Underwater laser ablation can be employed both as a means to produce nanoparticles and to texturize surfaces of various materials. In this approach, a stationary or flowing water layer above the target surface confines laser induced plasma which cools to form short lived cavitation bubbles, positively influencing the amount of removed material per laser pulse. Plasma and cavitation bubble evolution additionally give rise to bubbles which may persist in the water throughout the ablation process. These bubbles are known to have a detrimental effect on material removal rates particularly in stationary water, but the quantitative influence of bubble dimensions and position on removed material volume is currently unknown. Here we show the laser intensity profile changes induced by bubbles located at 0–0.4 Rayleigh lengths above a stainless steel surface and couple these changes to removed crater volume. Our results show that water flowing at Reynolds numbers in the range of 1–100 positively contribute to crater volumes for pulse frequencies up to 1 kHz. At 1 kHz, it was found bubbles have insufficient time to flow from the vicinity of the laser spot, regardless of the Reynolds number within the range investigated. These conclusions assist in selecting an appropriate combination of laser and flow conditions to optimize laser ablation material removal rate.

Under water laser ablation is a surface texturization method used to form micrometer-sized surface structures. Plasma confinement and cavitation bubble evolution play a critical role during the ablation process and their influence on material removal is strongly tied to liquid layer thickness. To influence the effects of these processes, such that material removal is at its maximum, an optimal layer thickness was found for various laser parameters. Specifically, for nanosecond pulsed laser ablation of stainless steel, however, the relation between layer thickness and volume removal is still unknown. Here, we show the relation between water layer thickness and removed material volume for a nanosecond pulsed laser. Results reveal that volume removal is at its maximum for a 1 mm water layer and drops by a factor of 2 when the layer thickness is increased to 2 mm. A further increase of layer thickness to 3 up to 10 mm shows a negligible effect on volume removal and removed volume amounts are shown to be similar to those obtained in ambient air in this water layer thickness range. This trend echo’s results obtained for nanosecond pulsed silicon ablation. The obtained results identify processing conditions which allow for faster and therefore more cost efficient texturization of stainless steel surfaces in the future.

In this paper a processing technique for sapphire is presented which combines laser-induced amorphization and subsequent selective wet etching of amorphized sapphire as well as anisotropic wet etching of single-crystalline sapphire (α-Al2O3). Using this technique, microstructures can be realized on the surface and in the bulk of sapphire substrates. By focusing ultra-short laser pulses inside sapphire, its structure can be transformed from crystalline into amorphous. The modified material can be selectively removed using etchants, such as hydrofluoric acid or potassium hydroxide (KOH), solely dissolving the amorphized part. In this work, however, an etchant consisting of a standard solution of sulphuric acid and phosphoric acid (96 vol% H2SO4: 85 vol% H3PO4, 3:1 vol%) at 180 °C is utilized. This method allows the realization of structures which are impossible to achieve when using conventional etchants which solely dissolve the amorphized sapphire. Ultrashort pulsed laser irradiation (230 fs) is used in this study as starting point for the subsequent anisotropic etching to form microstructures on the surface or in the bulk of sapphire that are terminated by characteristic crystal planes. In particular, the appearance of etching-induced patterns formed by stacks of rhombohedra is shown for structures below the surface, whereas triangular pits are achieved in surface processing.

Laser ablation of galvanized steel at a wavelength of 343, 515 and 1030 nm was compared for single as well as multiple picosecond laser pulses. The characteristics of ablated craters, such as ablation rate, crater shape and chemical composition, in relation to the processing parameters were studied. Surface morphology of the laser ablated craters were characterized with the help of confocal laser scanning microscopy and scanning electron microscopy. Chemical compositional and crystallographic changes were analyzed by energy-dispersive X-ray spectroscopy and electron backscatter diffraction respectively. Three ablation regimes were identified in the ablation process of galvanized steel. For equal amount of fluence, ablation rates are found to increase with decreasing laser wavelength. Analyzing the crater shape and the cross-sectional chemical composition, three possible applications are identified for three different wavelengths when processing galvanized steel with picosecond pulsed lasers, namely coating removal, surface texturing and micro-drilling.

