Volume||5||Issue||12||December-2017||Pages-7749-7753||ISSN(e):2321-7545 Website: http://ijsae.in
Index Copernicus Value- 56.65 DOI: http://dx.doi.org/10.18535/ijsre/v5i12.01
Enhancement in surface morphology by Pulsed electro deposition of pure Silver on Al
alloy
Authors
Vaishali Vaghela1, Dr. N.K. Shah2, V.V. Limaye3 1,3
Space Applicationscentre, (ISRO), Ahmedabad- 380015, India 1,2
Department of Chemistry, Gujarat University, Ahmedabad- 380015, India [email protected]
ABSTRACT
Pulsed electro deposition of pure silver is mainly focused at the enhancement of surface morphology by improving some surface properties like grain size, hardness, anti-tarnishing, porosity, and brightness. The pure silver has been deposited on aluminium alloy from cyanide bath having alkaline pH by applying pulse duty cycles ranging from 9 to 90%, having frequency of 10 Hz to 1000 Hz with peak current densities from 4 ASF to 1.2 ASF. In the present study the impact of pulse duty cycle, peak current density and pulse frequency on the thickness, hardness of silver deposit, and current efficiency of the plating process has been investigated. Enhancement in surface morphology that is good quality deposits (lower porosity, finer grains and increased brightness) has been obtained at lower peak current density, lower duty cycle and lower frequency. Characterization is done by Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM).
Keywords: Pulse electrodeposition, Pure Silver, Morphology, Aluminum alloy
INTRODUCTION
It is well known that direct current (DC) current plating is traditional method of electroplating. This can be modified by pulse current (PC) or pulsed reverse current (PRC) electrodeposition by current interruption as well as current reversal. There is only one variable current in DC plating [1-3]. Porosity and rough deposits are inadequacy of coatings formed by DC plating. On the other side, in PC and PRC plating more variables exist for example pulse duty cycle, pulse duration and peak current density. Various deposit properties such as porosity, ductility, hardness, electrical conductivity, plating thickness distribution, finer grain size and anti-tarnishing property can be improved by PRC plating. Reverse pulse current electrodeposition of silver is targeted for improvement in various properties listed above [4]. Electrodeposition of precious metal by using PRC is preferable so as to obtain the deposits of higher quality and secondly because of huge saving in the consumption of raw material which compensate for higher cost of the equipment [5-7]. A systematic study of silver PRC electrodeposition from an alkaline cyanide bath has been undertaken in the present study. Mainly square wave pulse current is applied and its influences on hardness of silver deposit, thickness as well as current efficiency of the plating process have been studied.
EXPERIMENTAL
cyanide 50 g/l and the proprietary brightener (make: Growel) are used [9–11]. The anode employed is stainless steel 304. Pulse duty cycles of various frequencies ranging from 10 Hz to 1000 Hz with peak current densities ranging from 4 ASF to 1.2 ASF are employed. The influences of pulse duty cycle, peak current density and pulse frequency on the thickness, hardness of silver deposit and current efficiency of the plating process are studied. The experiments are carried out with a constant time of 20 minute. Reverse current density is almost double than forward current density in all the cases [12-13]. Total 50 number of specimens were prepared for this study with wide ranging parameters [14-17].
3. RESULTS AND DISCUSSION 3.1 Effect of pulse duty cycle
3.1.1 Influence on thickness of the silver deposits obtained at various frequencies
As the pulse duty cycle increases, thickness of the silver deposit decreases. As the duty cycle increases, current on-time increases and off-time decreases. At a lower duty cycle, the peak current is flowing for less time and hence the overall amount of deposition is lesser than that obtained at higher duty cycle. At very high duty cycles and at high frequencies the pulse current is very low. Therefore, a reduced thickness is obtained. As frequency increases, thickness of silver deposit reduces.
