Extraordinary Adhesion of Niobium on Sapphire Substrates

We have investigated the adhesion between a metal and a ceramic interface, in particular between Nb(110) films and ${\mathrm{Al}}_2{\mathrm{O}}_3\left(11\overline{2}0\right)$ substrates. We have tested the adhesion properties by hydrogen loading of the metal film. The resulting strains were measured by in- and out-of-plane lattice parameter measurements. In contrast to the bulk behavior, we find for hydrogen in thin Nb films a highly anisotropic in plane strain. Comparing the in-plane expansion with the free case, we observe a tensile stress exceeding the yield stress of bulk Nb by at least a factor of 10. This unusually large resistance against expansion is due to giant adhesive forces at the metal/ceramic interface.