Lubricants, Volume 12, Issue 6 (June 2024) – 33 articles

This study investigates the effect of graphene nanoplatelets (GNPs) and milling duration on the microstructure, mechanical properties, and wear resistance of the AA7020 alloy reinforced with Fe₃O₄ and GNP. The composites were prepared with a fixed 10 wt.% Fe₃O₄ and varying GNP contents (0.5 and 1 wt.%) using high-energy ball milling for 4 and 8 h, followed by hot pressing. The aim was to enhance the performance of the AA7020 alloy for potential use in defense, automotive, aviation, and space applications, where superior mechanical properties and wear resistance are required. The results showed that the incorporation of 0.5 wt.% GNP and optimized milling significantly improved the composite’s performance. The AA7020 + 10 wt.% Fe₃O₄ + 0.5 wt.% GNP composite achieved the highest density (99.70%) when milled for 4 h. Its hardness increased with both the inclusion of GNP and extended milling duration, with the composite milled for 8 h exhibiting the highest hardness value (149 HBN). The tensile strength also improved, with the composite milled for 4 h showing a 28% increase (292 MPa) compared with the unreinforced alloy. Additionally, the friction coefficient decreased with GNP content and milling duration, with the composite milled for 8 h showing a 26% reduction. Wear resistance was notably enhanced, with the composite milled for 8 h exhibiting the lowest specific wear rate (7.86 × 10⁻⁷ mm³/Nm). Full article

Electric potential controlled lubrication, also known as triboelectrochemistry or electrotunable tribology, is an emerging field to regulate the friction, wear, and lubrication performance under charge distribution on the solid–liquid interfaces through an applied electric potential, allowing to achieve superlubrication. Electric potential controlled lubrication is of great significance for smart tunable lubrication, micro-electro-mechanical systems (MEMS), and key components in high-end mechanical equipment such as gears and bearings, etc. However, there needs to be a more theoretical understanding of the electric potential controlled lubrication between micro- and macro-scale conditions. For example, the synergistic contribution of the adsorption/desorption process and the electrochemical reaction process has not been well understood, and there exists a significant gap between the theoretical research and applications of electric potential controlled lubrication. Here, we provide an overview of this emerging field, from introducing its theoretical background to the advantages and characteristics of different experimental configurations (including universal mechanical tribometers, atomic force microscopes, and surface force apparatus/balances) for electric potential controlled lubrication. Next, we review the main experimental achievements in the performance and mechanisms of electrotunable lubrication, especially using ionic lubricants, including electrolyte solutions, ionic liquids, and surfactants. This review aims to survey the literature on electric potential controlled lubrication and provide insights into the design of superlubricants and intelligent lubrication systems for various applications. Full article

The requirement for high-performance and energy-saving materials motivated the researchers to develop novel composite materials. This investigation focuses on utilizing aluminum alloy (A383) as the matrix material to produce hybrid metal matrix composites (HMMCs) incorporating boron carbide (B₄C) and multi-walled carbon nanotube (MWCNT) through a cost-effective stir casting technique. The synthesis of HMMCs involved varying the weight fractions of B₄C (2%, 4%, and 6%) and MWCNT (0.5%, 1%, and 1.5%). The metallographic study was carried out by field emission scanning electron microscopy (FESEM) mapped with EDS analysis. The results indicated a uniform dispersion and robust interfacial interaction between aluminum and the reinforced particles, significantly enhancing the mechanical properties. Micro-hardness and wear characteristics of the fabricated HMMCs were investigated using Vickers microhardness testing and the pin-on-disc tribometer setup. The disc is made of hardened chromium alloy EN 31 steel of hardness 62 HRC. The applied load was varied as 10N, 20N, 30N with a constant sliding speed of 1.5 m/s for different sliding distances. The micro-hardness value of composites reinforced with 1.5 wt% MWCNT and 6 wt% B₄C improved by 61% compared to the base alloy. Additionally, the wear resistance of the composite material improved with increasing reinforcement content. Incorporating 1.5% CNT and 6% B₄C as reinforcements results in the composite experiencing about a 40% reduction in wear loss compared to the unreinforced aluminum alloy matrix. Furthermore, the volumetric wear loss of the HMMCs was critically analyzed with respect to different applied loads and sliding distances. This research underscores the positive impact of varying the reinforcement content on the mechanical and wear properties of aluminum alloy-based hybrid metal matrix composites. Full article

