by Ni 66 wt.% Co alloy (a litter lower than Ni and Ni-rich alloys) under identical wear test conditions. In addition, the friction coefficients of Co-rich alloys were much more stable than that of Ni-rich alloy deposits (see Fig. 6). Combined with the XRD analysis, the close friction coefficient for Ni and Ni-rich alloy can be attributed to the same fcc crystal structure they have. In case of Ni 66 wt.% Co alloy, a mixed fcc/hcp phase with smaller ratio of hcp phase structure led to the gradual reduction of friction coefficient. Furthermore, as for the Ni 81 wt.% Co alloy, dramatic reduction of friction coefficient was observed due to the higher ratio of hcp phase structure. Hence, we can conclude that the reduction in friction coefficient of Co-rich alloys with Fig. 6. The comparison of friction coefficients vs. sliding time between Ni-rich and Co-rich alloy deposits. the increase of Co content can be associated with the change of crystal structure from fcc to hcp crystal phase. The variation of the wear rates of Ni Co alloys as a function of Co content and microhardness of alloys are shown in Fig. 7. It is observed that all Ni Co alloy deposits in this study have lower wear rates when compared with pure Ni deposit. Moreover, the wear rate of Ni Co alloys slowly decreased with the increase of Co content from 6 to 49 wt.%. It is clear that when the Co content is lower than 49 wt.%, the gradual decrease of wear rates with the increase of Co content was attributed to the microhardness increase from 315 to 462 HV. Above improvement of wear resistance with hardness increase, in this study due to the grain size reduction, could be expressed using Archard s law mostly used in adhesive wear conditions [22,23], since the wear mechanism of Ni and Fig. 5. Friction coefficient as function of Co content in the Ni Co alloy deposits. Fig. 7. Wear rates as function of Co content in the Ni Co alloy deposits.

6 L. Wang et al. / Applied Surface Science 242 (2005) Ni-rich deposits is mostly the adhesive wear as evidenced by SEM morphology of worn surface in Fig. 8a and b. Thus, the Archard s law can be expressed as: Q ¼ K LN (2) H where Q is the volumetric wear loss, N the applied load, L the total sliding distance, K the wear coefficient and H the hardness of the wear surface. Under the same wear conditions, the wear rate is proportional to the inverse microhardness of materials. The data of wear rate for microcrystalline Ni Co alloys with Co content lower than 49 wt.% fit Archard s law very well. However, with further increase in Co content above 49 wt.%, the wear rates of Co-rich alloys decreased rapidly in spite of the fact that the hardness also decreased. The wear rate of Co-rich alloy with approximately 81 wt.% Co content is more than one order of magnitude lower than that of pure Ni and Nirich alloys. This reverse-archard law may be caused by special hcp crystal structure of Co-rich alloys. This agree well with the reduction in friction coefficient for Co-rich alloys, namely, the lower and stable friction coefficient of Co-rich alloys caused by hcp phase structure resulted in the less wear loss, while the higher friction coefficient of Ni-rich alloys due to fcc phase structure led to the more wear loss. More important is the fact that the Co-rich alloys exhibited excellent wear resistance and anti-friction behavior. The difference in the wear behavior of Ni Co alloys can be further verified by the worn surface morphologies of Ni-rich and Co-rich deposit as shown in Fig. 8a c. For the pure Ni deposit and Ni-rich alloy with completely fcc crystal structure, the wear track (Fig. 8a and b) shows the larger extent of adhesion wear and severe deformation in the sliding direction under the combined stresses of compression and shear, which results in larger wear rate of pure Ni and Ni-rich alloys. Furthermore, larger tendency for plastic deformation, this in turn increased the probability of formation of asperity junctions resulting in higher and unstable friction coefficient for Ni and Ni-rich alloys. Compared with pure Ni and Ni-rich alloys, a densification of the worn surface of Co-rich alloy seems to take place, the worn surface of Co-rich alloy with hcp crystal structure revealed slight adhesion wear and rather smooth surface with smaller damaged regions, only some light grooves and scars are noted on the worn surface (Fig. 8c). This resulted in the better wear resistance of Co-rich alloy than Ni-rich alloys. That is also the reason why the friction Fig. 8. Worn surface of Ni Co alloy deposits: (a) 0% Co; (b) 27% Co; (c) 81% Co.

