Tribology Letters最新文献

Flexible surface textures are often utilized in the design of robots that need to manipulate objects requiring a strong frictional force. In this study, we designed and prepared flexible silicone rubber films with surface textures inspired by groove patterns found at the tips of human fingers. These designs included loop, whorl, and arch patterns, as well as horizontal and vertical stripe textures as a control group. On the basis of surface morphology analysis, we established a relative sliding test platform to collect coefficient of friction (COF) through relative sliding tests of soft surface textures and rigid plane contact pairs. The friction coefficient guides the characterization of the contact properties in the finite element simulation process. According to the results of friction testing, the loop, whorl, and horizontal stripe exhibit a higher friction coefficient under variable contact stress, while the arch and vertical stripe display a lower coefficient. The variation patterns of the contact surfaces between a rigid surface and five distinct types of soft surface textures were analyzed by simulating the friction process using Abaqus explicit dynamic analysis. The deformation of the soft surface textures under different contact stresses is subsequently described in terms of elastic strain energy. Compared to the vertical stripe texture, loop, whorl, and arch exhibit greater recoverable strain energy during the relative sliding stage, which means a larger average elastic displacement. Subsequently, different soft surface textures are integrated onto the fingertip of a soft robotic hand, and the grasping ability is evaluated within lubrication-related medical scenarios. The texture perpendicular to the movement direction exhibits a higher friction-producing capability compared to the texture aligned parallel to it. Due to the intricate surface texture patterns, it demonstrates greater adaptability for relative motion in all directions. This research proposes a soft robotic hand incorporating a surface texture resembling fingerprint-like surface texture. By employing experimentation and finite element simulation, this study utilizes surface engineering design to comprehend the contact characteristics involved in the grasping process of a soft robotic hand.

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In recent years, bio-lubricants have received a growing interest for industrial applications. Still, a full-scale implementation in machinery lubrication requires a thorough evaluation of their performance through tribological and operational tests to stand upon their performance. Additionally, the promising outcomes achieved by nanoadditives in improving the performance of synthetic lubricants have prompted research efforts to identify suitable nanoadditives for bio-grease. This paper introduces a bio-grease from a hybrid vegetable oil and glycerol monostearate as a thickener for the lubrication of rolling bearings. Activated carbon nanoparticles (ACNPs) as nanoadditives were synthesized, characterized, and incorporated into the bio-grease at concentrations of 0.5, 1, and 2% by weight. Tribo-tests were conducted on these bio-grease blends, and running tests were carried out using 6006 ball bearings on a custom test rig. Throughout a 30-min test run under a radial load of 10% of the bearing’s dynamic load rating, mechanical vibrations and power consumption were measured and analyzed for each bearing. The bio-grease with ACNPs exhibited a substantial reduction in wear scar diameter (WSD) and coefficient of friction (COF), achieving improvements of up to 73.6 and 65%, respectively, in comparison to lithium grease. Furthermore, the load carrying capacity was enhanced by 200%. The study revealed a strong correlation between measured vibration amplitudes and the viscosity of the bio-grease. The absence of high frequency resonant bands in vibration spectra indicated that the test grease samples satisfied the conditions of elastohydrodynamic lubrication, and these findings were corroborated through calculations of the minimum oil film thickness.

Do two different and independent methods of estimating the wear rate of a test sample yield the same numerical result? Numerical values of specific wear rates estimated on the basis of alternative methods using a set of dry sliding rotary-pin-on-disk experiments are presented. Wear rates of brass and aluminium alloy pins were estimated using gravimetric and wear scar area methods. Gravimetric and linear displacement methods were used to assess wear rates of ABS plastic and machinable wax pins. Scepticism about the estimated nominal values of wear rates is reduced when alternative assessment methods result in comparable numerical values, or values having the same order of magnitude. This is particularly useful when ranking competing materials for wear rates, when the differences in these rates are small. Uncertainties in individual test sample wear rates, and dispersion in the nominal values of wear rates are also computed to support the aforementioned observations.