We proposed a numerical method to investigate the thermo-fluid–structural coupled characteristics of a mechanical seal of a reactor coolant pump (RCP), especially during extended loss of AC power (ELAP) operation. We developed a finite element program for the general Reynolds equation, including the turbulence effect to calculate the pressure, opening force, and leakage rate of fluid lubricant and the two-dimensional energy equation to calculate the temperature distribution of the fluid lubricant. We verified the accuracy of the developed program by comparing the simulated temperature distribution and leakage rate of this study with those of previous research. Heat conduction and elastic deformation due to pressure and temperature changes at the seal structure were analyzed using an ANSYS program. The results showed that temperature more significantly affected the elastic deformation of the seal structure near clearance than pressure both under normal and ELAP operating conditions. High temperature and pressure of the coolant under ELAP operating conditions deform the seal structure, resulting in a much smaller clearance of the fluid film than normal operating condition. However, even with a small clearance under ELAP operation, the leakage rate slightly increases due to the high internal pressure of the coolant. This research will contribute to the development of robust mechanical seals for RCPs by accurately predicting the characteristics of mechanical seals, especially when the RCP is operating under ELAP. Full article

Electromechanical energy conversion systems (EECSs) are widely used in vehicles to combine the double-row planetary gear system (DRPGS) with high transmission efficiency and high-performance motors. The integrated structure of the ring gear and motor rotor have put forward higher demands for the vibration performance of the DRPGS. This paper establishes a multibody dynamic model of the DRPGS for an EECS. Based on the kinetic relationship between the gear pairs and bearing components, the dynamic equations of the DRPGS are derived. The DRPGS model is simulated under different operating conditions. The results are compared to reveal the relationships between the system vibration and the operating speed and load torque. The typical conditions are selected to study the effectiveness of the structural parameters in reducing the DRPGS vibrations. The structural parameters, including the bearing clearance, the ball numbers, the gear tooth modification amount, and length, are comprehensively discussed. Several suggestions for the low-vibration design of the DRPGS for the EECS are provided. Full article

Traditional aero-engine bearings rotate simultaneously with their inner and outer rings, which makes the temperature rise prediction model computationally large with low accuracy, and it cannot be accurately verified due to the means of testing. This paper presents a method for predicting the temperature rise characteristics of aero-engine bearings under composite load conditions. Firstly, the local method is used to calculate the heat generation from heat sources such as bearing spin, lubricant drag, and the differential sliding of steel ball and collar, respectively, then finite element modelling and steady-state thermal analysis are carried out for aero-engine bearings under the simultaneous action of axial and radial external loads, a double-rotor test setup is designed and the predictive model is validated, and finally, the influences of rotational speed and load on the temperature rise characteristics of the bearings are investigated. The study shows that the aero-engine bearing prediction model proposed in this paper has high accuracy; with the increase in the rotational speed of the inner ring of the bearing, the temperatures of both the inner and outer rings of the bearing increase significantly; the temperatures of the inner and outer rings of the bearing increase with the increase in the axial load, and the effect of the radial load on the temperature of the bearing is not obvious. Full article

