Lithium Grease Research Articles

Development of wear and pitting curves with vibration analysis for lubricating grease under contamination conditions

The application of grease under contaminated environmental conditions typically leads to its degradation and subsequent damage to lubricated surfaces. This study examines the effects of various concentrations of environmental particles and water content, both separately and in combination, on the variation of tribological characteristics of lithium-based grease during wear. Wear and pitting curves are established for grease contaminated with varying weight percentages of water and particles. The introduction of water into grease reduces wear by absorbing friction heat but increases pitting, while particles extend the running-in period and destabilize surface roughness. In mixed conditions, particles exert a greater influence on the tribological behavior than water. To improve pitting monitoring, vibration indicators FM4 and NA4 are modified by incorporating the cumulative effect of the friction coefficient over time, accounting for the progressive accumulation of wear and vibration energy. The modified FM4 indicator proves more effective in detecting pitting variations under contaminated conditions.

The impact of water contamination on the performance failure of lithium grease

The impact of water contamination on the performance failure of lithium grease

Regulatory mechanism of grafted alkyl chain length of carbon quantum dots as additives on the tribological properties of lithium greases

Carbon quantum dots (CQDs) exhibit exceptional structural properties and minimal environmental impact, garnering significant interest in the field of lubrication. In this study, four types of modified CQDs with varying alkyl chain lengths were synthesized through hydrothermal methods and subsequently mixed with lithium grease (LG) to formulate mixed greases. The influence of the alkyl chain length on the CQDs used as additives in the mixed greases was investigated with a four-ball friction and wear tester, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). Specifically, compared to pure LG, the modified CQDs with the shortest alkyl chain length reduced the coefficient of friction (COF), the diameter of abrasive spots, and the depth of abrasive scars by 33.65%, 42.11%, and 51.87%, respectively. This improvement can be attributed to the ease of these shorter-chain CQDs entering the contact area and forming a stable film containing carbon, nitrogen, and oxygen elements through friction. The tribological properties of modified CQDs can be effectively controlled by adjusting the length of the carbon chain, providing a theoretical basis for the design of environmentally friendly and sustainable grease additives in engineering applications.

Synthesis of bio-based multi-purpose lithium grease from different vegetable oils: Drop point enhancement approach

Bio-based greases are considered as viable sustainable alternatives for mineral greases. This study investigates the synthesis of bio-based lithium grease from the trimethylolpropane ester obtained from different vegetable oils, namely sunflower, rapeseed, and soybean oils. The effects of the thickener concentration, reaction time, and type of vegetable oil on the drop point of the grease were investigated using response surface method. The results showed that the concentration of the thickener had the most significant effect on the drop point, followed by the type of vegetable oil. Increasing the thickener concentration in sunflower oil can increase the drop point about 20 %, in soybean oil about 26 %, and in rapeseed oil by about 6 %. The optimal conditions for the synthesis of bio-based lithium grease with a drop point of 187 °C were found to be a thickener of 30 %, a reaction time of 60 min, and sunflower oil as the base oil, with a desirability of 0.995. The bio-based lithium greases produced from sunflower oil in this study and measured for its physicochemical properties including NLGI grade, penetration, water washout resistance, and drop point. The results showed that the greases had an NLGI grade of 1, a penetration of 327 (0.1 mm), a water washout resistance of 9 %, and a drop point of 187 °C.

Study on the sliding tribological behavior of ZIF-8 under lubricating grease conditions

ZIF-8 is widely applied in lubrication, adsorption, and catalysis owing to its unique physicochemical properties. Previous experimental studies have demonstrated its feasibility as a lubricant additive. In the present work, the lubricating performance of ZIF-8 as an additive to lithium-based grease is quantitatively and dynamically analyzed at the atomic scale using molecular dynamics simulations. Friction wear experiments are also conducted to elucidate the lubrication mechanism of ZIF-8. The simulation and experimental results indicate that the incorporation of ZIF-8 effectively enhances the antifriction and antiwear characteristics of lithium grease. The most significant improvement in the lubrication performance of the grease is obtained at a mass fraction of 2.0 wt. % ZIF-8, which reduces the friction factor μ of the grease by about 17.0% and the wear by ∼40.0%. Furthermore, the molecular dynamics simulations reveal that ZIF-8 primarily functions as a ball bearing under low-load conditions. However, under high-load conditions, ZIF-8 undergoes significant deformation and primarily acts as a filler. This explains the experimentally observed significant reduction in friction coefficient after the addition of ZIF-8. The results of this study provide a theoretical foundation for the development of new environmentally friendly grease additives.

