Tribology Community Research Articles

Sliding on Slide-Ring Gels

Recent investigations have pointed to physical entanglements that greatly outnumber chemical crosslinks as key sources of energy dissipation and low friction in hydrogel networks. Slide-ring gels are an emerging class of hydrogels described by their mobile crosslinks, which are formed by rings topologically constrained to slide along linear polymer chains within the network. These materials have enjoyed decades of study by polymer chemists but have been underexplored by the tribology community. In this work, we synthesized a pseudo-rotaxane crosslinker from poly(ethylene glycol) diacrylate (PEG-diacrylate) and α-cyclodextrin-acrylate followed by hydrogel networks by connecting the sliding crosslinks with polyacrylamide chains. The mechanical and tribological properties of slide-ring hydrogels were investigated using a custom-built microtribometer. Slide-ring hydrogels exhibit unique behavior compared to conventional covalently crosslinked polyacrylamide hydrogels and offer a vast design space for future investigations.Graphical

The Invention of Tribology: Peter Jost’s Contribution

Peter Jost is known to the international tribology community as the father of the subject—‘the man who made the world spin a little easier’. He was the author of the eponymous ‘Jost Report’, the report of the Working Group set up in 1964 to investigate the state of lubrication education and research in the UK. Over the following five decades, up to his death in 2016, he was tireless in extolling the importance of tribology to manufacturing efficiency, energy conversion and environmental sustainability. He was instrumental in establishing the International Tribology Council and its prestigious gold, silver and bronze medals. Jost was himself the recipient of numerous national and international awards. In 1976, he was the President of the Institution of Production Engineers, and in 2013, he received the Sustained Achievement Award from the Royal Academy of Engineering. What is less well known is the story behind the Jost report—what was it that led the recently elected Labour government and its newly established Ministry of Technology to investigate lubrication and to choose Jost to lead the investigating panel? The story illustrates the combination of serendipity and determination on which success can so often depend.

Study of Tribological Properties and Evolution of Morphological Characteristics of Transfer Films in PTFE Composites Synergistically Reinforced with Nano-ZrO2 and PEEK Particles.

The materials tribology community has identified that the transfer film attached to the surface of the counterpart metal during the friction process is not only closely related to the filler modification material but also a key factor affecting the tribological properties of polymer composites; however, there is a lack of feasible methods to quantify the characteristics of the transfer film. In this study, Nano-ZrO2 and polyetheretherketone (PEEK) were filled into a PTFE matrix in order to enhance the wear resistance of polytetrafluoroethylene (PTFE). The tribological properties of the modified PTFE composites were tested using a linear reciprocating friction and wear tester, and the entire friction experiment was designed in seven separate stages. Morphological features were extracted and analyzed from photographs of the transfer film acquired by optical microscopy at each friction stage using an image processing program. The thickness and roughness of the transfer film sections were measured using a non-contact profilometer. Abrasive debris were collected, and their morphological features were observed with an electron microscope. The results showed that the synergistic addition of soft PEEK and hard Nano-ZrO2 particles effectively inhibited interlayer slippage between PTFE molecular chains, dramatically reducing the size and yield of abrasive debris, and facilitated the improvement of the thickness and firmness of the transfer film, which significantly enhanced the wear resistance of the PTFE composites (the lowest volumetric wear rate for Nano-ZrO2/PEEK/PTFE was only 1.76 × 10-4 mm3/Nm). Quantitative analyses of the morphological characteristics of the transfer films revealed that the coverage and roundness of the transfer films gradually increase with the friction stroke, while the aspect ratio and texture entropy subsequently decrease gradually. The coverage, area, mean, third-order moments, and consistency of the transfer film strongly correlated with the volumetric wear rate (correlation coefficient |r| > 0.9).

Exact Model Order Reduction for the Full-System Finite Element Solution of Thermal Elastohydrodynamic Lubrication Problems

The derivation of fast, reliable, and accurate modeling procedures for the solution of thermal elastohydrodynamic lubrication problems is a topic of significant interest in the Tribology community. In this paper, a novel model order reduction technique is introduced for the analysis of thermal elastohydrodynamic lubrication problems. The method uses static condensation to reduce the size of the linear elasticity part within the overall matrix system, followed by a splitting algorithm to avoid the burden of solving a semi-dense matrix system. The results reveal the exactness of the proposed methodology, which does not introduce any additional model-reduction approximations to the overall solution. They also reveal the reduction in computational times, which is in the order of 10–20% for line contacts, while it is in excess of 50% for circular contacts. The robustness of the proposed method is displayed by using it to model some relatively highly loaded contacts whose numerical solution is known to be rather challenging.

