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This work was supported by Russian Science Foundation project grant 18-19-00645 "Adhesion of polymer-based soft materials: from liquid to solid-., Polydimethylsiloxane (PDMS) is the most widely used silicon-based organic polymer, and is particularly known for its unusual rheological properties. PDMS has found extensive usage in various fields ranging from microfluidics and flexible electronics to cosmetics and food industry. In certain applications, like e.g. dry adhesives or dry transfer of 2D materials, adhesive properties of PDMS play crucial role. In this review we focus on probing the mechanical and adhesive properties of PDMS by means of atomic force microscopy (AFM). Main advantages and limitations of AFM-based measurements in comparison to macroscopic tests are discussed., Russian Science Foundation 18-19-00645; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

11th International Fluid Power Conference, 11. IFK, Aachen, Germany, 19 Mar 2018 - 21 Mar 2018; Aachen : [Mainz], 562-573 (2018)., Published by [Mainz], Aachen

Väitekirja elektrooniline versioon ei sisalda publikatsioone., Doktoritöö käsitleb tänapäeva füüsika ja materjaliteaduse üht aktuaalset uurimisvaldkonda – süsteemi omaduste sõltuvust süsteemi suurusest, mis on eriti oluline nanoskaalas süsteemide juures, kus pindala/ruumala suhe on oluliselt suurem kui makroskoopilistes süsteemides. Väga paljud nanostruktuuride omadused sõltuvad otseselt struktuuri suurusest ja arusaadavalt on triboloogilised omadused (s.t. hõõrdumisega seotud omadused) üheks suurusest sõltuvate omaduste klassiks. Töö eesmärgiks oli käsitleda teatavate nanoosakeste ja nanotraatide struktuurilisi ja triboloogilisi omadusi, luua nende kirjeldamiseks teoreetilised mudelid ning töötada välja mudelitel baseeruvad eksperimentaalsed meetodid. Eriline tähelepanu oli suunatud pentagonaalsete nanokristallide uurimisele. Pentagonaalsed nanostruktuurid pakuvad huvi eeskätt seetõttu, et maksomaailmas sellise sümmeetriaga kristalle üldjuhul ei leidu, mistõttu on alust nanokristallide puhul oodata nanoefektide eriti suureulatuslikku esiletõusu. Töö tulemusena töötati välja mitmed mudelid, kuidas antud sümmeetriast tingitud „üleliigne“ energia jaguneb nanokristalliidis ja milliseid erinevaid kujusid võib energia jagunemine põhjustada. Praktilisest sisukohast on eriliselt huvipakkuvad nö kaksikkiht-struktuurid ja teatavad „nõel“struktuurid, kus struktuursete pingete tõttu kasvab osakese pinnale nõelakujuline teravik. Töö teises osas käsitleti nanoosakeste triboloogiliste omaduste modelleerimist ja mudelite eksperimentaalset kontrolli. Mudelid aitavad ennustada, millistel tingimustel nanoosakesed liikuvate pindade vahel veerevad või libisevad, kui palju erineb nanoosakeste „kleepumine“ pinna külge makroskoopiliste pindade vastavatest näitajatest ning millises ulatuses nanoosakeste mehaanilised omadused erinevad makroskoopilistest materjalidest. Koostöö käesoleva ja mitme teise projekti raames tehtavate uurimistööde vahel on võimaldanud TÜ Füüsika Instituudis välja töötada unikaalne nanomanipulatsiooni eksperimentaalne kompleks, mille abil on võimalik väga täpselt mõõta nanoosakeste mehaanilisi omadusi ja visualiseerida nende reaktsiooni mitmesugustele mõjutustele., From the point of view of modern physics, size of the physical system under the investigation is crucially important and in many cases essentially determines the properties of the system. For a nanoscale material, when surface/volume ratio is much higher compared to a bulk material. Structure and related properties are strongly affected by the presence of large surface area. Tribology, i.e. frictional properties have interfacial nature and the tremendously differ at the nanoscale. The goal of this dissertation is consider some of structural and tribological properties of nanoparticles, nanorods and nanowires, propose corresponding theoretical models and demonstrate several experimental methods based upon those. In particular, structural properties of pentagonal nanocrystals concerned with the presence of intrinsic stresses and their stress relaxation are investigated and described in the first part of the thesis. Tribological properties of nanoparticles and nanowires on a flat surface are studied and described in the second part. Pentagonal nanocrystals, such as nanoparticles and nanorods possess internal mechanical stresses that may result in various structural transformations in them. The author had proposed a theoretical model of stress relaxation in pentagonal nanocrystals, such as nanoparticles and nanorods, related to formation of shell layer with crystal lattice mismatch. The shell layer of that kind can diminish intrinsic energy of the nanocrystals, and therefore stabilize it. Furthermore, a theoretical model of growth of metallic whiskers in a material with internal stresses was proposed. Extruding of nanoscale whiskers can be another way of stress relaxation in pentagonal nanocrystals. The author had developed and implemented a theoretical model for numerical simulations of a nanoparticle manipulated by external forces on a flat surface. In one case, nanoparticle is affected by a concentrated force that may original from a sharp tip. In the other case, the nanoparticle is clutched between 2 surfaces, when the top surface is vertically loaded and pulled with a certain velocity. For the both cases rolling and sliding motion of the nanoparticle were distinguished depending on the parameters. An experimental setup for real-time manipulation of nanoscale objects inside a scanning electron microscope had been elaborated in the University of Tartu. The author had developed the software for controlling and measurements in the nanomanipulation setup. Then, experimental methods for measurements of friction of nanoparticles and nanowires were proposed and demonstrated. The author had elaborated theoretical models for interpretation of the experimental data and measurement static and kinetic friction between the nanowire and flat surface.

