Wear analysis of tools in cold forging: PVD versus CVD TiN coatings

This paper proposes a new approach to the degradation of cold forging tooling. First, a mechanical analysis of a given forming process is performed. Contact pressure, plastic strain, sliding velocity and temperatures are computed at tool-workpiece interface. These contact conditions are then simulated on a specific friction test. The friction test involved in this work is dedicated to the simulation of hot and cold metal forming tribology. It involves a contactor, which creates a given plastic strain along the sample surface under given contact pressure, sliding velocity and temperature. The main results of that friction test are the Coulomb's coefficient of friction, contactor and sample surface roughness, chemical composition of the third body. In order to study the industrial degradation of the tooling in laboratory conditions, samples come from actual workpiece and contactors come from actual tools at various stages of their lifetime. This new approach is applied to quantify wear of PVD and CVD TiN coated AISI M2 tools used to form a screw head. Friction tests highlight the drift of the friction conditions at the contact interface due to tool surface deterioration. The results show that the CVD coating has to be used to improve the production of the screw head forging sequence. (c) 2005 Elsevier B.V. All rights reserved.