Fig.1. (a) Effect of pulse duty cycle on thickness
Fig.1. (b) Effect of frequency on thickness 3.1.2 Influence on current efficiency
It has been found that current efficiency of silver plating increases slightly as pulse duty cycle increases. Maximum current efficiency has been observed at 10% duty cycle and frequency of 55.56 Hz. The current efficiency is lower at higher frequencies.
3.1.3 Influence on hardness
It is observed that as pulse frequency increases the hardness is also found to increase. During short pulses at higher frequency, a very thin pulsating diffusion layer has been formed leading to an enhanced nucleation rate and surface coverage with denser building up of fine grained deposits [4]. This leads to lower porosity and correspondingly higher hardness values. The hardness of the silver deposit has been found to increase with increase in duty cycle. A lowest hardness value is obtained at 9 % duty cycle and 82.64 Hz frequency. At lower pulse duty cycle, a higher peak current is passed and this produces a powdery or burnt deposit with
0 1 2 3 4 5 6 7 8
0 10 20 30 40 50 60 70 80 90 100
Th ic kn es s µ
Pulse Duty cycle %
0 1 2 3 4 5 6 7 8
0 50 100 150
poor adhesion and considerable porosity. This porosity leads to a decrease in hardness of the deposit. However, at higher duty cycles, the peak current is lower almost nearing the optimum average current resulting in the formation of a smooth fine grained deposit. Improved surface coverage with denser build-up of grains is to be expected. In case of 10 % duty cycle and 55.56 Hz frequency, hardness value is 106 Hv.
3.2 Effect of pulse frequency 3.2.1 Influence on thickness
As frequency increases, the thickness of silver deposit decreases in general. Desired thickness of 6 µ is obtained at a frequency of 55.56 Hz and a duty cycle of 10%. As frequency increases, the on-time decreases and so thickness of the deposit also decreases.
3.2.2 Influence on current efficiency
As frequency increases, the current efficiency almost decreases. In most cases, the maximum current efficiency is obtained at 55.56 % duty cycle. This may be due to the effect that as frequency increases, the on-time decreases and also during short pulses a very thin pulsating diffusion layer is formed which makes the transport and diffusion of metal ions from bulk electrolyte to the cathode surface difficult.
3.2.3 Influence on hardness
It is observed that as pulse frequency increases the hardness also increases. At higher frequency during short pulses, a very thin pulsating diffusion layer has been formed leading to an enhanced nucleation rate and surface coverage with denser building up of fine grained deposits [12]. This leads to lower porosity and correspondingly higher hardness values. Maximum hardness is obtained at a frequency of 98 Hz for the current density of 4 ASF.
3.3 Effect of average current density
For an average current density of 4 ASF, the current efficiency and thickness of silver deposit is maximum at 33% duty cycle and 167 Hz frequency. As average current density increases, thickness of the silver deposit also increases. For an average current density of 1.2 ASF and 21% duty cycle, the sample has a burnt appearance. Good quality deposits are mostly obtained at an average current density of 4 ASF.
3.4 Effect of peak current density
As duty cycle increases, the peak current density decreases. Current efficiency is maximum for a peak current density of 4 ASFfor 10 % duty cycle at 55.56 Hz frequency. Higher peak current density especially at lower duty cycle produces burnt deposits with poor adhesion and considerable porosity resulting in lower hardness [13]. The highest peak current density beyond which burnt deposits formed is 1.2 ASF.
3.5 Scanning electron microscope (SEM) analysis
The surface topography and perviousness of the deposit shaped by various current are evaluated by SEM analysis. On analysing the three microstructures revealed in Fig. 2(a), (b), and (c), it is shown that the deposit formed by PRC at 10 % duty cycle and frequency of 55.56 Hz is having fewer holes in comparison with other parameters. Also the grain size formed at higher duty cycle and lower frequency is bigger than that formed at lower duty cycle and higher frequency. By using PRC, particle size can be reduced from 1.5 µ (in case of DC) to 40 nm. SEM micrograph of various silver plated specimens are shown in Fig.2.
