Comparison of ablation rate and surface quality of in-air and underwater picosecond laser processing of tungsten carbide cobalt

Comparison of ablation rate and surface quality of in-air and underwater picosecond laser processing of tungsten carbide cobalt | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Comparison of ablation rate and surface quality of in-air and underwater picosecond laser processing of tungsten carbide cobalt Michał Ćwikła, Maurycy Kempa, Robert Dziedzic, Kacper Marciniak, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8504989/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract This study investigates the influence of the processing environment and laser parameters on the picosecond laser ablation of tungsten carbide–cobalt (WC–Co), with a specific focus on underwater ultrafast laser processing. Experiments were conducted using 10-ps pulses to evaluate the effects of water-layer thickness, focal-plane position, laser fluence, and burst-mode operation on ablation rate and surface morphology. Underwater processing significantly improved surface quality by suppressing plasma expansion, enhancing the ejection of molten material, and reducing the formation of heat-affected zones and debris. The water layer thickness (1–5 mm) was found to have only a minor effect on the ablation rate. In contrast to in-air processing, underwater ablation produced smooth and more regular craters across all fluence levels, with no indications of thermal damage. Furthermore, underwater processing increased the ablation rate by up to ~ 75% compared with air processing, attributed to prolonged plasma confinement and enhanced plasma-assisted material removal. Burst-mode operation exhibited opposite trends in the two environments: in air, increasing the number of pulses per burst improved the ablation rate due to heat accumulation and plasma reheating, whereas underwater, pulses in a burst interacted with the long-lived ablation plume and expanding cavitation bubble, reducing the effective fluence and thereby diminishing the ablation rate. Overall, the results provide new insights into ultrafast laser ablation mechanisms in liquids and demonstrate that underwater processing offers a superior balance between ablation performance and surface integrity for hard, refractory materials such as WC–Co. Materials Engineering underwater laser ablation picosecond laser ablation tungsten carbide cobalt surface morphology ablation rate cavitation bubbles Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. Introduction Extending the lifespan and enhancing the performance of cutting tools remain significant challenges in the tooling industry. One promising approach is micro-texturing of the tool's active surface by means of pulsed laser ablation. For instance, the fabrication of shallow blind holes (dimples) has been shown to enhance machinability and tribological performance by reducing the effective contact area and improving lubrication through the storage of lubricants within the dimples [ 1 – 3 ]. Laser ablation using ultrashort pulses offers high precision, repeatability, and superior surface quality; however, it is typically characterized by relatively low processing rates [ 4 ]. Moreover, operating in the high-fluence regime often leads to thermally induced surface damage [ 5 ]. A potentially effective strategy to improve both quality and ablation rate involves performing laser ablation in a water environment instead of air. It has been suggested that the presence of a water layer prolongs the boiling phase of the material at the surface [ 6 ] and increases the pressure exerted on the substrate [ 7 ] volume of ablated material. Moreover, water facilitates the efficient ejection of molten material, thereby reducing the amount of resolidified melt in the ablation zone [ 8 , 9 ]. On the other hand, the confinement of the ablation plume may lead to the redeposition of ablated material on the irradiated surface, which suppresses the ablation rate [ 10 – 12 ]. Overall, due to the complex nature of the process, several factors—including material properties, liquid characteristics, and water layer thickness—can influence the total volume of material removed by a single pulse and the quality of the ablated surface. Experimental studies conducted by various research groups have produced inconclusive results regarding whether the presence of water enhances or reduces the ablation rate and surface quality, as these effects depend strongly on the material type and processing conditions. For ultrafast laser processing, an increase in ablation rate has been reported for silicon [ 13 , 14 ], glass [ 15 ], and copper [ 16 ], whereas a decrease has been observed for stainless steel [ 14 ], nickel [ 17 ], titanium [ 18 ], and zinc [ 19 ]. It has been suggested that the presence of a water layer prolongs the boiling phase of the material at the surface [ 6 ] and increases the pressure exerted on the substrate [ 7 ]. Moreover, water facilitates the efficient ejection of molten material, thereby reducing the amount of resolidified melt in the ablation zone [ 8 ]. However, due to the complex nature of the process, several factors—including material properties, liquid characteristics, and the thickness of the water layer—can influence the total volume of material removed by a single pulse. The influence of the water environment on surface quality during ultrafast laser processing is also highly material-dependent. For example, processing silicon with 120 fs pulses resulted in the suppression of self-organized structures, reduced debris formation, and improved regularity of hole edges [ 13 ]. Processing nickel with 8 ps pulses led to a decrease in debris, recast layer, and spatter formation [ 20 ]. In the case of zinc, irradiation with 30 fs pulses produced more homogeneous, debris-free crater morphologies, although porous structures appeared at high fluences [ 19 ]. Similarly, for zirconia, processing with 8 ps pulses reduced microcracks and surface roughness, along with a smaller heat-affected zone (HAZ) and thinner recast layer [ 21 ]. Processing silicon nitride ceramics with 273 fs pulses improved both surface smoothness and hole roundness [ 22 ]. Conversely, for stainless steel processed with 7 ps pulses, arc-like and spike-like surface features were formed [ 23 ]. Similar effects were observed for silver processed with 300 fs pulses, where highly rough and porous surfaces developed [ 24 ]. To date, no comprehensive studies have been reported on the underwater ultrafast laser processing of tungsten carbide, a key material in the field of mechanical engineering. Therefore, the effects of water assistance and laser processing parameters on ablation rate and surface quality remain unexplored. Furthermore, to our knowledge, the influence of burst-mode operation on underwater ablation behaviour has not yet been investigated for any material. This parameter is crucial for optimizing the ablation rate, as it can significantly increase material removal efficiency [ 25 – 28 ]. However, studies have also shown that applying a high number of pulses per burst may deteriorate surface quality due to heat accumulation [ 29 , 30 ]. Thus, its experimental verification constitutes an important contribution to the advancement of laser micromachining technologies. In this study, we investigate the picosecond laser processing (10 ps pulses) of tungsten carbide immersed in water. The laser was operated in a single-spot irradiation regime and at a low pulse repetition rate to isolate multi-pulse ablation effects and avoid phenomena related to cavitation-bubble shielding. We compare the results with those obtained for in-air processing. The experiments include variations in water layer thickness, focal plane position, laser fluence, and the number of pulses per burst, to evaluate their effects on surface quality and ablation rate. 2. Material and methods 2.1. Experimental setup The target material was tungsten carbide–cobalt (WC–Co) substrates supplied by the Fraunhofer IST Institute (Braunschweig, Germany), fabricated as discs with a diameter of 18 mm and a thickness of 3 mm. A detailed description of the material composition and surface properties was provided in Chapter S1 of the Supplementary Material. Figure 1 a shows a diagram of the setup used in experimental research as part of the underwater laser processing. Laser source was picosecond laser Duetto (Lumentum, USA). System enabled the emission of pulses in the form of pulse packets (so-called burst mode, Fig. 1 b) with a frequency equal to the frequency of the 82 MHz oscillator (FixBurst®/FlexBurst®, Lumentum, USA). One pulse packet can emit from 1 to 8 pulses, and the energy of the pulse packet is approximately equal to the energy of a single pulse in basic mode. The dominant wavelength of the emitted radiation is 1064 nm at 10 ps laser pulses. The maximum pulse energy, generated at 50 kHz, was 150,6 µJ per pulse in basic mode. Precise output power control and pulse gating were possible on the Pulse on Demand acousto-optic module (AOM, Lumentum, USA) placed after the optical output of the Duetto laser generator. The laser beam had a Gaussian distribution and a beam width (1/e 2 ) of 1.0 mm and 0.9 mm in the X and Y directions, respectively measured after 1 meter from generator. The laser beam was directed to the processing area using three mirrors with anti-reflective coatings (omitted from the diagram) and then focused on the target material using a AL2520 focusing lens (Thorlabs, USA) characterized by effective focal length of 20 mm. Setup resulted in propagation of laser beam characterized by radius (in-air propagation) in waist of 8.5 um (Chapter S3 in Supplementary material). The precise positioning of the laser beam's focal point relative to the workpiece was achieved by translational movement of the process lens, which was mounted on a translation table with an 8MT167-25BS1 stepper motor (Standa, Lithuania). The motor was controlled using an 8SMC5-USB controller (Standa, Lithuania) and XILab software (Standa, Lithuania). The beam focus position on the substrate was changed using an automated MAX343/M table with a BSC-103 controller (Thorlabs, USA), to which a water tank was mounted. For in-air processing setup was the same, but the cover-glass window was removed and water vessel was empty. In addition, high-speed imaging using the shadowgraphy technique was employed to verify that successive laser pulses did not interact with plasma-induced cavitation bubbles. This ensured the independence of individual pulses and eliminated potential effects related to pulse repetition frequency. Moreover, the high-speed observations confirmed the absence of interactions between the cavitation bubble and the cover glass for all investigated process parameters, thereby excluding bubble-induced liquid-jet erosion as a possible damage mechanism for the glass window. The procedure for bubble detection and the obtained results are described in detail in Chapter S6 of the Supplementary Material. The design of the liquid tank, which allowed for the adjustment of water layer thickness due to the changeable spacing between the glass-cover slip and the target, was developed and modelled in a CAD (Computer-Aided Design) environment. An isometric projection of the tank model is shown in Fig. 2 a, while its cross-section is presented in Fig. 2 b. Additionally, the tank was equipped with an inspection window made of 1 mm thick BK7 glass with an anti-reflective coating. 