Friction最新文献

In this paper, a semi-analytical calculation model for the coefficient of friction (COF) of single spherical protrusions is presented. It allows the prediction of the deformative friction part (μ_def) and adhesive friction part (μ_adh) of the friction pairings steel ∣ polyethylene with ultra-high molecular weight (PE-UHMW) and polyoxymethylene (POM) ∣ PE-UHMW. The experimental studies included unlubricated friction tests, which served to determine the total COF (μ_tot), as well as tests being lubricated with silicone oil, from which μ_def is obtained. Based on the verification tests, it could be shown that both states of lubrication result in the same deformation and that the relationship between the rear angle (ω) and μ_def postulated in the calculation model is valid. Therefore, friction tests with segmented spheres were carried out, which allow a specific variation of the ω.

It can be concluded that for both pairings the μ_def is generally of minor significance (approx. 1/3 μ_tot) and the influence of the μ_adh predominates (approx. 2/3 μ_tot) the friction process. Furthermore the μ_tot decreases with increasing contact pressure especially in the low pressure range and depends on the form of motion (continuous and discontinuous).

Tribological properties of femoro-tibial leg joints in two beetles, darkling beetle Zophobas morio and Congo rose chafer Pachnoda marginata were studied. Very low friction of 0.004 was revealed by the direct measurements in the joint. It is assumed that semi-solid lubricant functioning as in technical bearings is one of the leading factors of the friction minimization. Dependence of the surface texture and physical chemical properties (hydrophobicity) on the cuticle friction was analysed. Contribution of the surface texture to the tribological properties of contacting surfaces was examined by the measurement in the tribosystem “contacting surface/glass”. It is supposed that coefficient of friction (COF) decreases with decrease of surface roughness. At the same time, no statistically significant correlation was found between the hydrophobicity of the surface and the value of the friction coefficient.

High-temperature lubrication has always been a hot topic in the lubricant and grease industry, and is also an essential concern in the high-end equipment sector to be addressed. Carbon quantum dots (CQDs) are an emerging material widely applied in the field of lubrication, owing to their exceptional lubricity and high load-bearing capacity. However, the vulnerability of CQDs to oxidation in air and reduced stability dramatically restrict their high-temperature application capability. In this study, a nanocomposite of amphiphilic polyvinyl pyrrolidone (PVP) homopolymer with excellent lubricating properties and thermal stability, which is hydrogen bonded to CQDs (CQDs@PVP), was designed to achieve low friction and wear of lubricants at high temperatures. The CQDs@PVP are consistently dispersed in both PEG400 and water, and exhibit superior lubricity compared to unmodified CQDs at high temperatures (ranging from 200–150 °C and 90.50 °C). Meanwhile, the dense carbon film on the wear surface and the chemically reactive film of iron compounds directly contribute to the enhanced lubrication performance. These analytical results demonstrate the powerful candidacy of CQDs@PVP as a lubrication additive and promote future high-temperature applications of CQDs in industrial production.

Organic polymer coatings have been commonly used in biomedical field, which play an important role in achieving biological antifouling, drug delivery, and bacteriostasis. With the continuous development of polymer science, organic polymer coatings can be designed with complex and advanced functions, which is conducive to the construction of biomedical materials with different performances. According to different physical and chemical properties of materials, biomedical organic polymer coating materials are classified into zwitterionic polymers, non-ionic polymers, and biomacromolecules. The strategies of combining coatings with substrates include physical adsorption, chemical grafting, and self-adhesion. Though the coating materials and construction methods are different, many biomedical polymer coatings have been developed to achieve excellent performances, i.e., enhanced lubrication, anti-inflammation, antifouling, antibacterial, drug release, anti-encrustation, anti-thrombosis, etc. Consequently, a large number of biomedical polymer coatings have been used in artificial lungs, ureteral stent, vascular flow diverter, and artificial joints. In this review, we summarize different types, properties, construction methods, biological functions, and clinical applications of biomedical organic polymer coatings, and prospect future direction for development of organic polymer coatings in biomedical field. It is anticipated that this review can be useful for the design and synthesis of functional organic polymer coatings with various biomedical purposes.

Ultra-low friction is crucial for the anti-friction, anti-wear, and long-life operation of nanodevices. However, very few two-dimensional materials can achieve ultra-low friction, and they have some limitations in their applications. Therefore, exploring novel materials with ultra-low friction properties is greatly significant. The emergence of ternary two-dimensional materials has opened new opportunities for nanoscale ultra-low friction. This study introduced nickel phosphorous trisulfide (NiPS₃, referred to as NPS), a novel two-dimensional ternary material capable of achieving ultralow friction in a vacuum, into the large nanotribology family. Large-size and high-quality NPS crystals with up to 14 mm × 6 mm × 0.3 mm dimensions were grown using the chemical vapor transport method. The NPS nanosheets were obtained using mechanical exfoliation. The dependence of the NPS nanotribology on layer, velocity, and angle was systematically investigated using lateral force microscopy. Interestingly, the coefficient of friction (COF) of NPS with multilayers was decreased to about 0.0045 under 0.005 Pa vacuum condition (with load up to 767.8 nN), achieving the ultra-low friction state. The analysis of the frictional dissipation energy and adhesive forces showed that NPS with multilayers had minimum frictional dissipation energy and adhesive forces since the interlayer interactions were weak and the meniscus force was excluded under vacuum conditions. This study on the nanoscale friction of a ternary two-dimensional material lays a foundation for exploring the nanoscale friction and friction origin of other two-dimensional materials in the future.

