Flash talks

First Session September 30th 2022

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Big (Open) Data in Tribology. Big (Open) Data in Experimental Science.

Nick Garabedian, Ilia Bagov, Christian Greiner, Karlsruhe Institute of Technology (KIT), Institute for Applied Materials (IAM-ZM), Karlsruhe, Germany

Data’s role in a variety of technical and research areas is undeniably growing. This can be seen, for example, in the increased investments in the development of data-intensive analytical methods such as artificial intelligence (Zhang et al. 2022), as well as in the rising rate of data generation which is expected to continue into the near future (Rydning and Shirer 2021). Academic research is one of the areas, where data is the lifeblood of generating hypotheses, creating new knowledge, and reporting results. Unlike proprietary industry data, academic research data is often subjected to stricter requirements regarding transparency, and accessibility. This is in part due to the public funding which many research institutions receive. One way to fulfil these requirements is by observing the FAIR (Findability, Accessibility, Interoperability, Reusability) principles for scientific data (Wilkinson et al. 2016). These introduce a variety of benefits, such as increased research reproducibility, a more transparent use of public funding, and environmental sustainability.
Serially-produced FAIR data is the key ingredient to enabling tribological results for scalable machine-learning-based analyses, and thus, it can potentially solve tribology’s greatest challenges. In this presentation we will, first, address the challenges of implementing big-data techniques in the inherently interdisciplinary tribology research, and second, propose a framework for scalable non-intrusive techniques for FAIR data collection. In this light, we will describe the engineering and application of controlled vocabularies, ontologies, virtual research environments (electronic lab notebooks), data collection, FAIR data publication, using experimental tribology as a testbed showcase for their application.

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Give Tribological Data a Meaning: VocPopuli and PIDs

Ilia Bagov, Nick Garabedian, Christian Greiner; Karlsruhe Institute of Technology (KIT), Institute for Applied Materials (IAM-ZM), Karlsruhe, Germany

Sharing tribological data is often done with a specific recipient in mind. However, when data is meant to be shared with the general tribology community it is best practice to observe the FAIR (Findability, Accessibility, Interoperability, Reusability) data principles. Tribology is a peculiar scientific field as the major phenomena of interest are usually system properties: friction, wear and lubrication are always a function of numerous circumstances present in a test and hard to monitor. Therefore, the precise descriptions of our experiments are a challenge standing in the way of adopting FAIR data. However, an intuitive (and required) starting point is the composition of controlled vocabularies. A base requirement for the further applicability of these FAIR vocabularies to machine learning application is the robust scheme of persistent identifiers (PIDs).
A controlled vocabulary is a collective that denotes a controlled list of terms, their definitions, and the relations between them. In the framework presented in this contribution, the terms correspond to the metadata fields used in the data annotation process. Formally, the type of controlled vocabularies used in the framework is a thesaurus (National Information Standards Organization 2010). Thesauri consist not only of the elements mentioned previously, but also allow for the inclusion of synonyms for every defined term. This eliminates the ambiguity which can occur when using terms with similar definitions. The most important feature of our framework, however, is that the controlled vocabularies can be developed in a collaborative fashion by the domain experts of a given research field.
The components described above are being implemented in the form of multiple software tools related to the framework. The first one, a controlled vocabulary editor written as a Python-based web application called VocPopuli, is the entry point for domain experts who want to develop a metadata vocabulary for their field of research or lab. The software annotates each term, as well as the entire vocabulary, with the help of the PROV Data Model (PROV-DM) (Moreau and Missier 2013) – a schema used to describe the provenance of a given object. Finally, it assigns a PID to each version of the terms in the vocabulary, as well as the vocabulary itself. It is curious to note that the generated vocabularies themselves can be seen through the prism of FAIR data: they contain data (the defined terms) which is annotated with metadata (e.g., the terms’ authors) and provided with a PID.

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Fingerprint of a structural phase transition during superlubric sliding

Ebru Cihan, TU Dresden

Although the fundamental concept of structural superlubricity (i.e. ultra-low friction observed between clean and atomically flat, incommensurate surfaces) is very straightforward, the effective energy barrier for lateral motion still depends on the exact structural dynamics at the sliding interface. In fact, it can be computationally predicted that the superlubricity of amorphous structures is less effective than that of crystalline structures, however this is not always easy to demonstrate experimentally. But we now overcome this challenge by measuring the friction of antimony nanoparticles on highly oriented pyrolythic graphite in the high temperature regime, i.e. between 300 K and 750 K, where the interface can be restructured. At about 450 K, we trigger a phase transition in antimony nanoparticles, which also allows us to establish a direct link between friction and the interface structure. More specifically, our experiments reveal that the friction level decreases in the more crystalline state where the collective force cancellations are more effective. Due to the irreversible character of the phase transition, a reduced friction level can then also be observed after cooling back to room temperature. The reduction of friction can be associated with a decrease of the characteristic scaling factor of about 16%, as theoretically anticipated from the ‘scaling law’ for superlubricity.

