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Abstract
Femtosecond lasers have been applied to machining of zirconia (ZrO2) ceramics because of their ultrashort pulse duration and high peak power. However, an unclear understanding of the ultrafast laser–material interaction mechanisms limits the achievement of precision processing. In this study, a pump-probe imaging method comprising a focusing probe beam integrated with a high-speed camera was developed to directly observe and quantitatively evaluate the multi-timescale transient processing phenomena, including electron excitation, shockwave propagation, plasma evolution, and hole formation, occurring on the picosecond to second timescales, inside a ZrO2 sample. The variation mechanism in the shapes, lifetimes, and dimensions of these phenomena and their impacts on the drilling performance under different laser parameters were explored. The clear imaging and investigation of the above phenomena contribute to revealing the ultrafast laser–material interaction mechanisms and precision processing in the laser-drilling of zirconia ceramics.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Zirconia (ZrO2) ceramics are characterized by their high melting point, low thermal conductivity, wear resistance, corrosion resistance, chemical inertia, good mechanical strength, and biocompatibility [1]. They have been widely used in aeroengine structural parts, intelligent terminal precision components, bone repair, and other advanced manufacturing fields [1,2]. Owing to the characteristics of high hardness and brittleness, the traditional processing techniques of zirconia ceramics involve a large cutting force and crack damage, leading to low machining efficiency, low precision, and high cost [3–5]. A femtosecond laser has been applied for ZrO2 ceramics machining because of its ultrashort pulse duration with high peak power, which can locally melt, vaporize, or ablate samples in an extremely short laser–material interaction time [6]. However, the precision processing of ZrO2 ceramics using ultrashort pulse lasers is still challenging because of the formation of laser-induced processing defects, including spatter, heat-affected zones, microcracks, and unexpected lattice transformations around the processed area [7,8].
Various studies have been performed to minimize these problems by exploring the effects of the processing parameters on machining performance. To reduce the surface roughness and heat-affected zone of the machined channel, Adbo et al. [9] investigated the influence of key laser processing parameters, including scanning speed, repetition rate, and pulse intensity, on machining performance. Daskalova et al. [10] achieved defined surface texturing, smaller than 100 µm, in alumina-toughened zirconia using a femtosecond laser to induce microstructures, which reduced the thermal side effects, such as cracks and melted zones. Stanciuc et al. used femtosecond laser micromachining to fabricate multi-patterned zirconia ceramic components for rapid screening of cell–surface interactions [1]. Pits with well-defined edges and micrometric precision in pit diameter, depth, and spacing were produced, but the formation of microcracks, sub-micrometric pores, and sub-superficial pores up to a depth of approximately 5 µm was also observed. Han et al. [7] investigated the influence of the alumina content on the ablation behavior of ultrashort pulse lasers to improve the manufacturing processability of zirconia-alumina composite ceramics; however, the formation of thermal cracks and lack of a fundamental understanding of the material removal process continue to be a problem.
The abovementioned studies suggest that parametric investigation leads to a limited improvement in machining performance, and it cannot be used to completely eliminate the laser-induced damages. To achieve high precision and quality processing of ZrO2 ceramics, ultrafast laser–material interaction mechanisms must be deeply understood and revealed by the observations of multi-time scale processing phenomena. In particular, related studies have been conducted on transparent materials, such as glass materials. Grossman et al. [11] investigated the visualization of excited electrons and their effect on the formation of defects using pump-probe microscopy to explore the fundamental laser–material interactions and identify the micromachining processes of transparent materials using ultrashort pulse laser radiation. Sun et al. [12] presented a novel numerical model for laser ablation and laser damage in glass, and the experimental and simulated results demonstrated that the damage beneath the crater wall was mainly induced by high electron density. Ito et al. [13] combined the pump-probe shadowgraphy and high speed camera to capture the pulse number dependence and the time dependence of the propagating pressure waves in glass in the process of femtosecond laser drilling and revealed the damage mechanisms induced by the stress wave and thermal stress around the generated hole. The shockwave evolution on sapphire was also explored by Jo et al. [14], and the results indicated that shockwaves propagated from the position where the material was ejected and their propagation caused damage generation. Zhang et al. [15] investigated the plasma evolution and variation under different magnetic fields in laser induced plasma micro-machining of silica, and found that the characteristics of laser induced plasma plume could significantly affect the surface integrity and geometric characteristics of microstructures. Zhang et al. [16] analyzed the developmental process of elongated keyhole formation and the flow characteristics of the vapor and molten materials using a high speed camera during laser drilling GG17 glass; his study contributes to reveal the material removal mechanism.
