Femtosecond Laser Machining Research Articles

Machine vision-based high-precision and robust focus detection for femtosecond laser machining

We propose a machine vision-based focus detection method (MVFD) for femtosecond laser machining. By analyzing the laser focus pattern, the defocus direction and distance are obtained simultaneously. The proposed technique presents high precision with an average error of 0.047 µm and a root mean square error (RMSE) of 0.055 µm. Moreover, the method is robust and is less affected by the tilted sample. For the curved surface sample, the average error and RMSE are 0.093 and 0.145 µm, respectively. Thus, the proposed focus detection method can be easily combined with laser processing equipment, which is widely used in large-range and high-precision femtosecond laser processing.

In Situ Collection of Nanoparticles during Femtosecond Laser Machining in Air.

Nanoparticles generated during laser material processing are often seen as annoying side products, yet they might find useful application upon proper collection. We present a parametric study to identify the dominant factors in nanoparticle removal and collection with the goal of establishing an in situ removal method during femtosecond laser machining. Several target materials of different electrical resistivity, such as Cu, Ti, and Si were laser machined at a relatively high laser fluence. Machining was performed under three different charge conditions, i.e., machining without an externally applied charge (alike atmospheric pulsed laser deposition (PLD)) was compared to machining with a floating potential and with an applied field. Thereby, we investigated the influence of three different charge conditions on the behavior of laser-generated nanoparticles, in particular considering plume deflection, nanoparticle accumulation on a collector plate and their redeposition onto the target. We found that both strategies, machining under a floating potential or under an applied field, were effective for collecting laser-generated nanoparticles. The applied field condition led to the strongest confinement of the nanoparticle plume and tightest resulting nanoparticle collection pattern. Raster-scanning direction was found to influence the nanoparticle collection pattern and ablation depth. However, the laser-processed target surface remained unaffected by the chosen nanoparticle collection strategy. We conclude that machining under a floating potential or an applied field is a promising setup for removing and collecting nanoparticles during the machining process, and thus provides an outlook to circular waste-free laser process design.

Femtosecond Laser Trimming with Simultaneous Nanostructuring to Fine Piercing Punch to Electrical Amorphous Steel Sheets.

A CVD (Chemical Vapor Deposition) diamond coated tungsten carbide (WC) and cobalt (Co) sintered alloy punch was trimmed by the femtosecond laser machining to sharpen its edge with about 2 μm and to simultaneously make nanostructuring to its side surface. In addition to the sharpened edge, its edge profile was formed to be homogeneous enough to reduce the damage layer width by piercing the electrical amorphous steel sheet stack. Each brittle sheet in the stacked work was damaged to have three kinds of defects by piercing; e.g., the droop-like cracking in the thickness and at the vicinity of hole, the wrinkling in peak-to-valley with partial cracking on the peaks, and the circumferential cracking. When using the WC (Co) punch with the inhomogeneous edge profile in the sharpened edge width, these three damages were induced into each sheet and the maximum damage width exceeded 80 μm. When using the punch with the sharpened edge and homogeneous edge profile, the wrinkling mode was saved and the total affected layer width was significantly reduced to less than 20 μm. Through the precise embossing experiments, this effect of punch edge profile condition to the induced damages was discussed with a statement on the nanostructuring effect on the reduction of damaged width in electrical amorphous steel sheets. The developed tool with the sharpened edge and homogenous edge condition contributes to the realization of a low iron loss motor with a reduced affected layer width.

