
==== Front
ACS Omega
ACS Omega
ao
acsodf
ACS Omega
2470-1343
American Chemical Society

10.1021/acsomega.4c05018
Article
Improved Femtosecond Laser Welding of Nonoptical Contact Glass by Pressure and a Water Intermediate Layer
https://orcid.org/0009-0006-4818-4920
Chen Hao †
Li Zhaoxu †
Han Mingyang †
Yang Xiao †
https://orcid.org/0000-0002-1701-1166
Bai Shi *†‡
† Hebei Key Laboratory of Materials Near-Net-Forming Technology, School of Material Science and Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China
‡ Advanced Laser Processing Research Team, RIKEN Center for Advanced Photonics, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
* Email: shi.bai@riken.jp.
04 09 2024
17 09 2024
9 37 3887838886
28 05 2024
30 08 2024
23 08 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

Ultrafast laser welding has emerged as a significant technology for the joining of transparent materials and has been extensively researched for the welding of glass materials in recent years. However, achieving a robust connection requires optical contact between the samples during laser processing, which complicates the welding process and increases its cost. To achieve high-strength welding of glass samples without optical contact, the samples are typically pressed together to minimize the gap, allowing the molten materials to completely fill it. In this study, two approaches are employed to facilitate high-strength welding of silica glasses: femtosecond (fs) laser welding with the assistance of pressure using a fixture and femtosecond laser welding aided by a water intermediate layer. The welding quality achieved by both methods is thoroughly examined and compared. The findings reveal that femtosecond laser welding with water assistance outperforms other methods in terms of welding uniformity and quality. Furthermore, the principle behind achieving high-quality welding is discussed to elucidate the success of this method.

National Natural Science Foundation of China 10.13039/501100001809 52305354 Natural Science Foundation of Hebei Province 10.13039/501100003787 E2023208010 Natural Science Foundation of Hebei Province 10.13039/501100003787 BJK2024021 National Natural Science Foundation of China 10.13039/501100001809 52375422 document-id-old-9ao4c05018
document-id-new-14ao4c05018
ccc-price
==== Body
pmc1 Introduction

Glass, as a transparent material, is extensively used in various industrial products. Its exceptional physical and chemical properties have garnered significant interest in the realms of medicine, optics, biochips, photonic packaging, and microelectronic systems in recent years.1−3 With the advancement of silicon photonics, the fabrication of glass components often entails the joining of multiple pieces of glass.4,5 Traditional methods of glass joining, such as gluing, diffusion bonding, optical contact bonding, and continuous wave laser welding, exhibit limitations, including aging of the intermediate layer, the necessity for high-temperature heating, low joint strength, and the occurrence of welding cracks.6−8 Since 2005, the field of ultrafast laser welding of glass has attracted considerable attention due to the unique capabilities of femtosecond (fs) laser processing. The focusing of a fs laser inside glass induces ultrahigh pulse energy, leading to nonlinear absorption phenomena at the focus, such as photon ionization, tunnel ionization, and avalanche ionization.9 Subsequently, the energy is deposited at the focus and the glass material is melted to modify or ablate the glass. In this process, the molten glass material is resolidified by femtosecond laser radiation, and thus, the glass materials are connected. This technology overcomes the problem of aging of the intermediate layer and inhibits heat diffusion and thermal damage of glasses using traditional welding methods.4,10−12

As early as 2005, Tamaki et al. reported on femtosecond laser welding of silica glass, indicating the initial successful application of ultrafast lasers for glass joining and introducing a novel approach to transparent material connection.13 Subsequently, the technology of ultrafast laser welding for glass materials has undergone extensive research and development by numerous groups. In 2007, Isamu et al. discovered that high repetition rates of fs lasers were beneficial for the glass connection. Their findings indicated that the nonlinear absorption triggered by the ultrafast laser increased with increasing pulse repetition frequency, leading to a higher rate of electron excitation to the conduction band and intensified avalanche ionization. As avalanche ionization increased the temperature in the focal zone, a high pulse repetition frequency ultimately enhanced material melting, yielding a stronger connection effect and higher efficiency in less time.3 To achieve a sealing effect, Huang et al. introduced a strategy involving multiple laser scans on silica glass in 2012.14 Most studies on ultrafast laser welding necessitate optical contact (the gap between two glass plates being less than 1/4 the wavelength15) between glasses for optimal connection quality,16−19 as a larger gap can lead to welding cracks and residual tensile stress, resulting in a fragile connection. However, achieving optical contact in industrial settings is complex and increases the cost of welding.

