Wetting of calcium fluoride by liquid metals

The results of wetting experiments for the CaF2/Me and CaF2/Me-Ti systems (Me = Cu, Ge, Al, In, Ga, Sn, and Au) are presented and discussed. It was found that pure metals do not wet the CaF2 substrate, while a small quantity of Ti added to the melt improves the wetting. The effect of Ti depends on its thermodynamic activity in the melts. According to the thermodynamic analysis and experimental observations, Ti dissolved in the metals does not react with the substrate to form any new condensed phase at the interface and its effect cannot be attributed to the "reactive wetting" phenomenon. Density functional theory (DFT) was applied to focus on the nature of chemical bonding between the atoms in the melt and the surface of the substrate in these systems. It was demonstrated that partly filled d-states of Ti stimulate its adsorption onto F ions. Ab initio calculations show that Ti may segregate to the interface, decreasing the energy of CaF2/Me-Ti system. Based on the results of thermodynamic and DFT analyses, it is proposed that Ti segregation at the interface may be considered as the source of the improved wetting.