2.2. Experimental plan The experimental plan assumed the evaluation of the influence of three processing parameters, namely the water-layer thickness, laser fluence, and the number of pulses in burst mode (Table 1 ). To assess the effect of water-layer thickness, experiments were conducted at various focal-plane positions in order to identify the optimal focus at which the ablation rate was maximized. Evaluation was conducted at five levels of water layer thickness, ranging from 1 to 5 mm. This range was selected based on previous studies on underwater laser processing of various materials and experimental configurations, which consistently reported optimal water-layer thicknesses within this interval [ 14 , 23 , 31 – 37 ]. Applying a water layer thinner than 1 mm is impractical, as it may lead to damage of the protective optical window for certain focal positions (see Chapter S4 in the Supplementary Material). Conversely, thicker water layers cause a significant reduction in the effective laser fluence at the target surface due to the elongation of the system's effective focal length, as discussed in detail in Chapter S5 of the Supplementary Material. Additionally, the evaluation was performed at different levels of laser fluence (controlled by pulse energy) to determine whether focal shifts caused by nonlinear effects occurred. The influence of laser fluence was investigated over the range of 1.2–133.1 J/cm² (determined in accordance with Chapter S1 in the Supplementary material) while keeping the number of pulses and the pulse repetition rate ( \(\:PRR\) ) constant. To evaluate the effect of the number of pulses in burst mode, an intra-burst repetition rate of 82 MHz was used, corresponding to an inter-pulse delay of 25 ns within each burst. Pulse series from 1 to 8 were tested, which corresponds to the typical capabilities of commercially available picosecond laser sources. In the experiment, the burst fluence was set at fixed levels of 10.9, 27.3, 57.7, 92.2 and 133.1 J/cm². This meant that the energy in a single pulse decreased with an increase in the number of pulses in the burst, according to the relationship \(\:{F}_{Burst}=n*F\) , where \(\:{F}_{Burst}\) – burst fluence, \(\:n\) – the number of pulses in the burst, \(\:F\:\) – the fluence of a single pulse. As part of the experimental plan, a series of four identical craters was made for each set of parameters, spaced at equal intervals of 100 µm. Crater depth and diameter were determined following the procedure described in Chapter S2 of the Supplementary Material. The arithmetic mean and standard deviation of the analyzed geometric features were calculated based on measurements taken for all four structures. Table 1 Experimental parameter sets used in the research. Parameter Unit Effect of focal position and water layer thickness Influence of laser fluence Influence of the number of pulses in burst mode Pulse duration ( \(\:\tau\:\) ) ps 10 10 10 Fluence ( \(\:F)\) Jcm ─2 ─ 1.2–133.1 10.9 – 133.1 Pulse energy ( \(\:{E}_{p}\) ) µJ 12.3–150.6 1.3–150.6 12.3–150.6 Pulse Repetition Frequency ( \(\:PRF\) ) Hz 5 5 5 Number of pulses ( \(\:N\) ) au. 25 25 5 Number of pulses in burst ( \(\:{n}_{b}\) ) a.u. 1 1 1–8 Water layer thickness ( \(\:H\) ) mm 1–5 5 5 Focal-plane position ( \(\:x\) ) mm ─2.0 × Rayleigh length – + 2.0 × Rayleigh length Focal position ( \(\:x\) = 0) Focal position ( \(\:x\) = 0) 3. Results and discussion 3.1. Effect of water thickness level The achievable crater depths as a function of laser beam defocus were compared for five different water layer thicknesses (1–5 mm) in the underwater picosecond laser processing configuration (), as well as for in-air processing (Fig. 3 ). A focal plane shift was observed depending on the water layer thickness, which resulted from changes in the effective focal length of the optical system. For both underwater and in-air configurations, the dependence of crater depth on the focal position exhibited an approximately symmetrical profile, with the focal point corresponding to the maximum crater depth. This indicates a weak or negligible influence of beam shielding effects caused by ionized liquid, or ablation products above the material surface, which could otherwise occur at negative focal offsets (i.e., when the focal point is located above the sample surface). The experiments revealed significantly greater crater depths for all tested water layer thicknesses compared to those obtained in air. This suggests that the positive effect of plasma confinement contributes to an enhanced volume of ablated material. However, no substantial differences were found between the maximum crater depths obtained for different water thickness levels. The highest depths were recorded for 1 mm and 4 mm water layers—2.68 ± 0.01 µm and 2.66 ± 0.03 µm, respectively—while the lowest value of 2.48 ± 0.09 µm was observed for a 3 mm layer. The obtained values were closely comparable, which makes it challenging to identify a clear correlation between water layer thickness and ablation rate. It should also be noted that the step size of focal position adjustment was 31.25 µm (controlled by an automated translation stage). Considering the relatively short Rayleigh length of 222 µm, this step size could have resulted in missing the precise focal position corresponding to the maximum crater depth. The influence of defocus on the ablation rate for four different pulse energy levels is shown in Fig. 4 . For the two lower energy levels, the maximum ablation depth—corresponding to the focal position—occurred at zero focal offset (0 µm). In contrast, for higher pulse energies (65.3 µJ and 150.6 µJ), the maximum crater depth was observed when the focal plane was shifted by + 31.25 µm toward the material bulk. This phenomenon can be interpreted as a shortening of the effective focal length of the optical system. A positive focal shift, corresponding to the translation of the focusing lens closer to the sample surface, effectively alters the beam convergence and the position of the focus. This effect arises from the nonlinear modification of the refractive index in the medium under the influence of a high-intensity electromagnetic field—a phenomenon known as Kerr self-focusing. 3.2. Effect of fluence In the case of irradiation in air at low fluence ( \(\:F\) = 1.8 J/cm², Fig. 5 a), which was close to the ablation threshold ( \(\:{F}_{th}\) = 0.67 J/cm², see Supplementary Information), the entire crater surface exhibited characteristic laser-induced periodic surface structures (LIPSS). The orientation of these structures was perpendicular to the polarization direction of the laser beam [ 38 , 39 ]. With successive laser pulses, the LIPSS became more pronounced and well-defined [ 40 ]. When the fluence was increased to 2.3 J/cm² (Fig. 5 b), a distinct change in morphology was observed in the central region of the crater. The LIPSS transformed into randomly distributed nanostructures resembling solidified droplets or column-like spikes with no preferred orientation. Under picosecond irradiation ( \(\:\tau\:\) = 10 ps), the material underwent melting in a very thin surface layer, followed by rapid solidification due to efficient heat conduction into the bulk, with cooling rates on the order of 10¹² K/s. This ultrafast process, occurring within hundreds of picoseconds, resulted in the formation of a highly supercooled liquid metal and its subsequent solidification into a nanocrystalline structure, a phenomenon commonly referred to as material freezing in a transient state [ 41 ]. Such surface morphology suggests that ablation may proceed via a fragmentation mechanism, in which the crystal lattice of the superheated material disintegrates into delicate clusters during rapid expansion [ 42 ]. At a fluence of 5.3 J/cm² (Fig. 5 c), a pronounced heat-affected zone (HAZ) was observed around the crater. In this region, the pulse energy was insufficient to damage the tungsten carbide grains directly but high enough to promote thermally induced escape of unbound carbon atoms from interstitial lattice sites. These carbon atoms are subsequently oxidized in the presence of atmospheric oxygen, leading to local chemical composition changes within the laser-affected zone [ 43 ]. At a higher fluence of 33.0 J/cm² (Fig. 5 d), the ablation morphology was characterized by a radial ejection of molten material and its subsequent recrystallization into elongated, droplet-like structures. Microscopic spherical droplets of resolidified melt were also visible within the crater area. These phenomena indicate that the local radiation intensity exceeded the thermodynamic limit of liquid-phase stability, leading to rapid decomposition of the superheated material into a mixture of vapor and liquid droplets [ 42 , 44 ]. Additionally, the central region of the crater exhibited a transformation of the nanostructure into a porous, irregular layer resembling a foamy morphology, suggesting enhanced cooling of the superheated liquid and its rapid recrystallization [ 41 ]. A deterioration in the regularity of crater cross-sections accompanied such morphological changes. At the crater bottom, a volcano-shaped protrusion of resolidified material was observed. The central part of this feature, corresponding to the crater center, appeared smooth and lacked the previously observed nanoscale formations. Similar morphological characteristics—namely, the presence of a central uplift rather than rim elevation—have been reported in other studies on laser ablation of metals using pulse durations of 6.7 ps [ 45 ] and 10 ps [ 46 ]. The most plausible explanations for this behavior are thermocapillary (Marangoni) effects or plasma recoil pressure, both of which can induce lateral flow and redistribute molten material [ 45 ]. As a result, the resolidification process in this region likely occurred under slower cooling conditions, possibly involving heterogeneous melting and gradual crystallization. This behavior can be attributed to the lower temperature gradient characteristic of the central region, which is exposed to the highest laser intensity. For even higher fluence levels of 84.9 and 133.1 J/cm² (Fig. 5 e and Fig. 5 f, respectively), the crater bottoms exhibited pronounced irregularities. These were caused by repeated motion and redistribution of molten material under consecutive high-energy laser pulses. It is also important to emphasize that with increasing fluence, the contributions of phase explosion and fragmentation become dominant ablation mechanisms [ 42 ]. These processes enhance the ejection of molten droplets from the superheated zone, resulting in an increased number of solidified droplets deposited around the ablation area at the highest fluence levels. Crater morphologies obtained using underwater picosecond laser processing at a fluence of 1.8 J/cm² (Fig. 6 a) exhibited no characteristic LIPSS patterns at the crater bottom. Instead, a non-directional surface texture with a small number of pores was observed. The presence of pores was attributed to the nucleation of water vapor bubbles that interacted with the thin molten layer of target material, penetrating it before the solidification process was complete. The absence of well-ordered LIPSS formation can be attributed to the dynamic penetration of the liquid layer into the molten material, which disrupts the local interference field responsible for the self-organization of LIPSS. Similar suppression [ 31 , 45 , 47 – 49 ], or modification [ 50 , 51 ] of LIPSS formation under liquid-assisted ultrashort-pulse irradiation has also been reported by other authors for various materials. At intermediate fluences (2.3–5.3 J/cm², Fig. 6 b and Fig. 6 c, respectively), more pronounced LIPSS structures were observed only near the crater edges. This suggests that, in these peripheral regions, the interaction between the molten material and the surrounding liquid was sufficiently gentle to preserve the formation of the interference pattern. This effect may result from the occurrence of film boiling, which generates a thin vapor layer that separates the molten material from the liquid. Such a gaseous layer could allow for the smooth recrystallization of the melt without disturbing the optical interference, thereby producing LIPSS patterns analogous to those formed during processing in air. Moreover, at Fig. 6 b, arc-shaped features appeared within the ablation zone. These structures are associated with the scattering of the laser beam at moving persistent bubbles [ 23 ]. When the fluence exceeded 33.0 J/cm² (Fig. 6 d), the accumulation of a hot molten layer led to a partial reversal of the plasma expansion direction toward the target surface (the so-called "pushing-back effect" [ 52 ]). This phenomenon manifested as the splashing and redeposition of resolidified molten material on the target surface. At the highest fluence levels (89.4 and 133.1 J/cm², Fig. 6 e and Fig. 6 f, respectively), both the number and size of pores increased, which can be associated with a prolonged solidification time of the molten phase. The longer lifetime of the liquid state allowed for deeper penetration of water and enhanced nucleation of vapor bubbles. Additionally, an increase in crater edge ellipticity was noted, which may result from Kerr self-focusing effects and a corresponding upward shift of the focal plane above the sample surface. It is noteworthy that, in contrast to ablation in air, no pronounced morphological transitions were observed over the tested fluence range, indicating a uniform ablation mechanism under underwater conditions. The liquid environment significantly suppresses the phase explosion mechanism typical of ablation in air [ 53 ]. Expanding clusters of superheated liquid undergo fragmentation into smaller droplets, which is manifested by the presence of fine molten particles in the vicinity of the crater. This behavior contrasts sharply with that observed in air, where large molten droplets ejected during phase explosion redeposit on the surface [ 53 ]. Importantly, in underwater