Formation Of Surface Structures Research Articles

Surface Structure Formation in Plasma Cutting of Aluminum and Titanium Alloys Using Direct Current Straight and Reverse Polarity

Abstract The structural features and phase composition are examined in near-surface layers of specimens of Al-Mg, Al-Cu-Mg alloys and commercially pure titanium obtained by plasma cutting using direct current straight polarity (DCSP) and direct current reverse polarity (DCRP). It is found that the flows of molten metal ejected by the gas stream from the cut cavity during cutting form the fusion and heat-affected zones, whose structural morphology, phase composition, and thickness depend on both the selected material and the cutting mode. The fusion zone is thicker in specimens cut using DCRP than in those cut with DCSP. The thickness of the adjacent heat-affected zone is also the largest in the mode that provides a thicker fused layer. Aluminum alloy specimens cut in ambient air are characterized by the presence of oxygen in the near-surface layers. The lowest degree of oxidation is observed in Al-Mg alloy. Oxygen penetrates into the fused layer to a depth of 350–500 μm in Al-Cu-Mg and up to 200–250 μm in Al-Mg alloy. In titanium alloy, the thickness of oxide layers does not exceed 100–150 μm during straight polarity cutting and 200–250 μm during reverse polarity cutting. A thin brittle layer of TiO and TiO2 oxides is formed on the titanium alloy surface. It is shown that the generation of “water mist” around the plasma jet when cutting materials of all types with DCRP leads to a more intensive oxidation of metal, less thermal effect on the material, and reduced roughness of the cut face.

Comparative study of plasma, laser, and flame induced activation of HDPE liner surfaces of type 4 hydrogen vessels

ABSTRACT In our study, we compare the surface modifications of coarse HDPE surface induced by atmospheric air plasma, CO2 laser, and flame treatments. An order of WCA decrease of air plasma>flame>CO2 laser methods was detected. The improvement in adhesion strength measured 5 days after the activations followed the same trend. FT-IR and EDS proved that different oxygen compounds were formed after treatments, resulting in increased polar component of the total surface energy. Flame activation showed a saturation character regarding the O-moiety. Hydrophobic recovery showed a linear behavior, with a larger decreasing slope for the polar component than the total surface energy. Plasma treatments induced higher recovery rate; however, restore was not complete here either. The effects of the different CO2 irradiation intensities were nonlinear; SEM and roughness measurements revealed surface ablation or structure formation on the different samples, while after plasma and flame treatment a hilly microtexture appeared. With different mechanisms and intensities, all the tested methods are suitable for increasing the adhesion strength on HDPE surfaces.

Ageing of commercial austenitic, martensitic and duplex stainless steel surfaces after formation of laser-induced periodic surface structures by fs-laser irradiation

The time-dependent wetting behavior was investigated for commercially available austenitic, martensitic and duplex stainless steel surfaces after femtosecond (fs) laser irradiation. For this purpose, highly regular LIPSS with almost identical geometric properties were generated on the stainless steels in an air environment by near-infrared fs-laser processing (λ = 1025 nm, τ = 300 fs, frep = 100 kHz, v = 0.67 m/s, Δx = 6 μm, F = 1.1 J/cm2) and compared to the polished initial surfaces as reference. The wetting with distilled water was evaluated as a function of aging time by contact angle measurements using the sessile drop method. The laser-induced morphological, topographical and chemical alteration of the surface was characterized by scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, glow-discharge optical emission spectroscopy as well as high-resolution imaging and scanning transmission electron microscopy. Despite an almost identical LIPSS-based topography alteration, the stainless steels revealed very different ageing behavior. For permanently used samples, it was shown that the ageing effect is less pronounced than for samples that were stored in ambient air following fs-laser irradiation until the wetting measurement. The results obtained make an important contribution to understanding and controlling the ageing effect of metal surfaces and thus facilitate the transfer of LIPSS-based functional surface engineering and design to industrial processes.

