Knowledge

Welcome to our knowledge centre. Here you can find a selection of resources and articles on our products and industries we are involved with.

Paper

Friction and Temperature Behavior of Lubricated Thermoplastic Polymer Contacts

This work focuses on the friction and temperature behavior of thermoelastohydrodynamically lubricated (TEHL) contacts under rolling-sliding conditions. For this purpose, …

This work focuses on the friction and temperature behavior of thermoelastohydrodynamically lubricated (TEHL) contacts under rolling-sliding conditions. For this purpose, a twin-disk test rig is used with a hybrid setup of plain and fiber-reinforced polyamide (PA) 66 and polyetheretherketone (PEEK) disks paired with case-hardened steel disks and three different lubricants. Experimental investigations include various lubrication regimes by varying sum velocity and oil temperature as well as load and slip ratio. The measured friction in thermoplastic TEHL contacts is particularly very low in the area of high fluid load portion, which refers to the large deformation of the compliant polymer surface. Newtonian flow behavior mainly determines fluid friction. The low thermal effusivity of polymers insulates the contact and can further reduce the effective lubricant viscosity, and thus the fluid friction. For low sum velocities, solid friction influences the tribological behavior depending on the solid load portion. Although the interfacial contact friction is comparably small, material damping strongly contributes to power losses and increases bulk temperature, which in turn affects the TEHL contact. Thus, loading frequency and the resulting bulk temperature are identified as one of the main drivers of power losses and tribological behavior of lubricated thermoplastic polymer contacts.

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Paper

Influence of pH on Fluid Gels Produced from Egg and Whey Protein Isolate

Food producers are coming under increasing pressure to reduce fat content of foods. Fat forms a major structuring component in …

Food producers are coming under increasing pressure to reduce fat content of foods. Fat forms a major structuring component in many foods responsible for the desirable texture of foods which are rich in fats. Consumers want healthier foods whilst maintaining desirable sensory properties of these foods and using ‘natural’ ingredients. In this work we present suspensions of soft gelled protein particles produced by heating induced gelation in shear of proteins. We present egg white fluid gels and compare them with previously characterized WPI fluid gels. Understanding the effects of pH on proteins is important owing to the net charge influencing gelation and gel properties. Soft tribology and rheology were used to investigate textural properties of fluid gels produced and relate these to potential mouthfeel of these systems. Fluid Gels at the IEP were shown to produce aggregated particles of less than 1 μm diameter. These systems produced at the IEP demonstrated greater friction values in the mixed and boundary regimes of lubrication.

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Paper

Effect of organic friction modifiers on lubrication of PEEK-steel contact

The rapid adoption of the advantageous PEEK/steel pairing in many tribological applications has prompted intense research to optimize its lubrication. …

The rapid adoption of the advantageous PEEK/steel pairing in many tribological applications has prompted intense research to optimize its lubrication. Thus, the role of organic friction modifiers (OFMs) in improving the lubrication of PEEK-steel contacts has been studied and their mechanism explained. Their effect on friction and wear depends on the type of contact motion (i.e. sliding or sliding-rolling) and the steel surface roughness. N-oleoyl sarcosine had a significant effect on tribological properties due to its ability to absorb strongly on both materials, inhibit the formation of PEEK transfer films on steel and thus exert either a positive or negative effect depending on the test conditions.

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Paper

The Composition and Friction-reducing Properties of Leaf Layers

Every autumn rail networks across the world suffer delays, accidents and schedule changes due to low friction problems caused by …

Every autumn rail networks across the world suffer delays, accidents and schedule changes due to low friction problems caused by leaves landing on the rails. These leaves form a layer that can reduce the friction between the wheel and the rail to a similar level as that between ice and an ice-skate (µ = 0.01 − 0.05). Previous works have generated several hypotheses for the chemical reactions and low friction mechanism associated with these layers.
In this work, the reaction between an aqueous extract of sycamore leaves and metallic iron is investigated. This reaction has been shown to produce a black precipitate, which matches field observations of leaf layers, while friction tests with these extracts produce characteristic ultra low friction. The reaction is investigated through FTIR, XPS, CHNS and ICP-MS analysis as well as wet chemical testing. The impact of the reaction on friction is investigated through three rounds of tribological testing.
The results indicate that the black precipitate produced is iron tannate, formed by complexation of tannins with dissolved iron ions. Friction testing showed that eliminating tannins from the leaf extract resulted in a significant increase in friction coefficient compared to the control.