processing, a substantial reduction in the amount of residual molten material was observed after ablation. The absence of geometrical crater deformations and the stability of crater morphology across the entire fluence range indicate highly efficient removal of the molten phase, even at the highest applied energy levels. The average crater depth per single pulse, obtained for both in-air and underwater processing over a fluence range of 1.8–133.1 J/cm², is presented in Fig. 7 . For ultrashort-pulse laser ablation, a logarithmic relationship between crater depth and fluence is typically observed—a dependence first described in [ 54 ] and subsequently confirmed in numerous experimental studies. This correlation originates from the two-temperature heat diffusion model and remains valid as long as no additional phenomena alter the local fluence distribution, such as plasma shielding, medium ionization, or photomechanical ablation [ 38 ]. In both in-air and underwater processes, a clear logarithmic dependence of crater depth on fluence was observed. However, the slope of the linear regression shows a distinct breakpoint, indicating a transition in the dominant ablation mechanism. For processing in air, this transition occurs at a fluence of 1.8 J/cm², whereas for underwater processing, it shifts to 3.1 J/cm². In the case of in-air processing, the fluence of 1.8 J/cm² corresponds to the threshold at which morphological features characteristic of phase explosion first appeared in the central region of the crater (Fig. 5 a). This marks a shift in the material removal mechanism and is reflected in a change in the slope of the logarithmic dependence. Below this value, the ablation process was governed primarily by gentler, non-explosive mechanisms. In contrast, during underwater processing, exceeding a fluence of 3.1 J/cm² signifies the onset of a more violent ablation regime. However, due to the suppression of phase explosion in the water environment, the dominant mechanism was thermodynamic fragmentation, i.e., the decomposition of the superheated liquid phase into smaller fractions [ 42 ]. Nevertheless, the overall logarithmic trend between crater depth and fluence remains preserved: for in-air processing in the range of 1.8–133.1 J/cm² and for underwater processing in the range of 3.1–133.1 J/cm². The observed increase in processing efficiency for underwater processing, even at relatively low fluences (≈ 2.0 J/cm²), suggests effective confinement and deceleration of the plasma plume. This phenomenon maintains elevated temperatures and pressures within the ablation zone [ 54 ], thereby supporting favourable thermodynamic conditions for efficient material removal. Such interpretation is widely accepted as one of the principal reasons for the enhanced ablation rate observed under liquid-assisted processing conditions [ 6 ]. The interaction of the laser beam with the tungsten carbide surface using five bursts, each of 10.9 J/cm² fluence and containing a single pulse per burst ( B1 , Fig. 8 a), resulted in the formation of laser-induced periodic surface structures ( LIPSS ) at the crater periphery and a non-directional, melted structure in the central region, which was an evidence of a phase explosion [ 46 ]. Increasing the number of pulses per burst to two ( B2 , Fig. 8 b) did not significantly affect the overall crater geometry in cross-sections but led to noticeable changes in surface morphology. The LIPSS pattern at the edges became more pronounced, while the crater center exhibited a smoother, resolidified layer. This observation suggests slower cooling dynamics, potentially allowing for heterogeneous melting and gradual recrystallization of the material. The first pulse initiates the phase explosion and plasma emission, whereas the second pulse arrives during the plasma lifetime (or within the expanding ablation plume) and becomes partially absorbed by it. This absorption leads to plasma reheating, an extended plasma lifetime, and reduced plume expansion [ 27 , 55 ]. Consequently, additional ablation may occur due to energy transfer from the plasma back to the surface, while the slower cooling facilitates surface smoothing and recrystallization in the crater center. When a third pulse is introduced ( B3 , Fig. 8 c), the plasma generated by the first pulse—sustained by the second—partially dissipates before the next pulse arrives, enabling another phase explosion and efficient ablation. The presence of disordered surface features characteristic of rapid resolidification supports this behavior. Processing with four pulses per burst ( B4 , Fig. 8 d) again produced a smooth crater surface, following a mechanism analogous to that of B2 . The third pulse induces effective ablation through phase explosion, while the fourth pulse interacts with the plasma or ablation plume formed by the preceding pulse ( B3 ). This results in plasma reheating and delayed material solidification. In the case of five pulses per burst ( B5 , Fig. 8 e), the sequence of events observed for odd-numbered bursts repeats—surface morphology reveals distinct features of intense phase explosion. For the six pulses ( B6 , Fig. 8 f), the crater center again exhibits a smooth, recrystallized layer, indicative of gradual solidification. It is noteworthy that with an increasing number of pulses per burst, the crater depth systematically increased, despite the total fluence per burst remaining constant. For seven pulses per burst ( B7 , Fig. 8 g), the crater morphology differed markedly from that of other odd-numbered cases. The surface was covered with a uniform, smooth molten layer, lacking the disordered structures typically associated with phase explosion. The suppression of explosive features can be attributed to the significant reduction in individual pulse energy, which shifts the dominant ablation mechanism toward trivial thermomechanical fragmentation [ 42 ]. Nevertheless, despite the lower per-pulse energy, the resulting crater was deeper than that obtained with six pulses ( B6 , Fig. 8 f), indicating effective heat accumulation in the material and a reduction in the ablation threshold for subsequent pulses. Although no clear morphological evidence of phase explosion was observed, isolated spherical particles were detected on the crater surface, likely originating from redeposited ablation products reflected toward the target [ 55 ]. Further increasing the number of pulses to seven and eight per burst ( B7 , Fig. 8 g and B8, Fig. 8 h) resulted in an even deeper crater, accompanied by a pronounced increase in the amount of molten material and the formation of rim elevations around the crater edge. These features indicate radial expulsion of molten material, driven by plasma recoil pressure acting upon the liquid phase of the target material. Figure 9 presents the effect of the number of pulses per burst (ranging from 1 to 8) on the surface morphology and cross-sectional crater profiles obtained underwater processing at a fluence of 10.9 J/cm². Processing with a single pulse per burst ( B1 , Fig. 9 a) resulted in craters with a maximum depth of not more than 0.5 µm and a diameter of approximately 30 µm. The crater surface exhibited a porous structure, originating from the dynamic penetration of the molten layer by the supercritical liquid in direct contact with the rapidly cooled substrate. At the crater periphery, a radial splash pattern of resolidified material was observed, attributed to the so-called push-back effect—the recoil of the ablation plume [ 52 ]. In the case of two pulses per burst ( B2 , Fig. 9 b), the crater depth increased to approximately 0.75 µm, while the morphological features remained unchanged: the central region exhibited a disordered porous structure, and the periphery retained the characteristic radial splash pattern. For three pulses per burst ( B3 , Fig. 9 c), the maximum crater depth was comparable to that obtained for a single pulse ( B1 ). No significant differences in surface morphology were observed; however, arc-shaped features appeared within the ablation zone. Processing with four to eight pulses per burst ( B4–B8 , Fig. 9 d-h) did not lead to any substantial changes in either the surface morphology or crater geometry. All resulting craters exhibited a consistent maximum depth of approximately 0.5 µm, indicating that the ablation efficiency becomes saturated with an increasing number of pulses per burst under the examined fluence conditions. A quantitative analysis of the average crater depth per burst (ablation rate) for in-air processing, obtained over a fluence range of 10.9–133.1 J/cm², is presented in Fig. 10 . Across the entire investigated fluence range, a clear improvement in ablation efficiency was observed with an increasing number of pulses per burst—the highest material removal rates were achieved for eight pulses per burst ( B8 ), regardless of the total burst fluence. The maximum ablation rate was recorded for a burst fluence of 133.1 J/cm², reaching 1329 nm/burst, whereas for single-pulse operation ( B1 ) the corresponding rate was only 152.4 nm/burst at the same fluence. The relationship between average crater depth and the number of pulses per burst exhibited an approximately linear trend for all examined fluence levels, with one notable exception: for four pulses per burst (B4) at 133.1 J/cm², the crater depth per burst was nearly identical to that obtained with three pulses ( B3 ). This deviation from the otherwise linear trend was consistent with the previously discussed morphological similarities observed between configurations B3 and B4 . A quantitative analysis of the average crater depth per burst for the underwater processing was shown in Fig. 10 . The results reveal the highest ablation efficiency for processing with a single pulse per burst ( B1 ), with average depths ranging from 105.3 to 157.7 nm/burst for burst fluences of 10.9 J/cm² and 133.1 J/cm², respectively. The only exception was observed for the two-pulse configuration ( B2 ) at a fluence of 10.9 J/cm², where a slightly greater average crater depth was recorded compared to B1 ; however, the significant standard deviation prevents a conclusive assessment of its effectiveness. Irrespective of the number of pulses per burst, the crater depths obtained at each fluence were comparable to, or lower than, those achieved with single-pulse operation. This suggests that, unlike in-air processing, the burst strategy provides limited benefits for enhancing the ablation rate under underwater conditions. The highest per-pulse ablation efficiency was achieved in single-pulse mode (B1), while increasing the number of pulses resulted in a systematic decrease in ablation efficiency, reaching a minimum of approximately 15 nm/pulse for eight-pulse bursts ( B8 ). The observed decline in ablation efficiency with an increasing number of pulses per burst can be attributed to the interaction of subsequent laser pulses with the delayed ablation plume front formed by the first pulse in the sequence (low number of pulses in burst) and interaction with the early stage of cavitation bubble expansion (high number of pulses in burst). In liquid environments, plume disintegration occurs significantly later than in gaseous media, resulting in a reduction in the effective laser energy reaching the target surface due to reflection and absorption by the expanding plume [ 43 , 46 ]. Furthermore, within approximately 1 ns after pulse emission, vapor bubble nucleation occurs at the plume–liquid interface, followed by their coalescence into a hemispherical cavitation bubble [ 6 , 56 ]. At the bubble boundary, strong light reflection and refraction occur [ 56 ], resulting in beam defocusing and a corresponding reduction in effective fluence at the target surface. This effect becomes more pronounced with each subsequent pulse in the burst, since the bubble radius grows logarithmically over time during the initial phase of its expansion [ 57 ]. Moreover, it is worth noting that the application of a higher number of pulses in a burst resulted in only a minor effect on the cavitation bubble, lifespan, and sphericity (Chapter S6 in Supplementary material). 