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

Film-Forming Properties of Zinc-Based and Ashless Antiwear Additives

A progressive reduction in the permitted level of phosphorus in lubricating oils, coupled with concern to maintain engine and transmission …

A progressive reduction in the permitted level of phosphorus in lubricating oils, coupled with concern to maintain engine and transmission durability, means that it is becoming increasingly important to understand the detailed mechanism of antiwear additive behavior.

This paper describes a new experimental technique, which is able to measure both the thickness and distribution of antiwear additive films in rolling/sliding contacts. This enables the kinetics of antiwear film build-up to be investigated and the influence of the reaction film on friction and wear to be monitored. In the current paper, this technique is used to compare the film-forming behavior of ash-containing and ashless antiwear additives.


Keywords: Lubricating oils, wear, transmissions, logistics, durability

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Paper

Friction and Wear Behaviour of Zinc Dialkyldithiophosphate Additive

A method has been developed for monitoring the film-forming properties of antiwear additives in rolling-sliding, lubricated contacts. This makes it …

A method has been developed for monitoring the film-forming properties of antiwear additives in rolling-sliding, lubricated contacts. This makes it possible to study both the kinetics of reaction film growth and also the evolution of the film morphology as a function of rubbing time. The technique has been applied to investigate the behavior of a zinc dialkyl-dithiophosphate (ZDDP) additive solution and to correlate this with simultaneous friction and wear measurements.

The results show that ZDDP forms a thick, solid-like, reaction film in the rubbing tracks, with negligible film growth outside of the track. This film is extremely effective in preventing metal-metal contact. However the film is unevenly-distributed, with its roughness oriented in the direction of sliding. This directional roughness inhibits the entrainment of fluid film in the mixed lubrication regime, increases the proportion of load supported by solid-solid contact and consequently results in the high friction often associated with the use of ZDDP additives.


Keywords: Antiwear Additives, Zinc Dialkyldithiophosphate, Friction, Wear, Optical Interferometry, Spacer Layer

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Paper

In Situ Observation of Phosperous and Non-phospherous Antiwear Films using a Mini Traction Machine with Spacer Layer Image Mapping

Since 1940s the principal source of an antiwear additive in crankcase applications has been due to a family of additives …

Since 1940s the principal source of an antiwear additive in crankcase applications has been due to a family of additives known as zincdialkyithiophosphate (ZDDP). In this study, we have applied a novel technique, the mini traction with spacer layer image mapping (MTM SLIM) to study film formation characteristics of monoblend oils containing only basestock and ZDDP and also fully formulated oils containing a boron antiwear additive. The purpose of this study was initially to understand the build up of ZDDP film and then to establish whether boron compounds alone could you provide significant antiwear films under mixed rolling and sliding contact.


Keywords: Zinc dialkydithiophosphate, Mini traction machine with spacer layer image mapping

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Paper

The Elastohydrodynamic Friction and Film Forming Properties of Lubricant Base Oils

The stringent and often competing requirements of high fuel economy and low emissions are placing increasing emphasis on the selection …

The stringent and often competing requirements of high fuel economy and low emissions are placing increasing emphasis on the selection of appropriate base oils for modem engine lubricants. Two properties now recognized as important in engine oil design are the elastohydrodynamic (EHD) traction coefficient and the pressure-viscosity coefficient. The former determines the level of friction in high pressure contacts such as cams, while the latter plays a major role in determining the EHD film thickness. Unfortunately, for many fluids there is a broad correlation between the two properties so that a low traction coefficient implies a low pressure viscosity coefficient and thus film thickness. This paper measures the traction and film-forming properties of a wide range of base oil types at realistic engine oil temperatures in order to both explore the extent of this correlation and to determine the dependence of EHD lubricant properties on base oil compositions.


Keywords: Lubricants, Elastohydrodynamic Lubrication, Friction

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Paper

Cosmetic Powder Suspensions in Compliant, Fingerprintlike Contacts

Cosmetic powders are regularly employed in skin creams and cosmetic formulations to improve performance and enhance skin feel. A previous study investigated …

Cosmetic powders are regularly employed in skin creams and cosmetic formulations to improve performance and enhance skin feel. A previous study investigated the effect of particle concentration and size on the lubricating properties of powder suspensions in smooth, compliant contacts [Timm ., Tribol. Int. (2011)]. In this paper the tribological properties of cosmetic powder suspensions are investigated in compliant contacts having model fingerprintlike surface topography. Friction coefficients were measured for a series of powder suspensions with varying particle size and concentration in apolydimethylsiloxane (PDMS)/PDMS contact. A commercial tribometer (MTM, PCS Instruments) was employed to measure friction as a function of rubbing time (20 min), under pure sliding (50 mm/s) and low load (0.5 N) conditions. Compared to results using smooth surfaces, it was clear that surface topographyhas a pronounced affect on the timedependent tribological behavior of the cosmetic powder suspensionsstudied. A two-stage friction coefficient versus time curve was observed. By varying the particle size and concentration it was shown that the duration and magnitude of each stage can be controlled.


