The effect of B and C addition on microstructure and mechanical properties of TiN hard coatings grown by chemical vapor deposition

Incorporation of additional elements into TiN grown by chemical vapor deposition (CVD) provides an opportunity to enhance the performance of hard coatings used for the protection of cutting tools in the demanding metal processing industry. Within the present work, the hardness, Young's modulus and fracture behavior of CVD coatings within the quaternary system Ti(B,C)N are correlated with their chemical composition and microstructure. TiN, TiBN, TiCN and TiBCN coatings were prepared in an industrial-scale thermal CVD plant by variation of the precursors. X-ray diffraction revealed the majority phase to be face-centered cubic within all investigated coatings, accompanied by small amounts of hexagonal TiB2 in both B-containing coatings and amorphous TiB in TiBN. Transmission electron microscopy unveiled grain refinement and an increment of structural defects, when B, C or a combination thereof is added to TiN. Complementary nanoindentation and micro-mechanical bending tests identified the quaternary TiBCN as the optimum choice with respect to hardness (32.2 +/- 1 GPa), Young's modulus (587 +/- 29 GPa) and fracture stress (8.5 +/- 0.4 GPa). Fracture toughness was superior in the B containing coatings compared to TiN and TiCN with the highest value observed in TiBN (5.0 +/- 0.3 MPa m(1/2)). A post-mortem elevation profile of micro beams' fracture cross-sections verified, that within the B containing coatings the crack is deflected more effectively normal to the crack propagation direction in comparison to TiN and TiCN.