Structural Changes in High-Entropy Alloys CoCrFeNi and CoCrFeMnNi, Irradiated by He Ions at a Temperature of 700 °C.

High-entropy alloys (HEA) are promising structural materials that will successfully resist high-temperature irradiation with helium ions and radiation-induced swelling in new generations of nuclear reactors. In this paper, changes in the elemental and phase composition, surface morphology, and structure of CoCrFeNi and CoCrFeMnNi HEAs irradiated with He2+ ions at a temperature of 700 °C were studied. Structural studies were mainly conducted using the X-ray diffraction method. The formation of a porous surface structure with many microchannels (open blisters) was observed. The average diameter of the blisters in CoCrFeMnNi is around 1.3 times smaller than in CoCrFeNi. It was shown that HEAs' elemental and phase compositions are stable under high-temperature irradiation. It was revealed that, in the region of the peak of implanted helium, high-temperature irradiation leads to the growth of tensile macrostresses in CoCrFeNi by 3.6 times and the formation of compressive macrostresses (-143 MPa) in CoCrFeMnNi; microstresses in the HEAs increase by 2.4 times; and the dislocation density value increases by 4.3 and 7.5 times for CoCrFeNi and CoCrFeMnNi, respectively. The formation of compressive macrostresses and a higher value of dislocation density indicate that the CoCrFeMnNi HEA tends to have greater radiation resistance compared to CoCrFeNi.

Study on bulge structure formation mechanisms of laser remelting in air atmosphere

As a key feature of a part or assembly, surface structure can effectively improve the surface properties of the workpiece. Laser remelting is a promising method for surface structure formation, but sensitive to ambient gases. Therefore, this paper presents a numerical model that incorporates the contribution of air atmosphere on surface bulge structure formation. For this, the model couples a concentration field to reflect the oxygen mass transfer in the melt pool and introduces a semi-empirical function to describe the contribution of oxygen to the surface tension. Based on the model, it discusses the surface structure formation mechanisms, the heat-mass transfer characteristics, the melt flow patterns, and the surface force transient evolutions. The results reveal that the action of air atmosphere on melt flow manifests itself through two mechanisms, namely, the enhancement of inward thermocapillary forces and the introduction of outward solutocapillary forces. Furthermore, combined with the laser remelting experiments on 304 stainless steels, the established model is validated concerning the surface bulge structure size, the melt pool depth, and the oxygen distribution.

Reversible Laser Imprinting of Phase Change Photonic Structures in Integrated Waveguides.

Formation of laser-induced periodic surface structures (LIPSS) is known as a fast and robust method of functionalization of material surfaces. Of particular interest are LIPSS that manifest as periodic modulation of phase state of the material, as it implies reversibility of phase modification that constitute rewritable LIPSS, and recently was demonstrated for chalcogenide phase change materials (PCMs). Due to remarkable properties of chalcogenide PCMs─nonvolatality, prominent optical contrast and ns switching speed─such novel phase change LIPSS hold potential for exciting applications in all-optical tunable photonics. In this work we explore phase change LIPSS formation in thin films of Ge2Sb2Te5 (GST) integrated with planar and rib waveguides. We demonstrate that by fine-tuning laser radiation, the morphology of phase change LIPSS can be controlled, including their period and fill factor, and investigate the limitations of multicycle rewriting of the structures. We also demonstrate the formation of phase change LIPSS on a 1D waveguide, which has potential for use as tunable Bragg filters or structures for on-demand light decoupling into the far-field. The presented concept of applying phase change LIPSS offers a promising approach to enable fast and simple tuning in integrated photonic devices.

