Brush Plated Multilayered Cu/Ni Coating from a Single Electrolyte and its Fretting Wear Behaviors

Yaochuan Qian1,a, Jun Tan2,b, Hongjun Yang1,c, Zhongxiang Hu1,d 1National Key Laboratory for Remanufacturing, Academy of Armored Forces Engineering,

Beijing 100072, China 2PLA Key Laboratory of Surface Engineering, Beijing 100072, China

ay.qian05@gmail.com, btanjuncn@sina.com, cyanghongjun@sina.com, dhzxbx@yahoo.com.cn

Keywords: Brush Plating, Cu/Ni Multilayers, Electrodeposition, Fretting Wear.

Abstract. Brush plated of Cu/Ni multilayer coating from a single sulfate electrolyte was prepared. The influence of applied voltage on the composition of the deposit and deposition rate was investigated to optimize the plating parameters. Wear tests were carried out in ball-on-plate geometry unlubricated. Wear scar morphology was observed using scanning electron microscopy and optical microscopy. It was found that the brush plated multilayer coating was highly dense and free from porosity. The multilayer coating showed less wear than Ni coating brush plated from the same solution. The increase in the wear resistance of the multilayer coating was attributed to lubricating effect of the transform film formed by Cu wear particles.

Introduction

In engineering applications, failure often starts from free surface of materials because of wear, fatigue, corrosion, erosion and so on. Durability against such damages could be improved by various coating techniques. Metallic multilayer coatings have aroused interest in last two decades [1]. With abundant interlayer interfaces and sub-layer thickness near atomic dimension, these coatings often show improved magnetic, electrical and, in some cases, mechanical properties. Cu/Ni multilayers has long been known due to enhanced hardness, strength [2] and wear resistance [3-5] compared to its corresponding pure metals. However, previous work was most focused on electrodeposition of such coatings using potentiostatic/galvanostatic methods rather than from industry-used devices. On the other hand, the brush plating technique has been used extensively for repairing, strengthening and resizing purposes [6]. Although it showed great flexibility to acquire coatings with nano structures [7,8], unfortunately, little work has been conducted on preparing coatings with a multilayered structure using brush plating.

In this paper, the authors aimed to brush plate Cu/Ni multilayer coatings on a common used 45% carbon steel substrate from a single sulfate solution. This was achieved by applying a dual-pulse voltage. Microstructure and wear resistance of the multilayer coating were studied.

Experimental

The multilayered coatings were brush plated from a single solution having the composition as follows: NiSO4·6H2O 254 g·l-1, CuSO4·5H2O 2.5 g·l-1, complexing agent 56 g·l-1, aqua ammonia 150 ml·l-1, and brighteners 23 g·l-1. The substrates used for the deposition were 0.45% carbon steel, machined into flat pieces, polished to an average surface roughness ~0.03 µm. To get the adherent coating, a three-step pretreatment, including electrical cleaning, electro etching and Cu pre-coating, was adapted prior to the brush plating process. The element content of the deposits and deposition rate at different applied voltage were examined to determine the parameters for dual-voltage pulse waveform [1], which is used to deposit Cu and Ni sub-layers alternatively. The voltage waveform was given by the DGN-3 rectifier (AAFE, China). Samples for cross-sectional observation were cut and polished, then dipped in aqua ammonia to etch Cu selectively. The individual layer thickness was

Advanced Materials Research Vols. 97-101 (2010) pp 1467-1470Online available since 2010/Mar/02 at www.scientific.net© (2010) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.97-101.1467

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measured using scanning electron microscopy (SEM) and the composition in the deposit was characterized with an energy dispersive X-ray spectrometer (EDS). The brush plated area was fixed to ~5 cm2. The movement of the anode was controlled carefully to make sure that the whole surface to be plated in good contact with the anode all the time. Deposition rates at different voltages were calculated to decide the current duration needed to fabricate coatings with various layer periodicities.

