Synthesis of hard nitride coatings by ion beam assisted deposition

Ion beam assisted deposition (IBAD) has been used to deposit chromium nitride coatings using 1200 eV nitrogen ions from an RF-type ion source and thermally evaporated chromium. The ion/atom arrival ratio R was varied from 0 to 8 to modify the coating composition, microstructure, growth rate and stress state in order to optimize the properties of the material for use as a possible substitute for electroplated chromium in a number of anti-corrosion and tribological applications. The coatings were examined using Rutherford backscattering spectrometry and x-ray photoelectron spectroscopy, and contained up to 44 at % nitrogen in a mixture of bcc Cr-N, Cr₂N, and CrN. The microstructure of the coatings was examined by scanning electron microscopy, and the tribological behavior of the coatings was examined using an automated scratch testing and nanoindentation. XPS examination of the coatings indicates that nitrogen near the surface was bound to the metal as CrN and Cr₂N in most of the coatings studied by XPS, which is expected to significantly affect their corrosion behavior. The high R values needed to form large amounts of the CrN phase in the bulk of the coating causes significant sputtering during deposition. This study indicates that it is not possible to form a coating consisting solely of cubic CrN by IBAD under these experimental conditions (room temperature substrate and partial pressure of nitrogen of 1.8×10^-2 Pa). Nevertheless, the IBAD coatings produced were hard, in a compressive stress state, and highly adherent, all properties that make them candidates for use in selected Army applications.