Friction Reduction Mechanisms Research Articles

A Review on Tribological Considerations in the Transition from IC Engines to Electric Vehicles

The shift from internal combustion (IC) engines to electric vehicles (EVs) marks a significant transformation in the automotive industry, prompting a comprehensive reassessment of various engineering considerations. Among these, tribological factors play a critical role in ensuring the performance, reliability, and longevity of vehicle components. This review examines the tribological challenges and opportunities posed by the transition to EVs, focusing on key components such as bearings, gears, and braking systems, which face unique operating conditions in electric powertrains compared to their IC counterparts. The paper addresses how electric vehicles encounter distinct tribological scenarios, such as lower operating temperatures but higher torque loads, which demand new materials and lubrication strategies. It also explores how the near absence of internal combustion in EVs affects component wear and the mechanisms of friction reduction. Additionally, the tribological challenges in IC engines are revisited to provide a comparative understanding of how they differ from those in EVs, particularly regarding energy efficiency and frictional losses. This review emphasizes the importance of minimizing wear and friction to maximize energy efficiency, which is crucial for extending vehicle range and improving performance in EVs. By synthesizing the latest research findings and industry advancements, the review offers valuable insights for researchers and engineers involved in the design and optimization of tribological systems for the next generation of electric vehicles.

In-situ vibrational spectroscopic observation for thermally activated structural changes of 100% cellulose nanofiber molding with ultralow friction

This paper reports the thermally activated ultralow friction of 100% cellulose nanofiber (CNF) molding. The mechanism of friction reduction was investigated using a laboratory-built in-situ Raman tribometer. Our experimental results showed that a CNF molding exhibited an ultralow friction coefficient of below 0.04 in a CNF ring and steel disk tribopair under high-temperature conditions (T > 100°C). The results of the temperature-rise friction test showed that the friction coefficient of the CNF molding strongly depended on the temperature and decreased linearly with increasing temperature. The in situ tribo-Raman monitoring results, during friction, indicated a change in the structure of the CNF molding. Therefore, the crystallinity indices and lengths of the CNF fibers gradually changed as the temperature increased. Moreover, transfer tribofilms were observed on the counter-steel surface against the CNF rings. When the CNF molding exhibited thermally activated ultralow friction, the tribofilm was mainly composed of cellulose and graphitic carbon. Our results suggest that the thermal and friction-activated structural transformations of CNF molding and CNF-derived transfer film formation are pivotal factors contributing to the ultralow friction phenomenon observed in CNF molding at high temperatures.

Tribological Properties of Polytetrafluoroethylene (PTFE)/Aramid Fabric Composite Liner Modified by PAO40/Polysulfone/SiO2 Hybrid Shell Microcapsules

Hybrid shell microcapsules have been incorporated into polytetrafluoroethylene (PTFE)/aramid fabric composite liners to enhance the tribological behavior of self-lubricating spherical plain bearings. PAO40/polysulfone/SiO2 hybrid shell microcapsules were synthesized utilizing a combination of Pickering emulsification technology and solvent evaporation. The modified liner with 9 wt% microcapsules exhibited the highest reduction in the average friction coefficient and wear rate, which were 31.1% and 47.9% lower than the unmodified liner. Furthermore, the tribological behavior of modified liners was investigated under different loads and sliding speeds. The friction process of the modified liner and the formation process of the transfer film were analyzed. Investigations into the friction reduction mechanism revealed that the PAO-40# released from microcapsules transitioned the friction type from dry to mixed. SiO2 particles are conducive to forming a denser transfer film by the PTFE abrasive. The results of this study have important scientific significance.

