Ablation Hole Research Articles

Numerical study of multi-phase flow in ultrashort pulse laser processing

Ultrashort pulse laser processing is highly valued due to its enormous potential for high micromachining quality while it comprises complicated physical mechanisms especially when the pulse width falls in the regime about 10 ps. This study presents a complete mathematical modeling on multi-phase phenomena in ultrashort pulse laser processing, such as melting, vaporization, and solidification. The volume of fluid (VOF) technique is used to capture the interface during phase transition. Based on this hybrid model, the transport of energy and mass are investigated to study the influence of laser parameters on the formation of laser ablation hole. This work develops the incompressible multiphase laser ablation solver based on PIMPLE algorithm. The calculation results are validated by different references under different laser processing parameters which emphasize the importance of mechanism on thermal ablation and hydrodynamics in picosecond laser processing. More calculation results about the trends in hole depth over pulse laser width, laser fluence, and the number of pulses show the intricate influence of laser parameters.

Damage mechanism and bending properties degradation of aviation composite honeycomb sandwich structures under laser irradiation

Damage mechanism and bending properties degradation of aviation composite honeycomb sandwich structures are studied under different laser conditions in this paper. Firstly, the damage test of composite honeycomb sandwich structures is conducted. Temperature field and ablation process during laser irradiation are analyzed. Damage morphology and mechanism are verified. Secondly, the influence of laser damage on the bending properties of composite honeycomb panels is studied. Progressive damage process and failure mechanism of honeycomb structures during loading are explored by digital image correlation (DIC) and strain trend. The results show that the temperature at the center point of front surface reaches more than 2000 °C within 1 s under the laser irradiation, while the temperature of back surface exhibits hysteresis. The ablation hole is funnel-shaped and has severe delamination. In the heat affected zone (HAZ), resin matrix has been basically sublimated and oxidized, leaving only carbon fiber bundles and some residual carbon. The honeycomb core in this area has been completely carbonized and deformed. During bending process, the honeycomb core breaks firstly, and strain appears peak at both crack ends, leading the crack extend along both sides. The ultimate load of damaged specimen is 2.35kN, which has a 42.26 % drop compared to that of healthy specimen 4.07kN. Bending properties of specimen are significantly degraded by laser damage.

Processing characteristics and ablation mechanism of CMC-SiCf/SiC by rotational thirteen-beam coupling nanosecond laser

Silicon carbide fiber reinforced silicon carbide ceramic matrix composite (CMC SiCf/SiC) has the advantages of high temperature resistance, corrosion resistance, high strength, low density, and oxidation resistance. It is a high-performance advanced material in aerospace, energy and other fields. However, due to its high hardness, brittleness and anisotropy of fiber structure, the research on application-oriented high-quality processing technology has become a hot issue. In this paper, a new process of rotational thirteen-beam coupling nanosecond laser processing of CMC-SiCf/SiC was studied. The rotational thirteen-beam coupling ablation experiments were carried out on CMC-SiCf/SiC workpiece. A laser beam is divided into thirteen beams and rotated around the beam center by using a beam splitter element. The morphology, structure and composition of ablation holes at different speeds were observed and analyzed by laser confocal microscope, scanning electron microscope, and X-ray energy dispersive spectrometer. It is found that the characteristics of rotational beam coupling have a significant effect on the laser ablation morphologies. The results show that the ablation region of CMC-SiCf/SiC is elliptical due to the fast heat transfer of fibers. The energy density distribution of the waist section of the Gaussian like beam after beam coupling is similar to the “W" shape, and the central energy is lower than the surrounding energy, and the ablation holes also show a similar “W" shape. With the increase of rotational speed, the hole depth first decreased and then increased, the hole diameter and roundness first increased and then decreased, and the hole section was “W" shaped. The ablation area can be divided into five parts: the white granular SiO2 solid coverage area, the recast layer, the lower recast layer, the molten layer, and the heat affected zone. The rotational characteristics will make the reaction intensity of the whole ablation area more uniform in the circumferential direction. The formation mechanism and element distribution of each part were explored. The research work in this paper will provide a new technical way for the high-performance application of CMC-SiCf/SiC.

