Fatigue properties of Ti-based hard coatings deposited onto tool steels

During cold forming processes like deep drawing of steel sheets, tools are subjected to various loads. Besides adhesion, abrasion and tribo-oxidation, successive impacts during forming induce Hertzian contact pressures and cause fatigue failure. The aim of this work is to evaluate different hard coatings deposited by PACVD and magnetron sputtering onto tool steels with respect to adhesion and fatigue. The adhesion of TiN, Ti(C,N), (Ti,Al)N and TiB2 coatings deposited after different pre-treatments of the tool steels was characterized using the VDI indentation test. The fatigue behavior of coated surfaces was investigated employing a push–pull testing machine where cyclic loads of up to 80 kN are applied to the tool surface by cylindrical indenters. The number of cycles was systematically varied between 103 and 105. The fatigue progress was characterized by metallographic examinations using scanning electron microscopy. The results obtained are correlated with the residual stresses of the coatings and a simulation of elastic stresses for a Hertzian pressure distribution on a coated substrate.