Multilayered PVD Ti/TiN, TiN/NbN and TiN/TaN coatings - a tribological and mechanical comparison

Thin, hard, single layered PVD nitride coatings have been utilised by the industry for many years. However, since wear of coated tools often is initiated by coating fracture, there is a strong driving force for development of tougher coatings. The most efficient way to obtain an increased coating toughness is to deposit multilayered coatings. The basic idea is that the lamella structure will act as crack inhibitor and thereby increase the coating fracture resistance. In the present work three different multilayered PVD coatings, Ti/TiN, TiN/NbN and TiN/TaN, were characterised with respect to coating thickness, morphology, residual stress, hardness, fracture resistance, adhesion and resistance to abrasive wear. The multilayered coatings were compared to each other and to single layered TiN, TaN and NbN coatings. All coatings were deposited on PM high speed steel, using a commercial PVD R&D equipment and deposition processes developed in house. In general, single layered coatings displayed higher residual stresses than multilayered. The highest hardness (approximate to 3900 HV) was found for the TiN/TaN multilayered coating. Furthermore, all coatings displayed good coating quality.