Ablation Morphology Research Articles

The Effect of Spot Size Combination Mode on Ablation Morphology of Aluminum Alloy by Millisecond-Nanosecond Combined-Pulse Laser.

Ablation morphology affects the quality of laser processing. Therefore, the control of ablation morphology is very important. The influence of spot size combination mode on the ablation morphology of aluminum alloy is studied for the first time. Experimental results show that when the nanosecond laser spot is larger, the ablation morphology looks like a bowl-shape, and there is little solidification near the edge. When the nanosecond laser spot is smaller, the shape of the ablation morphology is similar to a hole, and the protuberance is formed near the edge of the cavity. Through the analysis and simulation of the physical model, the physical mechanism, which describes the influence of the spot size combination mode on the molten pool, is discussed. The research results of this paper have important guiding significance for the control of laser processing effect.

High-power laser resistance of filled sandwich panel with truss cores: Ablation mechanisms and numerical simulation

A new function of filled sandwich panel with truss core, superior high-power laser resistance, is reported. When filled with carbon powder-reinforced silicone resin composite, which is an ablative material, the panel exhibits superior high-power laser resistance to monolithic plate with equal areal density. This paper revealed the detailed mechanisms and reproduced the process with numerical simulation. FIIR, SEM-EDS, and TG/DSC analyses on the virgin filler material and ablation residues are conducted to investigate the thermo-physicochemical process of the filler material during the laser ablation. Considering pyrolysis, oxidation, and phase change in the laser ablation process, a 3D numerical model is developed by using the finite element method. The temperature field and ablation morphology obtained from the numerical model agree with those from the experiment. The ablation evolution process, pyrolysis effect, and laser resistance mechanism of the filled sandwich panels with truss cores are evaluated based on the present model.

Experimental study of the morphological evolution of the millisecond-nanosecond combined-pulse laser ablation of aluminum alloy.

A comparative study of the interactions of the millisecond-nanosecond combined-pulse laser and millisecond pulse laser with aluminum alloy is presented. During the interaction between the laser and aluminum alloy, the coupling effect of the nanosecond pulse laser on the molten pool formed by the millisecond laser is analyzed. During the nanosecond laser irradiation in the combined approach, the expansion velocity of the plasma plume reaches 600m/s, while the laser ablation depth increased by approximately 9times. Based on experiments, the high-speed shadow imaging method, correlation of the temperature evolution characteristics, and ablation morphology are considered for a comprehensive analysis. The coupling effect between the millisecond-nanosecond combined-pulse laser and aluminum-alloy molten pool phase is explained.

Ablation morphology and ablation threshold of Ti-6Al-4V alloy during femtosecond laser processing

The research explores the use of femtosecond laser to clean the surface of Ti-6Al-4V alloy. The laser ablation threshold was determined from either depths or diameters of laser-induced craters. Both methods are expected to give the same results; however, it was found that ablation threshold can be determined more reliably from the diameters than from the depths. This can be understood from the study of the surface morphology. As the laser fluence increased, the ablated surface became increasingly rougher, making it difficult to measure the depths accurately. In contrast, diameters of laser-induced craters could be measured with much better certainty and thus enabled the ablation threshold to be determined more accurately. The ablation threshold was found to be 0.142 ± 0.010 J/cm2 for 100 laser pulses at 130 fs pulse duration and 790 nm wavelength and it tends to decrease with increase in number of laser pulses. The single-pulse ablation threshold was determined to be 0.272 ± 0.021 J/cm2.

Selective rear side ablation of thin nickel–chromium-alloy films using ultrashort laser pulses

In recent years, the selective laser structuring from the transparent substrate side plays an increased role in thin film processing. The rear side ablation is a highly effective ablation method for thin film structuring and revels a high structuring quality. Therefore, the rear side ablation of nickel–chromium-alloy thin films on glass substrate was investigated using femtosecond laser irradiation. Single and multiple pulses ablation thresholds as well as the incubation coefficient were determined. By irradiation from the transparent substrate side at low fluences a cracking or a partly delamination of the film could be observed. By increasing the fluence the most part of the film was ablated, however, a very thin film remained at the interface of the glass substrate. This thin remaining layer could be completely ablated by two pulses. A further increase of the pulse number had no influence on the ablation morphology. The ablated film was still intact and an entire disc or fragments could be collected near the ablation area. The fragments showed no morphology change and were still in solid state.

