Microstructure, mechanical properties and cutting performance of CVD Ti (B,N) coatings

TiB0.19N1.06, TiB0.70N0.66, and TiB1.23N0.19 coatings were deposited on cemented carbide via the chemical vapor deposition (CVD) method based on the calculated phase diagrams of the Ti-B-N system. The microstructures, mechanical properties and cutting performances of the Ti(B,N) coatings were systematically studied via X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), nanoindentation, etc. The coating compositions obtained via thermodynamic calculations show the same trend as the experimental values. The XRD and selected area electron diffraction (SAED) results of the coatings reveal that the phase assemblages transform from a dominant cubic structure of TiB(0.19)N(1.06 )to a hybrid of cubic and hexagonal structure of TiB0.70N0.66 and finally to a dominant hexagonal structure of TiB1.23N0.19. According to the XPS results, the TiB0.19N1.06 and TiB0.70N0.66 coatings consist of the crystalline phases TiN and TiB2 as well as amorphous BN and TiB, and the TiB1.23N0.19 coating mainly consists of the crystalline phases TiB2 and TiN. The hardness values of the TiB0.19N1.06, TiB0.70N0.66, and TiB1.23N0.19 coatings are 33.6, 35.1, and 39.1 GPa, respectively. The content of amorphous phase in the Ti(B,N) coatings decreases while the hardness increases as the B content increases and N content decreases. The TiB(0.19)N(1.06 )coating exhibits a composite structure of nanocrystallite embedded in amorphous matrix. The TiB1.23N0.19 coated tool prepared in this study demonstrates excellent performance in milling titanium alloy TC18 with a long cutting life up to 56.1 min, an increase of 9 % and 28 % compared to the TiB1.98 coated tool and commercial grade deposited with CVD TiB2 coating, respectively.