Influence of B content on microstructure, phase composition and mechanical properties of CVD Ti(B,N) coatings

Within this work the effect of the B content on the microstructure, phase composition and mechanical properties of CVD Ti(B,N) coatings is investigated. Ti(B,N) coatings with B contents from 0 (fcc-TiN) to ∼5, ∼15, ∼30, ∼45 and 66 (h-TiB2) at.% have been deposited by CVD. The elemental composition of the coatings was confirmed by ERDA and their microstructure was investigated using XRD and SEM. With increasing B content, a transition from a fcc to a h-dominated structure via dual-phase fcc/h-Ti(B,N) was observed, which was accompanied by a decreasing grain size from the µm to nm range. Combinatorial use of Raman spectroscopy, XPS and APT measurements indicated B-rich grain boundary segregations and the formation of increasing amounts of h-Ti(B,N)2 clusters embedded within an fcc-Ti(B,N) matrix up to B contents of ∼30 at.%, while for ∼45 at.% B the matrix was predominantly composed of h-Ti(B,N)2. Complementary ab initio calculations predicting the phase formation confirmed the interpretation of the experimental results. In terms of the mechanical properties, nanoindentation measurements and micromechanical testing revealed a rise in hardness from ∼18 to ∼41 GPa and an increasing fracture stress and toughness from ∼7 to ∼13 GPa and ∼4.6 to ∼5.5 MPam1/2, respectively, by increasing the B content up to ∼30 at.%. In contrast, a significant drop in hardness, fracture stress and fracture toughness was observed at ∼45 at.% B. Thus it can be concluded, that both h-TiB2 and dual-phase fcc/h-Ti(B,N) coatings with maximized B content yield superior properties over TiN and consequently improved performance.