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The rheological properties of lubricating greases are determined by the viscosity of the base oil, the interaction between base oil and thickener, and the interaction between thickener particles. The contribution of the oil–thickener interactions to the viscosity is well known, but the contribution of the thickener–thickener interactions has not yet been studied by employing theoretical or computational frameworks. In this paper, we use coarse-grained molecular dynamics to simulate a fibrous microstructure, and we show that the experimentally observed viscoelastic/plastic behaviour can be well reproduced. A parametric study shows that the apparent viscosity increases with increasing fibre length, fibre stiffness and thickener concentration. This is as expected, showing that this modelling approach is useful to study effects on grease rheology that are not accessible experimentally, such as impact of fibre entanglement or agglomeration.

This work investigated the temperature changes inside the bulk of lubricating greases under controlled high-shear stress conditions (250–500 s −1 ). For this purpose, a newly developed temperature-measuring cell called Calidus was successfully tested. The temperature changes (Δ T ) have been related to the greases’ components (thickener, base oil-type, and composition) and the structural degradation of the lubricating greases. Furthermore, a theoretical approach was proposed for calculating the internal temperature change of lubricating greases during shear stress. All greases showed an internal temperature profile characterised by a sudden rise in Δ T within the first 4 h from starting the test and subsequent Δ T decay until it reaches the steady state value. Furthermore, it was found that greases C1 and C5, formulated with lithium and calcium soap, respectively, with different soap content (16.1 wt% and 9.7 wt%, respectively), but the same base castor oil, showed the highest value of the maximum Δ T , c.a. 3.2 K, and the most drastic drop of Δ T . These greases showed both the highest specific densities and heat capacities. In addition, they showed the lowest ratio of expended energies ( R tee ), which means more structural degradation in the stressed grease. On the contrary, the grease C3, with 13 wt% of Li-soap but the lowest base oil’s viscosity, showed the lowest maximum Δ T and the temperature profile was characterised by a moderate variation of Δ T along the test. The biogenic grease B3 developed a low-temperature change in the group of pure bio-genic greases close to grease C3.

In the present work, electrospun Kraft lignin/cellulose acetate nanostructures were produced, assessed and proposed as structuring or thickening agents of castor oil for lubricating applications. Solutions of Kraft lignins (KL) derived from different sources (eucalyptus, poplar and olive tree pruning) and cellulose acetate (CA) were prepared and used as feed for electrospinning. The rheological properties (shear and extensional viscosity), electrical conductivity and surface tension of KL/CA solutions influence the morphology of the electrospun nanofibers, which in turn is affected by the chemical structure and composition of the Kraft lignins. Electrospun KL/CA nanostructures consisting of filament-interconnected nanoparticles, beaded nanofibers or uniform nanofiber mats were able to form gel-like homogeneous fine dispersions by simply mechanically dispersing them into castor oil. The swelling of KL/CA nanofibers in the percolation network was demonstrated. The rheological, tribological and microstructural properties of these oleogels are essentially governed by the morphological characteristics of the electrospun nanostructures, i.e. fiber diameter, number of beads and porosity. Rheological properties of the resulting oleogels may be tailored by modifying the lignin source and KL:CA weight ratio. According to their rheological and tribological properties, KL/CA electrospun nanostructures-based oleogels can be proposed as a sustainable alternative to conventional lubricating greases.

Herein, a series of Ag coatings with different micro-dimples were fabricated on copper surfaces by laser surface texturing (LST) and magnetron sputtering. Multilayer graphene lubricating grease (MGLG) was prepared using multilayer graphene as an additive. The textured Ag coatings and MGLG were characterized. Moreover, the tribological and electrical performances of the textured Ag coatings under MGLG lubrication were investigated in detail. Results demonstrated that the textured Ag coating with an appropriate dimple diameter could exhibit improved tribological and electrical properties when compared to the non-textured Ag coating under MGLG lubrication. The characterization and analysis of the worn surfaces suggest that the synergetic effect of LST and MGLG contributes to these excellent tribological and electrical properties.

This paper presents the results of a study on the effect of the dispersed phase on the lubricating and rheological properties of selected lubricant compositions. A vegetable oil base (rapeseed oil) was used to prepare vegetable lubricants, which were then thickened with lithium stearate, calcium stearate, aluminum stearate, amorphous silica, and montmorillonite. Based on the results of the tribological tests of selected lubricating compositions, it was found that calcium stearate and montmorillonite have the most beneficial effect on the anti-wear properties of the tested lubricating greases, while silica thickeners (amorphous silica and montmorillonite) provide the effective anti-wear protection in compared to the lubricants produced on lithium and aluminum stearate. The lowest structural viscosity was found for grease thickened with montmorillonite. Much higher values of this parameter were observed for composition, where aluminum stearate was the dispersed phase, while the highest value of structural viscosity was observed for composition, where aerosol–amorphous silica was the thickener. The composition thickened with amorphous silica had the highest yield point value, while the composition in which montmorillonite was the dispersed phase had the lowest value. Dynamic viscosity decreases with temperature, which is characteristic of lubricants. No significant differences in dynamic viscosity were found for the lubricating compositions tested at temperatures above 50 [°C]. The most favorable rheological properties were observed for composition, which was produced using calcium stearate, as it allows the lowest dynamic viscosity at −20 [°C]. Lubricants produced with lithium stearate or aluminum stearate were characterized by higher viscosity at low temperatures. For grease, in which the lithium stearate was used as a thickener, the value of the elasticity index determines the weak viscoelastic properties of tested grease and a greater tendency to change structure under the influence of applied forces. For vegetable grease thickened with aluminum stearate, more than 15 times lower values of the MSD function were observed, and the calculated elasticity index value proves the stronger viscoelastic properties of the aluminum stearate grease in relation to grease thickened with the lithium stearate. The elasticity index value for grease thickened with amorphous silica was lower than for greases thickened with lithium and aluminum stearate, indicating its stronger viscoelastic properties in relation to these two greases. For grease composition prepared on the vegetable oil base and thickened with montmorillonite. The value of the elasticity index was lower than most of the tested grease compositions, without the composition, in which the calcium stearate was used as a thickener. Such results testify to moderately strong viscoelastic properties, which leads to the conclusion that the produced lubricant was a stable substance on changes in chemical structure under the influence of variable conditions prevailing during work in tribological joints.

