Polyurea greases play an important role in the operation of electric vehicles, and it is understood that their performance is principally determined by their microstructure. Consequently, developing a complete understanding of the structural characteristics of polyurea greases is crucial for the future engineering of new greases. One of the most widely used techniques for urea grease analysis is scanning electron microscopy (SEM). However, this requires the removal of the base oil component prior to analysis and hence does not give a true picture of the grease microstructure. Alternative techniques, including polarized optical microscopy (POM) and cryogenic SEM (cryo-SEM), do not require base oil removal, but fail to provide the chemical information required to definitively discriminate between the base oil and thickener components. In this study, we pioneered the use of a novel combination of native- and near-native-state chemical imaging techniques, namely, confocal Raman microscopy (CRM) and cryogenic secondary ion mass spectrometry (cryo-SIMS), to characterize a commercial polyurea grease sample. It was possible to detect peaks diagnostic of polyurea in CRM, and the urea species known to be present in the grease in cryo-SIMS. In the chemical imaging analyses, distinct particles of polyurea (∼250–280 μm2) were observed, broadly consistent with observations from POM of the same material. Such findings increase confidence that POM, the most straightforward native-state technique applicable for polyurea grease analysis, can be reliably used to understand the key structure-performance relationships that will ultimately guide the development of future polyurea greases.
Wang et al. (Sun,) studied this question.