A review of dry particulate lubrication: Powder and granular materials

Research efforts related to dry particulates in sliding contacts are reviewed. In the tribology community, there are primarily two types of dry particulate lubricants that are studied-granular and powder. Granular lubricants usually refer to dry, cohesionless, hard particles that transfer momentum and accommodate surface velocity differences through shearing and rolling at low shear rates, and collisions at high shear rates. Powder lubricants refer to dry, cohesive, soft particles that accommodate surface velocity differences mostly by adhering to surfaces and shearing in the bulk medium, in a manner similar to hydrodynamic fluids. Spanning the past five decades, this review proposes a classification system for the scientific works in the dry particulate tribology literature in terms of theory, experiments, and numerical simulations. It also suggests that these works can be further categorized based on their tribosystem geometry-annular, parallel, and converging.