Combinatorial investigation of hardening mechanisms in compositional gradient and multilayer thin films

K. Thomas


A combinatorial experimental approach was used to investigate the mechanical properties of thin films deposited by sputtering. Nanoindentation and micropillar compression experiments performed at var- ious temperatures and strain rates were used to evaluate the operative deformation mechanisms with- in the films as a function of their architecture and the testing conditions. Three classes of films were in- vestigated: lateral compositional gradient films, periodic bilayer multilayer films, and nanoparticle- dispersion metal matrix composites. A number of different materials systems were investigated in ad- dition to the copper-tungsten combination used for all of the listed film architectures. Lateral compositional gradient films were prepared via co-sputtering and by annealing thickness- gradient multilayer films. Both binary (two-component) and ternary (three-component) compositional gradient films were deposited and investigated. Nanoindentation was used to map the hardness and reduced modulus of these films across large-area substrates. In general a simple rule-of-mixtures is sufficient to predict the modulus of a compositional gradient film, while the hardness can be evaluated using a superposition of various hardening mechanisms. Metastable solid solutions were observed in the Cu-W films. Laser ablation experiments and optical reflectometry experiments were performed on the Al-Cu-Ni compositional gradient film and demonstrate the utility of the combinatorial approach beyond mechanical characterization. Multilayer thin films with incoherent interfaces were also investigated. Material systems included Cu- W, Cu-TiN, and Mg-Nb. A combinatorial approach utilizing thickness-gradient films was applied to in- vestigate the role of both layer thickness and the ratio of film components on the mechanical proper- ties of the film. Previous experiments in the literature have addressed only the effect of layer thickness while largely neglecting the influence of non-equal volumetric ratios. The combinatorial approach used here highlights the potential for investigating a larger number of parameters influencing the mechani- cal behavior of multilayer thin films with only a modest increase in experimental effort. Elevated tem- perature micropillar compression experiments were performed on both Cu-TiN and Mg-Nb thin films. In the case of both materials systems diffusion-based deformation mechanisms are shown to dominate material plasticity at elevated temperatures, a novel finding in a field where deformation behavior has been classically explained using dislocation-based deformation mechanisms. Cryogenic tests were also performed on the Mg-Nb multilayer films and revealed the onset of brittle fracture within BCC- magnesium at low temperatures.