Morphologies of femtosecond laser ablation of ITO thin films using gaussian or quasi-flat top beams for OLED repair

This study compares the ablation morphologies obtained with a femtosecond laser of both Gaussian and quasi-flat top beam profiles when applied to indium tin oxide (ITO) thin films for the purpose of OLED repair. A femtosecond laser system with a wavelength of 1030 nm and pulse duration of 190 fs is used to pattern an ITO thin film. The laser fluence is optimized for patterning at 1.38 J/cm2. The patterned ITO thin film is then evaluated through both optical microscope and atomic force microscope. Ablations with a square quasi-flat top beam are demonstrated using slits with varying x–y axes. With the Gaussian beam, the pattern width of the ablated area is shown to range from 9.17 to 9.99 μm when the number of irradiation pulse increases from one to six. In contrast, when slit control is used to obtain a quasi-flat top beam, the ablated pattern width remains constant at 10 μm, despite the increase in the number of pulse. The improved surface roughness is correlated with the quasi-flat top beam through measured Ra values. Furthermore, when using the Gaussian beam, the minimum resolution of the controllable ablation depth on the ITO thin film is found to be 60 nm. In contrast, when the quasi-flat top beam is used, the minimum ablation depth decreases to 40 nm.

High fluence nanosecond laser machining of SiCp/AA2024 composite with high pressure assistant gas

This paper presents a feasibility study on machining SiCp/AA2024 composite using nanosecond fiber laser (λ = 1064 nm) with the rated average power of 20 W and the assistance of 1 MPa Ar gas. Ablation characteristics are investigated with variation of pulse repetition rate (f = 3 ∼ 10 kHz, τl = 200 ns) and pulse width (τl = 4, 13, 20, 30, 50, 100, 200 ns, f = 3 kHz). The ablation morphology measured by scanning electron microscopy (SEM) changed from “coated layer”, “partially coated layer” to “column array” appearance with decreasing pulse repetition rate. An effective ablation of the SiCp/AA2024 composite was demonstrated at 3 kHz repetition rate. By increasing pulse width with a fixed pulse repetition rate of 3 kHz, ablation depth and ablation rate linearly increased, meanwhile, both surface roughness and recast layer thickness saturated. The rate achieved was in the range from 27 nm to 157 nm per pulse, and the resultant roughness from 6.75 μm to 20.60 μm. Ablation mechanism was analyzed from the aspects of high pressure assistant gas, high pulse fluence and high beam overlap of 99.8%. Furthermore, the morphology formation mechanism was discussed considering the competition effect between the time for ablated material escaping from cavity and the pulse interval time of “Δτl=1/f” available for ablation.

Laser effects based optimal laser parameter identifications for paint removal from metal substrate at 1064 nm: a multi-pulse model

Paint removal by laser ablation is favoured among cleaning techniques due to its high efficiency. How to predict the optimal laser parameters without producing damage to substrate still remains challenging for accurate paint stripping. On the basis of ablation morphologies and combining experiments with numerical modelling, the underlying mechanisms and the optimal conditions for paint removal by laser ablation are thoroughly investigated. Our studies suggest that laser paint removal is dominated by the laser vaporization effect, thermal stress effect and laser plasma effect, in which thermal stress effect is the most favoured while laser plasma effect should be avoided during removal operations. Based on the thermodynamic equations, we numerically evaluated the spatial distribution of the temperature as well as thermal stress in the paint and substrate under the irradiation of laser pulse at 1064 nm. The obtained curves of the paint thickness vs. threshold fluences can provide the reference standard of laser parameter selection in view of the paint layer with different thickness. A multi-pulse model is proposed and validated under a constant laser fluence to perfectly remove a thicker paint layer. The investigations and the methods proposed here might give hints to the efficient operations on the paint removal and lowering the risk of substrate damages.

