This article features with the enhancement of the static coefficient of friction by laser texturing the contact surfaces of tribological systems tested under dry friction conditions. The high-rate laser technology was applied for surface texturing at unprecedented processing rates, namely using powerful ultrashort pulses lasers in combination with ultrafast polygon-mirror based scan systems. The laser textured surfaces were analyzed by ion beam slope cutting and Raman measurements, showing a crystallographic disordering of the produced microscopic surface features. The laser induced self-organizing periodic surface structures as well as deterministic surface textures were tested regarding their tribological behavior. The highest static coefficient of friction was found of µ20 = 0.68 for a laser textured cross pattern that is 126% higher than for a fine grinded reference contact system. The line pattern was textured on a shaft-hub connection where the static coefficient of friction increased up to 75% that demonstrates the high potential of the technology for real-world applications.
The currently available microchannel fabrication techniques ranging from various etching methods and micro electrical discharge machining to laser microfabrication have some apparent advantages and weaknesses when compared one to another. Manufacturing process should satisfy several important criteria: diversity of the working material, the minimal fabricated feature size, the capability of 3D structuring, the precision and surface quality, maximum aspect ratio, the production costs, etc. This study focuses on combining the benefits of dry etching and laser structuring of a silicon substrate in order to produce microchannels with a capability of an improved heat transfer during boiling. The microchannels with a minimal cross section of 50×50 μm were etched in silicon and afterwards laser structuring was employed in order to make surface topography more appropriate for boiling heat transfer. The laser treatment resulted in micron sized cavities at the bottom of the microchannels, which lowered the temperature of the onset of boiling and improved the heat transfer during flow boiling. The performed combination of manufacturing methods proved to be complementary and cost effective.
A pump–probe setup including a Robert-cell-type delay stage is calculated and built in the presented study. The goal is to visualize laser beam material interactions upon highly repetitive ultrashort pulse irradiations by shadowgraph imaging, which makes a valuable contribution to clarify the occurring interaction phenomena in this field. Ultrashort laser pulses ( λ = 1030 n m ; τ H = 400 f s ) are irradiated onto a bright-rolled stainless steel metal plate (AISI 316). The high-speed shadowgraph sequences are captured for the time-resolved imaging of plasma and shockwave evolution during material ablation. The captured time frame ranges from the time just before the next pulse irradiates the interaction zone until 2 µs after pulse irradiation. The first part of the experimental study features the shockwave dynamics and evolution of the laser plasma/ablation plume as induced upon single-pulse irradiations. It is shown that the expansion velocity of the shockwave decreases from 10 km/s shortly after pulse irradiation to 6.1 km/s at 41 ns after pulse irradiation. The second part deals with laser pulse trains by irradiating up to 10 pulses at 500 kHz pulse repetition frequency to the substrate. For increasing pulse numbers, the shadowgraphs show a steady increase in height and width of the laser plasma/ablation plume that were measured at 2.4 mm in height and 1.2 mm in width after the 10th pulse.
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