Nanomachining

Abstract

LIPSS (LASER Induced Periodic Surface Structures) is a term used to describe ordered or periodically structured nanostructures. Until newly, this term was not extensively researched or optimized for use in superhydrophobic self-cleaning applications. This machining method is one of the most sophisticated and cutting-edge ways to create nanofeatures like grooves and bumps without using any chemicals. Here we are trying to explore with polarization, machining speed, number of pulses, LASER fluence, shielding gas, and other parameters dependence on surface nanostructures and features that varied in size and orientation when milled by a LASER. Mainly, the aspect ratio of nano features are varied with axial spacing and along the horizontal diameter of the LASER beam. When polarization employed on nanomachining, the increased number of threshold LASER pulses also causes the structures to become LASER Induced Irregular Surface Structures, and the orientation and size of these features (between 200 and 400 nm) also affected by external interacts of material surface. The existing knowledge describes the nano feature generation is by the interaction of LASER beam and surface wavers. In light of this, one of the newest topics to emerge for the development of nanofeatures using femtosecond LASER.

1. Introduction

The concepts of LIPSS and laser-induced irregular surface structures need to be thoroughly investigated in order to justify their application at all levels of the pertinent femtosecond laser field. During the femtosecond laser machining, the surface molecules of the material are producing various sorts of self-assembled features ranging on few microns (200 nm - 400 nm) either in LIPSS or laser-induced irregular surface structures. The laser fluence’s transition from near to far ablation threshold typically changes the morphology from a periodic configuration to a complicated configuration. The increase in laser fluence deepens the grooves and intensifies the roughness of its produced feature.

When a linearly polarized femtosecond laser beam strikes a solid under ambient conditions, parallel and perpendicular LIPSS with a spatial frequency close to the laser’s wavelength are produced.

Early-career researchers have already discovered that the most significant influences on the development of LIPSS and laser-induced irregular surface structures are the processing environment, the laser beam’s wavelength, angle of incidence, pulse frequency, and laser intensity. Another important consideration is the processing material’s optical characteristics in the focused situation. On polished stainless steel, Stephan and Frank (2015) described the change of nano feature with linear polarization of a 300 fs laser at a wavelength of 1025 nm and the application is to optoelectronic and biological devices. He has done the experiment in the ambient condition and under controlled electric field vector and the outcome was LIPSS with a periodicity of 925 nm perpendicular to the electric field vector [1]. Picosecond laser with specific polarization on cold-worked tool steel for modified LIPSS manufacturing was also tested by Slovenian researchers Peter et al. in 2016. Additionally, the increment of the number of pulses deleting the previous direction of the ripples, as well as the generation of nano ripples perpendicular to the laser polarization, were observed [2]. Naoki et al. [3] provided evidence and proof of the material’s invariability during the creation of LIPSS under femtosecond laser irradiation. The size and groove depth depend on processing material and its ability to harden, such as nitride steel or stainless steel, with any aspects that were constant. An ambient environment will offer spatially oriented surface structures in the laser wavelength range that are perpendicular to the polarization in 30 to 150 fs. When LIPSS was introduced, the optical properties of semiconductor and dielectric materials were changed, and it’s possible that the pulse shaping will govern the LIPSS orientation [4]. By using femtosecond laser, it is possible to create grooves on a single crystal diamond that are even 40 nm in size. However, the aspect ratio features cannot be created until the laser energy is near to threshold. This approach can be suggested to develop the submicron feature in diamonds for various purposes [5].

In this study, we extrapolate the machining characteristics to standardize the fabrication parameters of LIPSS and laser-induced irregular surface structures through the laser parameter restrictions and working conditions functionalities. The complete assessment would be necessary to create accurate and preplanned LIPSS or laser-induced irregular surface structures, which might help it overcome the limitations of the existing studies. Here, we are attempting to gather data from laser polarization, laser energy density, scanning speed, and the number of pulses in relation to aspect ratio, groove size, width, depth, spacing, and orientation in different a gaseous environment. The approach has not yet been investigated directly by researchers but some relevant advancements are available, in a variety of materials for a variety of applications.