Seals are crucial machine elements in hydraulic devices. However, especially with regard to dynamic seals – for example in cylinder applications – the physical understanding of the sealing mechanism is still insufficient. In this paper a physically based, transient elastohydrodynamic simulation for translational hydraulic seals is presented. The deformation of the seal is calculated in a dynamic finite element (FE) simulation, hyper- and viscoelastic material properties are taken into account. For the numerical calculation of the fluid film and its influences on the seal deformation the FE simulation is coupled with an implementation of the transient Reynold’s equation. For a physically based calculation of the solid contact, the FE-model is coupled with Persson’s theory of rubber friction and contact mechanics. Both, normal force and solid friction are implemented. In a simulation study the influence of the relative velocity in the contact between the elastic, highly deformable seal and a hard cylinder is investigated. The initial phase of motion is investigated in detail. The simulation results are compared to experimental data of a lubricated sliding contact between an nitrile butadiene rubber (NBR) O-ring and a rough steel surface.

Väitekirja elektrooniline versioon ei sisalda publikatsioone, Väitekirja raames uuriti mitmeid olulisi küsimusi, mis käsitlevad 1D nanostruktuuride mehaanilisi ja triboloogilisi omadusi ja käitumist. Põhitegevused ja uudsed aspektid on esitatud allpool. 1DNS elemente manipuleeriti tasasel pinnal ja analüüsiti vastavaid triboloogilisi protsesse. Alljärgnevalt tuvastati: • Tasasel pinnal asetsev elastselt painutatud nanotraat on väga oluline nanotriboloogilistes mõõtmistes, kuna võimaldab nanotraadi profiilist lähtuvalt leida alusega seotud triboloogilised väärtused ilma välist jõusensorit kasutamata. Meetodi täpsuse parandamiseks töötati välja uudne analüütiline meetod, mis võtab arvesse staatilise hõõrde jaotuse tasasel pinnal asetseva elastselt painutatud nanotraadil. Erinevalt varasematest meetoditest pakub uus mudel realistlikuma jõuspektri ja arvesteb ääretingimusi. Meetodit rakendati edukalt staatilise hõõrdumise arvutamiseks ZnO nanotraatide korral, mis olid ränialusel AFM-i teravikuga manipuleerimisega suvalisse kujusse painutatud. • Uus FEM mudel töötati välja sellise konfiguratsiooni jaoks, kus osa nanotraadist toetub lamedale substraadile, samal ajal kui teine osa on vabalt üle serva. Üleulatuva vaba otsa painutatakse alusele fikseeritud osa nihkumiseni. Registreerides paindprofiili vahetult enne fikseeritud osa nihkumist, saame sisendi fikseeritud osa mõjutatava jõu arvutamiseks. Vanemate mudelite puhul võeti eelduseks staatilise hõõrdejõu ühtlast jaotust fikseeritud osale. Uue mudeli puhul näidati staatilise hõõrdumise ületamist väga lokaliseeritud protsessina, mis sarnaneb pragude tekkimisega. Näidati, et olemasolevad mudelid on staatilisest hõõrdumise rolli tunduvalt alahinnatud, samas kui uus mudel pakub reaalsusega paremat kooskõla. • Töötati välja dünaamiline FEM-mudel lamedal aluspinnal asetsevast mõlemast otsast sulanud Ag nanotraadi kirjeldamiseks. Näidati, et nanotraadis tekitatud mehaanilised pinged on tingitud asjaolust, et sulanud otsad moodustavad ümarad elemendid, mille tulemusena on võimalik ületada nanotraadi ja ränialuse vahelist adhesiooni. Selle tulemusena saavutatakse konfiguratsioon, kus ainult saadud nanoosakeste otsaelmemendid puutuvad kokku pinnaga, samas kui keskosa on pinna kohal. Selline “hantlisarnane” struktuur ja konfiguratsioon on tribologiliste mõõtmiste jaoks äärmiselt atraktiivne, kuna seda saab hõlpsasti manipuleerida väikese kontaktiala tõttu ja samal ajal säilivad kõik 1D geomeetria eelised. Lisaks uuriti alljärgnevid 1DNS mehaanilisi omadusi: • Karakteriseeriti paksude seintega torukujuliste 1DNS elastseid omadusi kasutades nii eksperimentaalseid kui ka teoreetili meetodeid. o SiO2 nanotorude elastset moodulit mõõdeti kolme erineva meetodi abil, kasutades konsooltala painutamist, nanoindetatsiooni ja kolme punkti paindekatseid. Tuvastati, et kolme punkti paindekatse on kõige täpsem meetod paksuseinaliste torukujuliste 1DNS elastusmooduli mõõtmiseks, A number of important issues concerning mechanical and tribological properties and behavior of 1D nanostructures were studied within the framework of the thesis. Main activity and the novelty aspects are summarized below. First, tribological aspects of 1DNS manipulated on a flat substrate were considered. In particular: • Nanowire elastically bent of a flat substrate is highly attractive for nanotribological studies as profile of nanowire can be used for extracting frictional data without using external force sensors. In order to improve accuracy of the method, a novel analytical method was developed for the calculation of distributed static friction in elastically bent nanowire