Chromium nitride coatings produced by ion beam assisted deposition

Chromium nitride is a candidate material for the replacement of electroplated chromium as a tribological coating in a number of Army applications. In this study, ion beam assisted deposition has been used to deposit chromium nitride coatings onto a variety of substrates, including carbon foil, silicon, and steel, using 1200 eV nitrogen ions from an r.f.-type ion source and thermally evaporated chromium. The ion/atom arrival ratio R was varied from zero to approximately 8 and was found to have a strong effect on the film composition, film growth rate and stress state. At a constant background pressure of nitrogen, the nitrogen content of the coatings increased from 0 to 44 at.% with increasing R, indicating the ion assist was necessary for the incorporation of nitrogen into the coatings. The increased R also resulted in a reduced deposition rate due to ion sputtering effects, however, so the formation of stoichiometric CrN films may be not be possible under these deposition conditions (room temperature substrate and a background nitrogen partial pressure of 1.8 x 10(-1): Pa). The microstructure and stress of the coatings varied from columnar and tensile (evaporated Cr with no ion assist) to compact and highly compressive (deposition with ion assist). The fracture behavior of the films was examined using an automated scratch tester and scanning electron microscopy; tensilely stressed Cr films showed a tendency to crack and delaminate from silicon substrates, while compressively stressed CrxNy films did not crack easily and were more adherent.