Effects of nitrogen pressure on arc-evaporated TiN coatings

Abstract

This article addresses both fundamentals, such as the generation of metal ions and macroparticles (droplets), as well as more practical aspects, e.g. how different process parameters affect microstructure and mechanical properties of TiN films grown on powder metallurgical steel substrates by reactive arc evaporation. A commercially available Multi-Arc equipment has been used to deposit coatings using different nitrogen pressures, substrate-source distances and bias voltages during the growth stage. The microstructural characterisation, regarding droplet number density, surface roughness, changes in the strain and grain size of the deposited coatings, have been performed by scanning and transmission electron microscopy and X-ray diffraction. The influence of the nitrogen pressure on the growth rate, surface roughness and mechanical properties, such as adhesion and scratch resistance, have also been studied by using standard techniques. Only minor changes in the grain size, compressive strain, preferred orientation and adhesion were observed while varying the nitrogen pressure by a factor of ten. In this large process window, general trends such as an increased growth rate and a decreased droplet number density with increasing nitrogen pressure has been established, although these variations are different depending on the substrate-source distances. The increasing nitrogen pressure results in higher deposition rates for the TiN film but in lower ion current densities extracted by the negative substrate bias. This increase in growth rate is discussed in terms of a decreased resputtering effect owing to the observed decrease in ion current density. The major part of the droplets can be concluded to be created during the initial etching/heating stage and their number density decreases linearly with film thickness. An initial discussion regarding the results obtained and a phenomenological explanation of the possible reasons for the dependencies found is proposed.