Design and fabrication of honeycomb micro-texture using femtosecond laser machine

ABSTRACT In advanced machining processes on hard metals, fabrication of textured insert is still a challenge. To improve the machinability of a textured cutting insert the most effective factors are the area covered, design pattern, and orientation of micro-texture. For the coated cutting inserts, the fabrication of micro-grooves must not remove the coating from the surface. Due to the generation of several micro-pools of reservoir for lubricant, the novel honeycomb micro-textured groove has been designed in 3D. The femtosecond laser machine is used to fabricate the micro-textures on TiCN (Titanium Carbo-Nitride) coated tungsten carbide insert having 20 microns of coating depth. Honeycomb micro-texture has covered 2.11 mm X 2.11 mm of the area on the rack face of the cutting insert. After fabrication, the average groove width and wall thickness obtained are 57.7 microns and 29.49 microns, respectively. Fabrication has been done up to the depth of the groove of 18.6 microns. EDS (energy dispersive spectroscopy) confirms 68.2% presence of TiCN coating inside the groove with an increase of 33.7% of carbon and a decrease of 9.9% of titanium content due to laser cutting.

Nonlinear optical properties of 6H-SiC and 4H-SiC in an extensive spectral range

Silicon carbide (SiC), which is the leading representative of the third-generation of semiconductors, possesses many excellent physical properties. However, its advantages also incur difficulties in processing, which calls for special processing techniques, such as femtosecond laser machining. In addition, SiC has shown unprecedented potential for optoelectronic applications. Knowledge of the nonlinear absorption coefficient and the nonlinear refractive index coefficient of SiC is required in both of the fields. In this work, the nonlinear absorption coefficient and the nonlinear refractive index coefficient of semi-insulating (SI) 6H-SiC and SI 4H-SiC, the most pervasive SiC polytypes, are measured in an extensive spectral range from 400 nm to 1000 nm with the Z-scan technique. Besides, the spectral dependence of the nonlinear optical properties is analyzed, facilitated by linear absorption spectrum. Especially, two-photon absorption (2PA) and three-photon absorption (3PA) coefficients of SI 6H-SiC and SI 4H-SiC are characterized in the respective spectral ranges. From the characterization of SiC, we can observe self-focusing phenomenon for nonlinear refraction. In the end, we unravel the potential of SiC for ultrafast all-optical switching based on the measured nonlinear optical properties.

Training AI-Based Feature Extraction Algorithms, for Micro CT Images, Using Synthesized Data

X-ray computed tomography (CT) is a powerful technique for non-destructive volumetric inspection of objects and is widely used for studying internal structures of a large variety of sample types. The raw data obtained through an X-ray CT practice is a gray-scale 3D array of voxels. This data must undergo a geometric feature extraction process before it can be used for interpretation purposes. Such feature extraction process is conventionally done manually, but with the ever-increasing trend of image data sizes and the interest in identifying more miniature features, automated feature extraction methods are sought. Given the fact that conventional computer-vision-based methods, which attempt to segment images into partitions using techniques such as thresholding, are often only useful for aiding the manual feature extraction process, machine-learning based algorithms are becoming popular to develop fully automated feature extraction processes. Nevertheless, the machine-learning algorithms require a huge pool of labeled data for proper training, which is often unavailable. We propose to address this shortage, through a data synthesis procedure. We will do so by fabricating miniature features, with known geometry, position and orientation on thin silicon wafer layers using a femtosecond laser machining system, followed by stacking these layers to construct a 3D object with internal features, and finally obtaining the X-ray CT image of the resulting 3D object. Given that the exact geometry, position and orientation of the fabricated features are known, the X-ray CT image is inherently labeled and is ready to be used for training the machine learning algorithms for automated feature extraction. Through several examples, we will showcase: (1) the capability of synthesizing features of arbitrary geometries and their corresponding labeled images; and (2) use of the synthesized data for training machine-learning based shape classifiers and features parameter extractors.

In-situ SEM based method for assessing fiber-matrix interface shear strength in CFRPs

