Tribology Letters最新文献

While ZDDP tribofilm formation has been widely studied, the mechanism of ZDDP tribofilm removal during rubbing is still unclear. The study employs a ball on disc tribometer to monitor ZDDP tribofilm development in rolling-sliding, mixed lubrication conditions. It is found that when ZDDP tribofilms are formed very rapidly, as is the case with short alkyl chain, secondary ZDDPs, a large proportion of the initially-formed tribofilm is suddenly lost during rubbing. By contrast, the tribofilms that form more slowly from primary ZDDPs and longer chain secondaries are not partially lost during rubbing. XPS analysis showed that a rapidly-formed tribofilm before its partial removal has a very small Zn/O ratio, and a high BO/NBO. This suggests that such tribofilm contains a significant proportion of ultraphosphate, which is likely to have a relatively weak structure due to lack of stabilising cations. This may result in the tribofilm being partially removed when it reaches a certain thickness. By comparison, the remaining tribofilm, and also tribofilms that form slowly, have high Zn/O and low BO/NBO. This suggests that they consist of short chain polyphosphates and are thus stronger and more durable.

Graphical Abstract

A sealed reciprocating tribometer has been used to study the influence of different gaseous environments on the friction and wear properties of AISI52100 bearing steel at atmospheric pressure and 25 °C. Helium, argon, hydrogen, carbon dioxide and nitrogen all give high friction and wear, suggestive of very little, if any tribofilm formation under the conditions studied. Dry air and oxygen also give high friction, slightly lower than the inert gases, but produce extremely high wear, much higher than the inert gases. This is characteristic of the phenomenon of “oxidational wear”. The two gases ammonia and carbon monoxide give relatively low friction and wear, and XPS analysis indicates that this is due to the formation of adsorbed ammonia/nitride and carbonate films respectively. For the hydrocarbon gases studied, two factors appear to control friction and wear, degree of unsaturation and molecular weight. For the saturated hydrocarbons, methane and ethane give high friction and wear but propane and butane give low friction after a period of rubbing that decreases with molecular weight. The unsaturated hydrocarbons all give an immediate reduction in friction with correspondingly low wear. Raman analysis shows that all the hydrocarbons that reduce friction and wear form a carbonaceous tribofilm on the rubbed surfaces.

Graphical Abstract

Hexagonal boron nitride is being considered as an additive for greases due to its structure and physical and chemical properties. In the context of the application of such lubricants in real tribological systems, it is important to recognise the effect of hexagonal boron nitride not only on tribological properties, but also on other functional properties of this group of lubricants. In the present study, tests including dropping point, penetration and mechanical stability were carried out. Additionally, particular focus was placed on the properties of the additive itself, including particle size distribution and adsorption properties, as determined by scanning electron microscopy and low-temperature adsorption isotherms. The introduction of hexagonal boron nitride particles into lithium and calcium greases resulted in enhanced resistance to high temperature and prolonged mechanical stress. This phenomenon was attributed to the type of base grease and the modifications in the configuration of the grease's spatial network that ensued as a result of the incorporation of solid particles. It was found that an additive with a smaller particle size and a significant proportion of nanoparticle fractions, and a more developed porous structure, was more effective. Microscopic observations of the structure of the greases confirmed that the solid particles were deposited in the spatial network of the greases. The distribution of hexagonal boron nitride in the grease structure was found to be contingent upon the physical and chemical properties of the additive. Furthermore, the type of base grease, including the arrangement of the soap fibre network, was identified as a contributing factor.

