Special Issue: Textured Surfaces

Lubricants, 2013

Interactions between surfaces are ubiquitous phenomena in living organisms. Nature has developed sophisticated strategies for lubricating these systems, increasing their efficiency and life span. This includes the use of water-based lubricants, such as saliva and synovial fluid. These fluids overcome the limitations of water as a lubricant by the presence of molecules such as proteins, lipids, and polysaccharides. Such molecules may alter surface interactions through different mechanisms. They can increase viscosity enabling fluid-film lubrication. Moreover, molecules adsorb on the surfaces providing mechanisms for boundary lubrication and preventing wear. The mentioned molecules have typical sizes in the nanometer range. Their interaction, as well as the interaction with the entrapping surfaces, takes place through forces in the range of nanonewtons. It is therefore not surprising that the investigation of these systems have been boosted by development of techniques such as scanning probe microscopies and the surface force apparatus which allow studying tribological processes at the nanoscale. Indeed, these approaches have generated an enormous amount of studies over the last years. The aim of this review is to perform a critical analysis of the current stage of this research, with a main focus on studies on synovial joints and the oral cavity. zone protein.

Physical Review B, 2008

With the help of a simple two-dimensional model we simulate the tribological properties of a thin lubricant film consisting of linear (chain) molecules in the ordinary soft-lubricant regime. We find that friction generally increases with chain length, in agreement with their larger bulk viscosity. When comparing the tribological properties of molecules which stick bodily to the substrates with others carrying a single sticking termination, we find that the latter generally yield a larger friction than the former.

8th International Colloquium, Esslingen "Tribology 2000", 1992

A boundary lubrication regime model in which almost all the friction and wear are due to physical interaction processes in the interface between tribological surfaces is introduced. In this model the operation conditions for the tribological system are related with its solid elements properties and the physical-chemical characteristics of the lubricant. This generic lubricant has been considered as formed by a modified base stock and an anti-wear/extreme pressure a.w./e.p. additive. In this picture, molecular sizes and adsorption energies play an important role in the interface where the macroscopic contact geometry is taken into account based in the Hertz's theory for the calculation of the surface-surface direct real contact area under applied load. The block on ring FALEX-TIMKEN tester was used for measurements of friction force at different temperatures as function of the sliding velocity and applied mechanical load. These tests were carried out using three different formulated lubricant oils. Also, measurements of wear as function of the applied load were done. Once the generic additive adsorption energy and the effective base stock molecular size were estimated, they were introduced in the calculation for simulation. The model reproduces the experimental wear trends at low temperatures and explanations of data features can be drawn. Chemical wear may be involved as shown by the estimated values for the energies of adsorption. However, the model under estimates the measure of wear at high temperatures owing to the explicit neglection of chemical wear processes.

Frictional losses are one of the main causes of reduced energy efficiency in all machines and mechanisms. In particular, there is mounting pressure upon manufacturers of all forms of vehicle to comply with increasingly stringent legislation and directives with regard to harmful emissions. Therefore, reduction of friction has become an imperative issue. The traditional approach of dealing with surface material and lubricant formulation in isolation has been replaced by a lubricant–surface system approach. This paper presents multi-scale experimentation from nano/meso-scale lateral force microscopy of ultra-thin surface adsorbed films through to micro-scale precision sliding tribometry to investigate lubricant–surface friction optimisation within the mixed regime of lubrication, using lubricants with different organic and inorganic friction modifying species. These affect the parameters of the system, commonly used as input to models for mixed and boundary regimes of lubrication. Therefore, the precise measurement of these parameters at different physical scales is important. The study also makes use of detailed numerical predictions at micro-scale through combined solution of the average Reynolds equation as well as interaction of wetted asperities in mixed and boundary regimes of lubrication. Good agreement is found between the predictions and measurements at micro-scale tribometric interactions. Furthermore, the same trends are observed in testing across the physical scales.

CENTRAL ASIAN JOURNAL OF THEORETICAL & APPLIED SCIENCES, 2022

This article discusses the phenomena occurring in the zone of interaction of surfaces during friction and wear of lubricants in the presence of various liquid and gaseous media, as well as the analysis of the effectiveness of lubrication on surfaces during friction.

