Shear-induced particle size segregation in composite powder transfer films

Transfer films have been engineered to act as solid lubricants between sliding surfaces in environments where fluids cannot exist or are not desired. Composite transfer films can be crafted to combine the low friction behavior of a powder lubricant, with another powder component that offers an alternative functionality, such as electrical conductivity or oxidation control. However, transient rises in friction often occur during sliding as shown by tribological testing of multi-component, powder lubricant transfer films both in this work and in the literature. In this work, the potential for an electrically-conductive composite film to maintain favorable tribological performance under shear is explored. The effects of the constituents' composition percentage and relative particle sizes are studied in thin, composite transfer films consisting of copper (Cu) and molybdenum disulfide (MoS2). The transfer films are formed through a shearing process and tested on a pellet and slider pad-on-disk tribometer. An interesting dependence of friction performance on equating the constituents' particle sizes is observed, which may be explained by the phenomenon of shear-induced particle size segregation often witnessed in granular flows. The effect of particle size segregation on the coefficient of friction (μ) in composite powder films is discussed within a quasi-hydrodynamic framework. Conclusions are drawn in regards to how particle size segregation would lead to the observed frictional phenomena for both the results obtained in this study, as well as results from prior studies within the literature.