Simulation of femtosecond laser-induced periodic surface structures on fused silica by considering intrapulse and interpulse feedback

The formation of laser-induced periodic surface structures (LIPSSs) on fused silica upon irradiation with plane wave, double pulse, spot processing, and scanning processing (pulse duration tp = 35 fs, center wavelength λ = 800 nm, low repetition rate ≈1 kHz) is studied theoretically with an improved three-dimensional finite-difference time-domain plasma model. The model covers both intrapulse feedback under single shot and interpulse feedback under multi-shots, thus enabling better prediction of transient responses during laser–material interaction and the evolution of the ablated morphology and accumulated defects’ density with more shots. In simulations of a single plane wave, a double pulse can modulate LIPSS periodicity. In simulations of spot processing with Gaussian beam, an increase in the number of shots results in a noticeable ablation pattern where high-spatial-frequency LIPSS surrounds low-spatial-frequency LIPSS at a fluence of 2.8 J/cm2. Moreover, simulations of scanning processing with Gaussian beam showcase the broad applicability of this model, revealing that the orientation of the LIPSS depends on the polarization direction rather than the scanning path. This new model provides a powerful tool to simulate the formation of LIPSS on silica, particularly when temporally modulated laser is involved or predicting the evolution of morphology dependent on the number of shots.

SUPERHYDROPHOBIC AND ANTIBACTERIAL COATINGS ON VARIOUS COTTON FABRICS USING ZNO AND AESO

Superhydrophobic coatings on cotton utilized in medical applications like hospital gowns and bed linens to offer a protective barrier against fluids and bacteria. Masks were worn with different types of materials. In this study, various cotton employed ZnO and AESO to effectively decrease the surface energy of cotton fabric via a Schiff base reaction. This chemical transformation resulted in the formation of a textured surface structure that exhibited robust adhesion qualities. The study demonstrates that the superhydrophobic coating on silk fabric increases 153. 59%. The coating on silk provides a reference for fabric types with ZnO and AESO coatings.

Efficient Ablation, further GHz Burst Polishing, and Surface Texturing by Ultrafast Laser

A large variety of ultrafast laser‐matter interaction regimes and different processed surface finishing qualities of stainless steel can be achieved by varying the laser processing parameters. The optimization of the laser fluence to the most efficient ablation also leads to the lowest surface roughness of the ablated material. High laser fluence together with a high pulse repetition rate leads to heat accumulation, which results in the formation of microstructures with various morphology and scales. The use of GHz bursts allows a rough stainless‐steel surface to be smoothed and even polished. In this work, the fast and high‐quality milling of 2.5D cavities is demonstrated. Laser beam spot optimization is used to increase the throughput of the high‐power femtosecond laser (67.8 W), resulting in an ablation rate of 13 mm3 min−1. The complex cavities are produced by laser milling and cutting in layers. The formation of surface structures on stainless steel because of laser irradiation has been demonstrated. The possibilities of further GHz burst polishing are examined on previously laser‐milled stainless steel using with lowest possible surface roughness. The ability to polish the surface with microstructures below the original roughness is demonstrated by bursts of ultrashort pulses with a repetition rate in the GHz range. It has been demonstrated that mold milling in stainless steel designed for LED diffusers can be fabricated using a modern femtosecond laser source together with beam size optimization technique for efficient ablation with low surface roughness, layer‐by‐layer milling techniques, and GHz burst polishing techniques.

Polarization and kinetics of optical emission during laser induced periodic surface structure formation on crystalline silicon

Polarization and kinetics of secondary optical emission, which occurs during the formation of laser-induced periodic surface structures (LIPSSs) on crystalline silicon by a femtosecond laser beam in air, were analyzed. It was found that the emission lines of ablated Si atoms and formed in plasma SiN molecules are unpolarized, and they decay on the nanosecond scale. The second harmonic generation (SHG) is enhanced by laser-induced surface plasmons, and the SHG polarization basically repeats the polarization of the laser. The results can be used for optical monitoring of LIPSS formation in real time, which is especially important for laser processing of large non-planar surfaces.

