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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

an intimate aluminium

an intimate aluminium oxide film. In contrast to this intimate contact, the dry side of the oxide film is not wetted by the liquid. The poor wetting behaviour is observed in similar way when examining the results of wetting angle of Al on Al2O3 in the sessile drop test. The large scatter of reported contact angle data in this system is reviewed by Li (82) and presented in Figure 2.8. Based on the reviewed literature, the largest deviations have been reported between the melting point and 1000°C and lie between θ = 50° and 170°. In preform infiltration, the temperature of the liquid aluminium ranged between the melting point and 900°C. In this range, the reported contact angles vary between 60° and 170°. Wetting 0 angle (°) θ /° 180 130 80 30 600 800 1000 1200 1400 Figure 2.8 Temperature T (°C) T / °C The reported temperature-dependent wetting angle scatter as a function of temperature (82) . As suggested by John and Hausner (83) , the scatter is attributed to different atmospheric conditions, in particular the oxygen content and the resulting thickness of the oxide layer on the melt. The different experimental conditions are expected to be the reason for this behaviour. The authors achieved low oxygen partial pressures using oxygen-gettering materials. The melt droplet resting on the plane substrate was covered with a zirconium crucible. Therefore the droplet was optically not visible while testing and the θ could only be measured after the test. The authors stated that the equilibrium oxygen partial pressure in the 29

system Al-Al2O3 is 10 -49 Pa at 700°C and at the same temperature that of Zr-ZrO2 is marginally lower. Therefore the surface of the aluminium melt droplet was assumed to be oxygen-free. Under these conditions the wetting angle was measured to be 90° and therefore exactly either wetting nor non-wetting. Since the solubility of oxygen in liquid Al is extremely low (7) , different oxygen partial pressures in a sessile drop experiment will influence primarily the oxide layer thickness on the surface of the Al. At high oxygen pressures the oxide layer will prevent the formation of the equilibrium shape of the metal droplet, because of the mechanical strength of the oxide skin. The results demonstrated that an oxygen partial pressure of ca. 10 -13 bar is the limit for the formation of a droplet by the liquid Al. Under the experimental conditions an oxide layer on the surface of the Al completely prevented the formation of a spherical droplet. Very low values for the wetting angle were obtained if a strong oxygen getter such as Zr was present in the system. At very low oxygen partial pressures in the region of 10 -44 Pa, the wetting angle at 700°C was as low as 90°. Typically θ is obtuse at small contact times but decreases rapidly to a constant value θeq at long times. Asthana et al. (84) analysed this time-dependent spreading behaviour of liquid metals on solids. The θ(t) experimental data for selected systems were fitted to an empirical relationship of the form: θ = θ + θ Equation 10 eq ( B A t ) eqe − where the empirical constants A and B were determined from the graph of ln((θ/θeq)-1) vs t as shown in Figure 2.9. It is therefore inappropriate to use equilibrium values of contact angles in modelling dynamic processes that proceed quickly. The instantaneous contact angle θ decreases progressively with contact time. The relationship appears to describe adequately the 30

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