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Non-equilibrium molecular dynamics
simulations of organic friction modifiers
James Ewen
PhD Student
Tribology Group (Shell UTC)
Imperial College London
Session 8B • Bronze 2
Lubrication Fundamentals
STLE Annual Meeting
19th May 2016
Contents
1. Introduction:
a) Molecular Dynamics (MD) in tribology
b) Organic Friction Modifiers
c) Research objectives
2. Methodology:
a) Simulation procedure
b) Force-Fields
3. Results:
a) Preliminary squeeze out simulations
b) Film structure
c) Friction coefficient
d) AA vs. UA
4. Conclusions
1. Introductiona) Molecular Dynamics (MD) in tribology
• Classical MD is the ‘cheapest’ atomic
scale simulation method
• But no reactivity information
(electrons not treated explicitly)
• In tribology, MD gives unique insight into:
» Nanoscale structure of lubricant and
additive molecule systems
» Complex friction behaviour
» Important tribological phenomena
(e.g. shear thinning, stick-slip)
Fig 1: Computational Simulation Methods - adapted from:
Kermode et al., Multiscale Simulation Methods in
Molecular Sciences, NIC Series, Vol 42, 215-228 (2009)
Length Scale
0.1nm 1nm 10nm >1μm
Tim
e S
ca
leS
tatic
1p
s1
ns
1μ
s1
ms
1 10 102 103 104 105 106 107 ∞
Continuum
(e.g. CFD)
QMC
DFT
Coarse
Graining
Atomistic
MD
QM/MM
Number of Atoms
• Boundary Lubrication (low sliding
speed/high pressure) high friction
• OFM polar head groups adsorb onto
surface
• Form monolayer – interchain Van Der
Waals forces between fatty tails
• Incompressible and prevents solid-solid
contact reduces friction
Polar Head Group
Fatty Tail
Fig 3. Schematic of action of model organic friction modifiers
Stachowiak & Batchelor, Engineering Tribology, Elsevier Inc.,
2005
Hardy Model
Fig 2. Generalised Stribeck Curve
1. Introductiona) Organic Friction Modifiers (OFMs)
• Confined NEMD simulations of OFMs
• Gain unique insight into:
Nanoscale film structure
Friction reduction mechanism
Relative performance of different OFMs
• Comparative studies, different:
Tail groups (Z-unsaturation)
Head groups (acid, amide, glyceride)
Surface coverages
Conditions – sliding velocity, pressure
Force-fields – AA vs. UA
1. Introductiona) Research objectives
55 A
• Surface (100) α-Fe2O3 (hematite) – harmonic potential (Berro 2010)
• Surface-OFM, Surface-Lubricant – Lennard-Jones and Coulombic potentials
• Lubricant and OFM molecules – (L-)OPLS All-Atom (Jorgensen 1996, Price
2001, Siu 2012)
OFM Film
OFM Film
Hexadecane
Fig 4. (a) NEMD system set up, (b) OFM molecules simulated
2. Methodologya) Simulation procedure
Three OFM coverages: 4.32, 2.88, 1.44 nm-2
(max = 4.55 nm-2)
Vtotal = Vstretch + Vbend + Vtorsion + VVDW + Vqq
Fig 5. Potentials included in classical empirically parameterised Force-Field
𝑉𝑉𝐷𝑊(𝑟𝑖𝑗 ) = 4𝜀𝑖𝑗 𝜎𝑖𝑗
𝑟𝑖𝑗
12
− 𝜎𝑖𝑗
𝑟𝑖𝑗
6
Fig 6. a) All-Atom and b) United-Atom force-field representation of n-hexadecane
𝑉𝑞𝑞 𝑟𝑖𝑗 =𝑞𝑖𝑞𝑗
4𝜋𝜀𝑟𝑟𝑖𝑗2
2. Methodologya) Force-Fields
• Vacuum added in x-y plane to allow hexadecane to be
squeezed out (Sivebaek 2003)
• Decreasing wall separation converges at equilibrium value
• Equilibrium wall separation increases with OFM coverage
• Equilibrium amount of hexadecane remaining inside
contact volume independent on OFM coverage (two layers)
Fig 7. Variation in; (a) wall separation, (b) number of hexadecane molecules inside contact, with time
3. Resultsa) Preliminary squeeze out simulations
Estimate
hexadecane
layer
thickness at
0.5 GPa
SA Medium (2.88 nm-2)
• High coverage solid-like films with well-separated confined hexadecane layer
• Medium coverage amorphous films which are significantly interdigitated
• Molecular tilt partially aligns with the sliding direction
• NEMD can gain unique insight into structure and friction of OFM the films
SA High (4.32 nm-2) 0.5 GPa, 10m/s
3. Resultsb) Film structure - NEMD videos
• Higher coverage lower tilt angle
• Tilt angle relatively independent of
head and tail group type
• Good agreement with SFA and in-situ
AFM experiments (Campen 2015)
• Higher coverage larger z-CoM
• Saturated and unsaturated tail-groups
similar z-extension
• GMS & GMO larger z-extension –
most significant at high coverage
Fig 8. Variation in; (a) zCoM, (b) tilt angle, with OFM coverage
3. Resultsb) Film structure – OFM zCoM and tilt angle
4.32 nm-2 2.88 nm-2 1.44 nm-2
• Layering of additive
and lubricant in z
• More interdigitation of
lubricant into OFM film
at lower coverage
• More interdigitation of
lubricant into OFM film
in acids than
glycerides
• Glyceride films slightly
thicker than acid
• Good agreement with
SFA and in-situ AFM
• Z-unsaturated tail
group similar structureFig 9. Atomic Mass Density Profiles for: (a) GMS/GMO (b) SA/OA
3. Resultsb) Film structure – atomic mass density profiles
• Higher coverage more solid-like film
(increased long-range order)
