Superhard NbB2 −x thin films deposited by dc magnetron sputtering

Abstract

We have deposited weakly textured substoichiometric NbB2−x thin films by magnetron sputtering from an NbB2 target. The films exhibit superhardness (42±4GPa), previously only observed in overstoichiometric TiB2 thin films, and explained by a self-organized nanostructuring, where thin TiB2 columnar grains hinder nucleation and slip of dislocations and a B-rich tissue phase between the grains prevent grain-boundary sliding. The wide homogeneity range for the NbB2 phase allows a similar ultra-thin B-rich tissue phase to form between thin (5–10nm) columnar NbB2−x grains also for films with a B/Nb atomic ratio of 1.8, as revealed here by analytical aberration-corrected scanning transmission electron microscopy. Furthermore, a coefficient of friction of 0.16 is measured for an NbB2−x film sliding against stainless steel with a wear rate of 5×10−7mm3/Nm. X-ray photoelectron spectroscopy results suggest that the low friction is due to the formation of a lubricating boric acid film.