resting on a flat substrate. Unlike previously available methods, new model provides more realistic force spectrum and comply with boundary conditions. The method was successfully applied for calculation of distributed static friction in ZnO nanowires bent into arbitrary shapes in AFM manipulations on a Si substrate. • A novel FEM model was developed for configuration in which part of the nanowire is resting on a flat substrate while other part is suspended over the trench. Measurements consist in bending the free end until fixed part is displaced. The bending profile prior the displacement of fixed part is used for calculation of force acting on a fixed part. In older models static friction was considered to be uniformly distributed in adhered part. The new model considered overcoming of static friction as a highly localized process similar to crack formation. It was shown, that existing models severely underestimated static friction, while novel model provides more realistic results. • Dynamic FEM model of Ag nanowire that is being melted from both ends while resting on a flat substrate was created. It was shown that mechanical stresses, generated in nanowire due to the fact that molted ends form rounded bulbs, are able to overcome the adhesion between nanowire and silicon substrate. As a result, a configuration is achieved where only the end-bulbs of the obtained nanodumbell are in contact with the surface while intact midpart is suspended above the substrate. Such structure and configuration is highly attractive for tribological measurements as it can be easily manipulated due to the small contact area and at the same time it preserves all benefits of 1D geometry. Further, mechanical properties of 1DNS were considered: • Elastic properties of tubular 1DNS with thick walls were treated both experimentally and theoretically. o Elastic modulus of SiO2 nanotubes was measured by three different methods including cantilever beam bending, nanoindentation and three-point bending tests. Three-point bending tests were found to be the most appropriate method for measuring the Young’s modulus of thick-walled tubular 1DNS. o FEM model was created to investigate the behavior of tubular 1DNS in nanoindentation test. It was shown there are both compression and indentation present. Thus, neither of existing models where walls of nanotube are considered either as a thin membrane or rigid wall cannot be used for given system as they underestimate the Young modulus. • FEM model of composite core-shell nanowire consisting of elastic core and viscous shell was created to simulate the behavior of Ag/SiO2 core-shell nanowire in bending test under electron beam irradiation. By fitting the experimental result with FEM model it was found that even at moderate current and voltage e-beam is capable of inducing glass transition in amorphous oxide shell. Finally, two variations of three-point bending test of Au nanowires were compared: freely sliding ends and rigidly fixed ends. The effect of different boundary conditions on experimental results was determined and the adhesion forces acting between Au and substrate were estimated using the FEM modeling. In total, it was demonstrated that FEM is a powerful method for studying mechanical and tribological properties of nanoscale systems when used in combination with experimental results

In the modelling of the leakage rate, friction force or contact pressure distribution of hydraulic seals is quite common to assume the mating surfaces to be characterized by a random isotropic roughness. However, due to different surface finishing methods, such as coating, grinding or polishing, roughness with anisotropic characteristics is often generated. In this paper a first experimental investigation of the influence of such anisotropic surfaces on the sliding friction is provided. For this purpose, a test rig has been designed and set up to investigate a soft, lubricated line contact representative of a generic reciprocating hydraulic seal. In particular, an O-ring cord is squeezed into contact with a steady rotating rigid cylinder. In order to adopt a cylinder-on-flat configuration, the diameter of the rigid cylinder is chosen to be significantly larger than the O-ring (cross-section) diameter. Furthermore, three cylinders with different surfaces are used: One (sandblasted) isotropic surface and two anisotropic surfaces roughness, scratched perpendicularly or along the azimuthal direction. Therefore, under temperature control, Stribeck curves have been measured at different squeezing loads and surface roughness, showing a neat influence of the surface roughness characteristics on the friction force. Finally, the experimental results are compared with the predictions provided by a recent mean field theory of soft contact (e.g. rubber) lubrication.

http://tartu.ester.ee/record=b2653173~S1*est