Engineering Bicontinuous Interfaces for Enhanced Mechanical Performance
Derby, Benjamin
2020
Abstract
In this work, we report on the evolution of phase-separated morphologies in co-sputtered Cu-Mo films as a result of varying substrate temperature and deposition rate during deposition. A transition from vertically-oriented concentration modulations to laterally-oriented modulations followed by randomly-oriented modulations was observed in the Cu-Mo films because of changing substrate temperature at constant deposition rate. A refinement of the Cu-Mo domains was observed at increasing deposition rate at a constant substrate temperature. A coarsening of phase-separated domains was also observed with increasing substrate temperature. While these morphologies have been observed in state of the art research and technology, no work has shown the transition between these morphologies for a given system. The phase separation kinetics that controlled film morphology were limited by the surface in terdiffusion of each species as the film grew in thickness. Control over the surface interdiffusion length was achieved by altering the deposition temperature and material flux. Additionally, a new processing route has been discovered through which far-from-equilibrium, metastable architectures with unprecedented properties are synthesized. This novel architecture contains many orders of hierarchy with multiple concentra- tion modulation wavelengths. At one length scale, the matrix consists of lateral modulations of BCC Mo and pseudomorphic BCC Cu with a wavelength of 10nm. FCC, Cu-rich islands of approximate 250nm diameter are woven in-between the Mo- Cu matrix and contain ordered arrays of pseudomorphic, FCC, Mo-rich, coherent precipitates of approximately 1.0nm diameter spacing. The resulting material exhibits unprecedented mechanical behavior of extensive plastic deformability at room temperature. Traditional phase separated thin film morphologies are monolithic in architecture with only one concentration wavelength. By carefully manipulating the self-assembly kinetics through the deposition rate, we processed a structure with multiple concentration modulation wavelengths. Furthermore, with the introduction of high power impulse magnetron sputtering (HiPIMS) as a new sputtering technique, the range of morphologies achieved during deposition has been expanded. In this work, we co-deposit thin films composed of immiscible Cu, Fe constituents via traditional DC magnetron sputtering (DCMS) and compare this to deposition of the same Cu, Fe system using the HiPIMS technique. Microstructural features, including porosity, columnarity, and roughness were altered as a function of the Cu and Fe metal ion current. The nanostructured phase morphology also evolved from lateral concentration modulations of Cu and Fe deposited via DCMS to a more randomized phase domain structure when the film was deposited using HiPIMS. This change in structure is reasoned through an interdiffusion model as a function of deposition conditions. Finally, this work reports on Helium (He) accumulation in these novel metal nanocomposites fabricated by phase separation in three-dimensions. Since, these nanocomposites are dense with semi-coherent interfaces that ultimately influence He implantation response. The effect of film morphology on three aspects of He response is investigated: size and distribution of He precipitates, He retention, and He-induced changes in mechanical properties. Transmission electron microscopy analysis showed that He bubbles agglomerated along the vertical phase boundaries in morphologies with lateral and random domains of Cu and Mo. Nuclear Reaction Analysis (NRA) and nanoindentation results show He retention and hardness is morphology dependent.Subjects
phase separation thin films bicontinuous HiPIMS mechanical behavior He outgassing
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