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Paper

In Situ Raman-SLIM Monitoring for the Formation Processes of MoDTC and ZDDP Tribofilms at Steel/Steel Contacts under Boundary Lubrication

In this study, the formation processes of molybdenum dithiocarbamate (MoDTC) and zinc dialkyldithiophosphate (ZDDP) tribofilms were investigated using an in …

In this study, the formation processes of molybdenum dithiocarbamate (MoDTC) and zinc dialkyldithiophosphate (ZDDP) tribofilms were investigated using an in situ Raman-space layer imaging method (SLIM) tribometer under boundary lubrication. According to the results, the ratio of the POx and the P-O-P bonds in the tribofilms was almost the same as the film thickness behavior calculated from the SLIM images. Hence, the time-dependence chemical composition change of the tribofilms of phosphate related bonds in the tribofilms is considered a factor in the growth of ZDDP tribofilms. Moreover, the presence of MoDTC in the ZDDP solution contributed to the formation of phosphate compounds with a relatively high ratio of P-O-P links, such as ultra-phosphate glasses. This mechanism is attributable to the friction-reduction effects of MoS2 on the tribofilm, which can contribute to maintaining a suitable condition for the formation of relatively high P-O-P-link-ratio tribofilms.

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Paper

Study of Surface Roughness on Friction in Rolling/Sliding Contacts: Ball‑on‑Disc Versus Twin‑Disc

Although the efficiency of a gear pair is currently high, a better understanding of surface/lubricant contribution on efficiency is critical. …

Although the efficiency of a gear pair is currently high, a better understanding of surface/lubricant contribution on efficiency is critical. Electrified drivelines will, for example, impose higher speed and alternate loading, and it is expected that these new conditions will, to a greater extent, rely on the surface/lubricant characteristics. Phenomena taking place in the gear contact is often measured using ball-on-disc and twin-disc tribometers. In this study, these two test set-ups were compared in order to assess differences in the behaviour of surface/lubricant interactions. Results showed that ball-on-disc and twin disc set-ups refect the same friction trends. However, the friction results differed by a factor of roughly two, even though the tribometers were set-up to run at the same contact pressure. The wear mechanisms also differed: micropits occurred on discs used in the twin-disc set-up, whereas normal or no wear was found on the ball-on-disc specimens. The contact conditions for the two test set-ups were also analysed using a numerical model. The comparison of these two machines may aid gear designers in selecting the proper experimental set-up for their purpose.

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Paper

In Situ Microscopic Study of Tribology and Growth of ZDDP Antiwear Tribofilms on an Al–Si Alloy

Zincdialkyldithiophosphates (ZDDPs) form protective antiwear tribofilms but the growth mechanism and their contribution to friction on non-ferrous alloys are not …

Zincdialkyldithiophosphates (ZDDPs) form protective antiwear tribofilms but the growth mechanism and their contribution to friction on non-ferrous alloys are not well understood. Using a recently developed in situ AFM technique, we monitor the growth of ZDDP based tribofilms, friction, and wear, over Al matrix and Si phase of ADC12, sliding against a nonferrous alumina probe. Sliding tests performed at elevated temperature (110 °C), simultaneously over Al and Si phase, reveal tribofilm growth on both Al and Si regions without noticeable wear. However, the tribofilm thickness, density, and coefficient of friction on the Si phase are higher than on the Al matrix. Significant wear and increase in the coefficient of friction was observed when sliding within the Al matrix region alone.

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Paper

Gels and Oil-in-Water Emulsions: a Stepwise, Rheology- and Tribology-Focused Approach

Gels and oil-in-water emulsions are widely used in food, pharmaceutical, and personal care applications. In the case of emulsions, they …

Gels and oil-in-water emulsions are widely used in food, pharmaceutical, and personal care applications. In the case of emulsions, they can be either stabilized by an amphiphilic molecule, forming classical emulsions, or by colloidal particles, forming a Pickering emulsion. These systems exhibit rich rheological and frictional characteristics and factors such as component concentration and/or interactions can affect their final properties. Thus, their characterization is fundamental to understanding their performance from product development to final use. This dissertation provides insights on how to manipulate properties of multicomponent gels and emulsions based on their components, guiding the formulation of products with desired rheological and lubrication properties. For that, we focus on two groups of systems: 1) classical oil-in-water emulsions containing microgel-forming polymers and phospholipids as well as several simplified versions of these systems, and 2) Pickering emulsions stabilized by nanodiamond particles.

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