Performance optimization of PERC solar cells based on laser ablation forming local contact on the rear

Abstract To improve the photoelectric conversion efficiency (η) of the solar cell, a green wavelength (532 nm) laser source in a nanosecond range was used to ablate the passivated emitter and rear cell (PERC) to form the contact holes. If the laser ablation hole opening process was not set properly, the diameter or the external expansion of holes would be too large, causing the decline of the PERC performance. The Gaussian distribution of the laser is regulated by the output power (P o) and the repetition frequency (f rep) of the incident pulse laser, so that the optimized morphology of holes is obtained on the back of the PERC solar cells. After the contact holes are screen printed by the aluminum paste, the local back surface field is finally formed. The experimental results showed that the outward expansion decreases obviously with the increase of laser P o. Second, the spacing of the holes decreases with the increase of the laser f rep. It was found that under the laser P o of 33.0 W and f rep of 1,400 kHz, the η of the industrial PERC solar cells was the highest. The Quokka simulations indicated that small outward expansion, small diameter, and long spacing of holes would further decrease the recombination parameter in the rear surface. With the optimized morphology of contact holes and the low contact resistance, the PERC cell’s calculated V oc and η improvements were 6.5 mV and 0.48%, respectively, which was verified with experimental findings.

Assessment of continuous laser ablation model for lightweight quartz fiber reinforced phenolic composite

AbstractA comprehensive investigation into the interaction between a continuous wave (CW) laser and lightweight quartz fiber reinforced phenolic (LQFRP) composite would provide valuable insights into the characteristics and performance of ablation. To simulate the CW laser ablation of LQFRP composite, a 2D symmetrical element model was developed, incorporating the heat conduction equation, resin pyrolysis mechanism, and deformed geometry module. This model was the first to examine the internal pyrolysis degree and gas flow field of LQFRP composite under CW laser irradiation, and its predictions were in reasonable agreement with experimental measurements. Additionally, the model was used to simulate CW laser ablation with varying spot sizes and power densities. The results revealed that during the laser ablation process, an internal high‐pressure zone formed on the side of the ablation hole. The volatilization of SiO2 was identified as the dominant dissipation mechanism for material removal. The width of the ablation hole was positively correlated with the laser spot size, while the depth of the ablation hole was positively correlated with the laser power density.Highlights The model first studied the internal pyrolysis degree and flow field of LQFRP composite under laser irradiation. The volatilization of SiO2 was identified as the dominant dissipation mechanism for material removal. An internal high‐pressure zone formed on the side of the ablation hole during the laser ablation process. The width of the ablation hole was positively correlated with the laser spot size, while the depth of the ablation hole was positively correlated with the laser power density.

Laser nanoprocessing via an enhanced longitudinal electric field of a radially polarized beam.

Single-shot laser ablation is performed on the surface of a transparent glass material using a radially polarized femtosecond beam. Theoretical and experimental investigation revealed the significant role of the material interface under high-numerical-aperture conditions. The longitudinal electric field at the focus was remarkably enhanced due to the total reflection on the interface when a radially polarized beam was focused on the back surface of the glass from the inside using an immersion lens. This focusing condition enabled the fabrication of a small ablation hole sized 67 nm. This study offers a novel, to the best of our knowledge, approach to realize laser nanoprocessing with radially polarized beams.

