Thin Friction The rubbing motion between two surfaces is always hindered by friction, which is caused by continuous contacting and attraction between the surfaces. These interactions may only occur over a distance of a few nanometers, but what happens when the interacting materials are only that thick? Lee et al. (p. 76; see the Perspective by Müser and Shakhvorostov) explored the frictional properties of a silicon tip in contact with four atomically thin quasi–two dimensional materials with different electrical properties. For all the materials, the friction was seen to increase as the thickness of the film decreased, both for flakes supported by substrates and for regions placed above holes that formed freely suspended membranes. Placing graphene on mica, to which it strongly adheres, suppressed this trend. For these thin, weakly adhered films, out-of-plane buckling is likely to dominate the frictional response, which leads to this universal behavior. A universal trend is observed for the friction properties of thin films on weakly adhering substrates. Using friction force microscopy, we compared the nanoscale frictional characteristics of atomically thin sheets of graphene, molybdenum disulfide (MoS2), niobium diselenide, and hexagonal boron nitride exfoliated onto a weakly adherent substrate (silicon oxide) to those of their bulk counterparts. Measurements down to single atomic sheets revealed that friction monotonically increased as the number of layers decreased for all four materials. Suspended graphene membranes showed the same trend, but binding the graphene strongly to a mica surface suppressed the trend. Tip-sample adhesion forces were indistinguishable for all thicknesses and substrate arrangements. Both graphene and MoS2 exhibited atomic lattice stick-slip friction, with the thinnest sheets possessing a sliding-length–dependent increase in static friction. These observations, coupled with finite element modeling, suggest that the trend arises from the thinner sheets’ increased susceptibility to out-of-plane elastic deformation. The generality of the results indicates that this may be a universal characteristic of nanoscale friction for atomically thin materials weakly bound to substrates.

Frictional Characteristics of Atomically-Thin Sheets

Zinc dialkyldithiophosphate additives are used to control wear and inhibit oxidation in almost all engine oils as well as many other types of lubricant. They limit wear primarily by forming a thick, protective, phosphate glass-based tribofilm on rubbing surfaces. This film formation can occur at low temperatures and is relatively indifferent to the chemical nature of the substrate. There has been considerable debate as to what drives ZDDP tribofilm formation, why it occurs only on surfaces that experience sliding and whether film formation is controlled primarily by temperature, pressure, triboemission or some other factor. This paper describes a novel approach to the problem by studying the formation of ZDDP films in full film EHD conditions from two lubricants having very different EHD friction properties. This shows that ZDDP film formation does not require solid–solid rubbing contact but is driven simply by applied shear stress, in accord with a stress-promoted thermal activation model. The shear stress present in a high-pressure contact can reduce the thermal activation energy for ZDDP by at least half, greatly increasing the reaction rate. This mechanism explains the origins of many practically important features of ZDDP films; their topography, their thickness and the conditions under which they form. The insights that this study provides should prove valuable both in optimising ZDDP structure and in modelling ZDDP antiwear behaviour. The findings also highlight the importance of mechanochemistry to the behaviour of lubricant additives in general.

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On the Mechanism of ZDDP Antiwear Film Formation

Yield stress materials are ubiquitous, yet the best way to obtain the value of the yield stress for any given material has been the subject of considerable debate. Here we compare different methods of measuring the yield stress with conventional rheometers that have been used in the literature on a variety of materials. The main conclusion is that, at least for well-behaved (non-thixotropic) materials, the differences between the various methods are significant; on the other hand, the scaling of the measured yield stress with the volume fraction of dispersed phase shows the same dependence independently of the way in which the yield stress is obtained experimentally. The measured yield strain is similarly found to depend on the method employed. The yield stress values obtained for a simple (non-thixotropic) yield stress fluid are only similar for Herschel–Bulkley fits and stress-strain curves obtained from oscillatory measurements. Stress-strain curves with a continuous imposed stress or strain rate differ significantly, as do oscillatory measurements of the crossover between G′ and G″ or the point where G′ starts to differ significantly from its linear response value. The intersection of the G′ and G″ curves as a function of strain consistently give the highest value of the yield stress and yield strain. In addition, many of these criteria necessitate some arbitrary definition of a crossover point. Similar conclusions apply for a class of thixotropic yield stress materials, with the stress-strain curve from the oscillatory data giving the dynamic yield stress and the Herschel–Bulkley fit either the static or dynamic yield stress, depending on how the measurement is carried out.

On different ways of measuring “the” yield stress

"Microplastic" is an umbrella term that covers many particle shapes, sizes, and polymer types, and as such the physical and chemical properties of environmental microplastics will differ from the primary microbeads commonly used for ecotoxicity testing. In the present article, we discuss the physical and chemical properties of microplastics that are potentially relevant to their ecotoxicity, including particle size, particle shape, crystallinity, surface chemistry, and polymer and additive composition. Overall, there is a need for a structured approach to the testing of different properties to identify which are the most relevant drivers of microplastic toxicity. In addition, the properties discussed will be influenced by and change depending on environmental conditions and degradation pathways. Future challenges include new technologies that will enter the plastic production cycle and the impact of these changes on the composition of environmental microplastics. Integr Environ Assess Manag 2017;13:470-475. © 2017 SETAC.

Ecotoxicity testing of microplastics: Considering the heterogeneity of physicochemical properties.

