Tribology Exchange Workshop

“Lecturers ”

Vasilios Bakolas

Global Energy Balance of Bearings

Two methods will be presented for estimating energy losses when using catalog bearings on a global scale. These methods, which are based on current standards, provide an approximation for determining the energy balance of bearings. The advantages and disadvantages of the proposed methods will be discussed.

Dr. Vasilios Bakolas was born in Thessaloniki, Greece. He studied Mechanical Engineering at the Aristotle University of Thessaloniki, where he also wrote his Ph.D. thesis. In 2000 he started working as an analytical engineer for the Schaeffler Group. For more than 10 years he was responsible for contact modelling and lubrication analysis of contacts. From 2011-2017 he was responsible for the Advanced Bearing Analysis Department where he continued to deal with tribology questions, but he was also heavily involved in innovation projects in the field of bearings. In 2017 he was appointed to Principal Expert for Bearings R&D. He is also an Associate Editor for Tribology Transactions since 2009. He has been a member of the STLE Board of Directors from 2012-2018 and was named STLE Fellow in 2020. Dr. Bakolas has written more than 60 articles and conference papers. He also holds more than 10 patents related to various aspects of rolling element bearings.

Thomas Koch

Titanium and its alloys represent a special class of materials. With a density of 4.81 g/cm³, a tensile strength of over 1.200 MPa, a fatigue strength greater than that of steel, a low modulus of elasticity and its self-passivating, inert surface makes it an ideal material for lightweight structures in aerospace, marine applications, the chemical industry and for medical implants. Although titanium is inert in its oxidized state, its nascent surface created in machining reacts with almost everything in its environment, including the tool. Moreover, its poor thermal conductivity results in high thermal stress on the tools. Overall, these properties lead to high wear rates and turns a process like milling in to a challenge. Such processes therefore require lubricants with well selected performance additives. However, most of these performance additives are based on mineral-oil and thus come from a non-renewable resource.

In the presented work environmental-friendly alternatives to conventional mineral oil-based performance additives were investigated. Due to the working mechanisms of performance additives in machining this work focuses on sulfur- and phosphorus-containing polysaccharides and proteins from microalgae. It has been successfully shown that lubricants using extracts from microalgae as performance additives can be used for high-speed milling (HSC) of TiAl6V4. The investigated extracts were able to reach the performance level of conventional additives in terms of tool lifetime and wear. The results obtained show that appropriate alternatives to mineral oil-based additives exist from renewable raw material sources.

- Study of Biology at the University of Bremen, diploma as microbiologist

- Doctorate in groundwater microbiology at the University of Bremen

- Over 10 years research associate at the Stiftung Institut für Werkstofftechnik, Bremen with focus on metalworking fluids microbiology and maintenance

- Over 6 years HSE-officer at the Fuchs-Wisura, Bremen

- Over three years research associate at the City University of Applied Sciences, Bremen

Daniele Savio

Predictive tribological simulations can pave the way for fast and optimized product development, compared to traditional trial-and-error based approaches. Moreover, they can help in the selection of sustainable materials, e.g. polymers and lubricants, adapted to the requirements of each tribosystem.

The first step towards an exhaustive tribological description of mechanisms is the quantitative modeling of tribometer tests. Here, the input parameters and operating conditions are carefully controlled, and one can “get the physics right” for all relevant phenomena. Then, one can tackle the tribology of machine elements, e.g., sealing solutions, and the challenges presented by each system in terms of operating conditions and surrounding environment.

In the talk, we apply this modeling methodology to the tribology of lubricated elastomers.

First, we describe the whole Stribeck curve for pin-on-disc experiments through the coupling of contact mechanics and the Reynolds equation. Then, first results of newly developed simulations capturing the main operation mechanisms of radial shaft seals will be shown. Finally, the outlook will focus on our current challenges in tribological modeling, e.g., predicting dry and boundary friction, and how multi-scale simulations may provide possible solutions.