A new technique for finishing the surfaces of friction pairs has been proposed, which, in combination with the original test method, has shown a significant influence of the initial roughness configuration (surface texture) on friction and wear. Two types of finishing processing of the shaft friction surfaces were compared, and it was found that the friction and wear coefficients differ by more than 2–5 and 2–4 times, respectively. Based on a new methodology for analyzing standard roughness parameters, the tribological efficiency criteria (in the sense of reducing friction and wear) are proposed for the initial state of the friction surface of a radial plane sliding bearing shaft relative to the friction direction, which is consistent with its frictional characteristics. Comparison of the laboratory test results with the surface tribological efficiency criteria showed that these criteria are very promising for controlling existing technologies and optimizing new technologies for friction surface finishing in various friction systems. Full article

Ultrasonic surface rolling process (USRP) has the potential to improve the surface mechanical properties of metal components with platelike or cylindrical macrostructure, but its effect on spherical surfaces remains to be studied in depth. In order to investigate the effect of USRP on the surface roughness, hardness and wear resistance of a spherical joint bearing made of GCr15 bearing steel, ultrasonic rolling strengthening was carried out on a spherical bearing surface under various conditions. The surface roughness and hardness variations of samples before and after strengthening were investigated. It was found that the USRP strengthening process can effectively enhance the surface properties of GCr15 spherical bearing materials, reduce the surface roughness by more than 45%, and increase the surface hardness by more than 10%. Friction and wear tests were carried out before and after ultrasonic rolling. The results show that the friction coefficient of the bearing surface can be reduced by 28%, and that the wear volume can be reduced by 29%. The variation in the friction coefficient correlated to the variance of wear volume as the reinforcement changes. Full article

Artificial turf provides a consistent and durable surface; however, it has historically been associated with a high skin injury risk, or a ‘friction burn’, when a player falls or slides. Second-generation surfaces feature a short carpet pile, whilst third generation (3G) carpet piles are longer, enabling the integration of a performance infill. 3G surfaces provide sufficient energy absorption characteristics to be approved as Rugby Turf; however, such pitches can still cause skin injuries, despite being assessed using a friction-based test. Reducing skin injury risk motivates this study to develop a more sensitive testing methodology. A new test apparatus and impactor are proposed, achieving kinematics representative of an elite male rugby tackle. A commercially available skin simulant is employed to ensure the collection of repeatable and valid data. Photography and thresholding were used to assess surface abrasion and material transfer, whilst a thermal camera captured surface temperature change. Accelerometers quantified the surface resistance during the impact and sliding phases. These metrics were compiled into the Maxwell Tribo Index (MTI), providing a single measure of skin injury risk. The results demonstrated good repeatability and validity when four teams tested four different 3G surfaces. These results compared favourably to an expert panel’s ranked order. Full article

Brake wear particles, as the major component of non-exhaust particulate matter, are known to have different emissions, depending on the type of brake assembly and the specifications of the vehicle. In this study, brake wear and wear particle mass emissions were measured under realistic vehicle driving and full friction braking conditions using current commercial genuine brake assemblies. Although there were no significant differences in either PM₁₀ or PM_2.5 emissions between the different cooling air flow rates, brake wear decreased and ultrafine particle (PM_0.12) emissions increased with the increase in the cooling air flow rate. Particle mass measurements were collected on filter media, allowing chemical composition analysis to identify the source of brake wear particle mass emissions. The iron concentration in the brake wear particles indicated that the main contribution was derived from disc wear. Using a systematic approach that measured brake wear and wear particle emissions, this study was able to characterize correlations with elemental compositions in brake friction materials, adding to our understanding of the mechanical phenomena of brake wear and wear particle emissions. Full article

A profound comprehension of friction and wear mechanisms is essential for the design and development of high-performance polymeric materials for tribological application. However, it is difficult to deeply investigate the polymer friction process in situ at the micro/mesoscopic scale by traditional research methods. In recent years, molecular dynamics (MD) simulation, as an emerging research method, has attracted more and more attention in the field of polymer tribology due to its ability to show the physicochemical evolution between the contact interfaces at the atomic scale. Herein, we review the applications of MD in recent studies of polymer tribology and their research focuses (e.g., tribological properties, distribution and conformation of polymer chains, interfacial interaction, frictional heat, and tribochemical reactions) across three perspectives: all-atom MD, reactive MD, and coarse-grained MD. Additionally, we summarize the current challenges encountered by MD simulation in polymer tribology research and present recommendations accordingly, aiming to provide several insights for researchers in related fields. Full article

