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Crafting interior holes on chemically strengthened thin glass based on ultrafast laser ablation and thermo-shock crack propagations
Journal Pre-proof Crafting Interior Holes on Chemically Strengthened Thin Glass Based on Ultrafast Laser Ablation and Thermo-Shock Crack Propagations C-F Chuang, K-S Chen, T-C Chiu, T-S Yang, M-C Lin
PII:
S0924-4247(19)31321-4
DOI:
https://doi.org/10.1016/j.sna.2019.111723
Reference:
SNA 111723
To appear in:
Sensors and Actuators: A. Physical
Received Date:
25 July 2019
Revised Date:
28 October 2019
Accepted Date:
6 November 2019
Please cite this article as: Chuang C-F, Chen K-S, Chiu T-C, Yang T-S, Lin M-C, Crafting Interior Holes on Chemically Strengthened Thin Glass Based on Ultrafast Laser Ablation and Thermo-Shock Crack Propagations, Sensors and Actuators: A. Physical (2019), doi: https://doi.org/10.1016/j.sna.2019.111723
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Crafting Interior Holes on Chemically Strengthened Thin Glass Based on Ultrafast Laser Ablation and Thermo-Shock Crack Propagations C-F Chuang1, K-S Chen1*, T-C Chiu1, T-S Yang1, and M-C Lin2 1. Department of Mechanical Engineering, National Cheng-Kung University Tainan, Taiwan, R.O.C. 2. Laser and Additive Manufacturing Application Center, Industrial Technology Research
*Corresponding Author: Email: [email protected]
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Graphical Abstract
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Institute, Tainan, Taiwan, ROC
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Figure 1. Schematics of the proposed glass crack propagation control and separation method
Figure 2. The SEM micrograph of the cleavage surface.
(a)
(b)
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Figure 3. (a) Successful interior hole cutting, (b) the surface quality of the cut
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surface, and (c) cutting surface quality by another separation method (a lot of chipping). Highlights
We propose a glass cutting scheme by integrating ultrafast laser drilling and subsequently sub-critical crack growth via thermo-shock
Essential fracture mechanics model is outlined to guide the development of processing
Successful interior holes are cut in chemically strengthened glass with fine surface roughness for demonstrating the feasibility
Associate thermo-mechanical simulations are conducted in conjunction with experimental measurements to explain the crack propagation phenomenon based on mechanics.
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Abstract To prevent glass cracking from external mechanical damages, chemically strengthened glass is usually employed. The cutting of strengthened glass becomes critical due to their residual tensile stress induced in the inner core of
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glass attributed to the ion exchange process. This paper describes a novel technique for dealing cutting interior holes and separation of strengthened ultrathin glass for the display unit of mobile communication and computational devices. This method integrates picosecond laser ablation with quenching induced thermo-shock for accomplishing such a task. Essential analyses are performed based on fracture mechanics and finite element method to provide the scientific basis of such an approach. Experimental results indicate that with a proper temperature control, the 2
proposed method could achieve successful separations for various enclosed shapes with the associated surface roughness satisfying the requirement. Keywords: Glass, Packaging, picosecond laser ablation, thermo-shock
I.
Introduction With the advance in mobile computing, portable mobile devices have already become necessary parts in daily life. Ultrathin glass plays both functional and structural supporting roles for the display unit of these devices. Recently, for improving structural reliability consideration, chemically
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strengthened ultrathin glasses have been widely used due to their inherent high strength for resisting possible scratching and damages [1]. Sodium ions are replaced by larger potassium ions at the glass surface regimes and thus compressive residual stresses are induced. Consequently, the hardness and resistivity to scratch are enhanced. However, such an advantage also brings challenges in fabrication and assembly of glass pads. For achieving good surface scratch resistivity in glass, a layer with high residual compressive stress is required but this also induces tensile residual stress beneath the glass surface [2]. That is, the surface compressive residual stress would significantly enhance the damage resistance but the inner tensile residual stress would
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make the structure prone to flaws. Since glass is basically a highly brittle material, any tiny flaws due to cutting could trigger uncontrollable crack propagations [3, 4].
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For these strengthened glass, the existence of the tensile residual stress actually deteriorates the situation since the tensile residual stress can easily trigger the crack propagation in an even less flaw condition. For cutting glass to form desired external shapes, both wheel cutting [5] and laser cutting [6, 7, 8, 9] have been investigated and applied extensively. However, although the cutting problem could be solved, the existence of inner tensile stress has indeed further added challenges in the surface quality of glass cutting. On the other hand, for shaping small arbitrary enclosed holes, the situation is completely different. The wheel cutting is basically out of consideration due to its fundamental limitation and laser processing seems to be the only feasible strategy for cutting strengthened glass in straight-line manner [10]. However, the requirements for creating these inner functional holes are completely different. The cracks must propagate under desirable paths and cannot penetrate into the surrounding structures for maintaining the structural 3
integrity of the final panel. Both the requirements and the much smaller allowed dimensions basically rule out the possibility of using mechanical cleavage approach and the well-behaved CO2 laser cleavage [8, 11] is also fail for controlling the propagation orientation. In addition, the quality of the cleaved surface is also critical. If the defect is too large, the fracture strength would be significantly reduced. Previously, we have developed techniques to remove the damage layer by means of CO2 laser peeling [12, 13, 14]. However, this technique may not be useable in this scenario and a new technique is desperately needed for solving the above-mentioned problem. Recently, with the advances in fast laser processing technologies, such as
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Bessel beam femtosecond lasers, laser ablation becomes a promising approach for handling strengthened glass [15, 16, 17, 18], and sapphires [19]. Special technique such as double pulse laser ablation was also proposed to enhance the cutting efficiency [20] and using femtosecond Bessel beam for creating underneath stealth dice layers and then with external applied mechanical loading [21]. Furthermore, [22] also demonstrated thin glass cutting using femtosecond laser with overlapped laser beams.
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Although the above techniques have demonstrated the ability and potential in glass cutting, several concern remains. First, there is a trade-off between overlap ratio and the cutting performance. For example, for a large overlap
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ratio, the thermal damage and the laser pulse used are the major concerns. For a smaller overlap ratio, although fewer laser pulses used and less thermal damage concern, the uncontrolled cutting flaw propagation may cause failure in glass separation [22].
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In this work, by integrating the knowledge of fast laser processing and fracture mechanics of glass materials, a technique is proposed to use non-overlapped laser beam drilling and controllable crack propagation to address the above trade-off. First, one can create deep micro trench arrays over glass using fast laser. If one can trigger stable crack propagation from
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the surface of those trenches on such perforate structures, these grown cracks may be arrested by the next corresponding trenches before being out of control [23]. That is, by this approach, it is then possible to create controllable sub-critical crack growth from those trenches for separating the glass to form hollow structures. However, such a process involves complicate pyrolytic interaction between laser and glass materials. In addition, extra auxiliary stressing processes should also be developed for achieving a reliable glass separation [24, 25]. As schematically shown in Figure 1, by utilizing 4
pico-second laser drilling and subsequent thermo-shock crack propagation.
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It is possible to successfully create inner holes with different sizes and shapes on chemically strengthened glass with different thickness and hardness. Subsequent efforts then provide mechanical forces to create stable crack propagation to fully cleaving the glass. This approach thus opens an alternative direction for future glass cutting technology. In this work, the entire process and key achievement are presented. For more technical detail, please refer to the master thesis of the first author [24]. Based on the earlier conference presentation [26], this work would concentrate on the development of the cutting and separation technique based on mechanics.
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II.
Approach
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The approach is to create perforated structures fabricated by pico-second laser drilling first and then crack propagation is induced to form an enclosed path by any means. With careful design on the size and density of these trenches and a reliable separation method, it is believed that the crack growth can be propagated in a controllable manner. That is, the crack can be initiated at the edge of a laser drilled trench and they propagated but would be eventually arrested by the next hole. By such an approach, enclosed shapes can be cut and scribed and this technique can be applied to more complicate inner shape cutting. There are two key processes should be paid attention with. I.e., the laser drilling and the glass separation.
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In laser drilling aspect, two major concerns are identified: First, on the quality of drilling: this includes the concern of the surface damage and micro cracks induced by laser machining as well as the material properties variation due to photolytic or pyrolytic effects since they could affect the crack growth and propagation. Second, on the density of drilled trench and the maximum allowed crack propagation distance before out of control. For this issue, it is desired to have a mechanics-based qualitative analysis to guide the process optimization and it would be briefly presented in Section III. Meanwhile, for dealing with the former concern, a preliminary study to address the damage
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of pico-second laser drilling on the surrounding glass materials has been presented elsewhere [24, 25] and this issue would not be further addressed in this work.
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On the other hand, for the glass separation issue, two approaches could potentially be used although they actually represents two extremes in fracture mechanics. The first approach utilizes fatigue crack growth for connecting these trenches. This approach requires a small external fluctuating loading, possibly achieved by employing a piezoelectric actuator, for generating cyclic
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stress. Meanwhile, the other approach is to use thermo-shock for generating fast fracture schematically shown in Figure 1b. To achieve this goal, the perforated structure is heated up to a certain temperature and is then quenched immediately. The resulted thermal shock induces sufficient tensile stress to cause crack propagation from the surface of those trenches [24, 27]. Both of the approaches and have been investigated in this work and the results indicate that both of them can achieve successful separation. However, based on our performance evaluation shown later, the fatigue crack 6
growth approach, as will be addressed in Section V, is finally abandon due to its unsatisfied resulted surface quality. That is, severe chippings are found possibly due to fretting fatigue and this approach is creased and the thermo-shock approach is finally recommended based on the test results shown in this work. However, a successful separation based on thermal shock approach also involves complicate thermo-mechanical coupling and related mechanics should also be investigated. In this article a simple while effective finite element simulation is also performed and addressed in Section VII in conjunction with the process development and for guiding the future
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optimization.
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III.
