Multi-pulse Laser Ablation Research Articles

Multi-pulse femtosecond laser ablation of SiC/SiC composites: Morphological evolution, oxidation mechanisms and component characterization

SiC fiber reinforced SiC matrix (SiosC/SiC) composites processed by femtosecond laser play a pivotal role in thermal protection systems. This paper reveals the ablation characteristics of multi-pulse femtosecond laser ablation of SiC/SiC composites, including morphological evolution, oxidation mechanisms and component characterization by heat accumulation simulation and advanced experimental analysis. The results show that the ablation depth induced by the heat accumulation effect is an exponential function of laser parameters. Morphological analysis and component characterization indicate that two types of ablation regions are formed, normal ablation region is an oxidized zone induced by passive oxidation mechanism, and central ablation region is a non-oxidized zone induced by active oxidation mechanism. The increase of single pulse energy and decrease of scanning speed enhance the driving force of chemical reactions, thus dramatically affecting the gas hole and graphite properties in the normal ablation region. Furthermore, the propagation path of thermal stress-induced cracks is determined which emerges from the center ablation region, extending through gas holes to the normal ablation region.

Subpicosecond laser ablation behavior of a magnesium target and crater evolution: Molecular dynamics study and experimental validation

Abstract The micro-ablation processes and morphological evolution of ablative craters on single-crystal magnesium under subpicosecond laser irradiation are investigated using molecular dynamics (MD) simulations and experiments. The simulation results exhibit that the main failure mode of single-crystal Mg film irradiated by a low fluence and long pulse width laser is the ejection of surface atoms, which has laser-induced high stress. However, under high fluence and short pulse width laser irradiation, the main damage mechanism is nucleation fracture caused by stress wave reflection and superposition at the bottom of the film. In addition, Mg[0001] has higher pressure sensitivity and is more prone to ablation than Mg [ 10 1 ¯ 0 ] . The evolution equation of crater depth is established using multi-pulse laser ablation simulation and verified by experiments. The results show that, under multiple pulsed laser irradiation, not only does the crater depth increase linearly with the pulse number, but also the quadratic term and constant term of the fitted crater profile curve increase linearly.

Using double pulse laser ablation in air to enhance the strength of laser-driven shocks

In the process of multi-pulse laser ablation, inter-pulse delay time, Δt, is known to be an important parameter for maximizing ablation efficiency as well as impulse imparted to the target. In this work, using photon Doppler velocimetry, we show that for single pairs of colinear pulses (1064 nm, 8 ns, ∼ 60 J cm-2 per pulse) in air, the peak free surface velocity of the back surface of an aluminum target (125 µm thick) is increased, by a factor of nearly 3, when Δt = 10 microseconds, compared with both pulses arriving simultaneously (Δt = 0). Fast imaging of the ablation process suggests this enhancement is due to rarefaction of the contiguous air in the passage of the leading shock produced by ablation, which then in turn allows a larger fraction of the energy of the second pulse to reach the target surface. This interpretation is strengthened by additional experiments in which the two pulses do not overlap on the target surface, but the shock strength is nevertheless enhanced. Given a fixed energy budget this work suggests a prescription for maximizing laser-driven shock strength by judicious choice of inter-pulse delay.

Experimental research on micro-drilling of refractory material tungsten by multi-pulse femtosecond laser ablation

Experimental research on micro-drilling of refractory material tungsten by multi-pulse femtosecond laser ablation

Parametric optimization of hole taper control in ultraviolet nanosecond laser micro-drilling of copper foil

While hole taper is inevitably formed in the laser micro-drilling of through micro-holes, the formation propensity is largely determined by the interaction between laser processing parameters. In the present work, we perform numerical simulations and experiments to investigate the ultraviolet nanosecond laser micro-drilling of 1 mm diameter through micro-holes on copper foil with a thickness of 280 μm, with an emphasis on the coupled influence of laser processing parameters on both the hole taper and circularity. Firstly, the characteristics of formed micro-hole quality are experimentally characterized, and the formation mechanisms of hole taper are further revealed by finite element simulations of multi-pulse laser ablation. Then, the relational degree of individual laser processing parameters in determining the hole taper and circularity is discovered by experimental investigation based on the gray relational analysis model. Finally, the coupled effect between significant parameters of laser power and repeat scanning number on the hole taper and circularity is further investigated by utilizing the response surface methodology, which derives the optimum laser processing parameters for manufacturing 1 mm diameter through micro-hole with a taper degree of 2.556° and a circularity of 0.992 on the copper foil.

