Neutron reflection studies of titanium segregation to metal-ceramic interfaces

Titanium is an extremely reactive element which is known to promote the wetting of ceramics by liquid metals. The wetting process is believed to be controlled by the segregation of a very thin layer of Ti to the metal/ceramic interface where it undergoes a localised redox reaction with the ceramic. This hypothesis is consistent with cross-sectional microanalysis studies of metal-ceramic interfaces where thin layers of substoichiometric titanium oxides have been identified. The study of titanium segregation is ideally suited to neutron reflection because of the negative scattering length of the dominant isotope. In this study we have characterised the chemical composition of three interfaces between metals and a sapphire (Al2O3) single crystal using specular neutron reflection. The interfaces are: Sn/sapphire, Sn/sapphire containing a thin (approximate to 20 nm) Ti interlayer introduced by physical vapour deposition (PVD), and an interface between sapphire and a commercial Ag-Cu-Ti alloy used to braze ceramics. We have evaluated the neutron reflection data collected from the CRISP reflectometer at the ISIS facility using a multilayer model of the interface. The results indicate that the 20 nm evaporated layer cannot be distinguished from interface roughness while the interface between sapphire and the commercial braze alloy contains a thin (approximate to 70 nm) titanium suboxide layer. (C) 1998 Elsevier Science B.V. All rights reserved.