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Abstract
Femtosecond (Fs) laser micromachining is the most effective and flexible method for edge-cutting or transforming the physical properties of various crystalline brittle materials. Fs-laser micro-machining produces slag on the residual surface of micro-structures that reduces the quality and processing efficiency of a machined residual surface. In order to overcome the challenges overlaid during the processing of brittle materials, Fs-laser assisted ultrasonic nitrogen jet micro-machining technique is proposed. The method was applied to quartz chips to investigate improvements in surface quality after laser processing. For conceptualizations, an ultrasonic nitrogen nozzle based on a piezoelectric transducer was designed and Finite Element Method (FEM) was employed to realize the transition of a flow field. An experiment was performed that differentiate the quality of micro-grooves into quartz chips, and the results promote the significance of Fs-laser-assisted ultrasonic nitrogen jet micromachining for the processing of brittle materials. Besides, the machining quality at residual surfaces of quartz chips after laser processing was substantially improved. The process provides an aid to break down the slag into further tiny nano-particles and prevent a recast layer, meanwhile, it enhances the surface quality and processing efficiency without implementing any extensive procedure.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Crystalline brittle materials such as quartz [1,2], lithium niobate [3], and sapphire [4], have been widely used in several key industrial applications in the field of aerospace, defense, and consumer electronics due to their exceptional physical and chemical characteristics, including low fracture toughness, high brittleness, and high hardness. The conventional machining procedures exhibit several flaws to attain the maximum benefits of crystalline brittle materials in which high-quality residual flat surfaces and precision accuracy are the most needed. Laser machining is frequently used as a progressive contactless technique for the micromachining of brittle materials. Typical laser machining is classified into three categories including continuous wave, pulsed and ultra-fast. Continuous wave laser machining presents advantages of stability and longevity, but it tends to generate higher thermal stresses due to the high energy flow density, which defects the properties of brittle materials such as chipping and micro-cracking [5]. On the other hand, Fs-laser functions with ultra-short pulses and shows the advantages of ultra-high peak power and ultra-short pulse duration that effectively reduce the impact of heat accumulation while interacting with different brittle materials. The entire micro-machining process significantly improves precision accuracy in a micron scale, and is found to be a best choice for the processing of crystalline brittle materials. In recent practices for the processing of brittle materials, the accumulated threshold during Fs-laser micro-machining generates slag at the process surfaces that appear in the form of a recast layer, which indeed ejected and deposited on the surface as quasi-liquid and aggregate under thermal explosion. The overlaying of a recast layer causes degradation in the surface quality of brittle materials [6]. In addition, the random dispersion and overlaying of slags also reduce the laser etching efficiency by obscuring the laser propagation [7].
To reduce the impact of residual slags during laser machining, several methods have been reported such as jet-assisted laser machining [8–14], vibration-assisted laser machining [15–19], ultrahigh repetition rate Fs-laser [20,21], and etching-assisted Fs-laser modification [22]. Few methods out of those can be applied through traditional Fs-laser equipment, and not required additional post-processing are discussed as follows. Tangwarodomnukun et al. [13] developed a laser-waterjet hybrid ablation technique where a water-jet was applied off-axially to expel the softened elemental material by laser radiation and cool down the material in close proximity to damage-free micromachining. Chen et al. [10] explored water jet-assisted laser etching of polysilicon materials, and investigated different levels of surface quality for etched micro-grooves by varying the incidence angle and velocity of water-jet. The process substantially improves the micro-machining procedure for eliminating cracks, slags, and recast layers after a procedure of an etched micro-groove surface. Xing et al. [23] reported a method that utilizes the flow of gases to improve the surface quality of micro-structuring during laser processing. Firstly, oxygen was exposed during rough engraving which increased the slag removal rate and simultaneously reduced the cost, and afterward, argon gas flowed during the final engraving steps to improve the engraved surface quality and mechanical properties of the process material.
In recent years, vibration and ultrasonic-assisted techniques for laser machining have been used to improve laser machining efficiency for brittle materials. Park et al. [16] reported a low-frequency (500 Hz) vibration technique using an objective lens to vibrate while laser machining, and obtained excellent results for surface wall finishing that improved the aspect ratio of the micro-holes with the assistance of external vibration. Kang et al. [15] reported an ultrasonic technique using a workpiece during the laser etching of mild steel. The results indicate that the ultrasonic technique substantially improved the surface quality after laser machining by accelerating the laser machined area's cooling rate and reducing the recast layer's formation. Several researches have been reported about jet-assisted [10–12], vibration-assisted [15,16], and ultrasonic-assisted laser machining [17,18]. Few research works were reported [19] to integrate ultrasonic-assisted (or vibration-assisted) with jet-assisted laser machining to enhance the surface quality and processing efficiency for brittle materials.