Fig.2. (b) 12% duty cycle and 91 Hz frequency, with PC, at 500 nm scale
Fig.2. (c) 10% duty cycle and 55.56 Hz frequency, with PRC, at 200 nm scale
3.6 Morphology analysis
The morphology of the deposit fashioned by various current are reflected by Atomic force microscope. On analysing the three morphologies revealed in Fig. 3(a), (b), and (c), it is established that the deposit fashioned by DC current has surface roughness: 900 nm, Particle size: 1300-2500 nm, with PC current has surface roughness: 200 nm, Particle size: 200-500 nm, with PRC current it has surface roughness: 10 nm, Particle size: 30-40 nm .
Fig.3. (a) with DC Fig.3. (b) With PC Fig.3. (c) With PRC
3.7 Enhancement of properties
Silver metal is noted for its extremely high electrical conductivity, 0.0162 Ω-cm2 m-1. It is somewhat unique in that even in its thin-film (4-6 µ) form it exhibits nearly the same property, 0.017–0.024 Ω-cm2 m-1. It has been observed that the plating thickness is not uniform in DC plating. The experimental difference in thickness of the deposit is lower for pulse plating compared to that of DC plating. The current efficiency is comparatively higher (95%) for pulse plating. This PRC plating is providing superior tarnish resistance than DC plating
4. CONCLUSION
Table 1: Parameters for Reverse pulse plating of silver
Forward Reverse
On time ms Off time ms Duration ms On time ms Off time ms Duration ms
1 9 120 0.8 7.2 8
Acknowledgements
The authors wish to recognize the technical support provided by the Space applications centre, (ISRO), Ahmedabad and Gujarat University for the research work.
REFERENCES
1. Orr MA. Electroplating. In: Butts Allison, Coxe Charles D, editors. Silver economics, metallurgy and use. Florida: Robert Krieger Publishing Company; 1982. p. 180–9.
2. Devaraj G, Guruviah S, Seshadri SK. Mater Chem Phys 1990;25:439–61. 3. Roos JR, Celis JP, Buelens C, Goris D. Proc Metall 1984;3:177.
4. C. Shanthi, S. Barathan, Rajasrisen Jaiswal. The effect of pulse parameters in electro deposition of silver alloy, doi:10.1016/j.matlet.2008.08.032, Materials Letters 62 (2008) 4519–4521.
5. Randin Jean Paul, Asulab SA. Surf Coat Technol 1988;34:253–75.
6. Knodler A. Pulsed electrodeposition of precious metals. In: Puippe Jean-Claude, Leaman Frank, editors. Theory and practice of pulse plating. Florida: American electroplaters and surface finishers society; 1986. p. 119–74.
7. Kanani Nasser. Electroplating basic principles: processes and practice. Berlin, Germany: Elsevier Ltd; 2004.
8. Sarkar S. Silver the science and technology. Calcutta: Calcutta Book house; 1971.
9. Duva Robert. Pulse plating. In: Durney Lawrence J, editor. Electroplating engineering handbook. New Delhi: CBS publishers and distributors; 2000. p. 684–9.
10.Canning W. The Canning handbook surface finishing technology. 23rd ed. New Delhi: CBS publishers and distributors; 2005.
11.Frederick A. Lowenheim. Modern electroplating. 2nd ed. New York: John Wiley & Sons; 1966.
12. Siri. Modern technology of electroplating anodizing and other surface treatments. 13.Delhi Small Industry Research Institute; 2006.
14.Mohan S, Raj V. Trans IMF 2005;83:194–8.
15.Mordechay Schlesinger , Milan Paunovic. Modern Electroplating. 5thed. Wiley Publication; 2014. Nathan Paul. Handbook of Electroplating. Clanrye international; 2015.
16.Anmin Liu, Xuefeng Ren, Maozhong An, et all. A Combined Theoretical and Experimental Study for Silver Electroplating, SCIENTIFIC REPORTS | 4 : 3837 | DOI: 10.1038/srep03837,
www.nature.com/scientificreports; 2014.
