Growth defects in CrN/NbN coatings deposited by HIPIMS/UBM techniques

In recent years, high power impulse magnetron sputtering (HIPIMS) has caught the attention of users due to its ability to produce dense coatings. However, microscopic studies have shown that HIPIMS deposited coatings can suffer from some surface imperfections even though the overall number of defects can be significantly lower compared to, for example, arc deposited coatings of similar thickness. Defects can degrade the coating performance thus any kind of defect is undesirable. To better understand the nature of these imperfections and the science of their formation, three sets of chromium nitride/niobium nitride (CrN/NbN) coatings were deposited using HIPIMS technique combined with unbalanced magnetron sputtering (UBM) by varying the deposition parameters, i.e. deposition time (t = 15 to 120 min), bias voltage (Ub = - 40 to - 150 V) and chamber pressure (P = 0.2 to 1 Pa). For each set, one parameter was varied and other two were kept constant. All these experiments were carried out with chamber conditions close to those found in industrial environment. The study revealed that the generated defects were similar for all the coatings and with the increase in deposition time/bias voltage/chamber pressure the surface area covered by optically visible defects (surface defect density) was increased. These defects were categorised as flakes related defects (nodular, open void and cone-like defects) and defects associated with substrate pits (pinhole defects). Depending on their types, the defects influenced the corrosion and tribological properties of the coatings. As the origins of most defects were flakes (generated from the chamber components), an additional study was conducted to understand the influence of chamber cleanliness on defect generation. As expected, surface defect density of the coating produced in a comparatively clean chamber was reduced noticeably (from 3.18 % to 1.37 % after cleaning). Coatings with lower surface defects performed significantly well during corrosion and tribological tests. However, the comparison between pure UBM and combined HIPIMS/UBM deposited coatings suggested that along with the defects, coating structure also had a major role in corrosion, wear and friction mechanisms. Even for deposition conditions where HIPIMS coatings showed higher surface defects, owing to their microstructures, their corrosion resistance and tribological behaviour were superior to the UBM deposited coatings.