Microstructural analysis on the compatibility of various dilution oils on magnetorheological grease performance

This research explores the effects of dilution oils on the storage stability of magnetorheological grease (MRG) by studying the effect of dilution oil viscosity on the microstructure of carrier fluids medium for MRG, which can help address practical challenges encountered in the development and deployment of MRG. Three samples of MRG with 70 wt% CIP are prepared; a control, and 2 samples diluted until 10 wt% hydraulic fluid and kerosene respectively. The resulting samples were analysed using a modular compact rheometer (MCR) for oscillatory strain sweep and rotational current sweep. Rheological analyses were repeated after one year in storage. Fourier-Transform Infrared (FTIR) spectroscopy and Atomic Force Microscopy (AFM) results show significant microstructural and performance deterioration of grease thickener in the sample with kerosene, which concludes that kerosene had a very significant effect on the degradation of the grease thickener. From this study, it was revealed that low viscosity oils disrupt the reconstruction of the thickener of lithium grease, which in turn causes deteriorating shear rheological performance of MRG at off-state conditions. This comprehensive analysis explains the relationships between MRG composition, microstructural characteristics, and performance parameters, offering a foundational framework for further exploration and advancement in this scientific field.

Influence of electric current on tribological performance of grease-lubricated steels

In the rapidly growing field of electrification of mechanical systems, such as electric vehicles (EVs), it is needed to understand how electric currents affect lubricated mechanical interfaces. In this research, we employed a ball-on-disc tribometer to analyze the friction and wear behavior under varying direct current (DC) conditions (0, 1, 2, 3 A) on the steel contact surfaces lubricated with lithium base grease. To study the influence of electrical parameters on the mechanical behaviors of the interface, the impedance of the contact was measured with an alternating current (AC). The results revealed 35 % increase in coefficient of friction and 670 % rise in wear under 3 A compared to non-electrified conditions. Furthermore, the effect of operational speeds on the pitting density and electrical impedance were analyzed. Lower impedance was measured in at low speed, which increases the likelihood of electrical discharges and subsequent wear. Consequently, as sliding speed increases from 0 to 5 cm/s, the pitting density on lubricated surfaces significantly decreases from 1.8 to 0.4 pits per square micrometer. This study laid a groundwork for future research aimed at reducing wear and extending the lifespan of lubricated mechanical systems under the influence of electrical currents.

Triboelectric Performance of Ionic Liquid, Synthetic, and Vegetable Oil-Based Polytetrafluoroethylene (PTFE) Greases

Within electrical contacts, poor electrical conductivity of lubricants can lead to triboelectric charging, causing electrostatic currents and thermal effects, which accelerate lubrication failure. This study aimed to address these challenges by producing and testing three greases with different base oils: ionic liquid ([Oley][Oleic]), synthetic oil (PAO4), and vegetable oil-based synthetic ester (trimethylolpropane oleate). Each grease was prepared with polytetrafluoroethylene powder as the thickener. The greases were tested using a custom-made tribometer, integrated with a grounded electrical current system, with friction tests conducted with up to a 2 A electrical current flow at a constant voltage supply of 4.5 V. Under triboelectric friction testing, [Oley][Oleic] grease outperformed a commercial perfluoropolyether grease by 27.7% in friction and 16.3% in wear. This grease also showed better performance than formulated lithium grease with extreme pressure additives. The study demonstrates that greases with low interfacial resistance can retain their lubrication capacity under triboelectric conditions. These results indicate that [Oley][Oleic] grease, with its ionic liquid base oil, offers a promising solution for applications involving electrical contacts. This study highlights the potential of using advanced base oils and thickeners to enhance the performance and sustainability of lubricants in demanding environments.

