Femtosecond Laser Surface Processing Research Articles

Unraveling the formation dynamics of metallic femtosecond laser induced periodic surface structures

Femtosecond laser surface processing (FLSP) is an emerging fabrication technique to efficiently control the surface morphology of many types of materials including metals. However, the theoretical understanding of the FLSP formation dynamics is not a trivial task, since it involves the interaction of various physical processes (electromagnetic, thermal, fluid dynamics) and remains relatively unexplored. In this work, we tackle this problem and present rigorous theoretical results relevant to low-fluence FLSP that accurately match the outcomes of an experimental campaign focused on the formation dynamics of laser induced periodic surface structures (LIPSS) on stainless steel. More specifically, the topography and maximum depth of LIPSS trenches are theoretically and experimentally investigated as a function of the number of laser pulses. Moreover, precise LIPSS morphology measurements are performed using atomic force microscopy (AFM). The proposed comprehensive simulation study is based on two-temperature model (TTM) non-equilibrium thermal simulations coupled with fluid dynamic computations to capture the melting metal phase occurring during FLSP. Our rigorous simulation results are found to be in excellent agreement with the AFM measurements. The presented theoretical framework to model FLSP under low-fluence femtosecond laser pulses will be beneficial to various emerging applications of LIPSS on metallic surfaces, such as cooling high-powered laser diodes and controlling the thermal emission or absorption of metals.

Femtosecond laser surface processing combined with thermal oxidation for enhancing corrosion resistance of AZ31B magnesium alloy in chloride solutions

A novel method of ultrafast femtosecond laser surface processing (FLSP) combined with the post thermal oxidation (TO) treatment was employed to improve the corrosion resistance of AZ31B magnesium (Mg) alloys. FLSP with laser powers (< 100 mW) mainly creates the nanostructures on AZ31B alloy surfaces, while the higher laser powers (> 200 mW) induce the hierarchical micro/nanostructures. According to the electrochemical results, the corrosion resistance of AZ31B alloys in Cl− solutions is weakened after the FLSP. However, an obvious enhancement in corrosion resistance is found when the femtosecond (fs) laser-processed samples subjected to the TO treatment in the temperature range from 100 °C to 280 °C. TO treatment actually supports the oxidation and growth of the nanostructure to form a dense film, considerably improving its protective ability and enhancing the corrosion resistance of Mg alloys in Cl− solutions. The uniform corrosion is evidently inhibited and localized corrosion emerges on laser-processed samples after the TO treatment.

Superior wear resistance in cast aluminum alloy via femtosecond laser induced periodic surface structures and surface hardening layer

Cast Al alloys have been increasingly used in aviation, shipping, aerospace, and automotive industries as important raw materials of engine parts. However, its poor wear resistance hampered their application in some tribological environments. To overcome the current issue, in present work, we proposed a femtosecond laser (fs-laser) surface processing strategy to fabricate pillar-shaped laser induced periodic surface structures (LIPSS), as well as a work-hardening layer (hardness increment reached 51.2% compared with that of substrate) with an affected depth of ∼ 75 μm, on the surface of cast Al alloy in one step. In this case, the wear performance of processed samples showed an obvious improvement with a 49.4% reduction of the cross sectional area of wear track. Our work broke the existing understanding about the strengthening mechanism of fs-laser surface modification technology, in which surface texture or shock wave induced by fs-laser were viewed as two isolated strengthening effects. For the first time, we achieved the combination of these two strengthening effects, and proved the effectiveness of this strategy. The present results provided theoretical bases for the further development of novel fs-laser surface technologies with better performance.

Corrosion characterization of NiTi alloy by femtosecond laser surface processing

AbstractThe study of the corrosion resistance of NiTi alloy surfaces with different wettabilities is important to achieve improvements in biocompatibility. In this study, a femtosecond laser was used to process different wettability surfaces on the NiTi alloy. The corrosion resistance of the surfaces was examined via potentiodynamic polarization and electrochemical impedance spectroscopy. Scanning electron microscopy and X‐ray photoelectron spectroscopy were used to analyze the morphology and chemical composition of the surfaces. The findings demonstrate that when an original oxide film is destroyed by femtosecond laser processing with a low laser fluence, hydrophilic or hydrophobic surfaces are more vulnerable to erosion. The corrosion resistance of superhydrophilic surfaces is improved to a certain extent, whereas superhydrophobic surfaces exhibit excellent corrosion resistance. Superhydrophilic surfaces are protected from further corrosion by the formation of a dense oxide film after corrosion, whereas superhydrophobic surfaces can inhibit corrosion because of their thicker oxide film and additional air film produced by air trapped in the micro/nanostructure.

