Microstructural study of TiN-coated threading taps

Since the early 1980s TiN-coated high-speed steel (HSS) tools have become generally available in various cutting applications. Thin TiN-coating layers are mostly deposited by various physical vapor deposition (PVD) methods including ion plating, reactive evaporation, arc erosion, or sputtering. In this work commercially available TiN-coated taps were studied. Three series of different TiN-coated taps were chosen for cutting tests where both cast iron and quenched and tempered steel were used as work materials. Microstructural characterization of the coatings was mainly carried out using transmission electron microscopy (TEM). Nitrogen depth profiles were determined accurately using nuclear resonance broadening (NRB). The PVD TiN-coated tools showed distinctly different coating structures and thicknesses. Two of the coatings studied exhibited essentially stoichiometric TiN compositions throughout the whole coating layers. In the third coating the nitrogen content increased gradually, reaching the stoichiometric TiN in the immediate vicinity of the coating surface. These differences were clearly reflected in the microstructures of the coatings studied. The microstructural variations between individual coatings were, however, not clearly reflected in the performance of these coatings.