Microstructure and properties of Ti60 alloy by laser solid forming and ultrasonic impact hybrid manufacturing

The hybrid manufacturing of laser solid forming (LSF) and ultrasonic impact treatment (UIT) was used to improve the structure and properties of Ti60 alloy by strengthening the surface layer-by-layer. The microstructure, mechanical properties, and burn resistance of LSFed Ti60 samples before and after UIT by SEM, TEM, and XRD were analyzed and compared. The results showed that the grains of LSFed Ti60 alloy were refined and metallurgical defects were reduced after the UIT process. Meanwhile, the deposited layer was severely deformed after UIT and nanocrystals were formed on the surface. In addition, the microhardness of the deposition layer surface increased and the residual stress changed from tensile stress to compressive stress. The area of the ablation hole was reduced by 21% and the proportions of Mo, Nb, and Zr elements increased. In summary, hybrid manufacturing of LSF and UIT can improve the microstructure and properties of LSFed Ti60 samples.

Ablation holes in tape targets induced by ultra-intense laser pulses

We present a theoretical modelling able to predict the dimensions of mm-sized through-holes produced in interactions of the 1PW high-power 30fs Ti:Sa laser VEGA-3 with tape targets. We find that sizes of through-holes can be calculated by assuming that the full heat transferred from the laser-heated electron population to the target by electron–electron collisions drives the evaporation of target material. We demonstrate the good reproduction of experimental results.

Study on deformations of gold film induced by ultrafast laser at GHz burst mode

Deformation structure is very common in ultrafast laser processing metal film, but it is undesirable in some application scenarios. In this study, ultrafast laser at burst mode is used to suppress the deformation of metal film induced by ultrafast laser. The experimental research of 40 nm gold film irradiated by ultrafast laser at GHz burst mode has been carried out, with different numbers of sub-pulses from 1 to 25. When the burst fluence is lower than the ablation threshold, the height of bump decreases with the number of sub-pulses increasing in the burst and Molecular Dynamics (MD) simulations are carried out to explore the mechanism of deformation behaviors. While the burst fluence is higher than the ablation threshold, the deformation behaviors around the ablation hole also change interestingly with different number of sub-pulses, which can be divided into three stages. The increase of width of burrs, the decrease of driving force and the molten film left at edge of ablation hole, induced by the number of sub-pulses increasing, cause different deformation behaviors of gold film, respectively.

Investigation on Ablation Characteristics and Material Removal Mechanism of Si3N4 Ceramics by a New Type of Coupled Laser Beam

Due to the excellent comprehensive properties, Si3N4 ceramics have been widely studied and applied in the field of engineering structures. Conventional processing of Si3N4 ceramics faces a series of challenges, and laser processing has become an effective processing technology for hard and brittle materials. In this paper, a new type of dual-beam coupled nanosecond pulsed laser was used to carry out a series of ablation experiments on Si3N4 ceramic at various powers, and the laser ablation mechanism was analyzed. The equivalent diameter method and the equivalent area method were used to calculate the corresponding laser ablation threshold, and influence of laser power on surface roughness of the ablation hole edge, hole depth, removal volume and hole taper were explored. The results show that when the repetition frequency f = 10 kHz, wavelength λ = 532 nm and ablation time t = 6 s, the laser ablation thresholds of Si3N4 ceramic calculated by the equivalent diameter method and the equivalent area method are 0.55 J/cm2 and 0.59 J/cm2, respectively. In a certain power range, surface roughness of the ablation hole edge, hole depth and removal volume increase with the increase of laser power, while the ablation hole taper decreases with the increasing laser power. This innovative work is helpful to expand the scope of laser processing technology for hard and brittle materials, and has guiding significance for the optimization of laser processing technology parameters of Si3N4 ceramics.

Research on the ablation mechanism and ablation threshold of CMC-SiCf/SiC by using dual-beam coupling nanosecond laser

