Physical vapour deposition duplex coatings for aluminium extrusion dies

Duplex coatings [plasma nitriding followed by physical vapour deposition (P VD) coating] have been given a lot of interest as surface treatment for for ming and cutting tools in recent years. The advantage, as compared to conve ntional PVD coating, is a higher substrate hardness due to the nitriding pr ior to the PVD coating. The present investigation aimed at studying the eff ect of duplex treatment as compared to conventional PVD coating and nitridi ng, respectively, for aluminium extrusion dies. The tested surface conditio ns were CrN, TiN and TiAlN, all single-layered and duplex-treated. A salt-b ath-nitrided specimen was investigated for comparison and reference purpose s. Wear tests were undertaken using an extrusion simulation device. Prior t o wear testing, coating properties were evaluated by scratch testing, surfa ce roughness, hardness and residual stress measurements. Wear tests indicat ed that a PVD coating reduced the wear rate by about one order of magnitude as compared with the salt-bath-nitrided specimen. Duplex treatment resulte d in a further reduction of wear rate by two to four orders of magnitude. T iAlN was shown to be the most wear-resistant of the tested coatings, both a s a duplex coating and as a single layer. In addition, a field study with C rN duplex-treated extrusion dies was performed. The tools were worn by dela mination of the coating followed by severe wear of the exposed substrate. ( C) 1999 Published by Elsevier Science S.A. All rights reserved.