Ultrashort Pulsed Laser Ablation Research Articles

Development of Laser-Structured Liquid-Infused Titanium with Strong Biofilm-Repellent Properties.

Medical implants are commonly used in modern medicine but still harbor the risk of microbial infections caused by bacterial biofilms. As their retrospective treatment is difficult, there is a need for biomedical materials that inhibit bacterial colonization from the start without using antibacterial agents, as these can promote resistance development. The promising concept of slippery liquid-infused porous surfaces (SLIPS) possesses enormous potential for this purpose. In the present study, this principle was applied to titanium, a common material in implantology, and its biofilm-repellent properties were demonstrated. To simplify prospective approval of the medical device and to avoid chemical contamination, surface structuring was performed by ultrashort pulsed laser ablation. Four different structures (hierarchical micro- and nanosized spikes, microsized grooves, nanosized ripples, and unstructured surfaces) and five infusing perfluoropolyethers of different viscosities were screened; the best results were obtained with the biomimetic, hierarchical spike structure combined with lubricants of medium viscosities (20-60 cSt at 37 °C, 143 AZ, and GPL 104). The surfaces exhibited extremely low contact angle hysteresis, as is typical for liquid-infused materials and a reliable 100-fold reduction of human oral pathogen Streptococcus oralis biofilms. This characteristic was maintained after exposure to shear forces and gravity. The titanium SLIPS also inhibited adherence of human fibroblasts and osteoblasts. Toxicity tests supported the explanation that solely the surface's repellent properties are responsible for the vigorous prevention of the adhesion of bacteria and cells. This use of physically structured and liquid-infused titanium to avoid bioadhesion should support the prevention of bacterial implant-associated infections without the use of antibacterial agents.

Synthesis of Magnetic Nanoparticles by Ultrashort Pulsed Laser Ablation of Iron in Different Liquids.

Magnetic nanoparticles were generated by ultrashort pulsed laser ablation of an iron target in water, methanol, ethanol, acetone and toluene. The relationship between ablation rate, liquid properties and the physical and chemical properties of the nanoparticles was studied. Composition, morphology and magnetic properties were investigated by TEM, XPS and vibrating-sample (VSM) and SQUID magnetometry. The properties of the generated nanoparticle ensembles reflected the influence of the liquid environment on the particle formation process. For example, the composition was strongly dependent on the carbon to oxygen ratio within the molecules of the liquid. In contrast to short pulsed laser ablation in liquids, the nanoparticles generated by ultrashort pulses had a higher level of polycrystallinity.

Ablation threshold dependence on incident wavelength during ultrashort pulsed laser ablation

Ultrashort pulse laser micromachining is an advanced materials processing technique that allows cold of almost any material. This is especially of interest in the semiconductor industry, where mechanical cutting of wafers generates large amounts of waste - if laser micromachining could be applied here, there is the potential for huge increases in the efficiency and flexibility of semiconductor manufacturing. The biggest barrier to industrial application of this technology is the cutting speed, however by tailoring the pulse properties, we hypothesise that machining speed can be increased greatly. The commonly used metric for evaluation of laser micromachining is the ablation threshold - the energy density required to cause material ablation. In this study the effect of incident laser wavelength on the ablation threshold for materials of interest for microfabrication (e.g., silicon) was investigated. This was achieved using a Ti:Sapphire pumped optical parametric amplifier (TOPAS-C) producing femtosecond pulses (τ = 110 fs, repetition rate = 1 kHz) with wavelengths ranging from 400 nm to 1200 nm using the D-Scan technique. Future work will employ advanced beam shaping technology to tailor pulses in both the spatial and temporal domains to further improve machining efficiency.

Ultrashort pulse laser ablation of dielectrics: Thresholds, mechanisms, role of breakdown.

