Frictional behavior of rolled surfaces coated with polymer films

Rolled aluminum sheet, coated with a lacquer or polymer film, is often subjected to forming operations. In some cases, the polymer coating incorporates an internal lubricant, which migrates to the surface during curing. The lubricants help to improve the runability of the coated sheet through forming operations such as stamping or deep drawing. In the present study, samples of rolled aluminum sheet were coated with polymer films of various thicknesses. The coating contained various concentrations of a wax lubricant additive. Two cooling regimes of coating application were studied. The cooling profile affects the mechanical properties of the coating, along with the amount of wax migrating to the surface. Friction testing was performed by means of the ball-on-flat friction test apparatus. It was found that the coating thickness has the most prominent effect on the coefficient of friction. In the case of thin coatings, an increase in wax content on the surface gave rise to the coefficient of friction. At intermediate polymer coating thicknesses, the coefficient of friction was found to be substantially independent of the wax level. However, for thicker coatings, increasing the wax level on the surface caused a drop in the coefficient of friction. These observations can be explained in terms of the indentation of the coating film by the ball, which affects the area of contact during the friction test. A model is proposed which takes into account both the coating thickness and wax lubricant level in a single analytical expression. This model is based on the concept of hydrodynamic lubrication. The modeling part of the present work was supported by study of the indentation of the coated sheet by a steel ball.