Liquid layer thickness dependence of 515 nm, 7 picosecond pulsed laser ablation of stainless steel 304 and silicon is analyzed. Ablated crater volume and diameter are compared to ablated craters in ambient air by means of a novel, objective numerical procedure. While silicon ablation under a water layer is found to be more efficient in terms of removed material volume per pulse than ablation in ambient air, an opposite trend is found for stainless steel 304. For both materials, the ablation efficiently drops when the liquid layer thickness is decreased to 1 milimeter. A probable reason for the ablation efficiency drop is persistent bubble formation.

This paper analyzes laser and etching parameters to fabricate open and continuous microchannels and stacks of such microchannels in the bulk of crystalline sapphire (alpha-Al2O3). The structures are produced using a two-step method consisting of laser irradiation and selective etching. Infrared femtosecond laser pulses are focused in the bulk to locally render the crystalline material into amorphous. The amorphous material is, then, selectively etched in hydrofluoric acid. Amorphous sapphire shows a high etching selectivity in comparison to its crystalline state, which makes this material very attractive for a use with this technique. However, some of its properties make the processing challenging, especially during the laser-induced amorphization phase. This paper studies the effect of laser parameters by a step-by-step approach to fabricate long structures (longest dimensions up to millimeters) of different shapes inside the bulk of sapphire. The minimum cross-sectional dimensions of the resulting structures (microchannels) vary from few hundreds of nanometers for the smallest channels to tens of micrometers for the largest stacks of microchannels. The effect of the variation of repetition rate, pulse energy and channel-to-channel distance on the microchannels and stacks of microchannels is studied. SEM micrographs of polished cross-sections are used for performing a quantitative and qualitative analysis of the morphology of the structures after laser irradiation and, subsequently, after selective wet chemical etching.

Zinc and zinc-coated steel is processed with a picosecond laser source at a wavelength of 1030 and 515 nm to investigate the effect of time between consecutive pulses, i.e. pulse repetition rate on the laser ablation efficiency, in terms of maximum achievable depth of the ablated crater, material removal rate and processing quality. With increasing pulse repetition rate up to 40 kHz and number of pulses on the same location, material removal rate increases due to heat accumulation, while the maximum achievable depth decreases due to particle shielding for both zinc and zinc coated steel. It was found that, unlike the zinc-coated sample, both material removal rate and achievable depth is enhanced for bulk zinc at high repetition rates, due to a greater degree of heat accumulation than particle shielding. Using a numerical heat flow model, it is shown that the difference between bulk zinc and zinc-coated steel stems from the steel substrate that effectively acts as a heat sink for the absorbed energy in the zinc coating, inhibiting a higher degree of heat accumulation.

Hot-dipped galvanized steel is processed with short- and ultrashort-pulsed lasers in air at near-infrared wavelengths with pulse durations ranging from 350 fs to 241 ns. The morphology of the ablated craters (processed over a range of laser fluence levels and a number of laser pulses) is analyzed by confocal laser scanning microscopy and scanning electron microscopy. The ablation threshold of galvanized steel is found to increase with laser pulse durations following a simple power Fth=A.τB law. Longer pulse durations in the nanosecond regime, as compared to pulse durations in the picosecond and femtosecond regime, result in higher ablation efficiency and energy penetration depth at the cost of surface quality.

Laser ablation of zinc is performed with a 6.7 ps pulsed laser source to investigate the ablation mechanism and resulting morphology of the irradiated surface. The data shows the changes in crater morphology, as well as chemical composition, for different number of pulses and laser fluence levels. We observed Laser Induced Preferential Crystal Orientation (LIPCO), as a result of ultra-short pulsed laser processing of Zn at a wavelength of 515 nm. Crystallographic data for other laser wavelengths, namely 343 and 1030 nm, as well as for Zn coated steel are also provided in support of this observation. Data presented in this article are related to the research article "Investigation of the ultrashort pulsed laser processing of zinc at 515 nm: morphology, crystallography and ablation threshold" [1].

Bulk polycrystalline pure zinc is ablated in air using a picosecond laser source to perform single pulse and multi-pulse processing at a wavelength of 515 nm. The geometries and surface morphologies of the resulting craters are determined by confocal laser scanning microscopy and scanning electron microscopy. Further, the ablation threshold and its corresponding incubation coefficient is determined from the ablated volume for multiple laser pulses. Two different thermal ablation regimes are identified. The single pulse ablation thresholds are found to equal 0.1 J/cm(2) and 0.68 J/cm(2) respectively for the two regimes. It is found that the incubation coefficients are larger than unity, indicating material removal becomes energy expensive for multiple pulse laser irradiation. It is also found that an irradiated area undergoes laser induced preferred crystal orientation. (c) 2019 The Authors. Published by Elsevier Ltd.

