Sedimentation of IF-WS2 aggregates and a reproducibility of the tribological data

Inorganic fullerene-like (IF) solid lubricant nanoparticles and nanotubes with extremely useful mechanical and tribological characteristics have been realized, offering a plethora of new applications for these nanomaterials. The IF nanoparticles were found to be in the aggregated state. It is expected that the size of the aggregates and their distribution determine the penetration and entrapping of IF nanopowder into the interface. The main goal of the present work is to elucidate the effect of the mixing time of IF-WS₂ nanomaterial in the oil on the size of the IF aggregates and their influence on the friction and wear. The fraction of small aggregates increases and that of the large aggregates decreases with longer mixing time. Consequently, the spread of the tribological results diminishes with the lengthening of the mixing time.The reproducibility of the friction results for the pairs lubricated with oil +IF nanoparticles is determined by distribution of the IF aggregates in the lubricant and the size of the solid lubricant aggregates.     

On the Efficacy of IF–WS<Subscript>2</Subscript> Nanoparticles as Solid Lubricant: The Effect of the Loading Scheme

Fullerene-like WS₂ (MoS₂) nanoparticles (IF) have been studied in the past as solid lubricants. Using the tribological ball-on-flat experiments, it was shown that the size of the aggregates and their distribution determine the penetration and entrapping of the IF nanoparticles at the interface. It is expected that the wedge clearance at the inlet of the contact, i.e., the oblique-angle entrance to the contact zone between the two mating tribological surfaces, as well as the average surface roughness, can limit the supply of the lubricant into the interface in, e.g., the block-on-ring experiment. In the present series of experiments, the Stribeck curve was designed first using a linear loading scheme and pure oil. It was concluded that a wedge clearance (oblique-angle) in the inlet of the contact zone leads to entrapment of the IF nanoparticles and their compaction, which hamper the supply of the fluid lubricant into the interface. A ball-on-flat and flat block-on-ring friction devices with wedge clearance in the inlet of the contact can distort the efficacy of IF. Procedures for improving the supply of the IF nanoparticles to the contact zone and improving thereby their efficacy are considered.     

A new way to feed nanoparticles to friction interfaces

Fullerene-like WS₂ (MoS₂) nanoparticles (IF) have been studied in the past. Their efficacy as additives for lubrication fluids has been demonstrated. It was shown that the IF nanoparticles are usually delaminated in the inlet of the smooth contact. Thin sheets of broken IF nanoparticles can be entrapped between the rubbed surfaces and thus favorably affect the friction and the wear. Friction pairs at real mechanical macrosystems are often subjected to friction-induced vibrations. It was shown that the mechanical excitations can improve the supplying and preservation of fluid lubricant film in the interface. It can be hypothesized that under vibrations in a definite range of frequencies and amplitudes, the probability for small IF aggregates to be entrapped into the interface is increased. The main goal of this work was to study the effect of artificial mechanical excitations on the friction and wear of contact pairs rubbed with nanoparticles. In order to avoid friction-induced excitations, a new ball-on-flat friction device was developed. The frequency and the amplitude of the ball were varied using a motion of miniature micromotor attached to the ball holder. The behavior of IF nanoparticles in friction tests with and without external mechanical excitations was compared with the tribological behavior of the contact pair lubricated with pure paraffin oil. It was found that the external mechanical excitation of the mechanical parts rubbed with nanoparticles allows a penetration of these nanoparticles into interface. This effect leads to a remarkable shortening of the run-in period and improves the tribological properties of contact pairs. From the present results it may be anticipated that the accidental friction-induced vibrations, which are determined by the stiffness and damping force of the device, lead to preferential penetration of the IF nanoparticles into the contact area, affecting thereby the tribological behavior of the interface.     

