Segregation of Aluminium at Nickel-Sapphire Interfaces

Sessile drop experiments of pure liquid Ni on the basal surface of pure sapphire were conducted under controlled atmosphere and temperature. This system has been traditionally considered as non-reactive, based on thermodynamic assessments. However, the results of this study demonstrate that a capillary driven interaction exists between the pure liquid Ni and the sapphire, which causes the dissolution of the sapphire substrate mainly at the triple junction. Oxygen and Al resulting from the dissolution process diffuse into Ni and segregate at its interfaces with the atmosphere and the sapphire (probably as Al_xO_y clusters), which reduces the interface energy. It is considered that this reduction is beneficial for the adhesion of both liquid and solid Ni on sapphire. The amount of Al introduced into the drop, and hence the segregation of Al that affects the interface energy (and adhesion), are related to the size of the sessile drop.