The pursuit of high-quality connections between glasses without optical contact has recently become a focal point in the ultrafast laser welding of glass materials.6,20−22 In 2015, Cvecek et al. observed that ultrafast laser scanning rendered the surface of a glass substrate irreversibly convex, and when the convexity exceeded the sample gap, two base glass plates could be effectively bridged and connected.5 In 2017, Richter et al. sought to enlarge the molten pool volume by using a pulse train, generating sufficient molten material to bridge the sample gap. They achieved a final connection strength of 73 MPa, corresponding to 85% of the strength of the original material.23 In 2023, Jia et al. introduced a multiple-scanning technique for welding glasses without optical contact, achieving high-quality welds. They noted that although the initial single scan caused the molten material to enter the gap, it was insufficient to fill it completely. During subsequent scans, the influx of molten glass material continued, increasing the modified zone until the gap was entirely filled.24

In Table 1, we summarize recent studies on the laser welding of glass. Continuous lasers suffer from drawbacks such as high thermal input and challenging control, frequently resulting in material damage, particularly in hard and brittle materials. Due to their ultrashort pulses, femtosecond lasers exhibit a greater propensity to reach nonlinear thresholds and possess greater adaptability to the absorption characteristics of materials. Consequently, femtosecond lasers are widely employed in the processing of transparent materials, providing an efficient approach to glass welding. Clipping devices are commonly used in ultrafast laser welding glass, which is a direct and simple method for reducing the sample gap for nonoptical contact.25−27 However, due to the uneven pressure induced by the fixture on the glass samples, the strength between the samples may decrease due to residual stress after welding. Additionally, the design and fabrication of versatile clamping devices are costly and inefficient for real-world applications. Therefore, in this paper, we investigated and compared two methods for realizing connections between nonoptical contact glasses via fs laser welding. In method 1, a homemade fixture is used to reduce the gap between the glass samples. In method 2, a water layer is introduced in the gap to achieve high-quality connections via femtosecond laser welding. In both instances, the gap was several times larger than the laser wavelength. The impacts of the focus position and pulse energy on the laser-induced modified zone and the connection strength of the glass were investigated. Moreover, the mechanical properties and morphologies of the welded samples prepared by the two methods were compared, and the underlying mechanisms were briefly discussed.

Table 1 Summary of Reported Studies on Ultrafast Laser Welding Glass in Recent Years

author	materials	laser type	pretreatment method before welding	sample connection strength and test method	
Tamaki et al.13	silica glass	fs laser	fixture preloading	---	
optical contact	
Isamu et al.3	borosilicate glass	fs laser	natural stacking	---	
Horn et al.28	borosilicate glass	fs laser	fixture preloading	---	
Huang et al.14	silica glass	fs laser	fixture preloading	---	
optical contact	
Richter et al.10	silica glass	fs laser	optical contact	three-point bending measurement	
optimum performance 54 N/mm2	
Wu et al.26	aluminosilicate glass	fs laser	fixture preloading	shear test	
optimum performance 13.36 MPa	
Jia et al.24	soda-lime glass	ps laser	fixture preloading	shear test	
optimum performance 6.5 MPa	
Zhang et al.6	aluminosilicate glass	ps laser	fixture preloading	shear test	
optimum performance 30 MPa	
Richter et al.29	silica glass	ns laser	natural stacking	three-point bending measurement	
optimum performance 72.91 MPa	
Chen et al.30	soda-lime glass	ps laser	natural stacking	shear test	
optimum performance 64 MPa	
Zhang et al.31	aluminosilicate glass	ps laser	natural stacking	shear test	
optimum performance 17.4 MPa	
Chen et al.32	transparent lithium–aluminum-silicate glass ceramics	fs laser	natural stacking	shear test	
optimum performance 30.41 MPa	
Yu et al.15	borosilicate glass	fs laser	optical contact	shear test	
optimum performance 20.85 MPa	
Zhang et al.33	soda-lime glass	cw laser	natural stacking	shear test	
optimum performance 5.65 MPa	