In this paper, we establish connections between the thresholds and mechanisms of the damage and white-light generation upon femtosecond laser irradiation of wide-bandgap transparent materials. On the example of Corning Willow glass, evolution of ablation craters, their quality, and white-light emission were studied experimentally for 130-fs, 800-nm laser pulses. The experimental results indicate co-existence of several ablation mechanisms which can be separated in time. Suppression of the phase explosion mechanism of ablation was revealed at the middle of the irradiation spots. At high laser fluences, air ionization was found to strongly influence ablation rate and quality and the main mechanisms of the influence are analysed. To gain insight into the processes triggered by laser radiation in glass, numerical simulations have been performed with accounting for the balance of laser energy absorption and its distribution/redistribution in the sample, including bremsstrahlung emission from excited free-electron plasma. The simulations have shown an insignificant role of avalanche ionization at such short durations of laser pulses while pointing to high average energy of electrons up to several dozens of eV. At multi-pulse ablation regimes, improvement of crater quality was found as compared to single/few pulses.

Analysis of energy consumption in micro-drilling processes

The paper analyzes energy consumption as a performance indicator for micro-manufacturing. A comparative analysis between micro-EDM and ultrashort pulse laser drilling is performed on the basis of similar process outputs like drilled thickness of 350 μm, hole diameter of 180 ± 3 μm, drilling time, micro-hole shape and inner surface quality. Experimental data revealed that ultrashort pulse laser drilling enabled root mean squared surface roughness of about 100 nm in a drilling time of 2.5 s, whereas using a conventional micro-EDM drilling operation with a 10 s duration yielded surface roughness of 380 nm.For ultrashort pulse laser ablation the specific energy consumption is increased, as the drilling time is decreased due to an increase of the adopted pulse energy up to 50 μJ but the surface roughness and the capability of obtaining very sharp edges remain unchanged. With a calculated energy intensity in the order of 1010 J/kg and a process rate in the order of 10−6 kg/h, ultrashort pulse laser drilling can be considered as a valid alternative from both technology point of view and specific energy consumption to similar drilling process, like micro-EDM which is characterized by a lower process rate and a higher energy intensity.

Comparative analysis of tangentially laser-processed fluted polycrystalline diamond drilling tools

Ultrashort-pulsed laser ablation is increasingly applied in various fields of science and technology. For the purpose of processing ultra-hard materials, such as diamond and cubic boron nitride (CBN) composites, lasers have the decisive advantage of wear-free material removal. The availability of high-powered ultrashort-pulsed laser sources enables the efficient applications of tangential processing strategies to generate complex 3D geometries. Compared to the conventionally applied 2.5D volume ablation strategy, the resulting workpiece form tolerance, repeatability, and surface quality is increased significantly and does not depend on the quality of the initial surface. This makes tangential processing an ideal choice for high-precision finishing processes.This paper presents a set of processes for the tangential ablation of characteristic twist drill features, such as helical grooves, flank faces and notches at the chisel edge. The processes have been implemented using a pulse duration of 12ps, infrared laser source with an average power of 35W average power for generating PCD tools. A comparative drilling study in zirconium dioxide (ZrO2) with diamond-coated tungsten carbide tools and solid PCD tools processed by electrical discharge machining is conducted to assess the performance of the laser-processed PCD tools.

Reduced bacterial adhesion on titanium surfaces micro-structured by ultra-short pulsed laser ablation

Abstract Implant-associated infections still pose serious problems in modern medicine. The development of fabrication processes to generate functional surfaces, which inhibit bacterial attachment, is of major importance. Sharklet™-like as well as grooves and grid micro-structures having similar dimensions were fabricated on the common implant material titanium by ultra-short pulsed laser ablation. Investigations on the biofilm formation of Staphylococcus aureus for up to 24 h revealed similarly reduced bacterial surface coverage on all micro-structures investigated compared to smooth titanium surfaces. This study is a prove-of-principle and could serve as basis for further investigations towards a structure-based biofilm-inhibiting implant.