Solid Lubrication Performance of Few‐ and Multilayer Ti3C2Tx Coatings

Multilayer Ti3C2TX nanosheets (MXenes) have gained notable attention in the tribological community due to their outstanding wear resistance induced by their ability to form beneficial tribolayers. Regarding delaminated, few‐layer Ti3C2Tx nanosheets, much less knowledge about their tribological performance is available. Therefore, this study aims at comparing the solid lubrication ability of spray‐coated few‐ or multilayer Ti3C2Tx coatings (dispersion concentrations of 2 and 5 mg mL−1). Linear‐reciprocating ball‐on‐disk tests are conducted using normal loads of 100 and 200 mN. The results clearly demonstrate the superior tribological performance of multilayer Ti3C2Tx nanosheets, inducing a low, stable, and reproducible evolution of the coefficient of friction. This holds particularly true for coatings fabricated with the lower concentrated dispersion, which are characterized by a reduced coating thickness and less lubricious material. In contrast, few‐layer MXenes, irrespective of the coating thickness and available amount of lubricious material (dispersion concentration), demonstrate continuously rising frictional values, which can be traced back to the nanosheets’ degradation.

Mass and Energy Balance of a Three-Body Tribosystem

In the context of sustainable development and under the impulse of continuous technological progress, tribology contributes to the improvement of the life span of parts in dynamic contact and to the efficiency of mechanical systems. However, even if successes are obtained in lubrication, the tribology community struggles to build generalised laws of friction and wear in the case of dry friction. Based on the thermodynamics of open systems, we suggest an adaptation of the conservation of mass and energy equations to the tribosystem. The latter is modelled using the concepts of tribological triplet, tribological circuit and accommodation mechanisms. The tribosystem is described with four control volumes: two of them represent the first two bodies in dynamic contact; a third one is the tribofilm produced by the debris emission from the first bodies; a fourth control volume is used as an interface between the third body and the external environment. A mass balance is applied to these four control volumes by considering their interactions. An energy balance is then derived by applying the first principle of thermodynamics. Two systems of interdependent equations that describe the circulation of matter and energy flows in the tribosystem are outlined. These equations can be considered as a basis for future experimental developments that would aim at simultaneously characterising the different modes of energy dissipation in dynamic contact, qualitatively and especially quantitatively.

Petrus van Musschenbroek (1692–1761), man of tribology

Petrus van Musschenbroek was a famous scientist and inventor, natural philosopher, experimental physicist, engineer, instrument builder, experimenter, in the continental Newtonian tradition of Boerhaave and ‘s Gravesande. And: a tribologist. He is one who deserves more fame than he has received so far, and which is well documented in this publication. Van Musschenbroek coined the name tribometer for his device to measure friction in a journal bearing, and some authors rightly refer to this. However, what seems to have remained unnoticed until now is that he also published quantitative results of his friction measurements and tried to arrive at general laws of friction based on them. He reported in detail on friction experiments on sliding, dry as well as lubricated, sliders and journal bearings, a novum in his time, as early as in 1734. When the data from Van Musschenbroek's tables are mapped into graphs, a method which was not in use at that time, two Stribeck curves for journal bearings emerge. Van Musschenbroek's work deserves much more acclaim in the tribology community than it has now.

Publishing Science in Tribology: The Past, Present and Future of Tribology Letters

The last 25 years have seen immense changes, both in the world generally and in scientific publishing. It is now hard to imagine that our original editorial responsibilities included managing filing cabinets full of manuscripts and making frequent trips to the post office! In this first Invited Viewpoint, we have invited ourselves to highlight some of the key breakthroughs that have been made on topics that are within the scope of Tribology Letters, i.e., breakthroughs in the science of tribology. We also bring your attention to some unique, existing features of the journal, as well as new ways in which Tribology Letters will be more functional for you in the future. Finally, we share our views on publishing tribology research more generally, with the aim of encouraging publication decisions that benefit the tribology community as a whole.

A Simple Analysis of Texture-Induced Friction Reduction Based on Surface Roughness Ratio

The effect of surface texture on friction reduction under fluid lubrication has been broadly acknowledged in the tribology community. However, the lack of understanding of the underlying mechanisms remains a challenge for the advancement of textured enhanced lubrication. Numerous models have been proposed, but they are almost all based on the hydrodynamic effect alone and have proven complex, system limited and unreflective of the beneficial secondary lubrication provided by residual lubricants within the texture. This paper presents a simple analysis of texture induced friction reduction based on the actual liquid–solid interface area and the secondary lubrication hypothesis. A simple model based on the surface roughness ratio (the ratio between the actual and projected solid surface area) of the textured surface was proposed which (1) is quantitative, straightforward, intuitive and sensitive to texture shape and area fraction; (2) directly reflects the proposed secondary lubrication mechanisms proposed in literature; (3) reflects the general data trend in the collected literature data. By focusing on the variations of key texture parameters, the proposed model combined with a sampling of independent studies in literature has demonstrated that (1) the effect of increased pit depth-to-diameter ratio (d/D) on friction reduction is most significant between 0.01 and 0.2; (2) further increase in d/D only marginally affects the friction coefficient; (3) texture’s area fraction plays a much weaker role than the depth/diameter ratio in friction reduction. This model may prove useful in gaining more insights into texture-enhanced lubrication by providing a tool to quantitatively studying the secondary lubrication mechanism often cited in the literature.Graphical Abstract