Mechanics of Separation
From mechanics of materials perspective, there are two approaches to describe the glass cutting and separation problems. The first approach is to address the problem based on a deterministic ultimate tensile strength criterion and the failure (i.e., cutting and separation) occurs once the applied stress exceeds the ultimate strength. In our previous work, the laser induced damage is treated as the equivalent strength reduction and the associated stress-based designs were discussed [28]. On the other hand, the second approach deals the problem using fracture mechanics. That is, if the stress intensity factor or the strain energy release rate of glass exceeds the toughness or the critical strain energy release rate of the processed material, crack I.e.,
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propagation occurs.
K1 K1c or
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In comparison with the first approach, this approach takes the surface quality (i.e., initial flaws) into consideration. By this approach, the separation process can be simplified as a mode-1 fracture mechanics problem.
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It has no exact solutions for addressing the issue on the density of drilling trenches. However, phenomenologically, it can be approximated as a mode-1 fracture mechanics problem with periodically patterned initial flaw schematically shown in Figure 2. That is, after forming the periodic perforated structure, microscopically, the structure can be treated as a structure with
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periodically flaw system. According to [29], the stress intensity factor K1 can be found as 2𝑤
𝜋𝑐
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𝐾1 = 𝑌𝜎∞ √𝜋𝑐 ∗ ( 𝜋𝑐 𝑡𝑎𝑛 (2𝑤)) .
(2)
Where Y is a shaper factor typically greater than unity containing all finite size effect and local defects and will be determined by experimental data later shown in section VII. If the K1 greater than the associated glass toughness K1C, then the flaw would growth. From Eq.(2) obviously, the magnitude of K1 is related to the initial flaw size (i.e., 2c) and separation (i.e., 2w-2c). This is 8
structurally analogous to trench size and separation.
With a larger c/w (i.e., a
higher density on drilled trenches), K1 would effectively increase. This tendency agrees with that observed physically. A successful separation depends on the resulted stress intensity factor. It is desired to have a stress intensity factor slightly larger than the fracture toughness of glass. If the resulted K1 less than K1C, no separation occur. On the other hand, if K1 is much larger than K1C, dynamic fracture is expected and the glass sample would be broken to piece. Based on Eq. (2), two control factors are clearly identified. First, the density of holes should be sufficient
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high to increase the c/w ratio for increase the stress intensity factor for fracture. Second, with the density of drilled hole determined, the magnitude of the remote stress should be adjusted adequately for providing a suitable K1. Technically, the remote stress level can be adjusted by either changing the fatigue mechanical loading or by adjusting the thermal quenching rate.
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Taking some typical numbers as an illustration example here. With a shape
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factor Y equaling 2 and for a hole with radius c of 3 m and a separation w of 3.5 m. With a remote stress of 100MPa, the stress intensity factor (by Eq.(2)) is calculated as 1.1MPam, which is close to the fracture toughness of typical
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glass (KIC ~ 1 MPam). Meanwhile, by Eq.(2), it can also be seen that the result is extremely sensitive to the c/w ratio. For example, if c = 3m and w = 4m, the estimated stress intensity factor will reduce to 0.8 MPam, which is less than the fracture toughness of glass.
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To make (2) effective, the shape factor Y must be determined. In Section VII, by integrating experimental and numerical simulation, Y can be determined. Once Y is obtained, Eq.(2) can then be used to provide fast evaluation for optimizing the laser ablation design in the future.
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IV.
Pico-Second Laser Drilling
Chemically strengthened Corning 2320 glass with thickness of 550m, serves as the test specimen in this study. By using a glass surface stress meter (FSM-6000E), the surface compression stress and the thickness of the compressive layer are characterized as 7175 MPa, and 380.1m, respectively. By force equivalent estimation, the average tensile residual stress inside is approximately 84 0.5 MPa. A TruMicro picosecond laser (wavelength 1030 nm, pulse duration around 10 ps) with an average Bessel beam laser power 50W and an instantaneous maximum power of 250W is used in this work. Specimens are placed on an
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X-Y table for positioning. Essential laser processing parameters are listed in Table I. The schematic setup of the laser and the optical path is shown in Figure 3a. On the other hand, as shown in Figure 3b, since the power of the Bessel laser beam is distance-dependent and is not uniform along the optical axis, the distance between the glass sample and the laser head is adjusted to be at the pick power distance and the power uniformity through the entire glass thickness is estimated as 1.5%, which is an acceptable level.
Micro trenches are then drilled by the laser. Figure 4 shows the top and cleavage cross-section of a typical cutting process. Trenches with a 2m
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diameter and a surrounding heat affected zone (HAZ) with a thickness of approximately 0.5-1 m are clearly identified. The view from the cleavage plane also indicates that the cutting can effectively through the entire thickness with surface roughness approximately 1m. Notice that there are some surface micro flaws generated due to laser drilling, however, their characteristic length is small and the surface laser is actually highly compressive (for strengthened glass), these tiny flaws would not propagate during drilling. However, such flaws are very important for triggering stable crack growth for the subsequent separation process. The basic concept for forming a scribing line with a micro trench array and then separates the perforated structure by applying a tensile stress to cleavage has been schematically shown in Figure 1. Demonstrations for straight and curved paths separations are then shown in Figure 5 and both of them are separated by hand. It can be seen that the separations are successful along the scribing line and the induced crack propagations are successfully arrested by the next hole but with a few chippings, which is believed to be caused by 10
the poor control ability of using hand separation. With the feasibility being validated, it is possible for investigating glass separation method. Notice that the drilling quality also strongly depends on the laser parameters and the motion control and vibration of the X-Y table. During the early stage of development, undesirable results are frequently reported. For example, as shown in Figure 6, a zigzag cutting path is resulted. On the other hand, in Figure 6b, undesired tiny cracks not parallel to the cutting path are also generated around these trenches. These issues are eventually solved by improving the positioning and anti-vibration performance of the associated X-Y table. Meanwhile, it is also found that although the laser can effectively
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cut through the entire thickness, the resulted surface topologies such as chipping and HAZ could have differences on the both surfaces. This phenomenon always exists even after performing parameter optimization. This imposes an imperfect factor to the subsequent glass separation process addressed in the next two sections.
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V.
Fatigue Glass Separation
One natural thought for performing glass separation based on the laser drilled scribing line is to utilizing fatigue crack growth. As shown in Figure 7a, based on fatigue mechanics such as Paris’s Law [30] by applying proper fatigue loading, it is possible to generate a stable crack propagation to reach the adjacent trench. Notice that if the initial crack orientation is not favored, this approach may immediate generate failed result. A testing system is constructed to generate tiny high frequency fatigue loading using a piezoelectric actuator is shown in Figures 7b and 7c. This piezo actuator can generate loading amplitude around 22m in 100 Hz. A
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load cell is also used to measure the forcing level. With this design, it is hope that the cracks can progressively propagate on the desired scribing line to form an enclosed path.
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The experimental results are shown in Figure 8. Some specimens actually fail to form enclosed separation (Figure 8a). By examining these specimen, the failure can be attributed to the quality of laser drilling. That is, tiny undesired flaws can observed around those trenches as already addressed in Figure 7a. Second, for these specimens with successful separation (Figure 8b), severe chippings are always observed. As shown in Figure 8c, chipping
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size is up to 300 – 400 m and this is not acceptable for subsequent glass assembly. It is believed that those chippings are generated by the friction, or simply the fretting fatigue during the loading process. Technically, this effect can be reduced by further increase the accuracy in positioning the actuator and with a better loading control during fatigue process. However, practically, this is very difficult to achieve. As a result, although the fatigue approach can potentially achieve successful separation of enclosed paths, the inferior surface quality eventually forces us to abandon this approach. Instead of using progressively crack growth, fast fracture based on
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thermo-shock is then considered and addressed in the next Section.
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VI.
Thermo-Shock Separation
As addressed earlier, the fatigue crack growth approach fails to achieve required performance and an alternative approach must be found. Here, fast facture based thermo-shock is investigated for fulfilling the task. Thermo-shock is a very common phenomenon in ceramic structure failure [27]. That is, when a hot ceramic structure subjects to a localized low temperature quenching, the severe local temperature gradient induces significant tensile stress to cause structure fracture. Based on the argument, it is expected that the glass separation can be performed by this method, at least in theory. The questions remained are the required temperature and
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quenching conditions for providing sufficient driving tensile stress. Meanwhile, to ensure crack propagation along the scribing line, the thermo-shock induced tensile stress should be perpendicular to the scribing line. This approach would generate a standard mode-1 crack. For enclosed path such as circular, an axisymmetric temperature distribution, which can be achieved by proper design of quencher, would satisfy this requirement.
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A self-designed experimental apparatus is realized for performing the test and for evaluating the achieved glass separation performance such as surface chippings. The system, shown in Figures 9a and 9b, consists of a hot plate
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and two nozzles for proving cold jets. The hot plate, capable for heating up to 500C, is responsible for overall heating the glass specimen and these two jets can provide CHF3 jets at -80C for quenching. A test bed consisted of a load cell (ESENSE, LRM-5) is adapted offline for correlating the quenching pressure force and the due jet impinging conditions. The design can provide single side or double side quenching. Two type T thermocouples are also used to monitor the temperature of the glass surface. Figure 9c is a typical temperature history of the glass sample. It is heated up to a steady temperature (200C and 250C for both top and bottom surfaces) and the
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upper jet is open for performing upper surface quenching. The temperature of the top surface drops to -30C within 1s. This should provide sufficient tensile stress. On the other hand, due to heat conduction, the bottom surface has a less temperature drop rate (i.e., dropping 80C in 2 s). For circular scribing path (diameter = 1cm), a typical optical micrograph of the final separation is shown in Figure 10. Basically, no visual chippings are found
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along the surface and the feasibility is thus confirmed and a more detail
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parametric study is then conducted for performance assessment.
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VII.
