Influence of subsurface properties on the adhesion strength of sputtered hard coatings

Today's research activities covering the field of improving the properties of cutting tools are concentrated on optimising manufacturing technologies and tool geometry. as well as on improved alloying of special cutting mater ials and coating of tools. There is still a high potential to enhance wear resistance, especially in the area of coated cutting inserts. As a result of the reduced machinability of new cutting tool materials, hig h mechanical and thermal loads during grinding influence subsurface propert ies heavily. Hence, residual stresses with strong gradients are induced in the subsurface of the tools during manufacturing. Even with optimised coati ng parameters, deposited PVD coatings fail due to insufficient subsurface p roperties of the substrates. In this paper, the influences of residual stress distribution in subsurface layers on interface strength of PVD-coated carbides have been investigated . The investigations were carried out with WC-based cemented carbides coate d by PVD-deposited TiN and (Ti,Al)N layers. Topics which are considered inc lude the influence of grinding, micro-blasting and Rater peening of carbide s on surface topography and surface integrity. Dependencies between stress distribution in subsurface layers and interface strength, as well as effect s on wear behaviour in machining are highlighted. Surface properties of the tools are characterised by SEM and X-ray residual stress measurements. X-ray measurements were carried out by the well known sin(2)Psi method. Dep th profiles of residual stress before and after coating the cemented carbid e were determined by using different lattice planes and wavelengths which c orrespond to different penetration depths. Micro-blasted and water peened tools show improved interface strength in co mparison to ground carbides. Due to the increased interface strength of mic ro-blasted tools in final cutting tests: a superior wear behaviour in dry m achining was observed. (C) 1999 Elsevier Science S.A. All rights reserved.