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

Temperature Dependence of Molybdenum Dialkyl Dithiocarbamate (MoDTC) Tribofilms via Time-resolved Raman Spectroscopy

Molybdenum dialkyl dithiocarbamate (MoDTC) is a friction reducing additive commonly used in lubricants. MoDTC works by forming a low-friction molybdenum …

Molybdenum dialkyl dithiocarbamate (MoDTC) is a friction reducing additive commonly used in lubricants. MoDTC works by forming a low-friction molybdenum disulphide (MoS2) film (tribofilm) on rubbed surfaces. MoDTC-induced MoS2 tribofilms have been studied extensively ex-situ; however, there is no consensus on the chemical mechanism of its formation process. By combining Raman spectroscopy with a tribometer, effects of temperature and shear stress on MoS2 tribofilm formation in steel-steel contacts were examined. Time-resolved Raman spectra of the tribofilm were acquired, together with the instantaneous friction coefficient. The tribofilm is constantly being formed and removed mechanically during rubbing. Increasing shear stress promotes MoS2 formation. The nature of the tribofilm is temperature-dependent, with high-temperature tribofilms giving a higher friction than lower temperature films. Below a critical temperature Tc, a small amount of MoS2 gives significant friction reduction. Above Tc, a patchy film with more MoS2, together with a substantial amount of amorphous carbon attributed to base oil degradation, forms. The composition of this tribofilm evolves during rubbing and a temporal correlation is found between carbon signal intensity and friction. Our results highlight the mechanochemical nature of tribofilm formation process and the role of oil degradation in the effectiveness of friction modifier MoDTC.

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Paper

Formulation and Tribological Behavior of Ultra-low Sulfur Diesel Fuels Microemulsified With Glycerin

The lubricity of diesel is a fundamental parameter, as it has a direct impact on the engine operation, working life …

The lubricity of diesel is a fundamental parameter, as it has a direct impact on the engine operation, working life and wear of the parts. Ultra-low sulfur diesel fuels (ULSD) have low lubricity due to the removal of polar compounds resulted from the desulfurization process. This study evaluated the impact of glycerin addition on the lubrication properties of ULSD microemulsion fuel. Lubricity tests were developed in High Frequency Reciprocating Rig (HFRR). The results showed that, when compared to the ULSD reference fuel with 10 ppm sulfur, the addition of glycerin at the highest concentration was capable to double the percentage of lubricant film formation, reduce by 48% the wear scar formed in the steel ball and promote the reduction in the values of roughness parameters by approximately 50%.

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Paper

Friction of Lubricated Hydrogels: Influence of Load, Speed and Lubricant Viscosity

Agarose hydrogels are used to study the tribological response of hydrogel materials. We demonstrate that the dynamic frictional behaviour of …

Agarose hydrogels are used to study the tribological response of hydrogel materials. We demonstrate that the dynamic frictional behaviour of agarose hydrogels is influenced by their mechanical properties, lubricant viscosity and applied load. Within the linear viscoelastic regime of the hydrogels, we find the Stribeck-type framework proposed previously for elastomers and other viscoelastic materials describes their tribological behaviour and dependency on lubricant viscosity. However, this breaks down when sufficient stress in the contact is applied so that the hydrogel undergoes yielding (i.e. elastic-plastic deformation). As a result, we find that the friction- normal load relationship differs from the classic trend (F ~ W2/3) observed in elastomers above finite loads. Conventional interpretations of the friction associated with hydrogels should be revised with consideration for the non-linear deformation of the hydrogels. We suggest that theories on viscoelastic lubrication are generally applicable to hydrogels within their linear viscoelastic regime, and propose a conceptual model that considers the effects of bulk mechanical properties and measurement conditions including lubricant viscosity, measuring geometry, speed and load.

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Paper

Probing Gels and Emulsions Using Large-amplitude Oscillatory Shear and Frictional Studies With Soft Substrate Skin Surrogates

Water swellable crosslinked polymers are widely used in oil-in-water emulsions for the healthcare and cosmetic industries due to their thickening …

Water swellable crosslinked polymers are widely used in oil-in-water emulsions for the healthcare and cosmetic industries due to their thickening properties. In this study, we investigate the rheological and lubrication behavior of a microgel-forming polymer, a lightly-crosslinked hydrophobically modified polyacrylic acid (HMPAA), in an aqueous medium and in an emulsion. Hydrogenated phosphatidylcholine, a class of phospholipids, is used as a surfactant in the emulsions composed of different oil content. Rheological behavior is probed both in the linear and non-linear regimes using small strain amplitude and large amplitude oscillatory shear (LAOS) experiments, respectively. We observe all systems to exhibit gel-like behavior with the elastic modulus (G’) dominating and being frequency independent. Lissajous-Bowditch plots and nonlinear parameters obtained under large deformation show that the emulsions can resist greater deformations with smaller increase in the viscous dissipation when compared to a HMPAA gel. For tribology experiments, friction curves in a range of entrainment speeds are examined using substrates to mimic the skin surface (PDMS and Bioskin®). The role of polymer hydrophobicity on the different substrates are also explored by comparing the behavior of HMPAA to that of its hydrophilic analog, a polyacrylic acid highly crosslinked. We find the friction coefficient to be dependent on the hydrophobicity of the substrate and the polymer as well as the substrate roughness. These results taken together provide insights in the formulation of skincare products with efficient lubrication properties for different skin characteristics.

