Tribological Characterization of PVD-Coatings

Abstract

In cold forging processes the high tribological and mechanical loads often lead to early tool failure. If the cause of failure is wear, an extension of tool life could be achieved by the application of PVD coatings. In industrial tools there is a great variety of load cases that causes wear. Therefore a lot of coatings with different properties are necessary, which need to be adjusted to the various applications. In order to adapt the coatings appropriately to the different processes, a sufficient characterization of the coatings is required. In this paper three new types of PVD coatings based on TiC, AlCrN and TiAlN and two reference PVD coatings based on TiN and TiAlCN are investigated. Since friction plays an important role for the tribological conditions between workpiece and tool, the friction factor m of the coatings is determined for different lubricants in the so called Double Cup Extrusion (DCE) test. A major influence of the lubricants and a minor influence of the coatings can be seen in the conducted tests. Furthermore wear behavior of the coatings is investigated with a so called combined Punching-Forward Extrusion (CPFE) test. In this test it is possible to test simultaneously coated dies and punches under realistic conditions. For the new coating (TiAlN), characterized by monolayer structure, there is wear debris visible in some regions of the die shoulder. The early occurrence of wear might be caused by the ductility of the coating or adherence problems. The other new coatings (TiC and AlCrN), which have a multilayer structure, are revealing excellent wear behavior. The high wear resistance of these two coatings can be explained with their high hardness in combination with a special layer composition.