Mechanisms Controlling Friction and Adhesion at the Atomic Length-Scale

196 Pages·2017·5.63 MB·English

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Publicly Accessible Penn Dissertations
2015
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Xin Zhou Liu
University of Pennsylvania, [email protected]
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Part of the Mechanical Engineering Commons, and the Nanoscience and Nanotechnology Commons
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Liu, Xin Zhou, "Mechanisms Controlling Friction and Adhesion at the Atomic Length-Scale" (2015).
Publicly Accessible Penn Dissertations. 1086.
https://repository.upenn.edu/edissertations/1086
This paper is posted at ScholarlyCommons. https://repository.upenn.edu/edissertations/1086
For more information, please contact [email protected].
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A lack of understanding of the fundamental mechanisms governing atomic-scale adhesion and friction
creates ongoing challenges as technologically-relevant devices are miniaturized. One major class of
failure mechanisms of such devices results from high friction, adhesion, and wear. This thesis presents
investigations into methods by which atomic-scale friction and adhesion can be controlled. Using atomic
force microscopy (AFM), friction and adhesion properties of graphene were examined. While friction
between the tip and graphene depends on thickness, as explained by the â??puckering effectâ??,
adhesion is independent of the thickness when measured conventionally. However, adhesion is
transiently higher when measured after the tip has slid over the graphene. This effect is caused by
increased adhesiveness between graphene and tip due to aging. Second, chemical modification of
graphene, specifically fluorination, affects friction strongly, with friction monotonically increases with
increasing degree of fluorination. As supported by molecular dynamics (MD) simulations, this
dependence is attributed to the fact that attachment of fluorine to graphene greatly enhances the local
energy barrier for sliding, thereby significantly altering the energy landscape experienced by the tip.
Finally, through matched AFM and MD, the speed dependence of atomic friction was explored within the
framework of the Prandtl-Tomlinson model with thermal activation (PTT). For the first time, experiments
and simulations are performed at overlapping scanning speeds. While friction was found to increase with
the log of speed in both AFM and MD, consistent with the PTT model, friction in experiments was larger
than in MD. Analysis revealed that the discrepancy was largely attributable to the differences in contact
area and tip masses used in experiments vs. in simulation. Accounting for the overall influence of the two
with the presence of instrument noise fully resolves the discrepancy. Through those novel studies and
findings, it has been demonstrated that atomic-scale friction and adhesion can be controlled and
understood, assisting the development of applications where variable or constant friction and adhesion
are desired.
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Dissertation
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Doctor of Philosophy (PhD)
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Mechanical Engineering & Applied Mechanics
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Robert W. Carpick
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adhesion, contact mechanics, friction, graphene, speed dependence, two-dimensional materials
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Mechanical Engineering | Nanoscience and Nanotechnology
This dissertation is available at ScholarlyCommons: https://repository.upenn.edu/edissertations/1086
MECHANISMS CONTROLLING FRICTION AND ADHESION AT
THE ATOMIC LENGTH-SCALE

Xin Zhou Liu
A DISSERTATION
in
Mechanical Engineering and Applied Mechanics
Presented to the Faculties of the University of Pennsylvania
in
Partial Fulfillment of the Requirements for the
Degree of Doctor of Philosophy
2015

Supervisor of Dissertation
______________________
Robert W. Carpick
Professor and Chair, Mechanical Engineering and Applied Mechanics

Graduate Group Chairperson
______________________
Prashant Purohit, Associate Professor, Mechanical Engineering and Applied Mechanics

Dissertation Committee
Kevin T. Turner, Associate Professor, Mechanical Engineering and Applied Mechanics
Robert W. Carpick, Professor and Chair, Mechanical Engineering and Applied Mechanics
Vivek B. Shenoy, Professor, Materials Science and Engineering
MECHANISMS CONTROLLING FRICTION AND ADHESION AT THE
ATOMIC LENGTH-SCALE
COPYRIGHT
2015
Xin Zhou Liu
To my beloved wife, my parents, and my grandmother.