Accurate assessment of fiber-matrix interface shear strength may play key role in understanding structural strength behavior of polymer-matrix composites, and provide additional flexibility for enabling high-performing material designs. This work presents a method for assessment of fiber-matrix interface strength in carbon fiber reinforced polymers (CFRPs). The method uses in-situ scanning electron microscopy (SEM) imaging while pushing individual fibers out of 20–30 μm thick membranes. Femtosecond laser machining has been utilized to address standing challenges associated with manufacturing high-quality fiber push-out specimens. To ensure that the specimen preparation method used did not affect the properties of the composite, a thorough study has been conducted using benchmark specimens. The benchmark specimens are prepared using a conventional lapping/polishing process. Results for two CFRPs compare fiber-matrix interface shear strength generated using free-standing samples (conventional lapping/polishing preparation) and femtosecond laser-machined specimens. Both specimen types exhibit similar strength. However, load-displacement response of the free-standing samples is overly compliant, representing specimen response rather than fiber-matrix interface material behavior. The excessive compliance is caused by inherent imperfections in the contact of the sample and the test fixture. The femtosecond laser-machined samples are free of such imperfections. Finite element analysis has been accomplished to better understand the load-displacement response.

Modeling adult skeletal stem cell response to laser-machined topographies through deep learning

The response of adult human bone marrow stromal stem cells to surface topographies generated through femtosecond laser machining can be predicted by a deep neural network. The network is capable of predicting cell response to a statistically significant level, including positioning predictions with a probability P < 0.001, and therefore can be used as a model to determine the minimum line separation required for cell alignment, with implications for tissue structure development and tissue engineering. The application of a deep neural network, as a model, reduces the amount of experimental cell culture required to develop an enhanced understanding of cell behavior to topographical cues and, critically, provides rapid prediction of the effects of novel surface structures on tissue fabrication and cell signaling.

Pool boiling heat transfer of saturated water on rough surfaces with the effect of roughening techniques

The effect of surface roughness on pool boiling heat transfer was experimentally investigated in the saturated water. Eight testing surfaces were fabricated on plain copper through three preparation methods including random polishing, unidirectional polishing, and femtosecond laser machining. The surface roughness in the present study is characterized by the areal (three dimensional, 3D) roughness parameter. In the two dimensional (2D) profile roughness analysis, we know that a profile can be separated into the large-scale component (waviness) and the short-scale component (roughness). The arithmetical mean height of the profile, the waviness and the roughness are denoted as Pa, Wa, and Ra, respectively. However, there are no such kind of parameters defined in the 3D roughness analysis. To distinguish them, we define Sap and Sa in this study, corresponding to Pa and Ra in the 2D roughness parameters. The pool boiling experiments show that a rougher surface has higher heat transfer coefficient than a smoother one only within the same surface preparation method, if the surface roughness is characterized by Sap. For the surfaces prepared by the different techniques, the laser processed rough surface with Sap=3.40 µm could have deteriorated heat transfer coefficient compared with the relatively smooth one polished by the 180 grit sandpaper with Sap=1.29µm. However, the different trend is observed when the surface roughness is characterized by Sa. By applying a standardized filtration process, the large-scale component of the surface can be removed, and the areal arithmetical mean height calculated from the remaining part is denoted as Sa. The boiling curves are found to shift monotonically to the left as Sa becomes larger, regardless of the surface preparation methods. The enhancement in heat transfer coefficient of rougher surfaces is attributed to a combined result of larger nucleation sites, smaller departure bubbles and higher bubble departure frequencies, according to the visualization study. On the other hand, the critical heat flux (CHF) does not show a clear trend with the surface roughness when the different surface preparation methods are involved, no matter Sap or Sa is used. The measurements of static contact angle infer that the CHF is not only dependent on surface roughness, but also highly related to surface wettability. A correlation is developed to well predict the heat transfer coefficient from the experiments as a function of Sa, especially in the fully developed nucleate boiling region.

Development of a metal-to-metal imprinting process: Transcription quality analysis and surface wettability characterization

Micro- or nano-engineered (MNE) metal surfaces have high industrial utility because they have high mechanical strength, and also possess the electromagnetic properties of the metal and structure-mediated surface characteristics. Therefore, there is a large demand for mass production technologies for fabricating metallic surfaces having micro- or nano-patterns. In this study, we propose a process technology comprising femtosecond (fs) laser machining for fabricating a stamp, and metal-to-metal (M2M) imprinting for replicating its MNE surface. The major process conditions affecting the transcription quality (TQ) of the micro-/nano-patterns were investigated utilizing the design of experiments (DOE) based on the Taguchi method. The TQ analysis was carried out based on the signal-to-noise (S/N) ratios of the geometric parameters according to the process conditions applied during M2M imprinting. In addition, the water contact angles (CAs) for the fabricated stamps and imprinted Al substrates were measured, and their surface wettability with respect to the TQ thus characterized.

