Structure modification of magnetron-sputtered CrN coatings by intermediate plasma etching steps

PVD coatings often show defects like pores and pinholes caused by the growth mechanism inherent to the deposition process. Such defects — when going through from the top surface to the coating/substrate interface — can result in a markedly decreased corrosion resistance of the coating/substrate system if a less noble substrate material is coated with a more noble material like hard nitrides. Hence, the corrosion resistance of the coating/substrate system can be improved by ‘densifying’ the morphology of the coatings. This was proved with a ∼3-μm thick CrN film deposited by magnetron sputtering on high-speed steel substrates. The coating process was interrupted several times for an intermediate plasma etching process (−1000 V, 5 min, 1–16 steps). The coatings showed a multiple-layer structure in which the interfaces can clearly be distinguished. The corrosion resistance as characterized by electrochemical potentiodynamic and potentiostatic measurements increased with increasing number of etching steps. The ‘interfaces’ were additionally characterized by AES depth profiling. Concerning the mechanical properties, the hardness was reduced when the number of intermediate etching steps increased, while the critical load stays almost unchanged.