Compression tests were carried out on UHV diffusion bonded single crystalline sapphire/Nb/sapphire joints to investigate their mechanical properties, the mechanisms that lead to the failure of the joints and the dislocation-interface interaction. The tests were performed for different orientation relationships (OR) at the interface to study the influence of different bond strength on the mechanical behavior. Additionally, the metal layer thickness was varied for each OR to alter the influence of the interface. The experimental results showed, that with decreasing metal layer thickness the stress needed to form a crack increases drastically, whereas for the Nb/sapphire system the bond strength at the interface seems to have no significant influence on the mechanical behavior of the joint. A theoretical model will be presented that explains the experimentally observed relationship between metal layer thickness and crack stress.