Experimental and numerical study of multi-pulse picosecond laser ablation on 316 L stainless steel.

An experimental and numerical study on 10 ps laser ablation of 316 L stainless steel up to 400 hundred pulse exposure has been carried out. In this simulation, the material removal threshold temperature has been carefully discussed depending on the different ablation driving mechanisms. The influence of the instantaneous material removal has also been considered which will affect the calculation of the next pulse's absorption. For single-pulse ablation, the simulated ablation threshold Fsim = 0.26 J/cm2 is close to the fitted experimental result F0th = (0.29 ± 0.01) J/cm2. For multi-pulse ablation, the simulated ablation rate Rsim = 11.4 nm/pulse is close to the fitted experimental result Rexp = (12.4 ± 0.1) nm/pulse under 0.9 J/cm2 fluence, while the simulated ablation rate Rsim = 19.8 nm/pulse is slightly larger than the fitted experimental result Rexp = (16.1 ± 0.7) nm/pulse at 2.7 J/cm2, providing good agreement between theory and experiment for both single and multi-pulse ablation. This study could be used to predict the multi-pulse laser processing performance, especially with the help of a machine learning method to find the best parameters automatically.

Investigations on the oxidation behavior and removal mechanism of SiC/SiC composites by multi-pulse femtosecond laser ablation

SiC/SiC (SiC fiber-reinforced SiC matrix) composites have superior performance, making them promising candidates as one of the best materials for high-temperature components in aerospace applications. This investigation analyzed the morphology, structure, composition, and plasma state of multi-pulse femtosecond laser ablated SiC/SiC composites from multiple perspectives using various static and in situ characterization methods. Further, the oxidation behavior and removal mechanism of SiC/SiC composites were studied qualitatively and quantitatively. The results indicated that the ablation products of multi-pulse femtosecond laser ablation of SiC/SiC composites were mainly SiO2 and plasma deposits. The heat accumulation effect and oxidation degree at the ablation locations were enhanced accordingly as the effective ablation number or energy density increased, and the main shape of the products also changed regularly. At the same time, the material removal mechanism changed from non-thermal removal dominated to the equilibrium stage and then to thermal removal dominated. The detailed analysis of the oxidation behavior of each component in SiC/SiC composites enabled the controlled ablation of femtosecond lasers targeting the oxidation process of SiC/SiC composites. Besides, 1 MPa N2 could improve the processing efficiency of femtosecond laser for the depth and width of the groove by a maximum of about 90.40% and 104.02% in this study, respectively. Ultimately, the range of laser process parameters corresponding to multi-pulse femtosecond laser ablation of SiC/SiC composites without thermal removal as dominant was determined.

Numerical simulation and investigation of ultra-short pulse laser ablation on Ti6Al4V and stainless steel

In ultra-short pulse laser machining and micro/surface processing, accurate simulation of laser ablation is important for understanding laser-target interaction and improving ablation performance, but it remains challenging. This work aims to develop a numerical model to improve the accuracy of the ablation depth calculation. A grid deformation scheme is proposed based on energy conservation and considering contributions to instant material removal from both the electron and lattice subsystems. By incorporating this scheme with the two-temperature model (TTM), a reasonable prediction of the instant target surface profile during laser ablation has been achieved. In the case of single-pulse femtosecond laser ablation of Ti6Al4V, the calculated ablation depth ranges from 0.06 to 0.56 μm for laser energy from 1.0 to 10.0 μJ. For single-pulse picosecond laser ablation of stainless steel, as laser energy increases from 6.0 to 18.5 μJ, the predicted ablation crater deepens accordingly from 40 to 87 nm. In addition, for multi-pulse picosecond laser ablation of stainless steel, a linear dependence of the ablation depth on the pulse number is observed up to a depth of about 803 nm at 6.0 μJ and 20 pulses. In all the above-mentioned cases, the calculation results are in better agreement with experimental measurements than conventional TTM or other material removal schemes, validating the accuracy of the proposed model.

