Thermally induced self-hardening of nanocrystalline Ti-B-N thin films

Abstract

Nanocrystalline films with high hardness have attracted increasing interest for wear resistant applications. Specifically, nanocrystalline Ti-B-N layers have been demonstrated to exhibit enhanced hardness and thermal stability. Here, we show that Ti-B-N films grown at 300°C and consisting of a high volume fraction, ∼50%, of a fully percolated disordered phase encapsulating 2-3 nm wide TiN and TiB2 grains, have a hardness of 37 GPa and an elastic modulus of 332 GPa which increase with annealing to 43 and 362 GPa, respectively, at Ta=800°C. The structural rearrangement which occurs during annealing results in the formation of compact interface boundaries which lead, in turn, to the observed hardness increase. Annealing at Ta > 900°C decreases the hardness, although the elastic modulus continues to increase, due to the combination of grain growth and B loss via the formation of volatile boron oxides and hydroxides. These conclusions, obtained based upon a combination of x-ray diffraction, nanoindentation, electron probe microanalysis, and transmission electron microscopy, are corroborated by calorimetric investigations. The overall results provide insight toward developing "design rules" for high-temperature superhard nanoscale based coatings. © 2006 American Institute of Physics.