Thermo-Shock Experimental Results
After validating the feasibility, a systematic investigation on thermo-shock glass separation is then conducted. Figure 11 shows typical edge defects observed on the scribing lines. For providing meaningful statistics analysis on those either isolated or continuous chippings, here two performance index, maximum chipping size (cmax) and chipping ratio (cr), are defined for overall assessment in the subsequent parameter studies. The maximum chipping size are the observed largest chipping in a successfully separated specimen. On the other hand, the chipping ratio is defined as the surface length containing chipping flaw normalized by the entire perimeter of the enclosed
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path. From engineering perspective, cmax should be less than 100m for avoiding stress concentration and cr should be as less as possible. Both of them would be used to evaluate the fabrication performance for process optimization.
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First, the effect of the top heating temperature is investigated. Table II shows the investigation results. Here only the top surface is quenched with a resultant loading of 0.31N (i.e., averaged air pressure = 3.95kPa). It is found that if the top surface temperature of glass samples over 200C, they are simply smashed after quenching. On the other hand, for the top surface
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temperature below 125C, no visible changes occurs. These observations are reasonable since the former represents the situation of over stressing and the later, on the other hand, does not provide sufficient driving force for triggering crack propagation. Based on this investigation, the required glass surface temperature for achieving a stable glass separation is confirmed to between 200 and 125C. Meanwhile, from Table II, it can also be found that cmax on the bottom surface is always greater than that on the top surface in all cases. Nevertheless, the
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maximum chipping size is less than 100m, which is much less than that observed in fatigue separation method and is acceptable for practical usage. On the other hand, the chipping ratio cr also exhibits the same trend. However, it is interest to point out that for a top surface temperature of 200C, the chipping ratio of both top and bottom surfaces are very small, less than 0.5%. As a result, for single side quenching, this should be the optimal processing condition.
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Based on this observation, the process is further optimized by adjusting the air pressure of quenching. Specifically, at the top surface temperature of 200C, instead of using a larger quenching pressure once, by quenching the specimen three consecutive times with a smaller quenching pressure (total force 0.11N instead of the original 0.31N or an averaged quenching pressure 1.4kPa), the quality of the separation surface can be further improved. Based on this improved recipe, enclosed paths beyond the original circular shape such as rectangular fillet shape can also be successfully separated as shown in Figure 12a and the images shown in Figure 12b indicate that there
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are no chipping observed on the top surface and the cmax of the bottom surface is also reduced to 10.8m from the original value of 80 m. Meanwhile, the
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chipping ratio cr of the bottom surface is also reduced to 0.01%. It is believed that a smaller quenching pressure also results in a gentler forcing manner during the moment near the final separation and this affect the possible resulted chipping. Therefore, it is suggested here that this recipe (quenching temperature 200C and a quenching pressure of 1.4kPa with 3 quenches) should be used for future study.
In parallel, double side quenching is also investigated and there are no further visual improvements for the current specimen. However, it is anticipated
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that as the thickness of specimen further increases, the importance of double side quenching would become more dominant. This would leave as our immediate future work.
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It is also important to point out here that in order to convert this feasibility demonstration and analytical mechanics modeling for dealing with more complicated situation in real processing, numerical simulation in temperature and thermal stress analyses would be required for precisely determining the required process parameters. With different specimen and scribing line
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geometries, thermal boundary conditions, and quenching design, it would be difficult to design a successful glass separation process without reliable numerical simulations. For addressing this issue, finite element coupled temperature-displacement analyses performed by using general purposed finite element solver Simulia ABAQUS [31] with coupled temperature-displacement scheme is currently investigated. Here the preliminary results are briefly shown. Two models, i.e., axisymmetric and 2D plane stress models are used for temperature and stress evaluation. To 16
compare with experimental results, the glass specimen is heated to 473K and then is quenched. To keep the model simple while effective, the perforated structure on the cutting path is not physically generated but the effect on thermal resistance is modelled simply with reduction of thermal conduction coefficient at this zone. Figures 13a and 13b show typical temperature and von Mises stress contours after 0.5 seconds of quenching. The detail temperature history of the process with different hole densities is plotted in Figure 13c. It can be seen that for the case with a 50% hole-density (the situation close to the reality), it takes approximately 3.5 seconds to reduce the center temperature of the upper surface to 350K (~80C). The results is
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actually similar to the experimental observation shown in Figure 9c.
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Meanwhile, the maximum tensile stress generated at the cutting path is also found as 105 MPa. By using Eq.(2), the induced stress intensity factor is 0.581Y MPam. Since the experimental results indicate that a stable crack propagation occurs, it is reasonable to assume the stress intensity factor is just slightly over the fracture toughness. Under such an assumption, the shaper factor Y is then estimated as 1.7. With the shape factor determined, in the future, it is possible to perform parametric study using this finite element model to determine appropriate combinations of process parameters and line
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density of hole for optimization. In addition, it is also possible to incorporate these coupled temperature-displacement analyses with fracture mechanics to study the dynamic fracture propagation problems.
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VIII. Discussions
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According to the above results, we believe that the main contribution of this work is on the proposing and validating the feasibility of this new glass separation technique. To the best of our knowledge, although there are many laser glass cutting researches, most of them are either focused on laser-material interactions or on the feasibility demonstrations only. No similar works are reported on technique development specifically guided with failure mechanism-based design. In addition, it is believed that the proposed method is a more versatile solution in comparison with others in the focused applications.
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However, the success of the proposed scheme relies on both the quality of picosecond laser drilling and subsequent crack propagation. Experimental results indicate that a poor X-Y stage positioning control could lead to poor quality on drilling holes and result in tiny undesirable micro cracks. Once these cracks form, the main crack propagation would deviate from the scribing line and the task would fail. On the other hand, with acceptable laser drilled sample, glass separation is still a challenging task. The fatigue crack growth approach is proven to be unacceptable due to its severe chipping generated in fretting process. Instead, the thermo-shock approach
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could potentially achieve success separation with good surface quality. However, there is a narrow region on its parameter setup map to produce acceptable results. If the substrate temperature is too high, it would be smashed due to overstress. On the other hand, substrate without sufficient temperature would not result in crack propagation due to insufficient tensile stress. Therefore, it is important to find the feasible parameter window for process optimization. It is also important to point out the limit of the current approach here for
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future studies. The success of this approach relies on high temperature gradient during quenching and can be described by the dimensionless parameter called Biot number (Bi) [32]. For in-plane direction, the temperature gradient should be maximized for yielding a high tensile stress to cause stable cracking and a larger Bi is desired. On the other hand, for maintaining the scribed surface quality, the temperature of the out-of-plane (i.e., thickness) direction should be uniform and a smaller Bi is desired. Based on length scale analysis [32], a larger enclosed hole on a thinner glass 18
substrate is preferred.
That is, once the size of the enclosed hole is further
approach with less restriction on cutting geometry.
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reduced, or the thickness of glass substrate increases, this approach would meet certain difficulties due to the unfavorable length scale issue and further engineering improvement is required. Nevertheless, the current results indicate that enclosed holes with diameters down to a few millimeters could be made by this approach. If the size of inner hole is reduced to only 1-2 mm or less, it could potentially fabricate simply by high speed drilling, not by cutting. Meanwhile, based on the cutting results on rectangular fillet specimen, it is also expected that this technique is also feasible for creating enclosed holes with arbitrary shapes if the local curvature is not too small. As a result, it is believed that the proposed scheme should be a more versatile
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Finally, it is also important and interesting to investigate the effect of pico-second laser irradiation on the mechanical properties of strengthened glass. Previously, the interaction between CO2 laser and the thermo-mechanical properties evolution of strengthened glass has been investigated by us using both a high temperature oven and micro- and nano-indentation characterizations [25, 33, 34, 35]. It was found that the residual stress, toughness, and modulus of strengthened glass varies once the laser irradiation induced temperature raise above a threshold. Meanwhile,
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the local damage, residual stress field, and the equivalent fracture strength variation due to the pico-second laser irradiation have also been studied by us. These information are essential for a root-cause optimization of related application. In the future, it is possible to integrate this obtained information into account for process design or other applications.
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IX.
Conclusion
Chemically strengthened glass has been widely used due to their capability in resisting possible damages during service. However, such an advantage also brings struggles in fabrication and assembly due to extra induced tensile stresses in inner cores and this causes difficulty in processing. By integrating picosecond laser ablation with quenching induced thermo-shock for providing sufficient driving force to allow crack propagation in a controllable manner, a feasible glass separation technique aiming for creating enclosed interior holes on glass has been developed and demonstrated. Experimental results indicate that the proposed method could achieve successful separations for various enclosed shapes with the surface roughness and
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chipping level satisfying the requirement. Essential experimental and numerical parametric studies are performed for characterizing and optimizing the process window. It is found that the temperature control is crucial for guaranteeing stable crack propagation for glass separation. Nevertheless, based on current experimental results and numerical modeling, the developed method is possible to provide an effective solution or guidance for related glass cutting tasks. In the future, with more sophisticated parameter adjustment, it is expected that the proposed method could be adapted by related industries.
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Acknowledgements This work is supported by Industrial Technology Research Institute (ITRI) and Ministry of Science and Technology (MOST) of Taiwan under contract numbers 105-2221-E-006 -100 -MY3, 105-2221-E-006-074-MY3, and 108-2622-8-006-014.
