Effect of chamber pressure on defect generation and their influence on corrosion and tribological properties of HIPIMS deposited CrN/NbN Coatings

It has been reported that compared to state-of-the-art technologies, High Power Impulse Magnetron Sputtering produces very dense and droplet free coatings due to the high plasma density and ionisation rate. However, thorough investigation of the coating morphology by Scanning Electron Microscopy, optical microscopy and other surface analysis methods revealed the existence of various types of coating defects. This study reports the influence of chamber pressure in particular on defect formation in CrN/NbN nanoscale multilayer coatings. The coating series was deposited using combined HIPIMS/UBM technique while varying the total chamber pressure from 0.2 Pa to 1 Pa. Four types of defects were identified, namely, nodular, open void, cone-like and pinhole. Defect density calculations showed that the coating produced at the lowest pressure, 0.2 Pa, had the lowest defect density of 0.84%. As expected coating corrosion properties improved linearly with decreasing defect density. Potentiodynamic polarisation corrosion studies revealed that in the potential range of - 300 mV to + 300 mV, the current density decreased with decreasing defect density (from 5.96% to 0.84%). In contrast, pin-on-disk tribology tests at room temperature demonstrated that the tribological properties of the coatings deposited at different chamber pressures were dependent on the crystallographic orientations and on the nature of the oxides formed at the tribological contact. Coatings with (200) crystallographic orientation had lower wear rates (  1.6×10-15 m3N-1m-1) whereas coating with (111) crystallographic orientation had the highest wear rate (2.6×10-15 m3N-1m-1). Friction properties were influenced by the tribolayer formed during the tribological tests. However, for the coatings deposited at same chamber pressure of 0.35 Pa but with different defect densities (due to the difference in chamber cleanliness), the friction behaviour was directly influenced by the coating defects. The friction co-efficient (μ) decreased by a factor of two from 0.48 to 0.25 when the defect density decreased from 3.18% to 1.37%.