Wear of cermet cutting tools coated with physically vapour deposited TiN

Abstract

Physically vapour deposition coated cutting tools presently take a large share of the cutting tool market. Recently, coated TiC-based cermets also appeared on the market, exhibiting cutting behaviour which makes them competitive with coated WC-based hardmetals. The present paper discusses the results of a study devoted to the wear mechanisms of TiN (physically vapour deposited) coated TiC-based cermets. Static diffusion experiments were conducted in order to simulate the diffusion process across the tool-workpiece interface at high temperature and pressure. Additionally, experimental uncoated and TiN-coated cermets were tested in machining steel, and the depth profiles of the worm edges were analysed. The diffusion couples and the worm cutting tools were analysed by scanning electron microscopy, energy-dispersive spectroscopy and Auger electron spectroscopy in order to identify the dominant wear mechanisms. The results of cutting tests with the experimental inserts confirmed the beneficial effect of TiN coating on the tool life of cermet cutting tools. At high cutting speeds the wear of uncoated and coated cermet tools is mainly determined by diffusion across the tool-workpiece interface. A TiN coating a high concentration of structural defects can only partially hinder diffusion. The association of defects enables the diffusion of carbon and nitrogen as well as metal atoms through the apparently dense TiN layer. However, outward nitrogen diffusion may prevent carbide grain decomposition, resulting in extended tool life.