Superhard NbB2-x thin films deposited by dc magnetron sputtering

We have deposited weakly textured substoichiometric NbB2-x thin films by magnetron sputtering from a NbB2 target. The films exhibit superhardness (42 ± 4 GPa), 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 – 10 nm) 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 a NbB2-x film sliding against stainless steel with a wear rate of 5*10 -7 mm 3 /Nm. Xray photoelectron spectroscopy results suggest that the low friction is due to the formation of a lubricating boric acid film.