For instance, the laser assisted nanomachining were already experimented on different materials like metallic elements like K890 cold work tool steel [2], plasma nitrided austenitic stainless steel 304 [3], Cu, Zn, Al, Pb, Mo, Ni, and Fe [6] and transparent dielectrics, single crystalline BaF2 [7], other insulating material, CaF2 (111) [8], transparent glasses, synthetic silica glass ED-H [9], ceramics, titanium nitride (TiN) and diamondlike-carbon (DLC) deposited austenitic stainless steel 304 [10], and semiconductors, InP, GaP, GaAs and InAs [11]. From the different researcher’s points of view, the mechanism of nanogrooves fabrication is also different, the Instability of surfaces [7], alteration of refractive index on different medium [12], harmonic generation [11], surface plasmon polaritons excitation [13] and plasma generation [14]. Vibrant and advanced fields of application made us work more on this field, for example, antibacterial effect on 45S5 bioactive glass [15] and 316 L stainless steel [16] controlled cell migration [17] and laser-induced colorization [18] biomimetic surface structuring [19] and superhydrophobicity on AISI 304 stainless steel [20].

Nowadays, hierarchical micro - nanostructured surfaces receive considerable attention from researchers worldwide because of their unusual properties [21, 22, 23, 24]. Natural or Artificial surfaces with micro-nano surface morphology are crucial in determining the wettability of a solid surface (Figure 1).

Figure 1.

Different bio surfaces (a) peacock feather (Pavo cristatus), (b) hibiscus, (c) grass leaf (Poaceae), (d) lotus leaf (Nelumbo nucifera).

2. LASER assisted nanomachining

Nanomachining is one the most advanced machining technology to remove nano levels of materials from the workpiece surface by using a sophisticated tool (Figure 2). To perform this kind of machining needs an advanced sophisticated tool and nano level resolutioned motion controller.

Figure 2.

Interference of LASER with surface plasmons.

All of our experiments employed on AISI stainless steel 304, which has a 1.01 mm thickness and a 0.07 mm level of roughness. We adhered to the 10-minute acetone-assisted ultrasonic cleaning procedure, both before and after the trial.

All of our research was conducted using a cutting-edge Ti Sapphire laser system. As shown in Figure 3, the system had five axes of motion and 100 fs pulsed lasers with an 800 nm wavelength and a 10 kHz repetition rate. For nanomachining study, we chose a 50 mm focal length and a 25-micron meter spot diameter.

Figure 3.

Laser system with work station.

We also took a class of metallic materials to conduct other experiments. There also used ultra-short pulsed laser micromachining system with an advanced workstation. Cleaned with ultrasonicator and etched with suitable etchants. Topography studies were conducted with Opto-Digital Microscope, Stereo Microscope, Scanning Electron Microscope, Field Emission Scanning Electron Microscope, Atomic Force Microscope. Moreover, another methodological characterization was done with Digital Goniometer, UV Visible IR Spectrometer, Microhardness Tester, and X-ray Photoelectron Spectroscopy. We have selected the Olympus DSX 510 optodigital microscope with a different objective lens, the Hitachi S3400N scanning electron microscope (SEM), and the Park Systems atomic force microscope for the topographical study.

2.1 Nanomachining tuning

2.1.1 Polarization

2.1.1.1 Linear polarization (‘↔’ Electric field vector)

The effect of linear polarization in nanogrooves generation was studied with a set of experiments in the horizontal, vertical, and inclined direction, represented in the below schematic diagram Figure 4. In linear polarization, the grooves are oriented perpendicular in X–direction machining and parallel in Y – direction machining. Figure 5 represents its processed SEM image and AFM image with line profile, here we evidenced the generation of grooves is in the same format as explained earlier. The fusion effects of electromagnetic interference and its melt hydrodynamics may the conditions of generation of grooves, and the hydrothermal waves from Marangoni shear also influenced [25]. Generally, we cannot easily explain the formation of pattern with the convection or van der walls force performed hydrodynamic instabilities. However, may the temperature instability with a surface profile explain the formation of nano features by evaporation velocity.

Figure 4.

Schematic representation of machining on its scanning direction with generated grooves (orientation), SEM image of corresponding biomimetic metallic surface [10.0 k SE magnification].

Figure 5.

Processed SEM image [10.0 k SE magnification] and AFM image of 300 nm groove width 100 nm depth [scales are in μm] of surface nanostructures and its line profile for corresponding biomimetic metallic surface.

1. Aspect ratio

   Aspect ratio means the ratio of the width of groove and length of groove has studied with the axial distance from the middle of laser path or the center of the laser pulse. Result reviles, under high scanning speed (~100 mm/s) of lower energy laser pulses (~40.74 mJ/cm²) at 10000 Hz, generate a very rich nanostructure with periodically good orientation.

