Ultrasonic Modification
of Aluminum Surfaces: Comparison
between Thermal and Ultrasonic Effects.
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Abstract
Ultrasound has become an increasingly popular tool in
the modification
of metal surfaces, imbuing them with various desired characteristics
and functionalities. The exact role played by ultrasound in such processes
remains largely speculative and thus requires clarification. In this
study, aluminum was taken as a model metal to probe the nature of
the surface modification, focusing on both chemical and physical changes.
Using metal plates as substrates, the formation of a characteristic
porous surface structure was ascertained to arise from a purely thermal
mechanism, with the ultrasound providing an inhibitory influence when
compared with controlled experiments matching the thermal conditions
of sonication. No beneficial effect was observed through sonication,
with regards to surface texture, porosity, and electrochemistry. However,
for metal powders, a pronounced change in the phase composition was
observed following ultrasonic exposure, largely attributed to the
growth of bayerite from the surface. The immobilization of the powder
on a thin epoxy film nullified such effects. This suggests that the
changes in phase composition are due to the effect of ultrasound-induced mechanical stirring and high speed particle motion on the dissolution and reprecipitation of the metal oxide and hydrated oxide species. This work is of significant value to researchers both in materials
science and in sonochemistry