Ablation Behaviour and Microstructure of Carbon/Carbon and Hybrid Carbon/Carbon Composites Based on Plasma Torch Heating

The ablation properties and morphologies of two kinds of fine Fine-woven pierced composites materials, carbon/carbon (C/C) and hybrid C/C with tungsten (W) filaments in z directional carbon fibre bundles, were investigated. A plasma torch was used to explore the ablative characteristics in terms of linear/bulk ablation rate and microscopic pattern of ablation. Surface and in-depth temperatures during ablation were measured by using optical pyrometers and thermocouples. The experimental results showed that the C/C composite presented the best ablation resistance performance, followed by the hybrid C/C composite, while that of graphite was the worst. It was found that the thermo-mechanical ablation resistance of carbon matrix is equal to that of carbon fibres. The existence of WC not only had a faster intrinsic ablation velocity, but also accelerated the ablation velocity of the carbon fibres and carbon matrix, and significantly improved the ablation velocity of the carbon fibres.

Numerical Simulations of Multiscale Ablation of Carbon/Carbon Throat with Morphology Effects

A multiscale thermochemical ablation model for the multidirection carbon/carbon composite throat was established in solid rocket motors. The model was established based on the reactivity of different carbon phases (fibers and matrix), the surface recession control mechanism, and spatial characteristics of the composites to understand the origin of the ablation behavior. The level set method and the weighted essentially nonoscillatory scheme were used to obtain the evolution of transient ablation morphologies on the microscale and the mesoscale. The multiscale thermochemical ablation model and numerical framework were validated by using experimental data from a small solid rocket motor and a submerged nozzle throat of large solid rocket motor. The recession rates and evolution progress of the carbon/carbon composite ablative morphology at various scales were obtained. The results were consistent with experimental data under practical operating conditions.

Material removal effect of microchannel processing by femtosecond laser

Material processing using ultra-short-pulse laser is widely used in the field of micromachining, especially for the precision processing of hard and brittle materials. This paper reports a theoretical and experimental study of the ablation characteristics of a silicon wafer under micromachining using a femtosecond laser. The ablation morphology of the silicon wafer surface is surveyed by a detection test with an optical microscope. First, according to the relationship between the diameter of the ablation holes and the incident laser power, the ablation threshold of the silicon wafer is found to be 0.227J/cm2. Second, the influence of various laser parameters on the size of the ablation microstructure is studied and the ablation morphology is analyzed. Furthermore, a mathematical model is proposed that can calculate the ablation depth per time for a given laser fluence and scanning velocity. Finally, a microchannel milling test is carried out on the micromachining center. The effectiveness and accuracy of the proposed models are verified by comparing the estimated depth to the actual measured results.

A high precision scheme for micro‐channel processing in quartz glass using femtosecond laser

AbstractUltra‐short pulse laser with unique advantages in micromachining, especially on the hard brittle material becomes the focus of study on micro‐structure processing. Theoretical and experimental researches are taken from the ablation properties of micro‐channel in the quartz glass by femtosecond laser. As the ablation depth of quartz glass has not relationship obviously with defocusing distance within LAV effective range, a new way to control precisely the ablation depth of quartz glass is present in the study. With fixed laser power, the ablation depth could be known easily by the length of the ablation streak while moving the femtosecond laser focus along the straight line with a 45‐degree angle from the quartz glass surface within the LAV range. Research is carried out to study laser pulse power, scanning velocity and step distance to do something with micro‐channel ablation quality. According to the parameters of test, the micro‐channel with rectangle cross section of the quartz glass is processed, and then we analyze the ablation morphology and processing efficiency. The results provided a important suggestion to process micro‐structure precisely for quartz glass by femtosecond laser.