Rheological modelling of carbonyl-iron particles (CIP) paraffin oil-based magneto-rheological fluids

New types of magneto-rheological fluids are increasingly being developed lately, but there is a dearth of information on the performance of commonly used rheological models for emerging MRFs such as carbonyl-iron particles (CIP) paraffin-oil-based MRF. This work aims to investigate the performance of some rheological models for application in predicting shear stress and yield strength in an emerging MRF suitable for flow-mode applications. CIP, low viscosity paraffin oil, and lithium grease were used as magnetic particles, carrier fluid, and additives, respectively, to prepare the MRF. Based on different mixing proportions determined with the Taguchi method of experimental design, sixteen samples were prepared following a standard procedure. For each sample, the values of viscosity and shear stress were determined using a viscometer and rheometer, respectively, with an incorporated self-developed magnetic device. By fitting the data and using the multi-objective nonlinear programming solver in Micro-soft Excel to determine optimum parameters for each model, the Bingham Model, Herschel–Buckley Model, Casson Model, Cross models, and Power-law were used to model the experimental data. Predicted shear stress values and yield strength were then analyzed using ANOVA at a 5% confidence level. The relative errors were determined using RMSE, Mean Square Error, and Mean Absolute Error. There was a significant variation in the predicted outcomes of all the models. Overall, all the models gave relatively acceptable results. However, the Herschel-Buckley model gave the best results, while the Casson model gave the worst results, judging by their values of errors. It is shown that the Herschel-Buckley model should be best used for predicting the rheological characteristics of CIP and paraffin oil-based MRF.

Study on the Tribological Properties of Acid-Doped Polypyrrole (PPY) as Conductive Grease Additive

This study investigated the acid doping of polypyrrole (PPY) using as dopants three solutions: hydrochloric acid (HCl), dodecylbenzene sulfonic acid (DBSA), and 1-butyl-3-methylimidazolium tetrafluoroborate (LB104). Four conductive lubricating greases were prepared with these dopants based on composite lithium-based grease (LCG). The research aimed to explore the physicochemical properties and tribological performance of conductive lubricating greases. Experimental results indicate that acid-doped PPY conductive lubricating greases exhibit lower resistivity and higher dropping points. Moreover, these lubricating greases can form stable boundary lubricating films, demonstrating excellent antifriction and antiwear properties. Among these, the additive with LB104-doped PPY shows superior antifriction and antiwear performance, as well as conductivity, suggesting broad prospects for application in the field of conductive lubrication.

Influence of ionic liquid/liquid metals on the lubrication properties of lithium grease

PurposeThis paper aims to use an ionic liquid (IL, [HMIM]PF6) to improve the lubrication performance of liquid metal (LM) as a lithium grease additive and to expand the application range of LM.Design/methodology/approachIn this paper, the different mass ratios of [HMIM]PF6/LM mixtures were added into the lithium grease on a four-ball tribo-meter to investigate the effects of its tribological behavior. Scanning electron mircoscope/energy dispersive spectroscopy and X-ray photoelectron spectroscopy were used to reveal the anti-wear and friction-reducing mechanism of the additives.FindingsWhen the load was used at 461 N, the average coefficient of friction (ACOF) and average wear scar diameter (AWSD) of steel ball Lubricated with grease with an optimal ratio of 2:3 ([HMIM]PF6/LM) were reduced by 32.8% and 30.5%, respectively. Friction and wear mechanisms are ascribed to friction-induced additive components that can simultaneously form a composite lubrication film consisting of FePO4, FeF3, Ga2O3, In2O3 and SnO2.Research limitations/implicationsCompared with the pure lithium-based grease, when [HMIM]PF6/LM was added with an optimal ratio of 2:3, the ACOF and AWSD were reduced by 12.4% from 0.097 to 0.085 and 23.8% from 552.117 µm to 420.590 µm under 392 N, respectively. When at 461 N, the ACOF and AWSD of steel ball were reduced by 32.8% from 0.122 to 0.082 and 30.5% from 715.714 µm to 497.472 µm, respectively. It was shown that the simultaneous addition of LM and [HMIM]PF6 can form a composite lubrication film consisting of FePO4, FeF3, Ga2O3, In2O3 and SnO2.Originality/valueIn this paper, [HMIM]P F6 is added with LM simultaneously to improve the lubrication properties of lithium grease, and expand the application scope of LM.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-01-2024-0017/

