INFLUENCE OF INTERFACE IMPURITIES ON THE FRACTURE ENERGY OF UHV BONDED NIOBIUM SAPPHIRE BICRYSTALS

Defined interface doping is used to study the influence of segregants on the fracture energy of metal-ceramic interfaces. In a doping chamber connected to a UHV diffusion boding facility, sputter cleaned surfaces were covered with thin films of dopant in the range between submonolayers and nanometers. A quartz oscillator monitors the film thickness. After surface doping of one of the two surfaces to be bonded, the interface is produced by pressure bonding. Fracture measurements and AES measurements on in situ fractured samples are used to obtain a relationship between the fracture energy and the interface concentration of the impurity. Niobium-sapphire bicrystals were chosen for the first experiments because of their similar thermal expansion behaviour. Bicrystals with three different crystallographic orientations were studied. The segregating elements were silver and sulphur. Both elements are insoluble in niobium and sapphire. With increasing layer thickness and increasing interface concentration of silver the fracture energy decreased. Depth profiling of UHV fractured samples showed that the concentration profiles are similar to those of natural segregation. Silver reduces the fracture energy according to the models of Nicholas and Seah and Hondros. Sulphur was present in the bulk of the niobium and was enriched at the metal surface by segregation anneals prior to bonding. It shows a stronger tendency to segregate to free niobium (100) surfaces than to niobium-sapphire interfaces. Sulphur behaves as proposed for cohesion-reducing impurities in the model of Rice and Wang.