Fretting test of the Cu/Ni multilayer coating was conducted under dry, unlubricated condition using a ball-on-plate reciprocating wear and friction machine (UMT-2MT, USA). Pure Ni coating brush plated from the same solution was used for a comparison. The total thickness of the coatings was kept at 20 µm. Steel balls (40 mm in diameter), made of GCr15 with a hardness of 63 HRC were used as counter body. Sliding experiments were carried out at 5 N load with 30 Hz frequency keeping amplitude at 50 µm and sliding speed 3 mm·s−1. Coefficient of friction (COF) was recorded simultaneously. Wear-scar morphology was investigated under SEM and optical microscope. The wear volume was calculated based on the measurement of the wear track area. All experiments were conducted under laboratory atmosphere (room temperature: 25 °C, the relative humidity: 50%–70%). Three replicating tests were done for both multilayer coating and pure Ni.

Results and Discussion

The effect of applied voltage on the element content and the deposition rate of the deposits are shown in Fig. 1. Solid curves show the effect of applied voltage on the element content in the deposit. In a typical brush plating system, a constant-voltage control [7] is more adapted rather than constant-current method. Generally, with stable solution supply and anode movement, resistance of the two-electrode brush plating system can be regard as a constant. Thus, the dual-voltage pulse was induced instead of current/potential pulse. In our experiments, no deposit was found at voltage lower than 1.4 V. When applied voltage from 1.4 V to 1.8 V, the deposit was mostly Cu, with Ni less than 5%. As a less noble metal, the deposition of Ni was restricted due to the low current density. However, the content of Ni grew dramatically with the increasing voltage, reaching 81.8% at 2.8 V. As the voltage continue to increase, the growing rate of Ni content slowed down, to 89.4% at 4.0 V.

Fig. 1. Effect of applied voltage on the element content and deposition rate of the deposits. Vertical dash lines show the voltage for Cu (1.8 V) and Ni (2.8 V) sub-layer deposition, respectively.

Fig. 2. Cross-sectional SEM view of the brush plated Cu/Ni multilayer coating with layer periodicity of 160 nm.

The effect of applied voltage on the deposition rate is shown by the dash curve in Fig. 1. At voltage lower than 1.8 V, the deposition rate was very low. The deposition of Cu, from Cu2+ which is in low concentration, dominated the process. With higher voltage applied, the deposition of Ni took part in the process and the deposition rate started to increase significantly, to a value of 16.6 nm·s-1 at 4.0 V. To produce sub-layers with fine purity, a low voltage for Cu deposition and a high voltage for Ni deposition are preferred. However, too low a voltage causes low deposition rate while high voltage tends to bring coarse grains in our experiments. Thus, an optimized applied voltage used for Ni and Cu sub-layer deposition of 2.8 V and 1.8 V, respectively, was adapted. Based on the deposition rate at

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both the voltage, by precisely controlling of the pulse duration for deposition of Cu and Ni, multilayer coatings with various periodicities and Cu/Ni ratios can be produced. In present work, only coatings with Cu/Ni thickness ratio = 1:1 were discussed.

A cross-sectional SEM view of brush plated coating with a layer periodicity of 160 nm is shown in Fig. 2. Dark bands represent the etched Cu sub-layers while the bright ones are the unattacked Ni sub-layers. It is obvious that all sub-layers are uniform in thickness, highly dense and free from porosity.

Fig. 3 shows the coefficient of friction (COF) vs. the sliding time for Cu/Ni multilayer coating with a layer periodicity of 80 nm and the pure Ni coating, at 5 N load and 30 Hz frequency. After establishing the initial sliding amplitude, for both coatings the COF increases and stabilized at a steady state value. During the stabilized phase no significant difference was found between the two coatings. However, for pure Ni, before entering the stabilized phase, the COF increased to a peak value of ~0.5 (4 min), followed by a decrease to the stabilized value of ~0.4 (7 min). Higher COF existed for pure Ni at the earlier phase.

