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

The Borderline of Elastohydrodynamic and Boundary Lubrication

There is a growing trend for lubricated systems to operate for much of their operating life with very thin lubricating …

There is a growing trend for lubricated systems to operate for much of their operating life with very thin lubricating films. This paper reviews our current understanding of such films, at the borderline between elastohydrodynamic and boundary lubrication. The nature and properties of these films are very complex, since the proximity of the solid surfaces influences the structure and rheology of thin liquid layers, while boundary films can, themselves, possess rheological characteristics that vary with thickness.

Novel experimental tools, such as atomic force microscopy and ultra-thin-film interferometry have greatly accelerated our understanding of this area in the last few years and it has recently become possible to map lubricant film thickness within rough surface contacts. These tools are beginning to provide the level of understanding of thin-film rough surface behaviour required to develop accurate numerical and simulation models. The next 5 years should see a very rapid progression of our understanding of this important regime.

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Paper

The Design of Boundary Film-Forming PMA Viscosity Modifiers

Previous research has shown that some viscosity modifier additives are able to adsorb from oil solution on to metal surfaces …

Previous research has shown that some viscosity modifier additives are able to adsorb from oil solution on to metal surfaces to produce thick, viscous boundary films. These films enhance lubricant film formation in slow-speed and high temperature conditions and thus produce a significant reduction in friction. This article describes a systematic study of this phenomenon, which makes use of the versatile nature of polymethacrylate (PMA) chemistry. Dispersant polymethacrylates with a range of different functionalities, molecular weights, and architectures have been synthesized using controlled radical polymerization techniques. The influence of each of these features on boundary film formation and friction has been explored using optical interferometry and friction versusspeed measurement. From the results, guidelines have been developed for designing PMAs having optimal boundary lubricating and, thus, friction-reducing properties.

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Paper

Friction Reduction and Antiwear Capacity of Engine Oil Blends Containing Zinc Dialkyl Dithiophosphate and Molybdenum-Complex Additives

The efficacy of oil blends containing zinc dialkyl dithiophosphate (ZnDTP) and molybdenum (Mo)-complex additives to improve the tribological properties of …

The efficacy of oil blends containing zinc dialkyl dithiophosphate (ZnDTP) and molybdenum (Mo)-complex additives to improve the tribological properties of boundary-lubricated steel surfaces was investigated experimentally. The performance of oil blends containing three different types of Mo-complex additives of varying Mo and S contents with or without primary/secondary ZnDTP additions were investigated at 100°C. The formation of antiwear tribofilms was detected in situ by observing the friction force and contact voltage responses. Wear volume and surface topography measurements obtained from surface profilometry and scanning electron microscopy studies were used to quantify the antiwear capacity of the formed tribofilms. The tribological properties are interpreted in terms of the tribofilm chemical composition studied by X-ray photoelectron spectroscopy. The results demonstrate that blending the base oil only with the Mo-compound additives did not improve the friction characteristics. However, an optimum mixture of Mo complexes and ZnDTP additive provided sufficient amounts of S and Mo for the formation of antiwear tribofilms containing low-shear strength MoS 2 that reduces sliding friction. In addition, the formation of a glassy phosphate phase due to the synergistic effect of the ZnDTP additive enhances the wear resistance of the tribofilm. This study shows that ZnDTP- and Mo-containing additives incorporated in oil blends at optimum proportions improve significantly the tribological properties of boundary-lubricated steel surfaces sliding at elevated temperatures.

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Paper

Study of Zinc Dialkyldithiophosphate Antiwear Film Formation and Removal Processes, Part II: Kinetic Model

Zinc dialkyldithiophosphate (ZDDP) film thickness measurements made using in situ ultrathin-film interferometry and described in Part I of this two-part …

Zinc dialkyldithiophosphate (ZDDP) film thickness measurements made using in situ ultrathin-film interferometry and described in Part I of this two-part paper (Fujita, et al.), have been used to develop and test kinetic models of antiwear film formation and removal. The main component of ZDDP film formation involves the gradual coverage of the surfaces by thick, discrete islands of film material. This process can be modeled by combining a simple coverage model in which the rate of film formation is proportional to the fraction of surface not yet covered, with an induction period. The process of film removal can be modeled by assuming that the rate of film loss is proportional to the fourth power of the coverage or film thickness. The combination of these film formation and removal rate equations is able to predict the complex, transient maximum film-forming behavior of secondary ZDDP as well as the process of film formation by primary ZDDP and the removal of antiwear film by dispersant additive.

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Paper

Synergistic Effects of Boron, Sulfur, and Phosphorus-Containing Lubricants in Boundary Lubrication of Steel Surfaces

The effectiveness of various combinations of borate-, sulfur-, and phosphorus-containing additives blended in gear oil to form antiwear tribofilms on …

The effectiveness of various combinations of borate-, sulfur-, and phosphorus-containing additives blended in gear oil to form antiwear tribofilms on steel surfaces sliding in the boundary lubrication regime was investigated experimentally. The formation of protective tribofilms and their tribochemical activity in the temperature range of 32–100°C were analyzed in terms of coefficient of friction and contact voltage results. The antiwear performance obtained with each blend was evaluated by wear volume calculations based on surface profilometry measurements. Scanning electron microscopy studies provided insight into the dominant wear mechanisms at various temperatures. The tribological performance of the tribofilms and associated wear mechanisms were found to depend strongly on the type of additive(s), tribofilm composition, and temperature. This study has revealed that sulfide-dominated tribofilms produce lower friction, whereas borate- and phosphate-dominated tribofilms are more effective in increasing the wear resistance of the steel surfaces.

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Paper

Time-Dependent Film Formation from ZnDTPs and Nonphosphorus Antiwear Agents

Electrical contact resistance (ECR) studies, X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and X-ray absorption near-edge structure spectroscopy (XANES) …

Electrical contact resistance (ECR) studies, X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and X-ray absorption near-edge structure spectroscopy (XANES) were carried out on specimens run with oils containing 0.05% phosphorus as either primary zinc dialkyldithio-phosphate (ZnDTP) or secondary ZnDTP in Group II base oil. A series of progressively longer ECR experiments were run on each ZnDTP. At the end of each run in the series, the ball was removed and preserved for surface analysis. The surface analyses were designed to observe chemical species deposited on the surface and within the deposited films. The observation of surface phenomena at different intervening times during the ECR experiment, allowing for characterization of the maturing antiwear film, was the distinct feature of these experiments. In general, short ECR experiments gave poorer films than long ECR experiments. Atomic concentrations versus depth were determined from AES. Quite strikingly, the antiwear films formed after only 10 min of the ECR experiment showed that both primary and secondary ZnDTPs form a thin film (∼70 Å) very rapidly. Those films are rich in Zn, P, and S. Auger and XANES analyses of the same specimens were not as revealing, most likely due to the small wear scar on the balls and the unfortunately relatively large beam cross section. ECR, XPS, and AES were then performed on oils containing nonphosphorus antiwear agents in American Petroleum Institute Group II base oil. Several nonphosphorus supplemental antiwear inhibitors were evaluated. These experiments showed separation in apparent performance among the various components.

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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.

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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.

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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, teh 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.

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