Special Issue on Tribology of Additive Manufacturing

Special Issue on Tribology of Additive Manufacturing

The Use of Artificial Intelligence in Tribology—A Perspective

Artificial intelligence and, in particular, machine learning methods have gained notable attention in the tribological community due to their ability to predict tribologically relevant parameters such as, for instance, the coefficient of friction or the oil film thickness. This perspective aims at highlighting some of the recent advances achieved by implementing artificial intelligence, specifically artificial neutral networks, towards tribological research. The presentation and discussion of successful case studies using these approaches in a tribological context clearly demonstrates their ability to accurately and efficiently predict these tribological characteristics. Regarding future research directions and trends, we emphasis on the extended use of artificial intelligence and machine learning concepts in the field of tribology including the characterization of the resulting surface topography and the design of lubricated systems.

Phase behaviour of (Ti:Mo) S2 binary alloys arising from electron-lattice coupling

While 2D materials attract considerable interests for their exotic electronic and mechanical properties, their phase behaviour is still largely not understood. This work focuses on (Mo:Ti) S2 binary alloys which have captured the interest of the tribology community for their good performance in solid lubrication applications and whose chemistry and crystallography is still debated. Using electronic structures calculations and statistical mechanics we predict a phase-separating behaviour for the system and trace its origin to the energetics of the d-band manifold due to crystal field splitting. Our predicted solubility limits as a function of temperature are in accordance with experimental data and demonstrate the utility of this protocol in understanding and designing TMD alloys.

Science Thriller: The dramatic destiny of Alexander Mohrenstein‐Ertel and the history of elastohydrodynamics

AbstractPresentation is devoted to the life and work of Alexander Mikhailovich Ertel, the founder of elastohydrodynamics. He was the first to recognize the importance of elastic deformation of partners in lubricated contacts as well as pressure dependence of lubricants. In his dissertation written in Moscow during the Second World War, he suggested a solution of this complicated problem, which was not changed essentially by the later research. However, due to his escape to West Germany and change of identity, the real authorship remained unknown over many years. In West Germany, Mohrenstzein‐Ertel continued his work by practical implementation of the ideas suggested in his dissertation. He could reveal his identity after the Stalin's death, and could witness the recognition of his contribution through the German Tribological Society and international tribological community.

Double frictional receding contact problem for an orthotropic layer loaded by normal and tangential forces

With the increasing research in the field of contact mechanics, different types of contact models have been investigated by many researchers by employing various complex material models. To ascertain the orthotropy effect and modeling parameters on a receding contact model, the double frictional receding contact problem for an orthotropic bilayer loaded by a cylindrical punch is taken into account in this study. Assuming plane strain sliding conditions, the governing equations are found analytically using Fourier integral transformation technique. Then, the resulting singular integral equations are solved numerically using an iterative method. The weight function describing the asymptotic behavior of the stresses are investigated in detail and powers of the stress singularities are provided. To control the trustworthiness and correctness of the analytical formulation and to compare the resulting stress distributions and contact boundaries, a numerically efficient finite element method was employed using augmented Lagrange contact algorithm. The aim of this paper is to investigate the orthotropy effect, modeling parameters and coefficients of friction on the surface and interface stresses, surface and interface contact boundaries, powers of stress singularities, weight function and to provide highly parametric benchmark results for tribological community in designing wear resistant systems.

Rapid Thermal Characterization of Graphene Oxide—Nanocalorimetry as a Pathway for Novel Insights in Tribology

The use of solid lubricants such as graphene, graphene oxide, and other nanoparticles have gained notable attention in the tribological community to reduce friction and wear thus aiming at improved energy efficiency and sustainability. Tribological experiments unify rather extreme conditions such as high contact pressures, small contact areas, relative sliding motion, and rapid heating. This combination leads to mechanically- and/or thermally induced chemical, structural and microstructural modifications of the lubricating nanoparticles during rubbing thus altering their material’s properties. Due to the high sensitivity, we propose nanocalorimetry as the method of choice to shed more light on the thermally-induced processes and changes. As a model material for solid lubricants, we explore the transitions of graphene oxide under heating with 1000 °C/s up to 600 °C using quasi-adiabatic nanocalorimetry. We identify a strong exothermic runaway reaction at 317 °C. This runaway is preceded by exothermic reactions between 75–125 °C, which are correlated with the release of intercalated species and the formation of CO and CO2.