However, it is difficult to obtain clear images of the processing phenomena for ZrO2 ceramics by using the common pump-probe method because of the low transmittance induced by significant scattering effects of the incident light at the grain boundaries than that observed in other transparent materials. Furthermore, ZrO2 ceramics have not been extensively studied despite their widespread application in the precision manufacturing industry, which is dominant in uncovering the ultrafast laser–material interaction mechanisms.
In this study, we propose a pump-probe imaging method comprising a focusing probe pulse integrated with a high-speed camera to observe the multi-timescale processing phenomena, including electron excitation occurring on the picosecond (ps) to nanosecond (ns) timescale, shockwave (SW) propagation on the ns timescale, plasma evolution on a 100 ns timescale, and hole formation on the millisecond (ms) to second (s) timescale, inside an opaque ZrO2 ceramic sample. Subsequently, the machining processes at multiple timescales and related laser–material interaction mechanisms that occur during the femtosecond laser drilling of ZrO2 ceramics were elaborated. Finally, the processing phenomena and related machining performance, including the aspect ratio, taper and profile of the drilled holes under different laser parameters, were analyzed.
2. Methods
2.1 Experimental setup
The ultrashort pulse laser having a wavelength of 1030 nm, pulse duration of 180 fs and repetition rate of 1 kHz was generated by a Yb:KGW laser system. The laser beam was divided into a pump beam for drilling the ZrO2 sample and a probe beam for observation using a beam splitter, as shown in Fig. 1(a). The pump beam was then focused onto the sample surface using an objective lens (Mitutoyo; M Plan Apo NIR 10×) with a 20 mm focal length. The probe beam was frequency doubled to 515 nm using a BaB2O4 (BBO) crystal. Subsequently, the probe beam was incident on the delay stage to determine the optical path difference between the pump and probe beams. This path difference corresponds to the different delay times between the two beams and can be used for the observation of electron excitation and shockwave propagation on the fs to ns timescale. To overcome the scattering effect of ZrO2 and obtain a high intensity scattered probe beam, the incident probe beam was focused by a lens with a 200 mm focal length in a direction perpendicular to that of the pump beam, creating a small beam spot with high intensity. The probe beam was then passed through the ZrO2 sample and was collected using an objective lens (Mitutoyo; M Plan Apo NIR 20×). The images were formed using a tube lens (Thorlabs; TTL200) with a 200 mm focal length. Thereafter, a bandpass filter was applied to decrease the disturbance caused by the pump beam and plasma emission during the observation of electron excitation and shockwave propagation, while it was removed during the observation of plasma evolution for slower phenomena occurring on the 100 ns to 600 ns timescale. The shadows of the phenomena were projected onto an image intensifier (Hamamatsu; C10880-03C), and the intensified images were captured by a high-speed camera (Keyence; VW-9000). The high-speed camera was operated at 1 kHz and was synchronized with the femtosecond laser system to observe the phenomena occurring on multiple timescales under the irradiation of each pump beam. The hole formation process that occurs on the ms timescale was also observed because of the synchronization. To observe the plasma evolution occurring on the 100 ns timescale, the bandpass filter and probe beam were removed as shown in Fig. 1(b), and the time delay between pump beam arrival time and camera shutter open time was adjusted through the electronical delay control using a delay generator. Moreover, the gate time of the image intensifier was set as 110 ns throughout the experiments.
Fig. 1. Experimental setup for multiple temporal scale observation ranging from picosecond to second: (a) Shadow of the phenomena. (b) Luminescence. (c) Actual ZrO2 sample with low transparency. (d) Schematic of the dimensions and scattering effect of ZrO2.
