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Abstract
Laser-induced plasma micromachining (LIPMM) is an advanced technology that utilizes the plasma generated from laser breakdown to remove material, thereby facilitating the fabrication of microstructures. This paper explores the use of LIPMM on 304 stainless steel surfaces parallel to the laser beam in different solutions, focusing on the impact of the liquid environment on the machining process. It presents a theoretical analysis of the material removal mechanisms unique to this orientation and experimentally investigates how water, a salt solution, and ethanol affect plasma shockwave characteristics. Notably, the plasma shockwave in the salt solution demonstrates the most significant peak pressure and energy, enhancing the micromachining efficiency. These findings suggest that varying the liquid environment can significantly influence LIPMM's effectiveness, offering potential improvements in precision and control. This study broadens the understanding of LIPMM applications, especially in orientations not commonly explored, and opens new possibilities for advanced micromachining techniques in various industrial applications.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Research in biomimicry has shown that the microstructure of surfaces is the most important reason for the special functionality of biological surfaces in nature [1]. For example, the dense microcolumn structure and the nanoscale villi on the microcolumn make the lotus leaf surface with superhydrophobic property and self-cleaning capability [2]. Shark skin is composed of oriented diamond-shaped dermal denticles which are each covered with five conical ridges. This ingenious microstructure dramatically reduces the resistance of the shark as it swims through the water [3]. Inspired by these natural organisms, researchers have begun to apply various microstructures to enhance a range of surface properties of materials, including tribological performance [4], wettability [5,6], optical properties [7,8], and biological characteristics [9]. However, due to the scale limitations of microstructures, issues such as precision, repeatability, and cost arise during processing [10]. Consequently, the development of manufacturing techniques for these structures remains a topic worthy of further investigation.
Laser processing, with its advantages such as high precision, good controllability, environmental friendliness, and non-contact, provided new possibilities for micro-manufacturing technology [11]. By adjusting laser processing parameters such as energy, wavelength, polarization, and repetition rate, we can utilize lasers to fabricate surfaces with specific functional microstructures on given materials. However, for some materials that are difficult to absorb laser energy directly, such as transparent materials and those with high reflectivity, the processing efficiency is low, and in some cases, processing may not even be feasible [12]. To address these limitations, laser-induced plasma micromachining (LIPMM) has been proposed. During LIPMM, although solid targets can also be used to induce plasma, their use is generally limited to processing the rear surface of transparent materials [13]. Similar to laser direct writing, it has similar limitations and lacks the universality required for diverse processing applications. Therefore, current research on LIPMM primarily involves processing materials submerged in liquid solutions. Pallav et al. [14] indicates that during LIPMM, the laser breaks down the liquid above the workpiece to form plasma. The plasma then explosively expands, generating heat and shock waves, which in turn remove the material through melting and vaporization.
Subsequently, researchers began an extensive study of LIPMM in liquids. Wang et al. [15] developed an axisymmetric model combining the effects of cascade, multiphoton ionization, and recombination and diffusion losses to simulate the spatial and temporal plasma profiles at various pulse energies in distilled water during LIPMM. Saxena et al. [16] conducted LIPMM using saline solutions instead of distilled water. This approach leveraged the availability of dissociated ions in salt, which enhanced the plasma energy density. Consequently, this led to an increased rate of material removal, illustrating the significant impact of ion presence in the machining medium on the performance of the LIPMM process. Wang et al. [17,18] found that a stable flowing water layer benefits the production of a consistent plasma in LIPMM. This stable water flow assists in removing ablation debris, bubbles, and excess heat. This process results in the fabrication of microchannels with reduced thermal impact, smoother surfaces, and higher aspect ratios. In addition, integrating magnetic field assistance in LIPMM has emerged as a key research focus. Tang et al. [19] discovered that in a repulsive magnetic field, the Lorentz force can constrain the volumetric expansion of plasma, resulting in narrower microchannels. Zhang et al. [20] examined the influence of magnetic field orientation on plasma plume characteristics. They found that a longitudinal magnetic field has a more significant impact on the geometric dimensions and intensity of the plasma compared to a transverse magnetic field. Furthermore, Zhang et al. [21] also noted that the addition of a magnetic field has the most significant impact on the machining of magnetic materials.