Enhanced Transepithelial Riboflavin Delivery Using Femtosecond Laser-Machined Epithelial Microchannels.

PurposeThis study describes a femtosecond laser (FS) approach to machine corneal epithelial microchannels for enhancing riboflavin (Rf) penetration into the cornea prior to corneal crosslinking (CXL).MethodsUsing a 1030-nm FS laser with 5- to 10-µJ pulse energy, the corneal epithelium of slaughterhouse rabbit eyes was machined to create 2-µm-diameter by 25-µm-long microchannels at a density of 100 or 400 channels/mm2. Rf penetration through the microchannels was then determined by applying 1% Rf in phosphate-buffered saline for 30 minutes followed by removal of the cornea and extraction from the central stromal button. Stromal Rf concentrations were then compared to those obtained using standard epithelial debridement or 0.01% benzalkonium chloride (BAK) to disrupt the epithelial barrier.ResultsMicrochannels formed using a 5-µJ/pulse at a density of 400 channels/mm2 achieved a stromal Rf concentration that was 50% of that achieved by removal of the corneal epithelium and imbibing with 1% Rf. Stromal Rf levels were also equal to that of debrided corneas soaked with 0.5% Rf, threefold higher than those soaked with 0.1% Rf, and twofold higher than corneas soaked in BAK without epithelial debridement. Organ culture of treated corneas showed a normal corneal epithelium following FS machining while BAK-treated corneas showed extensive epithelial and stromal damage at 24 hours posttreatment.ConclusionsFS corneal epithelial machining can be used to enhance penetration of Rf into the stroma for corneal CXL.Translational RelevanceThe creation of epithelial microchannels allows for stromal Rf concentrations high enough to perform true transepithelial crosslinking.

Correlation between laser spectroscopic studies and mechanical characterization of zirconia-based multiwall carbon nanotube ceramic composites

The hardness of zirconium oxide-based ceramic nanocomposites was correlated with the laser spectroscopic studies by analyzing the samples through nanosecond and femtosecond laser to see the viability of this technique as a fast and in situ for assessment of mechanical properties in nuclear industry. Zirconia incorporated with different vol% of multiwall carbon nanotubes were processed by the high frequency induction heated sintering. The composites were characterized by the nanosecond laser-induced breakdown spectroscopy (LIBS) with optimized delay time of 1, 2, and 3 µs and 200 and 300 mJ energies generated by laser Nd:YAG (λ = 1064 nm). The plasma temperature resulted by the ablation of different samples was estimated through intensity of selected zirconium lines using the Boltzmann plot method. The samples were mechanically characterized by the Vickers hardness test. The estimated plasma temperature and the ratio of Zr(II) with different intensities of Zr(I) emission lines show rather weak dependency and increase with surface hardness. The samples were scrutinized by the femtosecond laser micromachining through variation in depth and surface morphology of machined areas. It is found that deeper circular groove and enhanced erosion of disk shape by femtosecond laser machining are achieved for less hard materials and are in agreement with the LIBS analysis.

Modelling laser machining of nickel with spatially shaped three pulse sequences using deep learning.

Femtosecond laser machining is a complex process, owing to the high peak intensities involved. Modelling approaches for the prediction of final sample quality based on photon-atom interactions are therefore challenging to extrapolate up to the microscale and beyond. The problem is compounded when multiple exposures are used to produce a final structure, where surface modifications from previous exposures must be taken into consideration. Neural network approaches allow for the automatic creation of a model that accounts for these challenging processes, without any physical knowledge of the processes being programmed by a specialist. We present such a network for the prediction of surface quality for multi-exposure femtosecond machining on a 5µm electroless nickel layer deposited on copper, where each pulse is uniquely spatially shaped using a spatial light modulator. This neural network modelling method accurately predicts the surface profile after three, sequential, overlapping exposures of dissimilar intensity patterns. It successfully reproduces such effects as the sub-diffraction limit machining feasible with multiple exposures, and the smoothing effect on edge-burr from previous exposures expected in multi-exposure laser machining.

