Superhard NbB 2 −x thin films deposited by dc magnetron sputtering-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/)

Abstract

We have deposited weakly textured substoichiometric NbB 2−x thin films by magnetron sputtering from an NbB 2 target. The films exhibit superhardness (42 ± 4 GPa), previously only observed in overstoichiometric TiB 2 thin films, and explained by a self-organized nanostructuring, where thin TiB 2 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 NbB 2 phase allows a similar ultra-thin B-rich tissue phase to form between thin (5-10 nm) columnar NbB 2−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 NbB 2−x film sliding against stainless steel with a wear rate of 5 × 10 −7 mm 3 /Nm. X-ray photoelectron spectroscopy results suggest that the low friction is due to the formation of a lubricating boric acid film