New explanation for the brightness of electrodeposits produced by ultrasound R. WALKER and C. T. WALKER

The literature published on the brightness of metals electrodeposited in ultrasonically- agitated solutions is discussed together with the two main theories which explain the formation of bright deposits. The effect of ultrasound on the surface of the deposits is shown in a series of photographs. The increased brightening effect of ultrasound is explained by the production of shock waves followed by cavitation erosion and the removal of lateral growth of the deposit on the cathode surface.

The use of ultrasound in a plating bath has been found to improve surface appearance and is particularly beneficial in the production of good quality, bright electrodeposited coatings.‘-4 Ultrasound has decreased whisker growth, consequently giving smoother surfaces than produced con- ventionally and this application has been patented by Jafri.!j This form of agitation has been found to give smoother and/or brighter coatings of many metals including cadmium,6-9 chromium,lO$ r1 cobalt,4, l2 copper,2j 8, 12-r5y l6 gold,87 l7 iron ,4 lead,12 nickel,43 8,107 12,18,19 silver 8,9,16

tin,12 and zinc.8,20-24 For example, the reflectivity of nickel deposits has been found to increase by 300% in one instance 4 and in another lo the specular reflectivity changed from 28% to 51%. Kozan,25 however, found that nickel plated from the Watts bath became coarser and con- sequently duller with the application of ultrasound.

The brightness of a deposit often decreases as the current density is raised above a certain value called the critical current density. Ultrasonic agitation has been observed to considerably increase the critical current density and, for example, good quality deposits have been produced from the sulphate bath at a current density of 14 x lo3 A rne2, whereas spongey deposits are formed under ordinary con- ditions.26 Similarly, ultrasound (frequency f= 30 kHz, intensity I= 3 x lo3 W m”) increased the current den- sity by 300-400% at which bright nickel and copper deposits could be produced from the acid sulphate bath.27 An even greater effect was observed with copper 27 from the pyrophosphate bath, and good quality deposits were formed with ultrasound at a current density of 500 A ms2, whereas using conventional conditions the surface appear- ance deteriorated above 60 A rne2. Roll 1 9 found that the brightest coatings from a still nickel bath were produced at a current density of 27 A rnq2 and that ultrasound u= 34 kHz, I = 3 x 1 O3 W mm2) gave the same degree of bright-

Dr R. Walker is a lecturer in the Department of Metallurgy and Materials Technology, University of Surrey, Guildford, Surrey, UK, and Dr C. T. Walker is at the Kommission der Europaschen Gemeinschaften (Euratom), 75 Karlsruhe, West Germany. Paper received 4 July 1974.

ULTRASONICS. MARCH 1975

ness at 400 A mm2. The reflectivity of gold l 7 falls from 96% to 73% as the current density is raised from 20 to 100 A mm2 in the conventional bath but the application of ultrasound cf= 20 kHz, I = 3 x lo3 W mw2) maintained the reflectivity above 90% at current densities up to 180 A rnq2.

An increase in the intensity of ultrasonic agitation has been found to raise both the current density at which bright deposits are formed and also the degree of brightening. A considerable deterioration in the quality of silver deposits, however, has been noted 9 at high intensities cf= 20 kHz, I= 5 x lo4 Wmm2).

Haase 28 has discussed the two main theories that have been proposed to explain the formation of bright electro- deposits. These propose that the production of a bright deposit is because either the grain size is less than the wavelength of light (the fine-grain theory) or because a high degree of preferred orientation exists with the crystal faces being parallel in the deposit (the texture theory). The increase in brightness of electrodeposits obtained with ultrasonic agitation has been associated with the production of deposits with a fine grain size and higher grain-packing density.29

Experimental

The copper plating solution consisted of distilled water and commercial grade salts with a composition of 125 g 1-l CuSO4 .SHaO and 49 g 1-l Ha S04. The anode was analar grade copper foil and the cathode was thick copper sheet. Both electrodes were degreased in acetone, pickled in 50% nitric acid, washed in distilled water and then dried. Three plating cells, each containing 1 litre of plating solution were connected in series and one was ultrasonically agitated u= 20 kHz, I= 3.4 x lo3 W ms2), one magnetically stirred and one was still. A current equivalent to 500 A rns2 was passed for 1.5 minutes with the solutions at room tempera- ture (20°C + 1°C). The plated specimens were then removed from the cells, washed, air dried and photographed on the Stereoscan. This plating procedure was repeated

79

Fig.1 Effect of agitation on the production ofgowdwy deposits. Current density 500 A mm2, plating time 15 minutes: a -stirred; b - still; c - ultrasonic agitation (f = 20 kHz, I = 3.4 x 10 W m )

with a current density of 300 A mm2 and a plating time of 2 hours using different ultrasonic agitation v= 13 kHz, I = 928 W me*).

Copper was also electrodeposited from 4 litres of the acid bath subjected to ultrasonic agitation v= 13 kHz, I = 928 W mm*) using a current density of 200 A rnsz to give a deposit thickness of 7.8 x 10v5 m. The solution was then filtered under vacuum with a multipore filter of size 0.22 I.tm and a typical particle was examined using the JEOL JX 50 A micro-probe analysis equipment. The surface of the deposited copper was examined with a Stereoscan.