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Second Session, October 10th 2022

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Neutral hydrogel permeability as a function of fluid viscosity for robust low friction applications

  Nusrat Chowdhury, University of Illinois

Hydrogels are materials composed of solid and fluid components in their structure. The transport properties of hydrogels largely depend on their varying concentration as solutes can pass through the polymer mesh. The permeability values of various hydrogels have applications in biotechnology and biomedical sciences like drug delivery and replacement of biological tissue and articular cartilage. A flow pressure-controlled permeameter was developed to determine the permeability of hydrogels at various polymer concentrations ranging from 7% to 11%. The permeability was determined by measuring the flow rate of aqueous solutions as a function of pressure drop across the membranes using a biphasic model to evaluate intrinsic permeability. The outcomes are depicted using Darcy’s law. The permeability showed a negative power law relation as the volume fraction of solid increases in a hydrogel. This procedure enables the rapid assessment of hydrogels’ intrinsic permeability and can be used to determine the permeability of a variety of different types of hydrogels with varying charge and polymer concentrations, and cross-linking. The main work focuses to see if the permeability of hydrogel has effects on friction coefficient, and relation with the viscosity of fluid flowing through it.

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Molecular Mechanisms of Tribochemical Reactions: Reactive Molecular Dynamics Simulations of Cyclic Organic Molecules

F. H. Bhuiyan, UC Merced

Fakhrul H. Bhuiyan¹, Yu-Sheng Li², Seong H. Kim², Ashlie Martini^1
1Department of Mechanical Engineering, University of California Merced, 5200 N. Lake Road, Merced, California 95343, United States
²Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, United States

Tribochemical reactions determine the performance of lubricant additives that form friction and wear reducing tribofilms. However, mechanistic understanding of these reactions is still limited because the mechanochemical response of reactant species is a complex function of many variables, including the direction of applied stress and the chemical features of the reactants in non-equilibrium conditions. Here, we studied shear-activated reactions of simple cyclic organic molecules to isolate the effect of chemical structure on reaction yield and pathway. Reactive molecular dynamics simulations were used to model α-pinene, methylcyclopentane, cyclohexane, and cyclohexene subject to compressive and shear stress between silica surfaces. Results identified shear stress as the key driver of association reactions under tribological conditions. The trend of reaction yield observed in simulations was consistent with shear-driven polymerization yield measured in ball-on-flat sliding experiments. Analysis of the simulations showed the distribution of atomic sites where oxidative chemisorption occurred and identified the carbon-carbon double bond as highly susceptible to mechanochemical activation. Lastly, the most common reaction pathways for association were identified, providing insight into how the chemical structures of the precursor molecules determined their response to mechanochemical activation.

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Accurate multiscale simulation of frictional interfaces by Quantum Mechanics/Green’s Function molecular dynamics

Alberto Pacini (UNIBO)

Alberto Pacini^{1 *}, Gabriele Losi¹, Nobuaki Kikkawa², Seiji Kajita², and M. Clelia Righi^{1 *
1} Department of Physics and Astronomy, University of Bologna – Alma Mater Studiorum, 40127 Bologna, Italy
² Toyota Central R&D Labs., Inc., 41-1, Nagakute, Aichi, 480-1192, Japan
^*Corresponding author: alberto.pacini2(at)studio.unibo.it, clelia.righi(at)unibo.it

Ab initio molecular dynamics (AIMD) simulations provide a reliable description of the chemical and physical behavior
of the sliding interface although only small systems can be practically simulated due to its computational cost. This is a
problem for reliable estimates of frictional energy dissipation caused by phonons which cannot be captured due to the
limited size. Green’s function MD (GFMD) constitutes an elegant way to overcome these limitations because it
implicitly includes the influence of the semi-infinite bulk atoms on the dynamics of superficial bulk atoms [1], [2] and
thus allows to represent the phonon energy dissipation and thermo/baro-stats in a welldefined manner [3].
We present an advanced multiscale approach based on linked quantum mechanics and Green ’s function molecular
dynamics, which provides an accurate description of both the interface chemistry and the elastic properties of two semiinfinite bulks in contact.
By employing our hybrid scheme, we analyzed the kinetic friction of two H-terminated diamond semi-infinite bulks in
contact [4]. The friction coefficient is very sensitive to the degree of surface passivation and increases with the number
of unsaturated dangling carbon bonds at the surface. The simulations also highlight the difference between the static and
kinetic tribological properties. We observe that the path followed by the system during sliding in non-equilibrium
conditions deviates considerably from the minimum energy path as shown in the Figure.

These results are part of the ”Advancing Solid Interface and Lubricants by First Principles Material Design (SLIDE)”
project that has received funding from the European Research Council (ERC) under the European Union’s Horizon
2020 research and innovation program (Grant agreement No. 865633).
References
[1] Campana, C. et al., “Practical Green’s function approach to the simulation of elastic semi-infinite solids”,
Phys. Rev. B 74, 075420 (2006).
[2] Kong, L.T. et al., “Implementation of Green’s function molecular dynamics: An extension to LAMMPS”,
Comput. Phys. Commun. 180, 1004-1010 (2009).
[3] S. Kajita, “Green’s function nonequilibrium molecular dynamics method for solid surfaces and interfaces”,
Phys. Rev. E 94, 033301 (2016).
[4] S. Kajita, A. Pacini, G. Losi, N. Kikkawa, and M.C. Righi, in preparation.