2 Experiments

2.1 fs Laser Welding of Silica Glass

The schematic representation of the experimental setup for the nonoptical contact silica glass fs laser welding system is depicted in Figure 1. In the experiment, a Gaussian beam (Light Conversion, Pharos PH2–20W) with a wavelength of 1030 nm, a repetition frequency of 1087 kHz, and a pulse duration of 300 fs was utilized as the welding light source. The pulse energy was modulated by an attenuator consisting of a 1/2λ waveplate and a polarized beam splitter. An objective (M Plan Apo NIR, NA = 0.45) focused the laser beam to achieve a focal spot with a diameter of 3 μm.

Figure 1 Schematic diagram of the femtosecond laser welding system and schematic diagram of the fs laser welding methods. (a) Homemade fixture pretightening welding. (b) Water intermediate layer-assisted welding. (c) Natural stacking welding.

Figure 1 shows a schematic diagram of three distinct welding methods. The base material used for the samples was commercially available fused silica glass, measuring 20 × 20 × 1 mm3. In Method 1, to mitigate the impact of surface impurities on the welding quality, the glass plates underwent ultrasonic cleaning in an ethanol solution for 60 s and were subsequently dried with pure nitrogen gas. During the experiment, a custom-made fixture was used to apply pressure to the two glass plates. The actual fixture is shown in Figure S1. In Method 2, the surface of the glass was treated with a plasma cleaner to achieve a hydrophilic surface. Afterward, a specific volume of deionized water was introduced between the two glass plates using a pipet. It is important to note that in both methods, the glass samples were not in optical contact. The gap measured 6.5 μm in Method 1 and 2.6 μm in Method 2, as verified by a confocal laser scanning microscope. A three-dimensional (3D) translation stage facilitated control over the sample movement. The scanning speed was maintained at 1 mm/s, covering a scanning area of 20 × 1 mm2, with the spacing between adjacent scan lines set at 200 μm, and a total of six scans were recorded.

2.2 Characterization

The shear properties of the welded samples were evaluated by using a universal testing machine (ZGDR, DR-509A). The shear strength test method and equipment are shown in Figure 2. In the test, the operating speed of the shear strength measurement was 0.5 mm/min. The gap between the samples was determined before welding through measurements taken with a confocal laser scanning microscope (Leica, DCM8). The morphologies of the welds were examined by using an optical microscope (Leica, DMC4500). Following the welding process, all of the samples were left to stand in the air at room temperature for 24 h prior to the shear performance measurements.

Figure 2 (a) Schematic diagram of the welding sample shearing scheme. (b) Equipment for measuring the shear strength.

3 Results and Discussion

3.1 Fixture Pretightening-Assisted Femtosecond Laser Welding of Nonoptical Contact Silica Glass

Figure 3(a) illustrates the effects of varying the focus position inside the glass when a repetition rate of 1087 kHz and a pulse energy of 9 μJ. The red dashed line indicates the interface between the upper and lower glass plates. Due to avalanche ionization at the laser focal point, a plasma is induced, which shields the laser. Therefore, as the number of laser pulses increases, a plasma region forms within the glass, extending from the focal point toward the laser source. The plasma region transfers energy to the surrounding material through the thermal diffusion effect, which initiates the formation of an external melting zone. Consequently, when the inner plasma zone and outer melting zone converge, a teardrop-shaped laser modification zone is formed.34 The position of this modified zone shifts with different laser focus positions, while the size of the modified zone remains constant. A cavity is noticeable at the apex of the modified zone, attributed to the tensile stresses in the liquid glass resulting from rapid cooling.31,35 The use of a high repetition rate leads to an expected accumulated heat effect, which contributes to glass deformation.36 With a defocus of 0, the modified zone is formed within the lower glass plate, preventing the molten material from filling the sample gap, which results in welding failure. In the experiment, we defined the plane where the laser can be focused to the smallest spot size. In cases where the defocus ranged from +60 to +210 μm, the modified zone appeared within both glass plates and progressively shifted into the upper glass plate, enabling the molten materials to bridge the sample gap and establish an effective connection. However, when the defocus exceeds +240 μm, the modified zone is confined to the upper glass plate exclusively and the molten material fails to fill the sample gap, leading to a failure in welding.

