Investigations on temperature-activated coatings for friction reduction during turning of titanium alloys

• Applicants: Professorin Dr.-Ing. Kirsten Bobzin; Professor Dr. Wolfgang Hintze
• Subject Area: Coating and Surface Technology ; Metal-Cutting and Abrasive Manufacturing Engineering
• Term: from 2019 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 422345568

In the research project the oxidation and diffusion behaviour as well as the application behaviour of vanadium-containing physical vapour deposition (PVD) hard coatings on cemented carbide substrates during turning of the difficult-to-machine titanium alloy Ti6Al4V were investigated. The results showed that CrAlVN with vanadium as triboactive element enables the formation of easily shearable oxide phases at operating temperatures of ϑ = 800 °C in tribological model tests. Supplementary, the formation of friction-reducing oxide phases in the area of the cutting edge on the tool coating was proven despite the use of cooling lubricant after the machining of Ti6Al4V. However, the investigations indicate that the number of oxides formed in the cutting process is too small to cause significant tool life increases. The temperature determination of the cutting temperatures on the chip and main flank surface could not be achieved in the current project due to strong adhesive wear. Therefore, a temperature determination during turning TiAl6V4 is necessary. Initial investigations with CrAlMoN coated tools showed a friction-reducing effect already at ϑ ≤ 600 °C. In addition, an improvement in tool life was achieved, which for the first time surpassed the tool life of the uncoated substrate used as state of the art. The reduced activation temperature might be the reason for this. Therefore, the knowledge of the cutting temperature is important to correlate the diffusion and oxidation due to the machining with the observations from the model tests and thus to derive influences on the damage mechanisms during machining. This results in a considerable need for research on CrAlMoN coatings.Main aim of the proposed follow-up project is to gain knowledge on the cause-effect relationships that can lead to a reduction of tool load in the machining processes of the difficult-to-machine titanium alloy Ti6Al4V with self-lubricating CrAlMoN coatings. For this purpose, CrAlMoN coatings with varying element content are deposited, followed by a characterization of coating and composite properties. Furthermore, the oxidation and diffusion behavior as well as the friction behavior will be investigated in model tests. Moreover, the cutting temperature during the turning of titanium is measured by means of a novel temperature measurement setup. Furthermore, coated cutting tools are used in turning tests and subsequently subjected to a detailed damage analysis.

• DFG Programme: Research Grants