ZrO2 ceramics with dimensions of 30 mm×30 mm×0.4 mm was selected as the sample and all surfaces of the sample were polished, as displayed in Fig. 1(c). To further overcome the scattering effect of the low-transparency ZrO2 sample and acquire clear images of processing phenomena, the focus of pump beam was fixed near the edge of the sample surface (30 × 0.4 mm), and the probe beam was focused close to another sample surface (30 × 30 mm), as displayed in Fig. 1(d).
2.2 Experimental design and measurement
The evolution of multi-timescale processing phenomena, including the electron excitation (ps-ns scale), shockwave propagation (ns scale), plasma emission (100 ns scale), and hole formation (ms-s scale), is explored using same parameter combinations with a spot size of 5.7 µm on the sample surface. To further investigate the dependence of these parameters on above phenomena, different laser parameters, including pump pulse energy (E) of 50 µJ, 100 µJ, and 150 µJ, and the distance of the laser beam focus position from the sample surface (F) of -30 µm, 0 µm, and 30 µm, were selected for the experiments. A separation of -30 µm implies that the pump beams were focused inside the sample. The spot size of pump beam on the sample surface changes into 21.9 µm when its focus positions are -30 µm and 30 µm. The pulse energy at the sample position of the probe beam is fixed as 0.5 µJ. The fluence (defined as the ratio of the laser energy to the area of the laser spot) of the pump beam deposited on the material surface can be changed by changing the focus position. The fluence of the pump beam under 50 µJ pulse energy is approximately 391.9 J/cm2 with focus position of 0 µm, and 26.6 J/cm2 with focus position of -30 µm and 30 µm. After the experiments were performed, background subtraction was applied using a background image captured before the start of the experiments to cancel the noise in the images. Subsequently, the gray values of the electron excitation regions were utilized to evaluate the dimensional change of these regions at different time delays. The entrance diameter and shape of the drilled holes were analyzed using a laser microscope (Olympus LEXT OLS4100) after cleaning the sample for 5 min using an ultrasonic cleaning machine to eliminate any unwanted deposition, and their depths were obtained from the images captured by the high speed camera.
3. Results and discussion
3.1 Multi-timescale observation
3.1.1 Temporal evolution of electron excitation (ps-ns scale)
During the interaction between an ultrashort pulse laser and ZrO2 ceramics, the laser energy is first absorbed by the electrons of the material through nonlinear absorption processes, leading to their excitation. In the region where electrons are excited, the absorption coefficient is increased and this region is observed as a black shadow [17]. The temporal evolution of electron excitation from 1 ps to 1.5 ns inside the ZrO2 sample is shown using pump-probe method in Fig. 2(a). At the beginning of electron excitation, the free electrons in the conduction band induced by the pump beam dominate, as shown in Fig. 2(a), and the electron excitation process induced by pump beam will disappear rapidly due to the short duration of the pump laser beam (180 fs). This kind of excitation region is almost indistinct owing to the low free-electron density and scattering effect of ZrO2. During subsequent energy relaxation processes, the energy transfers from the electrons to the lattice vibrations owing to the electron-phonon interaction. The laser-material interaction with ZrO2 led to the introduction of lattice defects, i.e., zirconium vacancies [18]. Besides, defects can also be formed on the boundaries of grains due to the slight variation of the composition, structure, and shape from grain to grain. The lattice defects usually introduce localized energy levels within the band gap, which can produce locally charged regions that can be good traps for electrons [19–22] as shown in Fig. 2(c), inducing the dark and long-life excited electron region [23], as observed in 50 ps. As time evolves, this dark excited electron region can remain until 1.5 ns. It can be found that the excited electron region in the ZrO2 ceramics consists of two parts, namely concentrated region (upper part) and channel-like region (lower part). As time progresses, the size of concentrated region remains almost unchanged while the channel-like region experiences the process of emergence and dissipation. During the first pulse irradiation, a small portion of the pump beam energy may be absorbed into the target due to the Kerr lens self-focusing and plasma defocusing (i.e. filamentation), and the region where free electrons are excited by absorbed pump energy determined the size of the excitation region. The absorption depth corresponding to filament length can be tens of micrometers as reported by Grossmann et al. [11]. The size and lifetime difference between the two parts can be attributed to the following reasons. On the one hand, as the laser pulse propagates into the sample, the width of the excited electron region continues to decrease from the upper part to the lower part due to the strong self-focusing effect, which can also be observed in laser drilling glass [17]. On the other hand, the large scattering effect of crystallographic defects and micro-pores inside ZrO2 would decrease the absorbed laser energy with increasing depth, and once the peak power of the absorbed laser energy lower than the critical power for self-focusing, the excited electron terminates inevitably [24]. As the delay time evolves to 50 ps, the darkness of the excited electron region in the lower part is large enough to be observed. Subsequently, the channel-like region appears, which has a shape similar to that of the excited electron region in the glass [25].