Ultrashort laser pulse ablation of copper, silicon and gelatin: effect of the pulse duration on the ablation thresholds and the incubation coefficients

In this paper, the influence of the pulse duration on the ablation threshold and the incubation coefficient was investigated for three different types of materials: metal (copper), semiconductor (silicon) and biopolymer (gelatin). Ablation threshold values and the incubation coefficients have been measured for multiple Ti:sapphire laser pulses (3 to 1000 pulses) and for four different pulse durations (10, 30, 250 and 550 fs). The ablation threshold fluence was determined by extrapolation of curves from squared crater diameter versus fluence plots. For copper and silicon, the experiments were conducted in vacuum and for gelatin in air. For all materials, the ablation threshold fluence increases with the pulse duration. For copper, the threshold increases as τ 0.05, for silicon as τ 0.12 and for gelatin as τ 0.22. By extrapolating the curves of the threshold fluence versus number of pulses, the single-shot threshold fluence was determined for each sample. For 30 fs pulses, the single-shot threshold fluences were found to be 0.79, 0.35, and 0.99 J/cm2 and the incubation coefficients were found to be 0.75, 0.83 and 0.68 for copper, silicon and gelatin, respectively.

Nanosecond laser processing of Zr41.2Ti13.8Cu12.5Ni10Be22.5 with single pulses

In addition to their attractive mechanical properties, the amorphous structure of bulk metallic glasses (BMGs) leads to favourable conditions for their processing using micro machining operations. At the same time, the generally high hardness and strength of such amorphous metals make short or ultra-short pulsed laser ablation a fabrication technology of interest for generating micro scale features on BMG workpieces in comparison with mechanical material removal means. In spite of this, relatively little research has been reported on the prediction and observation of the thermal phenomena that take place when processing BMGs with pulsed laser irradiation for a range of delivered fluence values and pulse lengths. Such investigations are important however as they underpin reliable laser processing operations, which in turn lead to more predicable material removal at micro scale. In this context, this paper reports complementary theoretical and experimental single pulse laser irradiation analyses conducted in the nanosecond (ns) regime for possibly the most prominent BMG material due to its relatively high glass forming ability, namely Zr41.2Ti13.8Cu12.5Ni10Be22.5, which is also known as Vitreloy 1. To achieve this, different pulse lengths comprised between 15ns and 140ns and varied fluence values were considered when delivering single pulses on a Vitreloy 1 substrate using a Yb fibre laser system. A simple thermal model of the laser material interaction process for single pulses was also developed to support the observations and interpretations of the experimental data obtained. One of the main conclusions from this research, with respect to potential micro machining applications, is that shorter pulses, i.e. 25ns and less, could lead to the formation of relatively clean craters. For higher pulse lengths, the low thermal conductivity and melt temperature of this BMG substrate mean that laser irradiation easily leads to the formation of a relatively large melt pool and thus to the re-solidification of material ejected outside craters.

Influence of the Liquid on Femtosecond Laser Ablation of Iron

Ultrashort pulse laser ablation has become a very important industrial method for highly precise material removal ranging from sensitive thin film processing to drilling and cutting of metals. Over the last decade, a new method to produce pure nanoparticles emerged from this technique: Pulsed Laser Ablation in Liquids (PLAL). By this method, the ablation of material by a laser beam is used to generate a metal vapor within the liquid in order to obtain nanoparticles from its recondensation process. It is well known that the liquid significantly alters the ablation properties of the substrate, in our case iron. For example, the ablation rate and crater morphology differ depending on the used liquid. We present our studies on the efficiency and quality of ablated grooves in water, methanol, acetone, ethanol and toluene. The produced grooves are investigated by means of white-light interferometry, EDX and SEM.

Numerical Simulations on Laser-Ablative Micropropulsion with Short and Ultrashort Laser Pulses

Aiming for the generation of high-precision thrust in the µN range, focused high-intensity laser pulses are used employing the recoil of the jet of the ablated material. Whereas a single laser pulse yields an extremely low impulse bit in the range of several nNs, a broad thrust range can be accessed by the variation of the laser pulse repetition rate up to several hundreds of kilohertz. A detailed laser parameter study is carried out for aluminum and gold as propellant varying the pulse length from 100 fs to 10 ns and the fluence from 0.09 to 23.8 J/cm^2. Two different regimes of thruster operation with respect to laser pulse length and specific impulse are identified. Irrespective of the pulse length regime, optimum impulse coupling is found at laser spot fluences around 2 J/cm^2 for aluminum and 20 J/cm^2 for gold, respectively, with coupling coefficients in the range of 25 to 40 μN/W. For ultrashort pulses, jet velocities are rather small yielding a specific impulse in the range of 70 s to 200 s, whereas for longer pulses beyond ≈ 100 ps, Isp is found to be in the range of 500 to 1000 s and beyond enabling low propellant consumption. However, ultrashort-pulse laser ablation might be favorable since material can be removed very smoothly which might contribute to very low thrust noise.