Load bearing capacity of bronze, iron and iron–nickel powder composites containing fullerene-like WS2 nanoparticles

In the past few years, inorganic fullerene-like (e.g. IF) supramolecules of metal dichalcogenide MX₂ (M=Mo,W, etc.; X=S, Se), materials with structures closely related to (nested) carbon fullerenes and nanotubes have been synthesized. Recent experiments showed that IF possess lubricating properties superior to those of commercially available layered solid lubricant (2H–WS₂ polytype) in a wide range of operating conditions. It was shown before that the impregnation of a small amount of such nanoparticles into porous bronze matrix largely improve the tribological properties of bronze–steel contact pairs. In the present work, the effect of the PV (pressure–velocity) parameter on the transition to seizure for powdered bronze–graphite, iron–graphite and iron–nickel–graphite composites impregnated 2H and IF lubricant has been studied. The tribological tests were performed using a ring–block tester at loads of 150–1200 N and sliding speeds from 0.5 to 1.7 m/s. It was found that impregnation of IF into the pores improves the tribological properties of the powdered composites in comparison to 2H–WS₂ solid lubricant. Furthermore, it was established that the impregnation of oil together with the IF nanoparticles allows to provide very high load bearing capacity of the powdered materials.It is suggested that the transition to seizure occurs when the wear debrises accumulate in the pores on the surface. This process practically blocks the surface pores and limits the supply of the solid lubricant particles to the contact surface. The main advantage of the IF nanoparticles is attributed to: (a) slow release and supply of nanoparticles from the open pores to the surface; (b) sliding/rolling of the IF between the rubbing surfaces; and (c) prevention of the accumulation of the agglomerated wear particles in the pores. The model of third body was used in order to explain the effect of the wear particles, oil and solid lubricant particles on the friction and wear behavior of powdered composites.     

Tribological studies of rhenium doped fullerene-like MoS2 nanoparticles in boundary,mixed and elasto-hydrodynamic lubrication conditions

The tribological properties of rhenium-doped inorganic fullerene-like MoS₂ nanoparticles were studied in poly alpha olefin oil, and compared to inorganic fullerene-like (IF) MS₂ (M=Mo,W) nanoparticles and 2H-MoS₂ platelets. For this purpose a rotational disc tribometer was used. Different regimes in the Stribeck curve, i.e., boundary, mixed and elasto-hydrodynamic lubrication conditions were studied. A precipitous reduction in friction and wear of oil suspensions with nanoparticles were observed for the entire IF samples. However, Re-doped IF exhibited the best results, presumably due to their reduced tribocharging and agglomeration. The various tribological mechanisms of solid state additives are discussed.     

Wear and friction behaviour of CaCO3 nanoparticles used as additives in lubricating oils

For some years, reports have been published on adding solid lubricant powder to oil to improve the tribological properties of the latter, but the results have not been satisfactory. In this paper, we describe the preparation of CaCO₃ nanoparticles in a microemulsion consisting of sodium dodecyl‐sulphate (SDS)/isopentanol/cyclohexane/water, and assessment of the tribological behaviour of CaCO₃ nanoparticles as additives for lubricating oils. The CaCO₃ nanoparticles were characterised by transmission electron microscopy (TEM), and their tribological performance was tested in a four‐ball machine; the rubbing surface was analysed with X‐ray photoelectron microscopy (XPS). The results indicate that the size of CaCO₃ nanoparticles increased with the concentration of aqueous reactant, and that CaCO₃ nanoparticles exhibited good load‐carrying capacity, antiwear and friction‐reducing properties. The tribological properties of lubricating oils could be improved significantly by dispersing CaCO₃ nanoparticles in 500SN base oil containing dispersants such as polyisobutene‐butanediimide (T154), calcium alkylsulphonate (T101) and methyl‐tricaprylamine chloride (aliquat 336). The improvements in friction and wear were concluded to be due to the formation of a film containing CaCO₃ and CaO in the rubbing region, and the presence of nanoparticles, which may act in the same way as ball bearings, to facilitate sliding.     

