Wear resistance of multilayered PVD TiN/TaN on HSS

For a given substrate material the tribological performance of thin hard coatings is mainly governed by one or several of the following parameters: coating/substrate hardness/stiffness, coating fracture resistance, coating adhesion, the contact temperature and chemistry (in the prevailing tribosystem). Thus, for a given application, an improved tribological performance, i.e. an increased wear resistance, can often be accomplished by increasing the coating fracture resistance while retaining its hardness or vice versa. It has been shown that a possible way to achieve this is to give the coating a multilayered structure, which may act as crack inhibitor. In fact, such a structure can even cause an increase in hardness, simultaneously to the improvement of toughness. PVD TiN/TaN multilayer coatings with three different multilayer periods (thickness of one TiN lamella together with one TaN lamella) of 10, 50 and 200 nm thickness, were deposited on high speed steel substrates. The morphology, the coating hardness, Young's modulus, residual stress, cohesion and adhesion to the substrate as well as abrasive and erosive wear resistance of the coatings were determined. The wear mechanisms in abrasion and erosion are discussed. Single layered TiN and TaN were included for comparison. It can be concluded that multilayering of TiN and TaN is a possible means to obtain a very wear-resistant PVD coating. A prerequisite is, however, that the lamella thickness is kept thin (11 nm or possibly less). A smaller multilayer period resulted in a slightly higher coating hardness and acted beneficially on the abrasive and the erosive wear resistance. For all coatings, there was a strong correlation between the hardness and the abrasive wear resistance. In erosion, where a high coating toughness is necessary in order to achieve a good wear resistance, the multilayered coatings displayed a much better wear resistance than both homogenous TIN and TaN. (C) 1999 Elsevier Science S.A. All rights reserved.