Wetting phenomena of Al–Cu alloys on sapphire below 800 °C

Using a modified dispensed drop method, a decrease in contact angle on sapphire from pure aluminum to low-copper-containing Al alloys (7–12 wt.%) was found; with higher copper additions θ transitions to the non-wetting regime. Atomic force microscopy on long-term samples showed a significantly increased surface roughness beneath the drop. Using high-resolution transmission electron microscopy, the reaction product at the interface was identified as CuAl2O4 for Al–7Cu and Al2O3 for an Al–99.99 drop. X-ray photoelectron spectroscopy further confirmed the formation of CuAl2O4 under CuAl2 drops. Spinel formation is caused by reaction of the alloy with residual oxygen in the furnace that is transported along the interface as modeled by thermodynamic simulations. The formation of CuAl2O4 causes the reduced σsl and hence the improved wettability of sapphire by low-copper-containing alloys compared to pure aluminum. The main reason for the increase in θ with higher copper contents is the increasing σlv of the alloy.