4. Summary This study investigated the influence of the processing environment (in-air and underwater) and laser parameters on the ablation rate and surface morphology of tungsten carbide–cobalt (WC–Co) during picosecond laser processing. Systematic experiments were conducted using 10-ps laser pulses over a wide range of fluence levels, water layer thicknesses, focal positions, and burst-mode configurations. Based on the conducted research, the following statements can be made: Water layer thickness (1–5 mm) had only a minor effect on the ablation rate. Therefore, using the thickest tested water layer (5 mm) is most advantageous, as it minimizes the risk of damaging the protective glass window and reduces the likelihood of persistent bubbles adhering to its surface. In-air processing at fluences above 5.8 J/cm² resulted in the appearance of thermally induced features, ranging from heat-affected zones (HAZ) and large droplets to the formation of resolidified material at the crater bottom and edges at the highest fluences. The displacement of molten material by plasma-induced recoil pressure led to irregular crater-bottom geometries. In contrast, underwater processing effectively suppressed plasma expansion, enhanced the ejection of molten material, and reduced the redeposition of debris, resulting in smoother and more uniform craters across all fluence levels. However, a shallow layer of splashed molten material—typical for laser ablation in liquids and attributed to the back-reflection of the ablation plume during its constrained expansion—was observed. Ablation rate increased under underwater processing across the 2–133 J/cm² fluence range. The improvement, reaching approximately 75% (starting from F ≈ 3 J/cm²), was attributed to the suppression of plasma expansion by the water overlay. This suppression increased the temperature and pressure of the plasma for a longer duration, thereby enhancing plasma-assisted material removal. Burst-mode processing exhibited markedly different behavior in air and water. In air, the ablation efficiency increased nearly linearly with the number of pulses per burst, reaching a maximum for eight pulses, primarily due to heat accumulation and plasma reheating between successive pulses. Underwater, burst mode did not improve the ablation rate. Delayed pulses interacted with the long-lived ablation plume and expanding cavitation bubble, leading to beam scattering, defocusing, and a reduction in effective fluence at the target. Consequently, the highest per-pulse ablation efficiency was obtained for single-pulse operation. Overall, the findings confirm that underwater picosecond laser processing of WC–Co offers a favorable balance between surface quality and ablation performance. The liquid environment suppresses thermal damage and debris redeposition while maintaining efficient material removal at moderate and high fluence levels. However, the benefits of burst-mode operation are vastly diminished underwater due to enhanced plasma shielding and the dynamics of cavitation. These results provide new insights into the mechanisms governing ultrafast laser ablation in liquid environments, contributing to the optimization of processing strategies for hard, refractory materials such as tungsten carbide. The authors would like to thank Piotr Lampa for his valuable assistance in setting up the experimental setup, as well as for his advice related to the measurement procedure and image processing. The authors also gratefully acknowledge the Fraunhofer IST Institute (Braunschweig, Germany) for supplying the tungsten carbide samples used in this study. Declarations Acknowledgments The authors would like to thank Piotr Lampa for his valuable assistance in setting up the experimental setup, as well as for his advice related to the measurement procedure and image processing. The authors also gratefully acknowledge the Fraunhofer IST Institute (Braunschweig, Germany) for supplying the tungsten carbide samples used in this study. 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09:14:33","extension":"html","order_by":24,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":129793,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8504989/v1/1c405a4a2573ac8d24f66051.html"},{"id":99684088,"identity":"1aa8e0bf-ade8-44e0-8b32-1f3a2ee88cee","added_by":"auto","created_at":"2026-01-07 09:14:32","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":126101,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Schematic representation of the laser processing setup utilized in the research. (b) Relationship between pulse energy and number of pulses in a burst.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8504989/v1/61aec8272fc4e89100321cf9.png"},{"id":99795788,"identity":"bb572923-430f-4622-bd2f-383ea9c30d6e","added_by":"auto","created_at":"2026-01-08 13:39:42","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":150658,"visible":true,"origin":"","legend":"\u003cp\u003eCAD model of a tank for processing using underwater laser processing technology. (a) Isometric projection of the model with exploded tank components: (1,7) Externally threaded pressure ring, (2) O-ring seal, (3) cover glass, (4) spacer insert, (5) tungsten carbide substrate, (6) special sample holder, (8) aluminium housing. (b) Cross-section of components, including (9) threaded inlet hole for hydraulic connection, (10) threaded outlet hole for hydraulic connection, (11) inspection window for observing cavitation dynamics using a high-speed camera.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8504989/v1/a5e9cf5c10e0d7f8dd38145e.png"},{"id":99796638,"identity":"4d262c34-ec97-4f3c-a6c2-652834fe6141","added_by":"auto","created_at":"2026-01-08 13:43:02","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":93793,"visible":true,"origin":"","legend":"\u003cp\u003eAblation rate against focal position - in-air and underwater laser processing (UW) comparison for various water layer thickness (H=1 – 5 mm). The rest of the parameters were constant: pulse energy (30,8 µJ), pulse repetition frequency (5 Hz) and number of pulses (25).\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8504989/v1/df2fa3f4179fd390fcafa20f.png"},{"id":99797021,"identity":"dc7f926d-0909-46b4-95f3-6c6ee2e81ddd","added_by":"auto","created_at":"2026-01-08 13:44:23","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":100656,"visible":true,"origin":"","legend":"\u003cp\u003eInfluence of focal-plane position on the ablation rate for different pulse energies (12,3 µJ, 30,8 µJ, 65,3 µJ and 150,6 µJ) during underwater picosecond laser processing of tungsten carbide–cobalt. Laser-ablated craters were produced using 25 successive pulses at a pulse repetition rate of 5 Hz.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8504989/v1/e3016e726a4bf0fce34a9e71.png"},{"id":99684095,"identity":"955e8f17-4871-47b1-a61a-f56221dec051","added_by":"auto","created_at":"2026-01-07 09:14:32","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":2088843,"visible":true,"origin":"","legend":"\u003cp\u003eMorphology of the craters (top view) and corresponding cross-sections obtained from in-air laser processing (25 laser pulses at 5 Hz) and varying fluence of (a) 1,8 Jcm\u003csup\u003e─2\u003c/sup\u003e, (b) 2,3 Jcm\u003csup\u003e─2\u003c/sup\u003e, (c) 5,8 Jcm\u003csup\u003e─2\u003c/sup\u003e, (d) 33,0 \u0026nbsp;Jcm\u003csup\u003e─2\u003c/sup\u003e, (e) 84,9 Jcm\u003csup\u003e─2\u003c/sup\u003e, (f) 133,1 Jcm\u003csup\u003e─2\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8504989/v1/dd66760aea735f4f71bb1d5f.png"},{"id":99684092,"identity":"c76e3b50-f717-43c3-9a1a-5b6fc0cbfc3f","added_by":"auto","created_at":"2026-01-07 09:14:32","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":1196765,"visible":true,"origin":"","legend":"\u003cp\u003eMorphology of the craters (top view) and corresponding cross-sections obtained from underwater laser processing (25 laser pulses at 5 Hz) and varying fluence of (a) 1,8 Jcm\u003csup\u003e─2\u003c/sup\u003e, (b) 2,3 Jcm\u003csup\u003e─2\u003c/sup\u003e, (c) 5,8 Jcm\u003csup\u003e─2\u003c/sup\u003e, (d) 33,0 \u0026nbsp;Jcm\u003csup\u003e─2\u003c/sup\u003e, (e) 84,9 Jcm\u003csup\u003e─2\u003c/sup\u003e, (f) 133,1 Jcm\u003csup\u003e─2\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8504989/v1/80fd7f12f3d9fd0bffff462c.png"},{"id":99795427,"identity":"7d3ccded-d772-4dbf-9bf8-3aa3aed3440d","added_by":"auto","created_at":"2026-01-08 13:38:00","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":121031,"visible":true,"origin":"","legend":"\u003cp\u003eAblation rate as a function of laser fluence and laser pulse energy for craters produced with 25 constructive pulses at a pulse repetition frequency of 5 Hz.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8504989/v1/4d5d2de5a720466e95abe312.png"},{"id":99796563,"identity":"14b1dbee-06b2-44e9-8825-ef5a231ec794","added_by":"auto","created_at":"2026-01-08 13:42:45","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":1400778,"visible":true,"origin":"","legend":"\u003cp\u003eMorphology of the craters (top view) and corresponding cross-sections obtained from in-air laser processing (5 laser bursts at 5 Hz) and constant fluence of 10,9 Jcm\u003csup\u003e─2\u003c/sup\u003e with variable number of pulses in a burst: (a) one, (b) two, (c) three, (d) four, (e) five, (f) six, (g) seven, (h) eight\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-8504989/v1/6a3d045152393211e14de871.png"},{"id":99795305,"identity":"c56b1b61-1236-474c-8eb9-2ddf2a714cae","added_by":"auto","created_at":"2026-01-08 13:37:41","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":1355257,"visible":true,"origin":"","legend":"\u003cp\u003eMorphology of the craters (top view) and corresponding cross-sections obtained from underwater laser processing (5 laser bursts at 5 Hz) and constant fluence of 10,9 Jcm\u003csup\u003e─2\u003c/sup\u003e with variable number of pulses in a burst: (a) one, (b) two, (c) three, (d) four, (e) five, (f) six, (g) seven, (h) eight.\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-8504989/v1/ae00cd010f31eaf0b67fceac.png"},{"id":99795046,"identity":"fea4d565-65fa-4744-a466-a401d2314eb7","added_by":"auto","created_at":"2026-01-08 13:36:52","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":290195,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of ablation rate as a function of number of pulses in burst for (a) in-air and (b) underwater laser processing conditions. For each test, 25 constructive bursts were applied at a constant burst repetition rate of 5 Hz.\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-8504989/v1/5376b5f5787bfd6bcd83975e.png"},{"id":99805055,"identity":"85f1bb04-70aa-4187-8188-04f253305cd0","added_by":"auto","created_at":"2026-01-08 14:15:43","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6508862,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8504989/v1/235165c5-3e09-445d-a233-9318948f4a57.pdf"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003e\u003cstrong\u003eComparison of ablation rate and surface quality of in-air and underwater picosecond laser processing of tungsten carbide cobalt\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eExtending the lifespan and enhancing the performance of cutting tools remain significant challenges in the tooling industry. One promising approach is micro-texturing of the tool's active surface by means of pulsed laser ablation. For instance, the fabrication of shallow blind holes (dimples) has been shown to enhance machinability and tribological performance by reducing the effective contact area and improving lubrication through the storage of lubricants within the dimples [\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Laser ablation using ultrashort pulses offers high precision, repeatability, and superior surface quality; however, it is typically characterized by relatively low processing rates [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Moreover, operating in the high-fluence regime often leads to thermally induced surface damage [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eA potentially effective strategy to improve both quality and ablation rate involves performing laser ablation in a water environment instead of air. It has been suggested that the presence of a water layer prolongs the boiling phase of the material at the surface [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e] and increases the pressure exerted on the substrate [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e] volume of ablated material. Moreover, water facilitates the efficient ejection of molten material, thereby reducing the amount of resolidified melt in the ablation zone [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. On the other hand, the confinement of the ablation plume may lead to the redeposition of ablated material on the irradiated surface, which suppresses the ablation rate [\u003cspan additionalcitationids=\"CR11\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Overall, due to the complex nature of the process, several factors\u0026mdash;including material properties, liquid characteristics, and water layer thickness\u0026mdash;can influence the total volume of material removed by a single pulse and the quality of the ablated surface.