Of the medals awarded during the Winter Olympics Games, most are awarded for sports involving cross-country (XC) skiing. The Double Poling (DP) technique, which is one of the sub-techniques used most frequently in XC skiing, has not yet been studied using simulations of the ski–snow contact mechanics. This work introduces a novel method for analysing how changes in the distribution of pressure on the sole of the foot (Plantar Pressure Distribution or PPD) during the DP motion affect the contact between the ski and the snow. The PPD recorded as the athlete performed DP, along with an Artificial Neural Network trained to predict the geometry of the ski (ski-camber profile), were used as input data for a solver based on the boundary element method, which models the interaction between the ski and the snow. This solver provides insights into how the area of contact and the distribution of pressure on the ski-snow interface change over time. The results reveal that variations in PPD, the type of ski, and the stiffness of the snow all have a significant impact on the contact between the ski and the snow. This information can be used to improve the Double Poling technique and make better choices of skis for specific snow conditions, ultimately leading to improved performance.

Graphical Abstract

This study employs molecular dynamics simulation to examine the tribological behavior of nano zinc oxide (nano-ZnO) lubricated with decanol. The changes in electrostatic interaction energy, molecular structure, and chemical reactions during the friction process were analyzed. For ZnO-decanol-ZnO system, the simulation revealed a notable reduction in the coefficient of friction for nano-ZnO, decreasing from 0.49 (at 0.5 GPa and 100 m/s) to 0.18 (at 3 GPa and 20 m/s). This improvement is attributed to the enhanced adsorption ability and temperature stabilization provided by the decanol lubricant. Furthermore, an increase in velocity induces elastoplastic deformation and wear on the sliding surface, leading to a decline in tribological performance.

A systematic experimental investigation concerning the fretting-induced tribo-chemical state and its effect on the fretting wear behavior of titanium alloys under the vacuum atmospheres (4 × 10^–3 Pa and 4 × 10^–1 Pa) in different fretting regimes is reported. An in situ XPS analysis tester was developed to capture the real tribo-chemical state of worn surface for all test conditions. Results show that samples subjected to different vacuum atmospheres have varied tribo-chemical states depending on the fretting regime, which play significantly different roles in determining the associated damage mechanisms and the resulting fretting wear resistance. Under both vacuum atmospheres, in the partial slip regime (PSR) the worn scars were mainly covered by TiO₂, showing comparable levels of very slight damage, while in the mixed fretting regime (MFR), the tribo-layer is still mainly consisted of TiO₂, but with an evident peak of Ti metal for the high vacuum degree (4 × 10^–3 Pa) in MFR, showing a mild damage. In contrast, in the gross slip regime (GSR), Ti metal was prone to be oxidized to Ti₂O₃ and TiO on the worn scar, especially for the low vacuum degree (4 × 10^–1 Pa) having a highest content of Ti₂O₃. It might be inferred that the tribo-layer containing more Ti₂O₃ formed during fretting wear process is susceptible to be broken, hence showing a highest fretting wear volume in GSR for the low vacuum degree. The results suggest that for the vacuum environments, the Ti6Al4V may be more suitable to be used under the high vacuum atmosphere.

Low ionic concentrations and the chemical stability of the ions involved to water are considered necessary for hydrated ionic solution lubricants. Herein, an ultra-high concentration of chemically active ZnCl₂ aqueous solution (up to 20 mol L^− 1) is first reported to be used as a liquid superlubricant, with trace amounts of Zn²⁺ hydrolyzed to generate Zn(OH)₂ (i.e., hard clusters) in addition to Zn²⁺ hydrated with water (i.e., soft clusters), resulting in the formation of heterogeneous phases with a mixture of soft and hard clusters. The coefficient of friction (COF) inversely correlates with the molar concentration of ZnCl₂, where the COF of the optimized samples can be as low as 0.006 with a stable macroscopic superlubricated state; the particle size distribution of clusters and their corresponding Spans, however, are positively correlated with the molar concentration, suggesting that asymmetric contact between these unequal-sized soft and hard clusters contributes greatly to the reduction of interfacial shear resistances. This ultra-high-concentration aqueous solution superlubricant not only breaks the convention but also deepens the mechanism of liquid superlubricity from the perspectives of cluster size distribution and interactions between clusters, providing a new insight into the design of advanced water-based ionic solution superlubricants.

