Growth behavior of CVD Ti(C,N,O) coating on cemented carbide controlled by CO and the mechanism

In α-Al2O3-based chemical vapor deposition (CVD) coatings, Ti(C,N)- or Ti(C,N,O)-based layer exerts a significant influence on the crystal growth of α-Al2O3 and the performance of the coated cemented carbide cutting tools. To explore the reinforcement measures and the related mechanism of α-Al2O3-based coating, the effect of CO volume fraction from 0 to 8% on the growth behavior of Ti(C,N,O) on cemented carbide is systematically investigated. Both multilayer and monolayer Ti(C,N,O) coatings and Ti(C,O) intermediate were deposited at 830 °C in an industrial-scaled hot-wall CVD reactor. Experimental results show that CO reacts with TiCl4 in the H2 atmosphere, resulting in the formation of nanocrystalline Ti(C0.5,O0.5) with a slow growth rate. Thermodynamic calculation reveals that with the increase of CO fraction, the equilibrium partial pressure of HCN increases, which is a vital gas-phase for the formation of Ti(C,N,O). As a result, grain refinement and increase in the growth rate and oxygen content of Ti(C,N,O) coating are achieved by increasing the CO fraction and hence the amount of Ti(C0.5,O0.5) with strong reactivity. The growth mechanism of Ti(C,N,O) concerning the codeposition and their in situ reaction of Ti(C,N) and Ti(C,O) is proposed, and the related phenomena of preferential growth transformation and Cl residue in Ti(C,N,O) coating are discussed.