\u003c/p\u003e \u003cp\u003eExperimental studies conducted by various research groups have produced inconclusive results regarding whether the presence of water enhances or reduces the ablation rate and surface quality, as these effects depend strongly on the material type and processing conditions. For ultrafast laser processing, an increase in ablation rate has been reported for silicon [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e], glass [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e], and copper [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e], whereas a decrease has been observed for stainless steel [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e], nickel [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e], titanium [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e], and zinc [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. It has been suggested that the presence of a water layer prolongs the boiling phase of the material at the surface [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e] and increases the pressure exerted on the substrate [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Moreover, water facilitates the efficient ejection of molten material, thereby reducing the amount of resolidified melt in the ablation zone [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. However, due to the complex nature of the process, several factors\u0026mdash;including material properties, liquid characteristics, and the thickness of the water layer\u0026mdash;can influence the total volume of material removed by a single pulse.\u003c/p\u003e \u003cp\u003eThe influence of the water environment on surface quality during ultrafast laser processing is also highly material-dependent. For example, processing silicon with 120 fs pulses resulted in the suppression of self-organized structures, reduced debris formation, and improved regularity of hole edges [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Processing nickel with 8 ps pulses led to a decrease in debris, recast layer, and spatter formation [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. In the case of zinc, irradiation with 30 fs pulses produced more homogeneous, debris-free crater morphologies, although porous structures appeared at high fluences [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Similarly, for zirconia, processing with 8 ps pulses reduced microcracks and surface roughness, along with a smaller heat-affected zone (HAZ) and thinner recast layer [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Processing silicon nitride ceramics with 273 fs pulses improved both surface smoothness and hole roundness [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Conversely, for stainless steel processed with 7 ps pulses, arc-like and spike-like surface features were formed [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Similar effects were observed for silver processed with 300 fs pulses, where highly rough and porous surfaces developed [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eTo date, no comprehensive studies have been reported on the underwater ultrafast laser processing of tungsten carbide, a key material in the field of mechanical engineering. Therefore, the effects of water assistance and laser processing parameters on ablation rate and surface quality remain unexplored. Furthermore, to our knowledge, the influence of burst-mode operation on underwater ablation behaviour has not yet been investigated for any material. This parameter is crucial for optimizing the ablation rate, as it can significantly increase material removal efficiency [\u003cspan additionalcitationids=\"CR26 CR27\" citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. However, studies have also shown that applying a high number of pulses per burst may deteriorate surface quality due to heat accumulation [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Thus, its experimental verification constitutes an important contribution to the advancement of laser micromachining technologies. In this study, we investigate the picosecond laser processing (10 ps pulses) of tungsten carbide immersed in water. The laser was operated in a single-spot irradiation regime and at a low pulse repetition rate to isolate multi-pulse ablation effects and avoid phenomena related to cavitation-bubble shielding. We compare the results with those obtained for in-air processing. The experiments include variations in water layer thickness, focal plane position, laser fluence, and the number of pulses per burst, to evaluate their effects on surface quality and ablation rate.\u003c/p\u003e"},{"header":"2. Material and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Experimental setup\u003c/h2\u003e \u003cp\u003eThe target material was tungsten carbide\u0026ndash;cobalt (WC\u0026ndash;Co) substrates supplied by the Fraunhofer IST Institute (Braunschweig, Germany), fabricated as discs with a diameter of 18 mm and a thickness of 3 mm. A detailed description of the material composition and surface properties was provided in Chapter S1 of the Supplementary Material. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea shows a diagram of the setup used in experimental research as part of the underwater laser processing. Laser source was picosecond laser Duetto (Lumentum, USA). System enabled the emission of pulses in the form of pulse packets (so-called burst mode, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb) with a frequency equal to the frequency of the 82 MHz oscillator (FixBurst\u0026reg;/FlexBurst\u0026reg;, Lumentum, USA). One pulse packet can emit from 1 to 8 pulses, and the energy of the pulse packet is approximately equal to the energy of a single pulse in basic mode. The dominant wavelength of the emitted radiation is 1064 nm at 10 ps laser pulses. The maximum pulse energy, generated at 50 kHz, was 150,6 \u0026micro;J per pulse in basic mode. Precise output power control and pulse gating were possible on the Pulse on Demand acousto-optic module (AOM, Lumentum, USA) placed after the optical output of the Duetto laser generator. The laser beam had a Gaussian distribution and a beam width (1/e\u003csup\u003e2\u003c/sup\u003e) of 1.0 mm and 0.9 mm in the X and Y directions, respectively measured after 1 meter from generator. The laser beam was directed to the processing area using three mirrors with anti-reflective coatings (omitted from the diagram) and then focused on the target material using a AL2520 focusing lens (Thorlabs, USA) characterized by effective focal length of 20 mm. Setup resulted in propagation of laser beam characterized by radius (in-air propagation) in waist of 8.5 um (Chapter S3 in Supplementary material). The precise positioning of the laser beam's focal point relative to the workpiece was achieved by translational movement of the process lens, which was mounted on a translation table with an 8MT167-25BS1 stepper motor (Standa, Lithuania). The motor was controlled using an 8SMC5-USB controller (Standa, Lithuania) and XILab software (Standa, Lithuania). The beam focus position on the substrate was changed using an automated MAX343/M table with a BSC-103 controller (Thorlabs, USA), to which a water tank was mounted. For in-air processing setup was the same, but the cover-glass window was removed and water vessel was empty.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn addition, high-speed imaging using the shadowgraphy technique was employed to verify that successive laser pulses did not interact with plasma-induced cavitation bubbles. This ensured the independence of individual pulses and eliminated potential effects related to pulse repetition frequency. Moreover, the high-speed observations confirmed the absence of interactions between the cavitation bubble and the cover glass for all investigated process parameters, thereby excluding bubble-induced liquid-jet erosion as a possible damage mechanism for the glass window. The procedure for bubble detection and the obtained results are described in detail in Chapter S6 of the Supplementary Material.\u003c/p\u003e \u003cp\u003eThe design of the liquid tank, which allowed for the adjustment of water layer thickness due to the changeable spacing between the glass-cover slip and the target, was developed and modelled in a CAD (Computer-Aided Design) environment. An isometric projection of the tank model is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea, while its cross-section is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb. Additionally, the tank was equipped with an inspection window made of 1 mm thick BK7 glass with an anti-reflective coating.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Experimental plan\u003c/h2\u003e \u003cp\u003eThe experimental plan assumed the evaluation of the influence of three processing parameters, namely the water-layer thickness, laser fluence, and the number of pulses in burst mode (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). To assess the effect of water-layer thickness, experiments were conducted at various focal-plane positions in order to identify the optimal focus at which the ablation rate was maximized. Evaluation was conducted at five levels of water layer thickness, ranging from 1 to 5 mm. This range was selected based on previous studies on underwater laser processing of various materials and experimental configurations, which consistently reported optimal water-layer thicknesses within this interval [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan additionalcitationids=\"CR32 CR33 CR34 CR35 CR36\" citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. Applying a water layer thinner than 1 mm is impractical, as it may lead to damage of the protective optical window for certain focal positions (see Chapter S4 in the Supplementary Material). Conversely, thicker water layers cause a significant reduction in the effective laser fluence at the target surface due to the elongation of the system's effective focal length, as discussed in detail in Chapter S5 of the Supplementary Material.\u003c/p\u003e \u003cp\u003eAdditionally, the evaluation was performed at different levels of laser fluence (controlled by pulse energy) to determine whether focal shifts caused by nonlinear effects occurred. The influence of laser fluence was investigated over the range of 1.2\u0026ndash;133.1 J/cm\u0026sup2; (determined in accordance with Chapter S1 in the Supplementary material) while keeping the number of pulses and the pulse repetition rate (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:PRR\\)\u003c/span\u003e\u003c/span\u003e) constant. To evaluate the effect of the number of pulses in burst mode, an intra-burst repetition rate of 82 MHz was used, corresponding to an inter-pulse delay of 25 ns within each burst. Pulse series from 1 to 8 were tested, which corresponds to the typical capabilities of commercially available picosecond laser sources. In the experiment, the burst fluence was set at fixed levels of 10.9, 27.3, 57.7, 92.2 and 133.1 J/cm\u0026sup2;. This meant that the energy in a single pulse decreased with an increase in the number of pulses in the burst, according to the relationship \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{F}_{Burst}=n*F\\)\u003c/span\u003e\u003c/span\u003e, where \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{F}_{Burst}\\)\u003c/span\u003e\u003c/span\u003e \u0026ndash; burst fluence, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:n\\)\u003c/span\u003e\u003c/span\u003e \u0026ndash; the number of pulses in the burst, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:F\\:\\)\u003c/span\u003e\u003c/span\u003e\u0026ndash; the fluence of a single pulse. As part of the experimental plan, a series of four identical craters was made for each set of parameters, spaced at equal intervals of 100 \u0026micro;m. Crater depth and diameter were determined following the procedure described in Chapter S2 of the Supplementary Material. The arithmetic mean and standard deviation of the analyzed geometric features were calculated based on measurements taken for all four structures.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eExperimental parameter sets used in the research.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eParameter\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eUnit\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eEffect of focal position and water layer thickness\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eInfluence of laser fluence\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eInfluence of the number of pulses in burst mode\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePulse duration (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\tau\\:\\)\u003c/span\u003e\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eps\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFluence (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:F)\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eJcm\u003csup\u003e─2\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e─\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.2\u0026ndash;133.