Various MoS2-, WS2- and C-Based Micro- and Nanoparticles in Boundary Lubrication

Various solid lubricant particles have been experimentally evaluated as possible additives to oils. However, information in terms of a direct comparison of their tribological properties is still missing. In this study, we have compared the tribological properties of seven different solid lubricant micro- and nanoparticles as additives in polyalphaolefin (PAO) oil: MoS₂ nanotubes, MoS₂ platelets (2 and 10 μm), WS₂ nanotubes, WS₂ fullerene-like nanoparticles, graphite platelets (20 μm) and multi-walled carbon nanotubes. The experiments were performed in the boundary lubrication regime under a contact pressure of 1 GPa (Hertz, max) using a ball-on-disc tribotester. In general, the particles significantly decreased the friction and wear compared to the base PAO oil. We found that it was the material of the particles that largely determined their tribological performance. The effect of the size of the particles was much less important, and the morphology (shape) of the particles had little or no influence. We have also investigated the effect of ultrasonication during suspension preparation on particle damage and found that the solid lubricant particles were not notably affected, except the MoS₂ and WS₂ nanotubes, which became somewhat shorter.     

A study on the tribological characteristics of graphite nano lubricants

Many researchers have tried to improve the tribological characteristics of lubricants to decrease friction coefficients and wear rates. One approach is simply the use of additives in the base lubricant to change its properties. Recently, nanoparticles have emerged as a new kind of additive because of their size, shape and other properties. A nano lubricant is a new kind of engineering lubricant made of nanoparticles, dispersant, and base lubricant. In this study, graphite nanoparticles were used to fabricate nano lubricants with enhanced tribological properties and lubrication characteristics. The base lubricant used was industrial gear oil, which has a kinematic viscosity of 220 cSt at 40°C. To investigate the physical and tribological properties of nano lubricants, friction coefficients and temperatures were measured by a disk-on-disk tribotester. The surfaces of the fixed plates were observed by a scanning electron microscope and an atomic force microscope to analyze the characteristics of the friction surfaces. The results show that when comparing fixed plates coated with raw and nano lubricants, the plate coated with a nano lubricant containing graphite nanoparticles had a lower friction coefficient and less wear. These results indicate that graphite nanoparticle additives improve the lubrication properties of regular lubricants.     

Tribological Behavior of Deagglomerated Active Inorganic Nanoparticles for Advanced Lubrication

The tribological behavior of novel, deagglomerated, and active molybdenum disulfide (MoS₂) nanoparticles as additives in paraffin oil is presented. In a novel approach, the MoS₂ nanoparticles were activated by their intercalation with organic molecules, particularly triglycerides (canola oil) and lecithin (source of phosphorus). A four-ball tribological test setup was used to measure the wear scar diameter, the coefficient of friction, and the extreme pressure properties of such formulated paraffin oils. The results showed significant influence of this uniquely designed MoS₂ nanostructured additive on the coefficient of friction (0.07), the wear scar diameter, and the extreme pressure (315 kg) properties of the paraffin oil. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDS) were also used for investigating size, the surface morphology, and the elemental composition of the nanoengineered lubricant. The characterization revealed a particle size less than 100 nm and the elemental composition analysis of the wear track showed the presence of Mo, S, and P in the tribofilm, explaining the observed improvements in the tribological properties.     

Tribological Properties of Lubricating Films on the Al-Si Alloy Surface via Laser Surface Texturing

Dimpled textures were prepared by using a pulse solid laser on the surface of Al-Si alloy. The combination of laser surface texturing (LST) and MoS₂ solid lubricant as well as their tribological properties were investigated in this article. The obtained friction and wear data were critically analyzed to investigate how the parameters of texture influence the tribological performance of Al-Si alloy. Furthermore, morphological investigations of the transfer layers on the worn surfaces were performed and the wear mechanisms are discussed. The results show that the combination of LST and solid lubricant improves the tribological characteristics of Al-Si alloy. The friction coefficient of Al-Si alloy: steel friction pairs can be reduced to 0.15 under dry friction. The lubrication mechanism is attributed to a synergetic effect of providing solid lubricant and traps wear debris in the dimples. It was found that the optimum density of structure was 37% for excellent tribological properties.     