In addition to research and optimization in the LIPMM process, there are also numerous studies focused on its applications. Wang et al. [22] applied LIPMM to the processing of microchannels on Al2O3/TiC ceramic surfaces and succeed to produce microchannels with greater depth and superior surface morphology. Zhang et al. [23] employed LIPMM for processing silicon and conducted a thorough study on the formation, expansion, and collapse of bubbles in the liquid during the machining process. It was shown that the elastic modulus and surface hardness of silicon surfaces are reduced by the effects of explosion bubbles. Lu et al. [4] utilized the LIPMM technique to successfully create microstructures on the edge of surgical blades, mimicking the structure of miscanthus leaves. This innovation aimed to enhance the cutting performance of the surgical blades. Zhang et al. [24] machined an array of cone-shaped protrusions on the surface with a micro-nano hierarchical structure on the titanium alloy Ti6Al4V for the regulation of surface wettability and anticondensation via LIPMM.
The studies discussed above clearly demonstrate the immense potential of LIPMM in the field of microstructure fabrication. It's important to note that in all the discussed LIPMM research, the laser is vertically incident on the material surface. However, in practical applications, it's often not feasible to directly position the laser vertically above the material. For instance, machining microstructures inside narrow tubes to enhance their friction and corrosion resistance [25], or creating secondary structures on the inner walls of grooves for equipment requirements [26], presents challenges due to the inability of vertical laser incidence. This highlights the need for further research and development to adapt LIPMM for these complex scenarios. In our previous research [27,28], we developed a two-step micro-hole processing strategy involved punching through and modification based on backside-water-assisted laser drilling. This strategy used laser-induced plasma to modified the formed through-holes, addressing issues like non-circular exits caused by laser polarization and reducing the taper of the micro-hole. This study demonstrates that LIPMM doesn't necessarily require the laser to be vertically incident on the material surface. It showcases the feasibility and effectiveness of LIPMM in scenarios where the laser incidence angle varies.
To further explore the potential of LIPMM in extreme machining scenarios, this study investigated the machining performance of LIPMM when the laser incidence direction is parallel to the material surface. The material removal principle when using laser parallel to the material surface for LIPMM was elucidated through theoretical analysis. A combined approach of theory and experimentation was employed to investigate and compare the shockwave characteristics in three liquid solutions. Finally, LIPMM using laser beams parallel to the material surface in the three solutions was conducted, and an analysis and discussion of the resulting surface morphology were carried out. This research provides insights into the adaptability and effectiveness of LIPMM in non-standard orientations, potentially expanding its application scope in specialized machining contexts.
2. Theory
2.1 Laser-induced breakdown and plasma evolution
The optical breakdown mechanism plays a significant role in the processing characteristics of laser-induced plasma. This is not only due to its predictive capacity regarding plasma intensity but also involves the evolution of the plasma through laser-plasma interactions. These factors are crucial in influencing the processing performance of LIPMM. The general rate equation for the evolution of free electrons in an irradiated medium can be represented by [29]:
The laser intensity I reaching the breakdown region is determined by the Beer-Lambert law:
where I0 represents the laser intensity above the liquid solutions, and α, h are the absorption coefficients and travelling distances through water, respectively.2.2 Principle of material removal
In previous studies on LIPMM [16,19] where the laser is incident perpendicular to the material surface, it is generally considered that the laser should be focused a few tens of micrometers above the material. After the plasma is formed, it directly contacts the material, facilitating heat transfer, which leads to the removal of the material. However, as indicated by the literature [15,16], laser-induced plasma in water exhibits an elongated morphology, with the plasma scale along the laser propagation direction significantly greater than its transverse scale. Therefore, as shown in Fig. 1, in LIPMM where the laser is incident parallel to the material surface, achieving direct contact between the plasma and the material surface requires positioning the laser focus significantly closer to the surface, which is a notably challenging task. On the other hand, due to the gaussian distribution of the laser beam, which is simplified as a conical beam in Fig. 1 for ease of illustration, forcibly moving the laser towards the workpiece surface will inevitably lead to the increasing obstruction of laser energy by the edges of the workpiece. Furthermore, once the focus is level with or below the surface of the workpiece, the laser will no longer be able to induce breakdown in the solution. However, focusing the laser too far away from the material surface would inevitably result in the inability to transmit plasma energy to the workpiece's surface. Therefore, in LIPMM where the laser is parallel to the surface of the workpiece, achieving direct contact between high-energy plasma and the material is exceedingly difficult. This also implies that when using laser beams parallel to the material surface for LIPMM, the material removal efficiency is not as effective as traditional LIPMM methods. Taking all factors into consideration, in this study, the laser was focused 50 µm above the material surface.