The role of the thermal conductivity of steel in quantitative elastohydrodynamic friction

Abstract Numerical models for thermal elastohydrodynamic lubricated contacts of steel surfaces have always employed a value for the thermal conductivity of steel corresponding to its soft annealed alloy state. However, steel in elastohydrodynamic lubricated contacts is usually hardened. It has been known for more than a century now (not within the Tribology community though) that the thermal conductivity of steel could be reduced by a factor of more than two when it is hardened. Only recently did the Tribology community realize this “mistake”, of which the impact on friction predictions is investigated in this work. The mistake is found to lead to significant overestimations of friction in the thermo-viscous regime.

Sliding frictional contact analysis of a monoclinic coating/isotropic substrate system

Owing to innovative developments and the need to satisfy the requirements of engineering applications, new and improved materials are investigated by engineers to compensate for the drawbacks of conventional materials. There is an urgent need for the tribology community to properly investigate the behavior of these materials before utilizing them in real life. In this study, the sliding frictional contact between a monoclinic coating/isotropic substrate system and a rigid cylindrical punch is provided using an analytical formulation and finite element method (FEM). The analytical expressions for the stress and displacement fields are presented, and the resulting singular integral equations (SIEs) are solved numerically. Details of the analytical formulation and the discretization used in the solution method for the SIEs are also presented. For obtaining the numerical solution, an augmented finite element method is used with plane strain conditions and 6-node higher order finite elements. According to the results, the analytical formulation and FEM results are consistent. Using different lamina orientations, coefficient of friction, and geometric/material parameters, provides the tribology community with a highly qualified parametric study that informs surface wear and surface fatigue crack initiation in contact mechanics problems

Surface Texturing in Machine Elements − A Critical Discussion for Rolling and Sliding Contacts

Surface texturing has gained great attention in the tribological community since precisely defined surface features can help to reduce friction and/or wear irrespective of the acting lubrication regime. The ability to positively influence tribological performance under different lubrication conditions makes surface texturing particularly interesting for machine elements since they may experience different conditions over the lifetime or sometimes even over one cycle/stroke. However, despite the great effort by both researchers and industry to introduce surface texturing in machine elements, many questions remain unclear regarding the optimal design of surface textures, as well as the positive and negative effects on the component's performance. The aim of this review article is to critically summarize the state of the art of surface texturing applied to machine elements, with a special emphasis on piston rings, seals, roller bearings, and gears. After a brief introduction, the first section focuses on surface texturing in sliding components (piston rings and seals), whereas the second section deals with surface texturing in rolling components (roller bearings and gears). Based upon the main evidence from the literature, the final section provides more general design guidelines for surface texturing in machine elements.

Designing surface textures for EHL point-contacts - Transient 3D simulations, meta-modeling and experimental validation

Abstract Micro-textures applied on rubbing surfaces have gained tremendous attention in the tribological community. Numerous studies deal with the deduction and testing of beneficial micro-textures. Nevertheless, not all questions have been answered yet. Will texturing really deliver as good effects as frequently claimed? Are there optimum texture designs? Are there occasions where texturing could enhance the tribological performance but is questionable from other perspectives? Therefore, ball-on-disc experiments are carried out with a special emphasis on the relative position of micro-texture and counter body. Furthermore, transient EHL simulations are performed, addressing the effects of structural parameters and relative texture's position on the tribological performance. Finally, meta-modeling with subsequent optimization is applied to predict numerically derived optima and robust texture designs.

Superlubricity achieved for commensurate sliding: MoS2 frictional anisotropy in silico

In the past decades, MoS2 has received substantial attention from the tribology community due to its excellent frictional properties. However, a fundamental understanding of the mechanisms determining friction at the nanoscale is yet to be achieved. The general consensus is that one of the ingredients required for obtaining the superlubric regime when sliding layers of MoS2 atop each other is incommensurability. Herein, we report on a molecular dynamics study focused on studying the effects of the sliding direction on the frictional properties of commensurate MoS2. It is found that different types of dynamics are obtained with different sliding directions, with the stick-slip dynamics characterized by a highly dissipative behavior. Also, it is found that there is a relation linking the mismatch angle between the driving force and the actual trajectory and the load-dependence of the frictional force. We show, for the first time with a computational study, that incommensurability is not required for achieving the superlubric behavior of MoS2.