Diffusion of cobalt into a TiC layer during chemical vapour deposition and its effects on the cutting performance of TiC/Al 2O 3-coated cemented carbides

Abstract

It is assumed that during the chemical vapour deposition (CVD) process onto cemented carbides cobalt from the substrate can diffuse into the coated ceramic layer. It would be very interesting to determine the relation between the CVD conditions and the amount of diffused cobalt and how the cobalt influences the cutting performance of the coated carbides. CVD of TiC was carried out onto cemented carbides. The CVD temperature (1030 and 1100 °C), CH 4 concentration (0.9 and 1.81 min -1) and total gas pressure (140 and 760 Torr) were varied. Diffusion of cobalt from the substrate carbides into the TiC layer was measured with an ion microanalyser and an energy-dispersive X-ray analyser. Cobalt diffuses through grain boundaries in the TiC layer. The distribution of the cobalt across the TiC layer is uniform from the interface with the substrate to the top surface of the TiC layer. The mean cobalt content in a TiC layer about 5 μm thick ranges from 0.13 to 0.44 wt.% depending on the CVD conditions. It increases with increases in the temperature, the CH 4 concentration and the gas pressure. The cobalt content in the TiC layer is increased by subsequent Al 2O 3 coating by CVD while essentially no cobalt is detected in the Al 2O 3 layer. The effects of the diffused cobalt on cutting performance of carbides coated with TiC(5–6 μm)/Al 2O 3(1–2 μm) were investigated. Two types of carbide, W WC-6 wt.%TiC-9wt.%TaC-5wt.%Co(A) and WC-2wt.%TiC-2wt.%TaC-7wt.%Co (B) were used. The cobalt content in the TiC layer after Al 2O 3 coating varied from 0.71% and 0.38% for carbide A and from 0.60% and 0.45% for carbide B. In steel turning, flank wear of carbide A with a high cobalt content in the TiC (specimen AH) is less than half that of carbide A with a low cobalt content in the TiC (specimen AL). However, in the case of carbide B, equal flank wear was observed for high and low cobalt contents in the TiC layer (specimens BH and BL respectively). In cast iron turning, flank wear of the high cobalt specimens AH and BH is one-third to one-quarter of that for the low cobalt specimens AL and BL. A chipping test by interrupted turning with a feed-up method demonstrates that the high cobalt specimens AH and BH withstand a feed rate one step higher than the low cobalt specimens AL and BL do. The coating layers of specimens AH and BH show no visual damage after a peeling test by turning, whereas those of specimens AL and BL are peeled from the cutting edge. It is proposed that the reason the cutting performance of coated carbides is improved by diffused cobalt is that the cobalt is effective in increasing the mechanical and adhesive strength of the coating layer, while the negative effects of the cobalt on hardness, wear resistance etc. are negligible for the small amounts of cobalt in this experiment.