Standard free energy of formation ΔG 0 (kJ/mole O 2 ) Standard free energy of formation ΔG0 / kJ/mole O2 -600 -800 -1000 -1200 SiO 2 ZrO 2 MgO TiO 2 TiO Al 2 O 3 CaO 0 500 1000 1500 35 Ti 3 O 5 Ti 2 O 3 Temperature T / °C Temperature (°C) Figure 2.11 Ellingham-Richardson-Jeffes diagram of selected metal oxides (89) Even though heavy reactions were predicted in thermodynamic calculations in the Al-TiO2 system (94) , no improved wetting in the sessile-drop test was found (95) . The formation of Al2O3 along the interface led to a layered system of molten Al on reaction-formed Al2O3. The wetting angle was of the same order or even higher than that of the Al-Al2O3 system. Arpon et al. (96) showed similar behaviour in the Al-TiC system where TiO2 was formed on the surface of TiC as a result of preheating the carbide in an oxidizing atmosphere. It has been suggested (97,98) that the formation of the spinel phase MgO·Al2O3 affects wetting of Al2O3 by pure Al and AlMg-alloys. The system was investigated by Nakae et al. (99) with attention to the proposed three stage wetting regime. At relatively low temperatures of 900°C, no significant change from an initial θ of 130° could be observed. At higher temperatures of 1000°C and 1100°C, a steep decrease after 10 3 s resulted in wetting angles of 100° and 85° respectively. A transient minimum contact angle was not observed throughout the experiments even though reaction zones of MgO·Al2O3 were found. This agrees with Shen et
al. (100) who found non-wetting behaviour of aluminium melts on MgO indicated by θ of 104 to 121° in sessile drop tests. In reactive wetting of liquid metals on ceramic substrates, Zhou and Hosson (101) proposed the ceramic volume change as the key factor to improve wetting. They suggested that if the volume of a ceramic substrate decreased after the reaction, wettability was not improved by the chemical reaction. In contrast, if the volume of the ceramic substrate increased, the wettability was improved by the chemical reaction. As an example, they pointed out the Al- SiO2 system where a volume decrease due to formation of Al2O3 and solution of silicon in the melt pool could be observed. In contrast, the Ti-Al2O3 system represents an example of volume increase by the formation of TiO2 and titanium suboxides like TiO and Ti2O3. The simple criterion of volume change proposed by the aforementioned authors was challenged by Shen et al. (23) who investigated the Al-SiO2 system intensively. They observed an improved wetting resulting from the chemical reaction. The authors stated that wetting is at first a surface phenomenon. If the change in surface energy due to reaction (Δγr) is the dominating factor in reactive wetting, there are two possible configurations at the metal-ceramic interface as shown in Figure 2.12. Reaction layer original substrate 36 Reaction layer Figure 2.12 Different configurations in front of the triple line in reactive wetting after equilibration.