Wear performance of (nc-AlTiN)/(a-Si3N4) coating and (nc-AlCrN)/(a-Si3N4) coating in high-speed machining of titanium alloys under dry and minimum quantity lubrication (MQL) conditions

Dry machining and minimum quantity lubrication (MQL) machining which are two types of green manufacturing processes have vast potentials for machining of titanium alloys. Tool coating is one key factor to realize high speed machining and green manufacturing. However, the tool coatings used presently in machining of titanium alloys cannot meet with the requirement of high speed machining and green manufacturing processes. The nanocomposite coatings can be alternativly applied in high-speed machining of α+β phase titanium alloy Ti-6Al-4V. In this research, the wear rate, wear pattern and wear mechanism of two kinds of nanocomposite coatings, (nc-AlTiN)/(a-Si3N4) and (nc-AlCrN)/(a-Si3N4), in dry and MQL conditions were investigated. Results showed that (1) MQL condition, which has cooling and lubricating effects, was found to have more significant influence in improving the tool life as compared to dry condition. Furthermore, the (nc-AlTiN)/(a-Si3N4) coated tool was confirmed to be more suitable for machining of titanium alloy than (nc-AlCrN)/(a-Si3N4) coated tool under MQL condition, which emphasizes the significance of matching between cutting fluids and coating materials. (2) The slower wear rate of (nc-AlTiN)/(a-Si3N4) coated tool in MQL condition was obtained than that of the (nc-AlCrN)/(a-Si3N4) coated tool. As a result, MQL condition can greatly prolong the tool life of (nc-AlTiN)/(a-Si3N4) coated tool while has minor influence on improving the tool life of (nc-AlCrN)/(a-Si3N4) coated tool. (3) Adhesive wear was observed to be the main wear type. The MQL technique not only has cooling and lubricating effects on nanocomposite coated tool, but also helps to form powerful protective layer. In addition, in MQL condition, the (nc-AlTiN)/(a-Si3N4) coated tool only suffered adhesive wear while the (nc-AlCrN)/(a-Si3N4) coated tool suffered adhesive, diffusion and oxidation wear. In contrast, when turning tests were performed under dry cutting condition, adhesive wear and oxidation wear might be the main wear types for the two coated carbide tools.