Untersuchung der Anisotropie im Benetzungsverhalten flüssiger Al-Cu Legierungen auf einkristallinen orientierten Al2O3-Substraten

Kurzfassung

The wetting behaviour of liquid Al-Cu alloys and pure metals on oriented single crystalline Al2O3-substrates was examined, utilising the sessile drop technique. For this purpose, a sessile drop apparatus was built, which permits independent heating of the substrate and the sample material. Droplets are not created until the desired measurement temperature is reached and are only then deposited on the substrate. Thus the initial conditions of the experiment are clean and as accurately defined as possible. Measurements were performed at moderate temperatures of 1100°C, where the alloys are liquid. There, since this system is considered to be inert, chemical reactions at the interface are negligible, at least on short timescales. Different Al2O3-surfaces were studied, which are terminated by the crystallographic planes (0001), (11-20), and (1-102), also called C-, A-, and R-surfaces. After deposition, pure Cu-droplets show an exponential increase of the wetting angle to a value of about 115° for all investigated Al2O3-surfaces. The timescale of this increase is of the order of 100 s. The effect of surface- and interfacial energies on the wetting angle is discussed considering Young’s equation. The most probable reason for its time-dependence seems to be an increase of the interfacial energy due to deoxidation of the droplet. Therefore it is reasonable to regard the isotropic contact angle value as the intrinsic one of the Cu/Al2O3 system. In contrast, the wetting angle of pure Al metal with the different Al2O3-substrates shows a qualitatively different behaviour. In this system, it rises from about 90° to 115° roughly for C-substrates, twice as fast as in the Cu case but to a comparable value. On the other substrates a wetting angle of about 90° establishes immediately, and no pronounced time dependence is obvious. This anisotropic behaviour of the wetting angle must be due to surface-specific processes, and thus might indicate a surface reconstruction of the C-plane caused by loss of oxygen or enrichment of Al on this surface. In order to study changes in the wetting behaviour of Al-Cu-alloys, which is isotropic for Cu and anisotropic for Al-rich alloys, contact angles of Al50Cu50, Al30Cu70 und Al17Cu83 on Al2O3 were determined. Qualitatively their wetting behaviour resembles that of pure Al; in wetting of A- and R-surfaces, equilibrium is often reached immediately, a time dependence only arises during wetting of C-surfaces. For each alloy composition the wetting angle is about 120° after 300 s. The initial values on distinct surfaces hardly differ and become non-wetting with increasing Cu-content. Hence, anisotropy decreases. To determine the work of adhesion of the solid-liquid interface, the temperature- and composition-dependence of the surface tension of Al-Cu-alloys was also measured. It was deduced from the surface oscillation frequencies of electromagnetically levitated samples employing the oscillating drop method. Here, one utilizes the nature of surface tension as restoring force for surface oscillations of a levitated droplet. For each alloy composition the temperature-dependence of the surface tension can be described by a linear decrease, which is most pronounced for alloys with lower Al-contents and smallest for medium ones. The experimentally determined composition dependence is well described by thermodynamic calculations based on the regular solution approximation. From the surface tension data and wetting angles the concentration-dependence of the work of adhesion within the Al-Cu/Al2O3-system at 1100°C was calculated. On reconstructed C-substrates it decreases, when Al-contents rises up to 50 at.% and remains constant for higher Al-percentages. On all other surfaces, including non-reconstructed C-surfaces, the work of adhesion seems to be independent of alloy composition, but a slight increase might also be identified, again occurring mainly at lower Al-concentrations. The anisotropy of the work of adhesion is about 50% for Al-rich alloys and therefore significant. The observed behaviour of the wetting angle and work of adhesion suggests segregation of Al at surface and interface of the droplet. Due to the strong interaction of Al with O, segregation is most pronounced at interfaces with O-rich Al2O3-surfaces, which are A-, R-, and also non-reconstructed C-planes.