Friction最新文献

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.

Nano- and micro-particles are being increasingly used to tune interfacial frictional properties in diverse applications, from friction modifiers in industrial lubrication to enhanced biological fluids in human osteoarthritic joints. Here, we assessed the tribological properties of a simulated synovial fluid enriched with non-spherical, poly lactic-co-glycolic acid (PLGA) microparticles (µPL) that have been previously demonstrated for the pharmacological management of osteoarthritis (OA). Three different µPL configurations were fabricated presenting a 20 µm × 20 µm square base and a thickness of 5 µm (thin, 5H µPL), 10 µm (10H µPL), and 20 µm (cubical, 20H µPL). After extensive morphological and physicochemical characterizations, the apparent Young’s modulus of the µPL was quantified under compressive loading returning an average value of ∼ 6 kPa, independently of the particle morphology. Then, using a linear two-axis tribometer, the static (µ_s) and dynamic (µ_d) friction coefficients of the µPL-enriched simulated synovial fluid were determined in terms of particle configuration and concentration, varying from 0 (fluid only) to 6µ10⁵ µPL/mL. The particle morphology had a modest influence on friction, possibly because the µPL were fully squeezed between two mating surfaces by a 5.8 N normal load realizing boundary-like lubrication conditions. Differently, friction was observed to depend on the dimensionless parameter Ω, defined as the ratio between the total volume of the µPL enriching the simulated synovial fluid and the volume of the fluid itself. Both coefficients of friction were documented to grow with Ω reaching a plateau of µ_s ∼ 0.4 and µ_d ∼ 0.15, already at Ω ∼ 2×10⁻³. Future investigations will have to systematically analyze the effect of sliding velocity, normal load, and rigidity of the mating surfaces to elucidate in full the tribological behavior of µPL in the context of osteoarthritis.

The gradient nanostructure is machined on the aluminum (Al) alloy by the two-dimensional ultrasonic surface burnishing process (2D-USBP). The mechanism of why the gradient nanostructure enhances wear resistance is investigated. The mechanical properties and microstructure characterization for the gradient nanostructure are performed by operating a nanoindenter, transmission electron microscopy (TEM), and electron backscattered diffraction (EBSD). Dry wear tests are performed on the samples before and after machining to evaluate the wear resistance and mechanisms. The effect of the gradient nanostructure on the wear resistance is explored by developing the crystal plasticity (CP) finite element and molecular dynamics (MD) models. The characterization results show that the 2D-USBP sample prepared a gradient structure of ∼600 µm thick on the aluminum surface, increasing the surface hardness from 1.13 to 1.71 GPa and reducing the elastic modulus from 78.84 to 70.14 GPa. The optimization of the surface microstructure and the increase of the mechanical properties effectively enhance the wear resistance of the sample, with 41.20%, 39.07%, and 54.58% of the wear scar areas for the 2D-USBP treated samples to the original samples under 5, 10, and 15 N loads, respectively. The gradient nanostructure hinders the slip of dislocations inside the sample during the wear process and reduces the size and scope of plastic deformation; meanwhile, the resistance to deformation, adhesion, and crack initiation and propagation of the sample surface is improved, resulting in enhanced wear resistance.

Development of energy-efficient lubricants is a way to reduce energy consumption for transportation, with the tendency to design molecules that are beneficial in reducing the viscosity of synthetic oils. Oligoether esters (OEEs), as a low-viscosity ester base oil, have characteristics such as simple synthesis and excellent lubrication effect, however, the application of OEEs in tribology field has rarely been investigated. The objective of the present study is to investigate the effect of structure on the lubricating performance of OEEs and to develop a predictive model for OEEs based on quantitative structure-property relationship (QSPR) through a combination of experiment and statistical modeling. Results showed that glycol chains contribute positively to lubrication with the ether functional groups increasing the sites of adsorption. Compared to branched-chain OEEs, straight-chain OEEs exhibited reduced wear, which was mainly due to the thicker adsorption film formed by the straight-chain structure. Furthermore, carbon films were detected on lightly worn surfaces, indicating that OEEs underwent oxidation during the friction process. Based on the results of principal component analysis (PCA) and partial least squares (PLS), it could be found that the predictive models of viscosity–temperature performance, thermal stability performance, coefficient of friction (COF), and wear volume (WV) performed well and robustly. Among them, COF and WV can be best predicted with an R² of about 0.90.