Graphical Abstract

Antiwear additives permit energy-efficient lubrication of gearboxes, bearings, and other tribological interfaces. We study zirconia (ZrO₂) nanocrystal additives, which readily form protective tribofilms in tribological contacts. Our prior work demonstrated cooperative antiwear performance between ZrO₂ and the S- and P-based co-additives in fully formulated hydrocarbon gear oils. Here, we extend that work by examining the growth kinetics of the ZrO₂ tribofilms, including the influence of the co-additives. In the boundary lubrication regime for mixed rolling-sliding contacts, the initial phase of ZrO₂ tribofilm growth is soon overtaken by removal processes, phenomena whose importance has gone unnoticed in prior work. Tribofilm removal affects the steady-state thickness and morphology of the tribofilm as well as its growth kinetics. The S- and P-based co-additives are incorporated into the ZrO₂ tribofilm, and alter the competition between the growth and removal processes, increasing initial net growth rates per contact cycle and contributing to a more polished final interface. This work highlights the significance of removal processes in determining tribofilm antiwear performance, and suggests several routes for improving tribofilm growth kinetics using co-additives.

Graphical abstract

This work examines the effect of environmental humidity on rate-and-state friction behavior of nanoscale silica-silica nanoscale contacts in an atomic force microscope, particularly, its effect on frictional ageing and velocity-weakening vs. strengthening friction from 10 nm/s to 100 μm/s sliding velocities. At extremely low humidities ((ll 1% RH)), ageing is nearly absent for up to 100 s of nominally stationary contact, and friction is strongly velocity-strengthening. This is consistent with dry interfacial friction, where thermal excitations help overcome static friction at low sliding velocities. At higher humidity levels (10–40% RH), ageing becomes pronounced and is accompanied by much higher kinetic friction and velocity-weakening behavior. This is attributed to water-catalyzed interfacial Si–O-Si bond formation. At the highest humidities examined (> 40% RH), ageing subsides, kinetic friction drops to low levels, and friction is velocity-strengthening again. These responses are attributed to intercalated water separating the interfaces, which precludes interfacial bonding. The trends in velocity-dependent friction are reproduced and explained using a computational multi-bond model. Our model explicitly simulates bond formation and bond-breaking, and the passivation and reactivation of reaction sites across the interface during sliding, where the activation energies for interfacial chemical reactions are dependent on humidity. These results provide potential insights into nanoscale mechanisms that may contribute to the humidity dependence observed in prior macroscale rock friction studies. They also provide a possible microphysical foundation to understand the role of water in interfacial systems with water-catalyzed bonding reactions, and demonstrate a profound change in the interfacial physics near and above saturated humidity conditions.

Inaccurate modeling of rough surface contact still makes it difficult to predict adhesion, friction, wear, leakage, and electrical and thermal contact resistance, which often need to be managed in engineering practice. To address this challenge, a new model of contact between two rough surfaces described by their bearing ratio curves has been developed. This model is compared to a traditional equivalent surface model employing the combined roughness concept and is experimentally verified using the spectrometric analysis of the gap between two surfaces in contact. The results show that the model based on the bearing ratio curves provides a more accurate practical solution for the rough surface contact formed under relatively light load.

Inspired by the shark skin shield scale structure and the excellent elasticity of shark skin, an elastic texture composed of the arc grooves and the rectangular convex structure evenly arranged in the lower layer is constructed to improve the lubrication performance of the friction pair. Under different geometric parameters and speeds, the elastic deformation, stress distribution, friction coefficient, and oil film bearing capacity of the friction pair are compared to analyze the influence of sharkskin texture on the lubrication performance. Firstly, the fluid–solid coupled method establishes a 3D simulation model of the elastic hydrodynamic lubrication. Additionally, the elastomeric bearing specimens with sharkskin bionic texture are fabricated using 3D printing technology, and visualization experiments are performed to validate the simulation results. During the sliding friction process, the shark skin texture can appropriately intensify elastic deformation and uniform overall stress distribution. With the increase in the dimensionless width of the rectangular convex structures, the overall elastic deformation intensifies, the bearing capacity of the oil film thickens, and the friction coefficient decreases. In this study, when the depth-width ratio of the arc groove is 0.1 and the dimensionless width of the rectangular convex structures is 0.125, the friction coefficient of the elastic bearing is the minimum, and the maximum reduction percentage is about 15.3%.