Results

Copper deposited from the still, stirred and ultrasonically- agitated solutions under different plating conditions is shown in Figs 1 and 2. The surface of the deposit produced in the ultrasonically-agitated bath and shown in Fig.3 exhibits severe cavitation erosion. Fig.4 shows a large particle and several small particles of copper removed by ultrasound from the deposit in Fig.3 together with an analysis of the copper content.

Discussion

The deposits produced at a current density of 500 A In-* from the still and stirred baths were powdery and of poor quality, whereas that from the ultrasonic bath was less pow- dery and brighter in appearance. It is apparent from Fig.1 that ultrasound can reduce the tendency to form powdery or burnt deposits at high current densities. At a lower current density (300 A mm*) and with ultrasound of a

different frequency and intensity an improvement is again observed in Fig.2. The figure shows that the deposits from the still and stirred baths are crystalline and have a matt appearance whereas that from the ultrasonically-agitated bath is much smoother and brighter.

The effects of ultrasound on the surface of a deposit can be seen in Fig.3. Severe cavitation erosion has occurred and this may account for the work hardening mechanism which we have proposed and discussed elsewhere.30 Several particles removed from the surface of the deposit shown in Fig.3 are shown in Fig.4a. An analysis of the copper is these particles is indicated in the trace shown in Fig.4a and the Cu Ko distribution is given in Fig.4b in which the lighter areas are copper rich.

In the present work ultrasonic agitation has sometimes pro- duced deposits with a finer grain size but there has been no evidence of preferred orientation. Thus the texture theory to account for brightening cannot explain the increased brightening observed with ultrasound. Furthermore the change in grain size, which is not always a decrease, cannot account for the brighter surface observed in all the experi- ments.

The results of earlier work 3o have shown that during immersion of an annealed metal in water subjected to ultrasonic agitation the metal surface became covered with indentation caused by cavitation. This cavitation also pro- duced surface hardening to a considerable degree, as indi- cated by the fact that the value for copper 31 increased from 47 to 117 HV while that for nickel 31 changed from 110 to 225 HV. Cavitation pits were also observed on

Fig.2 Effect of agitation on the surface topography of deposits. Current density 300 A mb2, c - ultrasonic agitation (f = 928 W mw2)

plating time 2 hours: a - stirred; b - still;

80 ULTRASONICS. MARCH 1975

Fig.3 Severe cavitation erosion on the surface of a deposit formed in the ultrasonic bath at a current density of 200 A mm2 (I = 928 W m

copper electrodeposits 3o which increased in hardness from 95 to 120 HV.

The fact that in the present work the ultrasonic agitation of the plating bath removed particles of the deposit and increased the brightness of the resulting coating suggests a new theory to account for the brightening action. If the growing deposit is subjected to shock waves resulting from the implosion of cavities at the metal/electrolyte interface it is probable that any whiskers that would normally form in a still bath would by physically removed from the surface by the action of these local forces. This mechanism can explain the observation of Jafri s that ultrasound decreases whisker growth. Similarly, ultrasound may be considered to remove or inhibit growth of any local areas on the grow- ing deposit surface that protude into the electrolyte, so encouraging lateral or two-dimensional rather than three- dimensional growth. This would result in a smoother or more uniform surface which would appear to be brighter.

The removal of particles of metal from a growing surface may result in the production of a deposit with a smaller grain size than that formed in baths without ultrasound. A smaller deposit grain size from ultrasonically-agitated solution has indeed been widely observed l and the

reduction should depend upon the frequency and intensity of the ultrasound and the strength and growth mechanisms of the metal being deposited. There are, however, a few instances in which a larger grain size has been produced 2s with ultrasound. For example, Ginberg 32 found that the gain size of nickel deposits could decrease or increase depending upon the current density used.

The removal of particles of deposited copper from the cathode during plating would be expected to give a reduction in the cathodic current efficiency. Very little difference, however, was observed in this work on copper electro- deposited from the baths subjected to stirring or ultrasound. This absence of significant change may be explained if the particles removed from the surface are trapped on the depositing surface or if they are too light to give a significant change in the weight of metal deposited. The fatter explanation is considered to be more probable.

Conclusions

The application of ultrasonic agitation to the plating bath increases the surface smoothness and brightness of the electrodeposited metal. This improvement in brightness is considered to be due to the action of ultrasound producing shock waves and cavitation erosion on the growing surface of the deposit. Growth perpendicular to the surface is

‘2)

Fig. 4: a - Particle of crystalline copper removed by cavitation from the deposit shown in Fig.3. Superimposed is a distribution trace for copper along the indicated line; b - as in (a) but showing Cu K, distribution

ULTRASONICS. MARCH 1975 81

inhibited and this results in a smoother finish on the deposit. The reduction in grain size often observed with ultrasound is not considered to be a satisfactory theory to account for the increased brightness observed in all deposits.

Acknowledgements

The authors wish to thank Professor M. B. Waldron for his interest and for the provision of research facilities at the University of Surrey and to Mr A. Grange of Ultrasonics Ltd, Shipley, Yorkshire, for his interest and for supplying the ultrasonic equipment used in this investigation.

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