• Glyceride (green) more solid-like than acid (orange)
• Intermolecular hydrogen bonding (3 vs 1 HB per OFM)
• Explanation for lower interdigitation for glycerides films
4.32 nm-2 2.88 nm-2 1.44 nm-2
C
CTT
C
CTT
Fig 10. RDF for SA and GMS at high, medium and low coverage
3. Resultsb) Film structure – RDF and intermolecular hydrogen bonding
• High coverage: OFM molecules move with wall, clear slip planes between
OFM-hexadecane and hexadecane-hexadecane layers
• Medium coverage: slip plane less clear – viscous friction in interdigitated region
• Low coverage: more Couette-like velocity profile – similar to confined pure
hexadecane (Savio 2013)
4.32 nm-2 2.88 nm-2 1.44 nm-2
Fig 11. Velocity profile for SA at high, medium and low coverage
3. Resultsb) Film structure – Velocity Profiles
‘Liquid’‘Amorphous’‘Solid’
• High coverage: low friction – formation of solid-like film, interdigitation low,
facilitates slip plane between layers
• Medium coverage: high friction – interdigitation high, rearrangement slow
• Low coverage: intermediate friction – films more interdigitated, rearrangement fast
• Friction coefficient: OA ≈ SA > OAm ≈ SAm ≈ GMO > GMS (Campen 2012)
0.5 GPa, 10m/s 4.32 nm-2 2.88 nm-2 1.44 nm-2
Fig 12. Variation in friction coefficient with coverage
3. Resultsc) Friction coefficient – effect of OFM coverage
(Yoshizawa 1992)
• Friction increases linearly with logarithm of sliding velocity
• Predicted by shear-induced thermal activation theory (Briscoe 1982, He 2001)
• Observed experimentally for boundary friction of OFM films (Campen 2012)
• Medium coverage friction greater dependence on sliding velocity
• Experimental behaviour: saturated (high coverage) vs. Z-unsaturated (low coverage)
Fig 13. Variation in friction coefficient with sliding velocity at high, medium and low coverage
4.32 nm-2 2.88 nm-2 1.44 nm-2
3. Resultsc) Friction coefficient – effect of sliding velocity
3. Resultsd) AA vs. UA
• Compare SA film structure and friction results for AA vs. UA force-fields:
1. (L-)OPLS All-Atom (Jorgensen 1996, Price 2001, Siu 2012)
2. TraPPE United-Atom (Martin 1998, Clifford 2006)
• UA order of magnitude cheaper – lower sliding velocities accessible
• But UA known to under-predict viscosity of long-chain alkanes (Allen 1997)
Viscosity?
Film Structure?
Film Phase?
Friction?
• UA accurately represents OFM film
structure, however:
• UA much lower friction coefficient
than AA
• AA friction-coverage behaviour
agrees with experiment
(Yoshizawa 1993)
• UA friction-coverage behaviour
opposite of experimental trend
• For UA, interdigitation much less
critical to friction
• AA model necessary for accurate
simulations of OFM friction
Fig 14. Variation in SA friction coefficient with coverage – UA vs AA
3. Resultsd) AA vs. UA – effect of OFM coverage
• UA much lower friction coefficient at all speeds and coverages
• Larger difference between AA and UA at medium coverage - more interdigitation
• UA also captures logarithmic trend, but values well below experiments…
4.32 nm-2 2.88 nm-2
Fig 15. Variation in friction coefficient with sliding velocity – UA vs AA
3. Resultsd) AA vs. UA – effect of sliding velocity
1E+01 1E+021E-001E-01
(1ms-1) (10ms-1)
Experimental (Campen 2012)
• Experimental friction coefficients agree much better with AA simulations
• Further suggests that AA models necessary for OFM simulations
Stearic acid AAStearic acid UA
Fig 15. Variation in friction coefficient with logarithm of sliding velocity – UA vs AA LHS experimental (Campen 2012), RHS high coverage NEMD results
NEMD (high coverage)
3. Resultsd) AA vs. UA – experimental comparisons
• Constructed model to compare various
OFMs under different conditions
• Film structure varies significantly
depending on OFM type and coverage
• Substantial reduction in friction at high
coverage - slip plane
• Z-unsaturated OFMs equally low CoF to
saturated ones – experimental
differences due to lower coverage
• GMS outperforms other OFMs at all
coverages (H-bonding)
• Friction coefficient increases with
logarithm of sliding velocity
• AA force-fields critical to accurately
model OFM friction behaviour
4. Conclusions
Ewen, J., Gattinoni, C., Morgan, N.,
Spikes, H., Dini, D. Non-equilibrium
molecular dynamics simulations of
organic friction modifiers adsorbed on
iron oxide surfaces, Langmuir, 2016
Research funded by the EPSRC and Shell (CASE)
Many thanks to: Dr. D. Dini, Prof. H. Spikes, Prof. D. Heyes, Dr. C. Gattinoni
(Imperial), Dr. N. Morgan (Shell) and the computational chemistry group at Shell India
Private Markets Limited
All systems were constructed using the MAPS platform by Scienomics Inc.,
simulations were run in LAMMPS and visualisations were created using VMD.
Acknowledgements
References
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