Tribochemistry
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VIRTUALLY ALL industrial devices contain surfaces
that are in relative sliding motion. Wear and friction are two major factors that limit the performance
of such devices, with significant economic and environmental consequences. For instance, the cost of
friction and wear is estimated to be several percent of the GDP in industrialized nations. In addition,
the energy losses associated with friction place higher demands on finite energy resources, and the
need to replace prematurely worn equipment leads to increased waste. Addressing these significant
problems requires the ability to better control friction and wear through the development of improved
lubricants. Doing this requires a better understand of the fundamental chemical and physical processes
that contribute to friction, wear and lubrication (together a field termed tribology). We are using
chemical simulations to gain a better understanding of the fundamental details of friction and wear, and
to help develop strategies for developing lubricants that exhibit improved performance under tribological
conditions.
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FRICTION AND wear have been studied for ages,
with ancient civilizations recognizing that using animal fats as lubricants made it easier to slide
objects past one another and Leonardo da Vinci discovery the basic relationships between friction
forces and load in the 15th century. These historical observations were based on a macroscopic
pictures of objects in contact, with the implicit assumption that surfaces were flat, and that
two surfaces in contact interacted over the entirety of the their macroscopic contact area.
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IN REALITY,
however, surfaces are covered with nanoscopic hills called asperities, and contact between two
surfaces corresponds to the points at which asperities on opposing surfaces come into contact. The
resultant real contact area is generally much lower than the apparent macroscopic area of
contact. Any loads applied, e.g. during sliding, are supported by the small real contact area, which
leads to extreme stresses and temperatures. As such, understanding the physical and chemical processes
associated with friction and wear involves determining the behaviour of materials and molecules
under these extreme conditions.
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Asperity contacts: Loads applied across surfaces are supports by the microscopic
areas where asperities come into intimate contact, leading to extreme stresses and temperatures
during sliding.
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OUR GROUP uses chemical simulation methods to
explore the properties of sliding contacts with the goals of gaining both a better understanding of
the atomic-level details of friction and wear, and to gain practical insights that can aid in the
development of improved lubricants, with an emphasis on the rational design of functional lubricants.
To do this, we use a combination of static quantum chemical calculations, first-principles molecular
dynamics simulations, and force-fields. We also develop phenomenological models to help explain the
underlying factors that control friction and wear, and to help guide practical design efforts by
providing insights regarding the relationships between atomic-level behaviour and parameters that
can be controlled in experiments.
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Sliding contacts: Sliding contact composed of two (0001) surfaces of
alumina separated by aldehydes and polyethers. First-principle molecular dynamics simulations of
this system indicate that static friction forces depend on the size of molecules used as lubricants.
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THE SIMULATIONS we perform to investigate
fundamental atomic-level factors relating to friction and wear generally involve using
first-principles molecular dynamics to simulate sliding contact of various forms. The structural
and chemical details of the contacts are varied and the consequent effects on friction and
wear are evaluated. The resulting data are then used to develop models that relate friction and wear
to fundamental the structural and chemical details of the surface. We have used these types of
simulations to provide insight regarding the dependence of friction on adhesive interactions
between the sliding surfaces (leading to an extending friction law), the dependence of friction
on the size of lubricant molecules, and the abilities of different lubricants to protect surfaces
from wear. Ongoing efforts are aimed at developing models that relate friction coefficients
to structural and chemical properties of interfaces, developing better ways to model sliding
contacts with first-principles molecular dynamics and quantify wear in those simulations, and
investigating the relationship between energy dissipation from a sliding contact and friction.
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APPLIED SIMULATIONS performed in this area
involve trying to gain basic insights into functional lubrication - the use of lubricant molecules
that respond to the extreme conditions in sliding contacts to alter their properties in a way
that is beneficial. To do this, we explore how molecules representing different chemical
functionalities respond to extreme stresses and temperatures and how the resulting changes in
properties affect friction and wear. The goal is to develop a large collection of information
that can be used to guide the selection of functional groups used in lubricants to rationally
design functional lubricants that respond and adapt to the specific environment in which they
function.
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Tribochemical reactions: Pressure induces the transformation of aldehydes into polyethers,
which is useful in the context of lubrication. Analysis of the associated changes in electronic structure
(via examination of maximally-localized Wannier functions, represented by orange spheres) sheds light on
the underlying reaction mechanism.
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