2019-now: Tribology dept. at Freudenberg Technology Innovation

2014-2019: Postdoctoral activities at KIT (Karlsruhe Institute of Technology) and Fraunhofer IWM Freiburg (Institute for Mechanics of Materials) Topics: Molecular dynamics (MD) of nm-thin fluid films, complex lubricants, thermoplastics; Extension of continuum lubrication models to the nanoscale

2010-2013: PhD in mechanical engineering with INSA-Lyon, KIT and SKF Aeroengine Topics: MD of nanoscale phenomena in EHL contacts, coupling of MD and continuum equations

2004-2010: Mechanical engineering degree at INSA-Lyon, KIT

Stefanie Hanke, Philipp Sieberg

The wear resistance of metals and alloys can be partially related to general properties, e.g. oftentimes a high hardness will yield lower wear rates. Still, such correlations are not trivial and do not always hold. Understanding the acting wear mechanisms in many cases is key for predicting lifetime, developing models describing component behavior or for the improvement of performance of components. Microscopy and analytical techniques are typically applied to analyze wear appearances. If correct and relevant conclusions on the acting wear mechanisms are drawn from the obtained analytical data, targeted measures can be selected in order to mitigate or prevent wear, as well as reduce friction.

Classifying wear mechanisms requires effort and an expert evaluating the damage manually. By the use of artificial intelligence (AI), no expert knowledge would be required; instead the classification is done by a purely data-driven model. In addition to the requirement of sufficient amounts of representative labelled training data, key issues to this approach are the clear definition of specific wear appearance features and a common agreement on well-defined wear mechanisms.

This talk will introduce different wear mechanisms and their appearance and variants on different materials. The materials’ reaction and the properties decisive for the resistance against each mechanism will be discussed. Further, the current state of research as well as future potentials and challenges in the classification of wear mechanisms on microscopy images by artificial neural networks will be presented.

Stefanie Hanke is a researcher and lecturer at University of Duisburg-Essen, Germany, and head of the Chair of Materials Science in Mechanical Engineering. Her group is focused on the correlation of microstructural mechanisms of damage accumulation and failure initiation, both under fatigue and tribological loading. This includes the analysis of sliding wear and cavitation erosion behavior, both at a fundamental level and for engineering applications including a.o. metal deformation, marine equipment and materials for biomedical or energy applications. Dr. Hanke has published more than 40 peer reviewed papers, is editor and co-organizer of various conferences and secretary of the ASME Tribology Division Executive Committee.

Philipp M. Sieberg is a researcher and lecturer at the University of Duisburg-Essen, Germany, and Managing Director of Schotte Automotive GmbH & Co. KG. His research interests are primarily in the area of applied artificial intelligence. One focus is on the use of artificial intelligence for sustainability goals, especially for wear reduction by classifying and identifying the predominant wear mechanisms. Dr. Sieberg has published more than 15 peer-reviewed papers, is an associate editor of several conferences, and is a member of the Executive Committee of IEEE Germany and the IEEE ITSS German Chapter.

Nicola de Laurentis

Shielded and sealed electric motor ball bearings are usually lubricated for life, which implies that their life is often limited and determined by the life of grease. Choosing the right grease is therefore crucial to guarantee a long, maintenance-free and sustainable operation of electric motors. Grease life depends on the combination of operating conditions, bearing design and the type of grease. Usually, grease life is measured by running grease-lubricated bearing tests under some given, constant test conditions. Grease life is then expressed in number of hours, obtained under these specific conditions. This approach makes it difficult to evaluate life performance when the conditions employed to test different greases are not the same. Moreover, electric motors generally operate under a wide range of loads, speeds and temperatures. Consequently, expressing grease life in absolute terms, and independently of the operating conditions, would be much more convenient. The versatile R0F+ methodology was employed to test grease life under many operating conditions. A methodology was then developed to enable the interpolation and extrapolation of the results to any test parameters, hence in turn making it possible to predict grease life under any operating conditions pertinent to electric motors. The presentation will also include the latest efforts on improving grease life performance by employing hybrid bearings, and on investigating grease high-speed capabilities.

Nicola holds an MSc degree in Mechanical Engineering from the University of Bologna, Italy, and a PhD in Tribology from Imperial College London. Here, he was involved in a number of research projects dealing with grease/oil lubrication, coatings and damage mechanisms relevant to bearings and gears. In the past, he worked in the private sector as an Application Engineer in coatings and as a Key Account Manager in lubricants for the cement & materials industry. In 2020 he joined SKF, where he currently works on research, technology development and business support projects in the field of grease lubrication for the automotive and energy sectors.

