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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

4.2. Contact angle 4.2.1

4.2. Contact angle 4.2.1 Influence of heating method on droplet formation Various heating methods for the metal were examined in order to obtain an optimized droplet shape and minimum surface oxidation. First the metal sample was heated directly on the substrate as presented in Figure 4.4 a) but the surface of the metal oxidized during heating to the testing temperature and a skull of oxide prevented the formation of a droplet. With the AlMg9 alloy, which is referred to as IM in the following, and by using a method which consisted of heating the metal in a tube above the substrate and dropping the molten metal onto the substrate, a droplet with a shiny surface was obtained as shown in Figure 4.4 b). Prior to the impact, the surface oxide was removed by shearing the melt along an alumina sheet located inside the tube, as explained in 3.2.3. Figure 4.4 a) contact heating of IM cubic sample on substrate at liquidus temperature. b) droplet of IM instantaneously after impact. In contrast to IM, when the dropping method was applied to the AlSi12Fe alloy (IS) regular spherical droplets were not achieved. A typical example with non metallic lustre is shown in Figure 4.5. The shape of the droplet could not be used for contact angle measurements. No solution was found to prevent the oxidation of the IS alloy during droplet preparation. This restricted the measurements to IM in contact with alumina (AF), titania (TF) and magnesia substrates (MO). 95

Figure 4.5 Droplet formation of the melt of alloy IS with oxide skin removal prior to impact of melt on substrate. 4.2.2 Wetting kinetics The time dependency of the static contact angle θst was measured for a maximum holding time of 1800 s. The error in contact angle measurement was estimated to be ± 5° based on three measurements of each droplet image of the same sample. Static Contact wetting angle angle θ θ (°) st /° st 180 160 140 120 0 500 1000 1500 2000 96 AF-IM TF-IM MO-IM Process Time time t (s) t /s Figure 4.6 Time dependence of wetting angle of the aluminium-magnesium alloy IM on alumina (AF), titania (TF) and magnesia (MO) substrates. As shown in Figure 4.6, the wetting angle of IM on AF ranged from 148° to 155° with no significant trend over the holding period. Indicating a non-wetting behaviour. The same behaviour was observed in the systems of IM on titania TF and magnesia MO where the wetting angles θst ranged from 143° to 150° and 150° to 155°, respectively. Taking into

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