Wear protecting layers in cutting processes
S. Ulrich, C. Ziebert
Forschungszentrum Karlsruhe, Institut für Materialforschung I, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany
In order to increase the productivity since the 1960-ties coating materials are used in the machining industry, that are deposited by PVD (physical vapour deposition) and CVD processes (chemical vapour deposition). Despite this long period only a few well-established coatings such as TiN, TiC, TiCN, TiAlN, TiAlCN, CrN, CrAlN, TiAlCrN, Al2O3, amorphous carbon (a-C) and diamond as well as their combinations in classical multilayers cover a broad application spectrum in a quite general fashion. The friction coefficient of these coatings against steel is higher than 0.5 apart for the a-C and diamond coatings and the coating development was mainly focused on a single functionality, namely the enhancement of the wear resistance. However, the globalisation of the markets brings the need for new materials to increase productivity, cutting speed and tool lifetime, to improve the surface quality of the machined components and to develop dry cutting processes to omit environmental pollution by lubricants. Therefore new multifunctional nanolaminated coatings with tailored mechanical, tribological and chemical features should be developed, realized and optimized to dry cutting applications. Here, the challenge is to understand and control the mechanisms of wear, corrosion, phase changes and crack formation in the coatings and their interfaces in order to extend the lifetime of the cutting tools and to decrease pollution and increase productivity. This requires an interdisciplinary approach, which combines latest developments on multifunctional nanolaminated coatings with high performance analytics for determining composition, microstructure, residual stresses, nanoscale properties, and coating failure as well as with multiscale materials modelling. Therefore the most important goals in this field of R&D are i) the optimisation of new multifunctional nanolaminated coatings and their interfaces by an improved understanding of the interaction mechanisms between the layers in the interface regions and the influence of constitution and thermal stability of these interfaces on the structure, properties and machining behaviour of a coated tool in cutting applications. ii) the development of new nanoscale characterisation methods for residual stresses and defects with high lateral and depth resolution with the possibility of an in-situ measurement of this features during the cutting process and iii) a multiscale model that is able to cover all length scales from atomistic over mesoscopic to macroscopic scale and can describe both the coating deposition and properties and the local and global wear and defect formation during the cutting process.