Tribological Analysis of a Hydrodynamic Journal Bearing under the Influence of Synthetic and Biolubricants

This study investigated the tribological characteristics of journal bearings exclusively for automotive applications under the influence of a synthetic lubricant (SAE20W40) and chemically modified rapeseed oil (CMRO) as a biolubricant, dispersed with TiO₂, WS₂, and CuO nanoparticles used as antiwear additive. The effects of synthetic and nanobased biolubricants on the tribological behavior of the hydrodynamic journal bearing were examined using a journal bearing test rig by measuring the coefficient of friction, oil film thickness, and wear under a load of 10 kN and a speed of 3,000 rpm. The test results show that CuO nanoadditives that are added to the biolubricant exhibit outstanding wear and friction reduction behavior, better than that with synthetic lubricants as well as other nanobased biolubricants. The inclusion of CuO nanoparticles in the biolubricant decreased the coefficient of friction by 27% and wear by about 47% compared to a synthetic lubricant. Additionally, investigations were performed using atomic force microscopy (AFM) and scanning electron microscopy (SEM) to study the surface morphology and surface roughness behavior of the tested bearing surfaces.     

Friction and wear of powdered composites impregnated with WS2 inorganic fullerene-like nanoparticles

The impregnation of inorganic fullerene-like nanoparticles of WS₂ (IF) allows to improve effectively the tribological properties of powdered materials in comparison to the impregnation of oil or commercially available layered WS₂ (2H) particles. The main goal of this work was to determine the dominant lubrication regimes under friction of the bronze, iron, and iron–nickel porous matrixes impregnated with 2H and IF solid lubricants. The tribological tests were performed at laboratory atmosphere (humidity ∼50%) using a ring-on-block tester at the sliding speed of V=1 m/s, and the loads of 150–1000 N. The wear of the metal bodies was measured by an eddy current probe system and by weighting of the samples before and after the test. In order to evaluate the radial clearance, the profiles of wear blocks were analyzed by profile projector. Than these data were used in the calculation of the Sommerfeld reciprocal values. Friction and wear results were presented as the Stribeck curves. The critical Sommerfeld reciprocals were evaluated from these curves. The Stribeck curves were compared with the Morgan’s curves for different ranges of non-dimensional permeability, Ψ. These results are used then in calculation of the permeability, Φ. Three lubrication regimes as: quasi-hydrodynamic, boundary or mixed, and dry friction were revealed under friction of porous samples in the load-range studied. It was found that the impregnation of IF nanoparticles provides the regime of quasi-hydrodynamic lubrication in the widest range of loads in comparison to the reference sample and the sample impregnated with 2H–WS₂. Fe–Ni samples exhibited the highest wear resistance and provided the widest range of quasi-hydrodynamic lubrication in comparison to bronze and iron powdered composites. The effect of IF on the regimes of lubrication is explained on the base of the third-body model. It is expected that the sliding/rolling of the IF nanoparticles in the boundary of the first bodies and in between the wear particles (third-body) facilitate the shear of the lubrication film and thus provide the quasi-hydrodynamic regime of friction. It is supposed that the roll shape of IF nanoparticles allows to release the IF from the pores to contact surfaces.     