Utilizing photodeposition to fabricate micro-nano structured microcapsules for enhancing mechanical and high-temperature tribological performance of fabric composites

Fabric composite materials face significant challenges in maintaining excellent tribological properties at high temperatures. This study employed photodeposition to deposit gold (Au) and copper (Cu) nanoparticles on the surface of titanium dioxide-coated jojoba oil microcapsules (TiO2 @jojoba), yielding micro-nanostructured Au-TiO2 @jojoba and Cu-TiO2 @jojoba microcapsules. Subsequently, microcapsules participated in the in-situ polymerization of polyimide (PI), serving as crosslinking points in PEEK/PTFE fabric-reinforced composites. PEEK/PTFE-Au composites containing Au-TiO2 @jojoba microcapsules exhibited outstanding tribological and mechanical properties. Compared to microcapsule-free samples, PEEK/PTFE-Au composites showed a 30.26 % reduction in friction coefficient at 250 °C and a 24.28 % increase in tensile strength. In addition, mechanistic studies indicated that TiO2 underwent a friction chemical reaction converting to TiNF during friction accompanying by friction-induced phase transitions. The release of jojoba oil during friction and the occurrence of friction chemical reactions were the main friction reduction mechanisms of PEEK/PTFE-Au composites at high temperatures.

Reduction in Powder Wall Friction by an a-C:H:Si Film

The wall friction angle is an important parameter in powder flow. In a recent study for various powders, a reduction in the wall friction angle for steel was demonstrated by the application of an a-C:H:Si film on the steel surface. This work presents the results of a study of this effect in more detail regarding the influence of the powder material, the wall normal stress and the particle size of the powder for mass median diameters from 4 µm to approximately 150 µm. The wall friction angles were measured using a Schulze ring shear tester for three different powder materials: aluminum oxide, calcium carbonate and silicon carbide. The results showed little difference with respect to powder chemistry. For the coarser powders, the reduction in the wall friction angle due to the a-C:H:Si coating was highest (10° to 12°) and rather stress-independent, while for the fine and medium-size powders the reduction was lower and stress-dependent. With increasing wall normal stress, the reduction in the wall friction angle increased. These results can be explained by the friction reduction mechanism of a-C:H:Si, which requires a certain contact pressure for superficial graphitization.

In-situ tribo-induced formation of superb lubricious carbon-based tribofilms on the catalytical active MoN-Ag film surfaces

The establishment of environmentally friendly lubrication systems is of significant importance to the sustainable development. Here, we demonstrated an environmentally friendly approach, where superb lubricious carbon-based tribofilms formed in situ originating from base oil through tribochemical reactions, on the surfaces of catalytically active MoN-Ag film. We focused on the study of the tribological behavior of the MoN-Ag/oil solid-liquid system under varying loads. Through detailed characterization analysis of the carbon-based tribofilms, we elucidated the mechanism of friction reduction and wear resistance of the MoN-Ag film in the base oil, as well as the formation mechanism of carbon-based tribofilms. Furthermore, we provided a thermodynamic perspective to clarify the influence of load on the tribo-induced tribochemical reactions within the MoN-Ag/oil solid-liquid system.

Adaptive TiN-Cu/PAO composite lubrication system: The tribocatalysis-induced PAO6 transferring to amorphous carbon

A friendly solid-liquid lubrication system, which is the key to solve the environmental harm caused by phosphorus and sulfur additives in lubricants, has attracted the attention. In this paper, the TiN-Cu/PAO solid-liquid composite system was constructed in situ by tribochemical reaction based on the performance of nitride film and base oil. The results show that when rubbing in PAO, the carbon-based films deriving from PAO will form at the contacting interface. From the perspectives of surface energy, friction products and simulation calculation, the mechanisms of friction and wear reduction of the solid-liquid composite system was systematically investigated. Meanwhile, the degradation process of PAO molecules during friction was revealed from the molecular level, including adsorption, dehydrogenation, C-C bond breaking and reconstruction.

Material system and tribological mechanism of plasma sprayed wear resistant coatings: Overview

