In this contribution, chemical, structural, and mechanical alterations in various types of femtosecond laser-generated surface structures, i.e., laser-induced periodic surface structures (LIPSS, ripples), Grooves, and Spikes on titanium alloy, are characterized by various surface analytical techniques, including X-ray diffraction and glow-discharge optical emission spectroscopy. The formation of oxide layers of the different laser-based structures inherently influences the friction and wear performance as demonstrated in oil-lubricated reciprocating sliding tribological tests (RSTTs) along with subsequent elemental mapping by energy-dispersive X-ray analysis. It is revealed that the fs-laser scan processing (790 nm, 30 fs, 1 kHz) of near-wavelength-sized LIPSS leads to the formation of a graded oxide layer extending a few hundreds of nanometers into depth, consisting mainly of amorphous oxides. Other superficial fs-laser-generated structures such as periodic Grooves and irregular Spikes produced at higher fluences and effective number of pulses per unit area present even thicker graded oxide layers that are also suitable for friction reduction and wear resistance. Ultimately, these femtosecond laser-induced nanostructured surface layers efficiently prevent a direct metal-to-metal contact in the RSTT and may act as an anchor layer for specific wear-reducing additives contained in the used engine oil.

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Corrosion is a major obstacle to a safe implementation of geotechnical applications. Using a novel approach that includes vertical scanning interferometry (VSI) and electrochemical impedance spectroscopy (EIS) we discuss timedependent carbon steel corrosion and film formation at geothermally relevant temperatures (80-160°C) in CO2saturated mildly acidic Na-Cl brine. Iron dissolution kinetics follows a logarithmic rate at 80 and 160°C and a linear rate at 120°C. At 80°C, high initial corrosion rates (first 24 hours) generate H2 at a minimum rate of 12 µmol h-1 cm-2 and lead to the formation of a continuous ~100 µm thick porous corrosion film. It exhibits a duplex structure with a crystalline outer FeCO3 layer and an inner layer composed of a skeletal network of Fe3C impregnated with FeCO3. Being an electrical conductor we hypothesize the Fe3C to strongly enhance corrosion rates by providing additional cathodic sites. Pseudo-passivity due to an anodic film-forming reaction (presumably Fe-oxide) was observed at 120 and 160°C, soon followed by the initiation of pitting at 120°C. Steady-state corrosion rates at 160°C are at least one order of magnitude lower than for 120°C. Our experimental approach demonstrated potential for general applicability in studying corrosion-related phenomena.