Direct observation by in situ transmission electron microscopy of the behaviour of IF‐MoS2 nanoparticles during sliding tests: influence of the crystal structure

Direct observation of the behaviour of individual inorganic fullerenes (IF)‐MoS₂ nanoparticles in a sliding interface is essential for the understanding of the influence of the intrinsic characteristics of the nanoparticles on their lubrication mechanisms, when they are used as additives in lubricant oil. In this work, in situ transmission electron microscopy sliding tests were performed on two different types of MoS₂ nanoparticles synthesised by two different methods. It is shown that the IF‐MoS₂ nanoparticles having perfect structure with a high crystalline order and without defects are able to roll and to slide under the combined effect of pressure and shear stress, whereas the IF‐MoS₂ nanoparticles containing many defects exfoliate immediately in the same conditions to deliver MoS₂ layers covering the mating surfaces. A link between these results, the lubrication mechanisms of the nanoparticles and their tribological properties at the macro‐scale was established, proving that the lubrication mechanisms of fullerenes depend on their intrinsic characteristics. Copyright © 2013 John Wiley & Sons, Ltd.     

Tribological behavior of vegetable oil-based lubricants with nanoparticles of oxides in boundary lubrication conditions

This work studied the development of vegetable based lubricants and the addition of oxides nanoparticles (ZnO and CuO) as additive for extreme pressure (EP), exploring the EP and oil base influence in tribological behavior. The results showed that with the addition of nanoparticles to conventional lubricant, the tribological properties can be significantly improved. A smoother and more compact tribofilm has formed on the worn surface, which is responsible for the further reduced friction and wear. Also, lubricants developed from modified vegetable oil can replace mineral oil, improving the tribological and environmental characteristics. However, the addition of nanoparticles in vegetable base lubricants is not beneficial to wear reduction.     

Formation of Low-Friction Particle/Polymer Composite Tribofilms by Tribopolymerization

Inorganic fullerene-like (IF) solid lubricant particles based on MoS₂ or WS₂ have recently gained attention in various tribological applications, for instance incorporated in coatings, porous materials, greases and dispersed in oils. However, their effect in oil can be limited often due to inadequate penetration into the contact zone. Meanwhile, tribopolymerization of monomer additives in oil have proven to be efficient to reduce wear, but without significantly reducing the friction. This investigation combines these two lubrication techniques in order to form particle/polymer composite tribofilms, aimed to give low friction and wear in high-pressure sliding contacts. A cyclic amine, caprolactam, was used as the monomer together with IF as well as normal 2H WS₂ particles. Zn-DTP was used as a reference antiwear additive and poly-alfa-olefin (PAO) was used as the base oil. Reciprocal ball-on-flat sliding test results found that monomer plus particles reduced the friction by 10–50%, depending on sliding speed and type of particles, compared with particles or monomer alone. And the scattering between different tests was also considerably lower. The wear rate was also substantially reduced to a level similar to that of Zn-DTP. The particle/polymer composite tribofilms were generally much larger than the actual contact area and its thickness varied from below a few nanometres in the contact centre to several micrometers in the outer parts. Consequently, the reduced friction is believed to come from two effects: one being a trapping of particles in the contact zone, which reduces the boundary friction level; and the other a shift in lubrication regime towards EHD-lubrication due to reduced contact pressures accompanied with the formation of the composite tribofilm.     

微织构光固化填充h-BN的巴氏合金表面摩擦学性能

为了提高巴氏合金在油润滑条件下的摩擦学性能,在巴氏合金表面加工凹坑微织构并利用光固化填充方法填充六方氮化硼(h-BN)固体润滑剂,制备出h-BN与表面微织构相结合的复合润滑结构。研究复合润滑结构在油润滑条件下的摩擦学性能及其减摩润滑机制。结果表明：复合润滑结构的摩擦学性能远高于未织构面和纯织构面;当凹坑微织构直径较小时,织构密度为10%～20%时,复合润滑结构摩擦因数较小,而凹坑直径较大时,随着织构密度的增加,复合润滑结构摩擦因数逐渐减小;当织构密度小于20%时,凹坑直径较小的复合润滑结构摩擦因数小,当织构密度达到30%时,随着凹坑直径的增加,复合润滑结构摩擦因数减小。复合润滑结构能够改善巴氏合金表面摩擦学性能,是因为h-BN固体润滑剂的释放在巴氏合金表面形成了固体润滑薄膜,避免了润滑油膜较薄处的巴氏合金表面直接与45钢表面接触,且释放h-BN固体润滑剂后的微织构凹坑可以起到收集磨粒,储存润滑油的作用。     