Analysis of anti-reflection mechanisms of the black aluminum alloy made by femtosecond laser processing

Mitigating the optical reflection of aluminum alloy over a broad spectral range from 0.45 μm to 15 μm is vital for many applications. This can be realized by introducing efficient light-absorbing textured surfaces via femtosecond laser surface processing. However, a clear analysis of antireflection performance has not been reported yet. This paper proposes a numerical model of anti-reflective structures is proposed based on SEM and EDS characterization. Multiple anti-reflective mechanisms were revealed intuitively through FDTD simulation.

Reflection Coefficient Estimation of Femtosecond Laser Surface Processing Using Support Vector Regression

An image analysis-based Reflection Coefficient (RC) estimation method of femtosecond laser surface processing for the blackening of X-ray imaging sensor shell is proposed. The Support Vector Regression (SVR) is used for RC computation and both an offline and an online steps are considered in this method. Regarding offline step, the typical laser process parameters are set to perform surface processing and Scanning Electron Microscope (SEM) images are recorded. Then a series of image features are computed and both the computed image features and typical laser parameters are used to train SVR: the training dataset includes the laser line space, laser beam diameter, natural logarithm of laser power divided by laser frequency, and image features of Gray-Level Co-occurrence Matrix (GLCM); the supervising data are laser ablation diameters. As for online step, when SEM image data are recorded after laser processing, the trained SVR is used to predict laser ablation diameter and then the RC can be computed by laser ablation model. Many experiment results have verified the effectiveness of our proposed method, and the RC estimation accuracy can be better than 90.0%.

Emissivity prediction of functionalized surfaces using artificial intelligence

Tuning surface emissivity has been of great interest in thermal radiation applications, such as thermophotovoltaics and passive radiative cooling. As a low-cost and scalable technique for manufacturing surfaces with desired emissivities, femtosecond laser surface processing (FLSP) has recently drawn enormous attention. Despite the versatility offered by FLSP, there is a knowledge gap in accurately predicting the outcome emissivity prior to fabrication. In this work, we demonstrate the immense advantage of employing artificial intelligence (AI) techniques to predict the emissivity of complex surfaces. For this aim, we used FLSP to fabricate 116 different aluminum samples. A comprehensive dataset was established by collecting surface characteristics, laser operating parameters, and the measured emissivities for all samples. We demonstrate the successful application of AI in two distinct scenarios: (1) effective emissivity classification solely based on 3D surface morphology images, and (2) emissivity prediction based on surface characteristics and FLSP parameters. These findings open new pathways towards extended implementation of AI to predict various surface properties in functionalized samples or extract the required fabrication parameters via reverse engineering.

Drag reduction in minichannel laminar flow past superhydrophobic surfaces

We present friction drag reduction in minichannel laminar flow past superhydrophobic metallic surfaces. Femtosecond laser surface processing (FLSP), a one-step scalable method, was used to create permanent microscale and nanoscale structures on 316 stainless steel plates. The resulting superhydrophilic plates were covered by tall (215 μm) or short (27 μm) structures. The FLSP plates were then transitioned to superhydrophobic by evaporative deposition of a fluorinated silane, a low surface energy coating. Using purified water, the friction factor was obtained by measuring pressure drop along a minichannel with a rectangular cross section for flow rates corresponding to Reynolds numbers from 45 to 250. The superhydrophobic FLSP plates with the tall or short microstructures were used on the bottom surface of the minichannel. Pressure drop reduction was observed when compared to a smooth unprocessed stainless steel surface at the same flow rate. Compared to the drag experienced by the smooth unprocessed surface, the superhydrophobic FLSP surfaces exhibited a drag reduction over the entire range of Reynolds numbers tested. The drag reduction was attributed to the slip velocity created by the presence of an air layer (plastron) between the water and the superhydrophobic surfaces. The superhydrophobic FLSP surface with the tall microstructures exhibited drag reduction that was almost constant at approximately 15% for the entire range of Reynolds numbers tested. The drag reduction for the surface with the short microstructures decreased monotonically with Reynolds number and ranged from about 31% at Reynolds number of 45 to about 25% at Reynolds number of 250.