Lucas Fourel

Experiments and Modelling on Rolling Contact Fatigue

L. Fourela,b,c,*, J.-P. Noyelb, X. Kleberc, P. Sainsota, J. Cavoreta, F. Villea,**

a. Univ Lyon, INSA Lyon, CNRS, LaMCoS, UMR5259, 69621 Villeurbanne, France b. Univ Lyon, ECAM Lyon, LabECAM, F-69005 Lyon, France c. Univ Lyon, INSA Lyon, CNRS, MATEIS, UMR CNRS 5510, 69621 Villeurbanne, France * This email address is being protected from spambots. You need JavaScript enabled to view it. ** This email address is being protected from spambots. You need JavaScript enabled to view it.

Rolling contact fatigue (RCF) is one of the most problematic failure mechanisms for many components such as rolling element bearings, gears and railways. RCF is characterised by surface damage called spalling or pitting which results from a crack initiation followed by a crack propagation phase. Depending on the contact stresses and materials, cracks can initiate on the surface or subsurface of the component. Many parameters are involved in the crack initiation mechanism due to the complexity of the physical phenomena. For example, manufacturing affects the initial surface state which may be subsequently indented by debris passing through the contact. Surface treatments and running-in also have an impact on RCF by changing material properties.

Several experiments using twin-disc machines have been conducted to analyse the microstructural changes that occur in RCF. Intergranular carbide networks and white etching cracks are two examples of detrimental phenomena. These mechanisms occur at the microstructural scale and play an import role in the RCF performance.

These experimental studies show a strong need to model RCF at the microstructural scale in order to better understand and predict the different physical phenomena. A new numerical model based on dislocation accumulation in polycrystalline materials has been developed to simulate fatigue crack initiation. The stresses induced by a moving contact pressure are computed using FEM on a polycrystalline aggregate with randomly oriented cubic elastic properties. Then, the accumulated energy is calculated based on the dislocation slip bands in each grain of the material. The Voronoi’s tessellations used to generate the polycrystalline aggregate and the random orientations make the model stochastic allowing statistical distributions of the fatigue lives. The influence of different contact and material parameters is analysed.

Lucas Fourel has a master degree in mechanical engineering from INSA Lyon, France. He did a 6-months internship at SKF Aerospace Valence on the wear simulation of spherical plain bearing. He is now doing a PhD on the modelling of crack initiation in rolling contact fatigue at LaMCoS and LabECAM, Lyon. His supervisors are Fabrice VILLE and Philippe SAINSOT from Lamcos, Jean-Philippe NOYEL from LabECAM and Xavier KLEBER from Mateis.

Mitjan Kalin

Surface-lubricant boundary slip and wetting strategy for energy-efficient and greener tribology

In this work we discuss a mechanism for the friction reduction in EHL contacts based on boundary slip and wetting phenomena. In recent years, this was presented and discussed using the DLC coatings, where up to 50 % friction reduction was obtained. While a potential complementary mechanism of friction reduction due to thermal isolating effect of DLC, and consequent viscosity decrease was proposed, current work achieves this goal with steel/steel contacts, so eliminating the thermal isolating effect. Here we show that due to the formation of oleophobic boundary layers from common boundary-lubrication additives, namely simple organic additives with different molecular structures, and with no further optimisation, we obtain different EHL friction, with up to 22 % reduction compared to base oil. It was found that the mechanism is more effective at 100 oC than at 25 oC. A small variation in the additive’s molecular structure results in notable changes to the friction, indicating that tailored green additives that form oleophobic layers could be used in future EHD lubrication technology, providing a sustainable and green friction reduction for a majority of lubricated contacts.

Dr Kalin’s areas of research are the wear and friction mechanisms of advanced materials, nanoscale interface phenomena, and boundary films for novel green-lubrication technologies, including his recognized contribution to the lubrication of DLC coatings. He has given 55 invited lectures world-wide and has published over 170 peer-reviewed journal papers with 4000 citations and an H-index of 36. He also published 10 book chapters, 3 books and holds 11 patents, including USA and EU patents. In his career he has led 40 large, 3-year projects, half of them international. He also collaborated in industrial projects with renowned companies in Europe, Japan and the USA in over 150 R&D projects. He has received several awards, including the ASME Burt L. Newkirk Award (2006) and Fellow of STLE (2012). He is a member of International Engineering Academy and Slovenian Academy of Engineering. Since 2012 he serves as the Editor of Lubrication Science. He is a Full Professor and was a Dean at the Faculty of Mechanical Engineering, University of Ljubljana, in term of 2017-2021, where he is also the Head of the Laboratory for Tribology and Interface Nanotechnology. He is also elected the Executive Board Member and Deputy President of the International Tribology Council (ITC).