Flow-resistance analysis of nano-confined fluids inspired from liquid nano-lubrication:A review

How to reduce flow resistance of nano-confined fluids to achieve a high flux is a new challenge for modern chemical engineering applications, such as membrane separation and nanofluidic devices. Traditional models are inapplicable to explain the significant differences in the flow resistance of different liquid–solid systems.On the other hand, friction reduction in liquid nano-lubrication has received considerable attention during the past decades. Both fields are exposed to a common scientific issue regarding friction reduction during liquid–solid relative motion at nanoscale. A promising approach to control the flow resistance of nano-confined fluids is to reference the factors affecting liquid nano-lubrication. In this review, two concepts of the friction coefficient derived from fluid flow and tribology were discussed to reveal their intrinsic relations. Recent progress on low or ultra-low friction coefficients in liquid nano-lubrication was summarized based on two situations. Finally, a new strategy was introduced to study the friction coefficient based on analyzing the intermolecular interactions through an atomic force microscope(AFM), which is a cutting-point to build a new model to study flowresistance at nanoscale.

Extra low friction coefficient caused by the formation of a solid-like layer:A new lubrication mechanism found through molecular simulation of the lubrication of MoS_2 nanoslits

Monolayer molybdenum disulfide(MoS2) is a novel two-dimensional material that exhibits potential application in lubrication technology. In this work, molecular dynamics was used to investigate the lubrication behaviour of different polar fluid molecules(i.e., water, methanol and decane) confined in monolayer Mo S2 nanoslits. The pore width effect(i.e., 1.2, 1.6 and 2.0 nm) was also evaluated. Results revealed that decane molecules exhibited good lubricating performance compared to the other two kinds of molecules. The friction coefficient followed the order of decane b methanol b water, and decreased evidently as the slit width increased, except for decane. Analysis of the spatial distribution and mobility of different confined fluid molecules showed that a solid-like layer was formed near the slit wall. This phenomenon led to the extra low friction coefficient of confined decane molecules.

Self-Powered,Long-Durable,and Highly Selective Oil-Solid Triboelectric Nanogenerator for Energy Harvesting and Intelligent Monitoring

Triboelectric nanogenerators(TENGs)have potential to achieve energy harvesting and condition monitoring of oils,the“lifeblood”of industry.However,oil absorption on the solid surfaces is a great challenge for oil-solid TENG(O-TENG).Here,oleophobic/superamphiphobic O-TENGs are achieved via engineering of solid surface wetting properties.The designed O-TENG can generate an excellent electricity(with a charge density of 9.1μC m^(−2) and a power density of 1.23 mW m^(−2)),which is an order of magnitude higher than other O-TENGs made from polytetrafluoroethylene and polyimide.It also has a significant durability(30,000 cycles)and can power a digital thermometer for self-powered sensor applications.Further,a superhigh-sensitivity O-TENG monitoring system is successfully developed for real-time detecting particle/water contaminants in oils.The O-TENG can detect particle contaminants at least down to 0.01 wt%and water contaminants down to 100 ppm,which are much better than previous online monitoring methods(particle>0.1 wt%;water>1000 ppm).More interesting,the developed O-TENG can also distinguish water from other contaminants,which means the developed O-TENG has a highly water-selective performance.This work provides an ideal strategy for enhancing the output and durability of TENGs for oil-solid contact and opens new intelligent pathways for oil-solid energy harvesting and oil condition monitoring.

Mechanisms behind the environmental sensitivity of carbon fiber reinforced polytetrafluoroethylene(PTFE)

