Physical vapour deposition duplex coatings for aluminium extrusion dies

Duplex coatings [plasma nitriding followed by physical vapour deposition (PVD) coating] have been given a lot of interest as surface treatment for forming and cutting tools in recent years. The advantage, as compared to conventional PVD coating, is a higher substrate hardness due to the nitriding prior to the PVD coating. The present investigation aimed at studying the effect of duplex treatment as compared to conventional PVD coating and nitriding, respectively, for aluminium extrusion dies. The tested surface conditions were CrN, TiN and TiAlN, all single-layered and duplex-treated. A salt-bath-nitrided specimen was investigated for comparison and reference purposes. Wear tests were undertaken using an extrusion simulation device. Prior to wear testing, coating properties were evaluated by scratch testing, surface roughness, hardness and residual stress measurements. Wear tests indicated that a PVD coating reduced the wear rate by about one order of magnitude as compared with the salt-bath-nitrided specimen. Duplex treatment resulted in a further reduction of wear rate by two to four orders of magnitude. TiAlN was shown to be the most wear-resistant of the tested coatings, both as a duplex coating and as a single layer. In addition, a field study with CrN duplex-treated extrusion dies was performed. The tools were worn by delamination of the coating followed by severe wear of the exposed substrate. (C) 1999 Published by Elsevier Science S.A. All rights reserved.