Due to the excellent properties of high hardness, oxidation resistance and high temperature resistance, silicon carbide fiber silicon carbide ceramic matrix composite (CMC-SiC f /SiC) is a typical difficult-to-process material, and is a high-performance advanced material in the aerospace field. In this paper, two groups of ablation experiments (experiment 1 and experiment 2) were performed on CMC-SiC f /SiC using a dual-beam coupling nanosecond laser, and the ablation morphology was observed by confocal laser microscope. The dual-beam coupling angle of experiment 2 is obtained by experimental method. And through the method of calculation, we get the dual-beam coupling angle of experiment 1 and experiment 2. According to the dual-beam coupling ablation mechanism, based on the theoretical calculation model of non-destructive method D 2 -ln P 0 , combined with the Equivalent Diameter Calculation Method (EDCM) and Equivalent Area Calculation Method (EACM), the laser ablation threshold corresponding to different beam waist size was calculated and compared. The results show that the ablation region of CMC-SiC f /SiC surface can be divided into three parts: ablation boundary, recast layer area and SiO 2 coverage area. When the pulse energy increases gradually from 300 μJ to 1500 μJ, the variation trend of hole depth is first increase, second decrease, increase again, and finally decrease. The angle between two laser beams affects the waist radius, which in turn affect the laser ablation threshold. The waist of the dual-beam coupling is elliptical, and the orifice of the ablation hole is elliptical. When the waist radius of nanosecond laser is 57 μm, the laser ablation threshold is calculated to be 3.12 J/cm 2 . The main factors affecting the laser ablation threshold are laser pulse repetition frequency ( f ), beam waist radius ( ω 0 ), laser pulse width ( τ ), minimum laser power ( P th ), and laser wavelength ( λ ).

Numerical and experimental analysis of nanosecond laser ablation of SiC

This work presents a fundamental study on the melting threshold, ablation morphology and removal mechanism of single-crystal SiC during nanosecond laser processing. A thermal model was established to investigate the thermal behavior of single-crystal SiC at different wavelengths and pulse durations, and to predict the size and cross-sectional profile of ablation hole. The phase transitions and thermophysical parameters variation of silicon carbide with temperature were considered in the numerical analysis. To verify the reliability of the established model, the simulation data were compared with experimentally measured ablation depth under the same operating parameters. The investigation results indicated that the melting threshold of crystalline material increased with the increase of wavelength under the studied condition and was approximately linear distribution. Surface evaporation should be the dominant removal mechanism for nanosecond laser pulse ablation of SiC. By comparing the diameter and depth of ablated hole obtained from experiment and simulation calculation, it is found that the experimental/modeling coupling method had good consistency. Therefore, the established model can accurately simulate the interaction process between nanosecond laser pulse and silicon carbide.

Patterning the surface structure of transparent hard-brittle material β-Ga2O3 by ultrashort pulse laser

Femtosecond laser direct writing technology offers a powerful and advantageous tool for precise three-dimensional (3D) micro/nanofabrication of transparent hard and brittle materials. In this paper, micro-craters, micro/nano-rectangular columns, and microchannels structures with different spatial characteristics are achieved after irradiation of crystalline (100) β-Ga2O3 via an optical-fiber femtosecond laser (515/1030 nm, 285 fs, 100 kHz) with different processing parameters in air ambient. Laser scanning confocal microscope, atomic force microscopy, and Raman spectroscopy are employed to characterize the roughness and constitutive property of the surface structure under tightly focused laser conditions. The influences of different pulse numbers and different pulse energies on the morphology of the pits were investigated. Through precise control and optimization of processing parameters, the formation of cracks and chipping can be effectively avoided. Crack-free ablation holes with smooth edges were fabricated under low-energy (E = 22 nJ) single-pulse conditions. Furthermore, the effects of different laser processing parameters (single-pulse scanning interval, pulse number and scanning repetition times) on the microchannel morphology were also explored. From this work, high-precision micro-hole arrays, micro-nano rectangular column arrays, and microchannels were successfully fabricated on (100) β-Ga2O3 substrates. Due to the interaction of single-pulse femtosecond laser with β-Ga2O3 materials, this fast fabrication method of 3D submicron structures has potential applications in sensing, photonics, or electronics.