Fig. 2. (a) Evolution of electron excitation inside the ZrO2 in single pump pulse (time label means the time difference between pump beam and probe beam arriving at the sample surface). (b) Evolution of electron excitation after different pump pulse number (pulse number means the sequence number of pump beam delivered to the sample). (c) Electron trapping by defect states.
The electron excitation at 200 ps under different pulse numbers is shown in Fig. 2(b), which demonstrates that no large excited electron region is formed when the number of pulses increase. The excited electron region becomes almost invisible after fifth pulse and this result indicates that the excited electron region does not permanently modify the interior of the ceramic, particularly in the channel-like region. This is because the femtosecond laser pulse is diverged by a dimple, which is generated by the previous pulses and acts as a concave lens. This excited-electron phenomenon observed in ZrO2 ceramics shows the potential and possibility of utilizing it for processing, such as filament cutters [26], or transient and selective laser processing [25], which will be beneficial to further improve the efficiency and quality of the precision machining of ZrO2 ceramics.
3.1.2 Propagation of shockwave in air (ns scale)
As the laser energy is absorbed by the free electrons of the material, electron-phonon coupling becomes stronger, leading to the rapid expansion of plasma plume exceeding the acoustic speed in the air, which forms the shockwave. After the incidence of a femtosecond laser pulse on the ZrO2 sample, shockwaves (on the nanosecond timescale) propagating in air were observed using the same setup and laser parameters. As shown in Fig. 3(a), a pressure wave was not observed at 0.2 ns, but a plasma channel was formed due to the laser-induce air breakdown. Interaction of femtosecond laser pulses with the ZrO2 surface induced a plasma plume with compressed pressure and extreme high temperature. As the plasma plume expanded rapidly faster than the acoustic speed in the air, shockwave 1 (SW1) is formed [27,28], and the interior of SW1 was black from 0.3 ns to 0.7 ns, which implies that there was a small transmittance in this region. The hemispherical shaded area inside SW1 may be the interferometric fringe. The SW1 expanded rapidly at a time delay from 0.3 ns to 1.2 ns; thereafter, a contact front was observed at 1.2 ns between the SW1 and the ionized plasma channel, and another pressure wave (SW2) appeared. This contact front is induced by the high temperatures in the ionized plasma channel and fast expansion speed of SW1. The pressure wave SW2 continued to expand at a time delay from 3 ns to 4 ns, and the shape of SW2 was not spherical owing to its large longitudinal propagation velocity because of the high temperature in the plasma channel. Moreover, the material ejection can also be captured at a time delay of 4 ns, as shown in Fig. 3(a); this is attributed to the recoil effect induced by the thermal explosion of the samples [29].
Fig. 3. (a) Propagation of shockwave produced in air in single pump pulse (ns scale). (b) Longitudinal expansion of shockwave SW1 and SW2.
As illustrated in Fig. 3(b), the average propagation speed of the pressure wave SW1 in air between 0.3 ns and 0.8 ns is 35.16 km/s, and the average velocity of SW2 between 1.5 ns to 4 ns is 14.26 km/s. The average expansion velocity of shockwave SW1 from 1.5 ns to 4 ns is 10.73 km/s, which is smaller than that from 0.3 ns to 1.5 ns; this is because of the decreasing particle speed as the shockwave collides with the air. It can be observed that the average expansion velocities of SW2 from 1.5 ns to 4 ns is larger than that of SW1 in the same range, leading to the generation of distortion in the front of SW1, as shown in Fig. 3(a). Based on these two types of shockwave propagation phenomena induced by a single pump beam, the material removal rate can be indirectly reflected by the propagation velocities and front sizes of shockwaves, particularly for SW1. The large propagation velocity and front size of shockwaves usually correspond to a high material removal rate.