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with high surface compressive stress,” In Proc. of SPIE, 7920, pp. 79200U-1, 2011. [12] T. S. Yang, G. D. Chen, K. S. Chen, R. C. Hong, T. C. Chiu, C. D. Wen, C. H. Li, C. J. H., K. T. Chen, M. C. Lin, "Thermal analysis of a laser peeling technique for removing micro edge cracks of ultrathin glass substrates for
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web processing," Proc. ASME 2015 International Mechanical Engineering Congress & Exposition, Houston, Texas, USA, 2015. [13] T. C. Chiu, C. A. Hua, C. H. Wang, K. S. Chen, T. S. Yang, C. D. Wen, C. H. Li, M. C. Lin, C. J. Huang, and K. T. Chen, "On the mechanics of laser peeling for ultra-thin glasses." Engineering Fracture Mechanics, vol. 163, pp. 236-47, 2016. [14] K-S Chen, T-S Yang, R-C Hong, T-C Chiu, and M-C Lin, “Thermo-mechanical analysis of laser peeling of ultrathin glass for 21
removing edge flaws in web processing applications,” Microsystem Technologies, vol. 24, pp. 397-409, 2018. [15] M. Kumkar, L. Bauer, S. Russ, M. Wendel, J. Kleiner, D. Grossmann, K. Bergner and S. Nolte, "Comparison of different processes for separation of glass and crystals using ultra short pulsed lasers." Proc. SPIE, vol. 8972, 897214, 2014. [16] M. Bhuyan, F. Courvoisier, P. Lacourt, M. Jacquot, R. Salut, L. Furfaro, and J. Dudley, "High aspect ratio nanochannel machining using single shot femtosecond bessel beams." Applied Physics Letters, vol. 97, 081102, 2010. [17] K. Wlodarczyk, A. Brunton, P. Rumsby, and D. Hand, “Picosecond laser
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Opto-Electronic Advances, vol. 1, 180014, 2018. [21] K. Liao, W. Wang, X. Mei, J. Cui, M. Li, and X Li, “An analytical model to predict the sizes of modified layer in glass with femtosecond Bessel beam”, Optik, vol. 185, pp. 232-241, 2019. [22] H. Shin and D. Kim, “Cutting thin glass by femtosecond laser ablation,”
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Optics and Laser Technology, vol. 102, pp1-11, 2018. [23] C. Makabe, A. Murdani, K. Kuniyoshi, Y. Irei, and A. Saimoto, "Crack-growth arrest by redirecting crack growth by drilling stop holes
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and inserting pins into them." Engineering Failure Analysis, vol. 16, pp. 475-83, 2009.
[24] C. F. Chuang, Effect of Laser and Thermal Processing on the Material Properties and Manufacturability of Ultra-Thin Glass, M.S. Thesis, Department of Mechanical Engineering, National Cheng-Kung University, Taiwan, 2017. [25] K. S. Chen, C. F. Chuang, T. C. Chiu, T. S. Yang, and M. C. Lin, “On the Separation of Chemically Strengthened Ultrathin Glass Based on Fast 22
Laser Induced Subcritical Crack Growth,” Proc. 12th International Symposium on Advanced Science and Technology in Experimental Mechanics (ISEM’17), Kanazawa, Japan, Nov. 2017. [26] C. F. Chuang and K. S. Chen, “A New Technique for Creating Curved Interior Holes on Ultrathin Glass Based on Picosecond Laser Drilling and Thermo-Shock Separation,” In Proc. 20th Symposium on Design, Test, Integration, and Packaging of MEMS and MOEMS (DTIP 2018), Rome, Italy, May 2018. [27] D. Hasselman, “Unified theory of thermal shock fracture initiation and crack propagation in brittle ceramics,” J. Am. Ceram. Soc., vol. 52, pp. 600-604, 1969.
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[28] T. Y. Lee, C. F. Chuang, S. Q. Lin, T. C. Chiu, K. S. Chen, T. S. Yang, and M. C. Lin, “Analysis of Stress in Ultrathin Glass Modified with Picosecond
Laser,” Proc. 12th International Symposium on Advanced Science and
Technology in Experimental Mechanics (ISEM’17), Kanazawa, Japan, Nov. 2017.
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[29] T. L. Anderson, Fracture Mechanics, Fundamentals and Applications, 4th Ed., CRC Press, 2017. [30] P. Paris and F. Erdogan, “A Critical Analysis of Crack Propagation Laws,” J. Basic Eng., vol. 85, pp. 528-533, 1963. [31] ABAQUS v. 6.10, Simulia, 2010
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[32] K. S. Chen, Materials Characterization and Structural Design of Ceramic Micro Turbomachinery, Ph.D. Thesis, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA, Feb. 1999. [33] K. S. Ou, H. Y. Yan, and K. S. Chen “Mechanical Characterization of
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KMPR by Nanoindentation for MEMS Applications," Strain, Vol.44, pp. 267-271, 2008. [34] I. K. Lin, P. H. Wu, K. S. Ou, K. S. Chen, and X. Zhang, "Mechanical property characterization of sputtered and plasma enhanced chemical
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deposition (PECVD) silicon nitride films after rapid thermal annealing"
Sensors and Actuators A: Physical, Vol. 117, pp. 117-126, 2011. [35] T. Y. Zhang, L. Q. Chen, and R. Fu, "Measurements of residual stresses in thin films deposited on silicon wafers by indentation fracture." Acta Materialia, vol. 47, pp. 3869-3878, 1999.
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Chiao-Fen Chuang earned her B.S. and M.S. degrees from Department of mechanical Engineering, National Cheng-Kung University (NCKU), Tainan, Taiwan, in 2015 and 2017, respectively. She is currently working for Syntec as a machine tool control engineer.
Ms Chuang’s research interests are in
mechatronics, fabrication, and control technologies.
Kuo-Shen Chen received the B.S. degree in power mechanical engineering from National Tsing-Hua University, Taiwan, in 1989, and the M.S. and Ph.D. degrees in mechanical engineering from Massachusetts Institute of Technology (MIT), Cambridge, in 1995 and 1999, respectively. Since August 1999, he has been with the Department of Mechanical Engineering, National Cheng-Kung University,
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Taiwan, where he is currently a Professor. His research interests include
mechanical analysis and material characterization in MEMS, system dynamics and control of mechatronics and MEMS, and structural control of smart structures.
Tz-Cheng Chiu received the Ph.D. degree in mechanical engineering from Lehigh
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University in 2000. He is an Associate Professor in the Department of Mechanical
Engineering at National Cheng Kung University. Prior to joining the university in
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2006, he was with Texas Instruments at Dallas, TX, and worked on the reliability simulation and material characterization for semiconductor packages. His current research interests include fracture mechanics, nonlinear mechanical behaviors,
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finite element simulation, and electronic component and system reliabilities.
Tian-Shiang Yang received his B.S. and M.S. degrees in Mechanical Engineering
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from National Taiwan University in 1987 and 1989, respectively, and his Ph.D. degree in Mechanical Engineering from Massachusetts Institute of Technology (MIT) in 1996.
Since February 1999, he has been with the Department of Mechanical His research interests include
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Engineering of National Cheng Kung University.
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fluid mechanics, applied mechanics, and applied mathematics.
Mao-Chi Lin received his B.S. degree in Department of Mechanical and Computer-Aided Engineering from Feng Chia University, Taichung, Taiwan, R.O.C., in 2003 and M.S. degree in Mechanical Engineering from Chung Hsing University in Taichung, Taiwan, R.O.C., in 2005, respectively.
Since
Dec. 2008, he has been with the Department of Laser Manufacturing System of Laser and Additive Manufacturing Technology Center(LAMTC) in Industrial Technology Research Institute (ITRI)., where he is currently a Project Manager and responsible for laser glass cutting and processing systems research. 24
List of Table
Table I. Essential laser processing parameters 100 kHz
Average laser power
50 W
Laser pitch
4 – 5 m
Pulse energy
140 – 200 J
Cutting speed
10 -30 mm/s
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Laser pulse frequency
Maximum
TemperatureC
cmax (m)
Top
Chipping
Bottom
Chipping Ratio cr (%)
Top
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Top
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Table II. Summary of thermo-shock specimen release performance with a pressure of 3.95 kPa
Bottom
250
smashed smashed /
/
200
7
0.08%
0.36%
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200
18
0
79
0%
0.46%
4
44
0.09%
~50%
0
82
0%
~80%
8
28
0.14%
~50%
150
0
58
0%
~50%
125
0
58
0%
~30%
125
Fail
Fail
/
/
175 175
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150
25
List of Figures
(a)
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Figure 1. Schematics of the proposed glass crack propagation control and separation method. (a) pico-second laser micro-drilling, and (b) thermo-shock separation.
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Figure 2 Schematic plot of a periodic crack distribution with crack size 2c and a pitch length 2w.
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Figure 3 Laser-glass distance setup. (a) the schematic plot of the laser setup and the light path, and (b) the Bessel beam power analysis and the optimal placement of the glass sample.
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(a)
10m
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(b)
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Figure 4. The SEM micrograph of the cleavage surface (a) the top views and (b) the cleavage surface
(b)
(a)
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94m 222m
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61m
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Figure 5. Glass cleaving results (a) straight and (b) curved external scribing lines
Figure 6. Defective laser drilling results (a) zig-zag scribing line and (b) micro cracks generated around the drilled trenches 28
(a)
(c)
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(b)
10 cm
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Figure 7. (a) Concept of glass separation based on fatigue crack growth and the fatigue testing system: (b) schematic plot and (c) the realized system
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(c)
100 m
Figure 8. Fatigue based glass separation results: (a) failed specimen, (b) specimen with successful separation, and (c) severe chipping observed for these successful separation specimens
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(a)
(b)
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10 cm
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(c)
Figure 9. Test system for performing thermo-shock glass separation: (a) schematic plot and (b) a picture of the system, and (c) the temperature
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histories of upper and bottom surfaces during quenching
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(a)
(b)
Figure 10. Typical thermo-shock separation results of circular holes: (a) global view and (b) enlarged edge image
chipping size = 39m
chipping size = 27m
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(a)
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Figure 11. Enlarged views of representative edge defects observed in separation: (a) isolated chipping and (b) continuous chipping.
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2 cm
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(a)
(c)
chipping size = 8.3m
Figure 12. (a) Glass separation for rectangular fillet holes and the quality of the separated surfaces with a quenching pressure of 1.4kPa: (b) top and (c) bottom surfaces. 31
(a)
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(c)
Figure 13. Typical contours from thermo-shock finite element analyses (a) temperature distribution of the axisymmetric model and (b) maximum stress
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contour of the in-plane 2D model and (c) the temperature histories of te front side with different drilling percentages.
