Tribochemical investigation of micropitting in rolling-element bearing applications: the influence of lubricant additives and water contamination

Bearings as a machine element are designed to facilitate relative motion. The expected life span for bearings in every each application, depending on operation conditions, is different. Whilst numerous efforts have been focused on increasing the bearing life-time, over the last few decades, premature failures have been reported especially in automotive and wind turbine gearbox bearings i.e.e.g. hydrogen embrittlement and micropitting surface fatigue. Furthermore, Zinc Dialkyl Dithio-Phosphate (ZDDP) as an anti-wear additive, which is crucial to supress destructive wear, accelerates micropitting. Moreover, water contamination is common in wind turbine gearboxes especially in off- and near-shore turbines which can further promote micropitting occurrence. Therefore, it is significantly important to understand the influence of ZDDP and water on micropitting and investigate the effect of low Sulphated Ash, Phosphorus and Sulphur (SAPS) additives capable of suppressing the micropitting-enhancing behaviour of ZDDP.
In this study a modified PCS instrument MicroPitting Rig (MPR) is utilised to perform micropitting experiments. The influence of ZDDP on micropitting is investigated over a range of different contact cycles and the tribochemical phenomena involved in the micropitting is addressed using X-ray Photoelectron Spectroscopy (XPS) and Energy-Dispersive X-ray (EDX) in a Transmission Electron Microscope (TEM). Employing wWhite lLight iInterferometry (WLI) a promising procedure for micropitting mapping is implemented in order to achieve a micropitting surface area in the wear scar. Experiments are undertaken in order to understand the impact of water as a contaminant on micropitting where ZDDP was used in the lubricant formulation. The influence of free and dissolved water on tribocorrosive micropitting is differentiated and clarified. A mechanistic understanding of effect of water and relative-humidity on micropitting is elucidated using XPS.
A range of organic Nitrogen-Containing-Additives (NCA) with a friction reduction capability is proposed to diminish the micropitting-enhancing behaviour of ZDDP and evaluated using MPR. The affinity of the functional group of NCAs, nitrogenN, to ZDDP which brings about a delay in the tribofilm formation, their friction modification properties, changes in the tribofilm composition and being an additive with no SAPS content were the rationale behind employing NCAs in combination with ZDDP. The tribofilm formation and frictional properties of the different lubricant formulations are examined using a mMini tTraction mMachine (MTM-SLIM). To gain a full understatingunderstanding of NCAs behaviours in combination with ZDDP, tribofilms are studied using XPS and EDX-TEM and aAtomic fForce mMicroscopy (AFM).
The results in this study showed that interaction of ZDDP with the steel surface under severe rolling-sliding contacts is the a influential derivativeprofound factor to induce micropitting. While dissolved-waterdissolved water increases micropitting nucleation and expands the nucleation across the wear scar, in free-waterfree water condition micropitting appearance is suppressed due to a dominant action of the mild wear. The chemical structure of NCAs significantly influences the tribological results. A careful selection of NCAs with a tailored concentration successfully reduces micropitting and protects the surface from wear simultaneously and thus NCAs are desirable additives in rolling-element bearing applications under certain conditions.