Tribomechanical properties of individual 1D nanostructures: experimental measurements supported by finite element method simulations

Abstract

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