Liquid Lubricants Research Articles

Sensor for a Solid–Liquid Tribological System

Solid–liquid lubrication systems have been widely used to enhance tribological behaviors. Alongside offering exceptional lubrication and wear-resistance performance, the active control of the tribological behavior of lubrication systems in accordance with service conditions is equally critical. To achieve this goal, accurately monitoring the condition of the lubrication system is fundamental. This review article aims to provide a fundamental understanding of different sensors for monitoring the condition of lubricants, as well as the friction and wear properties. Specifically, the sensors suitable for engineering applications are detailed introduced. Through this review, we wish to provide researchers in mechanical engineering with a clear technical overview, which can guide the design of intelligent lubrication systems with suitable sensors.

Increasing the Wear Resistance of Stamping Tools for Coordinate Punching of Sheet Steel Using CrAlSiN and DLC:Si Coatings

The punching of holes or recesses on computer numerical control coordinate presses occurs in sheets at high speeds (up to 1200 strokes/min) with an accuracy of ~0.05 mm. One of the most effective approaches to the wear rate reduction of stamping tools is the use of solid lubricants, such as wear-resistant coatings, where the bulk properties of the tool are combined with high microhardness and lubricating ability to eliminate waste disposal and remove oil contaminants from liquid lubricants. This work describes the efficiency of complex CrAlSiN/DLC:Si coatings deposited using a hybrid unit combining physical vapor deposition and plasma-assisted chemical vapor deposition technologies to increase the wear resistance of a punch tool made of X165CrMoV12 die steel during coordinate punching of 4.0 mm thick 41Cr4 carbon structural steel sheets. The antifriction layer of DLC:Si allows for minimizing the wear under thermal exposure of 200 °C. The wear criterion of the lateral surface was 250 μm. The tribological tests allow us to consider the CrAlSiN/DLC:Si coatings as effective in increasing the wear resistance of stamping tools (21,000 strokes for the uncoated tool and 48,000 strokes for the coated one) when solving a wide range of technological problems in sheet stamping of structural steels.

An experimental and modelling approach to proclaim sustainable machining using avocado oil-based nano-cutting fluids

Higher-end science and technology facilitate the human community with a sophisticated life despite it curses by abundant pollution. The alarming demand for sustainability pressurizes the manufacturing sector to ensure sustainable manufacturing. Since Molybdenum di sulfide (MoS2) and avocado oil are known solid and liquid lubricants respectively, hence, it is a worthwhile attempt to implement the bio-based degradable avocado oil enriched with nano Molybdenum di sulfide (nMoS2) particles as a potential machining fluid for CNC-end milling. Different proportions of avocado oil and nMoS2 were used to synthesise four distinct machining fluids to assess the individual impact of avocado oil and nMoS2 particles. The emulsification and sonication were employed to synthesise the fluids. A hybrid Grey Relational Analysis (GRA) coupled with Principal Component Analysis (PCA) was followed to scrutiny the effect of novel machining fluid on machining objectives. The experimental results of physio-chemical properties revealed that avocado-rich 0.5% nMoS2 excels among others. The L16 orthogonal array experiments associated with statistical analysis explored the developed machining fluid (A6W4/0.5) that significantly impacts the machining objectives. The experimental results manifest that nearly 64.87% of surface roughness and 93.3% of tool wear have been reduced during machining in the presence of A6W4/0.5 fluid than A4W6/0.75. The improved performance of the novel machining fluid upholds its potential to replace conventional fluids and ensure green manufacturing.