Figure 3 (a) Cross-sectional morphology of the modified zone at different focus positions at a pulse energy of 9 μJ, repetition rate of 1087 kHz, and scanning speed of 1 mm/s. (b) Cross-sectional morphology of the modified zone in glass under different pulse energies.

In detail, the shear strength of the welded samples was assessed at various focal points within a defocusing range of +60 to +240 μm, with increments of 60 μm. As illustrated in Figure 4(a), the shear strength increases as the defocus position shifts from +60 to +120 μm, where the highest shear strength recorded is 13.1 MPa. With further increments in defocus, the shear strength of the sample progressively diminishes to 0 MPa at +240 μm, aligning with the observations related to the modified zone depicted in Figure 3(a). To quantitatively demonstrate the effect of defocusing on connection quality, the width (L) of each modified zone at the interface between the two glass plates, as shown in Figure 3(a), was measured and is presented in Table S1. At a defocus of 0, L is 64.8 μm and increases to a maximum of 97.8 μm at +120 μm, corresponding to the optimum welding strength. As the focal position continues to increase, the width of the modified zone at the interface diminishes until the modified zone is entirely confined within the upper glass plate (at +240 μm).

Figure 4 (a) Width of the modified zone at the interface (L) and shear performance of the welded samples under different defocus positions. (b) Height and width of the modified zone with different pulse energy components and shear strength of welded samples with different pulse energy components.

Since the modified zone is critical for the quality of welding, the modified zone inside the glass under different pulse energies was investigated, as shown in Figure 3(b). With increasing pulse energy, the area of the modified zone was clearly increased. To analyze the influence of pulse energy, the height (H) and width (W) of the modified zone were measured, as shown in Table S2. When the pulse energy is 3 μJ, the entire modified zone is created in the lower glass plate, and the modified zones are relatively small, with H = 76.7 μm and W = 47.6 μm. When the pulse energy was >6 μJ, the modified zone gradually crossed the interface, which means that the two glass plates started to connect. When the pulse energy is 15 μJ, the H and W of the modified zone reach 274.2 and 182.4 μm, respectively, which are approximately four times greater than those at 3 μJ. In addition, L increases from 0 to 178.4 μm. Although the area of the modified zone increased with increasing pulse energy, the aspect ratio was approximately 1.6 (H/W). The shear strength of the samples prepared at different pulse energies was tested, and the results are shown in Figure 4(b). The shear strength reaches a maximum value of 14.99 MPa when the pulse energy is 15 μJ since it shows the largest modified zone. Increasing the pulse energy leads to significant ablation of the upper glass, which restricts the modified zone formation in the lower glass due to plasma shielding. Therefore, a large cavity is created in the glass, and the laser is scattered by the cavity to generate cracks near the cavity, leading to a failed connection.37

In the conducted experiments, as the number of scanning lines increased, a greater volume of molten material was able to fill and narrow the sample gap, corroborating findings from previous studies.29 As depicted in Figure 5, the molten materials generated during the initial scans are insufficient to completely fill the gap, leading to connections occurring solely within the laser focusing zone. Consequently, the gap remains visible outside the focusing zone. With an increase in the number of scans, the sample gap progressively diminishes until it is entirely eliminated and sealed. This phenomenon is attributed to the initial scans creating connections between the upper and lower glass plates, which effectively reduce the sample gap. Subsequently, the molten material from the later scans is capable of filling the entire sample gap.