Titanium alloys are widely used in various fields, but milling titanium alloy materials often leads to problems such as high milling forces, increased milling temperatures, and chip adhesion. Thus, the machinability of titanium alloys faces challenges. To improve the milling performance of titanium alloy materials, this study analyzes the effective working area on the surface of the milling cutter through mathematical calculations. We design micro-grooves in this area to utilize their friction-reducing and wear-resisting properties to alleviate the aforementioned issues. The effective working area of the ball end milling cutter’s cutting edge is calculated based on the amount of milling and the installation position between the milling cutter and the workpiece. By observing the surface structure of seashells, micro-grooves are proposed and designed to be applied in the working area of the milling cutter surface. The impact of the micro-groove area on the milling cutter surface and spindle speed on milling performance is discussed based on milling simulation and experimental tests. Experimental results show that the cutting force, milling temperature, and chip resistance to adhesion produced by micro-groove milling cutters are superior to conventional milling cutters. Milling cutters with three micro-grooves perform best at different spindle speeds. This is because the presence of micro-grooves on the surface of the milling cutter improves the friction state, promoting a reduction in milling force, while the micro-grooves also serve as storage containers for chips, alleviating the phenomenon of chip softening and adhesion to the cutter. When conducting cutting tests with a milling cutter that has three micro-grooves, the milling force is reduced by 10% to 30%, the milling temperature drops by 10% to 20%, and the surface roughness decreases by 8% to 12%. Full article

Understanding and predicting the friction between a steel runner and an ice surface is paramount for many winter sports disciplines such as luge, bobsleigh, skeleton, and speed skating. A widely used numerical model for the analysis of the tribological system steel-on-ice is the Friction Algorithm using Skate Thermohydrodynamics (F.A.S.T.), which was originally introduced in 2007 and later extended. It aims to predict the resulting coefficient of friction (COF) from the two contributions of ice plowing and viscous drag. We explore the limitations of the existing F.A.S.T. model and extend the model to improve its applicability to winter sports disciplines. This includes generalizing the geometry of the runner as well as the curvature of the ice surface. The free rotational mechanical mounting of the runner to the moving sports equipment is introduced and implemented. We apply the new model to real-world geometries and kinematics of speed skating blades and bobsleigh runners to determine the resulting COF for a range of parameters, including geometry, temperature, load, and speed. The findings are compared to rule-of-thumb testimonies from athletes, previous numerical approaches, and published experimental results where applicable. While the general trends are reproduced, some discrepancy is found, which we ascribe to the specific assumptions around the formation of the liquid water layer derived from melted ice. Full article

This research investigates the micro-mechanical and tribological properties of injection-molded parts made from polypropylene. The tribological properties of polymers are a very interesting area of research. Understanding tribological processes is very crucial. Considering that the mechanical and tribological properties of injected parts are not uniform at various points of the part, this research was conducted to explain the non-homogeneity of properties along the flow path. Non-homogeneity can be influenced by numerous factors, including distance from the gate, mold and melt temperature, injection pressure, crystalline structure, cooling rate, the surface of the mold, and others. The key factor from the micro-mechanical and tribological properties point of view is the polymer morphology (degree of crystallinity and size of the skin and core layers). The morphology is influenced by polymer flow and the injection molding process conditions. Gained results indicate that the indentation method was sufficiently sensitive to capture the changes in polypropylene morphology, which is a key parameter for the resulting micro-mechanical and tribological properties of the part. It was proven that the mechanical and tribological properties are not equal in varying regions of the part. Due to cooling and process parameters, the difference in the indentation modulus in individual measurement points was up to 55%, and the tribological properties, in particular the friction coefficient, showed a difference of up to 20%. The aforementioned results indicate the impact this finding signifies for injection molding technology in technical practice. Tribological properties are a key property of the part surface and, together with micro-mechanical properties, characterize the resistance of the surface to mechanical failure of the plastic part when used in engineering applications. A suitable choice of gate location, finishing method of the cavity surface, and process parameters can ensure the improvement of mechanical and tribological properties in stressed regions of the part. This will increase the stiffness and wear resistance of the surface. Full article