Anti-wear performance of human enamel in the mouth is closely related to the lubrication of salivary pellicle. It is well known that the inorganic hydroxyapatite (HA) of the enamel plays an important role in the adsorption and pellicle-forming of salivary proteins on the enamel, but the role of enamel matrix proteins remains unclear. In this study, the adsorption and lubrication behavior of salivary proteins on original, heated, and deproteinated enamel surfaces was comparatively investigated using an atomic force microscopy and nano-indentation/scratch techniques. Compared with that on the original enamel surface, the adsorption and lubrication behavior of salivary proteins remains almost unchanged on the heated enamel surface (where the enamel matrix proteins are denatured but the size of HA crystalline nanoparticles keeps constant) but exhibits an obvious compromise on the deproteinated enamel surface (where the enamel matrix proteins are removed and agglomeration of HA crystallites occurs). The HA agglomeration weakens the electrostatic interaction of enamel surfaces with salivary proteins to cause a distinct negative influence on the adsorption and pellicle-forming of salivary proteins. Further, the negative effect is confirmed with a quartz crystal microbalance with dissipation. In summary, by regulating enamel nanostructure for appropriate electrostatic interactions between salivary proteins and enamel surfaces, the enamel matrix proteins play an essential role in the adsorption and pellicle-forming of salivary proteins on human enamel, and then contribute to saliva lubrication, which provides the enamel with an anti-wear mechanism. The findings will promote and assist the design of enamel-inspired anti-wear materials.

Autonomous underwater vehicles (AUVs) have various applications in both military and civilian fields. A wider operation area and more complex tasks require better overall range performance of AUVs. However, until recently, there have been few unified criteria for evaluating the range performance of AUVs. In the present work, a unified range index, i.e., L*, considering the cruising speed, the sailing distance, and the volume of an AUV, is proposed for the first time, which can overcome the shortcomings of previous criteria using merely one single parameter, and provide a uniform criterion for the overall range performance of various AUVs. After constructing the expression of the L* index, the relevant data of 49 AUVs from 12 countries worldwide have been collected, and the characteristics of the L* range index in different countries and different categories were compared and discussed. Furthermore, by analyzing the complex factors affecting the range index, methods to enhance the L* range index value, such as efficiency enhancement and drag reduction, have been introduced and discussed. Under this condition, the work proposes a unified and scientific criterion for evaluating the range performance of AUVs for the first time, provides valuable theoretical insight for the development of AUVs with higher performance, and then arouses more attention to the application of the cutting-edge superlubricity technology to the field of underwater vehicles, which might greatly help to accelerate the coming of the era of the superlubricitive engineering.

Due to the increasing risk of obesity and cardiovascular diseases caused by high-fat diets, low-fat foods have become a priority demand for consumers’ health. However, the smoothness perception and scientific assessment methods of the existing low-fat foods should be improved. In this study, three food emulsions were prepared, and their lubrication characteristics, sensory evaluation of smoothness, and electroencephalogram (EEG) signals were assessed to preliminarily investigate the effects of food emulsion components on their above characteristics. The results showed that fat substitute (FSU) and fat could significantly reduce coefficient of friction (CoF) of the food emulsions, with average CoF reduced by 28% and 63% compared to the original food emulsions. In addition, fat-enriched food emulsions continued to exhibit excellent lubrication characteristics after adding artificial saliva, with an average CoF reduced by 31.1% compared to that of the food emulsions without artificial saliva. Both FSU and fat improved the smoothness of food emulsions, and the lubricating properties of fat were more pronounced, with fat-enriched food emulsion which could provide a substantial improvement in smoothness compared to the fat-free food emulsion. Comparison of subjects’ EEG signals revealed that food emulsion with lower CoF and higher smoothness triggered higher P3 amplitudes and longer latencies. These findings provide better insights into the scientific evaluation of food texture and the development of low-fat foods.

Efficient cooperative lubrication can be achieved via the introduction of core–shell structure lubricant additives with hard core and soft shell, for obtaining the expected anti-wear performance from the structural changes in the friction process. In this study, C@Ag microspheres with a core–shell structure were prepared by the redox method with carbon spheres as the core and Ag nanoparticles as the shell. Their tribological behaviors as base oil (G1830) additive with different concentrations were investigated in detail. Compared with base oil, the addition of C@Ag particles at 0.5 wt% can reduce the coefficient of friction (COF) and wear volume (Wv) up to 15.5% and 88%, respectively. More importantly, C@Ag particles provide superior lubrication performance to single additive (like carbon sphere (CS) and Ag nanoparticle). C@Ag core–shell particles contribute to the formation of tribo-film by melt bonding of flexible Ag and carbon sphere (CS) toward excellent self-repair performance and high-efficiency lubrication. Hence, core–shell structural nanoparticles with hard-core and soft-shell hold bright future for high-performance lubrication application.

Two-dimensional (2D) van der Waals layered materials have been widely used as lubricant. Penta-graphene (PG), a 2D carbon allotrope exclusively composed of irregular carbon pentagons has recently been predicted to have superlubricating property. In the present study, by combining the molecular dynamics simulation and first-principles calculations, we investigated the frictional property of PG in both commensurate and incommensurate contacts. Our calculations show the ultra-low friction at the interface of relatively rotated bilayer PG with twist angles of more than 10° away from the commensurate configuration. Meanwhile, our calculations demonstrate the isotropy of the ultra-low friction at the interface of incommensurate contact, in contrast to the anisotropic of the commensurate contacting interface. Additionally, the evolution of friction force and the fluctuation of potential energy along sliding path correlate closely with the interface’s structure. The energetics and charge density explain the difference between the friction at the interfaces of the commensurate and incommensurate contacts. Not only that, we found the correlation between the intrinsic structural feature and interlayer binding energy. Importantly, our findings on the retainment of the ultra-low friction under work conditions indicates that the superlubricating state of PG has good practical adaptability.