Tissue Phase Affected Fracture Toughness of Nano-Columnar TiB2+z Thin Films

Abstract

Despite being intensively investigated and already established in industry as protective coatings for aluminium machining [1] or diffusion barriers [2], Titanium-Diboride (TiB2) is still not fully understood. In particular, for the fracture behaviour and the corresponding material characteristic (intrinsic fracture toughness, KIC) the literature presents only little descriptions. So far it is known that the mechanical properties (H and E) of TiB2 are highly dependent on the crystal orientation [3] and the amount of tissue phase present [4], which in turn can be influenced by the choice of coating parameters. To figure out, if this also affects the fracture behaviour, we synthesized a broad stoichiometric variation from TiB2.07 up to TiB4.42 by DC magnetron sputtering and thoroughly determined their structural, morphological and mechanical properties. By using micro-mechanical test set-ups (in-situ microcantilever bending tests) the intrinsic fracture toughness was investigated in relation to the chemical and hence morphological variation. In addition, information about the stress states within the thin films were obtained by nanobeam (synchrotron) experiments. The presence of a B-rich tissue phase was confirmed by HR-TEM analysis, showing that the grain size is decreasing with increasing B content, which is accompanied by an increase in tissue phase fraction. The dominance of the tissue phase is progressing with increasing B, also forcing smaller column sizes (from ~10 to < 5 nm) [5]. A change in Boron content from TiB2.22 to TiB4.42 leads to a decrease in fracture toughness from 3.55 ± 0.16 MPa√m to 2.51 ± 0.14 MPa√m of these superhard films. In summary, this study underlines the influence of stoichiometry and the potential of tissue phase engineering on the mechanical properties of TiB2+z based thin films.