Challenging environments in aviation, automotive, electric power, and various industrial sectors are increasing industry and technological demands, resulting in machine damage and energy loss from friction and wear between interacting surfaces. Addressing this issue requires the development of new and advanced materials, processing techniques, and surface modification methods to enhance performance and reduce friction and wear. Plasma spraying technology is essential for material surface reinforcement and modification because of its low cost and stability. This paper reviews and summarizes the research progress on plasma spraying wear-resistant coating material systems and tribological mechanisms, covering the working process and principles of plasma spraying equipment. It also details wear-resistant coating material systems, including iron-based, nickel-based, copper-based, other metal materials, oxides, and monoxide ceramic materials.Furthermore, it addresses the wear-resistant and friction-reduction mechanisms of wear-resistant coatings, aiming to summarize the tribological strengthening mechanism concerning wear resistance and friction reduction. Moreover, this paper covers aspects such as powder design, sealing treatment, laser remelting, and other methods for regulating the wear resistance of coatings, examining their influence on the wear resistance of coatings in material design, spraying process, and post-spraying treatment. Plasma spraying wear-resistant coatings should be used along with doping modification and other methods to study the new spraying materials; furthermore, the optimization of the spraying process and post-treatment methods should be investigated to prepare a wear-resistant coating that meets the performance of the actual working conditions. At the same time, ultrahigh temperature wear-resistant coatings and super lubricating coatings are also future research priorities. In addition, when comparing plasma transfer arc spraying technology and plasma enhanced high-speed arc spraying technology with plasma non-transfer arc spraying technology, the jet temperature is higher, and the cost is lower; however, since the process is still in its infancy, the follow-up should be strengthened in this area of technical research. This paper provides a theoretical basis and reference for researching plasma spraying wear-resistant coatings in various fields.

Digging into the friction reduction mechanism of organic friction modifiers on steel surfaces: Chains packing vs. molecule–metal interactions

We present a study of five commercially available Organic Friction Modifiers (OFMs) that combines micro-Infrared reflection absorption spectroscopy (IRRAS), classical MD, and DFT/DFT-MD computational methods. We investigate the relation between the molecular structure of the OFMs and their temperature-dependent friction performance. Classical dynamics simulations show that at low temperatures, the OFM molecules, arranged in self-assembled monolayers, undergo collective torsional motion during a friction event. This becomes progressively easier as temperature increases, resulting in lower friction coefficients, until a temperature where these collective motions cannot be maintained anymore, leading to an increase of the friction coefficient. Static DFT calculations indicate that the strength of the packing between OFM molecules determines the friction performance. The temperature at which the collective behavior fails coincides with the temperature at which the packing-free energy is found to diminish.

Study on friction reduction performance of granular flow lubrication during the milling of Inconel 718 superalloy

PurposeThe purpose of this study is to reveal the friction reduction performance and mechanism of granular flow lubrication during the milling of difficult-to-machining materials and provide a high-performance lubrication method for the precision cutting of nickel-based alloys.Design/methodology/approachThe milling tests for Inconel 718 superalloy under dry cutting, flood lubrication and granular flow lubrication were carried out, and the milling force and machined surface quality were used to evaluate their friction reduction effect. Furthermore, based on the energy dispersive spectrometer (EDS) spectrums and the topographical features of machined surface, the lubrication mechanism of different granular mediums was explored during granular flow lubrication.FindingsCompared with flood lubrication, the granular flow lubrication had a significant force reduction effect, and the maximum milling force was reduced by about 30%. At the same time, the granular flow lubrication was more conducive to reducing the tool trace size, repressing surface damage and thus achieving better surface quality. The soft particles had better friction reduction performance than the hard particles with the same particle size, and the friction reduction performance of nanoscale hard particles was superior to that of microscale hard particles. The friction reduction mechanism of MoS2 and WS2 soft particles is the mending effect and adsorption film effect, whereas that of SiO2 and Al2O3 hard particles is mainly manifested as the rolling and polishing effect.Originality/valueGranular flow lubrication was applied in the precision milling of Inconel 718 superalloy, and a comparative study was conducted on the friction reduction performance of soft particles (MoS2, WS2) and hard particles (SiO2, Al2O3). Based on the EDS spectrums and topographical features of machined surface, the friction reduction mechanism of soft and hard particles was explored.