In this work, ultrasonic nitrogen jet-assisted Fs-laser micro-machining is reported for the processing of microgrooves structures in quartz chips. The mechanism of ultrasonic nitrogen jet-assisted laser machining was discussed and analyzed. For realizations, the ultrasonic nitrogen jet effects and improvement aspects have been experimentally characterized through morphology analysis and determining the surface quality of etched microgrooves structures in quartz chips.
2. Experimental
The schematic diagram of the ultrasonic nitrogen jet-assisted Fs-laser micro-machining apparatus is sketched in Fig. 1. Fs-laser (PHAROS 20W version Base Unit manufactured in Lithuania) is used for the demonstration of the experiment. Fs-laser parameters were set ∼290 fs pulse duration, ∼20 W maximum average power, and 1∼20 kHz repetition rate at a central wavelength of 1030 nm. The ultrasonic jet system comprises an ultrasonic generator, piezoelectric ultrasonic transducer, and gas nozzle. The ultrasonic transducer and nitrogen jet were directly coupled to assemble an ultrasonic nitrogen jet for Fs-laser system.
Fig. 1. Schematic diagram of the ultrasonic nitrogen jet-assisted Fs-laser micro-machining apparatus.
In the ultrasonic nozzle, the ultrasonic waves were generated by the ultrasonic transducer, and their propagation was directed toward the longitudinal axis of the nitrogen jet due to the compressibility of the nitrogen medium, and the sound pressure distribution P(r, t) along the axis of the nozzle as [24],
Assume that the velocity of the nitrogen medium near the bottom of the ultrasonic transducer is the same as used for the ultrasonic transducer, whereas the nitrogen density was set constant during the ultrasonic vibration. The initial sound pressure of the nitrogen can be estimated as [24],
where $\rho $ is the density of the nitrogen medium, c is the speed of ultrasonic in nitrogen medium,$A$ is the amplitude of vibration generated by the ultrasonic transducer, and $\varphi $ is the initial phase.MATLAB and FLUENT were employed to perform numerical simulation analysis for the determining of sound pressure distribution and velocity of the ultrasonic nitrogen jet, hence, a principle of working for ultrasonic nitrogen jet can be realized. Fig. 2(a) displays the sound pressure $P({r,t} )$ along the axis of the nozzle was varied periodically with the distance $r$, where the range of sound pressure amplitude was 0 ∼ 2${P_0}$. The sound pressure initially appeared to produce several oscillations and then shows a decrement monotonically after covering a certain distance. Fig. 2(b) demonstrates that the original steady-state nitrogen jet was transformed into a pulsed-state jet with an order of alternating high and low velocities while coupling with ultrasonic. The ultrasonic nitrogen jet appeared to have higher energy than the ordinal one based on the velocity distribution of the jet at the nozzle outlet, which tends to be concentric.
Fig. 2. Numerically simulated results (a) the pressure distribution along the axis of the nozzle at different moments and (b) the velocity distribution of the nitrogen jet in the nozzle
Thermal vaporization, melt spraying, and plasma removal are the main reasons for slags after laser processing, and none of these can prevent slag removal, and subsequently machined surface faces a challenge when a recast layer is formed [5,25]. Theoretically, during the Fs-laser processing of transparent crystalline materials, the thermal spread is low for the reason that most of the laser energy is absorbed by the electrons and then transferred to the lattice [26]. However, in practical Fs-laser ablation, there will be a noticeable thermal buildup for the high Fs-laser repetition rate. The resultant CFD (Computational Fluid Dynamics) displayed that the pulsed nitrogen jet has faster velocity and higher energy under the same gas input condition, and the entire process significantly enhances the convective heat transfer effect. The mechanism of ultrasonic nitrogen jet-assisted Fs-laser micro-machining primarily focuses on three aspects: (1) the pulsed nitrogen jet increases heat transfer, decreases heat accumulation, and creates a continuous blowing that removes the plasma and accelerates melt spraying [8], (2) the pulsed nitrogen jet breaks the larger slags into smaller segments, improving the surface quality compared to the original large nanoparticles, and (3) the pulsed nitrogen jet couple with ultrasonic energy that possesses higher kinetic energy, which easily breaks through the slit barrier and clears the inner wall surface.
As a result, compared to typical Fs-laser micro-machining, the surface quality of laser-machined microstructures can be improved using ultrasonic nitrogen jet assistance, as shown in Fig. 3.