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e10.9 \u0026ndash; 133.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePulse energy (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{E}_{p}\\)\u003c/span\u003e\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026micro;J\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e12.3\u0026ndash;150.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.3\u0026ndash;150.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e12.3\u0026ndash;150.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePulse Repetition Frequency (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:PRF\\)\u003c/span\u003e\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNumber of pulses (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:N\\)\u003c/span\u003e\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eau.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNumber of pulses in burst (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{n}_{b}\\)\u003c/span\u003e\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ea.u.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1\u0026ndash;8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eWater layer thickness (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:H\\)\u003c/span\u003e\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003emm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1\u0026ndash;5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFocal-plane position (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:x\\)\u003c/span\u003e\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003emm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e─2.0 \u0026times; Rayleigh length \u0026ndash;\u003c/p\u003e \u003cp\u003e+\u0026thinsp;2.0 \u0026times; Rayleigh length\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eFocal position (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:x\\)\u003c/span\u003e\u003c/span\u003e = 0)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eFocal position (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:x\\)\u003c/span\u003e\u003c/span\u003e = 0)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Effect of water thickness level\u003c/h2\u003e \u003cp\u003eThe achievable crater depths as a function of laser beam defocus were compared for five different water layer thicknesses (1\u0026ndash;5 mm) in the underwater picosecond laser processing configuration (), as well as for in-air processing (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). A focal plane shift was observed depending on the water layer thickness, which resulted from changes in the effective focal length of the optical system. For both underwater and in-air configurations, the dependence of crater depth on the focal position exhibited an approximately symmetrical profile, with the focal point corresponding to the maximum crater depth. This indicates a weak or negligible influence of beam shielding effects caused by ionized liquid, or ablation products above the material surface, which could otherwise occur at negative focal offsets (i.e., when the focal point is located above the sample surface). The experiments revealed significantly greater crater depths for all tested water layer thicknesses compared to those obtained in air. This suggests that the positive effect of plasma confinement contributes to an enhanced volume of ablated material. However, no substantial differences were found between the maximum crater depths obtained for different water thickness levels. The highest depths were recorded for 1 mm and 4 mm water layers\u0026mdash;2.68\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01 \u0026micro;m and 2.66\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03 \u0026micro;m, respectively\u0026mdash;while the lowest value of 2.48\u0026thinsp;\u0026plusmn;\u0026thinsp;0.09 \u0026micro;m was observed for a 3 mm layer. The obtained values were closely comparable, which makes it challenging to identify a clear correlation between water layer thickness and ablation rate. It should also be noted that the step size of focal position adjustment was 31.25 \u0026micro;m (controlled by an automated translation stage). Considering the relatively short Rayleigh length of 222 \u0026micro;m, this step size could have resulted in missing the precise focal position corresponding to the maximum crater depth.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe influence of defocus on the ablation rate for four different pulse energy levels is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. For the two lower energy levels, the maximum ablation depth\u0026mdash;corresponding to the focal position\u0026mdash;occurred at zero focal offset (0 \u0026micro;m). In contrast, for higher pulse energies (65.3 \u0026micro;J and 150.6 \u0026micro;J), the maximum crater depth was observed when the focal plane was shifted by +\u0026thinsp;31.25 \u0026micro;m toward the material bulk. This phenomenon can be interpreted as a shortening of the effective focal length of the optical system. A positive focal shift, corresponding to the translation of the focusing lens closer to the sample surface, effectively alters the beam convergence and the position of the focus. This effect arises from the nonlinear modification of the refractive index in the medium under the influence of a high-intensity electromagnetic field\u0026mdash;a phenomenon known as Kerr self-focusing.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Effect of fluence\u003c/h2\u003e \u003cp\u003eIn the case of irradiation in air at low fluence (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:F\\)\u003c/span\u003e\u003c/span\u003e = 1.8 J/cm\u0026sup2;, Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea), which was close to the ablation threshold (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{F}_{th}\\)\u003c/span\u003e\u003c/span\u003e= 0.67 J/cm\u0026sup2;, see Supplementary Information), the entire crater surface exhibited characteristic laser-induced periodic surface structures (LIPSS). The orientation of these structures was perpendicular to the polarization direction of the laser beam [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. With successive laser pulses, the LIPSS became more pronounced and well-defined [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. When the fluence was increased to 2.3 J/cm\u0026sup2; (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb), a distinct change in morphology was observed in the central region of the crater. The LIPSS transformed into randomly distributed nanostructures resembling solidified droplets or column-like spikes with no preferred orientation. Under picosecond irradiation (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\tau\\:\\)\u003c/span\u003e\u003c/span\u003e = 10 ps), the material underwent melting in a very thin surface layer, followed by rapid solidification due to efficient heat conduction into the bulk, with cooling rates on the order of 10\u0026sup1;\u0026sup2; K/s. This ultrafast process, occurring within hundreds of picoseconds, resulted in the formation of a highly supercooled liquid metal and its subsequent solidification into a nanocrystalline structure, a phenomenon commonly referred to as material freezing in a transient state [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. Such surface morphology suggests that ablation may proceed via a fragmentation mechanism, in which the crystal lattice of the superheated material disintegrates into delicate clusters during rapid expansion [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. At a fluence of 5.3 J/cm\u0026sup2; (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec), a pronounced heat-affected zone (HAZ) was observed around the crater. In this region, the pulse energy was insufficient to damage the tungsten carbide grains directly but high enough to promote thermally induced escape of unbound carbon atoms from interstitial lattice sites. These carbon atoms are subsequently oxidized in the presence of atmospheric oxygen, leading to local chemical composition changes within the laser-affected zone [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAt a higher fluence of 33.0 J/cm\u0026sup2; (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed), the ablation morphology was characterized by a radial ejection of molten material and its subsequent recrystallization into elongated, droplet-like structures. Microscopic spherical droplets of resolidified melt were also visible within the crater area. These phenomena indicate that the local radiation intensity exceeded the thermodynamic limit of liquid-phase stability, leading to rapid decomposition of the superheated material into a mixture of vapor and liquid droplets [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. Additionally, the central region of the crater exhibited a transformation of the nanostructure into a porous, irregular layer resembling a foamy morphology, suggesting enhanced cooling of the superheated liquid and its rapid recrystallization [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. A deterioration in the regularity of crater cross-sections accompanied such morphological changes. At the crater bottom, a volcano-shaped protrusion of resolidified material was observed. The central part of this feature, corresponding to the crater center, appeared smooth and lacked the previously observed nanoscale formations. Similar morphological characteristics\u0026mdash;namely, the presence of a central uplift rather than rim elevation\u0026mdash;have been reported in other studies on laser ablation of metals using pulse durations of 6.7 ps [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e] and 10 ps [\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]. The most plausible explanations for this behavior are thermocapillary (Marangoni) effects or plasma recoil pressure, both of which can induce lateral flow and redistribute molten material [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]. As a result, the resolidification process in this region likely occurred under slower cooling conditions, possibly involving heterogeneous melting and gradual crystallization. This behavior can be attributed to the lower temperature gradient characteristic of the central region, which is exposed to the highest laser intensity. For even higher fluence levels of 84.9 and 133.1 J/cm\u0026sup2; (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee and Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef, respectively), the crater bottoms exhibited pronounced irregularities. These were caused by repeated motion and redistribution of molten material under consecutive high-energy laser pulses. It is also important to emphasize that with increasing fluence, the contributions of phase explosion and fragmentation become dominant ablation mechanisms [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. These processes enhance the ejection of molten droplets from the superheated zone, resulting in an increased number of solidified droplets deposited around the ablation area at the highest fluence levels.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eCrater morphologies obtained using underwater picosecond laser processing at a fluence of 1.8 J/cm\u0026sup2; (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea) exhibited no characteristic LIPSS patterns at the crater bottom. Instead, a non-directional surface texture with a small number of pores was observed. The presence of pores was attributed to the nucleation of water vapor bubbles that interacted with the thin molten layer of target material, penetrating it before the solidification process was complete. The absence of well-ordered LIPSS formation can be attributed to the dynamic penetration of the liquid layer into the molten material, which disrupts the local interference field responsible for the self-organization of LIPSS. Similar suppression [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e, \u003cspan additionalcitationids=\"CR48\" citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e], or modification [\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e, \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e] of LIPSS formation under liquid-assisted ultrashort-pulse irradiation has also been reported by other authors for various materials.