Residual Heat in Ultra-Short Pulsed Laser Ablation of Metals

Residual Heat in Ultra-Short Pulsed Laser Ablation of Metals

Improved molding of micro structures using PVD-coated mold inserts

Abstract Micro structured optical plastics components are intensively used, i.e. in consumer electronics, for optical sensors in metrology, innovative LED-lighting, or laser technology. Injection molding has been proven successful for the large-scale production of these parts. However, the production of these parts still causes difficulties due to challenges in the molding and demolding of plastics parts created with laser-structured mold inserts. A complete molding of the structures often leads to increased demolding forces, which then cause a breaking of the structures and a clogging of the mold. An innovative approach is to combine physical vapor deposition (PVD)-coated, laser-structured inserts and a variothermal molding process to create functional micro structures in a one-step process. Therefore, a PVD coating is applied after the laser-structuring process in order to improve the wear resistance and the anti-adhesive properties against the plastics melt. In a series of molding trials with polycarbonate (PC) and polymethyl methacrylate (PMMA) using different coated molds, the mold temperature during injection was varied in the range of the glass transition and the melt temperature of the polymers. Subsequently, the surface topography of the molded parts was evaluated by digital three-dimensional laser-scanning microscopy. The influence of the molding parameters and the coating of the mold insert on the molding accuracy and the demolding behavior were analyzed. It was shown that micro structures created by ultra-short pulse laser ablation can be successfully replicated in a variothermal molding process. Due to the mold coating, significant improvements could be achieved in producing micro structured optical plastics components.

Defect elimination on laser milled geometries by means of error detection and correction

The present paper exposes the work carried out on the correction of geometrical errors generated during the ultra-short pulse laser ablation of AISI 304 stainless steel and yttria-stabilized zirconia. After the introduction of the error detection and correction methodology to be followed after the laser milling of a key geometrical feature, the different procedures for the experimental implementation of the methodology are detailed. Finally, after the application of the methodology to an artificially defective geometry on both analysed materials, the obtained results are exposed and discussed. The results show that the error detection methodology is able to properly identify defective zones and define a laser milling process for their correction, generating this way a geometry within the required geometrical tolerances in comparison to the desired one.

The influence of unshielded small cracks in the fracture toughness of yttria and of ceria stabilised zirconia

The fracture toughness, KIC of two 3Y–ZrO2 with different grain size (177 and 330nm) and 12Ce–ZrO2 were determined from a sharp micro-machined notch by Ultra-Short Pulsed Laser Ablation (UPLA) where a micro-cracked zone and non-transformed is generated in front of the notch. The notch plus the damage behaved as an unshielded edge surface crack. The fracture stress, σf of both 330nm-3Y–ZrO2 and 12Ce–ZrO2 with similar short crack sizes were found to be comparable in despite of their different published R-curves. The results of KIC were discussed in terms of the type of cracks induced and by using a simple R-curve model. It was concluded that for the development of high strength composites with 12Ce–ZrO2 as the matrix, the relevant KIC that controls the σf with surface unshielded short cracks is much closer to the intrinsic KIC than to the indentation KIC or to the plateau KIC of long cracks.

Mechanisms of femtosecond LIPSS formation induced by periodic surface temperature modulation

Here we analyze the formation of laser-induced periodic surface structures (LIPSS) on metal surfaces upon single femtosecond laser pulses. Most of the existing models of the femtosecond LIPSS formation discuss only the appearance of a periodic modulation of the electron and ion temperatures. However the mechanism how the inhomogeneous surface temperature distribution induces the periodically-modulated surface profile under the conditions corresponding to ultrashort-pulse laser ablation is still not clear. Estimations made on the basis of different hydrodynamic instabilities allow to sort out mechanisms, which can bridge the gap between the temperature modulation and the LIPSS. The proposed theory shows that the periodic structures can be generated by single ultrashort laser pulses due to ablative instabilities. The Marangoni and Rayleigh–Bénard convection on the contrary cannot cause the LIPSS formation.

