MT-CVD Ti(C,N,O) coatings controlled by CO: Microstructure evolution and its effect on the hardness

Incorporation of oxygen into Ti(C,N) coating by CO doping during moderate temperature chemical vapor deposition (MT-CVD) process is a promising strategy for enhancing the performance of cutting tools. In this work, Ti(C,N,O) coatings were prepared by MT-CVD in a hot-wall industrial-scale CVD reactor with increased CO volume fraction from 0 % to 8.0 % in the feed gas mixture. Microstructure evolution is revealed by high-resolution scanning electron microscopy, electron backscatter diffraction detector, and high-resolution transmission electron microscopy. Its effect on the hardness is investigated. The results show that as the CO fraction increases, the microstructure changes from columnar and large faceted grains to equilateral and nanoscale grains. An increasing density of plane defects including twins, especially Σ3 twin boundaries and stacking faults are identified in Ti(C,N,O) coatings with increasing CO fraction. The hardness of Ti(C,N,O) coatings increase from 26 GPa at 0 % CO to 34 GPa at 8.0 % CO, due to the grain refinement, solution strengthening, increased stacking faults, twins and {111} texture. A considerably lower rate of hardness increase is observed at CO fractions above 4.0 %, which is hypothesized to be related to grain boundary sliding, residual chlorine and excessive density stacking faults.