A Study of Green (Environmentally-Benign) Transfer Films through the Tribo-Comminution of Powder Pellets: Experiments and Modeling

Compacted pellets were formed from titanium dioxide (TiO2), molybdenum disulfide (MoS2), and tungsten disulfide (WS2) powders. Using a pellet-on-disk with slider pad tribometer, a series of tribo-comminution tests was conducted to ascertain the friction coefficient of a metal slider pad on a rotating metal disk covered with the powder transfer film. The purpose of the tests was to intentionally transfer third-body particulates to a tungsten carbide (WC) disk where its lubrication characteristics could be studied as it is depleted by a WC pad. It was shown that the pellet actually acted as a self-repairing, self-replenishing, oil-free lubrication mechanism. Results from the experiment were validated by developing an asperity-based fractional coverage (AFC) model, where the fractional coverage represents the fraction of the disk surface asperities that is covered by the lubricious transfer film. The AFC model was used to predict both (1) the friction coefficient at the pad/disk interface and (2) the wear of the powder pellet through comminution against the spinning disk. The disk surface topography was taken from an atomic force microscopy (AFM) image of a 50 microns by 50 microns patch. Using the AFC model, the wear rate (i.e., rate of pellet comminution) of the pellet and the coefficient of friction at the pellet/disk and slider pad/disk interfaces can be determined as a function of the pellet load, slider pad load, disk speed, and material properties. Results from the AFC model reasonably predict qualitatively and quantitatively the experimental behavior.