The 23rd International Colloquium Tribology in January 2022 was held for the first time as an online conference only, with about 300 participants. The organising team, speakers and participants therefore made the best of this traditional event in Germany in view of the Covid-19 pandemic, even though the “online conference is not a substitute for a conference on site”, as one participant stated aptly. In almost 140 talks, 8 of which were plenary talks, new findings were reported on the topics of lubricants and additives, measuring techniques, digitalisation, coatings and surfaces, transport and industry, and sustainability. Several presentations were dedicated to the EU project “i-Tribomat”, which will soon be launched as “The European Tribology Centre” that offers services for the tribological characterisation of materials and lubricants.

Cartilage damage typically starts at its surface, either due to wear or trauma. Treatment of these superficial defects is important in preventing degradation and osteoarthritis. Biomaterials currently used for deep cartilage defects lack appropriate properties for this application. Therefore, we investigated photo-crosslinked gelatin methacryloyl (gelMA) as a candidate for treatment of surface defects. It allows for liquid application, filling of surface defects and forming a protective layer after UV-crosslinking, thereby keeping therapeutic cells in place. gelMA and photo-initiator lithium phenyl-2,4,6-trimethyl-benzoylphosphinate (Li-TPO) concentration were optimized for application as a carrier to create a favorable environment for human articular chondrocytes (hAC). Primary hAC were used in passages 3 and 5, encapsulated into two different gelMA concentrations (7.5 wt% (soft) and 10 wt% (stiff)) and cultivated for 3 weeks with TGF-β3 (0, 1 and 10 ng/mL). Higher TGF-β3 concentrations induced spherical cell morphology independent of gelMA stiffness, while low TGF-β3 concentrations only induced rounded morphology in stiff gelMA. Gene expression did not vary across gel stiffnesses. As a functional model gelMA was loaded with two different cell types (hAC and/or human adipose-derived stem cells [ASC/TERT1]) and applied to human osteochondral osteoarthritic plugs. GelMA attached to the cartilage, smoothened the surface and retained cells in place. Resistance against shear forces was tested using a tribometer, simulating normal human gait and revealing maintained cell viability. In conclusion gelMA is a versatile, biocompatible material with good bonding capabilities to cartilage matrix, allowing sealing and smoothening of superficial cartilage defects while simultaneously delivering therapeutic cells for tissue regeneration.

The present investigation is aimed at understanding the nano indentation and nano scratch response of Mo-Se-C film. Towards that purpose, Mo-Se-C films with varying at.% of C were prepared employing magnetron sputtering technique. Detailed investigation reveals that the roughness data of the investigated films vary within 0.8 to 14.8 nm and the roughness does not exhibit any specific trend with carbon content. Hardness, elastic modulus, hardness to elastic modulus ratio and the ratio of hardness to the power three to elastic modulus to the power two increase with carbon content. These values decrease with increase in applied load except for very low load where slipping of indenter takes place. The friction coefficient and depth of scratch decrease with increase in carbon content up to 55 at.% C.

Economic and societal changes and technological development guide the focus of tribology research. “Classical” tasks, such as the improvement of materials or the tuning of a lubricant, have long been replaced by a function-oriented aggregate design, including specifications defined by needs arising from production and the environment. Tribology faces, among other remarkable changes, a paradigm shift according to the tendency to replace classic internal combustion engine (ICE) drivetrains with electric drives. How tribology will develop, and which research topics will prevail in the future, are being explored by several studies based on the experience of experts. The variety of contributions to journals and conferences provide an indicator of the importance of such tasks or topics. Here, a report on the ECOTRIB 2019—7th European Conference on Tribology held in Vienna, Austria, is presented. From the available information, an even stronger integration of other disciplines into tribology is noticeable, with certain hype in the fields of advanced material technology, sensor integration and the implementation of data science. Measures to rethink tribology from both an organizational and scientific point of view to cope with future tasks are being targeted and comprehensively implemented in the current research program “InTribology”, operated by the Austrian Center of Competence for Tribology (AC²T) in Wiener Neustadt, Austria.

Diamond-like carbon (DLC) films are known for thermal, chemical and mechanical properties. Hydrogen free and hydrogenated DLC films are deposited using pulsed direct current magnetron sputtering. The influence of sputtering conditions on the nano-scratch properties of these films is investigated. Raman spectra reveals that structural disorder in the a-C:H matrix decreases with increase in acetylene flow. Increased acetylene flow reduces frictions, residual stress and retains hardness. The friction responses of the deposited defects free ultra-smooth films are influenced by pull off forces at low load and by formation of carbonaceous layer at high load.

In the literature, wear damage of hip prostheses is typically expressed as volumetric wear. Details on wear distribution are rarely provided despite being fundamental to better understand the damaging process and improve implant design. This study presents a preliminary experimental investigation on the evolution of the wear map of hip replacements during wear testing. A ceramic-on-UHMWPE hip prosthesis was tested in-vitro and wear maps of the cup surface were measured throughout the test in a novel way, combining focus variation microscope inspections to the replica method. Wear maps revealed important characteristics of the damaging process not highlighted by the standard gravimetric procedure such as worn area location and shape as well as the progress of local damages.

