Self-organized nanocolumnar structure in superhard TiB_2 thin films

摘要

TiB_2 thin films are well known for their high hardness which makes them useful for wear-resistant applications. Overstoichiometric TiB_2 deposited at 300℃ by nonreactive sputtering has been shown to exhibit superhardness (H ≥ 40 GPa), while the hardness of their bulk stoichiometric counterparts is ～25 GPa. We show, using high-resolution transmission electron microscopy, that Overstoiehiometric TiB_(2.4) layers have a complex self-organized columnar nanostructure. The ～20 nm wide columns, encapsulated in excess B and oriented along 0001, consist of a bundle of ～5 nm diameter TiB_2 subcolumns separated by an ultrathin B-rich tissue phase. The nanocolumnar structure, which is thermally stable to postannealing temperatures up to 700℃, 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 ～60 GPa.