NEUTRON REFLECTION STUDIES OF TITANIUM SEGREGATION TO METAL-CERAMIC INTERFACES

Titanium is an extremely reactive element which is known to promote th e wetting of ceramics by liquid metals. The wetting process is believe d to be controlled by the segregation of a very thin layer of Ti to th e metal/ceramic interface where it undergoes a localised redox reactio n with the ceramic. This hypothesis is consistent with cross-sectional microanalysis studies of metal-ceramic interfaces where thin layers o f 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 stu dy we have characterised the chemical composition of three interfaces between metals and a sapphire (Al2O3) single crystal using specular ne utron reflection. The interfaces are: Sn/sapphire, Sn/sapphire contain ing a thin (approximate to 20 nm) Ti interlayer introduced by physical vapour deposition (PVD), and an interface between sapphire and a comm ercial Ag-Cu-Ti alloy used to braze ceramics. We have evaluated the ne utron reflection data collected from the CRISP reflectometer at the IS IS facility using a multilayer model of the interface. The results ind icate that the 20 nm evaporated layer cannot be distinguished from int erface roughness while the interface between sapphire and the commerci al braze alloy contains a thin (approximate to 70 nm) titanium suboxid e layer. (C) 1998 Elsevier Science B.V. All rights reserved.