Wettability of polycrystalline rutile TiO2 by molten Al in different atmospheres

Sessile drop experiments were performed to study the wettability of polycrystalline rutile TiO2 by molten aluminum in purified Ar, Ar–3% H2 and vacuum over a wide temperature range. The effects of atmosphere, temperature, time and testing method on the contact angles were thoroughly investigated and the resulting interfacial microstructures were examined. The atmosphere- and temperature-dependent reduction in the TiO2 substrates was manifested by (i) distinct color change of the crystals before and after annealing and (ii) color variation in the crystals produced in different atmospheres and at different temperatures. The wetting results were explained on the basis of TiO2 surface chemistry and competition reactions between Al surface oxidation and deoxidation. With regard to the contact angles, which are generally smaller than 90° in well-controlled Ar and vacuum, the Al–TiO2 system is of a partial wetting nature. However, this system can easily be non-wetting due to a considerable increase in the oxygen partial pressure as a result of TiO2 reduction, particularly in a hydrogen-containing atmosphere.