High temperature wear and friction behaviour of nitrided, PVD-duplex and CVD coated tool steel against 6082 Al alloy

The hot tribological contact between a surface engineered hot work tool steel and 6082 Al alloy was evaluated by means of a block on disc test configuration, under conformal contact. The steel was nitrided, duplex-PVD (CrN, TiAlN, TiCN) and CVD coated (TiC + TiN), following the typical processing conditions used for extrusion dies. Furthermore, different degrees of surface finishing were considered. The contact was characterized in terms of damage mechanism, friction behaviour and tendency towards Al pick up by the die material. The surface engineered steel showed better properties than base material in view of the higher load bearing capacity. This was particularly beneficial for duplex-PVD samples, combining high surface hardness with good stability against chemical dissolution by hot aluminium. The double layered CVD coating also showed excellent properties in view of the high interfacial adhesion, which was a limiting factor for duplex coatings. A lower resistance was highlighted by the compound layer in nitrided samples, due to the higher affinity of Fe to Al. This resulted in a heavy pick up and the formation of big hard particles on the die surface, very detrimental for the surface finishing of the extrudate. In coated samples, the friction diagrams evidenced a delayed transition towards an unstable contact regime, which could be associated to a lower tendency towards the formation of build up layer of aluminium. The positive influence of a better surface finishing was demonstrated, as well.