Fabrication of Microneedle from Stretched Biodegradable Polymer Sheet by 3D Laser Machining

Polylactic acid (PLA) microneedles have been usually fabricated by injection molding. Herein, we consider microneedles that mimic the maxillae of mosquito proboscises, which have sharp tips with jagged harpoon-like protrusions. In case of such microneedles, filling the melting PLA resin up to the tip of the mold trough without burrs is challenging. To address this issue, we have proposed a new microneedle fabrication method in this paper. In this method, the needle with the desired shape was obtained from a PLA sheet by femtosecond laser machining. The needle was turned by 90°, and its tip further cut obliquely with the laser for three-dimensional (3D) sharpening. Tensile and buckling tests were conducted by using a test piece cut out from the PLA sheet. It was experimentally established that the strength and Young’s modulus along the sheet’s stretch direction are higher than those along its perpendicular direction. The 3D sharpened PLA microneedle successfully penetrated an artificial skin made of polydimethylsiloxane. A pair of microneedles were alternately vibrated against each other, mimicking the motion of mosquito two maxillae. With this alternate vibration, the resistance force during insertion was found to be lower compared to that without vibration.

High speed and low roughness micromachining of silicon carbide by plasma etching aided femtosecond laser processing

Silicon carbide has a high hardness and corrosion resistance. Accordingly, high rate machining of this material is a big challenge to obtain good surface quality. In this work, plasma etching aided femtosecond laser micromachining was proposed to address this issue. A series of square and circular blind holes for forming SiC pressure sensor diaphragms were obtained by using a 50fs, 800 nm, 1000Hz Ti:Sapphire laser. Using the control variable method, the effects of laser machining power, scan line spacing and scanning rate on surface topography of the structure were investigated. As demonstrated by experimental results, surface roughness of the structure increases with the laser machining power within a certain range. While the scanning rate has negligible effects on surface quality of the structure. The maximum surface roughness after femtosecond laser micromachining is 2.867 μm measured by laser scanning confocal microscope, and this value is reduced to 2.143 μm after inductively coupled plasma etching. In addition, a Raman spectroscopy shows that this composite micromachining method can eliminate accumulation of amorphous carbon on SiC surface caused by the femtosecond laser processing. The reduction of surface roughness and removal of amorphous carbon can effectively decrease probability of crack failure of the sensor diaphragm and increase its deformation capability, thereby ensuring sensitivity of the sensor. The composite micromachining method fully combines the advantages of high efficiency of femtosecond laser machining and high surface quality of plasma etching, making it has high potential to be applied in precision machining of silicon carbide and fabrication of SiC sensors.

Fabrication of Superhydrophobic Stainless Steel Nozzles by Femtosecond Laser Micro-/Nano-Texturing

A dispensing nozzle is an essential mechanical element in inkjet, dot, and bioprinting. To improve the printing resolution, the inner diameter of the nozzle outlet must be as small as possible. A droplet dispensed through a hydrophilic stainless steel outlet expands on the whole outlet surface and along the side surface of the nozzle. This issue can be solved by physical surface modifications. In the present paper, a femtosecond laser micro-/nano-texturing method was developed to transform the originally hydrophilic stainless steel surface of a nozzle to a hydrophobic or superhydrophobic one. First, an AISI304 plate was used to demonstrate experimentally that, on its surface, the tailored micro-/nano-patterns were reproduced as micro-/nano-textures, making the surface superhydrophobic. Second, the technique was applied to the physical surface modification of an AISI304 stainless steel nozzle outlet by optimizing the femtosecond laser machining conditions. A high-speed camera was used to take a snapshot of the dispensed droplet from the surface-modified outlet. Finally, a line-printing experiment was performed to characterize the dispensing behavior of the stainless steel nozzles with and without physical surface modification.

