LAUSR

ABSTRACT Metal forming involves relative motion between a tool and a workpiece under very high contact pressures, thus possibly leading to severe tool wear, compromising the process stability and the quality of the final product. Despite being fundamental for forming, friction must be controlled to ensure stable forming conditions and reduced energy losses, often achieved by coating the tools. This work analyzed the tribological behavior of commercial thin hard PVD coatings used in metalforming tools. Wear mechanisms were identified via scanning electron microscopy (SEM) and 3D topographic characterization before and after strip drawing tests, which were then correlated with friction coefficient measurements. Eight tool materials and three different coatings were tested: VN, CrN/TiN and TiAlN. The tools, characterized by SEM, optical interferometry and microhardness, were classified into four groups according to their manufacturing process: sintered, rolled, cast and recast. The VN coating was rougher, whereas CrN/TiN and TiAlN followed the substrate topography. The strip drawing tests showed that the VN and CrN/TiN coatings presented lower and more stable friction coefficient values, which was traced back to the formation of relatively smooth and partially oxidized transfer layers on the tool, leading to more favorable tribological conditions. The TiAlN coating presented higher and very unstable friction values, apparently due to protruding transfer layers, which increased friction and eventually led to galling. The best performance was attributed to the tools coated with CrN/TiN since the friction values were very stable along the tests. The tools coated with TiAlN presented the worst tribological performance, despite their initially smoother surfaces.