Figure 5 Cross-sectional view of the welded sample after 11 scans. With an increasing number of scans, the sample gap is completely closed.

3.2 Water Intermediate Layer-Assisted fs Laser Welding of Nonoptical Contact Silica Glass

To investigate the practicality and process attributes of silica glass welding assisted by a water intermediate layer, the same parameters utilized in Method 1 were applied to the welding experiments. Specifically, the laser scanning speed was set at 1 mm/s, the defocus range extended from 0 to +240 μm, and the pulse energy varied between 6 and 15 μJ.

As depicted in Figure 6, the morphological characteristics of the modified zone within the glass were examined under various conditions following femtosecond laser welding. Table S3 presents the measured height and width of the modified zone. The modified zone progressively crosses the interface of the glass plates with an increasing defocus position and pulse energy. The glass can be connected when both glass plates are modified by a femtosecond laser, as mentioned above. When the defocus was 0 μm, the glasses were successfully modified and connected, which could not be achieved by using a fixture with the same laser parameters. This is attributed to the refractive index of the water intermediate layer (1.33) being larger than that of air (1), which leads to greater nonlinear absorption and the generation of a relatively large modified zone.

Figure 6 Morphologies of the zones modified by the water intermediate layer-assisted femtosecond laser welding in glass. (a) Cross-sectional morphology of the modified zone at different focal positions. (b) Cross-sectional morphology of the modified zone at different pulse energies.

In addition, we investigated the influence of the water volume on the shear strength of the sample connection, the results of which are illustrated in Figure S2. When the shear strength increases from 0 to 0.5 μL, the maximum shear strength (18.3 MPa) is obtained at 0.5 μL. In the volume range of 0.5 to 8 μL, the welding strength gradually decreases, while the shear strength of the welded sample remains above 10 MPa in this range, indicating reliable welding quality. However, when the water interlayer volume increases to 16 μL, the shear strength decreases to 0 MPa because the excess water increases the gap between the samples.

3.3 Mechanism of Water Intermediate-Assisted fs Laser Welding

To verify the improvement in the welding strength of the proposed methods for nonoptical contact glass, we compared the shear strength of the two proposed methods with that of the natural stacking of glasses. The macroscopic and microscopic morphologies of the welds generated by three distinct welding methods are depicted in Figure 7. During natural stacking, white welding lines were clearly observed. These opaque lines might be produced by the intense plasma during fs laser processing in glass samples, which led to significant ablation of the glass rather than reliable welding, indicating inferior welding quality.24 When pressure was applied to the glass plates, transparent welding lines emerged on both sides of the laser scanning area, suggesting an improved connection quality. Nonetheless, the central area of the welding lines remained opaque, possibly due to uneven pressure exerted by the fixture and inconsistent welding quality. In contrast, water intermediate layer-assisted femtosecond laser welding yielded continuous and transparent welding lines, demonstrating superior welding quality among the three methods. Specifically, the shear strength of the samples prepared by the three methods was measured, as shown in Figure 8. The samples welded by natural stacking exhibited very low shear strength with a maximum value of 0.7 MPa. However, the shear strength of the samples was enhanced to a maximum of 13.1 MPa by applying pressure on the glass plates, and this was further increased to 18.3 MPa through water intermediate layer-assisted fs laser welding. This suggests that the shear strength could be further improved by optimizing the laser parameters.

Figure 7 Macromorphology and microscopic morphology of the weld glass weld zone. (a, d) Natural stack welding. (b, e) Homemade fixture pretightening welding. (c, f) Water intermediate layer-assisted welding.

Figure 8 Comparison of the shear strength of samples prepared by different welding methods.