The specific material removal principle of LIPMM using laser parallel to the material surface is depicted in Fig. 1. The material removal principles [16,30] include the thermal energy transfer of the plasma as well as the mechanical energy generated by the plasma shockwave and bubble cavitation. Firstly, the shockwave generated during the initial expansion of the plasma, with pressures reaching up to GPa levels [31,32], impacts and damages the material surface. Subsequently, as the plasma expands to the workpiece surface, heat transfer occurs, leading to further removal of the softened material previously affected by the shockwave. Finally, the rapid pressure drop resulting from plasma bubble cavitation leads to the expulsion of debris assisted by high-speed jets. It is worth mentioning that, under the conditions of this experiment, the 50 µm distance results in a significant energy decay as the plasma expands to the workpiece surface due to the recombination of electrons within the plasma. At this point, the role of the shockwave at the front of the expanding plasma which is often overlooked in previous LIPMM studies becomes more significant in material removal. Therefore, in order to investigate the machining performance of LIPMM using laser beams parallel to the material surface, it is essential to conduct further in-depth research on the plasma shockwave.
As described in the literature [33], during the interaction between high-power-density lasers and materials, plasma absorbs a significant amount of laser energy. It then expands and propagates outward at a certain velocity, a process termed as laser-supported absorption wave (LSAW). LSAWs are typically categorized into two types which are linked to the laser's power density. The first type, which propagates at supersonic speed, is known as laser-supported detonation wave (LSDW). The second type, propagating at subsonic speed, is called laser-supported combustion wave (LSCW). When the laser's power density is sufficiently high, the ionized plasma expands outward at supersonic speeds. The forefront of this plasma expansion wave is referred to as LSDW, which is a principal factor in the removal of materials during LIPMM using laser parallel to the material surface. As a primary factor in material removal, the effective energy carried by the shockwave in the near field is given by [31]:
where Rs is the radius of the shockwave, ρ is the undisturbed density of liquid solutions, c0 is the sound speed in the liquid solutions, and Ps is the pressure of the shock wave front. Based on the conservation of momentum at a shock wave front, the following relationship can be derived [32]: where vs is the shock wave velocity and vp is the particle velocity behind the shock wave front. P0 represents the undisturbed pressure of liquid solutions. Additionally, vp can be further expressed as follows [32,34]: where c1 = 5190 m/s and c2 = 25306 m/s are constants. It should be noted that the values of c1 and c2 in this formula are derived from water environments. There will inevitably be errors when using this formula to calculate the particle velocity behind the shock wave front in other liquids environments, but these errors are within an acceptable range in this study. This is because the two main factors affecting particle velocity are solvent density and viscosity. The difference in density is already reflected in the formula. As for the influence of viscosity, the differences in viscosity among the three solutions studied in this paper, water (1.01 mPa·s), 2% salt solution (1.08 mPa·s), and ethanol (1.074 mPa·s), are very small. Therefore, the use of this formula in this study is relatively reasonable. Then, the pressure of the shock wave front can be given by:By taking the derivative of the shockwave radius vector, the modulus of the shockwave velocity vector as a function of time can be obtained, as follows:
In the following, by obtaining the relationship between the shockwave radius and time through experiments, the energy carried by the plasma shockwave can be derived using the series of equations mentioned above.
