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

In this dissertation, the results of experimental and theoretical studies exploring friction

and adhesion at the nanoscale are presented. Using a customized in situ transmission

electron microscopy nanoindentation methodology, it is observed that cohesion of silicon and

adhesion of silicon and diamond are strongly modied by the sliding speed and the normal

stress applied during sliding. This indicates that shear stress modulates the reactivity of the

surfaces. This is the rst time that tunable adhesion of hard contacts has been demonstrated

in situ.

If sliding experiments are performed in ultra-high vacuum and the interfacial shear stress

is low enough to avoid surface modication, the Multibond model of friction predicts that

adhesion will decrease with increasing sliding speed in experiments with simultaneous sliding

and retraction. Results from sliding of nanoscale silica asperities against highly-oriented

pyrolytic graphite (HOPG) and hydrogen-doped tetrahedral amorphous carbon (a-C:H) surfaces

are consistent with this model. This contrasts with the directly-proportional adhesion-speed

behavior observed in the in situ transmission electron microscopy experiments of

silicon and diamond.

When the number of available bonding sites increases with stress and speed, adhesion

will increase. This is the case for the silicon-silicon and silicon-diamond work. However, if

the number of available sites is constant, sliding faster will further reduce adhesion. This

is the case of the work of silica sliding against HOPG and a-C:H.

Existing popular reduced order models for friction, the Prandtl-Tomlinson with temperature

model and the Multibond model, are frequently used to explain the observed nanoscale

phenomena of friction increasing logarithmically with sliding speed. However, both models

contain overgeneralizing or unphysical assumptions. A new model, the modied Multibond

model, was developed and is consistent with experimental results. This dissertation

provides strong evidence that damping is a critical parameter and that the Fokker-Planck

equation is more suitable to describe friction-speed behavior than the Prandtl-Tomlinson

with Temperature and Multibond models. The modied Multibond model also predicts the

decrease of adhesion with increasing speed observed experimentally in the silica-HOPG and

silica a-C:H experiments.

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