A comparative high temperature wear study was conducted between two alloys: laser metal deposited vanadium-rich (V-rich) high speed steel (HSS) and spun cast carbide enhanced indefinite chilled double poured (CE-ICDP) iron. Laser Metal Deposition (LMD) of V-rich HSS alloy was performed by using a 4.0 kW Nd:YAG laser at three different laser scan speeds to investigate the effect thereof on the carbide size and morphology, phase constitution and mechanical properties (such as micro-hardness and wear resistance) of the laser metal deposits. A comprehensive microstructural characterization of these alloys revealed that the dendritic microstructure of the V-rich HSS alloy consisted of martensitic matrix and VC carbides. Increasing the laser processing speeds significantly changed the morphologies of VC carbides from square and round to angular and rod-like shapes. The micro-hardness of the V-rich HSS was improved from 760 HV to 835 HV by increasing the laser processing speed. During high temperature (500 degrees C) pin-on-disc wear tests, the V-rich HSS showed excellent wear resistance compared to CE-ICDP iron. It was found that V-rich HSS with square and round shape VC carbides (V-rich 10 mm/s) showed the most improved tribological performance with oxidative wear found to be the dominant wear mechanism at this temperature.

Directed energy deposition (DED) of two high-carbon high speed steel alloys Fe-bal-C-Cr-Mo-V and Febal-x-C-CrMo-V-W-x was performed by using a 4 kW Nd:YAG laser source. The purpose of additive manufacturing was design and evaluation of thermally stable - high temperature wear resistant alloys. High temperature (500 degrees C) pin-on-disc tests were conducted to investigate the effect of carbides phase fraction on friction and wear. Strain scanning of the powder and additively manufactured materials was carried out by Neutron diffraction. Microstructures of both alloys consisted of a martensitic matrix with networks of primary and eutectic carbides. Micro-hardness (0.5 HV) measurement of all multilayer laser deposits, showed a micro-hardness greater than 700 HV, with no detrimental effect of repetitive laser thermal cycling. Febal-x-C-Cr-Mo-V-W-x showed a better high temperature wear resistance due to greater phase fraction of VC and Mo2C carbides. Fracture surfaces of post-heat treated tensile samples of Fe-bal-C-Cr-Mo-V and Febal-x-C-Cr-Mo-V-W-x revealed brittle failures with minimal plasticity. Neutron strain mapping of the metal powders and the additively manufactured materials resulted in a weak diffraction signal and peak widening effect. These results could be explained either by an effect of strong crystallographic texture in the bulk or by the presence of nano- or semi-crystalline phases.

The single and multiple pulse laser ablation threshold of zinc and steel at picosecond laser pulse duration is studied as a function of initial surface roughness at laser wavelengths of 515 and 1030 nm. The initial surface topographies and the resulting crater morphologies are analyzed using confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM). Reflectivity measurements of the initial surfaces show increased absorptivity with increasing surface roughness. It was found that the single pulse ablation threshold increases with increasing effective surface area; the latter resulting from surface roughness. Rougher surfaces tend to have a higher degree of incubation as well. From the experimental and simulation results, it appears that the absorbed energy contributes more to residual heat than to material ablation when effective surface area increases.

Low spatial frequency Laser-induced Periodic Surface Structures (LSFL) have been created on single crystal silicon with picosecond laser pulses with a wavelength of lambda=1030 nm with varying laser spot diameters obtained by a defocused laser beam. The laser processing parameters have been adjusted theoretically and experimentally to obtain similar LSFL for all studied laser spot diameters. The periodicity and amplitude of the LSFL were measured by SEM and AFM analysis. It has been found that the periodicities of the LSFL do not change when LSFL were created with larger laser spot diameters. The amplitudes of the LSFL decrease with increasing laser spot diameters, although this correlation is not strong.