3. Experiment details
3.1 Experiment setup
The optical processing system and optical detection system employed in the experiment are depicted in Fig. 2. The laser used for processing is a Nd: YAG laser with a wavelength of 1064 nm and a pulse width of 10 ns. The laser beam first passes through an aperture, then through a reflective mirror, and finally focuses into the liquid through a focusing lens to induced plasma. On the other hand, the detection system consists of a Nd: YAG laser with a wavelength of 1064 nm and a pulse duration of 2 ns, along with a single-frame CCD camera, which is utilized to visualize the rapid process of laser breakdown. The KTP frequency doubling crystal is employed to convert the 1064 nm laser into visible light at 532 nm. Subsequently, the detection laser enters an aperture and a collimating beam expander system, achieving energy attenuation and generating wide-field illumination. Finally, the detection laser passes through the breakdown region, carrying the breakdown information, and enters the CCD camera. A digital time delay pulse generator is utilized to trigger both the laser and CCD camera. By capturing the time delay between the plasma flash detected by the photodiode and the detection light beam, precise timing for CCD image acquisition is achieved. The laser used for processing and the detection laser remain perpendicular to each other within the breakdown region. During the LIPMM process, the workpiece is placed on a support inside a glass container filled with a liquid solution, and the laser beam is directed parallel to the surface of the workpiece.
3.2 Experiment design
Based on the previous theoretical analysis, it is evident that when using laser beams parallel to the workpiece surface for LIPMM, the plasma shockwave plays a crucial role in the material removal process. Therefore, before conducting the machining experiment, a CCD camera was employed to capture the expansion behavior of the shockwave, which was then utilized to calculate the pressure of the shockwave front, and the energy carried by the shockwave. To achieve better machining performance, the three most commonly used solutions in LIPMM, namely water, salt solution, and ethanol, were employed for comparative research. The concentration of the salt solution was selected as 2% based on Ref. [16]. Calculation parameters for each solution are shown in Table 1. For the convenience of capturing the expansion behavior of the shockwave, no workpiece was placed during this time, and the laser was directly focused on the liquid solution. Subsequently, the workpiece was placed at a depth of 1 cm into each of the three solutions for LIPMM separately. The focusing point of the laser was about 50 µm above the workpiece. The workpiece selected was 304 stainless steel which is commonly used. This study exclusively investigates the fundamental impact of a single pulse, and the laser pulse energy remains constant at 45 mJ for both the study of shockwaves and the machining process in different solutions. Following the processing of the workpiece, it was subjected to ultrasonic cleaning, and then its surface morphology was observed using confocal laser scanning microscope produced by Olympus (OLS4100). To ensure experimental accuracy, all experiments under each set of conditions were repeated five times, and the results were averaged.
Table 1. Calculation parameters for each solution
4. Results and discussion
4.1 Plasma shockwave in different solutions
The shadowgraph images depicting the evolution of shockwaves in different solutions are shown in Fig. 3. Due to the resolution limitations of the CCD camera, the initial delay for capturing the shockwaves in all three solutions is set at 0.05 µs. At this point, the separation of the shockwaves is already quite discernible. In Fig. 3(a) and Fig. 3(b), it can be observed that the shockwaves in water and salt solution appear elliptical before 0.1 µs. This is because the plasma generated by breakdown forms a long, elongated shape along the direction of laser incidence. After 0.1 µs, the shadowgraph images of the shockwaves gradually transform into a more circular shape. While, the plasma shockwave formed in ethanol, as shown in Fig. 3(c), appears circular right from the initial captured shape, indicating a more concentrated optical breakdown in ethanol, devoid of multi-point or linear breakdown events. The reason for this phenomenon is speculated to be due to the higher ionization energy of ethanol, leading to a higher breakdown threshold. This makes breakdown in ethanol more difficult to occur, with breakdown only happening in a concentrated region where the laser energy is highest. In contrast, laser breakdown in water and salt solutions is relatively easy, resulting in breakdown even in areas with lower laser energy density, thus forming a longer elliptical breakdown region.