Wear topography is a significant indicator of tribological behavior for the inspection of machine health conditions. An intelligent in-suit wear assessment method for random topography is here proposed. Three-dimension (3D) topography is employed to address the uncertainties in wear evaluation. Initially, 3D topography reconstruction from a worn surface is accomplished with photometric stereo vision (PSV). Then, the wear features are identified by a contrastive learning-based extraction network (WSFE-Net) including the relative and temporal prior knowledge of wear mechanisms. Furthermore, the typical wear degrees including mild, moderate, and severe are evaluated by a wear severity assessment network (WSA-Net) for the probability and its associated uncertainty based on subjective logic. By integrating the evidence information from 2D and 3D-damage surfaces with Dempster–Shafer (D–S) evidence, the uncertainty of severity assessment results is further reduced. The proposed model could constrain the uncertainty below 0.066 in the wear degree evaluation of a continuous wear experiment, which reflects the high credibility of the evaluation result.

Bubbly oil lubrication is a type of lubrication method. However, the lubrication model of the bubbly oil has not been thoroughly considered. This paper aims to investigate the modelling for bubbly oil lubrication considering the interfacial effect and thermal effect, and a theoretical model is established based on the theory of multiphase mixtures. The interfacial and thermal effects on the static characteristics of a thrust bearing are analyzed. A test rig for the thrust bearing is developed to measure the static characteristics of the bearing under bubbly oil lubrication. The results show that the bearing static characteristics, i.e. bearing temperature rise, film thickness, friction torque, and volume flow, increase with consideration of three interfacial effects; the bearing temperature rise increases but the film thickness, friction torque, and volume flow rate decrease with consideration of the three thermal effects; the thermal effect on the bearing static characteristics is greater than the interfacial effect.

This study investigates the effect of contact surface curvatures on the friction response under varying tangential loadings using a finite element (FE) model. The results showed that the geometry of the surface influences the contact force at the interface and reduces the friction effect through an unsteady distribution of the contact force. The relationship between the friction effect, excitation, and contact surface shape was also examined, revealing a linear inverse relationship between the friction and curvature. The findings provide a comprehensive understanding of the frictional interactions between elastic bodies and highlight the role of curvature as a design parameter for regulating the friction effect.

A flow of silicon fluid in the gap between eccentric cylinders was studied experimentally. The condition of gaseous cavitation inception during the rotation of internal cylinder was considered. It was shown that at reduction of the gap between cylinders Saffman–Taylor instability appeared on surface of the internal cylinder and then gaseous cavitation was observed. Possibility of one uniform gas formation appearance under this type of instability was demonstrated.

Sliding motion has always been one of the major concerns when it comes to the analysis of viscoelastic contact problems. A new model simulating the transient sliding contact of smooth viscoelastic surfaces is developed in this paper. By taking the dry contact friction and the coupling between shear tractions and normal pressure into account, the effect of the early partial slip period, which is often neglected in the study of viscoelastic sliding contact problems, is investigated numerically. Compared with solutions based on the frictionless assumption, the steady-state pressure profile is found to be slightly different under the effect of the partial slip regime, including a lower peak pressure and the shift of the contacting region in the direction opposite to the sliding motion. Furthermore, the time required for the viscoelastic contact to reach its steady state is delayed owing to the partial slip period preceding the global sliding motion.

Inspired by the dynamic wet adhesive systems in nature, various artificial adhesive surfaces have been developed but still face different challenges. Crucially, the theoretical mechanics of wet adhesives has never been sufficiently revealed. Here, we develop a novel adhesive mechanism for governing wet adhesion and investigate the biological models of honeybee arolium for reproducing the natural wet adhesive systems. Micro-nano structures of honeybee arolium and arolium-prints were observed by Cryogenic scanning electron microscopy (Cryo-SEM), and the air pockets were found in the contact interface notably. Subsequently, the adhesive models with a three-phase composite interface (including air pockets, liquid secretion, and hexagonal frames of arolium), were formed to analyze the wet adhesion of honeybee arolium. The results of theoretical calculations and experiments indicated an enhanced adhesive mechanism of the honeybee by liquid self-sucking effects and air-embolism effects. Under these effects, normal and shear adhesion can be adjusted by controlling the proportion of liquid secretion and air pockets in the contact zone. Notably, the air-embolism effects contribute to the optimal coupling of smaller normal adhesion with greater shear adhesion, which is beneficial for the high stride frequency of honeybees. These works can provide a fresh perspective on the development of bio-inspired wet adhesive surfaces.