Micro- and nanotribological characterization of molybdenum oxide based coatings on 100Cr6 bearing steel surfaces

Abstract

Dry lubricated bearings are used in applications that are exposed to high temperatures or other ambient conditions that prohibit the use of lubricants. Examples can be found in the chemical or food industries. To handle such conditions, a molybdenum based layer system was developed for the dry lubrication of rolling contacts. The molybdenum oxide layers are generated by Physical Vapor Deposition (PVD). By using a magnetron sputter cathode, it is possible to produce a PVD molybdenum oxide multiple layer system. In the preinvestigation phase, various parameters (power, sputtering time, oxygen mass flow, etc.) were used for the preparation in order to achieve optimum adhesion and material strength. In the current project phase, the coatings were qualified by applying microtribological methods. In a first step of qualification, the nano hardness and Young's modulus were investigated via nanoindentation. Additional nano scratch tests allow conclusions regarding the friction and elastic properties of the coatings. In the second step of qualification, the coatings were qualified by micro wear and scratch tests by applying a milli-tribometer. The setup allows the measurement of the frictional properties of a 100Cr6 (AISI 52100) ball against a coated counterpart under oscillating motion. Scratch tests were performed by applying a Rockwell diamond tip with a radius of 5 µm with forces of up to 1 N and scratch lengths of up to 20 mm. The properties of the coatings regarding the transition from nano to micro scale are observed and taken into account for the layer development. With these results it is possible to characterize the generated layers and to define the wear and the optimal parameters for the PVD process. After qualification the coatings will be applied to radial bearing surfaces. The intended use of dry lubricant coating systems on rolling bearings will be presented.

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.…”

“…It must be investigated whether the boundary conditions on the nano level are sufficient to form an oxide layer in the wear surface or whether wear particles are enriched with oxygen and act as a transfer lubricant. The Mo-reservoir has to be more wear-resistant than the MoO 3 top layer, for example, a higher hardness or strength in order to minimize wear in the contact surface [ 35 , 36 , 37 ].…”

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.…”

“…Walia et al examined the change in electromechanical properties in MoO 3 and MoS 2 via nanoindentation [18]. Tribological investigations on Mo and MoO 3 were carried out on the micro-level by Konopka et al, via sliding tests [19]. Experiments in the FE8 test rig with a coating system of Mo and MoO3 were carried out by Konopka et al The axial bearing washers were coated and the cylindrical rollers were uncoated.…”