3.1.3 Evolution of plasma (100 ns scale)
The evolution of plasma, which is produced from materials in the vaporized or molten state by absorbing the energy of the laser, as a result of different laser pulse irradiations can be observed, as shown in Fig. 4. As shown the figure, the height of the plasma in air after the first pulse gradually increased from 110 ns to 250 ns reached the largest value of 411 µm at 250 ns and then continued to decrease until it vanished at 600 ns. An interesting phenomenon is that the shape of the plasma evolves into a dumbbell shape, particularly in the decay process occurring from 300 ns to 500 ns. This is due to that after the first pulse irradiation, the ions would be ejected on a very short time scale and moved upward at a high average speed of 3.74 km/s in the very intense field left by electron excitation and detachment [30], inducing the upper part of the plasma, while some part of the ions loses their momentum earlier (thin area), and the bottom part (slow) component would come from a subsequent thermal process, which needs more time to establish [31]. This upper part of the plasma consists mainly in ionized species, while the bottom part in neutral particles, forming the dumbbell shaped plasma. The similar phenomenon has been observed in a previous paper [32], which used intensified CCD imaging to study the dynamics of Al2O3 plasma plumes. It can also be found that the upper part of the dumbbell-shaped plasma shows larger brightness (intensity) and lifetime compared to lower part. No more vaporized materials were produced, and the lower part gradually disappeared at 300 ns. A more irregular shape of the plasma can be observed at the 30th pulse, which proves that the plasma is generated by the evaporated materials because of their random distribution. The ejected particles of the materials and the plasma inside the ZrO2 samples can also be observed at 550 ns. As the number of pump beam is increased (100 pulses and 500 pulses), the bottom part of the induced plasma gradually extends as the depth of hole, resulting from laser ablation, increases, and the shape of the plasma changes from conical into cylindrical. The presence of plasma in air can lead to attenuation and shielding of a non-negligible amount of laser energy from reaching the sample surface due to absorption, reflection and scattering, i.e. plasma shielding [33], resulting in negative effect on the drilling performance, such as the ablation rate [34,35]. Because of the shielding effect strongly depends on the expansion of the plasma, larger size and cone angle of the plasma induced by absorbing more energy from the incident laser usually corresponds to stronger plasma shielding effect [36–38]. The reason for the shape difference can be attributed to the vaporized material flow impinging on top of the hole. The ejected materials can easily escape from the surface of the sample during the first laser pulse drilling; as the depth of the hole increases, the vaporized or molten material is restricted by the hole walls, leading to different shape of plasma.
The plasma imaging can provide the direct information on material removal during the drilling process, and the transverse size of the plasma is related to the final diameter of drilled hole. Moreover, the clear vision created by the plasma inside the ZrO2 samples can be used for the evaluation of characteristics of holes in real-time, such as their diameter or shape. However, the plasma in air has a negative effect on the drilling process owing to plasma shielding, which can scatter the laser beam and absorb laser energy [39], leading to large heat-affected zone or low machining efficiency.
3.1.4 Process of hole formation (ms-s scale)
The progress of drilling on increasing the applied laser pulses under the same drilling parameters can be observed in Fig. 5, in which the generated plasma inside the materials is useful for observing the contour. Electron excitation can also be captured at the first pulse (0 ms) and disappears at the fifth pulse (4 ms), which has little effect on the drilling quality. The time delay between the pump and probe beams was 200 ps. The drilling velocity continued to increase from 0 ms to 30 ms and reached the largest value of 8.55 mm/s. Thereafter, it continued to decrease from 50 ms to 250 ms with the average value of 2.97 mm/s. A rapid drop occurred in the drilling velocity at 350 ms with a hole-depth of 139 µm, which may be because of the increase of the effective irradiated area which results in a decrease of the applied fluence below threshold [40]. Subsequently, the depth of the hole grew at a relatively low velocity (0.1 mm/s) until the 2500th pulse. Meanwhile, the diameter of the hole remained almost unchanged owing to the wall-reflected energy of the dominant hole inside the materials. As for the hole shape, an accentuated tapered shape with a large entrance diameter and sharp tip was created. The sharp tip comes from the reduced direct irradiation energy. A direct image of the hole-progress can contribute to the evaluation of the hole shape and material removal process.
