Lubrication Science最新文献

In this study, a dynamic friction testing apparatus was independently developed to investigate the frictional properties of tire-asphalt pavements under varying road conditions, tailored to complex working environments. This device underwent rigorous reliability analysis to ensure compliance with the experimental standards. The calibration of asphalt pavements with varying heating durations established regions of uniform temperature. Experiments were conducted using custom-designed equipment under conditions involving changes in particle size, temperature, and surface roughness. Results showed that both the maximum static friction and average friction forces increased with increasing temperature. Smaller particle sizes intensified stick–slip behaviour but reduced frictional forces, whereas greater tire surface roughness enhanced friction and accentuated stick–slip phenomena. An improved spring-slider model was proposed to simulate the frictional behaviour of asphalt pavements, yielding results consistent with the experimental data. Adjusting the roughness parameter in the model demonstrated that friction gradually decreases with reduced roughness, while stick–slip effects diminish, aligning qualitatively with the experimental observations.

Rolling contact fatigue (RCF) is the default mode of failure observed in Hertzian rolling contact elements, such as bearings. Experimentally, the current study examines the influence of the existence of solid lubricant dispersed in liquid base lubricant on RCF life. Polytetrafluoroethylene (PTFE), a solid lubricant additive, is selected for this purpose, and the RCF performance of varying percentages of PTFE (0.1–2.5 wt%) on the base lubricant has been examined. RCF tests were carried out on a two-disc on-cylinder test rig under pure rolling conditions within the mixed lubrication regime. The RCF life was enhanced for every concentration of PTFE-added lubricant composition. The base lubricant containing 2 wt% PTFE concentration exhibits the highest improvement in mean and L₁₀ RCF life, yielding a 2.3-fold increase in improvement over the base lubricant. The excellent RCF performance is attributed to the decreased wear rate, PTFE particle adsorption and tribo-chemical film formation, and there is a decreased likelihood of metal-to-metal contact and a delayed onset of micropitting.

In this study, lithium stearate and fumed silica co-thickened grease with po-ly(sodium-4-styrenesulfonate) (PSSNa) additive was synthesised and characterised for application as an automotive lubricant, including for the potential use in an electric vehicle (EV). Oscillatory shear and steady shear rheological tests confirmed that the sample exhibited appropriate shear thinning required in greases. Fumed silica was found to increase thermal stability. Additionally, the conductivity of the synthesised grease was higher than the threshold conductivity required to avoid electrical arcing and static charge buildup (4 × 10⁻¹² S/cm). The tribo-pair lubricated with LSFSPNa grease demonstrated a significant improvement in tribological performance, with the coefficient of friction reduced by approximately 65% compared to commercial grease, decreasing from 0.31 to 0.11. The wear volume showed a tenfold reduction, accompanied by a substantial decrease in surface roughness (Ra), which dropped from 0.81 μm with commercial grease to 0.17 μm with LSFSPNa grease. At the same time, the synthesised grease exhibited better copper corrosion resistance. Overall, the synthesised grease was found to be compatible for EV applications in terms of rheology, friction reduction, copper corrosion resistance, conductivity, and thermal stability.

Friction and wear are major challenges in automotive engines, and they can lead to significant operating issues such as increased fuel consumption, component damage, heat generation, loss of performance, etc. Friction and wear between piston rings and cylinder liner are significant issues in automotive engines, and researchers continually focus on reducing them to improve engine performance and efficiency. Numerous studies have been carried out to minimise friction and wear losses. However, coating is one of the promising techniques used in the automotive industry. PVD-deposited, CrN-coated piston rings demonstrated excellent tribological properties. However, CrN coatings become less effective under conditions of lubrication starvation, exhibiting a high coefficient of friction. In the present study, PVD-based magnetron sputtered CrN and carbon-doped CrCN coatings were applied to the surface of the piston rings. The structure, elemental composition, surface morphology, thickness, and hardness of the coatings were characterised systematically. The coated samples underwent tribological testing under loads of 50, 80, and 110 N to assess the coefficient of friction at the piston ring-cylinder liner interface under the boundary lubrication condition and evaluate the wear behaviour of the coated ring samples. The carbon-doped CrCN coating exhibits a low coefficient of friction at each load and exhibits a 16.59% reduction in the coefficient of friction compared to the CrN coating. This could be ascribed to the graphitisation effect of the CrCN coating. However, both coatings showed almost equal and minimal wear loss. Finally, the possible worn morphology of coated samples was proposed based on their microstructure, film deposition, and Raman's spectroscopy results.

This study aims to explore the potential application of LaF₃ nanoparticles as high-performance gear oil additives. Lanthanum trifluoride nanoparticles modified by di(2-ethylhexyl) phosphoric acid (D2EHPA) [denoted as LaF₃-D2EHPA], which exhibited excellent dispersion stability, were prepared using a liquid-phase chemical method. The tribological properties of LaF₃-D2EHPA nanoparticles in weakly polar mineral base oil (150 N), polyalphaolefin (PAO6), and strongly polar synthetic ester oil diisooctyl sebacate (DIOS) were investigated using a four-ball friction and wear tester. In combination with the adsorption experiments using a dissipative quartz crystal microbalance (QCM-D), the compatibility and action mechanisms of LaF₃-D2EHPA nanoparticles with extreme-pressure and anti-wear additives for typical gear oils were further investigated. The results indicate that when the mass fraction of LaF₃-D2EHPA nanoparticles is 0.2%, the tribological properties of all three base oils are improved to the greatest extent. When LaF₃-D2EHPA nanoparticles are mixed with gear oil additives T321 (sulfurized isobutylene) and T307 (ammonium thiophosphate), they exhibit high film-forming rates and anti-wear properties, forming a boundary lubricating film on the worn surface. This film consists of a tribochemical reaction film containing sulfur and phosphorus, as well as LaF₃-D2EHPA nanoparticles that have been deposited.

