Comparison between different test methods for evaluation of galling properties of surface engineered tool surfaces

In forming operations, galling is a tribological process associated with pick-up of formed material by the surface of the tool and subsequent damage of the work material surface. In lubricated operations, it is associated to lubricant film breakdown. Currently the main focus in development of forming tools is on improving their galling resistance, mainly by including new forming tool steels, improving surface finish, and applying surface treatments and coatings. However, in order to qualitatively rank the galling resistance of material couples, a proper test method for galling evaluation is needed. During the years several methods for determining the galling resistance have been reported in the literature. In the present investigation three different test methods were compared and evaluated as to their ability to assess the galling tendencies of coated and un-coated forming tool steel. The study included the pin-on-disc configuration, a modified cylinder-on-cylinder test with a spring loading system, and a new load-scanning test method, which involves two crossed cylinders. It was proved that the load-scanning test rig is both a very simple and the most suitable method to evaluate galling properties of tool steels. It gives the capability to compare different tool steels, tool surface finishes and treatments, and hard coatings in terms of friction level, amount of material transfer, and critical load for galling initiation.