(585c) Force Fields for the Prediction of Transport Properties of Lubricants at Extreme Conditions

Lubricants are exposed to extreme conditions in tribological contacts; e.g. pressures in rolling bearings can be as high as 4,000 MPa. For such conditions, no experimental data on the density and the transport properties of the lubricants are available. However, information of these properties is needed for the design of the lubricated contacts. The currently applied strategy to solve this problem is to extrapolate experimental data obtained at mild conditions, which is obviously risky. Therefore, in the present work molecular models of lubricants were investigated regarding their ability to predict data that is relevant for the design of lubricated contacts at extreme pressures. Five different hydrocarbons were studied: three linear alkanes (decane, eicosane and triacontane) and two branched isomers of triacontane: squalane and 1-decene-trimer. The latter is a common component of poly-α-olefine (PAO) based oils. Molecular models from the literature were selected and compared to experimental data of different thermophysical properties in the range where such data were available. Molecular models of different types were considered: all-atom, united-atom, coarse-grained, as well as reactive force fields. They cover a wide range of complexity of modeling molecular interactions. Equilibrium molecular dynamics simulations were carried out to obtain data for the density, shear viscosity, thermal conductivity and the self-diffusion coefficient. Some of the studied force fields yield remarkably good predictions. The study enables the choice of appropriate force fields to determine transport properties of long hydrocarbons.