Tribological studies of materials and lubricants used in EVs powertrains under electrified conditions

Abstract

Electric vehicles (EVs) are of major interest in the automotive industry since they represent a potential solution for reducing greenhouse gases and propitiate a cleaner sourcing by the energy production sectors. Also, EVs may be appealing to the consumer by their operating advantages and higher energy conversion efficiencies over internal combustion engine vehicles (ICEVs). However, the operation of electric motors in EVs produce undesirable shaft currents due to different electrical hardware sources which can potentially reduce their powertrain performance and reliability. In fact, previous research has shown that induced shaft currents can cause premature failure problems in electric motors for different industrial applications. The most studied failures have been centered in rolling bearings, which fail due to accelerated wear and/or fatigue by stray currents. Although this problem has been already addressed, related failures persist in electrical machines. In particular, with the proliferation of EVs in this decade, this problem became relevant, as it can significantly impact on the overall performance of the EVs. Therefore, this thesis is aimed to study experimentally the tribological behavior of materials and lubricants from EVs powertrains under the influence of stray currents considering different critical operating conditions yielding and measurement of the coefficient of friction, lubricant film thickness and electric contact resistance (ECR). In addition, advanced characterization techniques such as optical microscopy, scanning electron microscopy, Raman and FTIR spectroscopy were used to study the wear mechanisms resulting from by the effects of electrification. Outcomes from this thesis concluded that the presence of shaft currents on sliding/rolling powertrain materials caused CoF alteration and a noteworthy increase of wear volumes. Mainly, these were ascribed to accelerated oxidation on the steel surfaces and also caused thermal degradation and oxidation of the lubricant used in the contact interface. Hence, gears in EVs powertrains may be exposed to alteration of performance, and additional fatigue and wear due to shaft currents produced by the EVs electrical hardware.