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 sur face treated by nitriding. In the present study both commercial and experim ental physical vapour deposition (PVD) coatings are evaluated in a device w hich simulates the bearing wear in an extrusion die. The tested surfaces in clude hot work tool steel. coated with TiN, CrN, (Ti,Al)N and TiB2, respect ively. To simulate the extruded material an aluminium cylinder heated to 55 0 degreesC is used as counter body in a block-on-ring configuration. For co mparison, the tool steel was also tested in uncoated conditions. Prior to w ear testing, the test surfaces were characterized by roughness, hardness, t hickness and intrinsic stress. It is shown that coatings of CrN. and especi ally (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 gradu ally degenerated during this test, probably due to poor chemical inertness when exposed to hot aluminium. (C) 2001 Elsevier Science B.V. All lights re served.