Microstructure and mechanical properties of IBAD niobium films on sapphire substrates

Abstract

The effect of ion bombardment during film deposition on the microstructure, surface morphology, residual stress, hardness and interfacial fracture energy was determined for Nb films on sapphire substrates. In the present study, niobium films of nominal thickness 100 nm were deposited on (0001) sapphire substrates by Physical Vapor Deposition (PVD) and by Ion Beam Assisted Deposition (IBAD) at an ion energy E = 1000 eV and an ion-to-atom arrival rate ratio R = 0.4. The microstructures of the PVD and IBAD films were very similar, but the IBAD film had a much rougher surface. Ion bombardment also changed the residual stress state from tensile (224 MPa) for the PVD film to compressive (−400 MPa) for the IBAD film. The IBAD film had a lower hardness (5.7 GPa) than the PVD film (10.9 GPa) as determined by nanoindentation. An attempt was made to measure the fracture energy of both the PVD niobium-sapphire and IBAD niobium-sapphire interfaces by scratch test and nanoindentation. The PVD sample failed in a brittle manner in both tests and interfacial fracture energies were able to be estimated. By contrast, the IBAD sample failed in a ductile manner in both tests, precluding the determination of the interfacial fracture energy.