Nanoscale Friction Measurements on Manganite Thin Films: Which Material Properties are Important?
by Niklas Alfred Weber
Date of Examination:2022-12-20
Date of issue:2023-01-05
Advisor:Prof. Dr. Cynthia A. Volkert
Referee:Prof. Dr. Cynthia A. Volkert
Referee:Prof. Dr. Matthias Krüger
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EnglishAlthough the phenomenon of friction has been known to mankind for millennia, the underlying mechanisms that ultimately lead to the conversion of kinetic/mechanical energy into heat remain unclear. Based on Bowden and Tabor's findings, that the true contact area consists of numerous single asperity contacts, modern friction research thus focuses on deconvoluting the energy dissipation at these very contacts. In an attempt to decipher fundamental friction mechanisms, AFM-based friction measurements on manganite thin films were conducted in this thesis. The basic idea: if we observe a change in friction in connection with a change in material properties, we can reverse engineer the material properties that contribute to friction. To alter material properties in a controlled manner, friction measurements were performed at a temperature-driven phase transformation in a LaSrMnO film and as a function of layer thickness on [LaMnO]m/[SrMnO]n films. The experimental findings provide strong evidence that friction is governed by the damping of vibrational excitations or phonons near the surface. Based on the measurements, a model was developed that establishes a direct relationship between energy dissipation near the surface and the thermoelastic properties of the samples. From these considerations and the measurements on the superlattices, the depth to which material properties contribute to friction was established to be several nanometers.
Keywords: LaSrMnO; Friction; Tribology; Atomic Force Microscopy; Manganites; Superlattices; Phasetransitions; Thermal Expansion Coefficient; [LaMnO3]m/[SrMnO3]n; Lateral Force Microscopy