The structure of grease via electron microscopy and image analysis

Abstract

Many of the insolubles that are essential to the structure of lubricating greases are too small to be usefully imaged in the optical microscope. The higher resolution of the electron microscope is necessary to examine the thickener and additive morphology that contribute so much to the performance of modern greases. However, the high vacuum requirements of these instruments mean that the oil component of grease should not be introduced into the microscope at ambient temperatures. Techniques have been developed to remove the oil prior to examination, or to reduce its vapour pressure to a negligible level. A number of such techniques which have proved to be of value for both the scanning (SEM) and transmission (TEM) microscopes are described and illustrated. The distribution of the thickener fibres can now be measured and analysed by an image analysis procedure from the images of freeze‐fracture TEM replicas.

Graphene-Reinforced Lithium Grease for Antifriction and Antiwear

“…But the most common problem is linked to the vacuum environment of the mentioned equipment, where the grease sample is required to be submitted either to a cryogenic treatment or to oil removal. In many studies, the oil that obstructs the view of the underlying structure of soap is eliminated. − Since grease contains 80–95% oil, it is often argued that the thickener structure may be distorted if oil is washed out and that such a micrograph is ambiguous . On the contrary, the AFM technique allows us to define the arrangement of thickener structure in the presence of the base oil, thus considering the thickener–oil medium interactions, although generally with lower image resolution than SEM. , …”

“…Thickening agent plays an important role in both the rheological and tribological properties of the grease; thus a large number of thickeners such as fatty acid soap of lithium, sodium, calcium, barium, and aluminum are most commonly used and being developed. − That is why it is not always sensible to ascribe the frictional and antiwear behavior to a single component of grease, but it is usually determined by the interactive forces between grease elements, e.g., base oil, thickener, and additives, which in turn are strongly governed by the specific operating conditions, e.g., load and speed . Micro- and nanoscale structures of these thickeners are revealed by scanning (SEM) and transmission (TEM) electron microscopy methods in the form of fibers, spheres, and platelets, with each of them endowing different consistency and aimed at different mechanical applications. − …”

“…9 Micro-and nanoscale structures of these thickeners are revealed by scanning (SEM) and transmission (TEM) electron microscopy methods in the form of fibers, spheres, and platelets, with each of them endowing different consistency and aimed at different mechanical applications. 10 However, with the rapid development of industrial equipment, the performance of greases is still inadequate and fails to prevent direct contact in certain severe conditions, i.e., slowmoving heavily loaded or high-speed lightly loaded tribocomponents. Since the oil retention by the thickener is not always sufficient to withstand the external forces, additives have been introduced through decades to improve the tribological performance and to enhance the extreme pressure, antiwear, and friction-reduction capabilities of lubricants.…”

Open-Cell Rigid Polyurethane Foam Using Lithium Salt of 12-Hydroxystearic acid as a Cell Opening Agent

“…There is well-known by many researchers that, when Li-12HSA is properly synthesized as grease type by the neutralization reaction of LiOH with 12HSA in the base oil, it exists as a fine dispersion of crystallized lithium soap in the oil, usually in the form of long, twisted and well-entangled fibers. [28][29][30][31] More recently Sanchez and co-workers showed this fine dispersion state through atomic force microscopy (AFM). 31 Chemical structure of Li-12HSA is shown in Figure 1.…”

Microscopy of Polyurea Grease

“…The microstructure of greases in the absence of shear has been studied by electron microscopy in a number of ways (Shuff & Clarke, 1991b). The simplest is the washing of the base oil from the thickener network with a suitable solvent, leaving the thickener behind to be imaged with scanning electron microscopy (SEM) (Delgado et al, 2006).…”