The water intermediate layer-assisted femtosecond laser welding provided a value of 18.3 MPa, which is approximately 1.4 and 20 times greater than those achieved by applying pressure and natural stacking, respectively. The improvement in the shear strength is attributed to the decrease in the gap size, which was investigated by laser confocal scanning microscopy, as shown in Figure S3. The gap was 6.5 μm by applying pressure using the fixture, while the gap decreased to 2.6 μm by applying a water intermediate. According to Young’s equation,38 water droplets maintain equilibrium under the surface tension of the three phases of solid, liquid, and gas1

where γSV, γSL, and γLV represent the surface tension of the solid–gas interface, solid–liquid interface, and liquid–gas interface, respectively, while θ denotes the contact angle, as illustrated in Figure 9(a). In Method 2, we treated the glass surface with a plasma washer and obtained a hydrophilic surface.39 As shown in Figure 9(c), since the surface of the glass is hydrophilic, when two pieces of glass were stacked, a concave liquid surface at the edge side was generated.40

Figure 9 Schematic diagram of water intermediate layer-assisted laser welding. (a) Water droplet in equilibrium under the surface tension of the three phases of solid, liquid, and gas. (b, c) Water layer forming between stacked glass samples. (d) Stress on the concave liquid surface of the water intermediate layer. (e) Stress analysis of the glass–water–glass system.

Because the direction of surface tension follows the tangent direction of the liquid surface, when the liquid surface is concave, the attraction force F between water molecules at the interface will generate a combined force F1 toward the air side, resulting in an additional pressure Ps, as shown in Figure 9(d). In Figure 9(e), we analyze the forces acting on the glass system. Due to the presence of additional pressure Ps, the pressure P1 in the liquid layer should be2

where P1 is the actual pressure in the liquid layer and P0 is the atmospheric pressure. It is understandable that the pressure in the liquid layer is less than the external atmospheric pressure:3

Therefore, the whole glass–liquid–glass structure will be tightly pressed together under the action of external forces. The difference between P0 and P1, Ps, can be estimated by the Young–Laplace (Y–L) formula:394

5

where γ represents the surface tension coefficient, ΔP is the pressure difference of the system, and R1 and R2 are the principal curvatures of the water layer. Since R1 and R2 are orders of magnitude different, we can simplify the formulation as follows:6

7

where d represents the gap of the sample and θ represents the contact angle between the glass and water. A positive Ps indicates that the two glass pieces experience inward pressure from the external atmospheric pressure, leading to a decrease in the sample gap size from 6.5 to 2.6 μm.

4 Conclusions

In summary, nonoptical contact glass samples were successfully connected by applying mechanical pressure and utilizing a water intermediate layer. The effects of the focus position and pulse energy on the femtosecond (fs) laser-modified zone of the glass were investigated and analyzed. Through a comparison of shear strength using two different methods, it was found that a maximum shear strength of 18.3 MPa was achieved using water intermediate layer-assisted femtosecond laser welding, which is twice as high as that achieved through the use of fixtures to apply mechanical pressure. Finally, the principle of water intermediate layer-assisted fs laser welding was discussed. The findings indicate that water intermediate layer-assisted fs laser welding offers a convenient and reliable method for connecting nonoptical contact glass samples.

Data Availability Statement

The data underlying this study are available in the published article and its online Supporting Information.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c05018.Custom-made fixture used to reduce the gap between samples (Figure S1); shear strength of welding samples with different volumes of water intermediate layers (Figure S2); gaps between samples before welding using different methods (Figure S3); width of the modified zone at the interface under different defocusing amounts (Table S1); influence of pulse energy on the area change of the glass internal modification (Table S2); and height, width, and aspect ratio of the modified zone inside the glass at different pulse energies obtained by water intermediate layer-assisted fs laser welding (Table S3) (PDF)

Supplementary Material

ao4c05018_si_001.pdf

Author Contributions

H.C.: methodology; visualization; writing—original draft. S.B.: methodology; writing—review and editing; data curation; project administration; resources; funding acquisition. Z.L.: investigation; visualization. M.H.: investigation; validation. X.Y.: supervision; resources.

The authors declare no competing financial interest.

Acknowledgments

The authors thank the support from the National Natural Science Foundation of China (nos. 52375422 and 52305354) and the Hebei Natural Science Foundation of China (nos. E2023208010 and BJK2024021).
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