Grain boundary grooving in thin films revisited: The role of interface diffusion

Thin nickel (Ni) films were grown on c-plane-oriented sapphire substrates by electron-beam deposition. The as-deposited films exhibited a mazed bicrystal microstructure consisting of interwoven grains of two twinned orientations. Annealing of the specimens at 700 C resulted in the thickening of the Ni film. At the same time a mass deficit in the surface regions near grain boundary thermal grooves was observed. The shape of the observed grooves was very different from that predicted by the classical Mullins model. We developed a model of thermal grooving with simultaneous film thickening due to Ni diffusion along the Ni-sapphire interphase boundary, which agreed well with the experimental results. These findings demonstrate that self-diffusion of metal atoms along the metal-ceramic interface plays an important role in mass transport in thin films. Some implications of fast interphase boundary diffusion for the thermal stability of thin films are discussed.