Date of Award

8-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Materials Science and Engineering

Committee Member

Dr. M. Kennedy, Committee Chair

Committee Member

Dr. R. Bordia

Committee Member

Dr. V. Blouin

Committee Member

Dr. J. DesJardins

Committee Member

Dr. A. Poursaee

Abstract

As surfaces undergo sliding contact, their structure and mechanical properties can evolve or change relative to their as-fabricated state. These shifts can then influence the tribological performance of the system. To understand the extent and implications of these structural and property shifts, this dissertation examines the tribological performance of face centered cubic (f.c.c) metallic films and f.c.c. metallic/ceramic nanolaminates using nanoscale testing methods. Specifically, we sought to (1) understand the influence of deformation zone development to interpret nanowear testing results, (2) quantify the strain hardening exponent in metallic f.c.c. films with and without passivation layers, (3) quantify the impact of ceramic interlayers within f.c.c. metallic layers on the strain hardening relationship and the evolved structure during nano-sliding wear in metallic/ceramic nanolaminates, and (4) identify if (how) the wear mechanisms and wear rates of f.c.c. metallic/ceramic nanolaminates change primarily through altering interfacial density and ceramic composition.In this work, we quantified changes in mechanical properties (strain hardening, hardness) with nanoindentation when f.c.c monolithic films (Cu and Au) underwent linear reciprocating wear using a range of maximum contact pressures (4.1-14.1 GPa for Cu and 3.3-11.4 GPa for Au) and controlling the separate or overlay between deformation zones (contact and plastic) around parallel pass center points. The hardness for the Cu thin films decreased until the 100 µN wear boxes and increased as the contact pressure increased. The Au thin film wear boxes did not have the same effect. The hardness of the Au thin film remained similar to the as-deposited Au. The morphology of the Cu films did not have a significant change, but the Au film, with increased contact pressure during wear, there was an increase in wave-like texturing in the worn surface mirroring prior work on single crystal Ni films and gold thin films. Results highlighted the role of deformation zone interaction. As the spacing between parallel passes changed to transition from partial overlap of plastic zones to complete overlap of contact zones, there was also an increase in the worn film’s average hardness. There is a significant difference in the Cu film that is consistent with the empirical model, but there is no trend in the significant difference in the Au film. The percentage of overlap was estimated using empirical models, but not confirmed through structural characterization. This hardness difference between films undergoing sliding with the same contact pressure but different spacing between parallel passes could be due to the hardness test method sampling volumes of material. When the indentations were made within films containing heterogeneous deformation (passes separated by larger distances), the hardness would be lower since some of the volume could not have been modified during the sliding contact. Using only worn samples with the homogenous deformation along the surface (closely spaced passes), we calculated the strain hardening exponent (n) of the worn films to be 0.11 for the Cu.In addition to f.c.c. monolithic coatings, this work focused on f.c.c metallic/ceramic nanolaminate wear. The ceramic interlayers allowed us to better understand the impact of interfacial density, composition of the ceramic layers (TiN), and further investigate the influence of the H/E ratio on wear rate. Nanoindentation results displayed that the as-deposited nanolaminates with increased interfacial density had a higher hardness. Insteadof changing the line spacing, the cono-spherical tip diameter and load were changed to have a significant change, but the Au film, with increased contact pressure during wear, there was an increase in wave-like texturing in the worn surface mirroring prior work on single crystal Ni films and gold thin films. Results highlighted the role of deformation zone interaction. As the spacing between parallel passes changed to transition from partial overlap of plastic zones to complete overlap of contact zones, there was also an increase in the worn film’s average hardness. There is a significant difference in the Cu film that is consistent with the empirical model, but there is no trend in the significant difference in the Au film. The percentage of overlap was estimated using empirical models, but not confirmed through structural characterization. This hardness difference between films undergoing sliding with the same contact pressure but different spacing between parallel passes could be due to the hardness test method sampling volumes of material. When the indentations were made within films containing heterogeneous deformation (passes separated by larger distances), the hardness would be lower since some of the volume could not have been modified during the sliding contact. Using only worn samples with the homogenous deformation along the surface (closely spaced passes), we calculated the strain hardening exponent (n) of the worn films to be 0.11 for the Cu. In addition to f.c.c. monolithic coatings, this work focused on f.c.c metallic/ceramic nanolaminate wear. The ceramic interlayers allowed us to better understand the impact of interfacial density, composition of the ceramic layers (TiN), and further investigate the influence of the H/E ratio on wear rate. Nanoindentation results displayed that the as-deposited nanolaminates with increased interfacial density had a higher hardness. Instead of changing the line spacing, the cono-spherical tip diameter and load were changed to create wear boxes with increasing contact pressures and either one or ten passes. To measure the volume lost due to wear in the nanolaminate systems, AFM was used. For the wear boxes that displayed a distinct height difference between the as-deposited material and the worn region, it was determined that there was no difference in volume loss between the 1 and 10 cycle wear boxes created with the same tip contact pressure. Most of the wear boxes also had a wear rate coefficient > 0.001, which signifies severe wear. The specific wear rates for these nanolaminate samples were higher than the wear rates found by J. Lackner and M. Kot. For the indentation results in the worn regions, there was no statistical difference between the 1 and 10 cycle wear boxes for the samples at each contact stress. In addition, most of the hardness values were not significantly different from the as-deposited material except in the 1 µm/100 nm/RT sample. The flow stress and plastic radial strain were estimated using calculated mechanical properties. From the Hollomon's relation, it was found that the strain hardening exponent was most effected by the total thickness than any other parameter. The n for the 1 µm coatings was 0.50 and the n for the 3 µm coatings was 0.60 (RT) and 0.64 (500 °C). This signifies that the flow stress will increase higher and faster when more strain is applied to the 3 µm nanolaminates. The macrowear of the coatings were performed using linear reciprocating dry sliding tribological tests and the post-wear characterized. For coefficient of friction (COF), the steady-state COF decreased as the layer thickness increased. The dominant wear mechanism that was observed for the four nanolaminate coatings varied based on interfacial density. The 20 nm layered coatings displayed dominant wear mechanisms of plowing and wedging, while the 100 nm layered coatings exhibited many asperities relating to third-body abrasion. During post-wear characterization, it was determined using TEM and EELS that there was a formation of an amorphous titanium layer between the TiN layer below and the remnants of a recrystallized nanocrystalline titanium layer above in the worn region of the 100 nm layered nanolaminate. This effect had not been reported within published literature previously.

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