Evaluation of Wear Resistance of a Modified Radial Bearing Design Taking into Account Compressibility and Viscosity of the Lubricant

Introduction. The challenge problem of the quality of lubricants has led to both large-scale and narrow-focused theoretical and applied studies that relate to the operational properties of lubricants. In particular, the authors of the publications are interested in the interaction of bearing and lubrication, since numerous types of machinery and equipment contain these elements. In the literature, the composites used to strengthen the bearing surface are compared, the most effective compositions are determined, and the advantages and disadvantages of the components are analyzed. Mathematical models have been proposed and tested for some of the processes of the interaction under consideration, and acceptable adequacy has been proved for some of them. However, the improvement of such solutions requires taking into account the specifics of the tribosystem. This issue has been poorly worked out, and the presented article is intended to fill this gap. When evaluating the wear resistance of a radial bearing, the compressibility of a high viscosity lubricant is taken into account.Materials and Methods. The study was based on the tribocontact scheme, which included the radius of the polymer-coated shaft, the radius of the bearing sleeve, the height of the lubrication groove, and the thickness of the lubricating layer. To create new mathematical models that took into account the compressibility of the lubricant, the authors used three equations: motion of the liquid lubricant, continuity, and state. To verify the model, the results of calculations and laboratory tests were compared. In the experiments, a bearing with a groove to preserve lubrication was used. Its rotation speed, loads and temperature conditions were changed. Friction was measured using traditional methods and modern instruments.Results. The bearing design was modified to take into account an additional factor — the compressibility of the lubricant. The new model predicted the bearing capacity of the part by 8–10% more accurately, and the coefficient of friction — by 7–9%. Fluctuations in the coefficient of friction up to 45 MPa (equivalent to a five-fold increase in load) were detected and explained. This was due to dynamic changes in the surface contact conditions and the effects of external parameters. Optimal applications of antifriction coatings based on hybrid composite materials were determined. The possibilities of practical use of calculation models of a journal bearing were expanded. Its critically important operational characteristics were evaluated in practice.Discussion and Conclusion. The scientific research results described in this article make it possible to establish the performance characteristics of the bearing at the design stage. The significant potential of this approach has been identified in terms of increasing the reliability and durability of the studied part, and this seems to be an important step in the development of bearing and lubricant technologies. In the future, the authors intend to study such factors as temperature conditions, dynamic loads, and interaction with various lubricants. This will allow us to improve bearing designs and expand their application areas.

Peculiar Effect of Water on Tribological Properties of Natural Deep Eutectic Solvent.

Deep eutectic solvents (DESs) have emerged as promising green liquid lubricants for solving tribological problems due to their economic and excellent physicochemical and lubrication properties. However, a trace amount of water would affect their structure, physicochemical properties, and tribological properties. The effect of water on the tribological properties of DES is still unclear and needs further investigate. Herein, we carried out a systematic investigation into the chemical structure, rheological properties, and tribological performance of DES-water (DES-W) binary systems constructed by combining DES with varying contents of water. The results revealed that low levels of water in DES had a minimal impact on its chemical structure but affected its fluidity and viscosity. Frictional experiments demonstrated that DES-W binary systems displayed a reduced coefficient of friction from 0.094 to 0.025 compared to pure DESs and manifested outstanding antiwear properties under a high-load condition. This was attributed to the formation of hydration layers, adsorption layers, and tribochemical films at the tribointerface through physicochemical adsorption and tribochemical reactions. Our findings not only foster the design and development of green lubricating materials but also expand the engineering applications of DESs to solve wear-related mechanical failures in practical application.

Beyond Smoothness: The Art of Surface Texturing Battling against Friction

Abstract Leveraging surface texturing to realize significant friction reduction at contact interfaces has emerged as a preferred technique among tribology experts, boosting tribological energy efficiency and sustainability. This review systematically demonstrates optimization strategies, advanced manufacturing methods, typical applications, and outlooks of technical challenges toward surface texturing for friction reduction. Firstly, the lubricated contact models of microtextures are introduced. Then, we provide a framework of state-of-the-art research on synergistic friction optimization strategies of microtexture structures, surface treatments, liquid lubricants, and external energy fields. A comparative analysis evaluates the strengths and weaknesses of manufacturing techniques commonly employed for microtextured surfaces. The latest research advancements in microtextures in different application scenarios are highlighted. Finally, the challenges and directions of future research on surface texturing technology are briefly addressed. This review aims to elaborate on the worldwide progress in the optimization, manufacturing, and application of microtexture-enabled friction reduction technologies to promote their practical utilizations.