The replacement of mineral oils and non-renewable gelling agents is an imperative requirement for the lubricant industry in the near future. In this framework, cellulose pulp and castor oil are proposed as sustainable substitutes for these components. Biological treatment has been explored and evaluated to enhance the dispersing and thickening properties of cellulose pulp in oil media. Streptomyces sp. MDG147 and MDG301 strains were employed to modify agricultural wheat and barley straw residues from which cellulose pulp was obtained afterwards. In addition, an environmentally friendly process for the production of cellulose-pulp-/castor-oil-based polyurethanes was applied, in which neither catalysts nor harmful solvents were used, resulting in chemical oleogels. These oleogels were rheologically and tribologically characterized to evaluate their performance as lubricating greases. The enzymatic activity pattern developed was dependent on the raw material, the strain type, and the temperature, influencing the cellulose pulp’s composition, polymerization degree, and crystallinity. These modified characteristics tuned the rheological behavior of the different oleogels, providing a beneficial range of viscoelastic responses and viscosity values that were generally favored by the Streptomyces action. Furthermore, the friction coefficient and dimensions of wear scars measured in a tribological contact were comparable to, or even lower than, those found with commercial and other bio-based lubricating greases that have previously been studied.

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 and the effect of sample preparation protocol were analysed. 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 well-defined entangled and fibrous network was observed in both AFM and SEM techniques. As for the two biopolymer-based greases, fibre 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 analysed. 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.

The introduction of mechanical energy during a friction process stimulates the system to eliminate this disturbance and find ways for energy dissipation. There are two principal situations: the system is either near equilibrium or far from equilibrium. Near equilibrium, it can be expected that the disturbance will be damped after a certain time, and the system will settle in a stationary state at a level where it began. However, the situation could be entirely different when the system is far from equilibrium. After a phase of instability and crossing a critical parameter, there is a probability of a change in the order level. This means that a new structure will be formed. This paper describes some aspects of the criteria that lead a friction process inside the grease film to instability and examines the influence of different dependencies. In this publication, the dependencies are extended to verify the stability criterion. Finally, the rest phase of a thixotropic experiment is examined from the perspective of potential instability and, thus, the possibility of self-organizing processes occurring.

Purpose The purpose of this study is to reveal the significant contribution of MXene on enhancing tribological properties and to obtain the influence mechanism of various factors on friction characteristics of rolling bearing under extreme conditions. Design/methodology/approach Under extreme working conditions, the friction characteristics of rolling bearings directly determine the safety and reliability of the transmission system. In this study, MXene is added to the origin lubricating grease (OLG) of rolling bearing to enhance their friction characteristics. Then, the effects of inner ring speed, radial load, grease filling volume and other factors on the friction coefficient of rolling bearing are analyzed using the Taguchi method. Findings The results indicate that the ranking of various factors affecting the friction coefficient is: radial load, inner ring speed, MXene additive content in grease and grease filling volume. Especially, the radial load and inner ring speed exhibit extremely significant effects, while the MXene additive content in grease (P < 0.05) has a significant influence on the friction coefficient of rolling bearing. The optimal condition for rolling bearing lubricated with MXene additives lubricating grease (MALG) achieves the lowest friction coefficient of 0.0049 under 1,000 rpm, 9 kN and 35% grease filling volume. Originality/value This study could offer reference solution for utilizing MXene nano-lubrication to fufill the demands of precision, heavy-load, or long-lifespan bearings. Furthermore, the lubrication approach has the potential to be expanded into aerospace, defense, and various industrial fields, thereby significantly promoting its practial engineering applications.

The rapid demands of the engine industry for greases that can perform under a wide range of operating conditions focused on developing environmentally friendly, inorganic lubricating greases free from soap-based components. Utilizing bentonite clay as an innovative porous thickener, combined with two base oils blend with soft and hard paraffin wax additives, seeks to economically synthesize greases that are versatile and suitable for a wide range of service conditions. Acid post-treatment of raw bentonite was undertaken to activate the bentonite clay and alter its surface from hydrophilic to organophilic, enhancing its dispersion in organic base oil. The activated bentonite exhibited significant modifications in surface area, porosity, and crystal morphology, which directly influenced the grease's physical properties and performance. Notably, activated bentonite treated by nitric acid showed superior efficacy in homogeneously integrating with both base oil types and paraffin waxes compared to other acid post-treatments, producing greases with higher dropping points similar to lithium complex and organic clay grease, lower working penetrations as normal grease as classified by National Lubricating Grease Institute (NLGI) consistency grade. This study contributes to the development of inorganic grease formulations by systematically evaluating the rheological and thermal performance of modified bentonite. On the other hand, it highlights the flexibility of modified bentonite as a universal thickener for lubricating greases, demonstrating its efficiency as a multifunctional thickener that meets the evolving demands of various industrial engines.