A model is presented, which allows to predict the (in)homogeneity of large areas covered with Laser-induced Periodic Surface Structures (LIPSS), based on the laser processing parameters (peak laser fluence and geometrical pulse-to-pulse overlap) and experimentally determined material properties. As such, the model allows to establish optimal processing conditions, given the material properties of the substrate to be processed. The model is experimentally validated over a large range of geometrical pulse-to-pulse overlap values and fluence levels on silicon using a picosecond laser source.

Silver nanoparticle inks are among the key functional materials used in printed electronics. Depositing it by laser-induced forward transfer remains a challenging task because the non-linear rheological nature of these inks narrows the range of the laser processing parameters. Understanding, therefore, the influence of the laser parameters on the ejection dynamics and deposition quality is of critical importance. The influence of the laser pulse duration from pico- to nanosecond-laser-induced jet dynamics was investigated using time-resolved shadowgraphy imaging. Jet speed and surface area analyses showed that in the lower laser fluence level range, picosecond pulses induce higher surface area ejections which propagate at higher velocities. As the laser fluence levels were increased, the difference in jet velocity and surface area evolutions narrows. Deposition analysis showed a similar behavior with lower transfer thresholds and larger depositions at lower fluence range when picosecond-laser pulses were used.

The wear behavior of three laser clad high speed steel (HSS) alloys and one conventional spun cast HSS alloy was investigated by using a pin on disc tribometer at 25 degrees C and 500 degrees C. The wear mechanism was found to be the combined effect of abrasive, adhesive and tribo-oxidative wear, with the latter becoming more prominent at 500 degrees C. Due to refined microstructures with higher hardness, laser clad HSS alloys showed superior wear resistance at 25 degrees C. However, at 500 degrees C in-spite of the cracking of MC carbides, the cast HSS alloy showed the best wear resistance. With superior anchorage, the coarse MC carbides carried the load, enabling the matrix to form a stable oxide layer to resist the wear.

Laser surface texturing of polycrystalline zinc in bulk form and as galvanised coatings is performed with single, as well as multiple nanosecond and picosecond laser pulses at a wavelength of 1030 nm.By varying the laser fluence over a wide range, as well as varying the number of pulses on the same location, thousands of ablated craters can be fabricated and their morphology studied.From the analysis thereof, it can be concluded that, within the boundaries of our experimental (laser) conditions, ablated craters always show molten features.Depending on the pulse fluence and duration, the final solidifed morphology can feature jets with meso-and micro scale spherical endings, micro rims and protrusions, as well as nano-roughness or a rim around the ablated crater.If the laser fluence is high, one observes the evidence of surface waves and finger formation, resulted from different types of hydrodynamic instablity.In contrast to nanosecond laser processing, it is found that craters formed using picosecond pulses deviate significantly from the Gaussian shape of the laser intensity distribution at high laser intensity, due to different temperature gradients in the electronic and lattice subsystems.

Based on a literature review, it was concluded that Laser-induced Periodic Surface Structures (LIPSS) on polymers are produced when applying laser sources operating either in the ultraviolet wavelength and nanosecond pulse duration, or radiation of wavelengths ranging from 265nm to 1045nm and pulse durations in the femtosecond regime. LIPSS were not reported when using picosecond laser sources. The purpose of this paper is to study whether (and if so which) LIPSS form on polymers when picosecond pulsed laser source is applied. Low Spatial Frequency LIPSS (LSFL) and High Spatial Frequency LIPSS (HSFL) have been obtained on polycarbonate and on polystyrene when applying picosecond laser pulses at a wavelength of 343nm on single spots and on processed lines. When using a wavelength of 515nm, LSFL and HSFL have been produced only on polycarbonate, but also led to porousness of the structured area.

Ablation of bulk polycrystalline zinc in air is performed with single and multiple picosecond laser pulses at a wavelength of 1030 nm. The relationships between the characteristics of the ablated craters and the processing parameters are analyzed. Morphological changes of the ablated craters are characterized by means of scanning electron microscopy and confocal laser scanning microscopy. Chemical compositions of both the treated and untreated surfaces are quantified with X-ray photoelectron spectroscopy. A comparative analysis on the determination of the ablation threshold using three methods, based on ablated diameter, depth and volume is presented along with associated incubation coefficients. The single pulse ablation threshold value is found to equal 0.21 J/cm2. Using the calculated incubation coefficients, it is found that both the fluence threshold and energy penetration depth show lesser degree of incubation for multiple laser pulses.