Molecular modelling of lubricant-solid interactions that control friction and wear
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Date
04/11/2022Item status
Restricted AccessEmbargo end date
31/12/2100Author
Prentice, Iain J.
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Abstract
Lubricants are used widely in industry to reduce the effects of friction and wear.
Lubricant formulations for engine applications are typically composed of a base oil
and various additives, each having a different function. This complexity means that
their properties are not well understood, and it is difficult to study lubricants while
they are in operation in an engine. The rheology of the base oils, which make up
the majority of the lubricant formulation, is particularly important as the lubricants
are exposed to a wide range of pressures and shear rates.
Recent high-pressure experiments at Edinburgh indicate that model base oils –
squalane and a poly-α-olefin (PAO) mixture – are no longer hydrostatic media above
roughly 1 GPa. Diamond-anvil cell measurements show that the hydrostatic limit
for these base oils is in the range of 0.74 to 1.24 GPa, as they appear to undergo
solidification. X-ray diffraction shows no Bragg peaks, suggesting that a glassy
structure, and not a crystalline structure, is formed. In this work, the structure,
dynamics, and thermodynamics of model base oils were studied using molecular-dynamics (MD) simulations. The equation of state, self-diffusion coefficient, viscos ity, and radial distribution functions were calculated over a wide range of pressures,
from 0.0001 to 10 GPa. The results show that molecular diffusion is essentially
arrested above about 0.1 GPa, which supports the hypothesis that the samples are
kinetically trapped in metastable amorphous-solid states. The shear viscosity is immeasurably large at very high pressures, but the coefficient governing its increase
from ambient pressure is in good agreement with the available literature data. Subtle changes in the short-range real-space correlations are related to a collapse of the
molecular conformations with increasing pressure, while the evolution of the static
structure factor shows excellent correlation with the available X-ray diffraction data.
Next, oleamide, an industrially relevant organic friction modifier (OFM), was introduced to the base oil systems, where they formed clusters both with and without
the presence of water. Taking a step towards engine conditions, these base oil and
OFM systems were placed under confinement between iron oxide surfaces. It was
observed that the oleamide would adsorb onto the surfaces and form a layer. If water was present, it would out-compete the oleamide for surface coverage, even being
able to remove pre-existing oleamide from the iron oxide. The oleamide would then
form a layer on top of the water. The free energy of adsorption for oleamide onto
iron oxide surfaces was computed using umbrella sampling, and agreed well with
experimental measurements. Computing association constants for oleamide showed
that, at the experimental conditions and concentrations, it would adsorb onto the
surfaces as monomers rather than in a cluster. The association constants were also
in excellent agreement with experimental measurements.
The final part of this work involved adding shear to the confined base oil/OFM
systems to simulate the movement of the engine during operation. Initial frictional
measurements using pure base oil systems correlated well with literature data from
both experiments and simulations, and the effects of the OFM and water contamination on friction were then explored. Oleamide was found to reduce friction in
most cases, being particularly effective for thin lubricating films under relatively
low-shear conditions (< 107
s
−1
). Additionally, water was shown to have a larger
effect on reducing friction than oleamide under a wide range of conditions. These
effects showed good qualitative agreement with experimental measurements. High-pressure simulations showed some evidence that the base oils will undergo the same
pressure-induced solidification that was observed in the initial bulk simulations.
This work has made steps towards building up a comprehensive picture of how typical base oils and organic friction modifiers interact under engine conditions, starting
from simple bulk systems and gradually adding in complexity to get closer to engine
conditions. The simulations have been extensively compared and validated with experimental measurements, showing consistently strong agreement. The simulation
methods described here are applicable to other lubricant components to continue
elucidating how these complex formulations work.