New physical vapour deposition coatings applied to extrusion dies

The bearing surface of dies for extrusion of aluminium is subjected to very difficult wear conditions. To minimize wear and thereby obtaining a longer service life, these dies are typically made of hot work tool steel and surface treated by nitriding. In the present study both commercial and experimental physical vapour deposition (PVD) coatings are evaluated in a device which simulates the bearing wear in an extrusion die. The tested surfaces include hot work tool steel, coated with TiN, CrN, (Ti,Al)N and TiB2, respectively. To simulate the extruded material an aluminium cylinder heated to 550°C is used as counter body in a block-on-ring configuration. For comparison, the tool steel was also tested in uncoated conditions. Prior to wear testing, the test surfaces were characterized by roughness, hardness, thickness and intrinsic stress. It is shown that coatings of CrN, and especially (Ti,Al)N and TiB2 have the potential to effectively reduce the wear in extrusion dies. These coatings are mechanically, thermally and chemically stable when sliding against hot aluminium, in contrast with uncoated steel. It was also observed that the mechanical strength of the TiN coating gradually degenerated during this test, probably due to poor chemical inertness when exposed to hot aluminium.