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 19 th 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
Time Scale Static 1ps 1ns 1μs 1ms 1. Introduction a) Molecular Dynamics (MD) in tribology Classical MD is the cheapest atomic scale simulation method Length Scale 0.1nm 1nm 10nm >1μm 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 Coarse Graining Atomistic MD QM/MM Continuum (e.g. CFD)» Important tribological phenomena (e.g. shear thinning, stick-slip) QMC DFT 1 10 10 2 10 3 10 4 10 5 10 6 10 7 Number of Atoms Fig 1: Computational Simulation Methods - adapted from: Kermode et al., Multiscale Simulation Methods in Molecular Sciences, NIC Series, Vol 42, 215-228 (2009)
1. Introduction a) Organic Friction Modifiers (OFMs) Boundary Lubrication (low sliding speed/high pressure) high friction Polar Head Group Fatty Tail Hardy Model Fig 2. Generalised Stribeck Curve 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 Fig 3. Schematic of action of model organic friction modifiers Stachowiak & Batchelor, Engineering Tribology, Elsevier Inc., 2005
1. Introduction a) Research objectives 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
2. Methodology a) Simulation procedure 55 A Three OFM coverages: 4.32, 2.88, 1.44 nm -2 (max = 4.55 nm -2 ) OFM Film Hexadecane OFM Film Fig 4. (a) NEMD system set up, (b) OFM molecules simulated Surface (100) α-fe 2 O 3 (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)
2. Methodology a) Force-Fields V total = V stretch + V bend + V torsion + V VDW + V qq V VDW (r ij ) = 4ε ij σ ij r ij 12 σ ij r ij 6 V qq r ij = q iq j 4πε r r ij 2 Fig 5. Potentials included in classical empirically parameterised Force-Field Fig 6. a) All-Atom and b) United-Atom force-field representation of n-hexadecane
a) Preliminary squeeze out simulations Estimate hexadecane layer thickness at 0.5 GPa Fig 7. Variation in; (a) wall separation, (b) number of hexadecane molecules inside contact, with time 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)
b) Film structure - NEMD videos SA High (4.32 nm -2 ) 0.5 GPa, 10m/s 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
b) Film structure OFM z CoM and tilt angle Higher coverage larger z-com Fig 8. Variation in; (a) z CoM, (b) tilt angle, with OFM coverage Higher coverage lower tilt angle Saturated and unsaturated tail-groups similar z-extension GMS & GMO larger z-extension most significant at high coverage Tilt angle relatively independent of head and tail group type Good agreement with SFA and in-situ AFM experiments (Campen 2015)
b) Film structure atomic mass density profiles 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 Fig 9. Atomic Mass Density Profiles for: (a) GMS/GMO (b) SA/OA Z-unsaturated tail group similar structure
b) Film structure RDF and intermolecular hydrogen bonding 4.32 nm -2 2.88 nm -2 1.44 nm -2 C C CTT CTT Fig 10. RDF for SA and GMS at high, medium and low coverage 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
b) Film structure Velocity Profiles 4.32 nm -2 2.88 nm -2 1.44 nm -2 Fig 11. Velocity profile for SA at high, medium and low coverage 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)
c) Friction coefficient effect of OFM coverage 0.5 GPa, 10m/s 4.32 nm -2 2.88 nm -2 1.44 nm -2 Solid Amorphous (Yoshizawa 1992) Liquid Fig 12. Variation in friction coefficient with coverage 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)
c) Friction coefficient effect of sliding velocity 4.32 nm -2 2.88 nm -2 1.44 nm -2 Fig 13. Variation in friction coefficient with sliding velocity at high, medium and low coverage 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)
d) AA vs. UA Viscosity? Film Structure? Film Phase? Friction? 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)
d) AA vs. UA effect of OFM coverage 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
d) AA vs. UA effect of sliding velocity 4.32 nm -2 2.88 nm -2 Fig 15. Variation in friction coefficient with sliding velocity UA vs AA 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
d) AA vs. UA experimental comparisons Experimental (Campen 2012) NEMD (high coverage) Stearic acid AA Stearic acid UA 1E-01 1E-00 1E+01 1E+02 (1ms -1 ) (10ms -1 ) Fig 15. Variation in friction coefficient with logarithm of sliding velocity UA vs AA LHS experimental (Campen 2012), RHS high coverage NEMD results Experimental friction coefficients agree much better with AA simulations Further suggests that AA models necessary for OFM simulations
4. Conclusions 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 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
Acknowledgements 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.
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