Friction最新文献

Polymers are widely used in bearing applications. In the case of water-lubricated stern tube bearings, thermoplastic polyurethane (TPU)-based composites are used due to their excellent wear resistance, corrosion resistance, and tunable mechanical properties. Their tribological performance, however, depends on operating conditions. In this work, TPU was blended with carbon fiber, graphene platelet, and ultra-high molecular weight polyethylene (UHMWPE). Friction tests of TPU based-composites against copper countersurface were carried out in water to mimic the actual operating conditions of the bearing. Most of the resulting contacts were in the boundary lubrication regime, in which friction was attributed to both contact mechanics of asperities as well as water lubrication. Our results show that the viscoelasticity of TPU has a considerable impact on its tribological performance. Water lubrication at 50 °C promotes the softening of polymer surface material during sliding, resulting in higher fluctuation in the coefficient of friction and wear loss. This is attributed to the reduced thermomechanical properties. In addition, Schallamach waviness is observed on worn surface. The tribological properties of TPU are significantly improved by the inclusion of carbon fiber, graphene platelet, and UHMWPE. The formation of graphene transfer-layers and UHMWPE transfer film reduces friction and wear loss, while the inclusion of carbon fiber enhances wear resistance due to improved mechanical properties and load bearing capacity.

Durability and reliability have been studied for decades through intensive trial-error experimentation. However, there are numerous fields of application where the costs associated with this approach are not acceptable. In lubricated machines with severe dynamic loads, such as high-power-density engines, simulation tools offer clear advantages over intensive testing. Prototypes and multiple scenarios can be cost-effectively simulated to assess different lubricants and engine configurations. The work presented here details the study of wear based on a validated elastohydrodynamic (EHD) simulation model of the connecting rod journal bearing. This model accounts for elastic deformation through a connecting rod finite element model (FEM). In addition, multiple lubricant rheological and tribological dependences, determined by specific experimental tests, are applied in the model through their interaction with the simulation software. Correspondingly, a novel wear algorithm is proposed to predict wear depth over time evolution along a proposed wear cycle based on the typical working ranges of high-performance engines. A final assessment is presented to compare 4 different ultralow-viscosity lubricants in their protective performance under severe conditions. The results show the evolution of the wear load and wear depth over the wear cycle. This evaluation is key to describing a lubricant selection procedure for high-power-density engines.

To expand the use of metal–organic frameworks (MOFs) based self-lubricating composite, flexible MOFs MIL-88D has been studied as a nanocontainer for loading lubricant. In this work, the mechanism of oleamine adsorption and desorption by MIL-88D was investigated through molecular simulations and experiments. Molecular simulations showed that the oleamines can be physically adsorbed into open MIL-88Ds with the Fe and O atoms of MIL-88D interacting with oleamine NH₂-group. Higher temperature can cause Ole@MIL-88D to release more oleamines, while higher pressure on Ole@MIL-88D caused less oleamines released. Moreover the Ole@MIL-88D was incorporated into epoxy resin (EP) for friction tests. The optimum mass ratio of MIL-88D to EP is 15 wt%, and the EP/Ole@MIL-88D prefers light load and high frequency friction. This work suggests that flexible MOFs can be used as a nanocontainer for loading lubricant, and can be used as a new self-lubricating composite.

This work explores experimentally the effects of DC electrical currents on lubricant film thickness alteration in lubricated sliding steel contacts in the boundary and mixed regime as measured by ultrasound. The experiments were performed in a two-electrode cell-based pin-on-disk tester instrumented with ultrasonic transducers. Unelectrified and electrified tribological tests were conducted on steel flat-on-flat contacts under various speeds and loads using both a mineral base oil and a gear oil. Film thickness, coefficient of friction (CoF), and electrical contact resistance (ECR) were measured during short experiments (30 s) in unelectrified and electrified (1.5 and 3 A) conditions. The results suggest that film thickness, CoF, and all ECR are altered by passing DC currents through the contact. In particular, film thickness increased and decreased, respectively, by applying electricity at the different speeds and loads tested. These alterations were majorly ascribed to oil viscosity decrease by local heat and surface oxidation caused by electrical discharge and break down at the interface.

The good lubrication ability of articular cartilage holds significant importance in our daily lives. Osteoarthritis (OA), the most prevalent degenerative joint disease, causes cartilage damage, increased friction, and inflammation. However, the current clinical treatments for OA exhibit some defects. Recently, the sustained drug release systems with lubricating function have attracted considerable attention for treating OA. This review introduces the lubrication mechanism of cartilage, focusing particularly on the boundary lubrication mechanism. The research progress of boundary-lubricated biomaterials with drug delivery, including microcarriers, hydrogels, and nanoparticles in the treatment of OA by improving inter-articular lubrication and relieving inflammation is discussed and summarized. The efficacy and challenges of boundary-lubricated biomaterials with drug delivery in the treatment of OA are summarized, and the prospects are also discussed.