Investigation of through micropatterns fabrication on C194 copper foil by ultraviolet nanosecond pulsed laser microdrilling

While the lead frames of C194 copper alloy widely used in IC packages contain high density of through micropatterns, the method for realizing high precision manufacturing of the micropatterns on copper foil with thickness down to a few hundred micrometers is highly demanded. In the present work, we demonstrate the feasibility of manufacturing high density through micropatterns on C194 copper foil with high precision and high uniformity by using ultraviolet nanosecond pulsed laser microdrilling. Specifically, the absorption coefficient of C194 copper foil for 355 nm wavelength is experimentally derived, based on which the sublimation enthalpy of the material is calibrated by iteratively comparing predicted crater morphology by finite element simulations with experimental data. Then the multi-pulse laser ablation mechanisms of C194 copper foil are investigated jointly by finite element simulations and experiments, which derive the quantitative correlation of ablation crater depth with laser ablation parameters. Finally, high density arrays of through micropatterns with high dimensional accuracy and high uniformity are fabricated on C194 copper foil with a thickness of 100 μm by the proposed ultraviolet nanosecond pulsed laser microdrilling with theoretically predicted repeat scanning number. Furthermore, the effect of pickling on surface quality and resistance of the manufactured copper lead frame is addressed. This work provides a feasible method for manufacturing copper alloy lead frames with high precision through micropatterns in an environmentally friendly manner.

Threshold fluence and incubation during multi-pulse ultrafast laser ablation of quartz.

In this work, the incubation effect on the laser ablation threshold of quartz, after multi-shot irradiation with femtosecond pulses at 1030-nm-wavelength with different repetition rates, was investigated. A strong decrease of the multi-pulse ablation threshold with the number of pulses N was found due to incubation. Moreover, the influence of the repetition rate was negligible in the investigated frequency range which went from 0.06 to 200 kHz. A saturation of the threshold fluence value was observed at number of pulses N > 100 which has been found to be well fitted by an exponential incubation model. Using such model, we estimated the single-pulse ablation threshold value and the incubation coefficient for quartz, which were found equal to Fth,1 = 6.23 ± 0.23 J/cm2 and k = 0.058 ± 0.004.

Spatiotemporal evolution of the morphology of multi-pulse laser ablated metals considering plasma shielding

With the laser ablation of metals, ultrafast lasers have high peak power density and significant nonlinear absorption, but plasma shielding and large taper often exist during ablation, which seriously affects the quality and efficiency of ablation. In this paper, the heat conduction equation of the lattice system is rewritten into the dual-temperature model, the time and space terms in the femtosecond laser source equation are superimposed to calculate, and the plasma shielding effect is incorporated into the ablation model using multi-pulse laser ablation iterative calculations. The constructed 3D improved dual-temperature model uses the finite difference method to investigate the spatio-temporal evolution of the ablation morphology of the metal target under the influence of different laser parameters using the critical point phase separation mechanism. In the numerical simulation, the error of considering plasma shielding is controlled within 8.24% compared with that of not considering plasma shielding, the ablation process has obvious layering phenomenon, the actual ablation experimental results are basically consistent with the calculation results of the proposed model, and the prediction error of the ablation depth can be controlled within 13.28%, which indicates that the model proposed in this paper has the ability to more accurately describe the spatial and temporal evolution of metal ablation by femtosecond laser.

