Laser Surface Functionalization from Fundamentals to Application

Abstract

In the last decade stable ultra-short pulsed (USP) laser systems have become more widely available and are especially useful to ablate materials in a defined manner. Using small pulse energies mitigates heat input to the specimen useful for removing heat-sensitive materials. The exact ablation mechanism is still controversially discussed and as well the formation of sub-wavelength ripples. Utilizing laser radiation for microstructuring and cutting can potentially bridge the manufacturing feature-size gap of conventional techniques and clean room technology enabling innovative applications. However, there is a need in developing novel machine tool concepts and processing strategies with respect to a certain use considering this force-free process. In order to tailor the functionality governed by topography and chemistry, the ablation effects have to be unraveled exploiting the use of USP laser machining routines. Light-matter interaction mechanisms leading to ablation and evolution of self-assembled micro- and nanostructures have to be controlled transferring these unique patterns to application. This thesis presents experimental configurations developed to address the challenges just described. The test-beds consist of a combination using mechanical and optical axes encompassing a USP laser system, modifying optics and a beam delivery. Combined processes with radial and quasi-tangential irradiation condition are established for a fast production, where the controller speed is identified being the bottleneck for further acceleration. The impact of laser machining to the specimens is discussed based on microscopy and spectroscopy data using photons and electrons. Fundamental studies on single- and multi-pulse ablation reveals precursor ripple structures as origin of cone-like-protrusions (CLP) observed at steel samples. Stop-motion imaging combined with micro-structural assessments points to an evolution of topology from laser-induced periodic surface structures (LIPSS) - supra-wavelength ripples - to CLP after more than one laser pass. This evolution depends on the polarization state, fluence, total energy, and the grain orientation of the specimen. Moreover, the LIPSS spatial periodicity is tuned by wavelength being the major influence and the orientation controlled by wave plates. These findings enable a defined laser micro-machining of a wide range of materials from metals to dielectrics with the processing strategy being the key to success. Micrometer features are manufactured with high-precision paving the way for a rational surface design leading to a tailored function. The work proposes laser machining as a viable technology for many applications and especially for heat-sensitive and ultra-hard materials to introduce a distinct function. New laser marking strategies facilitate a coloration proven with oxide layers on steel and titanium substrates using interference and additionally LIPSS as diffraction gratings. Hierarchical structures on copper allow to control the wettability enabling passive droplet movement, mixing, and drop-wise condensation. Innovative hatched cutting and drilling strategies allow a production of high-precision hard shadow masks for lithography processing and chemical micro-reactors pointing to a selective production of hydrocarbons in electrocatalysis. Defined ablation of ceramics makes the USP laser machining of dental implants possible introducing customized biointerfaces to alter the osseointegration. However, quasi-tangential USP processing induces a sub-surface phase transition of the alumina-toughened zirconia being the origin of crack initiation. Using a customized scanhead-free setup enables the use of high average power for laser conditioning of super-abrasive grinding tools. This highlights the use of USP laser machining reaching a sub-micrometer precision on millimeter long contours with persistent diamond phase. Taking advantage of the selectivity between dissimilar materials with corresponding threshold fluence allows a laser sharpening to generate protrusions in favor of high-speed and precision grinding processes.