Laser Surface Functionalization from Fundamentals to Application

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

Observation of single ultrashort laser pulse generated periodic surface structures on linelike defects

The formation of self-assembled laser induced periodic surface structures (LIPSSs) after ultrashort pulsed laser ablation is still a matter of controversy in the literature. There is agreement that at least two different physical driving forces lead to ripples with distinguishable spatial periodicity. High spatial frequency LIPSSs with periodicity well below the incident wavelength are discriminated from low spatial frequency LIPSSs (LSFLs) revealing longer periodic structures. In general, both types of LIPSS appear after multipulse irradiation with the linear polarization direction on all material classes from metals to dielectrics. However, single-pulse induced LSFLs at 540 ± 35 nm periodicity with subpicosecond pulse are observed at linelike surface defects, e.g., scratches and grain boundaries. Depending on the difference in orientation between the electric field vector and the scratch direction, LIPSSs evolve upon ablation with 515 nm and 1 ps pulses near the threshold. This corroborates the theory proposed by Sipe et al. [Phys. Rev. B 27, 1141–1154 (1983)], where the impinging electromagnetic wave interacts with a collectively excited surface electron wave of the respective material at a surface defect. The observations on oxygenfree pure copper, zirconia, and a stainless steel substrate are discussed. Moreover, LSFLs generated with circular polarization at defects after single pulse ablation of wide bandgap zirconia ceramic are presented. In application, this phenomena affects the attainable surface quality, where LSFLs appear at defects such as scratches, grain boundaries, and, generally, material inhomogeneity. The absorptivity and ablation characteristic change leading to an altered material-laser interaction at the surface. This could be the root cause of conelike protrusion structures observed on stainless steel

Observation of Single Ultra-short Laser Pulse Generated Periodic Surface Structures on Line-like Defects

The formation of self-assembled laser induced periodic surface structures (LIPSS) after ultra-short pulsed laser ablation is still a matter of controversy in literature. It is agreed that there are at least two different physical driving forces depending on the spatial periodicity. High spatial frequency laser induced periodic surface structures (HSFL) with a periodicity well below the incident wavelength are discriminated from low spatial frequency structures (LSFL) revealing longer periodic structures. In general, both kind of LIPSS appear after multi-pulse irradiation with linear polarization direction on all material classes from metals to dielectrics. However, single-pulse induced LSFL at 540nm+/-35nm periodicity with sub picosecond pulse are observed at line-like surface defects, e.g. scratches. Depending on the difference in orientation between the electric field vector and the scratch direction, LIPSS evolve upon ablation with 515nm and 1ps pulses near the threshold. This corroborates the theory proposed by Sipe, where the impinging electromagnetic wave interacts with a collectively excited surface electron wave of the respective material at a surface defect. The observations on oxygen-free pure copper, zirconia and a stainless steel substrate are discussed

Ablation Characteristics of Alumina and Zirconia Ceramics on Ultra-short Pulsed Laser Machining

Technical ceramics are of high interest for several applications. However, due to their hardness ceramics are conventionally hard-to-machine. Here, ultrashort-pulsed laser manufacturing gives a unique route and enables little thermal impact. A comparative study on an alumina-toughened zirconia (ATZ) composite and pure alumina is presented. Single- and multi-pulse ablation of 1030nm and 515nm wavelength with sub picosecond pulses reveal an abrupt increase of the material removal rate. Pockets with 5μm beam distance and 15 layers rotated by 23° are generated to attain the multi-pulse ablation rate. The threshold fluence gives 0.09J/cm2 and 0.1J/cm2 for green wavelength and the ATZ and alumina, respectively. Single pulse ablation with 7J/cm2 energy density point to the sharp ablation limit and an electron microscopy assessment reveals a step-like behavior. Subsequent Raman spec-troscopy studies prove the persistence of the tetragonal morphology of zirconia. These findings enable laser manufacturing of microstructures on ceramic specimen with a 2.5D and quasi-tangential strategy

