Thermo-mechanical Stability and Strengthening Mechanisms of Ti/Ni Multilayer Thin Films

Abstract

In this work, systematic studies on mechanical and microstructural properties of Ti/Ni multilayer thin films were carried out to understand the coupled size and thermal effects on strengthening mechanisms of metallic multilayer thin films. The size effect comes from the individual layer thickness and total film thickness, and the thermal effect comes from either post-deposition annealing, in-situ high temperature deposition, or localized surface annealing by pulse-laser irradiation. For as-deposited metallic multilayer thin films, the deformation behavior was found to follow the traditional trend from dislocation mediated motions with dislocation pile up along interfaces with layer thickness from microns down to a few tens of nanometers, and single dislocation bowing between interfaces with layer thickness from a few tens of nanometers to a few nanometers, to grain boundary mediated motions with further reduced layer thickness. In addition, strong orientation dependent hardness was observed in as-deposited multilayer thin films. The anisotropic hardness is attributed to dominant deformation mechanism switch from dislocation pile-up against the interfaces to confined layer slip within the layers as the loading direction changes from perpendicular to parallel to the interfaces. A systematic study on Ti/Ni multilayers with layer thickness from 200 nm to 6 nm and annealing temperature up to 500 °C led to the establishment of a coupled layer-thickness and annealing-temperature dependent strengthening mechanism map. For annealed films, grain boundary relaxation is considered to be the initial strengthening mechanism with higher activation temperature required for thicker layers. Under further annealing, solid solution hardening, intermetallic precipitation hardening, and fully intermixed alloy structure continue to strengthen the thin layered films, while recrystallization and grain growth lead to the eventual softening of thick layered films. For films with intermediate layer thickness, a strong orientation dependent hardness behavior is exhibited under high temperature annealing due to mechanism switch from grain growth softening to intermetallic precipitation hardening when changing the loading orientation from perpendicular to parallel to the layer interfaces. Furthermore, deposition temperature induced texture evolution and mechanical strengthening were studied for Ti/Ni multilayer thin films deposited with elevated substrate temperatures up to 500 °C. An obvious substrate-temperature dependent texture strengthening was observed. After low temperature deposition, preferred crystallographic texture were detected for both Ti and Ni layers, and columnar structure was observed to extend through layers, leading to initial strengthening. The columnar structure became more distinct and complete with the increase of substrate temperature, and meanwhile more atomic diffusion and intermixing occur along the Ti/Ni interfaces, promoting the formation of Ti-Ni intermetallic precipitates, and the subsequent mechanical strengthening. After high temperature deposition, columnar Ti-Ni alloys were observed with disintegration of layered structure. Ti-Ni intermetallic was detected with preferred crystallographic texture. Recrystallization was observed with even higher deposition temperature, leading to even higher material strength. Finally, a picosecond pulse laser was utilized to treat Ti/Ni multilayer thin films to induce desired microstructure change and surface strengthening. It was observed that with the increase of pulse-laser energy, the surface morphology evolves from homogeneous grain surface, to a cross-hatched pattern surface, and then to a rough melted surface covered with bubbles, voids and cracks. And the cross-section morphology evolves from a multilayered structure to partially intermixed and eventually fully intermixed structure. Due to the precipitation of Ti-Ni intermetallic phase, laser treatment with high pulse-energy led to surface strengthening on Ti/Ni multilayer thin films. In addition, the film and layer thickness effects on microstructural and mechanical properties were investigated using layer thicknesses of 20 nm and 50 nm, and film thicknesses 500 nm and 1 µm. It was found that thinner film and larger layer thickness requires the least energy to produce the intermixing effect while thicker film and smaller layer thickness requires a lot more energy to produce the desired intermix and mechanical strengthening.