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

Oxidational Wear in Lubricated Contacts – or Is It?

This study examines the influence of inert gas atmosphere on the wear behaviour of rubbing steel-on-steel contacts lubricated by two hydrocarbon base …

This study examines the influence of inert gas atmosphere on the wear behaviour of rubbing steel-on-steel contacts lubricated by two hydrocarbon base fluids, isooctane and hexadecane. It is found that for both fluids, wear and mean friction in nitrogen and argon atmospheres are considerably lower than in dry air. As the oxygen content in nitrogen is increased, mean friction and wear both increase, to level out above about 10% oxygen (an O2 partial pressure of 10 kPa). Raman analysis of rubbed surfaces shows the presence of a carbon film on surfaces rubbed in inert gas and at low O2 levels. This film is not observed at high O2 levels.
These findings indicate that the prevailing model of oxidational wear in lubricated contacts, that states that wear is greater in air than in inert gas because of corrosion by oxygen, is largely incorrect. Instead, the deleterious effect of oxygen on lubricated wear is primarily due to it preventing the formation of a lubricious, carbon-based boundary film that is generated from hydrocarbon base fluids on rubbing steel surfaces in inert gas conditions.
The ability of organic fuels and lubricants to form carbon-based films on rubbing steel surfaces in inert atmospheres may provide a mechanism for reducing friction and wear of fuel- and oil-lubricated machine components. The study also provides a platform from which to design lubricant formulations for use in inert atmospheres.

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Paper

Mechanism of Oil-lubrication of PEEK and Its Composites With Steel Counterparts

The rapid adoption of the advantageous PEEK and its composites paired with steel counterparts in many tribological applications has prompted intense research …

The rapid adoption of the advantageous PEEK and its composites paired with steel counterparts in many tribological applications has prompted intense research to investigate their tribological properties under lubrication. This study investigated the effect of oil-lubrication and proposed a mechanism of action. Compared to the dry conditions, oil-lubrication with poly-α-olefin (PAO) base oils reduced friction, regardless of the type of polymer materials. However, the wear behavior depended on the polymer type; it increased for neat PEEK and decreased for PEEK composites. Additionally, the viscosity of the lubricant oils influenced the polymer behavior in distinct ways. These differences in tribological performance under oil-lubrication were explained by two important factors, the polymer transfer films on steel counterparts determined by Electron Probe Micro Analysis (EPMA) and the hardness modification of the polymer surfaces investigated by nanoindentation measurements. These factors were also related to each other, especially for PEEK composites.

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Paper

Boundary Lubrication Performance of Polymeric and Organic Friction Modifiers in the Presence of an Anti-Wear Additive

The effect of oleic acid, oleyl alcohol and oleyl amine based organic friction modifiers (OFM), ester and ethoxylated fatty ester based di-block polymeric friction modifiers …

The effect of oleic acid, oleyl alcohol and oleyl amine based organic friction modifiers (OFM), ester and ethoxylated fatty ester based di-block polymeric friction modifiers (PFM) and Zinc dialkyldithiophosphate (ZDDP) on the boundary lubrication of steel surfaces was studied using a ball-on-disc tribometer equipped with optical interferometry. Frictional performance of oil blends containing the OFM/PFM, with and without ZDDP, was investigated at a slide roll ratio (SRR) of 100% and 130 °C for a rubbing duration of 24 h. Surface topography measurements using 3D surface profilometer and scanning electron microscopy (SEM) were used to quantify the anti-wear performance. Comparison of optical interferograms and friction data showed that formation of a thicker tribofilm does not always translate to effective friction reduction. Our experiments showed that PFMs generally provide better friction reduction than OFMs with or without ZDDP by effectively eliminating the boundary regime. This was attributed to the ability of PFMs to form a tenacious low shear strength film on the metal surface due to its polymeric nature. In contrast, both the friction modifiers had a negative effect on the anti-wear performance of ZDDP. The observed tribological response is attributed to either preferential adsorption of the friction modifier over ZDDP or ZDDP decomposition products. This notion was further corroborated by results obtained from energy-dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS) with depth profile analysis, which showed lower concentration of ZDDP derived products for the formulation containing both the friction modifier and ZDDP.

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Paper

Functionalized Phosphate Ionic Liquids as Additives in PEG With Excellent Tribological Properties for Boundary/Mixed/Elastohydrodynamic Lubrication

The 2,4,6-triamino-1,3,5-triazin-1-ium dibutyl phosphate (NP44) and 2,4,6-triamino-1,3,5-triazin-1-ium diisopentyl phosphate (NP88) were prepared as additives of PEG. In terms of temperature, …

The 2,4,6-triamino-1,3,5-triazin-1-ium dibutyl phosphate (NP44) and 2,4,6-triamino-1,3,5-triazin-1-ium diisopentyl phosphate (NP88) were prepared as additives of PEG. In terms of temperature, slip-to-roll ratio and concentration, the effect of additives on tribological performance was explored. Excitingly, PEG with NP44 and NP88 presents outstanding lubricating property because of stable protective films formed in boundary/mixed lubrication. Furthermore, it was found that NP44 and NP88 can enhance film thickness in elastohydrodynamic lubrication when PEG was supplied with a limited quantity. It is attributed to the fact that adsorption of additives can reduce the surface energy to delay the oil depletion on the lubrication track.

