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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

The wetting behaviour

The wetting behaviour of Al2O3 by liquid Al has been widely studied, as reviewed by Rocha- Rangel et al. (75) . The principal problem that appears in this system is that wetting of Al2O3 by Al is a non-equilibrium phenomenon characterized by the continuous oxidation of Al. In fact the contact corresponds to the Al-Al2O3/Al2O3 system, rather than to the Al/Al2O3 couple. It appears that the alumina layer that coats liquid aluminium is the barrier impeding the wetting phenomenon. The critical factors affecting the wettability of α-alumina by molten aluminium were reviewed by Shen et al. (76) . The reported contact angles are so scattered, ranging from 160° to 40°, that it is difficult to get a clear picture of the intrinsic wettability in this system. The large scatter is primarily because of the presence of a protective surface oxide layer on the molten aluminium, which acts as a barrier inhibiting the formation of a true Al-Al2O3 interface, particularly below 1000°C. The ubiquitous oxide layer results in the much larger observed contact angles. At high temperatures (>1000°C) the scatter is relatively small, which is attributed to disappearance of the aluminium oxide film. A possible mechanism for elimination of the oxide film could be the reaction between it and liquid aluminium involving the formation of a gaseous suboxide according to the reaction (7,77) : (l) + Al O (s) → 3Al O (g) Equation 9 4Al 2 3 2 The vapour pressure of Al2O over liquid Al and solid Al2O3 at 1350°C is reported to be about 3·10 -4 bar (78) and that of liquid aluminium is reported to be 1·10 -4 bar at the same temperature. Thus evaporation of both Al and Al2O has to be assumed. The authors concluded that good wetting of aluminium on alumina is possible at elevated temperatures above 1000°C. The intrinsic wettability of α-Al2O3 by molten Al, corresponding to situations where the Al surface is free of oxidation and the Al2O3 substrate is not attacked by the aluminium liquid 27

and vapour, is difficult to evaluate. This is because of the viewpoint of thermodynamic equilibrium, surface oxidation of Al can only be prevented at extremely low oxygen partial pressures, lower than 10 -44 Pa at 700°C (6,75,76) . Apart from oxidation, the surface roughness influences the wetting behaviour (79) . Even though polished samples are used in most sessile drop tests, the surface is not totally smooth. It has been shown that when the roughness Ra is below 1 µm the influence on wettability is rather moderate. Shen et al. (76) proposed that, in the case of a clean Al melt surface, the effect of the substrate surface roughness is only noticeable when it exceeds 2 µm. The effect of temperature on wettability and shear strength of Al/Al2O3 couples was assessed by Sobczak et al. (80) . They proposed the "true" contact angle at 800°C in Al/Al2O3 to be 80°. They concluded that low contact angles correspond to high interfacial strengths. This was indicated by comparison of the shear stresses of two different Al-Al2O3 couples where the wetting tests were done at different test temperatures. The shear strengths were 50 MPa and 28 MPa at wetting angles of 90° and 120° respectively. The effect of an oxide layer on the contact angle has been revealed by Ksiazek et al. (81) through simultaneous measurement of the wetting angle θ and oxide thickness. The partial oxide breakdown on the surface in combination with open porosity allowed the metal to penetrate small pores even though the macroscopic wetting angle of the sessile drop was measured to be significantly larger than the threshold angle of spreading, which is 90°. Wetting behaviour is extremely sensitive to test conditions which are mainly atmosphere, material purity, temperature and surface finish. In oxidation of an Al melt the submerged oxide film is in perfect atomic contact with the Al supplying liquid (6) . After solidification, the metal-oxide bond is still strong and so the surfaces of all aluminium products are covered with 28

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