Use of Ultrasound to Modified Electrodeposited Silver and Silver-Tin Microstructures and Composition without Chemical Additives
Authors: Nevers Aymeric , Hallez Loic , Touyeras Francis , Hihn Jean-Yves 
Published: February 2020

Properties of electrodeposited coatings are directly related to their morphology and structure, which are strongly influenced, among others, by the presence of organic additives and electrodeposition parameters (current density, DC vs pulsed current). An attractive possibility results in the use of ultrasound to influence the microstructure without using chemicals, or at least, by reducing their quantity. Ultrasound is expected to have several effects on deposits, by heating the liquid or by generating solution movements that cause convection flows or cavitation bubbles, accompanied by chemical effects. Many works in the literature describe the beneficial effect of ultrasound in plating, reporting finer grains, reduced porosity and increased brightness. Even the penetration of nickel in complex geometric pattern has been reported. In the present study, conventional cyanide silver as well as silver-tin electrolytes were used to examine the effect of current density and ultrasound on the composition, structure and texture of the coatings. Silver-tin alloys were used for such materials as electric contacts because of the excellence in hardness and wear resistance. The goal was to determine the effect of ultrasound (20 kHz and 575 kHz) on sample microstructure and morphology. To insulate the effect of cavitation bubbles from convection flow, all coatings had to be elaborated exactly with the same agitation in absence or presence of ultrasound, at an “equivalent flow” condition. Plotting the silver structures identified by XRD vs. current density in still conditions, two main categories appeared: one poorly structured and the other following the [110] orientation. This is confirmed by SEM and EBSD measurements. When changing from still to mechanically stirred conditions, the value of the current density threshold moves from 2.5mA/cm² to 5mA/cm². This coating microstructure modification threshold is situated at higher current density values when coatings are produced under sonication (575 kHz or 20 kHz), while agitation is kept constant. In both cases, the shift is about 15 mA/cm². It is interesting to note that silver electrodeposits elaborated under 20 kHz ultrasound conditions appear to be less oriented than those obtained under high frequency conditions, with RTChklvalues dropping from 12 to 8 for the [110] orientation. Use of ultrasound during silver coating elaboration, irrespective of hydrodynamic conditions kept otherwise constant, makes it possible to modify significantly the microstructures and morphology of deposits without relying on chemical additives. In the case of silver-tin alloy electrodeposition, it is important to note that the current density variations coupled to the use of ultrasound leads to significant variations in properties of coatings (color, crystalline composition and alloy composition). For equivalent hydrodynamic agitation, silver is predominant at low current density and tin prevails at higher ones. If low frequency ultrasound is applied, Ag3Sn replace the tin, and for high frequency ultrasound, the coating is formed by an intermetallic compound of tin and silver irrespective of the current density. Finally, in all cases, agitation is always beneficial to widen the current density range favourable for structural modifications without the risk of coating deterioration due to burning. Moreover, at the same level of agitation, use of ultrasound offers more advantages because it ensures harder and less structured deposits in increased current density ranges, and offer modifications equally important as in the case of chemical additives use.

Figure 1: crystalline orientation of silver deposits at various current densities in presence of ultrasound or at an “equivalent” agitation

Figure 2:
composition of silver-tin alloy in presence of ultrasound or at an “equivalent” agitation

Figure 1