Performance of nanoscale metallic multilayer systems under mechanical and thermal loadin

Reports of nanoscale metallic multilayers (NMM) performance show a relatively high strength and radiation damage resistance when compared their monolithic components. Hardness of NMMs has been shown to increase with increasing interfacial density (i.e. decreasing layer thickness). This interface density-dependent behavior within NMMs has been shown to deviate from Hall-Petch strengthening, leading to higher measured strengths during normal loading than those predicted by a rule of mixtures. To fully understand why this occurs, other researchers have looked at the influence of the crystal structures of the component layers, orientations, and compositions on deformation processes. Additionally, a limited number of studies have focused on the structural stability and possible performance variation between as-deposited systems and those exposed to mechanical and thermal loading.;This dissertation identified how NMM as-deposited structures and performance are altered by mechanical loading (sliding/wear contact) and/or thermal (such as diffusion, relaxation) loading. These objectives were pursued by tracking hardness evolution during sliding wear and after thermal loading to as-deposited stress and mechanical properties. Residual stress progression was also examined during thermal loading and supporting data was collected to detail structural and chemical changes. All of these experimental observations were conducted using Cu/Nb NMMs with 2 nm, 20 nm, or 100 nm thick individual layers deposited with either 1 microm or 10 microm total thicknesses with two geometries (Cu/Nb and Nb/Cu) on (100) Si.;Wear boxes were performed on Cu/Nb NMM using a nanoindentation system with a 1 microm conical diamond counterface. After nano-wear deformation, the hardness of the deformed regions significantly rose with respect to as-deposited measurements, which further increased with greater wear loads. Additionally, NMMs with thinner layers showed less volume loss as measured by laser scanning microscopy. Strain hardening exponents for multilayers with thinner layers (2 nm: n ≈ 0.018 and 20 nm: n ≈ 0.022 respectively) were less than was determined for 100 nm systems (n ≈ 0.041). These results suggest that single-dislocation based deformation mechanisms observed for the thinner systems limit the extent of achievable strain hardening. This result indicates that both architecture strengthening and strain hardening should be considered if the coating will undergo sliding wear. Furthermore, the hardness of the worn 100 nm system was observed to exceed the as-deposited hardness of the 20 nm, a previously unreported finding, further indicating the interplay between the architecture- and strain-based strengthening mechanisms.;Residual stress has been identified as a potential mechanism to cause microstructural instability in NMM architectures. To understand the factors controlling thermal stress evolution for NMMs, the stress in Cu-Nb NMM systems was determined from curvature measurements collected as the sample was cycled from 25°C to 400°C. In addition, the stress within each of the component layers was assessed by using changes in primary peak position from X-ray diffraction (XRD). The thermoelastic slope of NMM systems was shown to not only depend on thermal expansion mismatch and elastic modulus. Analysis showed that layer thickness (interfacial density) affected the magnitude of thermoelastic slope while the layer order was observed to have minimal impact on the stress-response after the initial heating segment. When comparing the monolithic stress responses to those of the Cu-Nb NMM systems, the NMMs show a similar increase in stress magnitude above 200°C to monolithic Nb. This indicates that the Nb layers play a larger role in the development of initial stresses than the Cu layers. Localized stress measurements using in-situ XRD revealed that the stress response of the Cu and Nb layers within the NMM behave similarly to their monolithic counterparts by themselves, rather than the composite stress estimate from curvature measurements.;Although FCC Nb has been identified under very specific contexts (e.g. due to initial deposition conditions, appreciable impurity content), the transformation of pure Nb from BCC to FCC has not been previously observed. Through this work we identified that stress is a possible mechanism that allows this transformation to occur. During heating to 500°C, a sharp peak in the stress response of 1 microm monolithic Nb was observed at 475°C. Post-heating determination of structure revealed both the initial BCC orientation as well as peaks that coincide with a previously simulated FCC Nb structure. Due to the observation of both structures concurrently, the observed transformation did not progress to completion. The transformation coincided with an increase in the elastic modulus from 115 +/- 4 GPa to 153 +/- 4 GPa, another indication of a structural change within the Nb film. These findings have not been previously observed for pure Nb and are being further confirmed with high-resolution transmission electron microscopy (HRTEM) and selected area diffraction (SAD).