Self-organized nanocolumnar structure in superhard TiB2 thin films

Abstract

TiB 2 thin films are well known for their high hardness which makes them useful for wear-resistant applications. Overstoichiometric TiB2 deposited at 300 °C by nonreactive sputtering has been shown to exhibit superhardness (H⩾40GPa), while the hardness of their bulk stoichiometric counterparts is ∼25GPa. We show, using high-resolution transmission electron microscopy, that overstoichiometric TiB2.4 layers have a complex self-organized columnar nanostructure. The ∼20nm wide columns, encapsulated in excess B and oriented along 0001, consist of a bundle of ∼5nm diameter TiB2 subcolumns separated by an ultrathin B-rich tissue phase. The nanocolumnar structure, which is thermally stable to postannealing temperatures up to 700 °C, inhibits nucleation and glide of dislocations during hardness indentation measurements, while the high cohesive strength of the B-rich tissue phase prevents grain-boundary sliding. The combination of these effects results in the observed superhardness of ∼60GPa.