(626g) Lubrication Mechanism As a Function of Film Thickness for a Lubricant-Particle System

We study the influence of film thickness (which we assume corresponds to the gap size between a friction pair) on the lubrication mechanism involving a particle-lubricant system. We used carbon microspheres (CMS) dispersed in a glycerol/water mixture (60 wt.% glycerol) plus sodium dodecyl sulfate (SDS) as the model microlubricant. CMS have the ability to act as load bearers and ball bearings to reduce friction. After preparing the base lubricant (glycerol/water/SDS), varying amounts of the CMS particles (synthesized via the hydrothermal process using glucose as a precursor) were added to obtain lubricant formulations of particle concentrations ranging from 1-6 mg/ml. We then tested these formulations on a tribometer with a pin-on-disc configuration by shearing at different rotation speeds. A simplified Stribeck plot was produced to determine the lubricating behavior at different lubricating regimes. We find that in the boundary lubrication or BL regime (low speed) where the gap size is smaller than the particle size, the ability of the particles to act as rolling entities is enhanced, as evidenced by the reduction in the friction coefficient in the BL regime for the carbon-based lubricant formulations versus the base lubricant. However, in the mixed lubrication or ML regime (intermediate speed) the gap size increases and the particles begin to lose their effectiveness as rolling elements. With a further increase in speed, the system achieves full-film or hydrodynamic lubrication (HL). Here all the particles in the microlubricant dispersion become redundant. This is because the gap size is now effectively larger than the particle size, allowing the thick lubricant film to do the lubrication. In short, with steadily increasing speed from the BL to the HL regimes, there is a transition from a pure rolling mechanism to full-film lubrication. We also proposed a new parameter called the modified specific film thickness, that takes into account not only the surface roughness of the tribopair but also the intrinsic perturbation resulting from the dispersed particles in the lubricant. This parameter allowed us to distinguish the lubrication regimes. We hope that the findings from this fundamental study will prove useful in the superior design of particle-lubricant systems in specific applications.