Tribological properties of surface engineered hot work tool steel for aluminiumn extrusion dies

A tribological test aimed at the simulation of aluminium hot extrusion was performed. A 6082-Al disc, induction heated up to a maximum surface temperature of 350 degrees C, is allowed to rotate against a conformal hot work steel block, simulating the extrusion die. The test simulates the wear mechanism observed under in service conditions. After an initial period, the hot plasticised Al progressively adheres to the steel counterpart thus allowing direct Al-Al contact. The high shear stresses given by the strong adhesion produce the nucleation of deep cracks which strongly deteriorate the surface of the steel by delamination. Hot work tool steel (AlSl H11) is employed as extrusion dies. Salt bath nitriding (Tenifer) is usually carried out to improve wear resistance. The possible application of PVD (CrN, TiAlN) and CVD (TiC+TiN) hard coatings, exhibiting lower compatibility versus Al, has been evaluated in the present work. The occurrence of two distinct damage regimes was detected. Regime 1 shows minor excursions of the friction coefficient mu and is followed by regime 2 which is characterised by fully plastic Al-Al contact, with large fluctuations of mu. A time to transition t(T) has been identified as indicative of the ability of a surface layer in delaying the occurrence of regime 2. t(T) is correlated to the chemical compatibility of the surface layer (compound layer, PVD or CVD coating) versus Al. Furthermore, t(T) is related to the mechanical stability of the surface layer. A generalised and severe form of wear is displayed by nitrided steel while a localised and less severe form of wear is displayed by duplex treated, nitricling + PVD steel. No wear traces were observed by CVD TiC+TiN coated steel, providing the best performance among the surface treatment investigated.