Near-unity broadband omnidirectional emissivity via femtosecond laser surface processing

It is very challenging to achieve near perfect absorption or emission that is both broadband and omnidirectional while utilizing a scalable fabrication process. Femtosecond laser surface processing is an emerging low-cost and large-scale manufacturing technique used to directly and permanently modify the surface properties of a material. The versatility of this technique to produce tailored surface properties has resulted in a rapidly growing number of applications. Here, we demonstrate near perfect, broadband, omnidirectional emissivity from aluminum surfaces by tuning the laser surface processing parameters including fluence, pulse count, and the ambient gas. Full-wave simulations and experimental results prove that the obtained increase in emissivity is mainly a result of two distinct features produced by femtosecond laser surface processing: the introduction of microscale surface features and the thick oxide layer. This technique leads to functionalized metallic surfaces that are ideal for emerging applications, such as passive radiative cooling and thermal management of spacecraft.

Surface and microstructure investigation of picosecond versus femtosecond laser pulse processed copper

A comparison is made between the self-organized surface features that form on copper using femtosecond versus picosecond laser pulses. Modification of material surfaces (like metals and dielectrics) have shown to affect the properties for applications including enhancing heat transfer and producing anti-corrosive or antibacterial surfaces. Femtosecond laser surface processing is a precise and repeatable technique to produce uniform self-organized quasi-periodic structures on the surface for most metals. However, it is difficult to produce uniform quasi-periodic structures on noble metals (e.g. copper). Using femtosecond pulses in the processing of copper results in non-uniform structure formation with areas that have little to no mound development. Lengthening the pulses from femtoseconds to picoseconds is shown to result in uniform quasi-periodic structure formation with high repeatability. The subsurface microstructure formed using femtosecond versus picosecond pulses while keeping other processing parameters the same was investigated, providing insight into mound formation processes. Picosecond pulses resulted in finer-grained regions within the mounds, and after a critical pulse count threshold was reached, onion-like layers with alternating high-density copper and highly porous copper oxide regions formed near the surface and blanketed the mounds. Multiple formation mechanisms are proposed to help explain the microstructures that formed using picosecond pulses.

Single-shot femtosecond laser processing of Al-alloy surface: An interplay between Mbar shock waves, enhanced microhardness, residual stresses, and chemical modification

Ambient-air single-shot ablative femtosecond laser surface processing of an advanced Al-alloy AA5083 was characterized by non-contact broadband ultrasonics, scanning electron microscopy with energy-dispersive x-ray spectroscopy, x-ray diffraction, and Vickers microhardness tests. The characterization indicates that the generated Mbar-level shock waves induce not only a strong structural modification of the processed micron-thick surface layer with sub-GPa-level residual compressive and tensile stresses, raising its microhardness by almost 45%, but also significant depth-dependent chemical modification within the layer.

All-optical, self-focused laser beam array for parallel laser surface processing.

A single femtosecond laser beam can be focused to process material surfaces. However, the low processing efficiency by a single laser beam prevents broad applications of femtosecond laser surface processing. Here we have demonstrated an array of high-intensity femtosecond laser beams that self-focus in a piece of glass medium, enabling high-efficiency, millimeter scale parallel laser surface processing. Experimental and simulation results suggest that the laser fluence of each focused laser beamlet in the array is more than 1.2 J/cm2, which is sufficient for micro- or nano-structure laser processing. To prove the concept, nanoscale structures on silicon surface are generated by the self-focused laser beam array, which may improve the efficiency of black silicon fabrication for photovoltaic devices.

