Wetting of oriented sapphire surfaces by liquid Al-Cu Alloys

J. Schmitz1, I. Egry1 and J. Brillo1

1Institut für Materialphysik im Weltraum, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Köln, Germany

Keywords: wetting, contact angle
property: anisotropy
material: Al-Cu alloys, sapphire

The coexistence of a liquid with a solid and a gas causes a contact angle at the triple line of those phases and results in a work of adhesion. These properties were studied for liquid Al, Cu, and their alloys on differently oriented single crystalline sapphire surfaces, which were the C(0001)-, A(11-20)-, and R(1-102)-planes.

Measurements were performed at 1100°C and 300 mbar Ar atmosphere in a recently built sessile drop apparatus. In this machine the sample material is heated separately from the substrate within a drop dispenser, from which liquid metal is pressed out when the measurement conditions are reached.
For each sessile drop a digital CMOS-camera records a sequence of side view shadow images. From these images the time dependence of the contact angle is determined. Depending on the composition of the alloy the contact angle reaches a constant value within a certain time after the droplet touched the substrate.

In case of pure Cu the contact angle increases within about 200 seconds to a non-wetting equilibrium value, which is the same for each kind of wetted sapphire surface.For pure Al an anisotropy of the contact angle with regard to these surfaces is found: time evolution of the Al contact angle is only observed for wetting of C-surfaces. It is much faster than in the Cu case and equilibrium is already reached after an increase that takes about 50 seconds. Wetting of A- and R-surfaces shows no pronounced time dependence. In these cases, a smaller contact angle of about 90° is observed.Wetting of the different sapphire surfaces by Al-Cu alloys corresponds qualitatively to their wetting by pure Al: again, only for C-surfaces a time-dependent increase of the contact angle is observed. The time-independent wetting angle on A- and R-surfaces increases with Cu content of the alloy.

A relation with oxygen partial pressure, surface reconstructions of sapphire and adsorption of Al in the interfaces is discussed, also considering works of adhesion, which are determined by combining contact angle values with surface tension data measured separately in electromagnetic levitation.

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