Studies of a model metal-ceramic system: Nickel/sapphire interfaces

Nickel/sapphire (single crystal $\alpha$-aluminum oxide) was used as a model system for an in situ study of metal-ceramic interfaces under controlled conditions. The interface was prepared in ultra-high vacuum (UHV) by depositing nickel onto a sapphire substrate with control of initial starting surface and reaction environment. X-ray Photoelectron Spectroscopy (XPS) and Reflection High-Energy-Electron Diffraction (RHEED) were used to monitor simultaneously chemical composition and reaction at the interface, as well as structure of the growing nickel film on the sapphire. In the absence of oxygen at elevated temperatures, an interaction characteristic of a Ni-Al intermetallic interaction was observed between the deposited nickel and the sapphire substrate, as suggested by various evidence. The interaction appears to have extended over a few atomic layers beyond the surface. A possible mechanism has been proposed by which the interaction was attributed to the unsaturated surface aluminum bonds and consequent breaking of aluminum-oxygen bonds. It was found further that under the same UHV conditions nickel grew epitaxially on the sapphire substrate with an orientation relationship of $\{111\}\sb{\rm Ni}$ // (0001)$\sb{\rm sapphire}$ with $\langle 110\rangle\sb{\rm Ni}$ // $\langle 1120\rangle\sb{\rm sapphire}$, as evidenced by RHEED. The nickel film, which assumed approximately the bulk nickel lattice, was observed to grow from initially discrete microcrystallites to a coalesced continuous film as the nickel coverage increased. Effects of an oxidizing environment were studied on the mechanism of interfacial interaction and resulting structure. It was found that nickel aluminate spinel can be formed either by depositing nickel with oxygen onto the sapphire or by annealing the nickel film on the sapphire in oxygen. In the former case, the minimum oxygen pressure required is found to be the one that provides at least equal flux of oxygen to that of nickel, and in the latter, the minimum oxygen pressure was determined to be the one that maintains the stability of the oxidized nickel at the given temperature. Furthermore, the spinel formed in the former process was found to grow epitaxially on the sapphire with (111) spinel // (0001) sapphire, while the annealing of nickel on the sapphire substrate resulted in the formation of polycrystalline spinel