Modern advancements in lubricating grease technology

The aim of this paper was to understand the parameters influencing the grease film thickness in a rolling elastohydrodynamically lubricated contact under fully flooded conditions at medium speeds. Film thickness measurements were taken under pure rolling for six commercial greases and their bled oils. The grease film thickness was found to be higher than corresponding bled oil, suggesting the presence of thickener in the contact. No rheological properties (characterized by steady and dynamic shear) showed any direct relation to the film thickness of the studied greases. AFM measurements of the thickener microstructure, from which the dimensional properties of the thickener particles (fibers/platelets/spheres) were estimated, showed that the relative increase in the film thickness due to entrainment of the thickener was proportional to the ratio of thickener volume fraction to the size of the fibers/platelets/spheres. Hence, with the same concentration, smaller thickener particles lead to the generation of thicker films than larger thickener particles. Next, this relation was used to establish the percentage of the thickener particles passing through the contact. Depending on the grease type, between about 1 and 70 % of the thickener particles were found to travel through the contact.

/pdf/effect-of-thickener-particle-geometry-and-concentration-on-4d8a6e3mrd.pdf

Effect of Thickener Particle Geometry and Concentration on the Grease EHL Film Thickness at Medium Speeds

An experimental study using both a controlled stress and a controlled strain rheometer has been undertaken to characterize lubricating grease in shear, creep, stress relaxation, and oscillatory flow, with a main focus on determining the yield stress. The yield stress was examined using a cone–plate and parallel-plate system with smooth and rough surfaces. Clear discrepancies were observed in the yield stress values obtained using different techniques where oscillatory strain sweep measurements seem to be the best choice. This technique is less sensitive to wall slip, shows good reproducibility, and is relatively easy to perform. The method also shows that the yield stress is a function of the imposed frequency and therefore of the time domain. At lower values of shear—that is, in the linear viscoelastic regime—there is no structural breakdown and the rheology of the grease can be described by the Maxwell model where the stress and the strain are almost proportional to each other. Based on this observation, a novel method to determine the yield stress is proposed: “The yield stress can be determined from the point where this linearity no longer applies.” This method is compared to those that are commonly used. The yield stress was found to depend exponentially on temperature and linearly on frequency.

On a new method to determine the yield stress in lubricating grease

The operational life of bearings is often determined by the performance of the lubricating grease. The consistency of the grease prevents it from leaking out of the bearing and provides good sealing properties. The possible ingress of water into the bearing will have a considerable impact not only on this consistency but also on the lubricating ability of the grease. There are numerous applications where water ingress may occur, such as in the steel, food, pulp, and paper industries. Some greases are less sensitive to water than others. No specific guidelines are available to select the proper grease for bearings subjected to water ingress. The goal of the article is to contribute to the development of such guidelines for greases subjected to water ingress by studying the impact of water on grease rheology. Fully formulated, commercially available greases with the most common thickeners and base oils are used as model greases. It will be shown that water strongly influences rheological properties such as zero-shear viscosity, yield stress, and storage modulus. Calcium sulfonate greases were found to become stiffer after absorbing a considerable amount of water, leading to an increase in zero-shear viscosity and yield stress. However, lithium, lithium complex, and polyurea greases were found to soften, with appreciable changes in measured rheological properties

https://www.tandfonline.com/doi/pdf/10.1080/10402004.2015.1107929

Impact of Water on the Rheology of Lubricating Greases

This paper investigates the influence of water on the EHL film thickness of six commercial lubricating greases under fully flooded and starved conditions. Although grease can absorb large quantities of water, separation occurs due to pressure and shear, leading to free water. This does not have an impact on the film thickness under fully flooded conditions. However, water does have an effect on the film thickness under starved conditions where the differences are related to the change in oil bleed. In the presence of water, an increase in oil bleed was found for lithium, lithium complex and polyurea grease. These greases showed a reduction in the levels of starvation and, therefore, thicker films. Water contamination led to lower oil bleed for calcium sulfonate complex greases, which led to an increase in starvation, and therefore, thinner films compared to their uncontaminated counterparts.

/pdf/impact-of-water-on-ehl-film-thickness-of-lubricating-greases-9fnt5na7au.pdf

Impact of Water on EHL Film Thickness of Lubricating Greases in Rolling Point Contacts

A comprehensive study of the yield stress and start-up torque for six commonly used rolling bearing greases is presented. Both parameters were found to depend exponentially on temperature where the exponent changes below a low-temperature “break point.” This break point was found to be related to the pourpoint of the base oil, although the start-up torques of the greases were an order higher in magnitude than that of their corresponding bled oils. The start-up torque is mostly used to measure the low-temperature limit of a grease. It was found here that this temperature is much lower than the break point. The start-up torque criterion is measured using a particular bearing type and conditions. The low-temperature break point for the yield stress is a more universal grease parameter that gives useful information about the behavior of a grease at low temperatures and can be used as one of the guidelines for grease selection for low-temperature applications

/pdf/yield-stress-and-low-temperature-start-up-torque-of-1jx6xvg8bg.pdf

Febin Cyriac

Papers

Effect of Thickener Particle Geometry and Concentration on the Grease EHL Film Thickness at Medium Speeds

On a new method to determine the yield stress in lubricating grease

Impact of Water on the Rheology of Lubricating Greases

Impact of Water on EHL Film Thickness of Lubricating Greases in Rolling Point Contacts

Yield Stress and Low-Temperature Start-Up Torque of Lubricating Greases