Fig. 3. Concept of a laser machining (a) without ultrasonic nitrogen jet assistance and (b) with ultrasonic nitrogen jet assistance
Quartz is a typically hard and brittle material and a transparent dielectric, and its processing is extremely difficult [1]. The quartz samples used in the experiment, are known as quartz chips (AT tangential direction produced by the Shanghai Institute of Optics), whose specification parameters are given in Table 1. The dimension of the quartz chip was measured as 12 mm × 8 mm × 0.4 mm. The samples were set on the X−Y−Z translation stage with a spatial resolution of 100 nm.
Table 1. Quartz chip specification parameters
In this work, the ultrasonic nitrogen jet is reported to assist the Fs-laser in the micromachining of quartz chips. The influence of the ultrasonic nitrogen jet corresponding to morphology, geometry, and surface quality of etched quartz microgrooves was experimentally evaluated by comparing the microgrooves structures with and without ultrasonic nitrogen jet-assisted Fs-laser micro-machining. A scanning electron microscope (SEM), (Nippon Electron Corporation, JSM-IT300, Japan) and a metallographic microscope (Jiangnan Yongxin, MV5000, China) were used to analyze the fabricated etched microgrooves structures in quartz chips.
3. Results and discussion
3.1 Effect on the morphology and geometry
Firstly, we studied the evolution of the morphology and geometry of etched microgrooves structures into quartz chips by ultrasonic nitrogen jet-assisted Fs-laser processing. During the experiment, the Fs-laser was employed with laser input parameters of ∼20 kHz overlapping pulses at ∼50 µJ/pulse, ∼0.442 J/cm2 laser fluence, and 10 scanning times at ∼2 mm/s scanning speed. The nozzle was set at ∼1 mm above the quartz chip and the angle was set at ∼45° off-axially to the laser beam. The parameters of the ultrasonic jet system were set using a power of ∼300 W, frequency of ∼28 kHz, and gas inlet pressure of ∼0.6 MPa.
Fig. 4 shows the outcome that is more obvious and evident for the process improvement via using ultrasonic nitrogen jet-assisted laser processing, where the morphology and geometry of etched quartz microgrooves show significant improvement in surface quality after comparing the depth and width with and without nozzle-jet-assisted laser processing. The additional aid of ultrasonic increases the depth and width of the microgrooves by 18.4% and 35.7% than that without nozzle-jet assisted laser processing, respectively. The main reason for this phenomenon was that the nitrogen gas superimposes ultrasonic kinetic energy to blow away deep slags which might obscure the laser beam propagation, and then Fs-laser could further ablate materials at deeper locations.
Fig. 4. The morphology and geometry of the quartz microgrooves etched by Fs-laser (a) (c) using raw nitrogen jet assisted and (b) (d) with ultrasonic nitrogen jet assisted laser processing.
3.2 Effect on the surface quality
By using Fs-pulse irradiation, some nano-structures and recast layers were formed on the surface of the etched quartz microgrooves. Fig. 5 shows SEM images of the surface wall after processing quartz microgrooves which were obtained using Fs-laser parameters of ∼20 kHz overlapping pulses at ∼50 µJ/pulse, and ∼10 scanning times at ∼2.0 mm/s scanning speed, whereas the parameters for the ultrasonic jet system were set as, power ∼300 W, frequency ∼28 kHz, gas inlet pressure ∼0.6 MPa.
Fig. 5. SEM images of the wall surface after processing of etched microgrooves into quartz by Fs-laser: (a) (b) without any assistance; (c) (d) with raw nitrogen jet assistance; and (e) (f) with ultrasonic nitrogen jet assistance.
Fig. 5 (a) and (b) show the SEM images of the wall surface that show micro-grooves generated by Fs-laser micro-machining without assistance. It can be inferred from the results, the poor surface quality was caused by the aggregation of re-deposited nanoparticles that densely formed at the etched surface wall. In contrast, by using nitrogen jet-assisted Fs-laser micro-machining, the morphology and geometry of the re-deposited nanoparticles were smaller and more uniform, and the surface quality appeared better than the one without any assistance, as shown in the Fig. 5 (c) and (d). The process attributed that the nitrogen jet was effective to disperse the slag formed by plasma condensation and accumulated by heat transfer. Fig. 5(e) and (f) show that the aid of the ultrasonic nitrogen jet-assisted laser processing further breaks down the slag into tiny nano-particles, thereafter the process provides an ease to disperse these nano-particles (slag) from the surface wall of micro-grooves, and has substantially improved the surface quality. The primary reason was the pulsing effects of the ultrasonic nitrogen jet in addition to the jet-assisted blowing operation. The pulsed nitrogen jet scoured the machined surface with high frequency, which help to break down the relatively larger nanoparticles into smaller ones, further reducing the adhesion of slags on the machined surface wall.