Enhanced Imbibition in Liquid-Infused Coated Microchannels.

Spontaneous capillary imbibition has the potential to improve the performance of many micro and nanodevices since it does not require an external energy source to drive a fluid flow. Despite this advantage, controlling and reducing the friction exerted by the channel walls, which limits the speed of the liquid, remains a challenge. Here, we demonstrate experimentally that infusing the walls of a channel with a liquid lubricant substantially speeds up the imbibition process and reduces the overall viscous friction. By varying the viscosity of the lubricant, we observe a substantial reduction of the imbibition time of up to 50%. Our experimental results are in good agreement with previous theoretical predictions, providing a solid framework to study low-resistance spontaneous imbibition processes.

Effect of hydrostatic thrust bearing oil pad structure on performance

Background: The hydrostatic thrust bearing is a liquid lubrication technology based on the principle of hydrostatic pressure, characterized by high load-carrying capacity, low friction, and long lifespan. It has been widely applied in precision machinery and the spindles of high-speed, high-precision equipment. The oil pad of the hydrostatic thrust bearing is a crucial component, and its geometric structure significantly influences the bearing performance. During operation, the oil film of the hydrostatic thrust bearing is subjected to high loads and temperatures. If the oil film does not have sufficient strength, it can affect the normal operation of the bearing, leading to accidents. The oil pad structure is a key factor affecting the strength of the oil film. Therefore, the rational design of the oil pad structure is essential to ensure the performance of the hydrostatic bearing and prolong its service life. Objective: This study aimed to summarize the impact of different oil pad structures on the performance of hydrostatic thrust bearings, providing a basis for the future development of hydrostatic thrust bearings. Method: A search for journals, patents, conferences, and papers related to hydrostatic thrust bearings was conducted, and the impact of oil pad structures on hydrostatic thrust bearings was summarized and analyzed. Results: Based on the research findings of numerous scholars, this study summarizes the impact of oil pad structures on the performance of hydrostatic thrust bearings. It describes how the oil pad shape, oil pocket shape, oil pocket depth, and microtextures affect the overall performance of thrust bearings. The research methods employed are analyzed, and a concluding summary is provided regarding the influence of bearing oil pad geometric structures on bearing performance. Conclusion: This paper provides guidance for the design and optimization of the oil pad in hydrostatic thrust bearings. Finally, it outlines the future development trends of the oil pad in hydrostatic thrust bearings and provides prospects for future development trends and key areas of focus for hydrostatic thrust bearings.

Labyrinth seal design for space applications

Labyrinth seals, extensively used in space applications, serve to prevent the loss of liquid lubricants and shield satellite subsystems from contamination. These seals are essential for the reliable functioning of bearings and for protecting satellite subsystems from contamination. This study compares analytical predictions of lubricant loss against experimental measurements and computer simulations to optimize labyrinth seal configurations. Analytical models tend to overestimate mass loss by 5–8 times compared to experimental data, indicating limited reliability for complex seal geometries. Simulations using MolFlow+ and COMSOL Multiphysics align closely with experimental results, providing accurate mass loss predictions. Key findings highlight that labyrinth length, width, and surface roughness are critical factors in minimizing evaporative mass loss. Notably, stepped labyrinth seals with relief grooves and optimized step positioning effectively reduce molecular beaming effects and improve sealing performance compared to straight geometries. Effective sealing not only reduces mission failures but also helps to minimize space debris, thereby promoting safer satellite missions.

