Deposition and characterisation of multilayered PVD TiN/CrN coatings on cemented carbide

Abstract

In laboratory as well as in application tests, multilayered PVD coatings have shown enhanced mechanical and tribological properties as compared to today's single layered PVD coatings. A coating which has shown very interesting properties, such as high temperature stability and high fracture toughness, is multilayered PVD TiN/CrN. Our knowledge about the connection between the growth dynamics and the properties of this coating is, however, rather poor. Therefore, to further develop and optimise this coating, it is necessary to study the correlations between on the one hand the growth process, and on the other the microstructure, composition and mechanical properties of the coating.In this work growth rate, morphology, microstructure, chemical and phase composition, together with coating hardness, Young's modulus and fracture toughness of different multilayered PVD TiN/CrN coatings have been evaluated. All coatings have been deposited on cemented carbide substrates using a combination of reactive electron beam evaporation (Ti) and reactive d.c. magnetron sputtering (Cr). The influence of lamella thickness and different deposition parameters: substrate bias, magnetron sputtering power and nitrogen flow, on the above mentioned characteristics has been examined. In addition to the multilayered TiN/CrN coatings, homogeneous TiN and CrN have been included and compared.The investigation showed that a dense, fully cubic NaCl phase, multilayered PVD TiN/CrN coating with high fracture toughness can be deposited provided that the lamella thickness is kept less than 14 nm and 5 nm of TiN and CrN respectively. Thin lamellae seem to inhibit transformation from growth of the cubic NaCl phase to new phases, e.g. hexagonal β-Cr2N and metallic Cr. Furthermore, thin lamellae yielded a (200) preferred growth orientation while thicker lamellae generated a mixture of growth orientations. In addition, very thin lamellae must be deposited to obtain good fracture toughness. It was also found that it was necessary to use a negative substrate bias in order to obtain a high quality coating.