Graphical Abstract

The development of optimized lubricants is hindered by missing knowledge of fluid properties, in particular the viscosity, in the range of extreme pressures and temperatures relevant for application. Molecular dynamics simulations can be used to calculate viscosity, but the necessary computational effort imposes practical limits for high viscosities. In this study, the viscosity of PAO4 oil was extracted from equilibrium molecular dynamics simulations as a function of pressure and temperature reaching viscosities up to 20 Pas. Three calculation methods based on different microscopic expressions for the viscosity were used. The methods exhibit considerably different performance with respect to preciseness and computational efficiency. The highest viscosities were found to be calculated most efficiently via the Stokes–Einstein relation, by computing the diffusion coefficient from the velocity correlation function. This offers a new, more effective route to push viscosity calculations in equilibrium molecular dynamics simulations to higher pressure systems.

Graphical Abstract

O-rings made from foam rubber are often used in sealing applications. Foam rubber have low (macroscopic) elastic modulus (E_0) resulting in a low nominal contact pressure when squeezed against a countersurface. In most cases the foam rubber is covered by a thin surface film with the effective elastic modulus (E_1 > E_0). We show that the nominal contact pressure may not be high enough for the contact area to percolate and the O-ring seal will leak. For the leakage calculations we use the Persson multiscale contact mechanics theory, and the (modified) Bruggeman effective medium theory for the fluid flow conductivity. The experimental input for the theory are surface roughness power spectrum, which was obtained from stylus topography measurements, and the elastic properties ((E_0) and (E_1)) of the rubber O-ring. As an application of this calculation method, we have used the preliminary as well as the final results of the laboratory gas tightness tests of the 136 New Small Wheel Micromegas Quadruplets performed at CERN, from February 2019 to May 2021, in the framework of the ATLAS Experiment upgrade. In the integration quality control, a novel method for gas tightness measurement, that we have called “Flow Rate Loss”, has been used as a baseline method.

Graphical Abstract

Developing new lubrication concepts greatly contributes to improving the energy efficiency of mechanical systems. Nanoparticles such as those based on carbon allotropes or 2D materials have received widespread attention due to their outstanding mechanical and tribological performance. However, these systems are limited by a short wear life. Combining nanoparticle coatings with laser surface texturing has been demonstrated to substantially improve their durability due to the reservoir effect which prevents immediate particle removal from the contact. In this study, we investigate the high-load (20 N) tribological performance of AISI 304 austenitic stainless-steel substrates, which are line-patterned by laser interference patterning and subsequently coated with different carbon nanoparticle coatings (carbon nanotubes, carbon onions, carbon nanohorns) against alumina and 100Cr6 counter bodies. In addition to that, benchmark testing is performed with conventional solid lubricant coatings (graphite, MoS₂, WS₂). Electrophoretic deposition is used as the main coating technique along with air spraying (for WS₂). All coatings substantially improve friction compared to the purely laser-patterned reference. Among all coating materials, carbon nanotubes demonstrate superior lubricity and the longest wear life against 100Cr6 and alumina counter bodies. Detailed characterization of the resulting wear tracks by energy-dispersive X-ray spectroscopy, scanning electron microscopy, and confocal laser scanning microscopy provides insights into the friction mechanisms of the various solid lubricant particles. Further, material transfer is identified as an important aspect for effective and long-lasting lubrication.