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PII:
S0924-4247(19)31321-4
DOI:
https://doi.org/10.1016/j.sna.2019.111723
Reference:
SNA 111723
To appear in:
Sensors and Actuators: A. Physical
Received Date:
25 July 2019
Revised Date:
28 October 2019
Accepted Date:
6 November 2019
Please cite this article as: Chuang C-F, Chen K-S, Chiu T-C, Yang T-S, Lin M-C, Crafting Interior Holes on Chemically Strengthened Thin Glass Based on Ultrafast Laser Ablation and Thermo-Shock Crack Propagations, Sensors and Actuators: A. Physical (2019), doi: https://doi.org/10.1016/j.sna.2019.111723
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Crafting Interior Holes on Chemically Strengthened Thin Glass Based on Ultrafast Laser Ablation and Thermo-Shock Crack Propagations C-F Chuang1, K-S Chen1*, T-C Chiu1, T-S Yang1, and M-C Lin2 1. Department of Mechanical Engineering, National Cheng-Kung University Tainan, Taiwan, R.O.C. 2. Laser and Additive Manufacturing Application Center, Industrial Technology Research
*Corresponding Author: Email: [email protected]
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Graphical Abstract
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Institute, Tainan, Taiwan, ROC
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Figure 1. Schematics of the proposed glass crack propagation control and separation method
Figure 2. The SEM micrograph of the cleavage surface.
(a)
(b)
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(c)
Figure 3. (a) Successful interior hole cutting, (b) the surface quality of the cut
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surface, and (c) cutting surface quality by another separation method (a lot of chipping). Highlights
We propose a glass cutting scheme by integrating ultrafast laser drilling and subsequently sub-critical crack growth via thermo-shock
Essential fracture mechanics model is outlined to guide the development of processing
Successful interior holes are cut in chemically strengthened glass with fine surface roughness for demonstrating the feasibility
Associate thermo-mechanical simulations are conducted in conjunction with experimental measurements to explain the crack propagation phenomenon based on mechanics.
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Abstract To prevent glass cracking from external mechanical damages, chemically strengthened glass is usually employed. The cutting of strengthened glass becomes critical due to their residual tensile stress induced in the inner core of
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glass attributed to the ion exchange process. This paper describes a novel technique for dealing cutting interior holes and separation of strengthened ultrathin glass for the display unit of mobile communication and computational devices. This method integrates picosecond laser ablation with quenching induced thermo-shock for accomplishing such a task. Essential analyses are performed based on fracture mechanics and finite element method to provide the scientific basis of such an approach. Experimental results indicate that with a proper temperature control, the 2
proposed method could achieve successful separations for various enclosed shapes with the associated surface roughness satisfying the requirement. Keywords: Glass, Packaging, picosecond laser ablation, thermo-shock
I.
Introduction With the advance in mobile computing, portable mobile devices have already become necessary parts in daily life. Ultrathin glass plays both functional and structural supporting roles for the display unit of these devices. Recently, for improving structural reliability consideration, chemically
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strengthened ultrathin glasses have been widely used due to their inherent high strength for resisting possible scratching and damages [1]. Sodium ions are replaced by larger potassium ions at the glass surface regimes and thus compressive residual stresses are induced. Consequently, the hardness and resistivity to scratch are enhanced. However, such an advantage also brings challenges in fabrication and assembly of glass pads. For achieving good surface scratch resistivity in glass, a layer with high residual compressive stress is required but this also induces tensile residual stress beneath the glass surface [2]. That is, the surface compressive residual stress would significantly enhance the damage resistance but the inner tensile residual stress would
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make the structure prone to flaws. Since glass is basically a highly brittle material, any tiny flaws due to cutting could trigger uncontrollable crack propagations [3, 4].
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For these strengthened glass, the existence of the tensile residual stress actually deteriorates the situation since the tensile residual stress can easily trigger the crack propagation in an even less flaw condition. For cutting glass to form desired external shapes, both wheel cutting [5] and laser cutting [6, 7, 8, 9] have been investigated and applied extensively. However, although the cutting problem could be solved, the existence of inner tensile stress has indeed further added challenges in the surface quality of glass cutting. On the other hand, for shaping small arbitrary enclosed holes, the situation is completely different. The wheel cutting is basically out of consideration due to its fundamental limitation and laser processing seems to be the only feasible strategy for cutting strengthened glass in straight-line manner [10]. However, the requirements for creating these inner functional holes are completely different. The cracks must propagate under desirable paths and cannot penetrate into the surrounding structures for maintaining the structural 3
integrity of the final panel. Both the requirements and the much smaller allowed dimensions basically rule out the possibility of using mechanical cleavage approach and the well-behaved CO2 laser cleavage [8, 11] is also fail for controlling the propagation orientation. In addition, the quality of the cleaved surface is also critical. If the defect is too large, the fracture strength would be significantly reduced. Previously, we have developed techniques to remove the damage layer by means of CO2 laser peeling [12, 13, 14]. However, this technique may not be useable in this scenario and a new technique is desperately needed for solving the above-mentioned problem. Recently, with the advances in fast laser processing technologies, such as
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Bessel beam femtosecond lasers, laser ablation becomes a promising approach for handling strengthened glass [15, 16, 17, 18], and sapphires [19]. Special technique such as double pulse laser ablation was also proposed to enhance the cutting efficiency [20] and using femtosecond Bessel beam for creating underneath stealth dice layers and then with external applied mechanical loading [21]. Furthermore, [22] also demonstrated thin glass cutting using femtosecond laser with overlapped laser beams.
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Although the above techniques have demonstrated the ability and potential in glass cutting, several concern remains. First, there is a trade-off between overlap ratio and the cutting performance. For example, for a large overlap
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ratio, the thermal damage and the laser pulse used are the major concerns. For a smaller overlap ratio, although fewer laser pulses used and less thermal damage concern, the uncontrolled cutting flaw propagation may cause failure in glass separation [22].
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In this work, by integrating the knowledge of fast laser processing and fracture mechanics of glass materials, a technique is proposed to use non-overlapped laser beam drilling and controllable crack propagation to address the above trade-off. First, one can create deep micro trench arrays over glass using fast laser. If one can trigger stable crack propagation from
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the surface of those trenches on such perforate structures, these grown cracks may be arrested by the next corresponding trenches before being out of control [23]. That is, by this approach, it is then possible to create controllable sub-critical crack growth from those trenches for separating the glass to form hollow structures. However, such a process involves complicate pyrolytic interaction between laser and glass materials. In addition, extra auxiliary stressing processes should also be developed for achieving a reliable glass separation [24, 25]. As schematically shown in Figure 1, by utilizing 4
pico-second laser drilling and subsequent thermo-shock crack propagation.
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It is possible to successfully create inner holes with different sizes and shapes on chemically strengthened glass with different thickness and hardness. Subsequent efforts then provide mechanical forces to create stable crack propagation to fully cleaving the glass. This approach thus opens an alternative direction for future glass cutting technology. In this work, the entire process and key achievement are presented. For more technical detail, please refer to the master thesis of the first author [24]. Based on the earlier conference presentation [26], this work would concentrate on the development of the cutting and separation technique based on mechanics.
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II.
Approach
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The approach is to create perforated structures fabricated by pico-second laser drilling first and then crack propagation is induced to form an enclosed path by any means. With careful design on the size and density of these trenches and a reliable separation method, it is believed that the crack growth can be propagated in a controllable manner. That is, the crack can be initiated at the edge of a laser drilled trench and they propagated but would be eventually arrested by the next hole. By such an approach, enclosed shapes can be cut and scribed and this technique can be applied to more complicate inner shape cutting. There are two key processes should be paid attention with. I.e., the laser drilling and the glass separation.
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In laser drilling aspect, two major concerns are identified: First, on the quality of drilling: this includes the concern of the surface damage and micro cracks induced by laser machining as well as the material properties variation due to photolytic or pyrolytic effects since they could affect the crack growth and propagation. Second, on the density of drilled trench and the maximum allowed crack propagation distance before out of control. For this issue, it is desired to have a mechanics-based qualitative analysis to guide the process optimization and it would be briefly presented in Section III. Meanwhile, for dealing with the former concern, a preliminary study to address the damage
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of pico-second laser drilling on the surrounding glass materials has been presented elsewhere [24, 25] and this issue would not be further addressed in this work.
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On the other hand, for the glass separation issue, two approaches could potentially be used although they actually represents two extremes in fracture mechanics. The first approach utilizes fatigue crack growth for connecting these trenches. This approach requires a small external fluctuating loading, possibly achieved by employing a piezoelectric actuator, for generating cyclic
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stress. Meanwhile, the other approach is to use thermo-shock for generating fast fracture schematically shown in Figure 1b. To achieve this goal, the perforated structure is heated up to a certain temperature and is then quenched immediately. The resulted thermal shock induces sufficient tensile stress to cause crack propagation from the surface of those trenches [24, 27]. Both of the approaches and have been investigated in this work and the results indicate that both of them can achieve successful separation. However, based on our performance evaluation shown later, the fatigue crack 6
growth approach, as will be addressed in Section V, is finally abandon due to its unsatisfied resulted surface quality. That is, severe chippings are found possibly due to fretting fatigue and this approach is creased and the thermo-shock approach is finally recommended based on the test results shown in this work. However, a successful separation based on thermal shock approach also involves complicate thermo-mechanical coupling and related mechanics should also be investigated. In this article a simple while effective finite element simulation is also performed and addressed in Section VII in conjunction with the process development and for guiding the future
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optimization.
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III.
Mechanics of Separation
From mechanics of materials perspective, there are two approaches to describe the glass cutting and separation problems. The first approach is to address the problem based on a deterministic ultimate tensile strength criterion and the failure (i.e., cutting and separation) occurs once the applied stress exceeds the ultimate strength. In our previous work, the laser induced damage is treated as the equivalent strength reduction and the associated stress-based designs were discussed [28]. On the other hand, the second approach deals the problem using fracture mechanics. That is, if the stress intensity factor or the strain energy release rate of glass exceeds the toughness or the critical strain energy release rate of the processed material, crack I.e.,
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propagation occurs.
