Oxidation and wear behavior of CrAlMoN with varied Mo-content for cutting Ti6Al4V

Abstract

The cutting of difficult to machine materials such as titanium alloys is challenging for the machining industry. In case of the titanium alloy Ti6Al4V, the properties of the material cause high temperatures, mechanical loads as well as high frequency vibrations at the cutting edge, leading to premature tool failure. The use of uncoated carbide tools is very common for machining of Ti based alloys. However, temperature active, self-lubricating physical vapor deposition (PVD) coatings like CrAlMoN showed promising results to reduce friction and wear during turning of Ti6Al4V. In the present study, self-lubricating (Cr34Al41Mo25)N, (Cr29Al36Mo35)N and (Cr25Al31Mo44)N coatings were investigated on cemented carbide tools. These were deposited by a hybrid process combining direct current Magnetron Sputtering and High Power Pulsed Magnetron Sputtering. Coating morphology, thickness, chemical composition, indentation hardness and modulus at ϑ = 20 °C, ϑ = 200 °C, ϑ = 400 °C and ϑ = 600 °C as well as the oxidation behavior were analyzed. Moreover, wear development after cutting tests using a CNC-lathe was investigated. Independent of Mo-content, all coating variants possessed a dense morphology and a smooth surface topography, as well as a coating adhesion class of HF1 to the cemented carbide substrate in Rockwell indentation tests according to DIN 4856. With an increasing amount of Mo, heat treatment temperature and time, more self-lubricating molybdenum oxides such as MoO3 and Mo4O11 were detected by Raman spectroscopy. Therefore, the coating with the highest amount of Mo possessed the highest amount of molybdenum oxides. After cutting tests, molybdenum oxides were also found on the tool flank face by Raman spectroscopy. The level of flank wear land width decreased with increasing amount of Mo.