Crack-free femtosecond laser processing of lithium niobate benefited by high substrate temperature

Femtosecond lasers (fs-lasers) offer a powerful and advantageous tool for fabricating a very large variety of materials. When processing transparent dielectrics, structural defects, such as cracks and broken edges, are always present, thus restricting the precision of fs-laser processing. In this paper, the formation and suppression mechanism of fs-laser induced structural defects are systematically studied. We demonstrate a novel method to improve the processing precision of lithium niobate (LiNbO3) by elevating the substrate temperature. A crack-free ablation hole with a smooth edge was fabricated at the substrate temperature of 1000 °C. Our results show that the increase of absorptivity and the suppression of incubation effects are responsible for the high precision processing at a high substrate temperature, which not only inhibits the formation of defects, but also additionally increases the efficiency of fs-laser processing. This work provides a simple method to efficiently suppress the defect formation induced by fs-laser in LiNbO3 samples, paving the way for a new technique for high precision fs-laser processing.

Removal mechanism of 2.5-dimensional carbon fiber reinforced ceramic matrix composites processed by nanosecond laser

C/SiC composite material is widely used in aerospace fields because of its excellent properties; however, it is difficult to be removed and processed. In this paper, the 2.5-dimensional C/SiC composite material was ablated by nanosecond laser to explore the laser removal mechanism. Firstly, the laser ablation experiment was carried out to investigate the gradual process of ablation morphology with the increasing laser power density. Secondly, the characteristics of ablation holes were analyzed, and the effects of laser processing parameters and fiber orientation on the ablation morphology and size were studied. In addition, the ablation morphology of needle-punched fiber area was analyzed to expound the unique ablation removal characteristics of 2.5-dimensional C/SiC composites. At last, ablation removal mechanism of 2.5-dimensional C/SiC composites was summarized, and the model of ablation removal process was established. The results indicated that ablation morphology was evolved from nothing to ablation mark, to fiber expansion, to fiber shrinkage and tension, to resolidification, to removal holes, and to increasing holes with laser power density. Besides, the ablation morphology in needle-punched fiber area was different from elliptic ablation morphology in the 0° or 90° fiber area, the ablation holes in the needle-punched fiber area were smaller and deeper, and the ablation holes were approximately circular. The results would provide theoretical basis and technical support for processing functionalized structure of 2.5-dimensional C/SiC composites by nanosecond laser, such as blind holes, grooves, and square holes.

Microprocessing on Single Protein Crystals Using Femtosecond Pulse Laser.

Proteins with different micropatterns have various applications in biosensing, structural analysis, and other biomedical fields. However, processing of micropatterns on single protein crystals remains a challenge due to the fragility of protein molecules. In this work, we studied femtosecond laser processing on single hen egg white lysozyme protein crystals. Optimized laser parameters were found to achieve micropatterning without cracking of protein crystals. The ablation morphology dependence on the laser fluence and the pulse number was discussed to control the processing results. Under a laser fluence higher than 1 J/cm2, the ablation hole was formed. While multipulses with fluence lower than the ablation threshold were applied, the foaming area was observed due to the denaturation of protein. The numerical simulation shows that the ablation results were influenced by the ionization and energy deposition process. Micropatterns including lines, areas, and microarrays can be processed with a minimum size of 2 μm. Processed patterns on the crystal surface can be used for biosensing microarrays and the enhancement of crystal growth. The microprocessing method proposed in this study has potential applications in different fields including biodevices and biomedicine.

Laser ablative behavior of C/C modified by Si reactive infiltration

Carbon fiber-reinforced carbon matrix (C/C) composites were prepared by chemical vapor infiltration, and then they were subjected to silicon (Si) reactive melt infiltration to obtain C/C–SiC–Si composites. The ablation behavior of these two types of composites was evaluated by high-energy (500w) carbon dioxide laser irradiation under argon atmosphere. The depths and three-dimensional (3D) profiles of laser ablation holes were measured by laser confocal microscopy. The results showed that the depth of ablation hole of C/C composites gradually increased, and the ablation rate decreased with ablation time. The temperature distribution on C/C composite surface was simulated by 3D finite element analysis. The C/C–SiC–Si composites exhibited a rapid ablation rate during the ablation process within 7 s; however, no further ablation damage on carbon matrix was observed after 7 s. Si was found to dissipate energy through phase change, transpiration cooling, and smog attenuation. A laser ablation model was proposed based on the energy dissipation mechanism. C/C–SiC–Si composite showed better performance of laser ablation resistance in 100 s than pure C/C composite.

