Corrosion behaviour of post-deposition polished droplets-embedded arc evaporated and droplets-free HIPIMS/DCMS coatings

In this study, the effect of metal rich core of the droplets on the corrosion properties of TiN, CrN and ZrN arc evaporated nitride coatings has been investigated and the corrosion properties of such coatings have been compared with droplet free, highly dense coatings grown by combined high power impulse magnetron sputtering (HIPIMS) and direct current magnetron sputtering (DCMS) technique. An industrial size Hauzer HTC 1000-4 system enabled with HIPIMS technology was used for the deposition of combined HIPIMS/DCMS coatings. The corrosion behaviour of the coatings was studied by potentiodynamic polarisation test (ACM instruments Gill AC potentiostat, -1V to +1V) using 3.5% NaCl solution. Initially, as-deposited arc evaporated coatings with an exposed surface area of 1 cm2 were subjected to corrosion. Then, the coatings were gently polished to expose the metal rich core of the droplets. Subsequently, fresh un-corroded area of the polished coating was subjected to corrosion with previously corroded area masked. It has been found that mechanical polishing considerably deteriorated the corrosion performance of arc coatings by forming more than one galvanic couple between the two parts (metal rich, nitrogen rich) of the same droplet itself or between the metal rich part of the and the adjoining coating/exposed substrate. It has been further demonstrated that the droplet free highly dense HIPIMS/DCMS coatings exhibited superior corrosion resistance as compared to the arc-evaporated coatings. Raman analysis was used to study the constituents of the corrosion products. Scanning electron microscopy (SEM, planar view) was used to examine the as-deposited and corroded coating surfaces to define morphological differences. Energy dispersive X-ray (EDX) analysis was done to study the composition of the coatings. Cross-section SEM and ball cratering techniques (CSEM calo wear tester) were used to measure the thickness of the coatings.