Self-organized nanocolumnar structure in superhard TiB2 thin films -: art. no. 131909

TiB2 thin films are well known for their high hardness which makes them useful for wear-resistant applications. Overstoichiometric TiB2 deposited at 300 degrees C by nonreactive sputtering has been shown to exhibit superhardness (H >= 40 GPa), while the hardness of their bulk stoichiometric counterparts is similar to 25 GPa. We show, using high-resolution transmission electron microscopy, that overstoichiometric TiB2.4 layers have a complex self-organized columnar nanostructure. The similar to 20 nm wide columns, encapsulated in excess B and oriented along 0001, consist of a bundle of similar to 5 nm diameter TiB2 subcolumns separated by an ultrathin B-rich tissue phase. The nanocolumnar structure, which is thermally stable to postannealing temperatures up to 700 degrees 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 similar to 60 GPa. (C) 2005 American Institute of Physics.