Ablation and Patterning of Carbon Nanotube Film by Femtosecond Laser Irradiation

Carbon nanotube (CNT) film can be used as thin film electrodes and wearable electronic devices due to their excellent mechanical and electrical properties. The femtosecond laser has the characteristics of an ultra-short pulse duration and an ultra-high peak power, and it is one of the most suitable methods for film material processing. The ablation and patterning of CNT film are performed by a femtosecond laser with different parameters. An ablation threshold of 25 mJ/cm2 was obtained by investigating the effects of laser pulse energy and pulse number on ablation holes. Raman spectroscopy and scanning electron microscope (SEM) were used to characterize the performance of the pattern groove. The results show that the oligomer in the CNT film was removed by the laser ablation, resulting in an increase in Raman G band intensity. As the laser increased, the ablation of the CNTs was caused by the energy of photons interacting with laser-induced thermal elasticity when the pulse energy was increased enough to destroy the carbon–carbon bonds between different carbon atoms. Impurities and amorphous carbon were found at and near the cut edge while laser cutting at high energy, and considerable distortion and tensile was produced on the edge of the CNTs’ groove. Furthermore, appropriate cutting parameters were obtained without introducing defects and damage to the substrate, which provides a practical method applied to large-area patterning machining of CNT film.

Ablation morphology and redistribution layer of gold films with different substrates irradiated by femtosecond laser pulse

Gold micro/nanostructure is of great significance in many scientific and engineering fields for its unique optical, electrical and thermal properties. Gold film deposited through electron beam (EB) evaporation is a suitable raw material for the fabrication of gold micro/nanostructure. Femtosecond laser directing is one of the reported methods for high-efficiency and low-cost micro/nanofabrication. We present a comparative study of gold film ablation with different substrates (Si, SiO2, ZnO) under the irradiation of single femtosecond Gaussian pulse. The morphologies of ablation areas and redistribution layers are investigated by many characterization methods, such as scanning electron microscopy (SEM) and atomic force microscopy (AFM). In general, the ablation morphology and the ablation hole are mainly affected by the bandgap. Besides, the heat conductivity is the main factor affecting the width and height of the redistribution layer.

Surface treeing and segmented worm model of tracking behavior in MgO/Epoxy nanocomposites

Surface tracking experiments were conducted on MgO/epoxy nanocomposites according to IEC60112. Treeing phenomenon, which was commonly observed in bulk insulators, was detected on the surface of tracked samples via optical microscopy. Tree branches were further found to be segmented tracking channels by scanning electron microscopy, which consisted of a series of ablation holes along electric field, i.e. wormholes. According to distribution of surface trees, tracking area was divided into three typical regions. Region A, close to electrodes, was barely ablated. Region B, which directly connects electrodes, exhibited paralleled tracking channels. Region C, which is the outer annular ablated region, is full of crossed tracking channels. At the beginning, the initial tracking channels were demonstrated to appear at the triple junction of Region A, B and C, where electric field was supposed to be the highest. Then tracking regions spread from the junction to Region B and Region C with tracking channels segmentally growing longer and wider, indicating a segmented energy accumulation process. Tracking trees were finally formed connecting two electrodes, leading to sample failure. With addition of nano-MgO particles, wide and deep channels turned into slight but dense ones. Correspondingly, comparative tracking index (CTI) increased from 400 to 475 V with increasing nano-MgO from 0 to 10 wt%. Improved tracking resistance was obtained in nanocomposites while reduced thermal conductivity and pyrolysis characteristics were observed. Combining with results above, a segmented worm model was thus proposed to explain surface tracking performance in MgO/epoxy nanocomposites.