Periodic microstructures of Cr–O–N coatings deposited by arc ion plating

Coatings of chromium nitride (CrN) and Cr–O–N with varying O contents were prepared in a gaseous mixture of nitrogen and oxygen by arc ion plating, and their mechanical properties and microstructures were investigated. The oxygen content in the Cr–O–N coatings increased with the oxygen flow rate. When the substrate table was rotated during deposition, all Cr–O–N coatings exhibited a NaCl-type cubic CrN phase, up to an oxygen flow rate of 30%. Other phases such as chromium oxide (Cr2O3) were not detected. Field emission scanning electron microscopy observations revealed a columnar structure of the CrN coatings, whereas the Cr–O–N coatings have a very fine microstructure and develop nanometer-scale striped patterns, regardless of oxygen content. The period of the stripes depends on the rotation speed of the substrate table in the physical vapor deposition equipment. Transmission electron microscopy (TEM) clearly revealed the very fine periodic structure of the Cr–O–N coating. Moreover, scanning transmission electron microscopy and energy dispersive spectroscopy analyses revealed higher oxygen concentration in the striped patterns than in other areas. From TEM observations, the striped patterns were inferred to be amorphous Cr2O3. We determined that Cr2O3 formed when the substrate was largely separated from the vapor source, whereas cubic oxynitride formed when the substrate moved close to the vapor source.