Lubrication Science最新文献

We investigate flow and slip behaviour of Santotrac 50 molecules under high shear in Couette cell by means of Molecular Dynamic simulation to understand reduced friction force. Molecular chain stretches and oriented to shear direction, and move. Slip starts on metal surface at 2 × 10⁸ s⁻¹, and increases with shear rate. Slip length keeps scale at nanometre. Molecular conformation and occurrence of slip both indicate a reduced shear stress. Furthermore, when changing wettability, slip length increases in power law and thus decreases shear stress greatly. Occurrence of low-density region near surface can explain slip. And thus, we extended apparent slip model, which divided lubricant into liquid layers with different viscosities, to elucidate the relationship between molecule distribution inner layer and slip on surface influenced by shear velocity and wettability. Above all, our research sheds light on flow and slip behaviour of complex fluid and can be applied in improving lubrication property.

The aim of this study is to investigate the structural, mechanical and tribological behaviours of thermally oxidized MoS₂ films. MoS₂ coatings were deposited on D2 tool steel substrates using the closed field unbalanced magnetron sputtering method (CFUBMS). The thermal oxidation process was carried out at four different temperatures. Tribological properties were determined by pin-on-disc wear tests in the atmospheric environment. It was determined that thermal oxidation temperatures affected the chemical composition of MoS₂ films, but did not cause any change in film thickness. The wear rates of the samples differed depending on the oxidation temperature and the applied load. The lowest wear rate was determined as 1.97 × 10⁻⁸ mm³/Nm in the oxidized film at 350°C. In addition, the highest hardness value was obtained as 655 HV in the film oxidized at 400°C, and the lowest coefficient of friction was obtained as 0.01 in the film oxidized at 350°C.

Good dispersion stability of engine oil added with graphene is the premise of playing its superior tribology property and also the novelty of this study. As the tribological behaviour of a commercialised, fully synthetic 5W40 engine oil upon the incorporation of reduced graphene oxide in sample (0.02 wt%) like (5W40 engine oil + 0.02 wt% Reduced Graphene Oxide) called rGO6 presented superior tribological properties by exhibiting the lowest COF value in our previous work, the same sample was evaluated through a reciprocating tribometer, using steel ball (100 CR6) on a real polished gasoline engine cylinder liner with 5W-40 engine oil to investigate their wear and friction behaviour in boundary lubrication regime. It was found that a rGO6 nano-additive played an active role in lowering the coefficient of friction (3.29%) and increased surface protection by forming a protective layer on a smooth nanorough on rubbing surfaces.

A new stoichiometric FTIR Base Number (BN) method of significant utility and analytical benefit to the tribology sector has been developed, providing ASTM D4739-comparable BN results in lieu of titration in a fraction of the time. Thirty-six new/in-service oils analysed by both methods were linearly related with a between-method accuracy of ~±1.0 BN and a within-FTIR method reproducibility of ~±0.50 BN. Acid pK_a differences and the comparative similarity of the FTIR results to HCl titration are discussed, including analytical benefits. It provides a rapid means of producing quality ASTM-comparable results, taking ~1 min/sample for spectral analysis versus 30–40 min for potentiometric titration. Method protocols are best suited to an open architecture FTIR accessory but can be readily adapted to flow cell equipped FTIRs. As structured, ASTM-like results are obtained rapidly with a major analytical environmental/maintenance footprint reduction, being ideally suited for in-service lubricant or research labs analysing 20–50 samples/day.

Friction modifier additives play a crucial role in controlling friction and wear of lubricated tribological systems. Model experiments in a controllable atmosphere performed by integrating a tribometer into a system of in situ surface analytical methods in vacuum can give insights into the additives functionality. In this work, thin, well-defined layers of an organic friction modifier (OFM) are adsorbed onto an iron oxide surface by means of an effusion cell immediately before measuring friction and wear. The results show that contrary to the assumption that homogeneous layers are formed, this OFM accumulates in droplets on the surface. Droplet number and radius increase with evaporation time. In friction tests, the smallest friction values are found for a low coverage of droplets. For larger droplets, friction increases due to a capillary neck of additive that forms between the sliding surfaces and is dragged along during the friction test.