TEA CO2 Laser – Polymethyl Methacrylate Interaction: LIBS Hydrogen Analysis

The interaction of a Transversely Excited Atmospheric (TEA) CO2 laser with a polymer polymethyl methacrylate (PMMA) sample in a vacuum ambiance was studied. The main goal was to demonstrate the feasibility of laser-induced breakdown spectroscopy (LIBS) to detect hydrogen. The generation of plasma over the PMMA surface, using a low laser intensity of ∼ 48 MW/cm2 and fluence of ∼ 16.5 J/cm2, required the application of a metal sub-target. Besides hydrogen, the recorded spectra were dominated by atomic lines of carbon and oxygen and band emission of the C2 and CN molecules. The electron number density and temperature (ionic, vibrational, and rotational) were evaluated to characterize the laser-induced plasma. In addition, PMMA micro-damages (diameter ∼ 45 µm) created by a multipulse laser ablation could find potential applications in sensor technologies.

Heat accumulation and surface roughness evolution in CO2 nanosecond laser ablation of quartz glass

Quartz glass is frequently used for optical components, but traditional machining methods, particularly for aspherical optics, are often associated with long process times and high costs. One possible approach to achieve higher efficiencies and higher speeds in contactless processing is laser ablation using CO2 laser radiation. This study investigates the evolution of surface topography during laser ablation of quartz glass using nanosecond pulses from a Q-switched CO2 laser beam source. The special focus of a broad empirical study lay on ablation depth, ablation rate, ablation efficiency and resulting surface roughness for an ablation process using 300 ns laser pulses at a maximum laser power of 115 W and pulse frequencies up to 150 kHz. Ablation efficiency shows a characteristic logarithmic dependency on laser fluence. It was revealed that local heat accumulation significantly affects the optical penetration depth, which reduces the threshold fluence for multi-pulse laser ablation down to approx. 0.37 J/cm2. Ablation rates of up to 1.48 mm3 min−1 W−1 were achieved, which exceeds results achieved for laser ablation using cw laser radiation or ultra-short pulses. An ablation rate of up to 170 mm3/min is a particularly noteworthy result. Therein, a maximum ablation efficiency of approx. 64% shows that the largest part of the incident laser energy was used for material ablation. Finally, based on a simplified model, numerical calculations and experimental results, pulse stability was identified to have a decisive impact on the evolution of the resulting surface roughness. The resulting surface roughness Sa is typically increased in pulsed laser processing and depends linearly on standard deviation of ablation depth and the square root of the number of ablation layers.

Characterization of microcraters fabricated on the silicon surface by single and multi-pulse laser ablation at various laser intensities

Microcraters on the silicon surface were fabricated with the help of a nanosecond pulsed laser. The features of microcraters fabricated with single laser pulse and pulse trains of various laser intensities were analyzed. In case of single pulse, the diameter and depth of micro-crater rise with the laser fluence and achieve saturation at higher laser intensities. For the range of fluence 62–122 J/cm2, the mass of ablated material was in the range of 0.27 × 10−9–2.69 × 10−9 g/pulse. Numerically simulated temperature trends were compared with the measured crater depths. The influence of laser intensity on the target temperature and onset time to melt/boil were also investigated. In case of pulse trains, the reduction in material removal efficiency is attributed to the plume shielding and accumulation of ablated material on the crater walls. We demonstrated that the laser intensity and number of laser pulses can be used to control the morphology of a microcrater.

Time-resolved quantification of plasma accumulation induced by multi-pulse laser ablation using self-mixing interferometry

In this work a method based on self-mixing interferometry (SMI) is presented for probing the concentration of plasma plumes induced by multi-pulse laser ablation. An analytical model is developed to interpret the single-arm interferometric signal in terms of plasma electron number density. Its time dependence follows a power-law scaling which is determined by concurrent effects of plume accumulation and propagation. The model has been applied for the experimental study of plume formation at variable laser pulse frequencies on different materials. The plume expansion dynamics has been observed with high-speed imaging, and the SMI measurements allowed for a time-resolved estimation of the electron number density. The intrinsic advantages of the SMI technique in terms of robustness and low intrusiveness would allow for its usage as a fast diagnostic tool for the dynamical scaling of laser-induced plumes. Moreover it can be easily applied in laser-based manufacturing technologies where plasma concentration monitoring and control is important.