Second harmonic generation in zinc sulfide by femtosecond laser irradiation

Second harmonic generation (SHG) has been observed from zinc sulfide (ZnS) under linearly polarized and circularly polarized femtosecond laser excitations. The output power of SHG signal at 513 nm exhibits a quadratic relationship to the input power of femtosecond laser at 1026 nm, which can figure out the effective SHG response of 2.12 pm/V in ZnS. Additionally, the total, vertical, and horizontal SHG signals all demonstrate well defined cosine-like curves with approximate periods of 40°, indicating a textured crystallized material rather than randomly orientated crystals. Finally, the relative position between the laser focus point and ZnS surface could be detected by the relationship between the SHG power and sample step, due to the regulation of the SHG signal by the power density of incident laser, which is also determined by the relative position. This work may pave the way towards nonlinear optical architectures and femtosecond laser machining.

Effect of Laser Pulse Overlap and Scanning Line Overlap on Femtosecond Laser-Structured Ti6Al4V Surfaces.

Surface structuring is a key factor for the tailoring of proper cell attachment and the improvement of the bone-implant interface anchorage. Femtosecond laser machining is especially suited to the structuring of implants due to the possibility of creating surfaces with a wide variety of nano- and microstructures. To achieve a desired surface topography, different laser structuring parameters can be adjusted. The scanning strategy, or rather the laser pulse overlap and scanning line overlap, affect the surface topography in an essential way, which is demonstrated in this study. Ti6Al4V samples were structured using a 300 fs laser source with a wavelength of 1030 nm. Laser pulse overlap and scanning line overlap were varied between 40% and 90% over a wide range of fluences (F from 0.49 to 12.28 J/cm²), respectively. Four different main types of surface structures were obtained depending on the applied laser parameters: femtosecond laser-induced periodic surface structures (FLIPSS), micrometric ripples (MR), micro-craters, and pillared microstructures. It could also be demonstrated that the exceedance of the strong ablation threshold of Ti6Al4V strongly depends on the scanning strategy. The formation of microstructures can be achieved at lower levels of laser pulse overlap compared to the corresponding value of scanning line overlap due to higher heat accumulation in the irradiated area during laser machining.

Femtosecond Laser Ablation for Mesoscale Specimen Evaluation

Focused ion beam machining revolutionized the way samples are studied in materials science. The introduction of this tool enabled researchers to break into new frontiers of research. While this method focused on the nanometer length scale, the mesoscale manufacturing of samples has not seen such advances. It remains a challenge to manufacture sample geometries too large for focused ion beam milling yet too small for conventional machining. Femtosecond laser ablation opens this length scale of sample space and allows the fabrication of numerous useful geometries. This paper outlines a state-of-the-art femtosecond laser machining system that can be used for rapid, micro- and mesoscale sample preparation. To illustrate the utility of this system, stress–strain data are presented for single-crystal Cu micropillars and three microscale tensile test specimens prepared from physical-vapor-deposited Cu and Ni foils.

Design and fabrication of spiral triangular micro texture on chemical vapor deposition coated cutting insert using femtosecond laser machine

Machinability can be improved using an insert with a micro-texture groove but the size, shape, and orientation of micro-texture are very effective factors to be considered. It must not eradicate coating from the surface of a cutting tool. Micro-textured groove has been designed and fabricated using a femtosecond laser machine on tungsten carbide insert with (CVD) chemical vapor deposition coating of TiCN (Titanium Carbo-Nitride) and alpha-alumina up to 20 µm. The spiral triangular texture covers 1.12 mm × 1.12 mm of the area with grooved edges upright to the direction of the flow of chips. 10 µm of groove width, 20 µm of wall thickness and 10 µm of the depth of groove have been taken. Spiral triangular texture acts as a reservoir of lubricant and provides more area for heat dissipation rate. Topography shows that after laser grooving the coating is still evaded on cutting insert.