Atomic-Scale Insights Into Graphene/Fullerene Tribological Mechanisms and Machine Learning Prediction of Properties

Abstract Graphene/fullerene carbon–based nanoparticles exhibit excellent tribological properties in solid–liquid two-phase lubrication systems. However, the tribological mechanism still lacks profound insights into dynamic friction processes at the atomic scale. In this paper, the friction reduction and anti-wear mechanism of graphene/fullerene nanoparticles and the synergistic lubrication effect of the binary additive system were investigated by molecular dynamics simulations and tribological experiments. The friction performance was predicted based on six machine learning algorithms. The results indicated that in fluid lubrication, graphene promoted “liquid–liquid” interlayer sliding, whereas fullerene facilitated “solid–liquid” interface sliding, resulting in a decrease or increase in friction force. Under boundary lubrication, graphene/fullerene nanoparticles were adsorbed and anchored at the metal interface to form a physical protective film, which improved the bearing capacity of the lubricating oil film, transformed the direct contact between asperities into interlayer sliding of graphene and roll–slide polishing, filling, and repairing of fullerene, thus improving the frictional wear of the lubrication system as well as the friction temperature rise and stress concentration of the asperities. Furthermore, six machine learning algorithms showed low error and high precision, and the coefficient of determination was greater than 0.9, indicating that all models had good prediction and generalization capabilities, fully demonstrating the feasibility of combining molecular simulation and machine learning applications in the field of tribology.

Tribochemistry of mercaptoimidazoline as an additive in emulsion between self-mated GCr15 ball interfaces and its friction-reduction mechanism

The tribological properties of two mercaptoimidazolines, with amino (2-MBTA) and hydroxyl (2-MBTD) functional groups, as lubricating additives in a water-based rapeseed oil emulsion were investigated. The results of tribological tests on a four-ball tester showed that both mercaptoimidazolines had effective anti-wear and friction reduction properties. The optimal tribological performance of the 2-MBTD was found to be 98 N and could be scribed to the hydroxyl group, whereas the amino group in 2-MBTA created a more effective wear-reducing tribofilm on the rubbing surfaces at 294 N. EDS analysis on the worn surface areas showed that the difference in tribological properties between 2-MBTA and 2-MBTD was attributed to the different contents of the active elements N and S. In addition, XPS results indicated that sulfates were prevalent in the tribofilm for 2-MBTA, whereas sulfides that could reduce the coefficient of friction (COF) were present in the tribofilm for 2-MBTD. The friction mechanism of 2-MBTA and 2-MBTD is that different functional groups influence the composition of the boundary lubrication film formed, which in turn affects the tribological properties of the additives.

MoS2 Nanomaterials as Lubricant Additives: A Review

Improving the lubricating properties of base oils through additives is a crucial objective of tribological research, as it helps to reduce friction and wear of materials. Molybdenum disulfide (MoS2) is a 2D nanomaterial with excellent tribological properties that is often used as a lubricant additive. Several studies have been conducted on the preparation and utilization of MoS2 and its nanocomposites as lubricant additives. This paper reviews the research progress on MoS2 nanomaterials as lubricant additives. It firstly introduces various synthesis methods of MoS2 nanomaterials while focusing on the preparation of nano-MoS2 as lubricant additives. It then summarizes the dispersion stability of nano-MoS2 in lubricating oils which has been paid extensive attention. Moreover, this paper reviews and discusses the tribological properties of nano-MoS2 and its various composites as lubricant additives. The possible anti-wear and friction reduction mechanisms of nano-MoS2 and its composites are also discussed. Finally, this paper presents the challenges faced by nano-MoS2 additives in the field of lubrication and the prospects for future research in view of previous studies.

Preparation of ionic liquid@SiO2 nanocapsules for improving self-lubricating performance of PA6 composites

A novel high temperature resistance ionic liquid@SiO2 (IL@SiO2) nanocapsules were successfully prepared in order to meet the high temperature processing requirement. The average diameter and the loading capacity of as-obtained IL@SiO2 nanocapsules were about 420 nm and 44.7 wt% respectively. The IL@SiO2 nanocapsules showed excellent thermal stability with a 340 °C initial thermal degradation temperature. The frictional and wear properties of PA6 composites containing IL@SiO2 nanocapsules under different friction temperature were studied systemically. Self-lubricating PA6 composites with 3 wt% IL@SiO2 nanocapsules showed the optical tribological propertied. The corresponding friction coefficient of PA6 composites reduced by 38.9%, and the wear rate also showed a 60.7% decrease compared with pure PA6. Unlike most microcapsules, the addition of nanocapsuels did not cause an undesirable and obvious decrease in mechanical properties of PA6. Anti-wear and friction-reduction mechanism were depth analyzed, and results revealed that IL core and SiO2 wall in nanocapsules synergistically contribute to improve self-lubricating performance of PA6 composites.