The Effect of WS2 Nanoparticles on Friction Reduction in Various Lubrication Regimes

A study of the tribological behavior of nested inorganic fullerene-like (IF) nanoparticles of WS₂, as a potential additive to base oils is presented. Friction measurement results obtained from three different test rigs over a wide range of normal loads and sliding velocities are shown. Stribeck curves are used to reveal the lubrication regimes where the IF are most effective. It is found that the addition of IF-WS₂ nanoparticles to the base oils results in up to 50% reduction in friction coefficient in the mixed lubrication regime. The mechanism of improved friction and wear behavior with the IF additive is discussed.     

两种分散剂对IF-WS2纳米粒子分散性能和摩擦性能的影响

研究甲基萘和聚异丁烯丁二酰亚胺(PIBSI)两种分散剂对IF-WS 2纳米颗粒在基础油中的分散性能和摩擦性能的影响。使用722型分光光度计和四球摩擦试验机分别考察纳米WS 2润滑油在分散剂中的分散稳定性和摩擦性能,用激光粒度分析仪和红外光谱(FTIR)分析分散剂与WS 2纳米颗粒之间的作用机制。结果表明:甲基萘的分散效果和抗磨减摩效果最好,4%甲基萘分散的0.01%的纳米WS 2润滑油可以长期稳定保存,其纳米WS 2润滑油在490 N时的磨斑直径仅为0.540 mm,摩擦因数为0.11;甲基萘能够起到分散和抗磨减摩作用的原因是其对IF-WS 2纳米粒子具有一定的溶解作用。     

Effects of Nano-TiO2 Additive in Oil-in-Water Lubricant on Contact Angle and Antiscratch Behavior

Contact angle and scratch tests have been conducted to investigate the effects of nano-TiO₂ additive in oil-in-water (O/W) lubricant. The results show that the contact angle between high-speed steel with oxide scale and 1% (oil concentration) O/W lubricant decreases first and then increases as the concentration of nano-TiO₂ particle in the O/W lubricant increases. The smallest contact angle is obtained after an addition of 4% nano-TiO₂ additive to the O/W lubricant. This is because the nano-TiO₂ can enhance the surface excess of the oil when the nano-TiO₂ particles distribute throughout the surface of the oil droplets, and after saturation they can distribute throughout the water and also improve the surface excess of the water in the O/W lubricant. The scratch and hot rolling tests show that the nano-TiO₂ particles in the O/W lubricant can also reduce friction, improve scratch resistance, and reduce rolling force. A method for measuring the adhesion force of the oxide scale is proposed and the effect of nanoparticles is discussed. It is demonstrated that the effect of self-lubrication of nanoparticles in the O/W lubricant plays a more significant role in the tribological behavior during hot rolling than wettability.     

Preliminary Study of Nano- and Microscale TiO2 Additives on Tribological Behavior of Chemically Modified Rapeseed Oil

This study compared the tribological evaluation of chemically modified rapeseed oil as a potential biodegradable automotive lubricant with and without nano- and microscale titanium dioxide (TiO₂) particles, focusing on the influence of TiO₂ particles to improve the friction reduction and antiwear characteristics of chemically modified rapeseed oil. TiO₂ nano- and microscale particles of anatase phase and rutile phase were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Further, the analysis of chemically modified rapeseed oil with and without TiO₂ additives was carried out to determine its tribological behavior using a pin-on-disc tribometer. The experimental results showed that the addition of TiO₂ nanoparticles exhibited good friction reduction and antiwear properties compared with the addition of microscale TiO₂ and without TiO₂ additives to chemically modified rapeseed oil. Nanoscale TiO₂ is suitable as an antiwear additive in chemically modified rapeseed oil.