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ACKNOWLEDGMENTS
This thesis represents the work I performed in my more than five years at Penn. In
that time I have learned from and worked with many memorable individuals. I have spent
long hours and days in the laboratory, which was more than just a second home for me.
And I enjoyed it. With the completion of the thesis, I am glad to take this opportunity to
acknowledge people who have made the thesis and my research possible.
First and foremost I must thank my advisor, Professor Rob Carpick, for his
tremendous support, guidance, and mentorship for my research and during my graduate
studies. His positivity and words of encouragement kept me motivated in difficult times;
and his enthusiasm and curiosity for science made my times of success more joyful.
I would also like to thank Professor Kevin Turner and Professor Vivek Shenoy for
serving on my PhD dissertation committee, and for challenging me during the PhD
proposal exam and the final PhD defense. I would also like to thank my qualifying exam
committee members, Professor Paulo Arratia and Professor Pedro Ponte Castañeda, for
their time and help during my qualifying exam.
I am grateful to my collaborators for their useful discussions, Dr. Jeremy
Robinson and Dr. Paul Sheehan at the U.S. Naval Research Laboratory; Dr. Sang-Pil
Kim and Professor Vivek Shenoy at Penn and Brown University; Justin Ye and Professor
Ashlie Martini at the University of California Merced; and Professor Yalin Dong at the
University of Akron.
Many former and current group members have contributed to the work described
in this thesis. Particularly, I thank Professor Philip Egberts, Professor Qunyang Li, Dr.
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Prathima Nalam, Dr. Nitya Gosvami, Dr. Vahid Vahdat, Professor Tevis Jacobs, and Dr.
Graham Wabiszewski for helpful, useful, and interesting discussions. I would also like to
specially thank Professor Philip Egberts for his help with editing my thesis, and Dr.
Prathima Nalam and Dr. Rodrigo Bernal for their valuable comments on my scientific
presentations. I thank Zac Milne and Joel Lefever for his help with using the TEM. I
thank Qizhan Tam and James Hilbert for their help with MATLAB coding. I also
specially thank Qizhan Tam for his help with LabVIEW coding and machining of the D-
LFC fixture. I thank Kaiwen Tian for his interesting scientific discussions.
I would like to take the opportunity to thank the technical staff in the Department
of Mechanical Engineering and Applied Mechanics for their help of machining hardware
pieces for our instrument, Peter Szczesniak, Peter Rockett, Joe Valdez, and Terry Kientz.
I would also like to specially thank Peter Rockett for his great insights and promptness in
helping me.
I would also like to take the opportunity to thank the administrative staff in the
Department of Mechanical Engineering and Applied Mechanics for their help with
administrative requirements during my graduate studies, including Maryeileen Banford
Griffith, Sue Waddington-Pilder, Olivia Brubaker, Desirae Cesar, Nora Powell, and Peter
Litt. I would also like to thank Betty Gentner, Coordinator for Academic Affairs in the
School of Engineering and Applied Science, for her help my paperwork.
Use of the University of Pennsylvania Nano/Bio Interface instrumentation is
acknowledged. Use of the facilities of the Pennsylvania Regional Nanotechnology
Facility is acknowledged.
v
Finally, funding from the U.S. National Science Foundation under NSF/MRSEC
DMR-1120901, NSF/ENG CMMI-1068741, and CMMI-1401164 is gratefully
acknowledged.

vi
ABSTRACT

MECHANISMS CONTROLLING FRICTION AND ADHESION AT
THE ATOMIC LENGTH-SCALE

Xin Z. Liu
Professor Robert W. Carpick
A lack of understanding of the fundamental mechanisms governing atomic-scale
adhesion and friction creates ongoing challenges as technologically-relevant devices are
miniaturized. One major class of failure mechanisms of such devices results from high
friction, adhesion, and wear. This thesis presents investigations into methods by which
atomic-scale friction and adhesion can be controlled. Using atomic force microscopy
(AFM), friction and adhesion properties of graphene were examined. While friction
between the tip and graphene depends on thickness, as explained by the ―puckering
effect‖, adhesion is independent of the thickness when measured conventionally.
However, adhesion is transiently higher when measured after the tip has slid over the
graphene. This effect is caused by increased adhesiveness between graphene and tip due
to aging. Second, chemical modification of graphene, specifically fluorination, affects
friction strongly, with friction monotonically increases with increasing degree of
fluorination. As supported by molecular dynamics (MD) simulations, this dependence is
attributed to the fact that attachment of fluorine to graphene greatly enhances the local
energy barrier for sliding, thereby significantly altering the energy landscape experienced
by the tip. Finally, through matched AFM and MD, the speed dependence of atomic
vii
friction was explored within the framework of the Prandtl-Tomlinson model with thermal
activation (PTT). For the first time, experiments and simulations are performed at
overlapping scanning speeds. While friction was found to increase with the log of speed
in both AFM and MD, consistent with the PTT model, friction in experiments was larger
than in MD. Analysis revealed that the discrepancy was largely attributable to the
differences in contact area and tip masses used in experiments vs. in simulation.
Accounting for the overall influence of the two with the presence of instrument noise
fully resolves the discrepancy. Through those novel studies and findings, it has been
demonstrated that atomic-scale friction and adhesion can be controlled and understood,
assisting the development of applications where variable or constant friction and
adhesion are desired.

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