Investigating the Wettability, Rheological, and Tribological Properties of Ammonium-Based Protic Ionic Liquids as Neat Lubricants for Steel–Steel and Steel–Aluminium Contacts

This study aims to evaluate the tribological properties of two protic ionic liquids (PILs) under different tribological conditions as a sustainable alternative for mineral oil-based neat lubricants. The synthesis of PILs in this study uses a relatively simple and less expensive method. The Fourier transform infrared (FTIR) spectroscopy results help validate the synthesised PILs’ formation. Further, their physicochemical and tribological properties were investigated. The PILs as neat lubricants were tested on a ball-on-plate reciprocating tribometer using bearing steel–bearing steel and bearing steel–aluminium alloy friction pairs at 30 °C and 80 °C. The results show that the investigated PILs significantly reduced the coefficient of friction and wear. The dodecylamine-based PILs performed better in friction and wear reduction than the other investigated lubricants. The formation of the adsorption layer on the friction pairs was assumed to be the dominant friction and wear reduction mechanism.

Nanotribological Properties of Oxidized Diamond/Silica Interfaces: Insights into the Atomistic Mechanisms of Wear and Friction by Ab Initio Molecular Dynamics Simulations.

Controlling friction and wear at silica-diamond interfaces is crucial for their relevant applications in tribology such as micro-electromechanical systems and atomic force microscopes. However, the tribological performance on diamond surfaces is highly affected by the working environment where atmospheric gases are present. In this work, we investigate the effects of adsorbed oxygen on the friction and wear of diamond surfaces sliding against silica by massive ab initio molecular dynamics simulations. Different surface orientations, O-coverages, and tribological conditions are considered. The results suggest that diamond surfaces with full oxygen passivation are very effective in preventing surface adhesion, and as a result present extremely low friction and wear. At low oxygen coverage, Si-O-C bond formation was observed as well as atomistic wear initiated from C-C bond breaking at extreme pressure. The analysis of electronic structures of the configurations resulting from key tribochemical reactions clarifies the mechanisms of friction reduction and atomistic wear. Overall, our accurate in silico experiments shed light on the influence of adsorbed oxygen on the tribological properties and wear mechanisms of diamond against silica.

Turbulent boundary layer over porous media with wall-normal permeability

Porous walls are a widely used passive flow control technique, which shows potential in reducing skin friction and mitigating flow-introduced noise. In the present study, porous media with wall-normal permeability is applied to a flat plate to investigate its interaction with the turbulent boundary layer at the Reynolds number based on friction velocity of Reτ=225. Time-resolved planar and tomographic particle image velocimetry were employed to identify the impact on mean statistics and coherent structures. An overall skin friction reduction of 22% is achieved. The porous wall induces counter-rotating streamwise vortex pairs at the spanwise sides of each pore, leading to momentum transport and the generation of alternative low- and high-speed regions close to the wall. Slip velocity is obtained, associated with the reduction in turbulent fluctuations and Reynolds shear stress. The streamwise velocity streaks and the hairpin vortices are significantly distorted compared with the smooth wall condition due to the downwash and upwash motion, featuring a notable reduction in the number and scale of the coherent structures, in which the skin friction reduction mechanism is related to. The proper orthogonal decomposition analysis returns the most energetic unsteady modes. Although the wall-coherent mode type remains to dominate the production of turbulent fluctuations, the scale and energy content of wall-incoherent modes increase, confirming the modification of the distribution and scale of near-wall turbulent structures.

