AFM and SEM Assessment of Lubricating Grease Microstructures: Influence of Sample Preparation Protocol, Frictional Working Conditions and Composition

Abstract

The microstructure of lubricating greases greatly conditions their in-service performance. In that sense, optimal testing protocols are required in order to accomplish their correct morphological characterization. This study explores and compares the suitability of Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) techniques for imaging six different commercial metallic soap-based greases and two novel biopolymer-based formulations. Pros and cons of both techniques as well as the effect of sample preparation protocol were analyzed. The results revealed a wide variety of morphological characteristics depending on composition. Thus, the four anhydrous calcium-based greases demonstrated two clearly distinct microstructures (fibrous and granular) determined by the type of base oil employed. With regard to the lithium complex greases, the typically reported microstructure characterized by welldefined entangled and fibrous network was observed in both AFM and SEM techniques.As for the two biopolymer-based greases, fiber networks were also encountered.Besides this, selected greases were subjected to different tribological tests, and the effect of high-shear frictional working treatments on their microstructure was also analyzed. As a result of the friction and internal wear, the AFM results evidenced microstructural changes which depended on grease composition. Overall, the combined use of AFM and SEM techniques was demonstrated to be a powerful approach to microstructurally characterize lubricating greases.

Ionic Liquids as Grease Base Liquids

“…Decreasing base liquid wt % and thus increasing thickener wt % corresponds to decreasing values of tan(δ). This indicates that increasing thickener content produces a more extensive gelation network that is capable of exhibiting more pronounced elastic properties [1].…”

“…The thickener is either an emulsifying soap (e.g., lithium hydroxystearate) or powdered solid (e.g., polytetra-fluoroethylene (PTFE) particles) which acts to produce the semi-solid behavior of the grease. Grease thickeners form a three-dimensional gelation network (microstructure) which traps the oil and confers the appropriate rheological and tribological behavior to the grease [1]. Additives are frequently included to improve properties such as thermal or evaporative stability.…”

Tribological Stress of Lubricating Greases in the Light of System Entropy

“…Change of grease structure caused by increasing shear rates in a rotational rheometer test as described by [10]. Dark areas present the base oil and bright areas the thickener (here a Li-Ca-soap was used as a thickener).…”

Application of a Thermodynamic Concept for the Analysis of Structural Degradation of Soap Thickened Lubricating Greases

“…A number of SEM and AFM investigations [7,8] present important information about the degradation process. Delgado [7] shows the sensitivity of a grease structure withs SEM-pictures.…”

“…He observe the change of the structure influenced by composition and different conditions. Roman et al [8] investigate the degradation of thickener structure with AFM and show for selected samples the influence of shear rate and shear time.…”