Non-stoichiometry, Electrowetting and Contact Interaction of Zirconia, Titania or Hafnia with Metal Melts

Transition metal oxides, in particular titania, zirconia and hafnia, due to high anion mobility, can lose oxygen with the formation of non-stoichiometric phases and also passage of electric current at high temperature. This influences the properties of the materials, in particular the contact interaction with metals. This paper studies the wetting of zirconia, hafnia and titania with inert metal melts (Cu, Ni, Pt, Cu-Ga, Cu-Ge, etc.) in conditions where high non-stoichiometry of oxides was provided and where current passed through the interfaces. Sessile drop wetting in vacuum, microstructure analysis and x-ray studies were conducted. In wetting experiments with high non-stoichiometry, oxide samples contacted at the same time with active metal melts (Ti, Cu-Ti, Ni-Ti) to provide the oxygen deficit in substrates and with inert metal melts (Cu, Ni, Pt, Cu-Ga, Cu-Ge, etc.). In experiments with current, metal drops were situated between ceramic or ceramic and metal plates connected to current leads. When inert and active metals contacted the ceramic simultaneously, inert melts wet the zirconia and the hafnia but not the titania. This effect was explained by dissolution of surplus zirconium and hafnium in melts. Titania in contact with active metal was reduced to a lower oxide, and permanent oxygen removal from the sample volume was not provided. Dissolution of zirconium was confirmed for the nickel/zirconia/Ni-Ti samples through microstructure investigation and thermodynamic calculations. When a positive electrode was connected to a ceramic, wetting significantly improves for zirconia, titania and hafnia. It was also explained by oxygen depletion under the current and subsequent dissolution of surplus titanium, zirconium or hafnium in the melt. The results obtained were used to braze zirconia ceramics. Thus the non-stoichiometry caused by high anion mobility was found to influence the titania-, zirconia- and hafnia-to-metal interaction.