The oil-separation properties of lubricating greases are responsible for transporting base oil to the bearing contacts. Therefore, a good understanding of these properties is necessary to predict grease life based on physical grease properties. Currently, oil separation for small, aged grease samples collected from bearings, is studied using so-called maintenance tools. These tools give qualitative insight into the grease properties, e.g., increases or decreases in oil separation after ageing of the grease. In this work, a quantitative, lab-scale method to study oil separation is presented. Using this method, the amount of base oil transferred from a grease sample to a piece of blotting paper is measured based on the difference in light transmission through the oil stain and the dry paper. Translation of transmitted light intensity to oil concentration is accomplished using a modified Lambert-Beer’s law. This enables the determination of the instantaneous bleed rate and oil content. In combination with a physical model, this method can help to improve the understanding of the driving forces behind oil separation, e.g., the affinity pressure and permeability.

The protection of steel surfaces from wear under extreme pressure conditions is of major importance in several industries as it provides better performance and longer life of machinery. The motivation for this work was to study the lubrication of steel by ionic liquids (ILs), which have recently emerged as greener alternatives to commercial lubricants and additives. Three ILs based on sulfur-containing anions, used as 2-wt% additives in polyethylene glycol base oil (MW 200; PEG 200), were tested in the lubrication of ASTM 52100 bearing steel contacts in extreme pressure conditions (under mixed lubrication with a Hertzian pressure of 1.12 GPa) using a mini traction machine (MTM). Due to the poor resistance to corrosion of bearing steel, a semi-ester of succinic acid derivative corrosion inhibitor (Lanxess RC 4801) was added to the mixtures at a 1 wt% concentration. The ILs 1-hexyl-methylimidazolium trifluoromethanesulfonate ([C₆mim][TfO]) and 1-hexyl-4-picolinium trifluoromethanesulfonate ([C₆-4-pic][TfO]) revealed promising results in terms of surface protection of bearing steel. In contrast, 4-picolinium hydrogen sulfate ([4-picH][HSO₄]) as 2-wt% additive to PEG 200 + 1% RC 4801 did not show any improvement in wear performance compared to neat PEG 200 + 1% RC 4801. PEG 200 + 2% [C₆mim][TfO] + 1%RC 4801 allowed for a decrease in wear up to ~ 76% and PEG 200 + 2% [C₆-4-pic][TfO] + 1%RC 4801 up to ~ 46% when compared with neat PEG 200 + 1% RC 4801. Optical microscopy images suggest the formation of an adsorbed layer, which was further supported by chemical analysis via x-ray photoelectron spectroscopy (XPS) data for [C₆mim][TfO].

Graphical abstract

The impact of relative humidity on the formation of low-frictional interface in hydrogenated carbon nitride (CN_x:H) coatings sliding against Si₃N₄ balls and the formation continuity was elucidated through friction tests conducted in both air and nitrogen atmospheres with controlled relative humidity levels. In air atmosphere, a carbonaceous tribolayer with a transformed structure from the initial CN_x:H was formed on Si₃N₄ at less than the critical humidity that existed in 1.0–3.0% RH, resulting in low friction (μ < 0.05) and a low specific wear rate of the balls (< 2 × 10^–9 mm³/N·m). In contrast, this tribolayer failed to form above 3.0% RH. In nitrogen atmosphere, within the 0.25–1.0% RH range, the tribolayer continued to form concurrently with wear progression, maintaining low friction for over 50,000 cycles. However, in less than this humidity range, the lifetime of low friction was limited owing to the tribolayer’s structural alteration. Thus, relative humidity influences not only the formation of the low-frictional interface but also the formation continuity. On the CN_x:H friction surface, hydrogen, hydroxyl, and oxygen groups from environmental water and oxygen molecules continued to chemisorb owing to tribochemical reactions on the uppermost few nanometers during continuous low friction in a nitrogen atmosphere, while hydrogen content of CN_x:H desorbed. This study experimentally confirmed the critical role of controlling relative humidity in tribological systems using CN_x:H coatings to achieve low friction and improve its durability of low friction through the continuous formation of the low-frictional interface.