Influence of Copper Oxide on Femtosecond Laser Surface Processed Copper Pool Boiling Heat Transfer Surfaces

Pool boiling heat transfer with the use of femtosecond laser surface processing (FLSP) on copper surfaces has been studied. FLSP creates a self-organized micro/nanostructured surface. In the previous pool boiling heat transfer studies with stainless steel FLSP surfaces, enhancements in critical heat flux (CHF) and heat transfer coefficients (HTCs) were observed compared to the polished reference surface. However, this study shows that copper FLSP surfaces exhibit reductions in both CHF and HTCs consistently. This reduction in heat transfer performance is a result of an oxide layer that covers the surface of the microstructures and acts as an insulator due to its low thermal conductivity. The oxide layer was observed and measured with the use of a focused ion beam milling process and found to have thickness of a few microns. The thickness of this oxide layer was found to be related to the laser fluence parameter. As the fluence increased, the oxide layer thickness increased and the heat transfer performance decreased. For a specific test surface, the oxide layer was selectively removed by a chemical etching process. The removal of the oxide layer resulted in an enhancement in the HTC compared to the polished reference surface. Although the original FLSP copper surfaces were unable to outperform the polished reference curve, this experiment illustrates how an oxide layer can significantly affect heat transfer results and dominate other surface characteristics (such as increased surface area and wicking) that typically lead to heat transfer enhancement.

3D Electron microscopy characterization of Ag mound-like surface structures made by femtosecond laser surface processing

Laser processing of metal surfaces by ultrafast pulse lasers is a developing technology with many potential uses including applications in heat transfer, medical implants, and tribology. Laser processed silver surfaces has several potential applications such as biomedical devices, antibacterial surfaces, and chemical sensors. However, there is a lack of previous research on laser processing of silver is more difficult to process compared with other metals. A newly investigated dual-pulse femtosecond laser surface processing technique was capable of producing self-organized, micro/nanoscale surface features on silver where single-pulse techniques had previously failed. Three-dimensional (3D) scanning electron microscopy (SEM) was used to examine mound-like structures produced by this new method to determine their composition and formation processes. The interior microstructure revealed that the mounds were comprised mostly of resolidified Ag grains with approximately 1% porosity. Hydrodynamically-driven fluid flow was the primary process that forms these surface structures without significant oxidation.

Creation of micro/nano surface structures on silver using collinear double femtosecond laser pulses with different pulse separation

Self-organized mound-like micro/nanoscale structures are reported for the first time on silver using a dual-pulse femtosecond laser surface processing technique. The dual-pulse laser processing technique reported in this paper uses femtosecond laser pulse pairs with a controlled temporal delay between the leading and trailing pulses. Using dual pulses at higher fluence values, mound-like micro/nanostructures have been created on silver samples for the first time. Formation of the self-organized microstructures is shown to be dependent on the time delay between the leading and trailing pulses. Mound-like microstructures do not develop on silver for overlapped pulses or using single-pulse femtosecond laser surface processing for the parameter space studied. Subsurface microstructure characterization of a single mound-like surface structure is analyzed by cross-sectional analysis using focused ion beam milling followed by scanning electron microscopy and energy dispersive X-ray spectroscopy.

Effects of Femtosecond Laser Surface Processed Nanoparticle Layers on Pool Boiling Heat Transfer Performance.

An experimental investigation of the effects of layers of nanoparticles formed during femtosecond laser surface processing (FLSP) on pool boiling heat transfer performance has been conducted. Five different stainless steel 304 samples with slightly different surface features were fabricated through FLSP, and pool boiling heat transfer experiments were carried out to study the heat transfer characteristics of each surface. The experiments showed that the layer(s) of nanoparticles developed during the FLSP processes, which overlay FLSP self-organized microstructures, can either improve or degrade boiling heat transfer coefficients (HTC) depending on the overall thickness of the layer(s). This nanoparticle layer thickness is an indirect result of the type of microstructure created. The HTCs were found to decrease with increasing nanoparticle layer thickness. This trend has been attributed to added thermal resistance. Using a focused ion beam milling process and transmission electron microscopy (TEM), the physical and chemical properties of the nanoparticle layers were characterized and used to explain the observed heat transfer results. Results suggest that there is an optimal nanoparticle layer thickness and material composition such that both the HTCs and critical heat flux (CHF) are enhanced.