\u003c/p\u003e \u003cp\u003eAt intermediate fluences (2.3\u0026ndash;5.3 J/cm\u0026sup2;, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb and Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec, respectively), more pronounced LIPSS structures were observed only near the crater edges. This suggests that, in these peripheral regions, the interaction between the molten material and the surrounding liquid was sufficiently gentle to preserve the formation of the interference pattern. This effect may result from the occurrence of film boiling, which generates a thin vapor layer that separates the molten material from the liquid. Such a gaseous layer could allow for the smooth recrystallization of the melt without disturbing the optical interference, thereby producing LIPSS patterns analogous to those formed during processing in air. Moreover, at Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb, arc-shaped features appeared within the ablation zone. These structures are associated with the scattering of the laser beam at moving persistent bubbles [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eWhen the fluence exceeded 33.0 J/cm\u0026sup2; (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed), the accumulation of a hot molten layer led to a partial reversal of the plasma expansion direction toward the target surface (the so-called \"pushing-back effect\" [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e]). This phenomenon manifested as the splashing and redeposition of resolidified molten material on the target surface. At the highest fluence levels (89.4 and 133.1 J/cm\u0026sup2;, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ee and Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef, respectively), both the number and size of pores increased, which can be associated with a prolonged solidification time of the molten phase. The longer lifetime of the liquid state allowed for deeper penetration of water and enhanced nucleation of vapor bubbles. Additionally, an increase in crater edge ellipticity was noted, which may result from Kerr self-focusing effects and a corresponding upward shift of the focal plane above the sample surface.\u003c/p\u003e \u003cp\u003eIt is noteworthy that, in contrast to ablation in air, no pronounced morphological transitions were observed over the tested fluence range, indicating a uniform ablation mechanism under underwater conditions. The liquid environment significantly suppresses the phase explosion mechanism typical of ablation in air [\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e]. Expanding clusters of superheated liquid undergo fragmentation into smaller droplets, which is manifested by the presence of fine molten particles in the vicinity of the crater. This behavior contrasts sharply with that observed in air, where large molten droplets ejected during phase explosion redeposit on the surface [\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e]. Importantly, in underwater processing, a substantial reduction in the amount of residual molten material was observed after ablation. The absence of geometrical crater deformations and the stability of crater morphology across the entire fluence range indicate highly efficient removal of the molten phase, even at the highest applied energy levels.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe average crater depth per single pulse, obtained for both in-air and underwater processing over a fluence range of 1.8\u0026ndash;133.1 J/cm\u0026sup2;, is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. For ultrashort-pulse laser ablation, a logarithmic relationship between crater depth and fluence is typically observed\u0026mdash;a dependence first described in [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e] and subsequently confirmed in numerous experimental studies. This correlation originates from the two-temperature heat diffusion model and remains valid as long as no additional phenomena alter the local fluence distribution, such as plasma shielding, medium ionization, or photomechanical ablation [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. In both in-air and underwater processes, a clear logarithmic dependence of crater depth on fluence was observed. However, the slope of the linear regression shows a distinct breakpoint, indicating a transition in the dominant ablation mechanism. For processing in air, this transition occurs at a fluence of 1.8 J/cm\u0026sup2;, whereas for underwater processing, it shifts to 3.1 J/cm\u0026sup2;.\u003c/p\u003e \u003cp\u003eIn the case of in-air processing, the fluence of 1.8 J/cm\u0026sup2; corresponds to the threshold at which morphological features characteristic of phase explosion first appeared in the central region of the crater (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). This marks a shift in the material removal mechanism and is reflected in a change in the slope of the logarithmic dependence. Below this value, the ablation process was governed primarily by gentler, non-explosive mechanisms. In contrast, during underwater processing, exceeding a fluence of 3.1 J/cm\u0026sup2; signifies the onset of a more violent ablation regime. However, due to the suppression of phase explosion in the water environment, the dominant mechanism was thermodynamic fragmentation, i.e., the decomposition of the superheated liquid phase into smaller fractions [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. Nevertheless, the overall logarithmic trend between crater depth and fluence remains preserved: for in-air processing in the range of 1.8\u0026ndash;133.1 J/cm\u0026sup2; and for underwater processing in the range of 3.1\u0026ndash;133.1 J/cm\u0026sup2;. The observed increase in processing efficiency for underwater processing, even at relatively low fluences (\u0026asymp;\u0026thinsp;2.0 J/cm\u0026sup2;), suggests effective confinement and deceleration of the plasma plume. This phenomenon maintains elevated temperatures and pressures within the ablation zone [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e], thereby supporting favourable thermodynamic conditions for efficient material removal. Such interpretation is widely accepted as one of the principal reasons for the enhanced ablation rate observed under liquid-assisted processing conditions [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe interaction of the laser beam with the tungsten carbide surface using five bursts, each of 10.9 J/cm\u0026sup2; fluence and containing a single pulse per burst (\u003cem\u003eB1\u003c/em\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea), resulted in the formation of laser-induced periodic surface structures (\u003cem\u003eLIPSS\u003c/em\u003e) at the crater periphery and a non-directional, melted structure in the central region, which was an evidence of a phase explosion [\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]. Increasing the number of pulses per burst to two (\u003cem\u003eB2\u003c/em\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb) did not significantly affect the overall crater geometry in cross-sections but led to noticeable changes in surface morphology. The LIPSS pattern at the edges became more pronounced, while the crater center exhibited a smoother, resolidified layer. This observation suggests slower cooling dynamics, potentially allowing for heterogeneous melting and gradual recrystallization of the material.\u003c/p\u003e \u003cp\u003eThe first pulse initiates the phase explosion and plasma emission, whereas the second pulse arrives during the plasma lifetime (or within the expanding ablation plume) and becomes partially absorbed by it. This absorption leads to plasma reheating, an extended plasma lifetime, and reduced plume expansion [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e]. Consequently, additional ablation may occur due to energy transfer from the plasma back to the surface, while the slower cooling facilitates surface smoothing and recrystallization in the crater center.\u003c/p\u003e \u003cp\u003eWhen a third pulse is introduced (\u003cem\u003eB3\u003c/em\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ec), the plasma generated by the first pulse\u0026mdash;sustained by the second\u0026mdash;partially dissipates before the next pulse arrives, enabling another phase explosion and efficient ablation. The presence of disordered surface features characteristic of rapid resolidification supports this behavior. Processing with four pulses per burst (\u003cem\u003eB4\u003c/em\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ed) again produced a smooth crater surface, following a mechanism analogous to that of \u003cem\u003eB2\u003c/em\u003e. The third pulse induces effective ablation through phase explosion, while the fourth pulse interacts with the plasma or ablation plume formed by the preceding pulse (\u003cem\u003eB3\u003c/em\u003e). This results in plasma reheating and delayed material solidification.\u003c/p\u003e \u003cp\u003eIn the case of five pulses per burst (\u003cem\u003eB5\u003c/em\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ee), the sequence of events observed for odd-numbered bursts repeats\u0026mdash;surface morphology reveals distinct features of intense phase explosion. For the six pulses (\u003cem\u003eB6\u003c/em\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ef), the crater center again exhibits a smooth, recrystallized layer, indicative of gradual solidification. It is noteworthy that with an increasing number of pulses per burst, the crater depth systematically increased, despite the total fluence per burst remaining constant.\u003c/p\u003e \u003cp\u003eFor seven pulses per burst (\u003cem\u003eB7\u003c/em\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eg), the crater morphology differed markedly from that of other odd-numbered cases. The surface was covered with a uniform, smooth molten layer, lacking the disordered structures typically associated with phase explosion. The suppression of explosive features can be attributed to the significant reduction in individual pulse energy, which shifts the dominant ablation mechanism toward trivial thermomechanical fragmentation [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. Nevertheless, despite the lower per-pulse energy, the resulting crater was deeper than that obtained with six pulses (\u003cem\u003eB6\u003c/em\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ef), indicating effective heat accumulation in the material and a reduction in the ablation threshold for subsequent pulses. Although no clear morphological evidence of phase explosion was observed, isolated spherical particles were detected on the crater surface, likely originating from redeposited ablation products reflected toward the target [\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e]. Further increasing the number of pulses to seven and eight per burst (\u003cem\u003eB7\u003c/em\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eg and B8, Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eh) resulted in an even deeper crater, accompanied by a pronounced increase in the amount of molten material and the formation of rim elevations around the crater edge. These features indicate radial expulsion of molten material, driven by plasma recoil pressure acting upon the liquid phase of the target material.