Effects of (Cr,Al)N and (Cr,Al,Mo)N coatings on friction under minimum quantity lubrication

In today's mobility applications, efficiency is a key requirement. While in conventional mobility, increased efficiency contributes to reduce greenhouse gas emissions, in e-mobility every increase in efficiency translates also directly into increased battery range, which in turn contributes to reduced CO₂ emission. Therefore, different approaches like the reduction of lubricant viscosities and the reduction of lubricant quantities towards minimum quantity lubrication are pursued. A promising approach to meet the increasing demands are (Cr,Al)N and triboactive (Cr,Al,Mo)N coatings. Within this work (Cr,Al)N and triboactive (Cr,Al,Mo)N coatings were investigated at a twin-disk test rig in highly-loaded rolling-sliding contacts. The coatings were deposited by means of cathodic arc evaporation (CAE) on the case hardened steel (AISI 5115) and tribologically investigated under minimum quantity (MQL) lubrication with a low viscosity, sulphur and phosphorous doped, polyalphaolefine oil. Compared to the uncoated steel, the coatings lead to a higher system stability, which can be characterized by a stable friction behavior, and wear reduction. Tribochemical interactions between coatings and lubricant were analyzed by means of Raman spectroscopy and reveal the in situ formation of MoS₂. Furthermore, the coatings exhibit lower thermal conductivities compared to the uncoated steel, which might also affect friction reduction. The results show the high potential of (Cr,Al)N and triboactive (Cr,Al,Mo)N coatings under high tribological load.