Carbon fiber reinforced polytetrafluoroethylene (CF/PTFE) composites are known for their exceptional tribological performance when sliding against steel or cast iron in inert gas environments. Compared to experiments in humid air, about an order of magnitude lower wear rate and several times lower coefficient of friction have been reported for tests conducted in dry nitrogen and hydrogen. Moreover, trace moisture has been shown to affect the friction and wear significantly of this tribosystem, although a possible effect of oxygen cannot be ruled out due to uncertainties regarding the oxygen concentrations. While several studies have pointed out the environmental sensitivity of CF/PTFE, the understanding of the underlying mechanisms are very limited. The objective of this research is to investigate the individual and combined effect of oxygen and moisture on the tribological behavior of CF/PTFE sliding against steel. Additionally, this study aims to elucidate the underlying mechanisms that govern the environmental sensitivity of the system. Climate-controlled three-pin-on-disc experiments were conducted in nitrogen atmospheres at various concentrations of oxygen and moisture. The tribological results clearly demonstrate that both moisture and oxygen contribute to increased friction and wear. However, the adverse effect was much more pronounced for oxygen than moisture. A qualitative method was developed to estimate the tribofilm coverage on the CF/PTFE surface. Results showed strong correlation between high coverage of strongly adhered tribofilm and low wear rate. Moreover, a loosely adhered tribofilm was observed on top of the CF/PTFE surface in presence of moisture. FTIR analysis indicated that the loosely adhered tribofilm found in the moisture-enriched environment contained a significant amount of adsorbed water, which may explain the lower coefficient of friction in presence of moisture compared to oxygen. The adsorbed water in the loosely adhered tribofilm could be an indication of moisture-driven lubrication by the non-graphitic carbon in the tribofilm.

High temperature tribological behaviour of PVD coated tool steel and aluminium under dry and lubricated conditions

Aluminium alloys are commonly used as lightweight materials in the automotive industry.This non-ferrous family of metallic alloys offers a high versatility of properties and designs.To reduce weight and improve safety,high strength-to-weight ratio alloys(e.g.6XXX and 7XXX),are increasingly implemented in vehicles.However,these alloys exhibit low formability and experience considerable springback during cold forming,and are therefore hot formed.During forming,severe adhesion(i.e.galling)of aluminium onto the die surface takes place.This phenomenon has a detrimental effect on the surface properties,geometrical tolerances of the formed parts and maintenance of the dies.The effect of surface engineering as well as lubricant chemistry on galling has not been sufficiently investigated.Diamond-like carbon(DLC)and CrN physical vapour deposition(PVD)coated steel have been studied to reduce aluminium transfer.However,the interaction between lubricants and PVD coatings during hot forming of aluminium alloys is not yet fully understood.The present study thus aims to characterise the high temperature tribological behaviour of selected PVD coatings and lubricants during sliding against aluminium alloy.The objectives are to first select promising lubricant-coating combinations and then to study their tribological response in a high-temperature reciprocating friction and wear tester.Dry and lubricated tests were carried out at 300℃ using a commercial polymer lubricant.Tests using DLC,CrN,CrTiN,and CrAIN coated tool steel were compared to uncoated tool steel reference tests.The initial and worn test specimen surfaces were analysed with a 3-dimensional(3D)optical profiler,scanning electron microscope(SEM)and energy dispersive X-ray spectroscope(EDS)as to understand the wear mechanisms.The results showed formation of tribolayers in the contact zone,reducing both friction and wear.The stability of these layers highly depends on both the coatings'roughness and chemical affinity towards aluminium.The DLC and CrN coatings combined with the polymer lubricant were the most effective in reducing aluminium transfer.

Static/dynamic friction and wear of some selected polymeric materials for conformal tribo-pairs under boundary lubrication conditions

This work is aimed at investigating the friction and wear performance of different polymeric materials having potential for hydraulic system components under lubricated sliding conditions against a steel counter face.A pin-on-disc test configuration was used for the experimental study.The different polymeric materials selected for these studies were commercial polyimides(PI),polyether ether ketone(PEEK),and flouropolymers.Some of these materials were bulk materials whereas others were used as coatings applied on to the cast iron substrate.The tribological characteristics of the polymers were compared with a reference grey cast iron.The frictional characteristics were evaluated in both static and dynamic conditions.The results have shown that by using polymeric materials it is possible to reduce breakaway friction by an order of magnitude compared to grey cast iron.However,the breakaway friction increased significantly after the wear tests.The polymeric materials having lowest breakaway friction have shown the highest wear with the exception of the PEEK-PTFE coating which showed low wear.PI with graphite fillers also showed low wear but it resulted in relatively high friction.The carbon fibre reinforced materials resulted in unstable friction as well as higher wear compared to the PI materials with graphite fillers.