K1 K1c or
,
(1)
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G1 G1c
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In comparison with the first approach, this approach takes the surface quality (i.e., initial flaws) into consideration. By this approach, the separation process can be simplified as a mode-1 fracture mechanics problem.
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It has no exact solutions for addressing the issue on the density of drilling trenches. However, phenomenologically, it can be approximated as a mode-1 fracture mechanics problem with periodically patterned initial flaw schematically shown in Figure 2. That is, after forming the periodic perforated structure, microscopically, the structure can be treated as a structure with
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periodically flaw system. According to [29], the stress intensity factor K1 can be found as 2𝑤
𝜋𝑐
1 2
𝐾1 = 𝑌𝜎∞ √𝜋𝑐 ∗ ( 𝜋𝑐 𝑡𝑎𝑛 (2𝑤)) .
(2)
Where Y is a shaper factor typically greater than unity containing all finite size effect and local defects and will be determined by experimental data later shown in section VII. If the K1 greater than the associated glass toughness K1C, then the flaw would growth. From Eq.(2) obviously, the magnitude of K1 is related to the initial flaw size (i.e., 2c) and separation (i.e., 2w-2c). This is 8
structurally analogous to trench size and separation.
With a larger c/w (i.e., a
higher density on drilled trenches), K1 would effectively increase. This tendency agrees with that observed physically. A successful separation depends on the resulted stress intensity factor. It is desired to have a stress intensity factor slightly larger than the fracture toughness of glass. If the resulted K1 less than K1C, no separation occur. On the other hand, if K1 is much larger than K1C, dynamic fracture is expected and the glass sample would be broken to piece. Based on Eq. (2), two control factors are clearly identified. First, the density of holes should be sufficient
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high to increase the c/w ratio for increase the stress intensity factor for fracture. Second, with the density of drilled hole determined, the magnitude of the remote stress should be adjusted adequately for providing a suitable K1. Technically, the remote stress level can be adjusted by either changing the fatigue mechanical loading or by adjusting the thermal quenching rate.
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Taking some typical numbers as an illustration example here. With a shape
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factor Y equaling 2 and for a hole with radius c of 3 m and a separation w of 3.5 m. With a remote stress of 100MPa, the stress intensity factor (by Eq.(2)) is calculated as 1.1MPam, which is close to the fracture toughness of typical
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glass (KIC ~ 1 MPam). Meanwhile, by Eq.(2), it can also be seen that the result is extremely sensitive to the c/w ratio. For example, if c = 3m and w = 4m, the estimated stress intensity factor will reduce to 0.8 MPam, which is less than the fracture toughness of glass.
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To make (2) effective, the shape factor Y must be determined. In Section VII, by integrating experimental and numerical simulation, Y can be determined. Once Y is obtained, Eq.(2) can then be used to provide fast evaluation for optimizing the laser ablation design in the future.
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IV.
Pico-Second Laser Drilling
Chemically strengthened Corning 2320 glass with thickness of 550m, serves as the test specimen in this study. By using a glass surface stress meter (FSM-6000E), the surface compression stress and the thickness of the compressive layer are characterized as 7175 MPa, and 380.1m, respectively. By force equivalent estimation, the average tensile residual stress inside is approximately 84 0.5 MPa. A TruMicro picosecond laser (wavelength 1030 nm, pulse duration around 10 ps) with an average Bessel beam laser power 50W and an instantaneous maximum power of 250W is used in this work. Specimens are placed on an
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X-Y table for positioning. Essential laser processing parameters are listed in Table I. The schematic setup of the laser and the optical path is shown in Figure 3a. On the other hand, as shown in Figure 3b, since the power of the Bessel laser beam is distance-dependent and is not uniform along the optical axis, the distance between the glass sample and the laser head is adjusted to be at the pick power distance and the power uniformity through the entire glass thickness is estimated as 1.5%, which is an acceptable level.
Micro trenches are then drilled by the laser. Figure 4 shows the top and cleavage cross-section of a typical cutting process. Trenches with a 2m
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diameter and a surrounding heat affected zone (HAZ) with a thickness of approximately 0.5-1 m are clearly identified. The view from the cleavage plane also indicates that the cutting can effectively through the entire thickness with surface roughness approximately 1m. Notice that there are some surface micro flaws generated due to laser drilling, however, their characteristic length is small and the surface laser is actually highly compressive (for strengthened glass), these tiny flaws would not propagate during drilling. However, such flaws are very important for triggering stable crack growth for the subsequent separation process. The basic concept for forming a scribing line with a micro trench array and then separates the perforated structure by applying a tensile stress to cleavage has been schematically shown in Figure 1. Demonstrations for straight and curved paths separations are then shown in Figure 5 and both of them are separated by hand. It can be seen that the separations are successful along the scribing line and the induced crack propagations are successfully arrested by the next hole but with a few chippings, which is believed to be caused by 10
the poor control ability of using hand separation. With the feasibility being validated, it is possible for investigating glass separation method. Notice that the drilling quality also strongly depends on the laser parameters and the motion control and vibration of the X-Y table. During the early stage of development, undesirable results are frequently reported. For example, as shown in Figure 6, a zigzag cutting path is resulted. On the other hand, in Figure 6b, undesired tiny cracks not parallel to the cutting path are also generated around these trenches. These issues are eventually solved by improving the positioning and anti-vibration performance of the associated X-Y table. Meanwhile, it is also found that although the laser can effectively
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cut through the entire thickness, the resulted surface topologies such as chipping and HAZ could have differences on the both surfaces. This phenomenon always exists even after performing parameter optimization. This imposes an imperfect factor to the subsequent glass separation process addressed in the next two sections.
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V.
Fatigue Glass Separation
One natural thought for performing glass separation based on the laser drilled scribing line is to utilizing fatigue crack growth. As shown in Figure 7a, based on fatigue mechanics such as Paris’s Law [30] by applying proper fatigue loading, it is possible to generate a stable crack propagation to reach the adjacent trench. Notice that if the initial crack orientation is not favored, this approach may immediate generate failed result. A testing system is constructed to generate tiny high frequency fatigue loading using a piezoelectric actuator is shown in Figures 7b and 7c. This piezo actuator can generate loading amplitude around 22m in 100 Hz. A
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load cell is also used to measure the forcing level. With this design, it is hope that the cracks can progressively propagate on the desired scribing line to form an enclosed path.
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The experimental results are shown in Figure 8. Some specimens actually fail to form enclosed separation (Figure 8a). By examining these specimen, the failure can be attributed to the quality of laser drilling. That is, tiny undesired flaws can observed around those trenches as already addressed in Figure 7a. Second, for these specimens with successful separation (Figure 8b), severe chippings are always observed. As shown in Figure 8c, chipping
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size is up to 300 – 400 m and this is not acceptable for subsequent glass assembly. It is believed that those chippings are generated by the friction, or simply the fretting fatigue during the loading process. Technically, this effect can be reduced by further increase the accuracy in positioning the actuator and with a better loading control during fatigue process. However, practically, this is very difficult to achieve. As a result, although the fatigue approach can potentially achieve successful separation of enclosed paths, the inferior surface quality eventually forces us to abandon this approach. Instead of using progressively crack growth, fast fracture based on
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thermo-shock is then considered and addressed in the next Section.
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VI.
Thermo-Shock Separation
As addressed earlier, the fatigue crack growth approach fails to achieve required performance and an alternative approach must be found. Here, fast facture based thermo-shock is investigated for fulfilling the task. Thermo-shock is a very common phenomenon in ceramic structure failure [27]. That is, when a hot ceramic structure subjects to a localized low temperature quenching, the severe local temperature gradient induces significant tensile stress to cause structure fracture. Based on the argument, it is expected that the glass separation can be performed by this method, at least in theory. The questions remained are the required temperature and
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quenching conditions for providing sufficient driving tensile stress. Meanwhile, to ensure crack propagation along the scribing line, the thermo-shock induced tensile stress should be perpendicular to the scribing line. This approach would generate a standard mode-1 crack. For enclosed path such as circular, an axisymmetric temperature distribution, which can be achieved by proper design of quencher, would satisfy this requirement.
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A self-designed experimental apparatus is realized for performing the test and for evaluating the achieved glass separation performance such as surface chippings. The system, shown in Figures 9a and 9b, consists of a hot plate
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and two nozzles for proving cold jets. The hot plate, capable for heating up to 500C, is responsible for overall heating the glass specimen and these two jets can provide CHF3 jets at -80C for quenching. A test bed consisted of a load cell (ESENSE, LRM-5) is adapted offline for correlating the quenching pressure force and the due jet impinging conditions. The design can provide single side or double side quenching. Two type T thermocouples are also used to monitor the temperature of the glass surface. Figure 9c is a typical temperature history of the glass sample. It is heated up to a steady temperature (200C and 250C for both top and bottom surfaces) and the
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upper jet is open for performing upper surface quenching. The temperature of the top surface drops to -30C within 1s. This should provide sufficient tensile stress. On the other hand, due to heat conduction, the bottom surface has a less temperature drop rate (i.e., dropping 80C in 2 s). For circular scribing path (diameter = 1cm), a typical optical micrograph of the final separation is shown in Figure 10. Basically, no visual chippings are found
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along the surface and the feasibility is thus confirmed and a more detail
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parametric study is then conducted for performance assessment.
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VII.
Thermo-Shock Experimental Results
After validating the feasibility, a systematic investigation on thermo-shock glass separation is then conducted. Figure 11 shows typical edge defects observed on the scribing lines. For providing meaningful statistics analysis on those either isolated or continuous chippings, here two performance index, maximum chipping size (cmax) and chipping ratio (cr), are defined for overall assessment in the subsequent parameter studies. The maximum chipping size are the observed largest chipping in a successfully separated specimen. On the other hand, the chipping ratio is defined as the surface length containing chipping flaw normalized by the entire perimeter of the enclosed
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path. From engineering perspective, cmax should be less than 100m for avoiding stress concentration and cr should be as less as possible. Both of them would be used to evaluate the fabrication performance for process optimization.