Keywords: Cosmetics, Friction, Humans, Lubricants, Particle Size, Powders, Surface Properties,  Suspensions

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Paper

Rolling and Sliding Friction in Compliant, Lubricated Contacts

Friction is investigated in a rolling–sliding, lubricated, steel ball on elastomer flat contact. Two different types of friction are identified: rolling …

Friction is investigated in a rolling–sliding, lubricated, steel ball on elastomer flat contact. Two different types of friction are identified: rolling friction, which results from the movement of the surfaces relative to the contact, and sliding or interfacial friction, which arises from relative motion of the two contacting surfaces. A novel experimental technique is
described to measure these two types of friction simultaneously in a single test. This enables separate rolling and interfacial ‘Stribeck-type’ friction curves to be produced for Newtonian lubricants. These curves are compared with theoretical predictions of friction. The results show that rolling friction originates primarily from two sources: Poiseuille flow of lubricant in the contact and elastic hysteresis. There are also two main types of interfacial friction; due to Couette flow and solid surface adhesion. For compliant elastomer-on-steel contacts, rolling friction forms a significant proportion of the total friction even at quite high slide–roll ratios.


Keywords: Soft-elastohydrodynamic, Isoviscous-elastic, Friction, Rolling friction, Sliding friction, Interfacial friction, Couette friction, Poiseuille friction, Hysteresis

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Paper

The Frictional Properties of Newtonian Fluids in Rolling–sliding Soft-EHL Contact

A combined experimental and numerical study has been carried out to explore friction in rolling–sliding, soft-EHL contact. Experimental work has …

A combined experimental and numerical study has been carried out to explore friction in rolling–sliding, soft-EHL contact. Experimental work has employed corn syrup solutions of different concentrations in water to provide a range of lubricant viscosities and has measured Couette friction in mixed rolling–sliding conditions over a wide range of entrainment speeds. A Stribeck curve has been generated, ranging from the boundary to full film, isoviscous-elastic lubrication regime. In the latter regime, friction coefficient is approximately proportional to the product of (entrainment speed × viscosity) raised to the power 0.55. Numerical solution of the isoviscous-elastic lubrication regime has been used to derive predictive equations for both Couette and Poiseuille friction in circular, soft-EHL contacts. This shows that in soft-EHL the Poiseuille or “rolling” friction can have magnitude comparable to the Couette friction. The calculated Poiseuille friction coefficient can be predicted from non-dimensional load and speed using a simple power law expression similar to that used for film thickness. However accurate prediction of calculated Couette friction coefficient requires a two-term power law expression. Comparison of experimental and numerical Couette friction coefficients shows quite good agreement between the two, with a similar non-dimensional speed dependence, but slightly lower predicted than measured values.


Keywords: Soft-EHL, Isoviscous-elastic, Friction, Couette friction

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Paper

Particulate Lubricants in Cosmetic Applications

Polymer powders are commonly added to cosmetic formulations to improve product performance and skin feel. This study investigates the effect …

Polymer powders are commonly added to cosmetic formulations to improve product performance and skin feel. This study investigates the effect of particle concentration and size on the lubricating properties of powder suspensions. Results are reported for various particle sizes and concentrations.

When the tribological contact was fully immersed the addition of particles had no effect. However different behaviour was observed when the contact was only partially lubricated. In this case, a three-stage friction coefficient curve was observed. By varying the particle size and concentration it was shown that the duration and magnitude of each stage can be controlled.


Keywords: Sliding friction, Elastomer, Particulate, Real contact area

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Paper

Lubrication Properties of Non-adsorbing Polymer Solutions in Soft Elastohydrodynamic (EHD) Contacts

The in-use performance and processing of many consumer products in the food, home and personal care industries are dependent on …

The in-use performance and processing of many consumer products in the food, home and personal care industries are dependent on their tribological properties. A major component of these products is often a high molecular weight polymer, which is typically used to thicken aqueous systems. Polymer solutions tend to be non-Newtonian, and in particular their viscosity varies with shear rate, such that it is difficult to predict their friction or hydrodynamic film-forming behaviour. The present work relates the tribology of aqueous polymer solutions to their rheological properties in thin films in ‘soft’ contacts at high shear rates. The friction properties of three types of polymers in aqueous solution, polyethylene oxide, PEO; xanthan gum, XG; and guar gum, GG, have been studied as a function of polymer concentration over a wide range of entrainment speeds in a point contact formed between silicone rubber and steel. This has enabled the boundary lubrication and isoviscous-elastic lubrication properties of the solutions to be investigated using both hydrophilic and hydrophobic silicone surfaces.

It is found that the friction vs. entrainment speed dependence follows the shape of a classical Stribeck curve. In general, a lower friction is observed with increasing polymer concentration in the mixed-regime. Using scaling factors for the entrainment speed, we have shown that this decrease in friction is likely to be due to viscous effects and that the scaling factors represent effective high shear rate viscosities. In the case of PEO and XG, and GG at low concentrations, a good correlation is found between this effective viscosity and the apparent viscosity measured at the highest shear rates attainable with the available rheometer. However, for GG at concentrations above 0.2%, the effective viscosity decreases with increasing polymer content.

The three polymers do not significantly reduce friction in the boundary regime and in general give essentially the same response as water when an effective viscosity is taken into account. However, a slight increase in friction in comparison to pure water has been observed for XG and GG on hydrophobic surfaces. It is suspected that this may be due to a blocking of fluid entrainment, or possibly exclusion of polymer from the contact, due to the large hydrodynamic volume and rigid nature of the two biopolymers. Finally, for PEO solutions with full-film elastohydrodynamic conditions were reached, the measured friction coefficient of the film correlated quite well with the value calculated from the effective viscosity.


Keywords: Tribology, Lubrication, Soft contacts, Polymer solutions, Isoviscous-elastic, Soft EHD

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