3.2 Multiple time scale drilling phenomena under different parameters
During the interaction of an ultrashort pulse laser and ZrO2 samples, the laser energy is first absorbed by electrons through nonlinear absorption processes (electron excitation), and then transferred to the lattice through electron-phonon coupling, leading to the rapid expansion of plasma plume exceeding the acoustic speed in the air (shockwave). When irradiated by the laser, the materials are ionized through strong field ionization and impact ionization (plasma), leading to material ablation (hole formation). The multi-time scale drilling phenomena including electron excitation (ps-ns), shockwave propagation (ns), plasma evolution (100 ns), and hole formation (ms-s), under different laser parameters are depicted in Figs. 6 and 7, and the quantitative analyses under different parameters are discussed in the following sections.
Fig. 6. The drilling phenomena from ps to ns timescales under different parameters. (a) Variation trend of excited electron region (single pump pulse). E denotes pulse energy and F denotes focus position. (b) Images of electron excitation. (c) Variation trend of shockwave propagation (single pump pulse). (d) Images of shockwave.
Fig. 7. The drilling phenomena from 100 ns to s timescales under different parameters. (a) Variation trend of plasma length (single pump pulse). E denotes pulse energy and F denotes focus position. (b) Images of plasma. (c) Variation trend of hole depth. (d) Images of hole.
3.2.1 Dependence of beam focus position
The focus spot diameter, affected by the focus position, changes the laser fluence (the ratio of the laser energy to the area of the laser spot) deposited on the sample surface and overlaps the laser pulses, leading to different hole diameter, hole taper and drilling efficiency. The electron excitation and shockwave phenomena at different focus position are shown in Fig. 6. The electron excitation region and shockwave front (SW1) can be obtained with an increase of 25% and 11%, respectively, in their sizes at a focus position of -30 µm. The concentration region of the electron excitation was mainly located in the near-surface region because of the negative scattering effect of ZrO2 on the laser beam, with a length of approximately 30–40 µm. The growth in the size of the electron excitation and shockwave front indicates that the limitation of this scattering effect can be partly overcome at a proper internal focus position, leading to more sufficient multi-photon ionization, greater effective energy absorption of laser and stronger vapor pressure. Moreover, the expansion and dissipation times of electron excitation area are almost identical at different focus position. This means that the lifetime of electron excitation determined by the defect-states electrons almost unaffected by the focus position of pump beam, and it is due to the pump beam has already stopped when the defect-trapped electrons dominate.
The lifetime and size of the plasma in air increases at high focus position, as shown in Fig. 7(a), with the largest increase of 24% in length size, representing more negative effect on drilling process, such as plasma shielding on the subsequent pump beam. It can also be observed that existence time and length of the end region of the plasma are larger at a focus position of 30 µm at 500 ns than those at 0 and -30 µm, which is attributed to the large laser intensity distributed in this region, contributing to more energy absorption by the vaporized materials. A weaker laser propagation and absorption disturbance, such as plasma shielding, scattering effect, and laser beam reflected on the hole wall, can be obtained at a low focus position, particularly for the negative position (-30 µm), generating a more slender hole at 350 ms, as shown in Fig. 7(d). The drilling depth obtained at a focus position of -30 µm at 2.5 s is 11.64% higher, than that obtained at 0 µm.
Based on the above discussion, the focus position has a significant effect on the drilling phenomena at multiple time scales, and the determination of a proper internal focus position is beneficial for laser transportation and absorption, improving the drilling quality.
3.2.2 Dependence of pulse energy
The effect of pulse energy on shockwave propagation is illustrated in Fig. 6(c). It can be seen that larger propagation velocities and shockwave fronts of SW1 and SW2 appear in the transverse and longitudinal directions. More rapid expansion of plasma plume exceeding the acoustic speed in the air (SW1) and a high density of ionized plasma channels (SW2) can be produced due to increased laser energy deposition.
