Runner-Up Magnetron Sputtered CrN_x Coatings as Alternative to Electroplated Hard Chromium

Abstract

The coating of engineering components with electroplated hard chromium is a widely used practice. These coatings are reasonably hard (~1000 HV) and are deposited in thicknesses of ~20 μm, having a fine microcracked structure. Physical vapour deposition (PVD) is an alternative technique, replacing the limited performance and environmentally unfriendly electroplating process (hexavalent chromium). CrN_x is the most promising PVD coating to replace electroplated hard chromium. In contrast to other PVD hard coatings, it can be deposited in thicknesses up to 50 μm and at deposition temperatures below 250°C. The unbalanced magnetron sputtering technique allows reasonably high growth rates, typically several micrometers per hour. In contrast to cathodic arc evaporation, unbalanced magnetron sputtered coatings are smooth, without the arc typical droplets. The deposition equipment used in the present study allowed a wide range of deposition parameters leading to insights in the growth of bcc Cr–N, hcp Cr₂N, and fcc CrN coatings. The following modifications were observed: a fine grained solid solution bcc Cr–N (1800 HV at 18 at.-%N) with a Young's modulus similar to that of steel; an under stoichiometric hcp Cr₂N phase (2200 HV at 24 at.-%N); a soft single phase stoichiometric hcp Cr₂N phase (1600 HV at 33 at.-%N); the established {110} textured fcc CrN phase (1800–2000 HV) and finally, a new hard fcc CrN phase (2700–3100 HV) obtained with high bias voltages ( >- 200 V) and nitrogen flow rates in excess of the argon flow. The latter coating promises excellent properties with respect to wear, friction and corrosion.