Bilamellar 1T-MoS2/g-C3N4 Composites with Enhanced Tribological Performance as Additives in Poly-α-Olefin Greases

In our research described here gold-phase molybdenum disulfide (1T-MoS2)/graphite-phase carbon nitride (g-C3N4) composites were prepared by a simple solvent-thermal method to investigate their synergistic lubrication in poly-α-olefin lithium grease by the four-sphere mode. The growth of 1T-MoS2 on g-C3N4 nanosheets was demonstrated by scanning electron microscopy (SEM), X-ray diffractometry (XRD) and Fourier transform infrared spectroscopy (FTIR). Tribological tests showed that the best tribological performance of the poly-α-olefin grease was achieved when the concentration of 1T-MoS2/g-C3N4 was 3 wt%; the coefficient of friction and the diameter of the abrasion marks were reduced by 37.7% and 51.2%, respectively, compared with the pure grease. Investigating the wear marks by field emission scanning electron microscopy (FE-SEM) and elemental mapping analysis, it was found that the coupling between the liquid lubrication film and the solid friction film and the synergistic effect between 1T-MoS2 and g-C3N4 were important factors to improve the tribological performance of the poly-α-olefin grease. We, therefore, note this study is expected to promote the development of new, high-performance greases.

Development of industrial-grade grease from waste pyrolysis wax

The present research aims to deal with the synthesis and formulation of grease from waste pyrolysis polymeric wax. An attempt has been made to characterize and evaluate the effect of lithium-based thickeners on different properties of the grease. The comparison between polymeric waste wax-derived grease against grease synthesized from paraffinic oil and castor oil has been thoroughly studied. One of the key ingredients of grease preparation is formulated using a base oil, i.e., mineral oil, synthetic oil, or vegetable oil. Waste polymeric wax holds significant potential as a base fluid. With this objective, lithium grease was prepared by actuating the saponification reaction between a 12-hydroxystearic acid and hydrated lithium hydroxide within a waste wax medium in an open container. The saponification reaction takes place up to neutralization by stirring using an anchor impeller. The properties of this new formulation are studied comprehensively in the present work. The formulated grease was then compared with commercial grease by performing various tests such as drop point, four ball tear test and copper corrosion. It is observed that synthesized grease has identical properties to industrial grease. Therefore, produced lubricating grease is elementary and cost-effective.

Tribological characteristics and vibration response of grease lubricated contacts under environmental particles and water impact

In practical applications, the lubricating performance of grease gradually deteriorates owing to the inclusion of foreign contaminants. This study investigates the impact of environmental contaminants, including silicon dioxide (SiO2) particles and water content, on the degradation of lithium-base grease and its subsequent effect on the surface damage of bearing steel material. The experimental results reveal an interactive effect between water content and particles in grease. The presence of particles and water contamination in separated or mixed conditions leads to increased friction, wear, and pitting compared to the base grease. The wear mechanisms of the two contaminant factors are different as follows: the particles predominantly function as third bodies, which enhance the oil film thickness between contacting surfaces while inhibiting the formation and growth of a protective tribofilm from the base grease, resulting in elevated friction and wear, and severe surface damage. Additionally, the addition of suitable water content in grease can absorb friction heat produced by the particles, mitigating friction and wear. Vibration exhibits behavior similar to friction, where both friction and vibration increase proportionally in the absence of water contamination. This observation suggests that vibration can be used as a valuable indicator for monitoring friction in contaminated conditions.

Investigation of friction and vibration performance of lithium complex grease containing candle soot on aviation electrical machine

The prolonged operation of the motor in the all-electric aircraft can cause the fragmentation and shortening of grease thickener fibers, ultimately resulting in the lubricating grease becoming ineffective. In order to avoid grease failure causing motor winding burnout, this paper uses a four-ball friction testing machine to test the tribological properties of six composite greases containing candle soot with different concentrations, and analyzes the changes in friction coefficient and wear diameter. In addition, by building a test platform for motor vibration and noise, this paper studies the influence of lithium complex grease on vibration and noise characteristics of aviation electrical machine at different propeller speeds. The results show that the addition of 0.04 wt% candle soot lithium grease has the best tribological properties and vibration inhibition properties, because the candle soot in the oil film changes the sliding friction into rolling friction in the contact area of some friction pair. The temperature rise test of aviation electrical machine shows that the lithium complex grease plays an active role in the heat dissipation performance of the high-speed rotating motor.