It is apparent that a high laser intensity created by a large pulse energy can produce high density vaporized materials and more violent multi-photon ionization, causing the plasma to have a long lifetime and length size, as shown in Fig. 7(b). There is a 99.39% increase in the length of the plasma under a pulse energy of 150 µJ than that under 50 µJ. Similarly, more laser energy can be transferred and absorbed by the internal materials, resulting in the melting or vaporization of large volumes of materials which flow out under recoil pressure and generate deep holes with large diameters. A significant increase of 105.1% in the drilling depth can be observed under a pulse energy of 150 µJ than that observed under 50 µJ. Therefore, the direct visualization of multi-timescale drilling phenomena was revealed to further understand the drilling mechanism.
3.3 Hole quality analysis
To obtain the minimum value of the taper and the maximum value of the aspect ratio of the drilled holes, the effect of the laser parameters on the hole performance was investigated. As displayed in Fig. 8, a better hole quality, including a larger aspect ratio and smaller taper, can be achieved at a focus position of -30 µm regardless of the pulse energy than that achieved at 0 µm. This results from the weak plasma phenomenon in air and enhanced electron excitation phenomenon inside the ZrO2 samples, leading to a greater improvement in the depth direction than that in the diameter direction. More specifically, the largest improvements of 21.26% in aspect ratio and a maximum reduction of 18.73% in taper can be realized under a pulse energy of 50 µJ. With an increase in pulse energy, a large aspect ratio (maximum 10.47) and small taper (minimum 2.639) can be acquired at a fast growth velocity in depth. It is also obvious that the variation rates of the aspect ratio and taper and their difference rapidly decrease at high pulse energies, which means that these laser parameters have a limited effect on the hole quality.
Fig. 8. The multiple time scale drilling phenomena under different parameters. F denotes focus position.
A high circularity can be observed at the entrance of the hole, as shown in Fig. 8, with a change in the focal position from 0 µm to -30 µm. This is because of an internal focal position. The weak plasma and strong shockwave phenomena demonstrate more pronounced removal of molten material from the sample surface, inducing slight re-solidification, resulting in good edges of the hole at entrance. However, the laser beam converges at the hole entry while diverging at a positive focal position, which also produces greater circularity at the hole entrance.
4. Conclusions
In summary, a pump-probe imaging method comprising a focusing probe beam integrated with a high-speed camera was developed to directly observe and quantitatively evaluate the multi-timescale transient processing phenomena ranging from picoseconds to seconds, including electron excitation (ps-ns), shockwave propagation in air (ns), plasma evolution (100 ns), and hole formation process (ms-s), inside the ZrO2 samples. Electron excitation can be induced by the pump beam and the excited electrons can be trapped by the defects inside ZrO2, resulting in a lifetime larger than 1.5 ns. Two parts of the electron excitation area, namely the concentrated and channel-like regions, are produced owing to the strong self-focusing effect and scattering effect of ZrO2. The excited electron region becomes almost invisible after fifth pulse and this result indicates that the excited electron region does not permanently modify the interior of the ceramic. Two types of shockwaves can be generated in the nanosecond timescale, namely spherical SW1 and non-spherical SW2, and their propagation velocities and front sizes can be used for the indirect evaluation of the material removal rate. The plasma produced under one incident pump beam shows a shape change from a drop shape to a dumbbell shape with a lifetime of 600 ns. As the number of incident pump beams increased, the shape of the plasma changed from conical to cylindrical. A large focus position and laser energy had significant effects on the shape and size of the plasma in air, representing a more negative shielding effect on the subsequent pump beam. An accentuated tapered hole with a large entrance diameter and sharp tip is created by increasing the incident pump pulses, while a more slender hole can be obtained at an internal focus position of -30 µm. The drilling velocity was evaluated using these clear images of hole formation. Moreover, a proper internal focus position can partly overcome the scattering effect of ZrO2 and reduce the size of the plasma in air, improving the drilling quality aspect ratio, taper, and circularity, regardless of the pulse energy. The clear imaging and investigation of the above multi-timescale phenomena contribute to revealing the ultrafast laser-material interaction mechanisms and precision processing in laser-drilling zirconia ceramics.
Funding
Japan Society for the Promotion of Science (21K18667).
Acknowledgments
We would like to thank master student Wang Chao for microscope measurement.
Disclosures
The authors declare no conflicts 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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