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.
Highlights
The application of solid lubricants in tribological systems under fluid-free friction regimes enables a friction and wear reduction, through the separation of contacting surfaces and low shearing forces
A reason for this is the possibility of the in situ formation of a tribofilm composed of MoS2 and molybdenum oxides in the wear track
Based on tribological analysis in a PoD tribometer the distribution and the chemical composition of the tribofilm were analyzed by means of Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis
Summary
The application of solid lubricants in tribological systems under fluid-free friction regimes enables a friction and wear reduction, through the separation of contacting surfaces and low shearing forces. Erdemir divided solid lubricants into the five different groups metallic soaps and phosphates, polymers, oxide and fluoride compounds, soft metals and compounds with layered lattice structure [1]. A typical representative for the latter group is the transition metal dichalcogenides (TMD) molybdenum disulfide (MoS2) with a hexagonal lamellar crystal structure possessing strong covalent bonds within the planes and weak van der Waals forces between the planes. The use of M oS2 in technical applications can be realized by physical vapor deposition (PVD) coatings, powders or via lacquers. Analysis of lacquers and PVD coatings in tribological contacts in fluid-free friction regime showed an improved performance compared to the
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