Electrochemical and semiconducting properties of multifunctional smart conductive fabrics based on polypyrrole

AbstractWe have fabricated multifunctional conductive fabrics composed of polypyrrole/BaTiO3/poly(acrylonitrile‐co‐methylacrylate) (PPy/BaTiO3/P(AN‐co‐MA)(PBA) utilizing a simple process including dip coating and in situ chemical polymerization of pyrrole, which oxidized both ammonium persulfate (APS) and ammonium persulfate + iron (III) chloride (APS + FeCl3) for flexible and wearable textile applications. The influence of the oxidant nature, PPy content, and BaTiO3 on the surface structures, electrochemical, and semiconducting characteristics of the fabrics was examined using scanning electrochemical microscopy (SEM), electrochemical impedance spectroscopy (EIS), and the Mott–Schottky (M–S) studies. The study revealed that the size of the polymer nanostructures on fabric has an impact on the electrochemical impedance properties. Specifically, a decrease in diameter (when FeCl3 + APS is used) or the formation of more compact swelling surface structures (when only APS is used) is associated with changes in the conductivity of the coated fabric. Based on the EIS tests, the composite coating (S0.1, S0.2, and S0.3) with FeCl3 has a higher electrical conductivity compared to the coating with APS (PBA). The M–S tests indicate that the semiconducting characteristics of coated fabrics are dependent upon the kind of oxidant, the amount of PPy, and the presence of BaTiO3.

Investigating the transition from high to low spatial frequency periodic nanoripples on TiO2 (0 0 1) oriented surface upon irradiation with UV-femtosecond pulses

The formation of laser-induced periodic surface structures (LIPSS) on a TiO2 (001) oriented surface upon irradiation with UV-fs laser pulses was analyzed. Two systems of low spatial frequency LIPSS (LSFL) perpendicular to the direction of laser polarization are observed after N = 5 laser pulses. One was developed on the edges of the crater created by the first laser pulse, while the other appeared in the center of crater and spread towards the edges. When N was greater than 10, the two LIPSS systems merge. Both LSFL systems have a spatial period of approximately 200 nm and are due to the coherent superposition of surface waves scattered on inhomogeneities of a transient metallic surface. However they differ by the type of defects involved in the excitation process of surface waves. While a pronounced and localized rim triggered LSFL on the edge of the laser spot, LSFL in the central part nucleated at the shallow high spatial frequency LIPSS (HSFL), which are parallel to the laser polarization. A mechanism is proposed to explain the role of the HSFL in the formation of LSFL. Numerical simulations of the absorbed laser power density support the evolution from HSFL to LSFL as the number of laser pulses increased.

Impact of helium and hydrogen plasma exposure on surface damage and erosion of tungsten

The impact of helium plasma exposure on the tungsten surface damage structure development and erosion has been investigated by comparing the impact of hydrogen plasma exposure. Crystal orientation dependence of the undulating surface structure formation and erosion rate is observed on the plasma-exposed tungsten surface independently from the plasma species. The top surface of the plasma exposed tungsten has a tendency to {100} plane independently from the initial surface orientation. Although hydrogen and/or helium cause no erosion in tungsten under incident ion energy exposure conditions below the sputtering threshold, inevitable minute impurities, like oxygen, play an essential role in erosion, and significant erosion can be observed even at 30 eV.

Cost-effective superhydrophobic ZnO films with adjustable wetting behaviors deposited via solution precursor plasma spray process