Investigation of MoSe2 filled MAO Coating on TC6 alloy- tribological and corrosion characterization of multiple interface structure

To enhance the tribological properties and corrosion resistance of titanium alloys, this study proposes a novel tribological interface structure combining micro-arc oxidation ceramic coatings with MoSe2. Comprehensive analysis using X-ray diffractometry, profilometry, energy-dispersive spectroscopy, and scanning electron microscopy was conducted to evaluate the tribological characteristics of this structure under dry friction and liquid lubrication conditions. Electrochemical experiments assessed the corrosion resistance of the structure. The results indicated that under 3 N-2.5 cm/s, oil lubrication, the structure exhibited the best friction-reducing and lubricating performance, with the friction coefficient reduced by 75 %. The synergistic interaction between the low shear force MoSe2 particles and the TiO2-rich ceramic coating formed a lubricating layer on the surface, thereby achieving reduced friction and enhanced wear resistance. Additionally, this structure demonstrated excellent corrosion resistance. This study provides valuable insights for integrating micro-arc oxidation with new materials in engineering applications.

Tribological Properties of 7A04 Aluminum Alloy Enhanced by Ceramic Coating

The 7A04 Al alloy is a commonly used lightweight metal material; however, its low wear resistance limits its application. In this study, the wear resistance of this alloy was improved by preparing micro-arc oxidation (MAO) coatings, MAO/MoS2 composite coatings, and hard-anodized (HA) coatings on its surface. The friction and wear behaviors of these three coatings with diamond-like coated (DLC) rings under oil lubrication conditions were investigated using a ring–block friction tester. The wear rates of the coatings on the block surfaces were determined using laser confocal microscopy, and the wear trajectories of the coatings were examined using scanning electron microscopy. The results indicated that, among the three coatings, the MAO/MoS2 coating had the lowest coefficient of friction of 0.059, whereas the HA coating had the lowest wear rate of 1.47 × 10−6 mm/Nm. The MAO/MoS2 coatings exhibited excellent antifriction properties compared to the other coatings, whereas the HA coatings exhibited excellent anti-wear properties. The porous structure of the MAO coatings stored lubricant and replenished the lubrication film under oil lubrication. Meanwhile, the introduced MoS2 enhanced the densification of the coating and functioned as a solid lubricant. The HA coating exhibited good wear resistance owing to the dense structure of the amorphous-phase aluminum oxide. The mechanisms of abrasive and adhesive wear of the coatings under oil lubrication conditions and the optimization of the tribological properties by the solid–liquid synergistic lubrication effect were investigated. This study provides an effective method for the surface modification of Al alloys with potential applications in the aerospace and automotive industries.

A Different Perspective on the Solid Lubrication Performance of Black Phosphorous: Friend or Foe?

Black phosphorous (BP), a promising 2D material with exceptional electronic and optical properties, has shown remarkable potential in tribology as an additive in liquid lubrication and a composite in solid lubrication. However, its potential as the standalone solid lubricant is still at its early stage. This study evaluates BP's solid lubrication performance as deposited coating (by drop casting) on a variety of metallic substrates (polished AISI 52 100 steel, aluminum, copper, and iron) under different contact pressures using a ball‐on‐disc linear‐reciprocating test machine in dry conditions. The results demonstrate that BP does not systematically reduce friction and wear. Depending on the contact pressure and the characteristic of the substrate material (particularly surface roughness), its friction and wear behavior vary a great deal. The best results observed are a 33% reduction in friction with increased surface roughness on iron and a 23% reduction in wear on aluminum. While no general trend is observed for contact pressure effects, increased substrate roughness proves beneficial, enhancing lubricant retention and exploiting BP's low interlayer shear mechanism. Therefore, this study demonstrates that while promising, BP's solid lubrication performance is not exceptional. It also highlights the importance of optimizing test conditions and materials for enhanced lubrication.

Effect of CeO2 morphology and Ru impregnation method on CH4 selectivity reduction in polyethylene waste conversion to liquid fuels and lubricants

Hydrogenolysis of polyolefins offers a sustainable pathway by giving plastic waste a second life as valuable resources, transforming it into fuels and lubricants. Supported Ru catalysts have shown potential for depolymerizing polyolefins under mild conditions; however, depolymerization via terminal C-C bond cleavage can lead to the excessive formation of CH4. This study investigated the effects of CeO2 morphology and Ru impregnation method on the suppression of CH4 formation and increasing liquid and wax proportions (C5-C41) in the production of liquid fuels and lubricants through polyethylene hydrogenolysis. Controlling the morphology of CeO2 and impregnating Ru via electrostatic adsorption can enhance the formation of oxygen vacancies and strengthen the interaction between Ru and CeO2. The presence of defects in CeO2 also correlated with smaller particle size of Ru and a higher hydrogen reservoir site, which effectively suppress CH4 formation.