Ultraviolet laser cleaning and surface characterization of AH36 steel for rust removal

Laser cleaning of rusted surfaces has broad applications in marine ship industrial manufacturing. An ultraviolet nanosecond laser cleaning method was proposed to strip the rust layer from the surface of the AH36 steel to reduce surface roughness and increase corrosion resistance. Multi-pulse laser ablation was established to determine the single-pulse threshold fluence and pulse incubation coefficient of the substrate. Based on the experimental results, the surface cleaned at a fluence of 3.17 J/cm2 has the best combination of corrosion resistance and roughness. The chemical composition and microstructure of the sample surface were measured via x-ray photoelectron spectroscopy and scanning electron microscopy. Moreover, the corrosion resistance of the samples was evaluated with electrochemical measuring. The results showed that the laser-cleaned surface has approximately five times greater corrosion resistance than the rusted surface. The proposed method is an environmentally friendly approach that effectively increases corrosion resistance, which may subsequently extend the service lifetime of a ship. Hence, this study may contribute to the development of laser manufacturing.

Ultrafast laser ablation of copper by GHz bursts

The ablation of copper, using a 10 GHz burst ultrafast laser with a wavelength of 1030 nm, a pulse duration of 1 ps, a variable total laser fluence, and a number of sub-pulses per burst, is investigated theoretically. A two-temperature model with an extended Lorentz–Drude model for dynamic optical properties is used to simulate the melting and ablation process. Due to the heat accumulation from the preceding pulses, multipulse laser ablation could be advantageous over a single pulse. Moreover, the ablation performance can be maximized by properly selecting the pulse number, separation time, and energy in a laser burst. The numerical result shows that the present prediction is in fairly agreement with existing experimental result. Under the same total laser fluence of 32 J/cm2, a 10 GHz burst laser with an optimized 128 sub-pulse can significantly enhance the ablation depth, 4.2 times that a single pulse does. It is found that the optimized ablation depth is a linear function of the total fluence of ultrafast laser bursts.

Nanosecond Laser Ablation of Ti–6Al–4V under Different Temperature

Multi-pulse nanosecond laser ablation of Ti–6Al–4V is a complex process. In this study, the effect of substrate temperature on the nanosecond laser ablation of Ti–6Al–4V was investigated. Morphology, diameter and depth of ablation craters were observed; ablation efficiency ω (μm3/mJ) was proposed to analyzes the ablation process. The results showed that, with the increasing of substrate temperature, the ablation craters’ diameter increased and depth decreased, while ω initially increased, but then decreased rapidly. Furthermore, with increasing pulse number, the depth of ablation crater increased linearly, while the growth of the diameter gradually slowed down and tended to be stable after the 16th irradiation. The above changes were different in details at different substrate temperatures.

Thermal accumulation at kilohertz repetition rates inside fused silica under ultrafast laser irradiation.

Thermal accumulation effect has proved to reduce ablation threshold and improve the ablation rate during multi-pulse ultrafast laser ablation. It was widely believed that this effect cannot be triggered until the laser repetition rate is raised to the megahertz range. In this Letter, we experimentally discover strong thermal accumulation in fused silica at kilohertz repetition rates and its significant contribution to enhance ablation rate. It is found that the threshold repetition rates to trigger thermal accumulation are intrinsically determined by material thermal diffusivity and insensitive to ambient conditions. We observe two-fold enhancement of the ablation rate and clearly discriminate the contribution from thermal and non-thermal accumulation effects by 35% and 50%-70%, respectively. A multi-physics model is developed to assist the understanding of the process. This Letter promotes the fundamental understanding of thermal/non-thermal accumulation effects and opens the door to low-repetition-rate thermal accumulation for low thermal diffusivity materials.

Spectral features of colloidal solutions of elongated gold nanoparticles produced by laser ablation in aqueous solutions

The extinction spectra of colloidal solutions of gold nanoparticles produced by laser ablation in water and aqueous solutions of salts using two near-IR lasers with pulse durations of 200 ns and 1 ps are experimentally investigated. The extinction spectrum of the particles formed by ablation in aqueous solutions is characterised by enhanced optical density in the red and IR regions. This feature is due to the formation of elongated gold nanoparticles, as confirmed by transmission electron microscopy. The surface images of a gold target subjected to multipulse laser ablation exhibit micron and submicron structures.