Addition of Solid Oxide Particles for Friction Reduction

The paper carries out the smoothed particle hydrodynamics simulations of lubrication for the micronscale iron contact by the 20 types of the spherical solid oxide particles. The iron slabs and oxide particles of micrometer sizes are modeled by the elastic particle lattice coarse-graining and discrete element method, respectively. Motion of the iron particles is presented by the governing equations of the smoothed particle hydrodynamics approach. The interactions between the particle pairs such as iron-oxide and oxide-oxide ones are presented by the Hertz repulsive contact and van der Waals attractive noncontact forces. Furthermore, the oxide particle dissipates its kinetic energy into the environment containing it through the Stockes damping force. It is found that stability and shear of the tribofilm are the main factors influencing friction reduction in all the behaviors. The interactions between the oxide-oxide particles and between the oxide-asperity iron particles mainly contribute to formation and maintainability of the tribofilm. The oxide with the higher density or the heavier oxide particle results in the lower friction coefficient. The friction reduction also strongly depends on the water/diesel environment containing the particles; however, the relative lubrication ability among the oxides is almost independent of the environment. It is also found that in the contacts with the highly condensed situation of the oxide lubricant particles the van der Waals force can be neglected due to its very slight contribution to the detected forces and the rolling of the lubricant particles almost has no contribution to friction, in contrast to the strong influence of temperature of the environment.