Manipulation of coating and subsurface properties in reconditioning of WC–Co carbide cutting tools F. Klocke, T. Schroeder, E. Bouzakis ⁎ , A. Klein Laboratory for Machine Tools and Production Engineering (WZL) of RWTH Aachen University, Steinbachstr. 19, D-52074 Aachen, Germany Available online 23 June 2007 Abstract The physical properties in the surface layer of WC–Co cemented carbide tools have a significant influence on the tool wear behavior. In gear hobbing with carbide tools, the phenomenon that the tool life decreases with increasing number of reconditioning cycles has been identified. A significant correlation between residual stress in subsurface and tool life is shown as well as a steady decrease of residual compressive stress from cycle to cycle. This decrease can be linked to the thermal load of the cemented carbide substrate during the PVD coating process. Through micro blasting, the residual stress can be readjusted and tool lives improved. Besides these successes, disappointments have also been encountered. Since many parameters can and must be adjusted in the micro blasting process, blasting is likely to have no or even a negative effect if the set of parameters is not set properly. In order to get a better understanding of the aerosol micro blasting technology, fundamental studies on the influence of blasting parameters on the energy content of the aerosol have been conducted. In these studies, several phenomena have been identified. Finally, blasting of coated tools has also been applied successfully with regard to tool life increase. Through micro blasting compressive residual stress and increased hardness can be established in the cemented carbide substrate and the coating. Blasting of carbide tools prior to coating can improve the substrate properties through better clasping of the tungsten carbide grains. Moreover, blasting of the coating can improve its residual stress with regard to fatigue strength quality and cutting performance of the coated tool. © 2007 Elsevier B.V. All rights reserved. Keywords: Cemented carbide; PVD coatings; Surface layer; Tool reconditioning; Residual stress; Micro blasting 1. Introduction Unlike most cutting tools for standard milling and turning operations which are equipped with indexable or even single- use cutting inserts, full-carbide tools with larger volume and high degree of geometric complexity have a much longer life cycle, since they are reground and recoated many times before they are disposed. One of the most frequent applications of such tools is gear hobbing, where full-carbide cutting tools have started to compete with HSS tools some years ago [1]. Due to the beneficial influence of hard coatings in terms of obtainable tool life, the tools are recoated in every recondition- ing cycle despite the high additional effort that is required for shipping, decoating and coating. With WC–Co cemented carbide tools, however, an unfa- vorable phenomenon has been found in industrial processes. It is known that in many cases, there is a tendency of decreasing tool life with increasing number of reconditioning cycles [1,2]. This problem, which has been documented in industrial manu- facturing, has also been found in laboratory trials. Fig. 1 (upper part) shows the achieved tools lives of two carbide gear hobs for different reconditioning cycles. These hobs have been used for manufacturing of an automotive gear. The applied coating is a TiN film which has been deposited in an arc-PVD process. Laboratory trials by means of a bar turning test (lower part of Fig. 1) with coated cemented carbide tools could confirm the tendency of decreasing tool life. 2. Correlation between residual stress and tool life By analyzing the wear mode thoroughly and also consider- ing the technological system including tool reconditioning, the Available online at www.sciencedirect.com Surface & Coatings Technology 202 (2007) 1194 – 1198 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. Tel.: +49 241 8028471; fax: +49 241 8022293. E-mail address: E.Bouzakis@wzl.rwth-aachen.de (E. Bouzakis). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.06.023