Fig. 3. Coefficient of friction of pure Ni coating and multilayer coating with a layer periodicity of 80 nm versus sliding time, at 5 N load and 30 Hz.

Fig. 4. Wear scar morphology of (a) Ni coating and (b) the Cu/Ni multilayer with a periodicity of 80 nm, at 5 N load and 30 Hz. The inset figure is a part of the whole scar morphology.

Optical micrographs of wear scar are shown in Fig. 4. The inset is a typical magnified SEM morphology of part of the scar. Abrasive wear marks are more dominant in pure Ni rather than the multilayer coating. In case of the latter, lower COF was observed at the increasing phase. Copper being a softer and ductile material, initially asperities of steel ball penetrate the Cu surface and leads to ploughing action, creating lots of Cu wear particles. These particles then fill the asperities and valleys of the steel ball, forming a transform film. This leads to smoothing action during further wear process. However, for pure nickel, the effect is much different because of its higher hardness and lower ductility. The abrasive interaction causes generation of worn out particles of larger size from both surfaces. These leads to work-hardened wear particles along with ball asperities cause ploughing action in the coating resulting in higher COF and lower wear resistance at the earlier phase. Along with the work-hardening effect on the worn scar, it became harder for wear particles to penetrate in the coatings. Some particles with larger sizes were driven to the edge of scar while others, underwent further grinding by rolling between the two surfaces, and thus reduced the COF to a value similar with that of the multilayer coating. Wear scar was measured using optical microscope. We assume the wear scar in the coatings have a regular profile. Wear volume can thus be calculated with formula:

]41

21

)2arcsin([ 222 dRdRdRLV −−=∆ . (1)

Advanced Materials Research Vols. 97-101 1469

where L is the length of the wear track, d the width of the wear track, R the radius of the ball. Wear volume of the scar was used to calculate wear rate using following formula [9],

Wear rate (K) = Wear Volume in mm3 / (Load in N × Sliding Distance in m). (2)

The calculated wear rate for Cu/Ni multilayer coating is 5.8×10-9 mm3·N-1·m-1 while for pure Ni is 1.0×10-8 mm3·N-1·m-1. Multilayers showed better fretting resistance as compared to pure Ni. We also noticed that both calculated wear rates are much lower than those reported by previous work [4,5]. This could be attributed to the higher frequency and much lower amplitude in our experiment conditions for a fretting wear test.

Conclusions

By controlling the waveforms of applied voltage, Cu/Ni multilayer coatings with periodicities in nano-scale were brush plated from a single solution. The brush plated multilayer coating was highly dense and free from porosity. The coating with a periodicity of 80 nm showed better fretting wear resistance than pure Ni coating brush plated from the same solution. This was explained in terms of lower COF at the earlier phase induced by lubricating effect of Cu.

Acknowledgements

The support of the research fund by National Key Laboratory for Remanufacturing of China (No.9140C8503010604) is gratefully acknowledged.

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Manufacturing Science and Engineering I 10.4028/www.scientific.net/AMR.97-101 Brush Plated Multilayered Cu/Ni Coating from a Single Electrolyte and its Fretting Wear Behaviors 10.4028/www.scientific.net/AMR.97-101.1467

DOI References

[3] W. Zhang, Q. Xue: Thin Solid Films Vol. 305 (1997), p. 292

doi:10.1016/S0040-6090(97)00164-8 [5] S.K. Ghosh, P.K. Limaye, S. Bhattacharya, et al: Surf. Coat. Technol. Vol. 201 (2007), p. 7441

doi:10.1016/j.surfcoat.2007.02.014 [6] B. Subramanian, S. Mohan, Sobha Jayakrishnan, et al: Curr. Appl. Phys. Vol. 7 (2007), p. 305

doi:10.1007/s10800-006-9236-6