A nonlinear dynamic analysis of skidding behavior in rolling bearings using lubricant traction coefficients and cage flexibility

Skidding is a primary cause of rolling element bearing (REB) failure, which is influenced by operating conditions, lubricant properties, and bearing design. This paper presents an improved nonlinear dynamic model of REBs to analyze the impact of lubricant characteristics on REB skidding at different speed states. The model considers time-varying traction coefficient, cage flexibility, and cage pocket clearance. The selected tested lubricants include lithium grease (LGT2), compound calcium sulfonate (CCS) grease, and 4109 Chinese aviation oil. Simulation results show that CCS grease effectively reduces cage skidding, with the lowest load limits (300 N at 2000 RPM) under constant speed conditions and the highest stable limit (2932 RPM) during acceleration. LGT2 grease exhibits minimal sensitivity to acceleration and fluctuation speed parameters change.

Study of the changes in the microstructures and properties of grease using ball milling to simulate a bearing shear zone on grease

The effects of shear degradation on the microstructures and properties of grease were investigated using a planetary ball mill to simulate a bearing shear zone on grease. The microstructure, cone penetration, colloidal stability, rheological properties noise properties, water washout characteristics and low-temperature torque of lithium grease were characterized. The microstructure of the initial lithium grease is a three-dimensional network structure formed by the uniform fibers. The entanglement level is high. As the ball milling shear time increases, the network structure of lithium grease is destroyed and the fibers are sheared to become short. Eventually all of them become short fibers. The performance test of lithium grease reveal that the cone penetration increases, colloidal stability, structural strength, noise properties, water washout characteristics of lithium grease gradually decreased with the increase of ball milling shear time. Additionally, the low-temperature starting torque and running torque of the grease gradually decrease. This phenomenon occurs due to changes in the microstructure of lithium grease. The shear degradation of lithium grease was mainly divided into two stages: the rapid stage was the destruction of the thickener network structure and the fibers being shortened by shearing. The slow stage was the process in which short fibers were sheared into shorter fibers.

Effects of Spectrum Loading and break in period using molybdenum disulphide (MoS2) and zinc dialkyl dithiophosphate (ZDDP) as EP additives in lithium based grease

PurposeThe purpose of variable loading conditions (392 N-785N-392N-785N) with break-in period were used to study interactions between zinc dialkyl dithiophosphate (ZDDP) 0.1 P% (phosphorus) and fine-grade molybdenum disulfide (MoS2) 3%, in different mixtures of NLGI 2 lithium stearate grease. Four-ball wear tests were used to evaluate the tribological properties of different grease mixtures such as coefficient of friction and wear. ASTM 2266 as reported by earlier studies is useful, but it is not representative of real-life applications where variable loads and speeds and different break-in periods play a role and could change the results and the nature of tribofilms.Design/methodology/approachIn this study, chemical and mechanical properties of tribofilms were examined. Moreover, design of experiment was used to examine the data and shorten experimentation time. Research described here is investigating variable loading conditions for real-life applications by using a break-in period of 2 min at the start to minimize asperities and establish a clean surface. Design expert (DOE) analyzes responses to reveal those variables that are single factor and those that are multifactor whether synergistically or antagonistically.FindingsThe results indicated that spectrum loading with break-in period showed reduction in wear when tested in greases with ZDDP/MoS2 combinations. Ramping up or down the load every 7.5 min for a rotational speed of 1,200 rpm and a total of 36,000 revolutions or 30-min time slowed the wear properties of lithium-based grease under different MoS2 and ZDDP concentrations. Experiments indicated that wear was largely dependent on the loading condition and ZDDP additives during specific break-in period at 1,200 rotational speed. It is believed that MoS2 greases perform better under spectrum loading and under constant loading when mixed with ZDDP phosphorus.Originality/valueThis research indicates that there is a synergistic interaction between ZDDP, MoS2 and variable loading especially when a break-in period is applied. The results indicated that wear was largely dependent on the specific speed used with spectrum loading as presented in the energy dispersive spectroscopy and the Auger electron spectroscopy analysis, and thus a 3% MoS2 grease with ZDDP (phosphorus: 0.1 Wt.%) are needed to improve the wear resistance and improve the friction characteristics.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-01-2024-0016/