Evolution of Microstructures on Stainless Steel induced by ultra-short pulsed Laser Ablation

Ultra-short pulsed efficient laser ablation of stainless steel is accompanied by the evolution of different microstructures. The regimes of cones from impurities, laser induced periodic surface structures (LIPSS) and cone-like protrusion (CLP) are studied in more detail. Here, the identified region shows a small process window for defined 515nm sub 1ps ablation. A pre-cursor structure to the CLP shows a grain orientation and polarization dependent growth. The CLP structures could be beneficial for tribology, where micrometer features can lead to a reduced coefficient of friction under lubrication

Laser marking and coloration of Ti-6Al-4V with ultrashort pulses

© 2020 Author(s). A routine for color marking using oxide layers and laser-induced periodic surface structures is presented. Titanium and alloys thereof are marked with pixelated graphics at a high resolution with tempering colors. A computational approach for the laser path calculation enables a fast-forward marking of complex designs. The color map attained from a laser parameter studies enables vivid coloration. The minimal color pixel size is given by the optical setup and laser wavelength at near-infrared and green radiation to the focal spot size. A pixel size of 35 and 20 μ m was reached within this study, and no cross talk and distinctness between adjacent colors were observed. The oxide layer growth is sensitive on the applied laser strategy and parameter setting; however, a set of stable coloration conditions is conceived. Hitherto, fine color nuances in blue unravel the unique potential of this approach by oxidizing the substrate leading to a defined oxide layer thickness at a high repetition rate below the threshold fluence. Additionally, ultrashort pulsed laser pulses below 10 ps enable the generation of laser-induced periodic surface structures. In the low spatial frequency regime, these structures are correlated with the polarization direction of the laser light. Afterward, diffraction gratings with rotated spatial periodicity are manufactured using a half-wave plate. This allows forgery-proof marking strategies, where both mechanisms could be superimposed to increase the information density and complicating counterfeit product labeling

Geometry assessment of ultra-short pulsed laser drilled micro-holes

Ultra-short pulsed laser ablation enables a defined generation of micro-holes. A parameter study on the ablation characteristics of copper clearly reveals a benefit for green wavelength with lower threshold fluence, simultaneously increasing the Rayleigh length. The use of a circular drilling method allows a defined manufacturing of micro boreholes and micro through-holes with 35 mu m diameter of up to 165 mu m and 300 mu m length. Introducing high-resolution micro-computed X-ray tomography studying the micro-hole evolution and adjacent geometrical transformations reveals micrometer resolution and high usability. The conical geometry evolving up to an aspect ratio of 5:1 fits well to established models known for percussion drilling. However, increasing the number of pulses leads to non-conical geometry evolution, and this resulting geometry is studied for the first time. Henceforth, the exact geometrical evolution from conical to cylindrical shape upon laser drilling can be resolved revealing the impact of multiple reflections at the generated steep flanks.ISSN:0268-3768ISSN:1433-301

Laser Marking and Coloration of Ti-6Al-4V with Ultra-Short Pulses

A routine for color marking using oxide layers and laser induced periodic surface structures is presented. Titanium and alloys thereof are marked with pixelated graphics at high resolution and tempering colors. A computational approach for the laser path calculation enables a fast-forward marking of complex designs. A color map attained from laser parameter studies enables vivid coloration. The minimal color pixel size is given by the optical setup and laser wavelength at near infrared and green radiation utilized. A pixel size of 35μm and 20μm was used within this study and no cross talk and distinctness between adjacent colors observed. Additionally, ultra-short pulsed laser systems enable the generation of laser induced periodic surface structures. These are dependent on the polarization direction and diffraction gratings with rotated spatial periodicity are generated. This allows forgery-proof marking strategies and both mechanism can be superimposed

Ultra-short pulsed high-power laser conditioning of super-abrasive grinding worms Norbert

ISSN:0277-786