In this work, ZnO films featuring hierarchical micron−/nanometer-sized structures were deposited for adjusting wettability by a simple solution precursor plasma spray (SPPS) process using cost-effective zinc acetate solution. The effects of solution flow rate on the surface microstructures, surface roughness, crystalline orientation, and wetting behaviors of the films were investigated comprehensively. The results showed that high solution flow rate facilitated the formation of apparent hierarchical surface structures featuring more uniformly-distributed micron-sized protrusions and higher surface roughness. The increase of solution flow rate also promoted the crystalline growth along the non-polar (100) plane. With the solution flow rate increasing from 10 ml/min to 40 ml/min, the synergy of the surface structures and crystalline growth orientation resulted in a sharp decrease of the surface energy of the films from ~74.4 mJ/m2 to ~12.6 mJ/m2 and a dramatic elevation of water contact angles from ~130° to ~155°. The wetting states of different films were elucidated by correlating the wetting behaviors with the surface structures. The SPPS-deposited ZnO films showed high adhesion to water droplets, similar to the wetting behaviors of the petal effect, which could be used as a “mechanical hand” for non-loss microdroplet transfer. The work shows that SPPS represents a promising method to deposit cost-effective superhydrophobic films with adjustable wetting behaviors.

Shallow Subsurface Structure of the Moon: Key Questions for Future Exploration

Lunar shallow subsurface structure is important in revealing the formation and evolution of the Moon. Therefore, a review of key issues in the lunar shallow subsurface structure will help deepen our understanding of the Moon. From a global perspective, lunar shallow subsurface structure is formed by endogenic and exogenic geological processes such as volcanic activities, tectonic activities, meteorite impacts, and space weathering. Its morphological characteristics and stratigraphic structure record the evolution of these geological processes. Recent lunar exploration missions have returned new samples and high-resolution data that have greatly enriched our knowledge. On the basis of reviewing the research progress of radar detection, crater-based excavation analysis, material inversion, and heat flow measurement, we also discuss the processes that contribute to the formation of the lunar shallow surface structure, such as volcanoes, impacts, tectonics, and space weathering. The main hot issues were sorted out and focused on 3 areas: transformation of lunar shallow subsurface structure by geological processes, environment and material composition of the lunar shallow surface structure, and physical properties of lunar shallow surface structure. Overall, existing research on the lunar subsurface has made significant progress, but it has also brought more new unsolved mysteries. It is necessary to introduce new applied payloads such as synthetic aperture radar (SAR), orbiter subsurface investigation radar (OSIR), or time-domain electromagnetic sounding (TDEM) to provide higher-resolution subsurface data, and develop better interpretation methodologies, to further deepen the understanding of the lunar shallow subsurface structure and indeed reveal the mechanism of lunar geological evolution.

Surface blackening of titanium alloy by laser nanosecond pulses

The possibility of developing and blackening the surface of titanium alloy by nanosecond pulses of an ArF laser, a Nd:YAG laser, and a copper vapor laser for use in space engineering is studied in this work. For various irradiation regimes, the optimal parameters for the formation of the necessary surface structures were determined. The possibility of applying a reliable color layer without the use of any paints on the surface of a titanium alloy as a result of treatment with an Nd:YAG laser beam is shown. In particular, this method of laser processing will make it possible to obtain black color on the surface of devices used in outer space to absorb and dissipate solar energy, and it is expected to increase the radiation resistance of such coatings and their service life.

Oxidation-driven 2D LIPSS on amorphous Si films atop metal layers: The interplay of near-field and far-field effects

We investigate the formation of Laser-Induced Periodic Surface Structures (LIPSS) on amorphous Si films coated on Al using circularly polarized femtosecond lasers. By controlling laser fluence, we induce oxidation-based surface patterns, with their regularity governed by the interplay between near-field and far-field contributions. Achieving the right balance between these effects results in a consistent hexagonal pattern. Varying the underlying metal type further revealed its impact on pattern quality: strong far-field contributions led to irregular patterns, while dominant near-field effects resulted in extensive oxidation. Our study reveals the critical balance between near-field and far-field effects in optimizing 2D LIPSS patterns, emphasizing the role of the underlying metal.