An investigation on 800 nm femtosecond laser ablation of K9 glass in air and vacuum

Ablation rates of K9 glass were studied as a function of femtosecond laser fluences. The central wavelength was 800nm, and pulse durations of 35fs and 500fs in air and vacuum were employed. Ablation thresholds of 0.42J/cm2 and 2.1J/cm2 were obtained at 35fs and 500fs, respectively, which were independent with the ambient conditions and depend on the incident pulse numbers due to incubation effects. The ablation rate of 35fs pulse laser increased with the increasing of laser fluence in vacuum, while in air condition, it slowly increased to a plateau at high fluence. The ablation rate of 500fs pulse laser showed an increase at low fluence and a slow drop of ablation rate was observed at high fluence in air and vacuum, which may due to the strong defocusing effects associated with the non-equilibrium ionization of air, and/or the shielding effects of conduction band electrons (CBEs) produced by multi-photon ionization and impact ionization in K9 glass surface. The typical ablation morphologies, e.g. smooth zone and laser-induced periodic surface structures (LIPSS) were also presented and illustrated.

Periodic disruptions induced by high repetition rate femtosecond pulses on magnesium-oxide-doped lithium niobate surfaces

In this paper, we demonstrate the periodic disruption formation on magnesium-oxide-doped lithium niobate surfaces by a femtosecond fiber laser system with wavelength and repetition rate of 1040 nm and 52 MHz, respectively. Three main experimental conditions, laser average power, scanning speed, and orientation of sample were systematically studied. In particular, the ablation morphologies of periodic disruptions under different crystal orientations were specifically researched. The result shows that such disruptions consisting of a bamboo-like inner structure appears periodically for focusing on the surface of X-, Y- and Z-cut wafers, which are formed by a rapid quenching of the material. Meanwhile, due to the anisotropic property, the bamboo-like inner structures consist of a cavity only arise from X- and Z-cut orientation.

Experimental and Numerical Evaluation of the Ablation Process of Carbon/Carbon Composites Using High Velocity Oxygen Fuel System

The ablation process of carbon/carbon (C/C) composites was tested under hypersonic flowing propane flame. The microstructures of C/C composites were characterized and the numerical analysis was performed. Two typical ablation morphologies of the carbon fibers, which are drum-like and needle-like shapes, were observed depending on the alignments of fibers to the flame directions. Temperature fields in the composites were analyzed using finite element method, and the mechanisms that govern the formation of different ablation behaviors were elucidated. For paralleled fiber bundles, the highest temperature situates in the middle parts underlying the ablation pits, where the drum-like shape is formed. For perpendicular fiber bundles, the highest temperature appears at the turning point between the transverse section and the surface of fiber, which leads to the gradual ablation from the fiber surface toward the axis, and eventually the formation of the needle-like shape.

Microstructure and properties of ablative C/ZrC–SiC composites prepared by reactive melt infiltration of zirconium and vapour silicon infiltration

C/ZrC-SiC composites with a density of 3.09g/cm3 and a porosity of 4.8% were prepared by reactive melt infiltration and vapour silicon infiltration. The flexural strength and modulus were 235MPa and 18.3GPa, respectively, and the fracture toughness was 7.0MPam1/2. The formation of SiC and ZrSi2 during vapour silicon infiltration, at the residual cracks and pores in the C/ZrC, enhanced the interface strength and its mechanical properties. The high flexural strength (223MPa, c. 95% of the original value) after oxidation at 1600°C for 10min indicated the excellent oxidation resistance of the composites after vapour silicon infiltration. The mass loss and linear recession rate of the composites were 0.0071g/s and 0.0047mm/s, respectively and a fine ablation morphology was obtained.

