Control of texture in niobium film and interface toughness of niobium-sapphire system.

Modification of niobium film properties, such as microstructure, surface roughness, texture, residual stress, and hardness by ion bombardment during film deposition were studied. Focus was put on the control of in-plane texture using ion bombardment, and the understanding of texture development in IBAD films. Experimental results indicate that the development of texture in films with ion bombardment during deposition is a growth phenomenon instead of a nucleation phenomenon. A phenomenological growth model was proposed for the first time to directly link the surface roughness evolution and texture development in niobium films under ion bombardment. Both surface roughness and texture are controlled by the same mechanism based on the preferential growth of aligned grains due to reduced ion sputtering as a result of ion channeling. A critical thickness was introduced in the model at which bridge-over of the faster-growing grains occurs, and the growth of the misaligned grains is suppressed. For film thickness beyond the critical thickness, grain alignment (or texture) is fully established, and the surface roughness starts to depart from the prediction by the growth model. Experimental results of both the surface roughness and the texture in niobium films substantiate the proposed model. The interface toughness of the niobium-sapphire system was controlled by the composition and the orientation relationship at the interface. A technique was developed to produce the interface modification of orientation relationship for the vapor deposited system through the establishment of in-plane texture in films using energetic ion bombardment during film deposition. The effects of such interface modifications (composition and orientation relationship) on the interface toughness were evaluated using the microscratch test, indentation test, and delamination technique, and a range of interface adhesion, from weak to strong, was obtained using these interface modifications. The presence of silver at the niobium-sapphire interface causes weak interface adhesion. The reduction in interface toughness increases with the amount of silver at the interface, and follows a power law relation between the interface toughness and the work of adhesion. On the other hand, establishment of orientation relationship at the interface using ion bombardment results in a strong adhesion between the niobium film and sapphire substrate. In addition, interface mixing, which is a result of ion bombardment, also promotes interface adhesion, and could be partly responsible for the strong interfaces in IBAD samples.Ph.D.Applied SciencesCondensed matter physicsMaterials sciencePure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/131003/2/9825259.pd