Solid-Film Lubrication of Galvanized Sheets

Drawbead simulation (DBS) tests were conducted on materials representative of cold-rolled, galvannealed, electrogalvanized, and hot-dip galvanized sheets used in the automotive industry, with paraffin wax, microcrystalline wax, stearic acid, tristearin, and Na-stearate solid films. Lubrication behavior did not follow a constant coefficient of friction model; friction declined with increasing sheet thickness but generally less than dictated by a constant shear stress model. Friction was affected also by the surface topography and hardness (deformability) of the sheet surface and was lower for surfaces of lower roughness and hardness. Microscopic valleys in the contact area of galvannealed and, particularly, electrogalvanized sheets indicated the development of micro-plastohydrodynamic lubrication. The effects of speed were attributable mostly to the strain-rate sensitivity of steel with a minor contribution by the lubricant film.