Abstract
The effect of ultrasonic agitation on the grain size, porosity, hardness, brightness and internal stress of copper deposits from an acid sulphate electrolyte was studied. The influence of ultrasound on cathodic polarization, limiting current density and current efficiency was also investigated. An acoustic plating tank was devised, and an acoustic calorimeter constructed. The calorimeter was used to measure the ultrasonic intensity in the electrolyte. The cavitation threshold intensity was determined with a piezoelectric probe. The properties of deposits from the ultrasonic bath were compared with those of deposits from still and stirred baths formed under identical plating conditions. Experimental techniques included optical, transmission and scanning electron microscopy and Spark Mass Spectrometric Analysis. Deposit grain size was measured by the linear intercept method and internal stress was determined with a Brenner Senderoff Spiral Contractometer. The porosity of deposits was calculated from their density. Ultrasonic agitation raised the limiting current density and, at high current densities, the current efficiency. Further, it increased deposit grain size, hardness, brightness and internal stress and decreased porosity. These changes are attributed to cavitation in the electrolyte. It is established that hardening results from a decrease in porosity and from work-hardening, and that increased brightness is due to cavitation erosion of the deposit surface. Work-hardening is caused by repeated impacts on the deposit which arise from cavitation collapse. Further, it is concluded that the decrease in porosity and increase in internal stress are due to the promotion of lateral deposit growth. The rise in limiting current density and increase in grain size result from a reduction in concentration polarization caused by stirring arising from cavitation. The industrial significance of ultrasonic agitation is discussed.