Date of Award

2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Mechanical Engineering & Applied Mechanics

First Advisor

Robert W. Carpick

Abstract

Hydrogenated amorphous carbon (a-C:H) films are a class of non-crystalline materials composed of hydrogen and carbon bonded in both sp2 and sp3 configurations. These films are notable for their high hardness (10–16 GPa), low roughness, chemical inertness, and good tribological performance (low friction and wear). This combination of favorable properties has led to promising applications in diverse fields, including automotive and aerospace components, biomedical devices, and computer hard drives. However, a-C:H becomes unstable above 150 °C, preventing its use in important technological applications, such as heat assisted magnetic recording (HAMR) disk drives. Also, the low friction and wear is only maintained in dry and vacuum conditions. To understand and address these limitations, the effect of adding silicon and oxygen to a-C:H films is considered, since prior experimental evidence shows that this can significantly increases thermal stability, and help maintain low friction and wear in humid environments. However, the mechanisms and extent of these improvements are unknown. Friction and wear testing were performed on a-C:H doped with Si and O (a-C:H:Si:O) in a range of environments. Friction coefficients varied from approximately 0.05 in dry environments (RH < 5%) to 0.15 in humid air, better than prior observations for undoped a-C:H films. The friction and wear behavior is controlled by adhesive interactions leading to the development of transfer films on the steel counterface. Possible mechanisms underlying this behavior are discussed. Annealing experiments showed significant improvements in thermal stability up to 450 °C. In order to understand the atomistic origins of this enhanced thermal stability, reactive molecular dynamics (MD) simulations using the ReaxFF potential were performed. The primary thermal degradation pathway in undoped a-C:H was observed to be the breaking of tensile strained C-C bonds. The presence of Si suppresses this mechanism by decreasing the frequency of highly strained C-C bonds in the unannealed structure. This is due to the longer C-Si equilibrium bond length compared to C-C bonds. The activation energy for rehybridization could be modeled using the same methods as in prior experiments and produced good agreement between the experimental and simulation results.

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