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Third Session October 14th 2022

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Friction control in extreme environments using laser-deposited self-lubricating alloys

Manel Rodríguez Ripoll, AC2T research GmbH

Self-lubricating materials are a broad class of compounds featuring the incorporation of one or more solid lubricants leading to decreased friction and wear during sliding contact. The main driving force behind their development was initially the need of reducing and controlling friction in applications for which conventional oils and greases were ineffective such as high temperatures (>300 °C) or in vacuum. More recently, the research focus has shifted towards the complete elimination of externally added lubrication in metal forming and machining processes, with the ultimate goal of having lubricant-free factories.

We devote our effort to the development of self-lubricating metallic alloys for laser deposition processes. Laser deposition processes such as laser metal deposition or direct energy deposition are additive manufacturing techniques that offer a great flexibility and efficiency compared to traditional subtractive manufacturing processes. However, the extreme thermal conditions during deposition and the rapid cooling times pose great challenges in the alloy design. This work illustrates how to overcome these challenges in order to design nickel, iron and even titanium-base self-lubricating alloys. The selected alloys are blended with lubricious compounds for achieving a
microstructure containing soft metals inclusions or metal sulfides. The goal is the development of metallic alloys able to provide low friction in metal-to-metal contacts operating in extreme environments such as high temperature or vacuum, without the aid of external lubricants.

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Design principles for crystalline interfaces directional locking, directional superlubricity and structural lubricity

Andrea Silva, SISSA

Emanuele Panizon^1,5, Andrea Silva^2,3, Xin Cao¹, Jin Wang³, Clemens Bechinger¹, Andrea
Vanossi^2,3, Erio Tosatti^3,2,5, and Nicola Manini^4
1Fachbereich Physik, University Konstanz, 78464 Konstanz, Germany
²CNR-IOM, Consiglio Nazionale delle Ricerche – Istituto Officina dei Materiali, c/o SISSA, 34136
Trieste, Italy
³International School for Advanced Studies (SISSA), Via Bonomea 265, 34136 Trieste, Italy
⁴Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy and
⁵International Centre for Theoretical Physics (ICTP), Strada Costiera 11, 34151 Trieste, Italy

The understanding of friction at nano-scales, ruled by the regular arrangement of atoms, is
surprisingly incomplete. Here we provide a unified understanding by studying the interlocking
potential energy of two infinite contacting surfaces with arbitrary lattice symmetries, and
extending it to finite contacts.
We categorise, based purely on geometrical features, all possible contacts into three different
types: a structurally lubric contact where the monolayer can move isotropically without friction, a
corrugated and strongly interlocked contact, and a newly discovered directionally structurally
lubric contact where the layer can move frictionlessly along one specific direction and retains
finite friction along all other directions. This novel category is energetically stable against rotational
perturbations and provides extreme friction anisotropy.
The finite-size analysis shows that our categorisation applies to a wide range of technologically
relevant materials in contact, from adsorbates on crystal surfaces to layered two-dimensional
materials [1,2] and colloidal monolayer [3].
Our categorisation opens the possibility to design atomic interfaces with arbitrary
commensuration starting from existing material databases in a high-throughput fashion [4].
[1] M. Liao, P. Nicolini, L. Du, J. Yuan, S. Wang, H. Yu, J. Tang, P. Cheng, K. Watanabe, T.
Taniguchi, et al., Nature Materials 21, 47 (2022).
[2] K. Wang, W. Ouyang, W. Cao, M. Ma, and Q. Zheng, Nanoscale 11, 2186 (2019).
[3] X. Cao, E. Panizon, A. Vanossi, N. Manini, and C. Bechinger, Nature Physics 15, 776 (2019)
[4] N. Mounet, M. Gibertini, P. Schwaller, D. Campi, A. Merkys, A. Marrazzo, T. Sohier, I. E. Castelli,
A. Cepellotti, G. Pizzi, and N. Marzari, Nature Nanotechnology 13, 246 (2018)

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Surface Structure Modulation of Agarose-Poly(acrylamide-co-acrylic acid) Double

Ming Jun Lee, University of Illinois Urbana-Champaign

 Hydrogels provide a versatile platform to create biocompatible and biodegradable soft materials for applications in biomedicine, among others. For many of these applications, the sensing capability and responsive ability of the material is strongly desired, while maintaining sufficient strength to survive stressors. Charged double network hydrogels could provide a solution to this conundrum. These hydrogels possess desirable mechanical strength similar to biological tissues such as cartilage and can be designed with biologically compatible networks. Furthermore, the inclusion of charged copolymer enables dynamic response to stimuli such as salt concentration, pH and electric field, which enables charged double network hydrogels as aqueous sensors and actuators. In this project, I will explore how the surface of agarose-poly(acrylamide-co-acrylic acid) double network hydrogel changes in response to salt solution through characterization methods such as atomic force microscopy, surface zeta potential and thin gel tensile test.

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