Physicochemical and tribological properties of gemini-type halogen-free dicationic ionic liquids

A series of new halogen-free dicationic ionic liquids(ILs)with different alkyl chain lengths were prepared,and the relationship between the alkyl chain length,physicochemical and tribological properties of ILs,and their role as neat lubricant for steel–steel friction pairs,was investigated.Evaluation of stability during hydrolysis and copper strip corrosion test results show that synthetic ILs are stable and not corrosive to metal contacts,due to the halogen-free anions.The friction and wear test results indicate that ILs with long alkyl chains have excellent friction-reducing and anti-wear properties,especially at high temperatures.Based on the surface three-dimensional(3D)profiles,electrical contact resistance,scanning electron microscopy(SEM),energy dispersive X-ray spectroscopy(EDS),and the X-ray photoelectron spectrometry(XPS)analysis of the worn surfaces of steel discs,we can conclude that the efficiency of ILs is due to the formation of high quality tribofilms that consist of both tribochemical reaction and ordered absorption films.

Nanolubricant additives: A review

Using nanoadditives in lubricants is one of the most effective ways to control friction and wear,which is of great significance for energy conservation,emission reduction,and environmental protection.With the scientific and technological development,great advances have been made in nanolubricant additives in the scientific research and industrial applications.This review summarizes the categories of nanolubricant additives and illustrates the tribological properties of these additives.Based on the component elements of nanomaterials,nanolubricant additives can be divided into three types:nanometal-based,nanocarbon-based,and nanocomposite-based additives.The dispersion stabilities of additives in lubricants are also discussed in the review systematically.Various affecting factors and effective dispersion methods have been investigated in detail.Moreover,the review summarizes the lubrication mechanisms of nanolubricant additives including tribofilm formation,micro-bearing effect,self-repair performance,and synergistic effect.In addition,the challenges and prospects of nanolubricant additives are proposed,which guides the design and synthesis of novel additives with significant lubrication and antiwear properties in the future.

Numerical micro-texture optimization for lubricated contacts-A critical discussion

Despite numerous experimental and theoretical studies reported in the literature,surface micro-texturing to control friction and wear in lubricated tribo-contacts is still in the trial-and-error phase.The tribological behaviour and advantageous micro-texture geometries and arrangements largely depend on the contact type and the operating conditions.Industrial scale implementation is hampered by the complexity of numerical approaches.This substantiates the urgent need to numerically design and optimize micro-textures for specific conditions.Since these aspects have not been covered by other review articles yet,we aim at summarizing the existing state-of–the art regarding optimization strategies for micro-textures applied in hydrodynamically and elastohydrodynamically lubricated contacts.Our analysis demonstrates the great potential of optimization strategies to further tailor micro-textures with the overall aim to reduce friction and wear,thus contributing toward an improved energy efficiency and sustainability.

Viscoelasticity and shear-thinning effects on bio-polymer solution and suspended particle behaviours under oscillatory curve Couette flow conditions

Formation of wear particles within total hip replacement is one of the main causes of its failure.In addition toimproving the lubrication and wear resistance of materials used as bearing surfaces,understanding of wear particledistribution patterns within lubricants inside an implant gap could be used to improve design parameters andimplants'lifespan.In this study,the behaviours of biolubricants(with compositions similar to human joint synovialfluid)and suspended particles were investigated by micro-particle image velocimetry in curved mini channels underoscillatory Couette flow conditions.The studied biolubricants had shear-thinning viscoelastic characteristics.Theauthors found that increasing shear-thinning,elasticity or motion frequency levels did not affect the trend behavioursof biolubricant flows due to the low strain values of the experimental conditions applied.However,suspended particlesformed strings along flow directions and exhibited cross-stream migration to channel walls.Motion frequency,fluidshear thinning and elasticity characteristics and channel dimensions strongly affected particle behaviours.