In the analysis of tribological contacts, the focus is often on a singular question or result. However, this entails the potential risk that the overall picture and the relationships could be oversimplified or even that wrong conclusions could be drawn. In this article, a comprehensive consideration of test results including component and lubricant analyses is demonstrated by using the example of rolling contact. For this purpose, thrust cylindrical roller bearings of type 81212 with unadditized base oils were tested in the mixed-friction area. Our study shows that by using an adapted and innovative surface analysis, a deeper dive into the tribo-film is feasible even without highly sophisticated analytical equipment. The characterization of the layers was performed by the three less time-consuming spatially resolved analysis methods of µXRF, ATR FTIR microscopy and Raman spectroscopy adapted by Schaeffler. This represents a bridge between industry and research. The investigations show that especially undocumented and uncontrolled contamination of the test equipment could lead to surprising findings, which would result in the wrong conclusions. Simple substances, like hydrocarbons, are demanding test specimens. Full article

Vacuum and air atmospheres impose very different requirements on tribological-loaded contacts, which usually require different surface materials. While hard tetrahedral amorphous carbon (ta-C) coatings provide good tribological properties in air, soft coatings such as molybdenum disulfide (MoS2) work well in a vacuum. Tribological performance in the respective other environment, however, is poor. In this work, the combination of laser microstructured (direct laser interference patterning) steel substrates and the deposition of ta-C and MoSx coatings with vacuum arc evaporation (LaserArc™) was studied, resulting in steel/DLIP, steel/DLIP/ta-C, steel/DLIP/MoSx, steel/DLIP/ta-C/MoSx, and steel/MoSx surface combinations. The tribological properties were studied using a ball-on-disk tribometer with a steel ball counter body in air and in a vacuum (p < 5 × 10⁻⁷ mbar). The type of the topmost coating governed their tribological properties in the respective atmosphere, and no general beneficial influence of the microstructure was found. However, steel/DLIP/ta-C/MoSx performed best in both conditions and endured the highest contact pressure, which is attributed to the mechanical support of the ta-C coating and MoSx reservoir in the remaining structure, as evidenced by Raman spectroscopy. The findings suggest that such combination allows for surfaces bearing a high load capacity that can be applied in both a vacuum and in air, for example, in multi-use space applications. Full article

This article presents the improved properties of GI (hot-dip galvanized) steel plates in combination with a special permanent surface treatment. The substrate used was hot-dip galvanized deep-drawn steel sheets of grade DX56D + Z. Subsequently, various surface treatments were applied to their surface. The coefficient of friction of the metal sheets without surface treatment, with a temporary surface treatment called passivation, and a thin organic coating (TOC) based on hydroxyl resins dissolved in water, Ti and Cr³⁺ were determined by a cup test. The surface quality and corrosion resistance of all tested samples were also determined by exposing them for up to 288 h in an atmosphere of neutral salt spray. The surface microgeometry parameters Ra, RPc and Rz(I), which have a significant influence on the pressing process itself, were also determined. The TOC deposited on the Zn substrate was the only one to exhibit excellent lubrication and anticorrosion properties, resulting in the lowest surface microgeometry values owing to the uniform and continuous layer of the thin organic coating compared to the GI substrate and passivation surface treatment, respectively. Full article