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e presents the effect of the number of pulses per burst (ranging from 1 to 8) on the surface morphology and cross-sectional crater profiles obtained underwater processing at a fluence of 10.9 J/cm\u0026sup2;. Processing with a single pulse per burst (\u003cem\u003eB1\u003c/em\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ea) resulted in craters with a maximum depth of not more than 0.5 \u0026micro;m and a diameter of approximately 30 \u0026micro;m. The crater surface exhibited a porous structure, originating from the dynamic penetration of the molten layer by the supercritical liquid in direct contact with the rapidly cooled substrate. At the crater periphery, a radial splash pattern of resolidified material was observed, attributed to the so-called push-back effect\u0026mdash;the recoil of the ablation plume [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e]. In the case of two pulses per burst (\u003cem\u003eB2\u003c/em\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eb), the crater depth increased to approximately 0.75 \u0026micro;m, while the morphological features remained unchanged: the central region exhibited a disordered porous structure, and the periphery retained the characteristic radial splash pattern. For three pulses per burst (\u003cem\u003eB3\u003c/em\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ec), the maximum crater depth was comparable to that obtained for a single pulse (\u003cem\u003eB1\u003c/em\u003e). No significant differences in surface morphology were observed; however, arc-shaped features appeared within the ablation zone. Processing with four to eight pulses per burst (\u003cem\u003eB4\u0026ndash;B8\u003c/em\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ed-h) did not lead to any substantial changes in either the surface morphology or crater geometry. All resulting craters exhibited a consistent maximum depth of approximately 0.5 \u0026micro;m, indicating that the ablation efficiency becomes saturated with an increasing number of pulses per burst under the examined fluence conditions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eA quantitative analysis of the average crater depth per burst (ablation rate) for in-air processing, obtained over a fluence range of 10.9\u0026ndash;133.1 J/cm\u0026sup2;, is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e. Across the entire investigated fluence range, a clear improvement in ablation efficiency was observed with an increasing number of pulses per burst\u0026mdash;the highest material removal rates were achieved for eight pulses per burst (\u003cem\u003eB8\u003c/em\u003e), regardless of the total burst fluence. The maximum ablation rate was recorded for a burst fluence of 133.1 J/cm\u0026sup2;, reaching 1329 nm/burst, whereas for single-pulse operation (\u003cem\u003eB1\u003c/em\u003e) the corresponding rate was only 152.4 nm/burst at the same fluence.\u003c/p\u003e \u003cp\u003eThe relationship between average crater depth and the number of pulses per burst exhibited an approximately linear trend for all examined fluence levels, with one notable exception: for four pulses per burst (B4) at 133.1 J/cm\u0026sup2;, the crater depth per burst was nearly identical to that obtained with three pulses (\u003cem\u003eB3\u003c/em\u003e). This deviation from the otherwise linear trend was consistent with the previously discussed morphological similarities observed between configurations \u003cem\u003eB3\u003c/em\u003e and \u003cem\u003eB4\u003c/em\u003e.\u003c/p\u003e \u003cp\u003eA quantitative analysis of the average crater depth per burst for the underwater processing was shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e. The results reveal the highest ablation efficiency for processing with a single pulse per burst (\u003cem\u003eB1\u003c/em\u003e), with average depths ranging from 105.3 to 157.7 nm/burst for burst fluences of 10.9 J/cm\u0026sup2; and 133.1 J/cm\u0026sup2;, respectively. The only exception was observed for the two-pulse configuration (\u003cem\u003eB2\u003c/em\u003e) at a fluence of 10.9 J/cm\u0026sup2;, where a slightly greater average crater depth was recorded compared to \u003cem\u003eB1\u003c/em\u003e; however, the significant standard deviation prevents a conclusive assessment of its effectiveness.\u003c/p\u003e \u003cp\u003eIrrespective of the number of pulses per burst, the crater depths obtained at each fluence were comparable to, or lower than, those achieved with single-pulse operation. This suggests that, unlike in-air processing, the burst strategy provides limited benefits for enhancing the ablation rate under underwater conditions. The highest per-pulse ablation efficiency was achieved in single-pulse mode (B1), while increasing the number of pulses resulted in a systematic decrease in ablation efficiency, reaching a minimum of approximately 15 nm/pulse for eight-pulse bursts (\u003cem\u003eB8\u003c/em\u003e).\u003c/p\u003e \u003cp\u003eThe observed decline in ablation efficiency with an increasing number of pulses per burst can be attributed to the interaction of subsequent laser pulses with the delayed ablation plume front formed by the first pulse in the sequence (low number of pulses in burst) and interaction with the early stage of cavitation bubble expansion (high number of pulses in burst). In liquid environments, plume disintegration occurs significantly later than in gaseous media, resulting in a reduction in the effective laser energy reaching the target surface due to reflection and absorption by the expanding plume [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]. Furthermore, within approximately 1 ns after pulse emission, vapor bubble nucleation occurs at the plume\u0026ndash;liquid interface, followed by their coalescence into a hemispherical cavitation bubble [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e]. At the bubble boundary, strong light reflection and refraction occur [\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e], resulting in beam defocusing and a corresponding reduction in effective fluence at the target surface. This effect becomes more pronounced with each subsequent pulse in the burst, since the bubble radius grows logarithmically over time during the initial phase of its expansion [\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eMoreover, it is worth noting that the application of a higher number of pulses in a burst resulted in only a minor effect on the cavitation bubble, lifespan, and sphericity (Chapter S6 in Supplementary material).\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Summary","content":"\u003cp\u003eThis study investigated the influence of the processing environment (in-air and underwater) and laser parameters on the ablation rate and surface morphology of tungsten carbide\u0026ndash;cobalt (WC\u0026ndash;Co) during picosecond laser processing. Systematic experiments were conducted using 10-ps laser pulses over a wide range of fluence levels, water layer thicknesses, focal positions, and burst-mode configurations. Based on the conducted research, the following statements can be made:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eWater layer thickness (1\u0026ndash;5 mm) had only a minor effect on the ablation rate. Therefore, using the thickest tested water layer (5 mm) is most advantageous, as it minimizes the risk of damaging the protective glass window and reduces the likelihood of persistent bubbles adhering to its surface.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eIn-air processing at fluences above 5.8 J/cm\u0026sup2; resulted in the appearance of thermally induced features, ranging from heat-affected zones (HAZ) and large droplets to the formation of resolidified material at the crater bottom and edges at the highest fluences. The displacement of molten material by plasma-induced recoil pressure led to irregular crater-bottom geometries. In contrast, underwater processing effectively suppressed plasma expansion, enhanced the ejection of molten material, and reduced the redeposition of debris, resulting in smoother and more uniform craters across all fluence levels. However, a shallow layer of splashed molten material\u0026mdash;typical for laser ablation in liquids and attributed to the back-reflection of the ablation plume during its constrained expansion\u0026mdash;was observed.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eAblation rate increased under underwater processing across the 2\u0026ndash;133 J/cm\u0026sup2; fluence range. The improvement, reaching approximately 75% (starting from F\u0026thinsp;\u0026asymp;\u0026thinsp;3 J/cm\u0026sup2;), was attributed to the suppression of plasma expansion by the water overlay. This suppression increased the temperature and pressure of the plasma for a longer duration, thereby enhancing plasma-assisted material removal.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eBurst-mode processing exhibited markedly different behavior in air and water. In air, the ablation efficiency increased nearly linearly with the number of pulses per burst, reaching a maximum for eight pulses, primarily due to heat accumulation and plasma reheating between successive pulses. Underwater, burst mode did not improve the ablation rate. Delayed pulses interacted with the long-lived ablation plume and expanding cavitation bubble, leading to beam scattering, defocusing, and a reduction in effective fluence at the target. Consequently, the highest per-pulse ablation efficiency was obtained for single-pulse operation.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eOverall, the findings confirm that underwater picosecond laser processing of WC\u0026ndash;Co offers a favorable balance between surface quality and ablation performance. The liquid environment suppresses thermal damage and debris redeposition while maintaining efficient material removal at moderate and high fluence levels. However, the benefits of burst-mode operation are vastly diminished underwater due to enhanced plasma shielding and the dynamics of cavitation. These results provide new insights into the mechanisms governing ultrafast laser ablation in liquid environments, contributing to the optimization of processing strategies for hard, refractory materials such as tungsten carbide.\u003c/p\u003e \u003cp\u003eThe authors would like to thank Piotr Lampa for his valuable assistance in setting up the experimental setup, as well as for his advice related to the measurement procedure and image processing. The authors also gratefully acknowledge the Fraunhofer IST Institute (Braunschweig, Germany) for supplying the tungsten carbide samples used in this study.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgments\u003c/h2\u003e \u003cp\u003eThe authors would like to thank Piotr Lampa for his valuable assistance in setting up the experimental setup, as well as for his advice related to the measurement procedure and image processing. The authors also gratefully acknowledge the Fraunhofer IST Institute (Braunschweig, Germany) for supplying the tungsten carbide samples used in this study.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eMao B, Siddaiah A, Liao Y, Menezes PL (2020) J Manuf Process 53:153\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eNanbu T, Ren N, Yasuda Y, Zhu D, Wang QJ (2008) Tribol Lett 29:241\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eVishnoi M, Kumar P, Murtaza Q (2021) Surf Interfaces 27:101463\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLi X, Guan Y (2020) Nanatechnol Precision Eng 3:105\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZheng B, Jiang G, Wang W, Mei X, Wang F (2017) Opt Laser Technol 94:267\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKanitz A, Kalus MR, Gurevich EL, Ostendorf A, Barcikowski S, Amans D (2019) Plasma Sources Sci Technol 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Experiments were conducted using 10-ps pulses to evaluate the effects of water-layer thickness, focal-plane position, laser fluence, and burst-mode operation on ablation rate and surface morphology. Underwater processing significantly improved surface quality by suppressing plasma expansion, enhancing the ejection of molten material, and reducing the formation of heat-affected zones and debris. The water layer thickness (1\u0026ndash;5 mm) was found to have only a minor effect on the ablation rate. In contrast to in-air processing, underwater ablation produced smooth and more regular craters across all fluence levels, with no indications of thermal damage. Furthermore, underwater processing increased the ablation rate by up to ~\u0026thinsp;75% compared with air processing, attributed to prolonged plasma confinement and enhanced plasma-assisted material removal. Burst-mode operation exhibited opposite trends in the two environments: in air, increasing the number of pulses per burst improved the ablation rate due to heat accumulation and plasma reheating, whereas underwater, pulses in a burst interacted with the long-lived ablation plume and expanding cavitation bubble, reducing the effective fluence and thereby diminishing the ablation rate. Overall, the results provide new insights into ultrafast laser ablation mechanisms in liquids and demonstrate that underwater processing offers a superior balance between ablation performance and surface integrity for hard, refractory materials such as WC\u0026ndash;Co.\u003c/p\u003e","manuscriptTitle":"Comparison of ablation rate and surface quality of in-air and underwater picosecond laser processing of tungsten carbide cobalt","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-07 09:14:21","doi":"10.21203/rs.3.rs-8504989/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"a8b38fb1-de0f-490d-8365-6e97eef55c19","owner":[],"postedDate":"January 7th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":60511841,"name":"Materials Engineering"}],"tags":[],"updatedAt":"2026-01-07T09:14:22+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-07 09:14:21","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8504989","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8504989","identity":"rs-8504989","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}