Multiscale study on the solid-liquid synergistic lubrication mechanism of graphite and liquid lubricant in polymer composites

Solid-liquid synergistic lubrication, in combination with the advantages of solid and liquid lubrication, has gained the great popularity in recent years. However, the mechanism of solid-liquid synergetic lubrication is still unclear. In this paper, graphite and poly α-olefin base oil (PAO) were used as self-lubricating additives to evaluate their synergistic effect. The results show that the composites possess good mechanical and excellent tribological properties with a nanoindentation modulus of 3.8 GPa, a coefficient of friction of 0.0998, as well as a wear rate of 1.3 × 10−14 m3/N·m. Both transfer film and lubricating oil film exist on the friction interface, and the lubricating oil film protects the counter ball from excessive oxidation. At the same time, with combination of molecular dynamics (MD) simulation and first-principles calculations, the synergistic lubrication mechanism of graphite and PAO was manifested. PAO destroyed the interlayer structure of graphite, making it easier to form a transfer film on the counter ball surface. The weak chemical bond between graphite and PAO facilitates more PAO molecules to gather at the interface while forming the transfer film, which benefits the formation of lubricating oil film. The synergistic effect of graphite and PAO molecules is critical in promoting the further reduction of COF.

Skiing-inspired robust lubricating composite coatings from thermally sprayed ceramic template

Inspired by the mechanism of skiing, herein, a novel composite coating with excellent lubrication properties was developed by incorporating solid-liquid reversible lubricating phases into the inherent microcracks and pores of thermally sprayed ceramic coatings. The results indicate that the crystalline-amorphous heterostructure formed within the micro-nano cracks, enhancing their cohesive strength and toughness. Compared to traditional ceramic coatings, the friction coefficient of the composite coating has been reduced by 80 %, while the wear rate decreased from 2.89 × 10−4 to 6.44 × 10−7 mm3∙N−1∙m−1. Under the applied load and friction-induced heat, a thin liquid lubrication film is formed. Importantly, the lubricating phase continuously overflows from defects under the induction of frictional heat to replenish the lubricating film, ensuring long-term lubrication of the coating.

Study on ultra-low friction performance of PTFE film combined with water-based ionic liquid lubrication at electrical contact interfaces

By polytetrafluoroethylene (PTFE) films on the surface of steel disc and using water-based ionic liquids as lubricants, the solid-liquid synergistic effect of PTFE films and water-based ionic liquids was systematically investigated. The results showed that PTFE films exhibited excellent tribological performance and maintained good insulation capabilities under electrical conditions in the sliding electrical contact interface. When using water-based lactate Ionic liquids as lubricants, the PTFE film achieved a COF of less than 0.01 under a load of 2 N, realizing ultra-low friction. The analysis of the lubrication mechanism revealed that two types of protective films were formed on the PTFE film and steel friction pair surfaces. These protective films effectively prevented direct contact between the friction pairs, significantly enhancing tribological performance.

Tribological properties of YG8/steel friction pair in an aqueous lubrication system with an ionic liquid: Experimental results and molecular dynamics simulations

In this work, an ionic liquid (G-RIC) with excellent lubricating performance was successfully prepared, and the tribological properties as an additive in glycerol-water solutions were explored. The results indicated that 2.0 % additive content can reduced the friction coefficient to below 0.1, leading to a decline in the wear volume of 76 %. Gray cast iron corrosion tests revealed that the additive exhibited excellent corrosion resistance. Quantum chemical calculations revealed that the anion RIC has a low energy gap (ΔE = 0.034 Ha), which implies that it is more prone to undergoing tribochemical reactions with the metal substrate under frictional heat. The synergistic action of the liquid lubrication layer, adsorption film, and friction protective film created an interface that easily shears.