Biobased greases are derived from renewable resources, are considered more environmentally friendly, and offer comparable performance to petroleum-based greases. In this study, lubricating greases from frying cooking oils were prepared, thus valorizing waste in order to obtain sustainable and environmentally friendly products. Twelve batches (500 g each) were produced from sunflower and palm frying oils, with 20% by weight calcium/lithium stearate soaps prepared in situ and filled with 15 wt.% cellulose or lignin sulfate. The greases were rheologically characterized. Their consistency was assessed by the penetration test performed before and after working the greases. Dropping point determinations offered information about the stability at higher temperatures, and oil bleeding tests were performed. The average values of the friction coefficient (COF), the contact resistance, and the wear scar diameter were measured through mechanical tests. The greases prove to be comparable to those obtained from mineral oils, with good rheological properties, soft consistency, and good antiwearing behavior, e.g., in open or total-loss lubricating systems, like in open gears and certain food processing machinery; they are thermally stable andprone touse in low-loading working mechanisms. Full article

With the increasingly demanding engine conditions and the implementation of “double carbon” policies, the demand for high-quality lubricants that are cost-effective and environmentally friendly is increasing. Additives, especially high-performance friction modifiers, play an important role in boosting lubricant efficiency and fuel economy, so their developments are at the forefront of lubrication technologies. In this study, 1,3-dioleoamide-2-propyloleate (DOAPO), which incorporates polar amide, ester, and nonpolar alkyl chains, was synthesized from 1,3-diamino-2-propanol to give an eco-friendly organic friction modifier. Nuclear magnetic resonance (NMR), high-resolution mass spectrometry (HR-MS), Fourier-transform infrared spectroscopy (FT-IR), and thermogravimetric analysis (TGA) were used to characterize the structure and thermal stability of DOAPO. Meanwhile, the storage stability and tribological behaviors of DOAPO in synthetic base oil were studied and compared with a commercial oleamide. The results show that DOAPO has better thermal stability and better storage stability in synthetic base oil. Additionally, 0.5 wt.% of DOAPO could shorten the running-in period and reduce the average friction coefficient (ave. COF) and wear scar diameter (ave. WSD) by 8.2% and 16.2%, respectively. The worn surface analysis and theoretical calculation results show that the ester bond in DOAPO breaks preferentially during friction, which can reduce the interfacial shear force and easily react with metal surfaces to form iron oxide films, thus demonstrating a better friction-reducing and anti-wear performance. Full article

The coefficients of friction (COFs) is one of the most important parameters used to evaluate the braking performance of a friction brake. Many indicators that affect the safety and comfort of automobiles are associated with brake COFs. The manufacturers of friction brakes and their components are required to spend huge amounts of time and money to carry out experimental tests to ensure the COFs of a newly developed braking system meet the required standards. In order to save time and costs for the development of new friction brake applications, the GRU (Gate Recurrent Unit) algorithm optimized by the improved PSO (particle swarm optimization) global optimization method is employed in this work to predict brake COFs based on existing experimental data obtained from friction braking dynamometer tests. Compared with the LSTM (Long Short-Term Memory) method, the GRU algorithm optimized by PSO avoids the accuracy reduction problem caused by gradient descent in the training process and hence reduces the prediction error and computational cost. The combined PSO–GRU algorithm increases the coefficient of determination (R²) of the prediction by 4.7%, reduces the MAE (mean absolute error) by 14.3%, and increases the prediction speed by 40.1% compared with the standalone GRU method. The prediction method based on machine learning proposed in this study can not only be applied to the prediction of automobile braking COFs but also for other frictional system problems, such as the prediction of braking noise and the friction of various bearing transmission components. Full article