Enhancing the Elastohydrodynamic Lubrication and Vibration Behavior of Rolling Bearings Using a Hybrid Bio-Grease Blended with Activated Carbon Nanoparticles

In recent years, bio-lubricants have received a growing interest for industrial applications. Still, a full-scale implementation in machinery lubrication requires a thorough evaluation of their performance through tribological and operational tests to stand upon their performance. Additionally, the promising outcomes achieved by nanoadditives in improving the performance of synthetic lubricants have prompted research efforts to identify suitable nanoadditives for bio-grease. This paper introduces a bio-grease from a hybrid vegetable oil and glycerol monostearate as a thickener for the lubrication of rolling bearings. Activated carbon nanoparticles (ACNPs) as nanoadditives were synthesized, characterized, and incorporated into the bio-grease at concentrations of 0.5, 1, and 2% by weight. Tribo-tests were conducted on these bio-grease blends, and running tests were carried out using 6006 ball bearings on a custom test rig. Throughout a 30-min test run under a radial load of 10% of the bearing’s dynamic load rating, mechanical vibrations and power consumption were measured and analyzed for each bearing. The bio-grease with ACNPs exhibited a substantial reduction in wear scar diameter (WSD) and coefficient of friction (COF), achieving improvements of up to 73.6 and 65%, respectively, in comparison to lithium grease. Furthermore, the load carrying capacity was enhanced by 200%. The study revealed a strong correlation between measured vibration amplitudes and the viscosity of the bio-grease. The absence of high frequency resonant bands in vibration spectra indicated that the test grease samples satisfied the conditions of elastohydrodynamic lubrication, and these findings were corroborated through calculations of the minimum oil film thickness.

Formulation and evaluation of bio-grease from the blend of chemically modified rice bran oil and Calophyllum inophyllum oil

Vegetable oils are a highly promising alternative to produce various lubricants, owing to their biodegradability and eco-friendliness. In comparison to mineral oil, these oils possess a higher flash point and viscosity index, along with superior lubricating properties. Additionally, most of the vegetable oils are easily accessible in local markets in India. However, their industrial application is limited by poor thermal and oxidative stability, which can be addressed through chemical modification, the addition of appropriate additives, or by blending these oils. Greases, characterized by their semi-solid consistency, are widely used lubricants. Most of the grease production is based on mineral oils as base oil and lithium soap as a thickener. These materials are not only non-biodegradable but also scarce and have health implications. Consequently, biodegradable grease represents an eco-friendly and healthy alternative. Grease made using vegetable oils with the required properties has the potential to bring about revolutionary changes. The present work focuses on the feasibility of using chemically modified blended rice bran oil (RBO) and Calophyllum inophyllum oil (CIO) as a bio-lubricant. The oils undergo a two-step modification process, involving a transesterification reaction followed by epoxidation. Significant improvements have been observed in the chemical properties (acid, peroxide, and iodine values) of transesterified epoxidized rice bran oil Calophyllum inophyllum oil mixture–50:50 (ETRCIO) when compared with Unmodified Rice bran oil Calophyllum inophyllum oil mixture–50:50 (RCIO). The acid value, peroxide value, and iodine value improved by 91.66%, 87.08%, and 15.78% respectively. The rheological, tribological, and chemical properties of the blended samples have been evaluated and compared to pure oils using American Society for Testing and Materials (ASTM) and Indian Standards (IS). Additionally, ETRCIO was used to develop a bio-grease, and its tribological properties were extensively analyzed. The mean coefficient of friction (COF) and wear scar diameter (WSD) of the ETRCIO grease sample improved by 10.20% and 29.32% respectively when compared with that of commercially available grease. These findings indicate that the ETRCIO bio-grease exhibits superior tribological properties in comparison to commercially available grease.