Mechanical properties of 12Ce–ZrO2/3Y–ZrO2 composites

12Ce–ZrO2/3Y–ZrO2 composites with different compositions have been prepared by presureless sintering at 1450°C and the resulting microstructures have been correlated with their mechanical properties in terms of hardness, elastic modulus and resistance to contact loading by sharp indenters. The composition with a better compromise between mechanical properties and resistance to hydrothermal degradation has been processed at different sintering temperatures in the range 1350°C and 1650°C and the fracture toughness have been determined by the single edge V-notch beam (SEVNB) method with a very small starting crack induced by ultra-short pulsed laser ablation. It is shown that the resulting grain size is sensitive to composition and sintering temperature while the elastic modulus is the same for all the compositions investigated. The hardness and the resistant to contact loading are also strongly affected by the addition of 3Y–ZrO2. Finally it is shown that despite the high resistance to fracture by contact loading encountered for some compositions rich in 12Ce–ZrO2, the fracture toughness determined from surface shallow sharp cracks is similar to that of 3Y–TZP. This is explained in terms of R-curve behaviour and crack dimensions.

Quasi-superhydrophobic porous silicon surface fabricated by ultrashort pulsed-laser ablation and chemical etching

A silicon surface with distinctive structures is fabricated by ultrashort pulsed-laser ablation and chemical etching with acidic fluoride solutions. The surface consists of micro/nanostructures that result in the quasi-superhydrophobicity of the silicon surface. By fine tuning a key process parameter (i.e., pulsed laser power), surfaces with different wettability are fabricated. The morphology and composition of the surfaces are characterized by scanning electron microscopy, which reveals nanopores. The contact angle of water on these surfaces was measured and found to be as high as 150° at optimized parameters. This work presents a novel process of fabricating a silicon-based quasi-superhydrophobic porous surface.

High power ultra-short pulse laser ablation of IN718 using high repetition rates

Laser ablation using ultra-short pulsed laser radiation with pulse durations below 10ps is known to be capable of high-precision machining with very small thermal load for the processed workpiece. Although the processing quality is excellent, the productivity is too small for many applications. Therefore, an upscaling of the achievable ablation rate is one major research topic in the field of ultra-short pulse laser ablation. In this study, we identify and investigate a novel high-speed ablation process for the ablation of Inconel 718 using ultra-short pulsed laser radiation. The formation of a thin layer of molten material avoids the formation of rough microstructures on the workpiece surface that usually appear for the processing with high average powers and high spatial pulse overlaps. Thus, processing with high average power of more than 50W using conventional, high efficient and high flexible beam deflection technology becomes applicable. The presented process is approximately 20 times faster than conventional ultra-short pulse laser ablation with average powers of less than 5W.The influence of the repetition rate, average power, pulse duration and hatch distance on the high-speed ablation process is investigated. The potential of an optional finishing process to remove the molten layer by melt-free ablation with the same laser system but different processing parameters is demonstrated.

Time-resolved microscopy reveals the driving mechanism of particle formation during ultrashort pulse laser ablation of dentin-like ivory.

In dental health care, the application of ultrashort laser pulses enables dental tissue ablation free from thermal side effects, such as melting and cracking. However, these laser types create undesired micro- and nanoparticles, which might cause a health risk for the patient or surgeon. The aim of this study was to investigate the driving mechanisms of micro- and nanoparticle formation during ultrashort pulse laser ablation of dental tissue. Time-resolved microscopy was chosen to observe the ablation dynamics of mammoth ivory after irradiation with 660 fs laser pulses. The results suggest that nanoparticles might arise in the excited region. The thermal expansion of the excited material induces high pressure in the surrounding bulk tissue, generating a pressure wave. The rarefaction wave behind this pressure wave causes spallation, leading to ejection of microparticles.