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

Hydrogenated Vegetable Oil in Injection Systems: A Tribological Study

The aim of this paper is the assessment of the possible impacts of eco-friendly fuels on injection systems by conducting …

The aim of this paper is the assessment of the possible impacts of eco-friendly fuels on injection systems by conducting tribological model tests. In this regard, lubricity (High-Frequency Reciprocating Rig, HFRR), scuffing load at different temperatures, and oxidation stability of different fuels B7, R33, pure HVO, and commercial-grade HVO diesel fuel have been deeply investigated.
As a result of our study, the HFRR wear scar diameter (WSD) shows no distinct temperature dependence for both fossil-based diesel fuels (B7 and R33). In contrast, vegetable-based ones (pure HVO and commercially available HVO-based fuel) reveal lower lubricity with a trend to higher HFRR value when the temperature is increased. The commercial HVO fuel shows, compared to the pure HVO, better HFRR values at all tested temperatures. Nevertheless, all HFRR values still stay within the limits set by the relevant fuel standards EN 590 and ASTM D975.
For all fuels, the scuffing load clearly depends on the temperature. B7 shows the highest and pure HVO the lowest scuffing load for all tested temperatures. At higher temperatures, commercially available HVO shows a similar, or even better, behavior compared to R33. The results indicate that there is no or only weak correlation between the HFRR and the scuffing load. This correlation obviously varies with fuel grades, additives, and other added substances.
HVO shows excellent oxidation stability due to its pure paraffinic character. Fossil fuels are less stable because aromatic hydrocarbons are much easier to crack than paraffins.
Combustion engines will continue to play an important role until the electrification of transportation is fully established. Our results show that alternative fuels like R33 and HVO represent good alternatives for fossil fuels in diesel engines.

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Paper

Tribofilm Formation, Friction and Wear-Reducing Properties of Some Phosphorus-Containing Antiwear Additives

The film-forming, friction and wear properties of a range of model and commercial ashless P and P/S antiwear additives have …

The film-forming, friction and wear properties of a range of model and commercial ashless P and P/S antiwear additives have been studied. A method has been developed for removing the tribofilms formed by such additives in order to effectively quantify mild wear. In general the P/S additives studied formed thinner tribofilms but gave lower wear than the S-free P ones. In extended wear tests, three P/S additives gave wear as low, or lower, than a primary zinc dialkyldithiophosphate (ZDDP). For almost all lubricants tested the wear rate measured in short tests was considerably higher than that in long tests due to the greater contribution of running-in wear in the former. This highlights the importance of basing antiwear additive choice on reasonably long tests, where running-in becomes only a small component of the wear measured. It has been found that for both P and P/S ashless additives the addition of oil-soluble metal compounds based on Ti and Ca boosts tribofilm formation and can lead to very thick films, comparable to those formed by ZDDP. However, this thick film formation tends to be accompanied by an increase in mixed friction and also does not appear to reduce wear but may even increase it.

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Paper

The Low Adhesion Problem: The Effect of Environmental Conditions on Adhesion in Rolling-Sliding Contact

Low adhesion problem is one of the major problems for railways all over the world because this phenomenon can occur …

Low adhesion problem is one of the major problems for railways all over the world because this phenomenon can occur anytime and anywhere. To investigate when poor adhesion conditions can be expected in real operation, a ball-on-disc tribometer with a climate chamber was employed to simulate rolling-sliding contact under various environmental conditions. Clean and contaminated discs with leaf extract were used to simulate different surface conditions. Results indicate that contact operating under rolling-sliding conditions is more prone to the occurrence of low adhesion than found by others for pure sliding contact. Very low adhesion (≤0.05) were identified for contaminated and oxidized specimens operating under humid and wet conditions. For clean surfaces, low adhesion (≤0.15) were found under dew conditions.

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Paper

Influence of Wear Surface Morphology and Phosphorus-Containing Tribofilm on Crack Initiation of Manganese Phosphate Coated Steel under Rolling–Sliding Contact

To improve the rolling–sliding contact fatigue strength of a case-carburized steel, the effect of the wear surface morphologies of manganese …

To improve the rolling–sliding contact fatigue strength of a case-carburized steel, the effect of the wear surface morphologies of manganese phosphate (MnP) coated steel and the growth and removal of phosphorus-containing tribofilm on surface-initiated crack formation was investigated. In order to modify the wear surface morphologies, two types of surface textures (ground and shot blasted) were prepared, followed by the MnP coating process. The tribological properties of the coated steel, tribofilm growth and removal, and surface-initiated crack formation were evaluated using a ball-on-disk tribometer with a rolling–sliding mode. The MnP coating on both the ground and shot blasted steel had nearly the same thickness and surface roughness. However, for the ground surface sample, the interface morphology between the coating and steel substrate was more irregular than the shot blasted surface sample, resulting in a larger number of exposed steel areas with smaller sizes after the MnP was almost worn away on tribological tests. During the running-in period, phosphorus-containing tribofilm growth and removal on the smaller exposed steel areas were observed. The surface-initiated crack formation on the smaller exposed steel areas was suppressed compared with larger exposed steel areas.

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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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