Simulation of Lubricant Properties and their Interactions with Surfaces

Abstract

The behaviour of lubricants at operational conditions, such as at high temperatures and pressures, is a topic of great industrial interest. In particular, viscosity and the viscosity-pressure relation are especially important for applications and their determination by computational simulations is very desirable.

In this thesis we evaluate methods to compute these quantities based on fully atomistic molecular dynamics simulations which are computationally demanding but also have the potential to be most accurate. We tested several molecules that are used as lubricants, such as 9,10-dimethyloctadecane, main component of PAO-2 base oil, which was used as the main lubricant for our simulations. The methods used for the viscosity simulations are the Green-Kubo equilibrium molecular dynamics (EMD-GK), the direct computation of viscosity from shear during non-equilibrium MD (NEMD) and the use of confined NEMD, where the fluid is confined within explicitly defined iron oxide wall surfaces, at pressures of up to 1.0 GPa and various temperatures (40-150 degrees Celsius). We present the theory behind these methods and investigate how the simulation parameters affect the results obtained, to ensure viscosity convergence with respect to the simulation intervals and all other parameters. We show that by using each method in its regime of applicability, we can achieve good agreement between simulated and measured values. NEMD simulations at high pressures captured zero shear viscosity successfully, while at 40 degrees Celsius EMD-GK is only applicable to pressures up to 0.3 GPa, where the viscosity is lower. In NEMD, longer and multiply repeated simulations reduce the standard deviation of viscosity, which is essential at lower pressures.

Additionally, by using confined NEMD simulations, it was demonstrated that the film thickness of the fluid affects viscosity, and as we increase the number of lubricant molecules, we approach the viscosity value of the bulk fluid derived from NEMD simulations.

Another aspect of these methods is the choice of the utilised force field for the atomic interactions. This was investigated by selecting three different commonly used force fields. We have explored several methods for calculating viscosity and we obtained results of particular industrial interest.

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Identifiers

ORCID for Dimitrios Mathas: orcid.org/0000-0003-0116-0156

ORCID for Chris-Kriton Skylaris: orcid.org/0000-0003-0258-3433

ORCID for Ling Wang: orcid.org/0000-0002-2894-6784

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