Fig. 3. Shadowgraph images of the shockwaves in different liquid solutions: (a) water, (b) salt solution, (c) ethanol
The radius-time relationship of shock waves in three different solutions is depicted in Fig. 4(a). It is observed that at the same delay, the radius of the shock wave in the salt solution is the largest, whereas it is the smallest in ethanol. The expansion velocity of shock waves in these solutions over time was calculated by determining the average velocity between each pair of adjacent time points, as illustrated in Fig. 4(b). It is noticeable that the expansion velocity of the shock wave in the salt solution is the fastest, reaching a peak of 1834 m/s, while the velocity in ethanol is significantly slower than that in water and salt solution, with a maximum of only 1536 m/s. In all three solutions, the shock waves exhibit the highest expansion velocities at the initial stages of measurement, followed by a rapid decrease, eventually converging to the sound speed of the respective solutions. This velocity trend aligns with the studies conducted by Yang [31] and Vogel [32]. This phenomenon is attributed to the continuous propulsion of the surrounding medium by the propagating shock wave, a process that dissipates energy due to the viscosity and compressibility of the medium. Moreover, as the shock wave expands, the surface area of its front increases, leading to a reduction in energy per unit area. This results in the deceleration of the shock wave. When the velocity of the shock wave approaches the sound speed, its characteristics begin to transform, evolving from a shock wave to a conventional sound wave. As sound waves do not generate intense compression and rarefaction effects during propagation, their velocity stabilizes at the sound speed of the medium.
Figure 5(a) presents the relationship between the pressure of shock waves and delay in different solutions. The trend observed in pressure variation is consistent with that of the shock wave expansion velocity, characterized by an initial sharp decrease followed by a gradual leveling off. Within the experimental delay range, the peak pressure of the shock wave in the salt solution is 265.1 MPa, significantly higher than that in water and ethanol. A higher peak pressure of the shock wave implies a more effective material removal in the process of LIPMM using laser parallel to the material surface. This correlation is based on the principle that greater shock wave pressures can enhance the impact force exerted on the material surface, thereby improving the efficiency and efficacy of material ablation. Theoretically, based on the findings of this calculation, the salt solution emerges as the most superior medium in terms of machining performance among the three selected solutions. Besides, the rate of pressure decay in the salt solution is the fastest in the early phase (t < 0.125 µs). This is attributed to the higher density of the salt solution compared to water and ethanol. In denser solutions, shock wave expansion requires more energy to propel the medium ahead. Notably, the peak pressure of the shock waves calculated in this study did not reach the GPa level, which is lower than the results calculated by Vogel [32]. The reason for this discrepancy is that the shock wave pressure calculations in this study commence from a delay of 0.075 µs, and the pressure decay is extremely rapid in the initial phase. This implies that the actual peak pressure of the shock wave (t < 0.075 µs) should be significantly higher than the calculated values in this study. This underestimation is attributed to the limitations in experimental precision. The methodology and analysis, although constrained by the onset of measurement and experimental precision, provide a reliable depiction of the relative trends and comparative behaviors of shock wave pressures across various mediums. This contributes valuable insights into the dynamics of shock wave interactions in different solutions, offering a significant understanding of the LIPMM using laser parallel to the material surface.
Fig. 5. (a) Variation of shock wave pressure with delay in different solutions, (b) energy of shock waves in different solutions
Figure 5(b) displays the energy of the shock waves in the three tested solutions, with the highest energy observed in the salt solution. The energy of the shock wave in the salt solution is quantified as 12.61 mJ, constituting 28.02% of the laser energy. Conversely, the shock wave energy in ethanol is the lowest at 9.78 mJ, accounting for 21.73% of the laser energy. This phenomenon can be attributed to the presence of Na+ and Cl− in the solution. These ions reduce the ionization energy (ΔE) required for the solution to undergo ionization. According to Eqs. (2) and (3), a lower ionization energy implies a lower breakdown threshold and results in the formation of a plasma with a higher electron density during the breakdown process. This dense plasma generates stronger shock waves, contributing to the enhanced performance of material removal. However, the study acknowledges that the calculated proportion of shock wave energy to the total laser energy is on the lower side. This underestimation is similar to the previously discussed pressure analysis. The primary reason for this discrepancy is attributed to the time point from which the shock wave calculations begin, set at 0.075µs. During the period before delay equals 0.075 µs, the shock wave has already expended a considerable amount of energy in its expansion. This means that the initial, potentially more energetic phase of the shock wave is not captured in the calculations, leading to a lower reported energy percentage. Despite the acknowledged calculation error in determining the shock wave energy, this discrepancy does not impede the further elucidation that salt solution is the most suitable medium for LIPMM using laser parallel to the material surface.
