Tribological performance of (Cr,Al)N+Mo:W:Sg in fluid-free friction regime

Highly loaded contacts in fluid-free-friction regime of uncoated gears lead to early system failures due to frictional heat and wear phenomena such as overheating, fretting as well as wear. A possibility for reducing friction and wear is the incorporation of solid-lubricants such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2) by applying physical vapor deposition (PVD) coatings. For this purpose, a (Cr,Al)N hard matrix was modified with Mo, W and S in a graded coating architecture (Cr,Al)N+Mo:W:Sg. Furthermore, coating analyses as well as tribological analyses of (Cr,Al)N+Mo:W:Sg coated 16MnCr5E specimens were conducted in fluid-free friction regime by varying the Hertzian contact pressure between 400 MPa ≤ pH ≤ 1300 MPa against uncoated 100Cr6 counter parts in a pin-on-disc (PoD) tribometer. The results show a high friction and wear reduction independent from Hertzian pressures in fluid-free friction regime. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses of the formed tribofilm prove a chemical composition of oxidic and sulfidic bindings of Mo and W. W forms sulfide-bonded mixed oxide WOmSn but not WS2 opposite to Mo which forms MoxOy and MoS2 within the tribofilm. Based on these results, synergistic effects between WOmSn, MoxOy and MoS2 were assumed. •Wear and friction reduction in dry contacts independent from Hertzian pressures.•Mostly oxidic bindings in tribofilm responsible for wear and friction reduction.•W forms sulfide-bonded mixed oxide WOmSn but not WS2 in tribological contact.