Microstructure and mechanical properties of metastable solid solution copper-tungsten films

A combinatorial science approach is utilized to study the microstructural and mechanical properties of metastable copper-tungsten solid solutions. Lateral compositional gradient (also called composition-spread) samples were deposited by simultaneously sputtering copper and tungsten targets positioned obliquely at opposite ends of a silicon substrate. The chemical composition of the film varies continuously along its length from 12 to 45 atomic % copper and has a nominal thickness of 1 mu m. Nanoindentation was performed to measure the hardness and elastic modulus of the film. Grain size and solid solution strengthening models are applied to interpret the hardness of the film, though a simple rule of mixtures is found to give a more satisfactory fit to the data. The elastic modulus of the film is consistently below that predicted by the rule of mixtures. X-ray diffraction revealed plane spacing less than that predicted by Vegard's law as well as three chemical compositions exhibiting enhanced long-range order. Transmission electron microscopy analysis confirms that the film consists of a single metastable body centred cubic solid solution where the lattice spacing and grain size depend on chemical composition.