Thesis defense of Henning Moldenhauer

This thesis focuses on advancing the knowledge on fundamental structural and physicalchemical processes in the contact of relative moving surfaces using Raman spectroscopy. With this advancement, a step towards monitoring wear and friction in real-time applications is performed. In this thesis, Raman scattering signatures are correlated with their spatial distribution at worn metallic surfaces and their microstructural properties are investigated. It is shown that tribological material typically contains oxides and is formed by micro- or nano-sized particles. Furthermore, the properties and the tribological behavior of, in particular, MoS2 and amorphous carbon films are sensitive to the deposition process, post-processing and the surrounding atmosphere.

In detail, the spatial distribution of hematite and magnetite Raman modes is used to detect heavily worn steel surfaces, the Raman signatures of incorporated oxygen in tungsten carbide coatings are exploited to determine the coating’s degree of wear and the different chemical behavior of third-body MoS2 particles from deposited MoS2 films are utilized as indicator for the degree of wear of the respective coatings. Furthermore, deposition parameters reducing the probability for sp3 formation in amorphous carbon films, deposition parameters shifting the S/Mo ratio in MoSx coatings and changing their bond lengths are identified. Additionally, influences on the MoSx inter-layer spacing, strain-relaxation and a basal reorientation process are investigated. The thesis closes with a chapter on tribofilm formation in steel-steel contacts, where nano-sized hematite, magnetite and amorphous carbon particles form the tribofilm. Here, the accumulation of amorphous carbon in pitting areas is observed and an estimation of the flash temperatures is given. Furthermore, atmospheric oxygen is identified to select the reaction pathway for tribofilm formation.

Taking these results into account, the tribological properties may be tailored to the desired application. In order to simplify the analysis of the spectral data, a software package is developed, which allows for a fast correlation of spectral information with surface properties. The software package also combines the Raman scattering data with chemometrical methods, providing a fast insight into huge sets of data and supporting the results obtained by conventional analysis.

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