Molecular Dynamics Simulation of Different Configurations of PAO Molecules in Shear Iron Plates†

doi:10.7503/cjcu20190043

Abstract

Abstract:

The viscosity, friction and distribution of linear and branched polyalphaolefin(PAO) lubricants molecules on sheared iron plates were investigated with molecular dynamics simulation at the molecular level. The simulation results show that compared to the linear PAO lubricants, the shear viscosity of branched PAO lubricants are less affected by the shear rate; the friction coefficient of branched PAO lubricants is less than that of linear PAO lubricant; due to the presence of side chains, the branched PAO molecules are difficult to spread on the surface of iron, resulting into the complex network structure between different adsorbed layers of lubricants and reducing the mobility of branched PAO molecules under the additional shear force. The network structure of lubricants on the surface represents one stable lubricant film, and prolongs the life of lubricants.

Key words: Lubricant molecular structure, Friction coefficient, Lubricant film stability, Molecular dynamics

CLC Number:

O641

TrendMD:

LIU Shasha, WANG Lin, YUAN Shiling, CAO Xiaorong. Molecular Dynamics Simulation of Different Configurations of PAO Molecules in Shear Iron Plates^†[J]. Chem. J. Chinese Universities, 2019, 40(7): 1472.

Figures/Tables 8

Fig.1 Side-view mode of lubricant oil confined between iron surfaces(A) and chemical structures of simulated lubricant molecules(B) The model shear speed is opposite for two iron surfaces.

Table 1 Forced field parameters between PAO and metal ions

Pairwise	σ/nm	ε/eV
Fe-CH₃	0.368	0.029
Fe-CH₂	0.350	0.021
Fe-CH	0.370	0.020

Fig.2 Snapshots of PAOpe(A), PAOpp(B) and PAOpd(C) after 500 ps of sliding at 10.0 m/sAdsorption layers are represented by gray balls.

Fig.3 Shear viscosity of lubricant oil changes with shear rate

Fig.4 Friction(A) and friction coefficient(B) between lubricant and iron changes with shear rate

Fig.5 Microscopic distribution of PAOpe(A), PAOpp(B) and PAOpd(C) molecules in systemsLeft: around ion surface. Number density percentage of central atom of the molecule PAO in three systems. The irons atoms are shown as purple CPK models, green balls for second ad-layer atoms, cyan balls for first ad-layer. Right: local amplifications of the adsorption conformation. The first ad-layer PAO molecules are shown as cyan sticks, green balls for second ad-layer atoms.

Fig.6 Top view distribution conformation of PAOpe(A, B), PAOpp(C, D) and PAOpd(E, F) molecules adsorption layer on iron surface (A), (C) and (E) The first adsorption layer conformations of PAO on iron surface(shown as cyan sticks); (B), (D) and (F) the second adsorption layer conformations of PAO on iron surface(shown as green balls).

Fig.7 Different conformation methods for adsorption of PAO on iron surface

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