Influence of ion-to-neutral flux ratio on the mechanical and tribological properties of TiN coatings deposited by HiPIMS

Abstract

High power impulse magnetron sputtering (HiPIMS), due to its dense plasma and high ionization degree of sputtered material, enables to deposit high-quality thin films, with fully dense structure, low root mean square (RMS) surface roughness and superior hardness and wear properties. Operating the HiPIMS discharge with ultra-short pulses opens the possibility to control the degree of metal ionization and to tailor the microstructure of the growing film.In order to study the influence of ion-to-neutral flux ratio on the films microstructure, nanocrystalline titanium nitride (TiN) thin films were deposited on silicon substrates, at different degrees of metal ionization and reactive gas dissociation, by adjusting the HIPIMS pulse duration and operation mode (single- or multi-pulse). Energy resolved mass spectrometry and quartz crystal microbalance in combination with a two-gridded energy analyzer were used to determine the ion energy distributions, composition and fraction of ionized species in the substrate's vicinity. The properties of deposition flux were correlated to the topological, structural, mechanical and tribological properties (hardness, H, Young's modulus, E, adhesion/cohesion, kinetic coefficient of friction) of TiN thin films. Atomic force microscopy, X-ray diffraction, scanning electron microscopy, nanoindentation, scratch and tribological measurements were carried out in order to study the previously mentioned properties of deposited TiN thin films.According to the mass spectrometry results, the contribution of low and intermediate energetic parts (ions with energies below 20eV) to Ti+ ion energy distribution functions significantly increases during ultra-short and multi-pulse HiPIMS discharge. The highest ion flux fraction (up to 52%) was measured for the multi-pulse HiPIMS operation mode. The hardest coatings, with the highest H/E and H3/E2 ratios and lowest kinetic coefficient of friction were found to be the TiN thin films deposited by multi-pulse HiPIMS.