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- Englisch (8)
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- Laser ablation (2)
- Acoustical measurement (1)
- Aluminium oxide (1)
- Laser spallation (1)
- Microphone (1)
- Silicon (1)
- Solid electrolyte interface (1)
- Water (1)
The removal and accelerated corrosion monitoring of metal coating systems is of general interest in materials science
and engineering technology. The femtosecond and nanosecond laser ablation and delamination at 800 nm and 532 nm,
respectively, of anodic oxide layers on aluminium alloys in electrolyte contact were investigated. Laser-induced
modifications of the oxide layer resulted in an ionic contact between electrolyte and metal, which produced a current
transient and thus allowed the in-situ electrochemical monitoring of the modification. Oxide coatings with different
optical properties, almost transparent and opaque, were examined. The transparent anodic oxides showed contrasting
ablation mechanisms in the nanosecond and femtosecond pulse duration regime: nanosecond pulses caused spallation,
whereas femtosecond treatment led to ablation. Current signals measured in the transparent film were consistent with
light absorption below the metal-oxide interface and with film spallation. Irradiation of the opaque coating yielded ionic
current transients within the so-called shock-affected-zone of the oxide layer. This investigation provided insight into the
role of the penetration depth of light and the heat-affected zone, the extent of the shock-affected zone, and the defect
formation in the coating and at the solid-solid interface between metal and oxide.
Near-ablation threshold investigations focusing on the generation of periodic nanostructures and their correlation with physico-chemical
properties of the solid phase such as e.g., the material-dependent surface energy, were conducted. Molecular dynamic modelling in the
sub-picosecond time domain was used to consider ultrafast opto-electronic processes triggering surface reorganization reactions.
Fluid containment of solid interfaces showed strong influence on the resulting micro- and nanostructures due to its drastic reduction of the
surface energy. The phenomena are discussed in respect to the minimization of the surface free energy in dependence of material composition
and interfacial structure.
Femtosecond laser interaction with silicon was investigated in water and in air, with 130-fs laser pulses at 800 nm wavelength. Under water confinement, higher modification thresholds, lower ablation depths and similar incubation factors were found in comparison to the dry experiment. Morphological features of the laser-induced cavities also differed. In contrast to air experiments, debris redeposition was negligible, while the ablated material remained suspended in the water layer phase. Underwater cavities obtained at high fluences and high number of pulses per spot showed anomalous profiles, consistent with a strong spatial deformation of the laser beam coupled into the target. Ripples formed at the edges of the modified area showed varying spacings: f100 and f700 nm for water and air experiments, respectively. Differences to the air experiment were related to a complex combination of fluence-dependent non-linear effects occurring in the water layer and to pulse-number-dependent shielding effects induced by cavitation bubbles and suspended ablated
material.
Single-pulse damage thresholds of hydrogenated amorphous carbon (a-C:H) films were measured for 8-ns laser pulses at 532-nm wavelength. Layer thicknesses from below the optical penetration depth to above the thermal diffusion length (60 nm13 µm) were examined. After correction of the damage-threshold values for the fraction of energy effectively absorbed by the material, the damage threshold was found to increase linearly with the layer thickness, also for film thicknesses below the optical penetration depth of a-C:H. The threshold fluence reached the bulk value for a layer thickness equal to the thermal diffusion length. The thermal diffusion coefficient was obtained from fitting the experimental data. Several phenomena like graphitization, blistering, exfoliation, and ablation were observed for different fluence regimes and film thicknesses.