Characterization of Molybdenum Based Coatings on 100Cr6 Bearing Steel Surfaces

Abstract

Energy losses and friction locking are decisive factors in the conceptual design and sustainable realization of machine elements. Thus, the improvement of the tribological properties of rolling bearings by ceramic coatings on bearing surfaces represents a promising approach. These coatings are to be optimally adapted to the load case by minimizing the slip and resulting wear by rolling elements. For this purpose, molybdenum-based coatings were applied by means of magnetron sputtering in a vacuum atmosphere at controlled and adjusted oxygen partial pressure on 100Cr6 axial bearing washers. The effect of diffusing oxygen at near surface areas can be achieved during the physical vapor deposition (PVD) process itself as well as under adequate loading cases, so that a regenerative separation layer prevents high tribological wear at running surfaces. The generated layers were then characterized by high-resolution analysis with regard to morphology, attachment to the substrate and stoichiometry. The adjusted process parameters yielded pure molybdenum, as well as molybdenum oxide, dioxide and trioxide as a function of corresponding oxygen partial pressure. Scanning electron microscopy (SEM) was used for topographical evaluation, X-ray diffraction (XRD) for the characterization of stoichiometry and focussed ion beam cutting (FIB) for coating thickness determination. From selected surfaces, additional energy-dispersive X-ray spectroscopy (EDX) mappings were performed to quantify local oxygen contents at the border area of generated molybdenum layers. To record tribological characteristics, the layers were analyzed for their mechanical properties subsequently. Therefore, nanoindentational studies were carried out, which could provide information on the wear behavior in point contact in the form of nanoclay experiments. The results showed lower coefficients of friction for oxidized surfaces and thus a better resistance against sliding wear than uncoated specimen surfaces.

Characterization and Modeling of Nano Wear for Molybdenum-Based Lubrication Layer Systems

“…In terms of a reproducible and successful deposition of the coating on the steel substrate surface, the experience gained from previous investigations was taken into account [ 2 , 34 , 35 ]. Hardened rolling bearing steel AISI 52100 was used as the substrate for the coatings as cylindrical plates with a diameter of 14 mm and a height of 5 mm.…”

“…The considered self-regenerative molybdenum layer system was built up as shown in Figure 1 . As the substrate, the hardened bearing steel AISI 52100 was used, on which a 2 µm thick molybdenum (Mo) reservoir was sputtered using the physical vapor deposition (PVD) process [ 2 ]. The top layer of the system was molybdenum trioxide (MoO 3 ), which was applied as a very thin layer of approximately 100 nm.…”

Investigation of the Hardness Development of Molybdenum Coatings under Thermal and Tribological Loading

“…A Mo target with a purity of 99.95% was used, whereby the process' partial pressure of argon was constantly set to 2.2 × 10 −2 mbar during the deposition of the coating, following Deambrosis et al's work in which the highest layer hardness was achieved at this pressure with no detection of pores [12]. Based on experience from previous investigations and the known deposition rates, a layer thickness of 2 µm was aimed for, as this achieved the best layer adhesion [9,10,19]. In addition, silicon wafers were added to the process.…”

“…The current research project investigates the use of Mo-based solid lubricants for rolling bearings. Therefore, Mo-layer systems were produced by means of physical vapour deposition (PVD), in which the solid lubricant molybdenum trioxide (MoO 3 ) was generated from a Mo layer via tribo-oxidation [9]. In addition, various coating systems have been developed with a MoO 3 top layer, so that MoO 3 is directly available in the run-in phase.…”

“…While MoO 3 takes over the lubricating effect, the Mo reservoir ensures a continuous lubricant supply through tribo-oxidation. The layer system consisted of a 2 µm thick Mo reservoir and a 500 nm thick MoO 3 top layer [10]. The mechanical characterisation of the thin layers was carried out via nanoindentation.…”