Investigation of the tribological behaviors for 4H–SiC substrate under different lubrication conditions

Lubrication conditions are an important factor affecting both the machining efficiency and quality of 4H–SiC. To investigate the tribological behaviors under different lubrication conditions, a series of scratching experiments are conducted under different loads and environments. The X-ray photoelectron spectroscopy (XPS) surveys and atomistic simulations are used to explain the different tribological behaviors. Both experimental and simulation results show that liquid lubrication can significantly reduce the coefficient of friction (COF) and minimize structural damage. Compared to pure water, the H2O2 solution is more conducive to the oxidation of SiC atoms and the modification of tribological behaviors. However, the difference in tribological behaviors between H2O2 solution and pure water diminishes as the load increases. The XPS surveys show that the liquids lead to higher-order oxidized species of SiC atoms, such as Si4C4-xO2 and Si-Ox-Cy, which are also observed in the simulation results. It is shown that the oxidized species can reduce the direct bonding between SiC and diamond indenter, which is an important reason for the lower COFs in the liquids. Since the liquids can reduce the direct bonding and mechanical interaction between 4H–SiC and diamond, the material removal rate is much lower under lubrication conditions.

Study on the Tribological Properties of Multilayer Concentric Hexagonal Laser Texturing on Rubber Surfaces of Screw Pumps.

Given the friction and drag reduction effects observed in various biological hexagonal structures in nature, a new design was implemented on the rubber surface of the stator of a submersible screw pump. This design featured a multilayer concentric hexagonal groove structure. Furthermore, a composite multilayer hexagonal structure integrating grooves and pits was also developed and applied. This study investigated the influence of groove layer number, groove depth, pit depth, and multilayer hexagonal groove texture arrangement on the rubber surface flow characteristics. Additionally, the pressure field state, the degree of influence on the oil film-bearing capacity, and the biomimetic and hydrodynamic lubrication theories were tested using the finite element analysis method. Tribological experiments were conducted on nanosecond laser-processed rubber textures under simulated liquid lubrication conditions, reflecting actual shale oil well experiments. These experiments aimed to investigate the influence of multilayer hexagonal shape parameters on the tribological characteristics of the stator-rotor friction pair of a submersible screw pump. The results indicated that with a constant overall size, a multilayer hexagonal structure with ~0.1 mm groove depth enhanced the oil film-bearing capacity, providing significant friction and drag reduction. For composite textures, a deeper pit depth within the study area enhanced the oil film-bearing capacity. Furthermore, a gradient arrangement of groove textures featuring wider outer grooves and shallower depth exhibited superior performance in terms of bearing capacity.

Effects of liquid lubricants on the surface characteristics of 3D-printed polylactic acid

In this study, 3D-printed Polylactic acid (PLA) specimens were manufactured and polished using various lubricants to assess their surface, friction, and wear characteristics. After polishing, the surface roughness decreased by approximately 80% compared with that before polishing, except when acetone was used as the lubricant. In particular, under deionized (DI) water and acetone lubrication conditions, the friction coefficient decreased by 63% and 70%, respectively, whereas the specific wear rate decreased by 88% and 83%, respectively, compared with the unpolished specimens. In the case of dry polishing, adhesion, friction, and wear increase owing to surface damage. Ethanol and IPA polishing resulted in hydrolysis and increased friction, but slightly decreased wear rates. The surface of the specimen polished with acetone dissolved and became very rough. Only the surface polished with DI water exhibited hydrophobic properties. When acetone and DI water were used as lubricants, the surface adhesion force, adhesion energy, friction coefficient, and wear rate were lowest. The finite element analysis results showed that the polished surface exhibited stable contact pressure and friction force, while the unpolished surface showed large fluctuations in contact pressure and friction force owing to the laminated pattern. These results suggest that the polishing process is crucial for improving the surface characteristics and mechanical performance of 3D-printed PLA parts.