Mechanisms of friction and wear reduction by nanosilver additives to base oil: Molecular dynamics simulation and experimental study

Although nanosilver particles are commonly used as oil lubrication additives, their mechanism for improving lubrication at the atomic scale remains unclear. This article explains how the performance of a pentaerythritol oleate lubrication system can be improved using silver nanoparticles through molecular dynamics simulation. Additionally, tribological tests were conducted using a reciprocating friction and wear testing machine. The relative concentrations and simulated shear conformation revealed that silver nanoparticles underwent deformation under shear stress and fractured at the interlayer slip. This resulted in the formation of a deposited film that spread over both the top and bottom Fe layers. We characterized the interaction between pentaerythritol oleate molecular chains and Fe layers by analyzing interfacial interaction energies, mean square displacements, and self-diffusion coefficients. Our findings indicate that the presence of silver nanoparticles improves both the adsorption of pentaerythritol oleate molecular chains onto the Fe layer and their diffusion behavior. The results of tribological tests indicate that adding silver nanoparticles significantly reduces friction coefficient and frictional wear across various lubrication conditions. The addition of silver nanoparticles at different loads and temperatures resulted in varying reductions in the coefficient of friction and wear. At a load of 20 N and a temperature of 298 K, the coefficient of friction decreased by 9%, and wear decreased by 31%. When the load was reduced to 2 N while maintaining a temperature of 298 K, the coefficient of friction decreased by 8% and wear decreased significantly by 84%. Finally, at a load of 20 N but with an increased temperature to 373 K, there was a larger reduction in the coefficient of friction (23%) compared to wear (50%). The film-formation mechanism of improved lubrication by silver nanoparticles was verified through tribological tests and simulations.

Novel Wood-Based Functional Material with Vibration and Noise Reduction.

As a kind of typical green material, natural materials tend to exhibit excellent performance in the engineering field because of their structure and special functions. The homogeneous vessels of red willow (RW) are potentially unique structures to store lubricants or reinforcing agents to present special functions for engineering applications. A series of novel red willow wood-based composites, which were infused with nano-MoS2 and then reinforced by the epoxy, were developed. Their self-lubricating, mechanical vibration and noise reduction performances were investigated, and the friction, vibration, and noise reduction mechanisms were disclosed. The infusion MoS2 treatment was very beneficial for improving the tribological properties of MoS2-curing epoxy/red willow (MCW), and the coefficient of friction (COF) was reduced by 65.8% under water-lubricated friction after infusing 24 times. Meanwhile, the mechanical performances of MCW were obviously enhanced through the curing treatment of the epoxy. The synergistic effects of the infusion and curing treatments significantly decreased the wear phenomena on the friction surfaces of MCW and weakened the COF and its fluctuation amplitudes, which resulted in the presented excellent vibration and noise reduction performance. The knowledge gained herein could not only develop a novel wood-based composite with low COF and good vibration reduction properties in the engineering field but also provide a new methodology for the design of artificial porous materials with stable and smooth friction processes.

A review on tribology, characterization and lubricants for water-based drilling fluids

The challenges associated with drilling large displacement horizontal wells are numerous, and their adverse effects can lead to severe damage or even shut down of the drilling operation. Frictional torque of the tubular column is one of the most common problems encountered when applying water-based drilling fluids to the drilling of extended reach wells. This is because during the drilling process, there is continuous friction between the drilling tool and the rock. When water-based drilling fluid cannot effectively reduce friction, it will cause difficulty in lifting or lowering the drilling tool, resulting in sticking, and thus affecting the drilling operation. Most petroleum engineers use different lubricants to reduce friction and wear during drilling.The purpose of this study is to present the friction and lubrication state during drilling, the mechanism of action of boundary lubrication, lubrication evaluation and analysis methods, and friction reduction and wear resistance related to water-based drilling fluid lubricants reported in literature for reducing friction.Based on the review, we recommend a broad evaluation of lubricants using different lubricity performance evaluation methods to determine the true lubricity performance in the coexistence of multiple frictions during drilling. In addition, we recommend that future researchers enhance the application of numerical simulations in the analysis of lubricant lubrication mechanisms to address the difficulty of direct experiments in boundary lubrication laboratories. Any experimental program requires at least five characterization techniques, including friction coefficient, wear resistance testing, scanning electron microscopy, energy dispersive X-ray spectroscopy, and electron spectroscopy, to provide a detailed understanding of the friction reduction mechanism of the lubricant used. Lubricants based on bionic technology with good water solubility, low biotoxicity, and easy biodegradability are highly encouraged because of their availability and good lubricating properties.