The thermo-penetrative spreading behaviours of oil in microgrooves on stainless steel 316L surfaces were studied experimentally under temperature gradient conditions. An interface of smooth surface and microgrooved surface was designed to comparatively investigate the penetrative spreading of oil in microgrooves and the apparent spreading of oil on microgrooves and smooth surfaces. Then, microgrooves with different depths but a same width were laser processed to analyse the relationship between the actions of surface texture and temperature gradient on oil directional spreading. Results showed that the apparent oil on microgrooves extended directionally along the temperature gradient, leading to the lubricant loss from the high-temperature region to the lower, while the penetrative oil in microgrooves could spread rapidly from the low-temperature region to the higher under the obstruction of the thermocapillary effect, providing a potential method to enhance the lubrication.

This study had investigated the main degradation mechanism of a roll in hot rolling process, to reduce the wear and oxidation, a type of low-melting glass fabricated by the fuse-polymerisation method was tested as a hot metal forming lubricant by a ball-on-disc tribometer at high temperature. The results revealed that the roll degrades mainly due to the cracks initiated at the interface of carbides/matrix on the roll surface, then propagate along the interface, and being sheared off when the cracks confluence in the subsurface. The high-speed steel (HSS) starts to be oxidised at 500°C, and grows heavily with increasing temperature. The recommended lubricant presented a desired lubrication behaviour for hot rolling process that generating a higher friction coefficient at low temperature and a lower one at high temperature. The friction coefficient, wear rate, and oxidation of HSS roll was reduced materially, which meets the requirements of hot rolling lubricant well.

Based on the lubrication angle field and the locus of lubricating weak point, the lubricating characteristic of the toroidal surface enveloping cylindrical worm drive is globally investigated. The nonlinear equation system to determine the lubricating weak point is established by utilising the lubrication angle. Generally speaking, the weak locus always exists roughly in the middle of the tooth surface of cylindrical wormgear and a zone with worse lubricating behaviour is around the locus. By increasing the number of worm thread, the locus of lubricating weak point can be completely removed from the wormgear tooth surface, and the locus approximately is in the shape of the inverse capital letter L. On this occasion, the entire wormgear tooth surface is half surrounded by the locus, and the overall lubrication performance of the worm drive is improved. The current research is an application of elastohydrodynamic lubrication (EHL) on worm drive.

A self-made soot capture device was applied to burn coal-to-liquids and collect its soot (CS), and then modified the CS with nitric acid or nitric acid and oleic acid to obtain nitric acid modified CS (CSN) or nitric acid and oleic acid modified CS (CSNO). The influence of CS, CSN and CSNO on the lubrication performance of 10^# white oil (WO) was investigated on a four-ball tester. The structural difference, surface topography, element composition and tribological mechanism of the three types soot were analysed by Fourier transform infrared spectrometer, X-ray photoelectron spectroscopy, scanning electron microscope, high resolution transmission electron microscope, and so on. From the obtained results, after further modification with oleic acid, the solubility of oil is significantly improved. Compared with 10^# WO, the average friction coefficient and average wear scar diameter of CS, CSN and CSNO are all reduced. Among them, CSNO has the best anti-friction and anti-wear effect. CSNO is expected to be used as carbon lubricating material from coal-to-liquids soot.

Surface designs and wetting mechanisms of Nepenthes pitcher plant provide useful guiding principles for achieving control over the wettability of solid surfaces as mimicked in slippery liquid-infused porous surfaces (SLIPS). Here, the effect of the over-layer thicknesses was investigated by gradual impregnation of porous silica coatings with squalane. Characterisation and wettability of the coatings at various stages of the impregnation were studied using fluorescence microscopy, gravimetric analysis and water contact/sliding angle measurements. The technique allowed us to progressively generate variable thicknesses of the oil over-layers and systematically tune the wetting behaviour of the coatings. The results clarify that very thin oil over-layer may not lead to slippery surface and the slipperiness of the coated surfaces was observed to increase with increase in the thickness of the over-layer. It is suggested that fabricated SLIPSs be accompanied with specified workable thickness of the oil over-layer in order to clearly evaluate their efficiencies.