Effects of oxidizing species on ablation behavior of C/C-ZrB2-ZrC-SiC composites prepared by precursor infiltration and pyrolysis

C/C-ZrB2-ZrC-SiC composites are fabricated by precursor infiltration and pyrolysis process with polycarbosilane, ZrB2 and ZrC polymeric precursors. The oxyacetylene and plasma torches are used as two oxidizing flames to investigate the ablative properties of C/C-ZrB2-ZrC-SiC composites at high temperatures. The C/C-ZrB2-ZrC-SiC composites exhibit good ablation resistance under the O2 and H2O+O2 dominated atmospheres. Due to the different oxidizing species and temperatures generated by the two flames, the composites reveal different ablation rates, morphologies and mechanisms. After the oxyacetylene torch test, a molten ZrO2-SiO2 film formed in the ablation center, whereas a porous stacking layer is present under the plasma flame. The formation of the protecting layers can effectively reduce the inward diffusion of oxygen and heat transfer into the substrate, enabling the C/C-ZrB2-ZrC-SiC composites to have good ablation resistance at 2300 and 3000°C.

Ablation resistance of wedge-shaped C/C-ZrB2-ZrC-SiC composites exposed to an oxyacetylene torch

The ablation resistance of wedge-shaped C/C-ZrB2-ZrC-SiC composites was studied under an oxyacetylene torch. Ablation morphologies and phase compositions were examined by SEM and XRD. As exposed to low heat flux of 2.38MW/m2, a porous and homogeneous ZrC-ZrO2 skeleton generated. With the increasing of heat flux, ceramic oxides at the tip became loose and fragmented instead of sintering together. Besides that, owing to the wedge-shape of the specimen, high gas pressure mainly concentrated at this region. The high gas pressure and strong shear scouring of flame resulted in the limited accumulation of ceramic oxides. This led to an inferior ablation of wedge-shaped C/C-ZrB2-ZrC-SiC composites in high heat flux of 4.18MW/m2.

Influence of the confinement on laser-induced dry etching at the rear side of fused silica

Laser-induced etching at the rear side of transparent material enables high-quality machining results. However, the mechanism is still not completely recognized which would allow further optimization. Therefore, multi-pulsed laser-induced backside dry etching with different thick photoresist films was studied experimentally for air (MP-LIBDE) and water confinements (cMP-LIBDE). The water confinement causes differences in photoresist ablation morphology and etching rate in dependence on laser fluence, film thickness and pulse number. Owing to the water confinement, the extent of photoresist film spallation and the etching rate slope difference in low and high fluence ranges are reduced. In particular, the etching rate of cMP-LIBDE keeps constant with different film thicknesses in contrast to MP-LIBDE. Two effects that are related to the water confinement, mechanical confinement and heat transfer alterations, are analysed and discussed in relation to the differences between MP-LIBDE and cMP-LIBDE.

The Effects of Liquid Environment on Ablation Efficiency and Morphology of Gold Nanoparticles Prepared by Laser Ablation Technique

Gold (Au) nanoparticles (NPs) were prepared by pulsed laser (Nd:YAG, 1064 nm) ablation of an individual target in deionized water (DIW), pure ethanol, and mixture of DIW and ethanol with different concentrations of ethanol. The effects of ambient liquid environment on ablation efficiency, stability, and size of Au NPs were experimentally investigated. The optical spectral characterization and morphological observations were determined by UV-Vis spectrophotometer and transmission electron microscopy, respectively. Particles are nearly spherical in shape in all liquid environments. The ablation experiment was initially performed by using DIW as the reference sample. The obtained Au NPs have an average particle size of about 18.71±7.50 nm. The comparison of the experimental results with the reference sample after a storage time of 48 h revealed that Au NPs in pure ethanol medium were not stable and exhibited much lower ablation efficiency and broader size distribution with an average particle size of 23.83±11.65 nm. Addition of ethanol with different volumes to the DIW significantly improved Au NPs properties in the mixture. The optimum ethanol concentration in the mixture was determined to be 70 vol%, which yielded the maximum ablation efficiency, rather narrow size distribution, and controllable size (12.43±8.10 nm) of Au NPs. A simple method for controlling the properties of Au NPs produced in the binary mixture of DIW-ethanol by laser ablation technique is presented.