Functional Surfaces with Enhanced Heat Transfer for Spray Cooling Technology

In this study the effects of nano/microstructuring and surface chemistry on wettability, evaporation rate and the Leidenfrost temperature are experimentally investigated. The functional surfaces with two alternative patterns were originally fabricated via direct femtosecond laser surface processing of polished silicon wafer in air at a fluence slightly above ablation threshold. The droplet lifetime method was used to measure the evaporation rate of a water droplet (4.5 μL) at surface temperatures of 25–350°C and to determine the Leidenfrost temperature. Generally, after processing the functional surfaces with hierarchical surface morphology demonstrate enhanced wetting behavior, evaporation rate enhancement and positive shifts in the Leidenfrost temperature. The functional surfaces with a microgrooved surface pattern, extensively covered by flake-like nanostructures, exhibit strong superhydrophilicity, resulted in a significant temperature-dependent enhancement of evaporation rate (up to 6 times) and an increase of about 30°C in the Leidenfrost temperature relative to the polished surface. The functional surfaces with a microcratered surface pattern being only hydrophilic demonstrate a nearly twofold temperature-independent enhancement of evaporation rate. Thermostability tests showed the heating of the functional surfaces above 340°C to be resulted in a drastically deteriorated wettability and a reduction of evaporative heat transfer performance under repeated experiments.

Condensation and subsequent freezing delays as a result of using femtosecond laser functionalized surfaces

In this paper, the authors report on the use of femtosecond laser surface processing (FLSP) to enhance the anti-icing properties of a commonly used aircraft alloy, Al 7075-O Clad. By changing the surface morphology through FLSP and the surface chemistry through siloxane vapor deposition, the wettability of Al 7075-O Clad was altered. Tall mound and short mound FLSP functionalized surfaces were created through two sets of laser parameters. Condensation and the subsequent freezing of condensates on FLSP Al 7075-O Clad was studied. Both structure height and surface wettability were shown to play a role in the delay of freezing. Freezing occurred on the FLSP superhydrophilic surface faster than on the unprocessed Al 7075-O Clad surface; however, freezing was delayed for all superhydrophobic FLSP surfaces. Tall structure height FLSP functionalized surfaces delayed freezing time longer than short structure height FLSP functionalized surfaces although all were superhydrophobic. It was shown that FLSP functionalized surfaces were able to delay freezing by up to 530 s compared with unprocessed Al 7075-O Clad. The authors also report on self-propelled condensate jumping on FLSP surfaces during the condensing process. The self-propelled jumping phenomena provide a means to promote anti-icing of materials, especially where jumping drops can be swept away in flow conditions.

Surface functionalization under water using picosecond and femtosecond laser pulses – first observations and novel effects

Functionalization by changing the topography of workpiece surfaces is interesting for many applications. Lasers – especially ultrashort pulse lasers – are flexible tools for this kind of tasks. Besides deterministic structures they allow for processing surfaces in a way that stochastic structures appear. Ripple and spike structures can be fabricated in air. In liquids, however, there are only a few investigations and many open questions. The idea of this paper is to check how water in contact to the workpiece surface affects femtosecond laser surface processing and what kind of structures appear. Results obtained with a picosecond laser are presented for comparison.

Effect of topology and material properties on the imprint quality of the femtosecond-laser-induced surface structures

Femtosecond laser surface processing (FLSP) is a technique that can be used to modify surfaces imparting many desirable properties, such as an improved wettability, heat transfer, and drag reduction. The morphology of the FLSP surface along with surface chemistry is critical factors that strongly influences the wetting behavior of surfaces produced. Reproducing the FLSP original surface over a large area in an economic manner is a challenge. In this paper, imprinting the FLSP surface on oxygen-free copper is investigated. Based on the topology of an FLSP titanium surface, a model has been established to investigate the topology of the mounds, which is defined by the roughness and period between the self-organized structures. This topology affects the imprint quality indicated by the plastic deformation in the blank and is defined as the target material specimen. Both oxygen-free copper and stainless steel 304 were used as blank target material in this study and represent both soft and hard material specimens. The stress–displacement relationship was used to determine the sensitivity of imprint parameters. The imprint model was verified by comparing an equivalent imprint model with nanoindentation experimental results. The model results showed that the die’s morphology, in particular the die roughness-to-period ratio (referring to the peak-to-valley height and the peak-to-peak distance), had greater influence than the blank material itself.