Acoustic emission (AE), i.e. elastic waves caused by rapid release of elastic energy, is sensitive to wear mechanisms. However, the nature of wear is complex and, in addition, reflections of AE waves at the boundaries of the propagation medium cause interferences and complicate the correlation of AE features and wear events. To overcome these problems, an experiment has been established which experimentally models temporally short two-body abrasive wear events. The aim was to reveal the relationship between wear and associated AE. The RMS value of AE showed a correlation with the contact area of the experimentally modelled wear events. Normalized mean values of specific frequency ranges of Fourier transformed AE are applicable for distinction of the type of modelled wear.

Aluminium is an important material for many industrial applications and often used in daily life for decorative reasons. In both cases scratch damage is an unwanted phenomenon reducing the quality of the product. In order to assess the scratch damage induced by a hard indenter, an experimental and numerical approach was chosen in this study. Topographies and microstructural changes resulting from experimental scratches are analyzed in detail, while numerical simulations were used to investigate the stresses within the material leading to these changes. Scratch experiments were carried out on a conventional aluminium alloy with a diamond indenter of Rockwell C geometry at various loads. Numerical scratch simulations made use of the mesh-free Material Point Method (MPM) implemented in the open-source code LAMMPS, which is well suited for simulating scratch phenomena [1]. We applied a Johnson-Cook visco-plastic material model, parametrized it according to [2], and refined it based on the experimental scratch results. A strong focus of this paper lies in the analysis of the microstructural changes. For this, cross-sections of the scratches were prepared for nanoindentation and EBSD analysis to investigate the hardening and grain refining behaviour of the aluminium under abrasive load. The results were compared to the stresses calculated with the numerical model. The numerical results suggest that stresses up to 1,000 MPa occur during scratching. An exemplary stress distribution is shown in Fig. 1. These stresses are believed to lead to the hardening behaviour detected using nanoindentation. Fig.1 Modelling of stress distribution during scratching ACKNOWLEDGMENTS This work was funded by the Austrian COMET Program (Project K2 XTribology, no. 849109) and carried out at the “Excellence Centre of Tribology”. The government of Lower Austria is gratefully acknowledged for financially supporting the endowed professorship Tribology at the Vienna University of Technology (Grant no. WST3-F-5031370/001-2017). REFERENCES [1] Varga, M., Leroch, S., Eder, S.J., Rojacz, H., Rodríguez Ripoll, M., Influence of velocity on high-temperature fundamental abrasive contact: A numerical and experimental approach, Wear 2019, in press. [2] Senthil, K., Iqbal, M.A., Chandel, P.S., Gupta, N.K., Study of the constitutive behavior of 7075-T651 aluminum alloy, Int. J. Impact Eng. 108 (2017) 171-190.

Abrasive wear of key components is a critical process limiting the lifetime of machinery for mining or farming. However, materials and process parameters are mostly based on empirical data since the underlying wear mechanisms in the field are not well known. In order to assess and predict the effects of abrasive wear on typical tillage machine components, the authors developed a combined experimental and numerical simulation procedure based on a time- and space-resolved version of the Archard wear equation. The applicability of the aforementioned procedure is demonstrated on a tooth of a circular harrow. The simulated worn geometry is in good agreement with the real geometry of a tooth worn in a soil bin.

Amorphous carbon (or diamond-like carbon, DLC) films have shown a number of important properties usable for a wide range of applications for very thin coatings with low friction and good wear resistance. DLC films alloyed with (semi-)metals show some improved properties and can be deposited by various methods. Among those, the widely used magnetron sputtering of carbon targets is known to increase the number of defects in the films. Therefore, in this paper an alternative approach of depositing silicon-carbide-containing polymeric hydrogenated DLC films using unbalanced magnetron sputtering was investigated. The influence of the C2H2 precursor concentration in the deposition chamber on the chemical and structural properties of the deposited films was investigated by Raman spectroscopy, X-ray photoelectron spectroscopy and elastic recoil detection analysis. Roughness, mechanical properties and scratch response of the films were evaluated with the help of atomic force microscopy and nanoindentation. The Raman spectra revealed a strong correlation of the film structure with the C2H2 concentration during deposition. A higher C2H2 flow rate results in an increase in SiC content and decrease in hydrogen content in the film. This in turn increases hardness and elastic modulus and decreases the ratio H/E and H3/E2. The highest scratch resistance is exhibited by the film with the highest hardness, and the film having the highest overall sp3 bond content shows the highest elastic recovery during scratching.