As part of a publicly funded cooperation project, novel high-performance lubricants (oils, greases, assembly pastes) based on ionic liquids and with the addition of specific micro- or nanoparticles are to be developed, which are adapted in their formulation for use in applications where their negligible vapor pressure plays an important role. These lubricants are urgently needed for applications in cleanrooms and high vacuum (e.g., pharmaceuticals, aerospace, chip manufacturing), especially when the frequently used perfluoropolyethers (PFPE) are no longer available due to a potential restriction of per- and polyfluoroalkyl substances (PFAS) due to European chemical legislation. Until now, there has been a lack of suitable laboratory testing technology to develop such innovative lubricants for extreme niche applications economically. There is a large gap in the tribological test chain between model testing, for example in the so-called spiral orbit tribometer (SOT) or ball-on-disk test in a high-frequency, linear-oscillation test machine (SRV-Tribometer from German “Schwing-Reib-Verschleiß-Tribometer”), and overall component testing at major space agencies (ESA—European Space Agency, NASA—National Aeronautics and Space Administration) or their service providers like the European Space Tribology Laboratory (ESTL) in Manchester. A further aim of the project was therefore to develop an application-orientated and economical testing methodology and testing technology for the scientifically precise evaluation and verifiability of the effect of ionic liquids on tribological systems in cleanrooms and under high vacuum conditions. The newly developed test rig is the focus of this publication. It forms the basis for all further investigations. Full article

The problem of normal impact of a rigid sphere on a Maxwell viscoelastic solid half-space is considered. The first-order asymptotic solution is constructed in the framework of Hunter’s model of viscoelastic impact. In particular, simple analytical approximations have been derived for the maximum contact force and the time to achieve it. A linear regression method is suggested for evaluating the instantaneous elastic modulus and the mean relaxation time from a set of experimental data collected for different spherical impactors and impact velocities. Full article

The role of industrial lubricants in machinery is to reduce friction and wear between moving components. Due to the United Nations’ tendency to reduce dependency on fossil fuel, a general awareness is strongly driven towards developing more eco-friendly lubricants. Palm oil possesses promising properties, which promote it to be a competitive alternative to the hostile mineral oils. Still, marginal oxidation stability, viscosity, and tribological properties remain critical issues for performance improvement. This paper presents an improved palm grease using reduced graphene oxide (rGO) and zinc oxide (ZnO) nano-additives at different concentrations. Oil and grease samples were tested for viscosity, oxidation stability, pour point, penetration, roll stability, dropping point, churned grease-oil release, copper corrosion, friction, and wear. ZnO additives enhanced the oxidation stability by 60% and shifted the pour point to 6 °C. Adding ZnO and rGO to the palm grease increased the load-carrying capacity between 30% and 60%, respectively, and reduced the friction coefficient by up to 60%. From the wear scar morphologies, it is believed that graphene 2D nanoparticles formed absorption layers which contributed to the increase in load-carrying capacity, while ZnO chemically reacted with the metallic surface layer, forming zinc compounds that resulted in a protective boundary lubricating film. Full article

This paper is the second part of a two-part report studying the responses of a typical point-contact elastohydrodynamic lubrication (EHL) system to multiscale roughness mimicked by wavy surfaces. The wavy surfaces are defined by three key parameters: amplitudes, frequencies, and directions. The previous Part I paper focuses on the full film lubrication condition, while the current paper focuses on the partial film regime where asperity contacts occur. A transient thermal EHL model simulates lubrication problems with different waviness parameters, loads, and speeds. The total number of simulations is 1600. Performance parameters, including the asperity contact ratio, minimum film thickness, maximum pressure, central point film thickness, central point pressure, mean film thickness, coefficient of friction (COF), and the maximum temperature rise, are obtained for each simulation. These performance parameters are post-processed in the same manner as those in the previous Part I paper. The influences of the waviness parameters, load, and speed values on the eight performance parameters are discussed. Full article

