Date of Award
Doctor of Philosophy (PhD)
Mechanical Engineering & Applied Mechanics
Robert W. Carpick
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
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.
Milne, Zachary Banks, "The Role Of Sliding Contact In Nanoscale Tribochemistry" (2018). Publicly Accessible Penn Dissertations. 3254.