Understanding the Tribological Behavior of Graded (Cr,Al)N + Mo:S in Fluid-Free Friction Regime

Abstract

Components running in fluid-free friction regimes are exposed to harsh conditions leading to increased friction and wear. Thereby, the use of the solid lubricant molybdenum disulfide (MoS2) via lacquers, powders or physical vapor deposition (PVD) coatings enables a friction and wear reduction. However, the tribological performance is limited to low mechanical loads. A promising coating concept already proven for high mechanical loads is the incorporation of the triboactive elements Mo and S in wear resistant hard nitride (Cr,Al)N matrix. In this study the supply mechanism and transfer of the tribofilm build out of the toplayer of the triboactive coating graded (Cr,Al)N + Mo:S under high mechanical loads at humid air were analyzed. Here, the chemical composition of the tribofilm was determined by a combination of Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) and correlated with tribological analysis. The results prove a friction and wear reduction independent of Hertzian pressure in the area of 400 MPa ≤ pH ≤ 1300 MPa due to the formation of a MoS2 and molybdenum oxide MoxOy containing tribofilm, whereby the share of MoxOy dominates compared to MoS2. Based on the results the understanding of the effect of the MoS2 + MoxOy containing tribofilm on the tribological behavior is increased.