Microstructure, mechanical properties and cutting performance of super-hard Ti(B,C,N) coatings prepared by chemical vapor deposition method

Ti(B,C,N) coatings with different compositions by changing the gas flow rate of BCl3 are prepared at 850 degrees C and 15 kPa on WC-Co based cemented carbide inserts from a gas mixture of Ti & Oslash;, BCl3, CH3CN, N2 and H2 using chemical vapor deposition (CVD) method in a commercial industrial-scale hot wall reactor. The surface morphology changes from coarse faceted of TiC0.39N0.61 to fine-grained feature of Ti(B,C,N) and the crosssectional fracture transforms from columnar to featureless characteristic. As the BCl3 gas flow rate improves from 150 to 950 mLPM, a continuous increase of B and decrease of N content is obtained, the deposition rate enhances from 1.5 to 1.9 mu m/h while the grain size reduces from 14.2 to 6.1 nm and lattice constant decreases from 0.4286 to 0.4275 nm. The Ti(B,C,N) coatings consist of polycrystalline Ti(C,N) and tissue phase. B exists in the form of BN and TiB2. The incorporation of B to Ti(C,N) coating results in lattice distortions. A maximum hardness of 43.3 +/- 4.1 GPa is obtained for TiB0.19C0.37N0.44 coating, which is 50 % higher than that of TiC0.39N0.61 (28.7 +/- 0.8 GPa). Furthermore, the TiB0.19C0.37N0.44 coating shows 20-55 % longer tool life than the typical commercial grades coated with CVD TiB2 during wet milling on titanium alloy TC18.