   In conclusion, we opted four regions and found an aspect ratio. These four are shown in the above Figure 6. Here, the aspect ratio is varied with axial outreach from the center of the laser pulse. The rise of aspect ratio may be by the energy influence, which means that it is getting varied as per spatial Gaussian beam delivery schematically shown in Figure 7. May the number of photons for absorption will get altered slightly on aspect ratio and axial distance but the trend remains same [26].

2. LIPSS to laser-induced irregular surface structures transformation

   The above-shown Figure 8 represents the alteration of LIPSS to laser-induced irregular surface structures by the increment of pulses in a specified area. Initially, in two numbers of pulse, the feature is perfectly linearly ordered, and in 10 numbers of pulses also showing somewhat same feature with slight modification under linear polarization. The entire feature transformed to laser-induced irregular surface structures or pillar format in many pulses.

3. laser-induced irregular surface structures to LIPSS transformation

   Figure 9 shows the random surface feature to periodic surface feature transformation by rising material interaction energy. Here the input energy is getting riced by the number of multiple thresholds of stainless steel under linear polarization.

   In linear polarization (↕ electric field vector), the polarization electric field vector is along the Y-axis and perpendicular to the X-axis, then the laser-induced structures perpendicular to the electric field. The nanogrooves alteration in a particular direction got influenced with the dense electron plasma by multyphoton excitation at the surface. The dense electron plasma generated by the front end of the pulse enhances the remaining electric field and breaks into different nano levels, which may be perpendicular to the direction or parallel to the direction shown in figs.

Figure 6.

Aspect ratio of periodic groove structure with axial spacing from the center of LASER spot on biomimetic stainless steel surface [12,481 X OM magnification].

Figure 7.

Schematic representation of laser pulses with energy variance.

Figure 8.

LIPSS to laser-induced irregular surface structures transformation of biomimetic metallic surface at 40.74 mJ/cm² energy dencity [9707 X OM magnification] (A) 2 pulses, (B) 10 pulses, (C) 100 pulses.

Figure 9.

Laser-induced irregular surface structures to LIPSS transformation of biomimetic metallic surface at 10 number of pulses [9707 X OM magnification] (A) 142.6 mJ/cm² (B) 101.86 mJ/cm² (C) 40.74 mJ/cm².

3. Circular polarization (‘O’ electric field vector)

For the study of circular polarization, we have exsiccated a single laser path experiment with different angular displacements like horizontal, 45° inclined, and vertical. The analyzed optodigital images incise the variation of nanogrooves.

A circularly polarized laser beam with rounded electric field vector making structure as to not in any particular orientation (Figure 10). It depends on the orientation of the impinging moment, may whether perpendicular or parallel or other.

Figure 10.

Schematic representation of machining on its scanning direction with generated grooves (orientation), enlarged focused optical image of corresponding biomimetic metallic surface [~9.0 k OM magnification].

3.1 Nano to micro machining

In circular polarization, the machining from nano to micro depends on the laser energy used. The above-represented Figure 11 shows the cubic micrometer level volumetric material variation data on different energy densities. Here, it shows the transition of the athermal region to thermal region of material removal. In the above, all LIPSS to laser-induced irregular surface structures study belongs to both optical penetration region and thermal penetration region. Here we explain machining precision from nano level to micro to millimeter level.

Figure 11.

Representations of material removal rate in (μm³) of different energy dencity at 10 mm/s scanning speed.

In 203.71 mJ/cm² energy laser pulses with circular polarization at 10000 Hz frequency generates laser-induced irregular surface structures but different roughness factors. The main influential factor for laser-induced irregular surface structures is high energy or under higher multiple threshold fluence. May the collective upshot of the pressure wave and shock wave causing roughened surfaces and corrosive or non-corrosive working index also alter the feature slightly. The working environment of helium is much clear on microscope images due to its inertness. We have noted no differentiable deviation in orientation due to the accordance of refractive index rice of working environment. The observable difference is that the features’ size varies with the working ambiance or the refractive index alteration on different mediums. The periodicity of the nanogrooves reduced. The threshold energy required to generate the LIPSS features on the different working ambiances are different [27], and the laser-material interaction angles are also a matter in these regards.” On the delivery of femtosecond laser may the medium drastically influence, or it may alter the focused beam profile in the air than inert gas like helium and argon [28].

In self-cleaning, the surface should be superhydrophobic, and the water droplet should carry a significant amount of dust in isotropic rolling or anisotropic rolling (Figure 12).

Figure 12.

Theories related to hydrophobicity.

The field with directional hydrophobicity is one of the emerging areas to furnish in its full depth. As we know, the lotus leaves with multi-directional or isotropic superhydrophobicity, and grass leaves with unidirectional or anisotropic superhydrophobicity are there to mimic physically [29].

