EFFECTS OF NITROGEN PRESSURE ON ARC-EVAPORATED TIN COATINGS

This article addresses both fundamentals, such as the generation of me tal ions and macroparticles (droplets), as well as more practical aspe cts, e.g. how different process parameters affect microstructure and m echanical properties of TiN mms grown on powder metallurgical steel su bstrates by reactive are evaporation. A commercially available Multi-A re equipment has been used to deposit coatings using different nitroge n pressures, substrate-source distances and bias voltages during the g rowth stage. The microstructural characterisation, regarding droplet n umber density, surface roughness, changes in the strain and grain size of the deposited coatings, have been performed by scanning and transm ission electron microscopy and X-ray diffraction. The influence of the nitrogen pressure on the growth rate, surface roughness and mechanica l 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 obse rved while varying the nitrogen pressure by a factor of ten. In this l arge process window, general trends such as an increased growth rate a nd a decreased droplet number density with increasing nitrogen pressur e has been established, although these variations are different depend ing on the substrate-source distances. The increasing nitrogen pressur e results in higher deposition rates for the TiN film but in lower ion current densities extracted by the negative substrate bias. This incr ease in growth rate is discussed in terms of a decreased resputtering effect owing to the observed decrease in ion current density. The majo r part of the droplets can be concluded to be created during the initi al etching/heating stage and their number density decreases linearly w ith film thickness. An initial discussion regarding the results obtain ed and a phenomenological explanation of the possible reasons for the dependencies found is proposed.