The effect of molten pool evolution on the formation of surface structures during laser polishing

As a novel surface treatment technology, in-situ laser polishing aims to improve surface finish of 3D printed parts, while laying the foundation for the integrated processing of 3D printing and laser polishing. However, the polishing process will introduce a few new surface structures (undercuts, step structures, ripples, etc.), which will affect the surface roughness. In this work, the surface roughness and surface structure after high power and low power polishing were compared respectively through experiments. In addition, a 2D transient numerical model was developed to explore the forming mechanism of different structures. The evolution of temperature field, velocity field and freedom surface of melting pool under different parameters were analyzed by the model. The results showed that the solidification induced surface structure with large roughness was generated after high power polishing. During its formation, the thermocapillary force was dominant and drove the tangential flow of fluid. While, low power polishing resulted in ripple structure with small roughness. The emergence of this structure was primarily due to the capillary force as the leading force in the melting pool flow process, which drove the normal flow of the melt, thus eliminating the large surface curvature. Furthermore, the width of the melting pool in the simulation was compared with the width of the single scanning track, and they were in good agreement. Finally, by adjusting the test strategy, it was found that the surface roughness could reach the lowest (0.88 ± 0.06 μm) only when the two structures coexisted.

Porosity Engineering of Dried Smart Poly(N-isopropylacrylamide) Hydrogels for Gas Sensing.

A recent study unveiled the potential of acrylamide-based stimulus-responsive hydrogels for volatile organic compound detection in gaseous environments. However, for gas sensing, a large surface area, that is, a highly porous material, offering many adsorption sites is crucial. The large humidity variation in the gaseous environment constitutes a significant challenge for preserving an initially porous structure, as the pores tend to be unstable and irreversibly collapse. Therefore, the present investigation focuses on enhancing the porosity of smart PNiPAAm hydrogels under the conditions of a gaseous environment and the preservation of the structural integrity for long-term use. We have studied the influence of polyethylene glycol (PEG) as a porogen and the application of different drying methods and posttreatment. The investigations lead to the conclusion that only the combination of PEG addition, freeze-drying, and subsequent conditioning in high relative humidity enables a long-term stable formation of a porous surface and inner structure of the material. The significantly enhanced swelling response in a gaseous environment and in the test gas acetone is confirmed by gravimetric experiments of bulk samples and continuous measurements of thin films on piezoresistive pressure sensor chips. These measurements are furthermore complemented by an in-depth analysis of the morphology and microstructure. While the study was conducted for PNiPAAm, the insights and developed processes are general in nature and can be applied for porosity engineering of other smart hydrogel materials for VOC detection in gaseous environments.

Chemical and Topographical Changes upon Sub‐100‐nm Laser‐Induced Periodic Surface Structure Formation on Titanium Alloy: The Influence of Laser Pulse Repetition Rate and Number of Over‐Scans

Titanium and its alloys are known to allow the straightforward laser‐based manufacturing of ordered surface nanostructures, so‐called high spatial frequency laser‐induced periodic surface structures (HSFL). These structures exhibit sub‐100 nm spatial periods – far below the optical diffraction limit. The resulting surface functionalities are usually enabled by both, topographic and chemical alterations of the nanostructured surfaces. For exploring these effects, multi‐method characterizations were performed here for HSFL processed on Ti–6Al–4V alloy upon irradiation with near‐infrared ps‐laser pulses (1030 nm, ≈1 ps pulse duration, 1–400 kHz) under different laser scan processing conditions, i.e., by systematically varying the pulse repetition frequency and the number of laser irradiation passes. The sample characterization involved morphological and topographical investigations by scanning electron microscopy (SEM), atomic force microscopy (AFM), tactile stylus profilometry, as well as near‐surface chemical analyses hard X‐ray photoelectron spectroscopy (HAXPES) and depth‐profiling time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS). This provides a quantification of the laser ablation depth, the geometrical HSFL characteristics and enables new insights into the depth extent and the nature of the non‐ablative laser‐induced near‐surface oxidation accompanying these nanostructures. This allows to answer the questions how the processing of HSFL can be industrially scaled up, and whether the latter is limited by heat‐accumulation effects.