Tribology International, 1997

Knowledge of the bulk viscosity provides little guidance to predict accurately the interfacial shear strength and effective viscosity of a fluid in a lubricated contact. To quantify these differences between bulk and thin-film viscosity, an instrument was developed to measure the shear of parallel single crystal solids separated by molecularly-thin lubricant films. The effective shear viscosity is enhanced compared to the bulk, relaxation times are prolonged, and nonlinear responses set in at lower shear rates. These effects are more prominent, the thinner the liquid film. Studies with lubricant additives cast doubt on the usefulness of the concept of a friction coefficient for lubricated sliding.

Tribology International, 2019

Tribofilms are activated using precision sliding strip microscale tribometry with a base and a fully formulated lubricant with a ZDDP anti-wear additive. The employed tribometry uses combined pressure, shear and temperature activation. The chemical compositions of the formed tribofilms are ascertained through use of Photo-electron X-ray Spectroscopy (XPS). Nanoscale frictional measurements of the tribofilms are reported using fluid cell lateral force microscopy (LFM). The measured coefficient of interfacial boundary shear strength is used with analytical contact mechanics to relate the in-situ conditions to the activation energy components of the Eyring potential cage model. The paper shows that combined LFM and the Eyring model can explain the variations in the frictional characteristics of formed tribofilms.

Tribology International, 2004

Lubrication is an art that has been practiced for thousands of years from the early days of our human civilization. The study of lubrication as a science began in the 17th century with the development of bearings and axles. In the early 21st century, the advent of automobiles and steam engines spurred the development of modern complex lubricants consisting of base oils and chemical additives. The development, however, has been mostly empirical in nature. The detailed mechanisms of the chemistry and why they worked were not understood. Rapid advancements in analytical instrumentations and techniques in the last several decades offer an unprecedented opportunity to analyze the complex chemistry and probe the surfaces for chemical evidence. Recent developments in nanotechnology provide further ability to examine phenomena and mechanisms at the nanometer level. As a result of these advances, our understanding of the complex lubrication system has improved significantly. This paper will attempt to provide a molecular basis of how lubricant and additives function in lubrication. Monomolecular thin films have been developed to investigate the fundamental mechanism of boundary lubricating films. Results provide additional insights of how antiwear films work in the lubrication system. Prospect for applying this know-how may result in a revolutionary change in our current lubricating technology.

Tribology Letters, 2018

Carboxylic acids are well known for their friction-reducing abilities driven by the formation of low-shear-strength films on the steel surface. However, understanding of the adsorption mechanisms especially in polar solvents is yet not well explored. In this work, atomic force microscopy was used to visualise the adsorption behaviour of various carboxylic acids in both polar and less-polar solvents. The work was continued with a tribological study of the lubricants additivated with carboxylic acids in a laboratory scale ball-on-disc tribometer. During this study, the effect of concentration and carboxylic acid chain length was studied in polar media (water-based lubricants) and compared with commonly used synthetic non-polar lubricant (polyα-olefin, PAO). It was observed that for both polar and less-polar lubricants, surface coverage of carboxylic acids increased with increasing length of hydrocarbon tail. In less-polar lubricants, carboxylic acids adsorbed to the surface by spreading on it evenly, whereas in polar lubricants, very dense multi-layered formation was promoted. Friction reduction achieved with the use of carboxylic acids in the non-polar lubricant was not as efficient as in the case of the polar lubricant. This was associated with the more pronounced multilayer formation of carboxylic acids in the polar lubricants, facilitating higher friction reduction as compared to the adsorption of carboxylic acids in a dense monolayer form seen in the less-polar lubricant.

International Journal of Precision Engineering and Manufacturing, 2012

NOMENCLATURE S = the Sommerfeld number or bearing characteristic number R = shaft radius C = the radial clearance µ = the absolute viscosity of the lubricant n = the speed of the rotating shaft in revs/s P = the load per unit of projected bearing area