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Paper

3D Enabled Facile Fabrication of Substrates With Human Tongue Characteristics for Analysing the Tribological Behaviour of Food Emulsions

The mouth-feel or oral perception of a food may vary in different oral surfaces or tongue-palate environment depending on the …

The mouth-feel or oral perception of a food may vary in different oral surfaces or tongue-palate environment depending on the physical characteristics of human tongue. Tribological measurements emulating the characteristics of tongue-palate environment will assist in the evaluation of oral perception of food materials. This study reports a 3D printing assisted approach for rapid fabrication of the soft elastomer substrates with different surface (roughness) and bulk characteristics (elastic modulus and thickness) for tribological measurements. The influence of substrates with various regularly patterned surfaces, elastic modulus, and thickness on the lubrication properties of four food emulsions (full-fat and skim milk, high-fat and low-fat yoghurt) has been investigated. It is observed that friction coefficient of food emulsions increases with a decrease in height or an increase in density of rectangular micro-scale pillars representing papillae on the surface of soft substrate. Interestingly, a decrease in the elastic modulus of the substrates causes an increase in the friction coefficient for the milk samples, whereas for yoghurt samples it causes a decrease. The variations in the thickness of the substrates have little effect on the friction coefficients of the food emulsions.

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Paper

Diblock Polymeric Friction Modifier (PFM) in the Boundary Regime: Tribological Conditions Leading to Low Friction

The aim of this paper is to investigate the tribological conditions required to obtain low friction with a diblock PIB-PEG …

The aim of this paper is to investigate the tribological conditions required to obtain low friction with a diblock PIB-PEG polymer friction modifier (PFM) blended in base oil (PAO4 + 1% wt PFM) under a severe lubrication regime. Two tribological conditions, rolling/sliding and reciprocating pure sliding, were investigated. A very low friction coefficient (μ ~ 0.035) was obtained at a temperature of 100 °C whatever the tribometer used. ECR measurements, ToF-SIMS characterizations of wear tracks and AFM analysis suggested the presence of an adsorbed polymer film on the rubbing surfaces. ToF-SIMS characterizations showed that the polymer bonds to the steel substrate through polar functions.

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Paper

Fluorescent Imaging of Razor Cartridge/skin Lubrication

Shaving comfort is linked to friction, and thus lubrication, in the razor cartridge/skin interface. It is a distinguishing factor when …

Shaving comfort is linked to friction, and thus lubrication, in the razor cartridge/skin interface. It is a distinguishing factor when a user evaluates the perceived quality of a shave. A novel Laser Induced Fluorescence (LIF) technique was developed to study lubrication in a razor cartridge/skin mimic contact under simulated shaving conditions. The experiments were conducted with a 5-blade cartridge, loaded and sliding against a transparent silicone elastomer. The lubricant was distilled water marked with fluorescent dye. The LIF technique measured in-contact film thickness at different sliding speeds (50, 150 mm s−1) and showed differences in lubricant distribution for each component (lubricant strip, blades, guard) interface. The measured calibration film thickness range was 2–135 μm. The guard and lubricant strip maintain relatively thick and consistent lubricant films across the speed range. Minimum films were measured at the blade cutting edges, although at sliding speeds of 150 mm s−1 the blade cutting edge film thickness increases towards the back of the cartridge; and film thickness under blades 1 and 2 decreases at high speeds, suggesting starvation of fluid flow. The new technique allows study of lubrication behaviour in complex geometries, multiphase fluids and hard/soft material combinations for example oral processing, contact lens/eyelid and personal care product application. These systems are not easily studied by conventional test methods or classical lubrication analysis.

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Paper

Tribology of Hard Particles Lubricating Soft Surfaces

Soft materials often have interesting and unexpected frictional behavior owing to their deformable nature. We use soft polydimethylsiloxane (PDMS) surfaces …

Soft materials often have interesting and unexpected frictional behavior owing to their deformable nature. We use soft polydimethylsiloxane (PDMS) surfaces lubricated by hard glass spheres to study how this deformability influences particle-based lubrication. For particles between 100 and 2000 μm in size, we observe a nontrivial rate dependence and three frictional regimes: (I) a rolling friction regime where the rolling particles keep the surfaces apart sufficiently to give low friction coefficients—this is mainly found for large particles and smooth surfaces; (II) a sliding friction regime with high friction coefficients where the surfaces are partially in contact, which is found for small particles, rough surfaces, and high normal forces; (III) a PDMS-PDMS contact regime where the particles are fully inserted into surfaces and the surfaces are in contact. We interpret the friction dynamics in terms of the Hertzian contact deformation effects in the indentation of the PDMS surfaces.

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Paper

Tribological and Sensory Properties

Eating, functionalized by mouth physiology, is performed through a series of processes which collectively helps in food ingestion, preparing the …

Eating, functionalized by mouth physiology, is performed through a series of processes which collectively helps in food ingestion, preparing the food for swallowing, ab-initio digestion, and food sensory perceptions. Sensory properties of food are typically defined by its texture, flavor, and color. Unlike flavor and color, characterizing texture perceptions remain a daunting task because of variegated in-mouth breakdown mechanisms of food depending on several influencing factors. Therefore, it always remains a persisting challenge to correlate instrumental outputs with texture perception. Over the recent decade, principles of tribology—the subject of friction, wear, and lubrication––have been recognized in food sensory research in order to adopt novel instrumental approaches for texture perceptions. This idea of incorporating tribological principles stems from the availability of friction that arises while the tongue manipulates food over the palate during oral processing. Eventually, the terminology such as oral tribology has been introduced, and the subject is rapidly gaining maturity for food sensory applications especially to demonstrate some highly specific sensory descriptions and to define a quantifiable metric for those sensory descriptions. This chapter will revisit the various principles and applications of tribology in pertinence to texture characteristics of food in general and edible hydrocolloids in particular while attempting to identify potential research gaps and future research scopes.

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