3.3 Effect by the parameters of the ultrasonic jet system
In order to investigate the effect of the ultrasonic jet system integrated with Fs-laser for the processing of etched microgrooves quartz, the gas pressure and the ultrasonic power were varied to examine the evolution of microgrooves structure. Fig. 6 shows the experimental results corresponding to the depth and aspect ratio of quartz microgrooves acquired with different gas input pressures, where the Fs-laser input parameters were set as, pulse energy of ∼50 µm/pulse, ∼0.442 J/cm2 laser fluence, a scanning speed of ∼2.0 mm/s, and an ultrasonic jet with an ultrasonic power of ∼300 W, and an ultrasonic frequency of ∼28 KHz.
Fig. 6. Influence of gas inlet pressure on the depth and aspect ratio of the quartz microgrooves etched by Fs-laser.
Through the aid of the ultrasonic nitrogen jet, the values of the depth and aspect ratio of etched quartz microgrooves were drastically increased by increasing the input gas pressure. Meanwhile, when the gas pressure reached at 0.3 MPa approximately, the peak values tend to decrease and stabilize. The presumed reason for this phenomenon was as follows: (a) The blowing effect of the nitrogen jet has reached its limit, and further increment of the gas pressure was unable to remove more slag from the processed surface wall; (b) some coupling and matching mechanism was observed between ultrasonic power and gas inlet pressure; (c) the pulse effect of the ultrasonic jet system was reached at the local extreme when input gas pressure was approaching to 0.3 MPa.
Fig. 7 shows the experimental results of the depth and aspect ratio of the quartz microgrooves obtained by the variation of different ultrasonic powers. The Fs-laser parameters were set: a pulse energy of ∼50 µm/pulse, ∼0.442 J/cm2 laser fluence, a scanning speed of ∼2.0 mm/s and scanning times 10, and an ultrasonic nitrogen jet with an ultrasonic frequency of ∼28 kHz, and a gas inlet pressure of ∼0.6 MPa.
Fig. 7. Influence of ultrasonic power on the depth and aspect ratio of the quartz microgrooves etched by Fs-laser.
The result indicates that the ultrasonic nitrogen jet-assisted Fs-laser processing has significantly improved the efficiency of micro-structuring such as etched microgrooves quartz, and showed analogous results obtained from the gas inlet pressure, where the values of the depth and aspect ratio tend to increase first and then decrease. Furthermore, the trend of the experimental values again affirms the existence of the coupling and matching efficiency through utilizing an ultrasonic jet system. The reason for this phenomenon may be related to the interaction between the purging and pulsating effects of nitrogen medium. Accessibly, when the jet velocity is too fast, the pulsation effect may be masked. On the other hand, when the jet velocity is too slow, the pulsation is relatively weak, and it is very difficult to penetrate the slit. In addition, the higher the frequency of the ultrasonic transducer, the greater the energy loss in the gas medium. Therefore, establishing a correct relationship between ultrasonic power and ultrasonic frequency is another important research point.
Although the coupling and matching mechanism of the proposed ultrasonic jet system was generically discussed, the results demonstrated herein substantially improve the machined surface quality and machining efficiency for the processing of quartz chips by integrating an ultrasonic nitrogen jet with an Fs-laser system.
4. Conclusion
In summary, a novel ultrasonic nitrogen jet-assisted Fs-laser system is experimentally demonstrated for processing and micro-machining of crystalline brittle materials. The ultrasonic nitrogen jet provides aid in laser processing and makes the system to generate additional kinetic energy therefore it penetrates into the slit barriers and further helps to remove the excessive slag from the inner surface wall of the processed micro-structure. FEM analysis shows promising results for the placement of an ultrasonic nitrogen jet-assisted laser processing system for brittle materials, as the laser system integrated ultrasonic nitrogen jet generates a pulsing effect that possesses higher velocity than the ordinary jet that subsequently provides aid for removing excessive slag from the processed surface area. The simulated results were further experimentally verified, and ultrasonic nitrogen jet-assisted Fs-laser processing far exceeds the surface quality of etched microgrooves quartz than that without any jet assistance. Therein, it was found that the depth and aspect ratio of the machined quartz microgrooves have drastically improved the surface quality with an aid of the ultrasonic nitrogen jet. The performance of the ultrasonic nitrogen jet-assisted Fs-laser micro-machining approach affirms the surface quality enrichment and machining efficiency for quartz-chips processing.
Funding
National Natural Science Foundation of China (51975442).
Acknowledgments
We appreciated the efforts of He Kuikui (Master student) in the designing of the ultrasonic nozzle.
Disclosures
There are no conflicts of interest for this work.
Data availability
Data underlying the results presented in this paper are available upon request by contacting the corresponding author.
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