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First, the effect of the top heating temperature is investigated. Table II shows the investigation results. Here only the top surface is quenched with a resultant loading of 0.31N (i.e., averaged air pressure = 3.95kPa). It is found that if the top surface temperature of glass samples over 200C, they are simply smashed after quenching. On the other hand, for the top surface
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temperature below 125C, no visible changes occurs. These observations are reasonable since the former represents the situation of over stressing and the later, on the other hand, does not provide sufficient driving force for triggering crack propagation. Based on this investigation, the required glass surface temperature for achieving a stable glass separation is confirmed to between 200 and 125C. Meanwhile, from Table II, it can also be found that cmax on the bottom surface is always greater than that on the top surface in all cases. Nevertheless, the
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maximum chipping size is less than 100m, which is much less than that observed in fatigue separation method and is acceptable for practical usage. On the other hand, the chipping ratio cr also exhibits the same trend. However, it is interest to point out that for a top surface temperature of 200C, the chipping ratio of both top and bottom surfaces are very small, less than 0.5%. As a result, for single side quenching, this should be the optimal processing condition.
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Based on this observation, the process is further optimized by adjusting the air pressure of quenching. Specifically, at the top surface temperature of 200C, instead of using a larger quenching pressure once, by quenching the specimen three consecutive times with a smaller quenching pressure (total force 0.11N instead of the original 0.31N or an averaged quenching pressure 1.4kPa), the quality of the separation surface can be further improved. Based on this improved recipe, enclosed paths beyond the original circular shape such as rectangular fillet shape can also be successfully separated as shown in Figure 12a and the images shown in Figure 12b indicate that there
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are no chipping observed on the top surface and the cmax of the bottom surface is also reduced to 10.8m from the original value of 80 m. Meanwhile, the
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chipping ratio cr of the bottom surface is also reduced to 0.01%. It is believed that a smaller quenching pressure also results in a gentler forcing manner during the moment near the final separation and this affect the possible resulted chipping. Therefore, it is suggested here that this recipe (quenching temperature 200C and a quenching pressure of 1.4kPa with 3 quenches) should be used for future study.
In parallel, double side quenching is also investigated and there are no further visual improvements for the current specimen. However, it is anticipated
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that as the thickness of specimen further increases, the importance of double side quenching would become more dominant. This would leave as our immediate future work.
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It is also important to point out here that in order to convert this feasibility demonstration and analytical mechanics modeling for dealing with more complicated situation in real processing, numerical simulation in temperature and thermal stress analyses would be required for precisely determining the required process parameters. With different specimen and scribing line
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geometries, thermal boundary conditions, and quenching design, it would be difficult to design a successful glass separation process without reliable numerical simulations. For addressing this issue, finite element coupled temperature-displacement analyses performed by using general purposed finite element solver Simulia ABAQUS [31] with coupled temperature-displacement scheme is currently investigated. Here the preliminary results are briefly shown. Two models, i.e., axisymmetric and 2D plane stress models are used for temperature and stress evaluation. To 16
compare with experimental results, the glass specimen is heated to 473K and then is quenched. To keep the model simple while effective, the perforated structure on the cutting path is not physically generated but the effect on thermal resistance is modelled simply with reduction of thermal conduction coefficient at this zone. Figures 13a and 13b show typical temperature and von Mises stress contours after 0.5 seconds of quenching. The detail temperature history of the process with different hole densities is plotted in Figure 13c. It can be seen that for the case with a 50% hole-density (the situation close to the reality), it takes approximately 3.5 seconds to reduce the center temperature of the upper surface to 350K (~80C). The results is
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actually similar to the experimental observation shown in Figure 9c.
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Meanwhile, the maximum tensile stress generated at the cutting path is also found as 105 MPa. By using Eq.(2), the induced stress intensity factor is 0.581Y MPam. Since the experimental results indicate that a stable crack propagation occurs, it is reasonable to assume the stress intensity factor is just slightly over the fracture toughness. Under such an assumption, the shaper factor Y is then estimated as 1.7. With the shape factor determined, in the future, it is possible to perform parametric study using this finite element model to determine appropriate combinations of process parameters and line
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density of hole for optimization. In addition, it is also possible to incorporate these coupled temperature-displacement analyses with fracture mechanics to study the dynamic fracture propagation problems.
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VIII. Discussions
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According to the above results, we believe that the main contribution of this work is on the proposing and validating the feasibility of this new glass separation technique. To the best of our knowledge, although there are many laser glass cutting researches, most of them are either focused on laser-material interactions or on the feasibility demonstrations only. No similar works are reported on technique development specifically guided with failure mechanism-based design. In addition, it is believed that the proposed method is a more versatile solution in comparison with others in the focused applications.
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However, the success of the proposed scheme relies on both the quality of picosecond laser drilling and subsequent crack propagation. Experimental results indicate that a poor X-Y stage positioning control could lead to poor quality on drilling holes and result in tiny undesirable micro cracks. Once these cracks form, the main crack propagation would deviate from the scribing line and the task would fail. On the other hand, with acceptable laser drilled sample, glass separation is still a challenging task. The fatigue crack growth approach is proven to be unacceptable due to its severe chipping generated in fretting process. Instead, the thermo-shock approach
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could potentially achieve success separation with good surface quality. However, there is a narrow region on its parameter setup map to produce acceptable results. If the substrate temperature is too high, it would be smashed due to overstress. On the other hand, substrate without sufficient temperature would not result in crack propagation due to insufficient tensile stress. Therefore, it is important to find the feasible parameter window for process optimization. It is also important to point out the limit of the current approach here for
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future studies. The success of this approach relies on high temperature gradient during quenching and can be described by the dimensionless parameter called Biot number (Bi) [32]. For in-plane direction, the temperature gradient should be maximized for yielding a high tensile stress to cause stable cracking and a larger Bi is desired. On the other hand, for maintaining the scribed surface quality, the temperature of the out-of-plane (i.e., thickness) direction should be uniform and a smaller Bi is desired. Based on length scale analysis [32], a larger enclosed hole on a thinner glass 18
substrate is preferred.
That is, once the size of the enclosed hole is further
approach with less restriction on cutting geometry.
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reduced, or the thickness of glass substrate increases, this approach would meet certain difficulties due to the unfavorable length scale issue and further engineering improvement is required. Nevertheless, the current results indicate that enclosed holes with diameters down to a few millimeters could be made by this approach. If the size of inner hole is reduced to only 1-2 mm or less, it could potentially fabricate simply by high speed drilling, not by cutting. Meanwhile, based on the cutting results on rectangular fillet specimen, it is also expected that this technique is also feasible for creating enclosed holes with arbitrary shapes if the local curvature is not too small. As a result, it is believed that the proposed scheme should be a more versatile
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Finally, it is also important and interesting to investigate the effect of pico-second laser irradiation on the mechanical properties of strengthened glass. Previously, the interaction between CO2 laser and the thermo-mechanical properties evolution of strengthened glass has been investigated by us using both a high temperature oven and micro- and nano-indentation characterizations [25, 33, 34, 35]. It was found that the residual stress, toughness, and modulus of strengthened glass varies once the laser irradiation induced temperature raise above a threshold. Meanwhile,
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the local damage, residual stress field, and the equivalent fracture strength variation due to the pico-second laser irradiation have also been studied by us. These information are essential for a root-cause optimization of related application. In the future, it is possible to integrate this obtained information into account for process design or other applications.
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IX.
Conclusion
Chemically strengthened glass has been widely used due to their capability in resisting possible damages during service. However, such an advantage also brings struggles in fabrication and assembly due to extra induced tensile stresses in inner cores and this causes difficulty in processing. By integrating picosecond laser ablation with quenching induced thermo-shock for providing sufficient driving force to allow crack propagation in a controllable manner, a feasible glass separation technique aiming for creating enclosed interior holes on glass has been developed and demonstrated. Experimental results indicate that the proposed method could achieve successful separations for various enclosed shapes with the surface roughness and
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chipping level satisfying the requirement. Essential experimental and numerical parametric studies are performed for characterizing and optimizing the process window. It is found that the temperature control is crucial for guaranteeing stable crack propagation for glass separation. Nevertheless, based on current experimental results and numerical modeling, the developed method is possible to provide an effective solution or guidance for related glass cutting tasks. In the future, with more sophisticated parameter adjustment, it is expected that the proposed method could be adapted by related industries.
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Acknowledgements This work is supported by Industrial Technology Research Institute (ITRI) and Ministry of Science and Technology (MOST) of Taiwan under contract numbers 105-2221-E-006 -100 -MY3, 105-2221-E-006-074-MY3, and 108-2622-8-006-014.
