WLI, XPS and SEM/FIB/EDS Surface Characterization of an Electrically Fluted Bearing Raceway

Abstract

Electrical bearing currents may disturb the performance of the bearings via electro-corrosion if they surpass a limit of ca. 0.1 to 0.3 A/mm2. A continuous current flow, or, after a longer time span, an alternating current or a repeating impulse-like current, damages the raceway surface, leading in many cases to a fluting pattern on the raceway. Increased bearing vibration, audible noise, and decreased bearing lubrication as a result may demand a replacement of the bearings. Here, an electrically corroded axial ball bearing (type 51208) with fluting patterns is investigated. The bearing was lubricated with grease lubrication and was exposed to 4 A DC current flow. It is shown that the electric current flow causes higher concentrations of iron oxides and iron carbides on the bearing raceway surface together with increased surface roughness, leading to a mixed lubrication also at elevated bearing speeds up to 1500 rpm. The “electrically insulating” iron oxide layer and the “mechanically hard” iron carbide layer on the bearing steel are analysed by WLI, XPS, SEM, and EDS. White Light Interferometry (WLI) is used to provide an accurate measurement of the surface topography and roughness. X-ray Photoelectron Spectroscopy (XPS) measurements are conducted to analyze the chemical surface composition and oxidation states. Scanning Electron Microscopy (SEM) is applied for high-resolution imaging of the surface morphology, while the Focused Ion Beam (FIB) is used to cut a trench into the bearing surface to inspect the surface layers. With the Energy Dispersive X-ray spectrometry (EDS), the presence of composing elements is identified, determining their relative concentrations. The electrically-caused iron oxide and iron carbide may develop periodically along the raceway due to the perpendicular vibrations of the rolling ball on the raceway, leading gradually to the fluting pattern. Still, a simulation of this vibration-induced fluting-generation process from the start with the first surface craters—of the molten local contact spots—to the final fluting pattern is missing.

Keywords

• bearing surface damage
• electric bearing currents
• rotor-to-ground current
• bearing voltage
• electrical wear
• electrically damaged bearing