Journal
SURFACE & COATINGS TECHNOLOGY
Volume 257, Issue -, Pages 295-300Publisher
ELSEVIER SCIENCE SA
DOI: 10.1016/j.surfcoat.2014.07.087
Keywords
Boride; Magnetron sputtering; Structure characterization; Mechanical properties; Friction; Tribological properties
Funding
- Vinnova (Swedish Governmental Agency for Innovation Systems) through the VINN Excellence Centre FunMat
- Swedish Foundation of Strategic Research through the Synergy Grant FUNCASE
- Knut and Alice Wallenberg Foundation
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We have deposited weakly textured substoichiometric NbB2-x thin films by magnetron sputtering from an 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 an NbB2-x film sliding against stainless steel with a wear rate of 5 x 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. (C) 2014 The Authors. Published by Elsevier B.V.
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