To mitigate the environmental hazards aroused by fossil-based lubricants, the development of eco-friendly internal lubricants is imperative. Siloxane-based internal lubricants, widely applied as plasticizers in polymeric compounds, are a promising option. However, their impacts on the tribological properties of polymeric tribocomponents are still unclarified. Therefore, in the current study, a siloxane-based internal lubricant with the product name ‘EverGlide MB 1550 (EG)’ was dispersed into a polybutylene terephthalate (PBT)-based tribological composite to investigate whether the tribological properties of the composite can be optimized. A block-on-ring (BOR) test configuration was used for this purpose. It was found that the addition of EG to the composite significantly improved the tribological behavior; the improvement was particularly significant under lower load conditions (pv-product ≤ 2 MPa∙m/s). Compared to the reference PBT composite, the addition of EG reduced the friction coefficient (COF) by about 30% and the specific wear rate by about 14%. An accompanying surface analytical investigation using photoelectron spectroscopy to elucidate the effective mechanisms at the molecular level showed the availability of tribologically effective and free EG after its addition to the composite in the relevant tribocontact. Full article

Over one billion rubber tires are disposed of worldwide annually as a major component of the solid waste stream, posing a significant environmental risk. Therefore, recycling and taking advantage of the rubber component in End-of-Life Tires (ELTs) presents an advantageous opportunity to produce environmentally friendly and cost-effective products. This work studied multiple properties of oil extracted from ELTs using thermal pyrolysis (i.e., pyro-oil) as a potential candidate for industrial lubrication applications. First, pyro-oil was characterized by studying its morphological and chemical properties. Then, rheological studies were conducted to explore the oil properties at different temperatures and shear rates. A tribometer was also used to assess pyro-oil’s tribological performance at different temperatures and speeds. Finally, wettability and thermal analyses were performed to understand the wetting and thermal stability properties. The results revealed that pyro-oil has chemical properties similar to conventional engine oil with slightly higher sulfur content. Furthermore, the pyro-oil exhibited lower viscosity and lubrication performance than conventional engine oil, but this difference was smaller at higher temperatures. Thermal stability and wetting properties of pyro-oil were found to be significantly lower than those of conventional engine oil. Based on the properties found and compared with engine oil, pyro-oil presents itself as a suitable liquid lubricant for low-speed, low-load applications operating in temperatures below 61 °C. This work presents a comprehensive study of pyro-oil properties extracted from end-of-life waste tires, offering a feasible route to obtain sustainable and low-cost products. Full article

The longevity of prosthetic hip implants is significantly influenced by wear. Surface textures of various length scales can reduce the friction coefficient and wear of lubricated bearing surfaces. The optimization of multiscale texture parameters, aimed at maximizing lubricant film thickness, was achieved through hydrodynamic lubrication simulations that solve the Reynolds equation with a mass-conserving cavitation model under various operating conditions. The outcomes indicate that adding “interstitial” texture features to a pattern of microscale texture features can further increase the lubricant film thickness. Additionally, the lubricant film thickness increases as the interstitial texture feature aspect ratio and texture density decrease. Pin-on-disc experiments align with simulation findings, demonstrating that multiscale texturing with ultra-fast laser ablation on Ti6Al4V discs significantly improves wettability and reduces the friction coefficient of ultra-high molecular weight polyethylene pins when compared to untextured and microscale textured surfaces. The multiscale surface texturing also changes the evident wear mechanisms on the pins, reducing the incidence of abrasive scratches and adhesive wear compared to both untextured and just microscale textured surfaces. Full article

Hydrogels have received extensive attention as functional lubricants because of their excellent anti-friction and anti-wear properties, tunable tribological performances, and effectiveness in alleviating lubrication failures caused by the creeping or leakage of conventional liquid lubricants owing to their semi-solid nature. This review summarizes the current research advances in hydrogel lubricants fabricated with various organic and/or inorganic gelators, including organic polymeric or supramolecular hydrogels, inorganic particles-based hydrogels, and organic polymer-inorganic particle hybrid hydrogels. We illustrate not only the design strategies for constructing high-performance hydrogel lubricants but also the tribological behavior and mechanism of different types of hydrogel lubricants and their potential applications in industrial and biomimetic fields. Corresponding outlooks and suggestions for future studies have also been proposed. Full article