Two-dimensional materials are excellent lubricants with inherent advantages. However, superlubricity has been reported for only a few of these materials. Unfortunately, other promising two-dimentional (2D) materials with different physical properties cannot be discovered or applied in production; thus, energy consumption can be greatly reduced. Here, we carry out high-throughput calculations for 1,475 2D materials and screen for low-friction materials. To set a standard, we propose, for the first time, a geometry-independent lubricating figure of merit based on the conditions for stick-slip transition and our theory of Moiré friction. For the efficient calculation of this figure of merit, an innovative approach was developed based on an improved registry index model. Through calculations, 340 materials were found to have a figure of merit lower than 10⁻³. Eventually, a small set of 21 materials with a figure of merit lower than 10⁻⁴ were screened out. These materials can provide diverse choices for various applications. In addition, the efficient computational approach demonstrated in this work can be used to study other stacking-dependent properties.

Wind power gears will be excessively worn due to lubrication failure during operation. Herein, the tribological properties of rubbing pairs are improved by filling solid lubricants into surface texture. In texture design, three types of topological textures (Circle (C), Hexagon (H) and Circle/Hexagon (CH)) were obtained by cell topology optimization, and then three cases with 20%, 30%, and 40% density were designed for each texture. Next, SnAgCu and TiC were deposited in texture of AISI 4140 steel (AS) to obtain 9 kinds of self-lubricating surfaces. Among them, AS with 30% CH density (AS-CH30) exhibits excellent mechanical and tribological properties. Compared with AS-C and AS-H, the maximum equivalent stress of AS-CH was decreased by 10.86% and 5.37%, respectively. Friction coefficient and wear rate of AS-CH30 were 79.68% and 78% lower than those of AS. The excellent tribological performances of AS-CH30 can be attributed to the synergistic effect of topological surface and solid lubricants. Topological surface can not only reduce fluctuation of equivalent stress, but also promote the stored lubricants to be easily transferred at the contact interface to form a 200 nm lubricating film containing solid lubricants (mainly), oxides and wear debris.

Sliding-mode triboelectric nanogenerator (S-TENG) is based on the coupling of triboelectrification and electrostatic induction, converting electrical energy from sliding motion. Introducing micro-textures into the sliding surface, and adjusting the angle between the texture and sliding direction (direction angle) may achieve performance anisotropy, which provides novel ideas for optimizing the tribology and electrification performance of S-TENG. To guide the performance optimization based on the anisotropy, in this paper, groove micro-textures were fabricated on the surface of S-TENG, and anisotropic tribology and electrification performance were obtained through changing the direction angle. Based on the surface analysis and after-cleaning tests, the mechanism of the anisotropy was explained. It is shown that the anisotropy of friction coefficient can be attributed to the changes of texture edge induced resistance and groove captured wear debris, while the voltage anisotropy is due to the variations of debris accumulated on the sliding interface and the resulting charge neutralization. Among the selected 0°–90° direction angles, S-TENG at angle of 90° exhibits relatively small stable friction coefficient and high open-circuit voltage, and thus it is recommended for the performance optimization. The open-circuit voltage is not directly associated with the friction coefficient, but closely related to the wear debris accumulated on the sliding interface. This study presents a simple and convenient method to optimize the performance of S-TENG, and help understand the correlation between its tribology and electrical performance.

Abstract

Lighter and more powerful next generation vehicles and other rotary machinery demand bearings to operate in harsher conditions for higher efficiency, and the continuous development of advanced low-wear and friction materials is thus becoming even more important to meet these requirements. New aluminium composites reinforced with high performance lubricate phases such as graphene nanoplatelets (GNPs) are very promising and have been vigorously investigated. By maintaining a low coefficient of friction (COF) and offering great strength against wear due to their self-lubricating capability, the solid lubricant like GNPs protect the bearing surface from wear damage and prevent change in metallurgical properties during temperature fluctuations. This paper first studies the high-temperature tribological performance of aluminium matrix composites reinforced with GNP, consolidated via powder metallurgy, then elucidates their tribological mechanism. We report that the best tribological performance is achieved by the composite containing 2.0 wt% GNP, with an extraordinarily low COF of 0.09 and a specific wear rate of 3.5×10⁻² mm³·N⁻¹·m⁻¹, which represent 75% and 40% reduction respectively, against the plain aluminium consolidated under identical conditions. The in-track and out-of-track Raman analysis have confirmed the role of GNPs in creating a tribofilm on the counterpart surface which contributed to the excellent performance.

Herein, we have prepared SiO₂ particles uploaded MXene nanosheets via in-situ hydrolysis of tetraetholothosilicate. Due to the large number of groups at the edges of MXene, SiO₂ grows at the edges first, forming MXene@SiO₂ composites with a unique core-rim structure. The tribological properties of MXene@SiO₂ as lubricating additive in 500 SN are evaluated by SRV-5. The results show that MXene@SiO₂ can reduce the friction coefficient of 500 SN from 0.572 to 0.108, the wear volume is reduced by 73.7%, and the load capacity is increased to 800 N. The superior lubricity of MXene@SiO₂ is attributed to the synergistic effect of MXene and SiO₂. The rolling friction caused by SiO₂ not only improves the bearing capacity but also increases the interlayer distance of MXene, avoiding accumulation and making it more prone to interlayer slip. MXene@SiO₂ is adsorbed on the friction interface to form a physical adsorption film and isolate the friction pair. In addition, the high temperature and high load induce the tribochemical reaction and form a chemical protection film during in the friction process. Ultimately, the presence of these protective films results in MXene@SiO₂ having good lubricating properties.