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with high surface compressive stress,” In Proc. of SPIE, 7920, pp. 79200U-1, 2011. [12] T. S. Yang, G. D. Chen, K. S. Chen, R. C. Hong, T. C. Chiu, C. D. Wen, C. H. Li, C. J. H., K. T. Chen, M. C. Lin, "Thermal analysis of a laser peeling technique for removing micro edge cracks of ultrathin glass substrates for
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web processing," Proc. ASME 2015 International Mechanical Engineering Congress & Exposition, Houston, Texas, USA, 2015. [13] T. C. Chiu, C. A. Hua, C. H. Wang, K. S. Chen, T. S. Yang, C. D. Wen, C. H. Li, M. C. Lin, C. J. Huang, and K. T. Chen, "On the mechanics of laser peeling for ultra-thin glasses." Engineering Fracture Mechanics, vol. 163, pp. 236-47, 2016. [14] K-S Chen, T-S Yang, R-C Hong, T-C Chiu, and M-C Lin, “Thermo-mechanical analysis of laser peeling of ultrathin glass for 21
removing edge flaws in web processing applications,” Microsystem Technologies, vol. 24, pp. 397-409, 2018. [15] M. Kumkar, L. Bauer, S. Russ, M. Wendel, J. Kleiner, D. Grossmann, K. Bergner and S. Nolte, "Comparison of different processes for separation of glass and crystals using ultra short pulsed lasers." Proc. SPIE, vol. 8972, 897214, 2014. [16] M. Bhuyan, F. Courvoisier, P. Lacourt, M. Jacquot, R. Salut, L. Furfaro, and J. Dudley, "High aspect ratio nanochannel machining using single shot femtosecond bessel beams." Applied Physics Letters, vol. 97, 081102, 2010. [17] K. Wlodarczyk, A. Brunton, P. Rumsby, and D. Hand, “Picosecond laser
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cutting and drilling of thin flex glass,” Opt. Lasers Eng., vol. 78, pp. 64-74, 2016. [18] J. Lopez, K. Mishchik, B. Chassagne, C. Javaux-Leger, C. Honninger, E. Mottay, and R. Kling, “Glass cutting using ultrashort pulsed Bessel
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beams,” In Proc. Int. Congress on Applications of Laser & Electro-Optics (ICALEO), pp. 60-69, 2015. [19] Amina, L. Ji, T. Y. Yan, and R. Ma, “Ionization behavior and dynamics of
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Opto-Electronic Advances, vol. 1, 180014, 2018. [21] K. Liao, W. Wang, X. Mei, J. Cui, M. Li, and X Li, “An analytical model to predict the sizes of modified layer in glass with femtosecond Bessel beam”, Optik, vol. 185, pp. 232-241, 2019. [22] H. Shin and D. Kim, “Cutting thin glass by femtosecond laser ablation,”
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Optics and Laser Technology, vol. 102, pp1-11, 2018. [23] C. Makabe, A. Murdani, K. Kuniyoshi, Y. Irei, and A. Saimoto, "Crack-growth arrest by redirecting crack growth by drilling stop holes
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and inserting pins into them." Engineering Failure Analysis, vol. 16, pp. 475-83, 2009.
[24] C. F. Chuang, Effect of Laser and Thermal Processing on the Material Properties and Manufacturability of Ultra-Thin Glass, M.S. Thesis, Department of Mechanical Engineering, National Cheng-Kung University, Taiwan, 2017. [25] K. S. Chen, C. F. Chuang, T. C. Chiu, T. S. Yang, and M. C. Lin, “On the Separation of Chemically Strengthened Ultrathin Glass Based on Fast 22
Laser Induced Subcritical Crack Growth,” Proc. 12th International Symposium on Advanced Science and Technology in Experimental Mechanics (ISEM’17), Kanazawa, Japan, Nov. 2017. [26] C. F. Chuang and K. S. Chen, “A New Technique for Creating Curved Interior Holes on Ultrathin Glass Based on Picosecond Laser Drilling and Thermo-Shock Separation,” In Proc. 20th Symposium on Design, Test, Integration, and Packaging of MEMS and MOEMS (DTIP 2018), Rome, Italy, May 2018. [27] D. Hasselman, “Unified theory of thermal shock fracture initiation and crack propagation in brittle ceramics,” J. Am. Ceram. Soc., vol. 52, pp. 600-604, 1969.
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[28] T. Y. Lee, C. F. Chuang, S. Q. Lin, T. C. Chiu, K. S. Chen, T. S. Yang, and M. C. Lin, “Analysis of Stress in Ultrathin Glass Modified with Picosecond
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[29] T. L. Anderson, Fracture Mechanics, Fundamentals and Applications, 4th Ed., CRC Press, 2017. [30] P. Paris and F. Erdogan, “A Critical Analysis of Crack Propagation Laws,” J. Basic Eng., vol. 85, pp. 528-533, 1963. [31] ABAQUS v. 6.10, Simulia, 2010
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[32] K. S. Chen, Materials Characterization and Structural Design of Ceramic Micro Turbomachinery, Ph.D. Thesis, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA, Feb. 1999. [33] K. S. Ou, H. Y. Yan, and K. S. Chen “Mechanical Characterization of
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KMPR by Nanoindentation for MEMS Applications," Strain, Vol.44, pp. 267-271, 2008. [34] I. K. Lin, P. H. Wu, K. S. Ou, K. S. Chen, and X. Zhang, "Mechanical property characterization of sputtered and plasma enhanced chemical
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deposition (PECVD) silicon nitride films after rapid thermal annealing"
Sensors and Actuators A: Physical, Vol. 117, pp. 117-126, 2011. [35] T. Y. Zhang, L. Q. Chen, and R. Fu, "Measurements of residual stresses in thin films deposited on silicon wafers by indentation fracture." Acta Materialia, vol. 47, pp. 3869-3878, 1999.
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Chiao-Fen Chuang earned her B.S. and M.S. degrees from Department of mechanical Engineering, National Cheng-Kung University (NCKU), Tainan, Taiwan, in 2015 and 2017, respectively. She is currently working for Syntec as a machine tool control engineer.
Ms Chuang’s research interests are in
mechatronics, fabrication, and control technologies.
Kuo-Shen Chen received the B.S. degree in power mechanical engineering from National Tsing-Hua University, Taiwan, in 1989, and the M.S. and Ph.D. degrees in mechanical engineering from Massachusetts Institute of Technology (MIT), Cambridge, in 1995 and 1999, respectively. Since August 1999, he has been with the Department of Mechanical Engineering, National Cheng-Kung University,
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Taiwan, where he is currently a Professor. His research interests include
mechanical analysis and material characterization in MEMS, system dynamics and control of mechatronics and MEMS, and structural control of smart structures.
Tz-Cheng Chiu received the Ph.D. degree in mechanical engineering from Lehigh
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University in 2000. He is an Associate Professor in the Department of Mechanical
Engineering at National Cheng Kung University. Prior to joining the university in
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2006, he was with Texas Instruments at Dallas, TX, and worked on the reliability simulation and material characterization for semiconductor packages. His current research interests include fracture mechanics, nonlinear mechanical behaviors,
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finite element simulation, and electronic component and system reliabilities.
Tian-Shiang Yang received his B.S. and M.S. degrees in Mechanical Engineering
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from National Taiwan University in 1987 and 1989, respectively, and his Ph.D. degree in Mechanical Engineering from Massachusetts Institute of Technology (MIT) in 1996.
Since February 1999, he has been with the Department of Mechanical His research interests include
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Engineering of National Cheng Kung University.
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fluid mechanics, applied mechanics, and applied mathematics.
Mao-Chi Lin received his B.S. degree in Department of Mechanical and Computer-Aided Engineering from Feng Chia University, Taichung, Taiwan, R.O.C., in 2003 and M.S. degree in Mechanical Engineering from Chung Hsing University in Taichung, Taiwan, R.O.C., in 2005, respectively.
Since
Dec. 2008, he has been with the Department of Laser Manufacturing System of Laser and Additive Manufacturing Technology Center(LAMTC) in Industrial Technology Research Institute (ITRI)., where he is currently a Project Manager and responsible for laser glass cutting and processing systems research. 24
List of Table
Table I. Essential laser processing parameters 100 kHz
Average laser power
50 W
Laser pitch
4 – 5 m
Pulse energy
140 – 200 J
Cutting speed
10 -30 mm/s
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Laser pulse frequency
Maximum
TemperatureC
cmax (m)
Top
Chipping
Bottom
Chipping Ratio cr (%)
Top
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Top
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Table II. Summary of thermo-shock specimen release performance with a pressure of 3.95 kPa
Bottom
250
smashed smashed /
/
200
7
0.08%
0.36%
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200
18
0
79
0%
0.46%
4
44
0.09%
~50%
0
82
0%
~80%
8
28
0.14%
~50%
150
0
58
0%
~50%
125
0
58
0%
~30%
125
Fail
Fail
/
/
175 175
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150
25
List of Figures
(a)
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Figure 1. Schematics of the proposed glass crack propagation control and separation method. (a) pico-second laser micro-drilling, and (b) thermo-shock separation.
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Figure 2 Schematic plot of a periodic crack distribution with crack size 2c and a pitch length 2w.
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Figure 3 Laser-glass distance setup. (a) the schematic plot of the laser setup and the light path, and (b) the Bessel beam power analysis and the optimal placement of the glass sample.
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(a)
10m
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(b)
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Figure 4. The SEM micrograph of the cleavage surface (a) the top views and (b) the cleavage surface
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61m
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Figure 5. Glass cleaving results (a) straight and (b) curved external scribing lines
Figure 6. Defective laser drilling results (a) zig-zag scribing line and (b) micro cracks generated around the drilled trenches 28
(a)
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Figure 7. (a) Concept of glass separation based on fatigue crack growth and the fatigue testing system: (b) schematic plot and (c) the realized system
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(c)
100 m
Figure 8. Fatigue based glass separation results: (a) failed specimen, (b) specimen with successful separation, and (c) severe chipping observed for these successful separation specimens
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(a)
(b)
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Figure 9. Test system for performing thermo-shock glass separation: (a) schematic plot and (b) a picture of the system, and (c) the temperature
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histories of upper and bottom surfaces during quenching
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(a)
(b)
Figure 10. Typical thermo-shock separation results of circular holes: (a) global view and (b) enlarged edge image
chipping size = 39m
chipping size = 27m
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Figure 11. Enlarged views of representative edge defects observed in separation: (a) isolated chipping and (b) continuous chipping.
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2 cm
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(c)
chipping size = 8.3m
Figure 12. (a) Glass separation for rectangular fillet holes and the quality of the separated surfaces with a quenching pressure of 1.4kPa: (b) top and (c) bottom surfaces. 31
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Figure 13. Typical contours from thermo-shock finite element analyses (a) temperature distribution of the axisymmetric model and (b) maximum stress
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contour of the in-plane 2D model and (c) the temperature histories of te front side with different drilling percentages.
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