7 332 L. Wang et al. / Applied Surface Science 242 (2005) coefficients of Co-rich alloys were much more stable and more than two times lower than that of Ni and Ni-rich alloys. It is evident that the high the amount of hcp phase structure, the better the friction and wear behavior will be. Above evidence suggests that the crystal structure is indeed a dominant factor, which influences the friction and wear behavior of Ni Co alloys. Hence, it clearly demonstrates that hcp crystal structure in Ni Co alloys contributed to the remarkable friction reduction effect and better anti-wear performance of Co-rich alloys. 4. Conclusions (1) The Co content in Ni Co alloys increased gradually with the increase of Co 2+ concentration in the electrolyte, which is confirmed by the anomalous codeposition of iron group metals. (2) Surface morphology of Ni Co alloys changed from regularly polyhedral crystallites into spherical cluster surface when increasing Co content from 7 to 49 wt.% and the morphology of the Ni Co alloys with 81 wt.% Co showed a rather regularly branched structure. Both the crystal structure and phase composition are mainly dependent on the Co content in alloys. The phase structure of Ni Co alloys gradually changed from fcc into hcp with the increase of Co content. (3) Microhardness of Ni Co alloys increased initially with Co content increasing from 0 to approximately 49 wt.%, and then gradually decreased as Co content increased further above 49 wt.%. The hardness change of Ni Co with grain size follows Hall Petch effect. (4) For the Ni-rich alloys, the improvement of wear resistance with hardness increase fits Archard s law. In addition, the Co-rich alloys exhibited much lower friction coefficient and higher wear resistance than Ni-rich alloys. It has been suggested that hcp crystal structure in Co-rich alloys contributed to the remarkable friction reduction effect and better anti-wear performance. Acknowledgements The authors gratefully acknowledge the National Natural Science Foundation of China (Grant No , and ), the 863 Program of China (No. 2003AA305670), and Top Hundred Talents Program of Chinese Academy of Sciences for financial support of this research work. References [1] A.N. Correia, S.A.S. Machado, Electrochim. Acta 45 (2000) [2] V.B. Singh, V.N. Singh, Plat. Surf. Finish. 7 (1976) 34. [3] D. Golodnitsky, Yu. Rosenberg, A. Ulus, Electrochim. Acta 47 (2002) [4] A. Bai, C.-C. Hu, Electrochim. Acta 47 (2002) [5] L. Burzynska, E. Rudnik, Hydrometallurgy 54 (2000) 133. [6] D. Golodnitsky, N.V. Gudin, G.A. Volyanuk, Plat. Surf. Finish. 85 (1998) 65. [7] H.R. Johnson, J.W. Dini, Plat. Surf. Finish. 70 (1983) 47. [8] E. Gómes, J. Ramirez, E. Vallés, J. Appl. Electrochem. 28 (1998) 71. [9] W.H. Safranek, Properties of Electrodeposited Metals and Alloys, New York London, [10] C.-C. Hu, Y.-S. Lee, T.-C. Wen, Mater. Chem. Phys. 48 (1997) 246. [11] A.N. Correia, S.A.S. Machado, L.A. Avaca, Electrochem. Commun. 1 (1996) 600. [12] E. Gómes, E. Valles, J. Appl. Electrochem. 29 (1999) 805. [13] S. Armyanov, Electrochim. Acta 45 (2000) [14] E. Gómes, E. Vallés, J. Appl. Electrochem. 32 (2002) 693. [15] D. Golodnitsky, N.V. Gudin, G.A. Volyanuk, J. Electrochem. Soc. 147 (2000) [16] N. Zech, E.J. Poklaha, D. Landolt, J. Electrochem. Soc. 146 (1999) [17] F.R. Morral, Met. Finish. 62 (1964) 82. [18] G. Hibbard, K.T. Aust, G. Palumbo, et al. Scripta Mater. 44 (2001) 513. [19] E. Arzt, Acta Mater. 46 (1998) [20] F. Dalla Torre, H. Van Swygenhoven, M. Victoria, Acta Mater. 50 (2002) [21] Kh. Saber, C.C. Koch, P.S. Fedkiw, Mater. Sci. Eng., A 341 (2003) 174. [22] J.F. Archard, J. Appl. Phys. 24 (1953) 981. [23] D.H. Jeong, F. Gonzalez, G. Palumbo, et al. Scripta Mater. 44 (2001) 493.