Lubricant composition plays a crucial role in reducing wear and friction between moving components, thereby ensuring the smooth operation and protecting critical components of industrial machinery. Friction modifiers, as key additives, play a significant role in lowering the coefficient of friction and improving the energy efficiency of lubricants. In this study, we investigated the influence of friction modifier chemistry and concentration on the physico-chemical and tribological properties of industrial lubricating oil. Results demonstrated that increasing the friction modifier concentration effectively reduces the friction coefficient in tribological tests and enhances the elastohydrodynamic film thickness. Furthermore, the study highlights the significant role of surface temperature in activating the friction modifier. These findings provide valuable insights for the design of energy-efficient lubricants with optimised performance characteristics.

Among atmospheric contaminants, soot is known to cause severe wear of mechanical components and degrade lubricating oils; however, its impact on lubricity grease remains largely unexplored. This study investigates the interactions between carbon black (as a soot surrogate) and other lubricant additives, including molybdenum disulfide (MoS₂) and anti-wear additive zinc dialkyldithiophosphate (ZDDP). Additionally, the effects of individual and combined additions of MoS₂ and ZDDP on the tribological properties of soot-contaminated lithium grease (LG) were compared. The lubricity of LG decreased progressively with increasing soot concentration. Conversely, incorporating MoS₂ improved the lubricity of LG, with optimal performance observed at 0.5 wt%. The individual addition of MoS₂ to LG minimally mitigated the abrasive wear caused by soot; however, the inclusion of ZDDP T202 enhances the effectiveness of MoS₂ in reducing the friction and wear. MoS₂ and ZDDP participate in tribochemical reactions to form protective films during sliding. MoS₂ contributes by reducing friction, while ZDDP reacts to form protective films that enhance wear resistance. However, the combined addition of ZDDP and MoS₂ demonstrates limited effectiveness in fully mitigating the adverse effects of soot on grease lubrication. Consequently, exploring more efficient methods to prevent lubrication failure caused by soot.

With the development of aviation engine technology, the temperature in the lubricating system continues to rise. It is expected that aviation lubricating oils should have excellent thermal-oxidative stability. The high-temperature deposit and oil degradation characteristic test is adopted to evaluate the thermal stability of aviation lubricating oils in the performance specifications. In this study, a high-temperature bearing test machine was established first, and two typical aviation lubricating oils were measured by this developed test machine. The thermal-oxidative processes and ageing mechanisms of the aviation lubricating oils in the high-temperature bearing test were analysed. Moreover, the extreme-pressure and anti-wear performances of the aviation lubricating oils before and after the bearing test were studied. Experimental results indicated that the established high-temperature bearing test machine had good discriminability, and the high-temperature bearing test could meet the standard method requirements. Viscosity and total acid number (TAN) of the two aviation lubricating oils gradually increased with the high-temperature bearing test time. Antioxidants and anti-wear and extreme-pressure additives were consumed during the bearing test. Compared with dioctyldiphenylamine (DODPA), the antioxidant additive n-phenyl-α-naphthylamine (α-NPA) exhibited higher antioxidant activity under test conditions. The thermal-oxidative degradation of the aviation lubricating oils involved complex physical and chemical processes. The effects of the thermal oxidation on the extreme-pressure and anti-wear performances of the two aviation lubricating oils were significantly different.

Here we provide atomistic insights into the intrinsic origin of friction from interfacial charge fluctuation by density functional theory (DFT) calculations of typical 2D vdWs systems, including graphene, h-BN and transition metal chalcogenides. We proposed a quantitative method to discriminate the redistribution of interfacial charge density in the sliding process. It is thus discovered that the synchronous evolution between sliding charge fluctuation and potential energy surfaces corrugation is necessary to surmount frictional resistance, giving a linear relation coefficient k for charge transfer and potential energy acting on the frictional sliding under different normal stress. This study implies that the inherent friction characteristic of the sliding systems could be traced back to the evolution of charge transfer, enabling prediction and design of tribological response.

The vibration response of locally defective ball bearings is closely related to their lubrication conditions. Previous bearing dynamics models mainly regarded the lubrication state as the ideal lubrication condition and failed to reveal the effect of insufficient lubricant on the vibration behaviour of defective bearings. In this paper, the interaction within the system under starvation condition was considered, and a dynamics model of defective Deep groove ball bearing (DGBB) with insufficient lubricant was established, and the accuracy of the model was verified experimentally. The results showed that the outer ring failure through frequency was modulated by the cage rotational speed, and the increase of the degree of starved lubrication significantly increased the friction, which led to the increase of cage rotational speed, and ultimately led to the increase of failure frequency. At the same time, the increase in rotational speed made the slipping phenomenon more serious, leading to an increase in the deviation rate of the failure frequency at the same starved degree. The results of the study are useful for fault diagnosis and condition monitoring of related equipment.