Finding methods to fabricate these complex structures is a vital challenge in surface engineering (Figure A1). As we understood the ultrafast laser-assisted micromachining is a method to develop these structures with full functionality. One of our work demonstrates the method and develops multifunctional surfaces with anisotropic and isotropic hydrophobicity using femtosecond laser machining alone. Researchers are already came with anisotropicity of wetting by liquid metals in the past, and the study explained through anisotropicity of interfacial energies on surfaces. The solid, liquid interfaces are much complex to reveal in anisotropic wetting. They demonstrate that the discrepancies occurred while deriving suitabilities in this regard. Hierarchical features by micro and nanostructures are already reliable in hydrophobicity, but the mode of development is getting versatile from time to time. The 130 femtoseconds pulsed laser-assisted hierarchical structure development was also reported earlier, but the mode of machining operations was to be defined. Our study appropriates to place directional hydrophobicity in new advanced dimensions from bio mimics.

4. Conclusion

By the existing knowledge, the nanogrooves generated with interference between the surface plasmon polariton and an incident laser beam means a coupling is happening with the surface. The surface plasmon directly correlated with surface roughness, so increasing the pulse number will alter the periodicity of the nano surface feature from LIPSS to laser-induced irregular surface structures. The dense electron plasma and its remaining enhancement drive a temperature instability on the normal surface, which directly prompts the evaporation velocity and nano surface feature generation. The rise of aspect ratio was influenced by beam profile and the above-mentioned hydrodynamic temperature instability. The generated surface is getting altered with the working environment also evidenced in this study. Laser energy, number of pulses, the effect of polarization and its angular movement, and normality of topographical material surface are main tuning factors to forge featured surface nanostructure.

We know from the Young Equation to the transition study of the theory of Cassie, Wenzel, continually updating the influential physical attributes to the hydrophobic surface development. Primary surface features and hierarchical surfaces such as two-level hierarchy and three-level hierarchy we demonstrated for our all studies. 30–40 μm sized air trapped hibiscus flower petal surface, Barbs with barbules of peacock feather sized 20–30 μm, 70–90 μm periodic grooves of grass leaves and lotus leaves surface spikes of 20–40 μm are the biological samples we were collected as the imitable evidence. For the optimization study of LASER processing parameters to spectral data, we use Box Behnken Design of experimental study; from these, we establish mathematical relationships for the effects of processing parameters on surface wettability and topography, and it verified with practical data. The BBD methodology allowed for better optimization of ultrafast laser processing parameters for superhydrophobic surfaces. The method with physical alteration will give as the durability of hydrophobic property on the material surface, and the alteration of this property is based on only by another machining process. The method with a fast process for surface alteration is the primary advantage of this applied processing technology on self-cleaning applications. Suitable material with specified texture may provide a foundation to self-cleaning surfaces for food processing components or equipment, hygiene in a food factory that we cannot compensate. It is a very fast method for the development of multifunctional surfaces using femtosecond laser for biomedical application. Hydrophobicity with super or ultra-featured contact angle very low sliding or rolling off-angle can instrument self-cleaning physical activities in solar panels, hydraulic equipments, agri machines for instant residue removal. In addition, to prevent ice formation on power transmission cables and aircraft wings and cockpit shields and corrosion resistance of structures water pipelines hydraulic components and other machineries in addition to biofouling resistance on ship basements structures and other pipelines and thermal barrier coating, enhanced broadband absorption. We have done a proper parameter control on the nanofeature control from nanogrooves to nano spikes.

Acknowledgments

The researchers, who are noteworthy, produced the manuscripts. Manuscript written by Mr. Srin K S with helpful advice from Prof. Ramkumar J, Dr. Ravi N Bathe, and the Late Dr. G Padmanabham.

All authors acknowledge the assistance of the Department of Science and Technology, Government of India (Grant No: DST/TSG/AMT/2015/628), ARCI Hyderabad, and Indian Institute of Technology Kanpur, India.

Conflict of interest

The authors have no conflict of interest to declare that are relevant to the content of this article.

Figure A1.

Elemental analysis (XPS) of modified and unmodified SS 304 surfaces.

Thanks

ARCI Hyderabad provided such a relaxable research environment to perform all my experiments to pursue my Ph.D. with all needed facilities. IIT Kanpur provided me the freedom to study all aspects of, which related to my Ph.D. research like a child. Government of India provided me with all the needed helps to pursue my Ph.D. nicely.