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Paper

Efficacy of Lead Naphthenate for Wear Protection in High Vacuum Space Mechanisms

The purpose of this research is to investigate the efficacy of lead naphthenate as a wear additive in a multialkylated …

The purpose of this research is to investigate the efficacy of lead naphthenate as a wear additive in a multialkylated cyclopentane (MAC) fluid for use in high vacuum space mechanism applications. The use of lead naphthenate in MAC lubricants has a spaceflight history of over thirty years. However, despite the history of use for this additive in a variety of rolling and sliding applications, little is known or understood about the tribochemical process by which these additives function...
The results of this work will help the design engineer understand how materials, including lubricants, play a critical role in the performance and life of space mechanisms in demanding high vacuum environments. A greater understanding of the relationship between lead content and tribological performance will be developed along with further understanding of the tribochemical degradation process. Data gathered from wear testing and application simulation work will provide mechanism design engineers with a better understanding of the tribological performance of this lubricant additive.

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Paper

Experimental Investigation Into the Effects of Diesel Dilution on Engine Lubrication

The dilution of lubricant due to contamination with diesel fuel is an increasingly prevalent, potentially important and poorly understood issue. …

The dilution of lubricant due to contamination with diesel fuel is an increasingly prevalent, potentially important and poorly understood issue. This study addresses two fundamental questions: 1) How does the change in lubricant rheology due to diesel dilution affect engine lubrication? 2) How is the chemical performance of lubricant components (base oil and performance additives) impacted by diesel dilution under different lubrication regimes (boundary/full film, hydrodynamic/elastohydrodynamic). This is achieved by testing three lubricant samples: 1) neat fully formulated 0W-30 engine oil, 2) fully formulated 0W-30 oil diluted with diesel at a concentration of 15%, denoted “0W-30D”, and 3) neat, fully-formulated 0W-16, with the same base oil components and performance additives as the 0W-30, but blended to give a viscosity equal to that of the diluted an equivalent “0W-30D”. Tribometer tests, including 1) low pressure, low shear viscosity, 2) Ultra-high Shear Viscosity (USV), 3) elastohydrodynamic film thickness, 4) Stribeck friction and 5) boundary friction and wear, are then conducted. To further emulate engine lubrication conditions, Stribeck curve measurements are performed on the three lubricants using a journal bearing test rig, fitted with a connecting-rod and commercial diesel engine shells. Results suggest that diesel dilution only slightly affects chemical additive performance (with friction modifiers being more inhibited than anti-wear additives) but does reduce both viscosity and film thickness. However, care must be taken in using viscometrics to predict dilution behaviour because 1) the pressure viscosity coefficient is also affected by diesel dilution which has implications for elastohydrodynamically lubrication contacts, 2) shear thinning means that viscosity modifier additives effects lose their functions at high shear rates; whereas diesel contamination affects viscosity behaviour throughout the whole shear rate range.

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Paper

Boundary Friction of ZDDP Tribofilms

The lubricant additive zinc dialkyldithiophosphate (ZDDP) has been used as an antiwear agent in engine oils for more than six …

The lubricant additive zinc dialkyldithiophosphate (ZDDP) has been used as an antiwear agent in engine oils for more than six decades. It limits wear by forming a protective, phosphate-based tribofilm on rubbing surfaces. In recent years its role as an antiwear additive in engine oils has become increasingly important because of the use of very low viscosity oils to improve fuel economy. Such low viscosities mean that engine components operate for longer periods in thin film boundary and mixed lubrication conditions where wear may occur. Unfortunately, as well as controlling wear, ZDDP is also believed to increase friction in both boundary and mixed lubrication conditions and consequently can be detrimental to fuel economy. It is therefore important to determine ways to reduce the friction of ZDDP tribofilms. This paper explores the boundary friction of ZDDP tribofilms and the influence of ZDDP molecular structure on this friction. The aim is to identify ZDDPs that are less harmful to fuel economy than those currently in use.

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Paper

Friction and Temperature Mapping of Environmentally Acceptable Gear Oils

In recent years, environmental issues have raised the demand to protect the environment against the pollution caused by the uncontrolled …

In recent years, environmental issues have raised the demand to protect the environment against the pollution caused by the uncontrolled spillage of lubricating oils. One solution is using Environmentally Acceptable Lubricants (EALs), however, these oils are more expensive than the common mineral oils. The consumers require to test the oil performance using test machines but testing in real machines is costly and time-consuming. Small test machines like ball-on-disc have been previously used for friction mapping and ranking gear oils. In this paper, the friction maps are measured from 0.65 GPa to 1.25 GPa, and temperature maps are devised to experimentally simulate the gear contact along the line of action. Results illustrate that EALs can provide up to 60 % better frictional efficiency that leads to 20 oC cooler oil temperature in high-pressure contacts operating under elastohydrodynamic lubrication (EHL) regime.

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Paper

Effects of Dispersant and ZDDP Additives on Fretting Wear

This paper examines the effect of dispersant and anti-wear additives on fretting wear in lubricated bearing steel contacts. Reciprocating sliding …

This paper examines the effect of dispersant and anti-wear additives on fretting wear in lubricated bearing steel contacts. Reciprocating sliding ball-on-flat fretting tests with a stroke length of 50 μm have been carried out on steel-to-steel contacts in both dry and lubricated conditions. Wear and friction coefficient have been measured, and surface characterisation has been carried out using optical techniques to investigate fretting wear. The presence of base oil reduces fretting wear markedly compared to dry conditions, but fretting damage is still observed at low reciprocation frequencies. As frequency is increased, there is a transition from oxidative to adhesive/scuffing damage. The anti-wear additive ZDDP is effective in forming a tribofilm on the surfaces and reducing visible oxidation and wear. A succinimide dispersant also reduces the accumulation of solid debris but does not alleviate wear damage. The combination of both ZDDP anti-wear additive and dispersant in base oil appears to provide significant protection against fretting wear.

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