4.2 Comparison of processing performance
Following the calculation of relevant shock wave parameters, practical machining experiments were conducted to validate the theoretical computations. This experimental phase is crucial in bridging the gap between theoretical predictions and real applications. It involves applying the insights gained from the shock wave analyses, such as peak pressure and energy levels in different solutions, to actual LIPMM on surfaces parallel to the incident laser. Figure 6 showcases the machining results in different solutions. It can be observed that the material removal results in pits that are approximately elliptical in shape. This morphology aligns with the initial form of the plasma and the plasma shock wave. The elliptical shape of the pits indicates the influence of the plasma's spatial distribution and the dynamics of the shock wave propagation during machining.
Fig. 6. Machining topography and topography after ellipse fitting in different solutions: (a) water, (b) salt solution, (c) ethanol
The machined surface topography has been fitted into an elliptical shape, with the major and minor axes of the ellipse used to describe the specific dimensions of the pits formed during the machining process. The detailed measurements are presented in Fig. 7. It is observed that the pits machined in the salt solution exhibit the longest major axis and the shortest minor axis, while the dimensions in the ethanol solution are the opposite. This observation aligns with the previously analyzed breakdown characteristics, indicating that breakdown in the salt solution more readily occurs, causing the plasma to elongate along the direction of the laser. In contrast, the breakdown points in ethanol are relatively more concentrated. Additionally, the pits machined in the salt solution demonstrate the greatest depth of 13.3 µm. This can be attributed to the fact that the plasma and plasma shock wave generated during the breakdown in the salt solution possess higher energy. This higher energy results in more efficient material removal, leading to deeper pits. Such results not only corroborate the theoretical analysis but also highlight the significant impact of solution properties, especially the breakdown characteristics, on the machining outcomes in LIPMM on surfaces parallel to the incident laser.
It is indeed important to acknowledge that the machining results obtained in this experiment are relatively coarse, primarily due to the study focusing exclusively on the machining characteristics of single laser pulses. For future research, the use of high repetition rate lasers can allow for a more nuanced and precise machining process. By coordinating the movement of the laser relative to the material, it is possible to fabricate microstructures with specific properties on the surface parallel to the incident laser.
5. Conclusions
This study innovatively applies LIPMM to the processing of material surfaces parallel to the direction of laser incidence. The study integrates theoretical calculations with experimental methods to discuss the machining performance of this new technique in various solutions. The specific conclusions drawn from this research are as follows:
- (1) In the application of LIPMM using a laser parallel to the material surface, the mechanism of material removal differs from that of traditional LIPMM. In this new process, the energy of the plasma has already attenuated to some extent by the time it expands to the material surface, meaning it cannot be considered the sole or absolute cause of material removal. Instead, the impact and softening effect of the plasma shock wave on the material becomes a critical factor that cannot be overlooked.
- (2) Among the three solutions used in this study, the plasma shock wave generated in the salt solution breakdown exhibited the highest peak pressure and energy. Water ranked second in these terms, while ethanol had the lowest peak pressure and energy in its plasma shock wave.
- (3) Using salt solution for LIPMM on material surfaces parallel to the laser incidence direction demonstrates superior machining performance compared to water and ethanol. This superiority is primarily reflected in the depth of the pits machined on the material. Besides, the morphology of the pits produced in each solution corresponds with the analyzed characteristics of the shock waves, which lends a certain degree of accuracy to the findings of this study.
Funding
National Natural Science Foundation of China (12374289).
Disclosures
The authors declare that there is no conflict of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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