A quantitative molecular dynamics (MD) study of nanoscopic wear under dry grinding and polishing conditions with multiple abrasive particles (grits) is presented. The initial topography of the monocrystalline iron surface has a pseudo-random Gaussian height distribution, and the 16 rigid abrasive grits have cuboid or spherical geometries. The grinding and the polishing process are differentiated via the kinematic constraints imposed on the abrasive grits. A post-processing scheme based on drift velocity analysis dynamically identifies atoms as either part of a wear particle, the substrate, or the sheared zone between the two. The knowledge of each atom׳s zone affiliation and a time-resolved, mesh-based evaluation of the substrate topography lead to a break-down of the asperity volume reduction into its constituents: pit fill-up volume, individual wear particles, shear zone, and sub-surface substrate compression. It was found that the initial geometric type of the abrasive grits as well as their kinematics strongly influences the quality of the final surface.

The layered structure of transition metal dichalcogenides makes them promising materials for self-lubricating films. Transition metal dichalcogenide films can be considered as substitute for carbon-based self-lubricating films in several varieties of environmental conditions. The tribological properties of these films at high load have been studied extensively. However, the tribological behaviour of these films in the milli-Newton load range relevant for micro-electromechanical systems has hardly been reported. In the present work, the microtribological response of W-S-C coatings deposited by reactive sputtering is investigated. For that purpose, W-S-C coatings with various concentrations of carbon were deposited on steel substrates using magnetron sputtering. The friction and wear of these coatings are determined as function of applied load and carbon content. The results show that, even though the wear of these films increases with applied load and decreases with carbon content, the friction coefficient is minimum for the films containing the highest amount of carbon at low and intermediate load. It is maximum for the films containing minimum carbon at the highest load. There is no evidence for a transfer layer on the worn surfaces.

Tungsten sulfide is a transition metal dichalcogenide (TMD) with excellent self-lubricating properties, and a potential candidate for coatings for MEMS applications. Its mechanical and tribological properties can be further improved by alloying it with carbon (W-S-C films). These films are commonly manufactured by sputter deposition. The present work investigates the influence of sputtering procedure on the microtribological performance of W-S-C films. For this purpose, carbon was incorporated in the films via three different ways: (1) by using a reactive gas (CH4); (2) by co-sputtering from two separate targets (WS2 and C); and (3) by sputtering from a composite target of graphite embedded with WS2 pellets. The films were characterized with scanning electron microscopy (SEM), nanoindentation, atomic force microscopy (AFM), and micro-Raman spectroscopy (RS). Reciprocating wear tests were performed on a microtribometer with steel balls as counterbodies. The worn surfaces were investigated with white light confocal microscopy, RS, and X-ray photoelectron spectroscopy (XPS). The results show that the total wear decreases with the hardness of the investigated films and increases with applied load of the tribological test. The friction coefficient at higher load is governed by the roughness of the films. At low load, the presence of graphitic carbon determines the friction coefficient. No transfer of material from the counteracting body is observed.

The presented research for the first time establishes the relations among nanotribological features, structural properties and deposition parameters (acetylene flow and bias voltage) of diamond like carbon films deposited by pulsed direct current magnetron sputtering. The nanotribological behaviour of hydrogen free and hydrogenated diamond-like carbon films (a-C:H) deposited by pulsed direct current magnetron sputtering was investigated using atomic force microscopy (AFM). A structural analysis with Raman spectroscopy showed that elevated acetylene concentrations in the process gas atmosphere favoured a decreased structural disorder in the a-C:H matrix, which was derived from the decreasing full with at half maximum of the G-band in the spectrum from 191±3cm−1 to 173±1cm−1. More ordered films showed friction coefficients of ~0.001, while having still relatively high hardness. Spectroscopic data could be associated with mechanical properties: higher structural disordering of the films showed highest hardness (23.3±0.3GPa). Coatings deposited only with argon as sputtering gas have low friction coefficients ~0.002 and low wear rates ~1.2nm. In general, the deposited films were found to be relatively free of defects and smooth, which was derived of an averaged roughness of the film surfaces of less than 1nm.

The objective of the present work is to evaluate the influence of carbon content on nanotribological response of reactively sputtered W-S-C thin film. Towards that purpose, W-S-C film containing varying amount of carbon was deposited on tool steel substrate using reactive sputtering. The nanohardness and lateral force microscopy of these films are determined using a nanoindenter and an atomic force microscope (AFM). The results show that the hardness and the pull off force are the maximum at intermediate carbon content. High carbon also appears detrimental for tribology at low load.

In this study, an advanced wear volume measuring technique making use of the radioisotope concentration method is applied to a ball-on-disc experiment, which allows highly precise online wear volume measurements and distinguishes among different wear regimes. Furthermore, the adhesive transfer of material from the activated specimen to the counteracting body was investigated with this method; it appears to be a promising technique for evaluating oil-protective capability. The experiments were carried out in a model tribometer following the ASTM D 6425 standard. A polyalphaolefin with zincdithiophosphate was used as lubricant. Two series of reciprocating sliding tests were conducted with different frequencies, 50 and 100 Hz. Results show a frequency-dependent running-in behaviour though the applied load was identical and constant. Using this precise online technique, a more accurate lifetime prediction can be estimated for applications like rolling bearings to which ASTM D 6425 refers.