|Ph.D Student||George Levi|
|Subject||Energy and Structure of Nickel-Alumina Interfaces|
|Department||Department of Materials Science and Engineering||Supervisor||Professor Wayne D. Kaplan|
The purpose of this research was to provide new insight into the mechanisms which govern interface wetting and adhesion, interface reactions, and interface strength in metal-ceramic systems. High temperature wetting experiments of liquid and solid Al, Ni, and Ni alloys on the basal surface of sapphire were conducted under controlled working conditions as a function of temperature and working gas, and alloying elements.
Dissolution/etching of sapphire in contact with liquid alloys was shown to be associated with capillary driven diffusion during ridging at the liquid-solid-vapour triple line junction, and not with a chemical reaction. Detection of FeAlO3 precipitates at the Al(31at.ppmFe)-sapphire interface showed that oxygen undergoes non-equilibrium segregation to the Al-Al2O3 interface, followed by the formation of an oxygen-rich interphase, improving wetting and adhesion at the Al-sapphire interface. The constituents of the sapphire substrate dissolve within the initially pure Ni, and segregate to the free surface/interface, reducing the surface/interface free energy. An (Al,O)-rich interphase forms at the liquid Ni-alumina interface, which decomposes during solidification, and leaves an Al excess at the interface, which improves interfacial wetting and adhesion.
A qualitative relation between interfacial strength and wetting and adhesion was determined through the analysis of the interfacial void(s) formation in the Ni-Al2O3 system. Aluminium is beneficial to the interfacial strength. Sulphur does not reduce the interfacial strength, but it severely limits the dissolution of Al (and oxygen) atoms diffusing from the sapphire substrate.
A new generalised Young’s equation was derived. The elastic strain induced at the triple junction affects the equilibrium contact angle in a non-metamorphic system. If the substrate is not inert, the elastic strain initiates ridging. However, for relatively large metal sessile drops on sapphire assessments based on the classical Young’s equation are still correct.