subsurface cracks by laser irradiation

Optics and Laser Technology 111 (2019) 497–508

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Full length article

Repair of ultrasonic machining induced surface/subsurface cracks by laser irradiation

T

⁎

Jingsi Wanga, , Pay Jun Liewb a b

Marine Engineering College, Dalian Maritime University, 1 Linghai Road, Ganjingzi District, Dalian 116026, China Fakulti Kejuruteraan Pembuatan, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100, Durian Tunggal, Melaka, Malaysia

H I GH L IG H T S

machining induced micro-cracks can be completely healed by laser irradiation. • Ultrasonic dimple-like micro-structures were generated as a result of the combined influences of USM and laser irradiation. • Periodic • Combination of micro-USM and laser irradiation presents a potential for efficient machining of complex micro-optics.

A R T I C LE I N FO

A B S T R A C T

Keywords: Ultrasonic machining Machining efficiency Crack repair Laser irradiation Glass

In ultrasonic machining (USM), a mirror-like surface cannot be readily obtained on hard and brittle materials owing to easily generated microcracks. This drawback greatly limits the utilisation of USM as a finish machining technology. Therefore, minimizing the crack size or even eliminating the cracks is necessary to improve surface quality. In this study, fine abrasive particles are first applied to minimize the crack size during USM. Material removal efficiency and surface quality are then evaluated. However, crack formation during this process is unavoidable, and microcracks left on the machined surface cannot be completely removed by merely decreasing the particle size. In addition, minimizing the particle size is not beneficial to maintaining a desired machining rate. Therefore, a new method to repair the cracks by laser irradiation is proposed. The USM machined surfaces of glass plates are irradiated by a carbon dioxide laser at a wavelength of 10.6 μm to repair the cracks because glass has a high absorption coefficient at this wavelength. The temperature field generated during this process is evaluated using the finite element method. Simulation and experimental results demonstrate that the scanning speed of the laser beam has a considerable effect on the temperature increase, thereby influencing the repair result accordingly. Finally, a smoothed surface without cracks is obtained under a scanning speed of 300 mm/ min and 5 W power. The combination of micro-USM and laser irradiation exhibits great potential for fabricating various micro-shapes and structures on hard and brittle materials, such as glass with high efficiency, high form precision and high surface quality.

1. Introduction

beneath the work surface extend deep into the material and may remain inside the workpiece, ultimately leading to surface/subsurface defects [4,5]. Lee and Chan [6] investigated the influence of vibration amplitude, static load and particle size on machining rate and surface quality. They suggested that the material removal rate will increase, whereas the machined surface quality will degrade with an increase in any of these parameters. High material removal efficiency generally results in more cracks on the machined surfaces. Such cracks will lead to strong variations in material strength [7] and thus influence the service life of micro-products. The easiest way to reduce potential cracks in USM is by using small

Abrasive-based ultrasonic machining (USM) is a completely mechanical process widely used to manufacture various structures on hard and brittle materials, such as glass [1]. USM is non-thermal, non-electrical and non-chemical and does not introduce significant levels of residual stress [2]. These characteristics make USM suitable for the stable and efficient micromachining of hard and brittle materials [3]. During USM, the brittle materials are removed by tiny crack generation and accumulation due to the impact of abrasive particles driven by the ultrasonically vibrated tool. The resulting median cracks generated

⁎

Corresponding author. E-mail address: [email protected] (J. Wang).

https://doi.org/10.1016/j.optlastec.2018.10.029 Received 22 June 2018; Received in revised form 9 October 2018; Accepted 14 October 2018 0030-3992/ © 2018 Elsevier Ltd. All rights reserved.

Optics and Laser Technology 111 (2019) 497–508

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Transducer Ultrasonic vibration spindle

Horn

Work stage

Tool

(b)

Slurry

Ultrasonic vibration

Workpiece

Abrasive grain

Fixture

Workpiece Load cell

(a)

(c)

Fig. 1. Schematic and photograph of USM experiments: (a) schematic view, (b) experimental setup and (c) load cell for machining force measurement. Table 1 Experimental conditions for blind-hole drilling experiments using spherical Al2O3 abrasive particles with different diameters. Vibration frequency Vibration amplitude Maximum tool feed depth Machining force Tool feed rate Tool material Flow rate of slurry Abrasive material Abrasive volume Workpiece material

Table 2 Viscosity and the corresponding temperature for soda-lime glass [26].

About 61 kHz About 4 μm (peak-to-peak) 500 μm Lower than 3 N 1–60 μm/s 304 stainless steel (∅ 1 mm) 50 mL/min Spherical Al2O3 (mean size: 2 μm, 4 μm, 6 μm) 10 wt% mixed with water Soda-lime glass

Viscosity dPa s

State

Temperature °C

105 108.6 1014 1014.3 1015.5

Working point Softening point Annealing point Transition temperature Tg Strain point

1040 720 540 530 506

z

Laser beam

abrasive particles, though this approach is ineffective for maintaining a desired machining rate. Therefore, post-process operations such as finish grinding are applied to remove the cracks after USM if necessary. However, in the manufacture of wafers and panels, such as glass panels for smartphones as thin as 0.3 mm, finish grinding is difficult to implement because the process will lead to a break. The difficulty of the finishing process also increases if the surface is curved at the same time [8]. Furnace annealing is a promising method of removing small surface and subsurface cracks in optical glass [9,10]. However, sample deformation may occur due to the heat effect in this process, which decreases the form accuracy [11]. For instance, an aspherical lens with a complex and specified surface profile is not suitable for treatment by furnace heating to remove defects because the transformation of the surface structure is not acceptable. Thus, finding a new method to effectively remove surface/subsurface cracks after USM and thus ensure high surface quality is important. At present, laser beams are widely used in materials processing, including cutting, drilling, milling, marking, welding, sintering and heat treatment [12]. Laser irradiation offers several advantages compared with conventional processing, such as high geometrical freedom, high lateral and vertical resolution and fast processing without contacting the material [13]. With carbon dioxide laser (CO2 laser), glasses can be repaired and polished [14–16] and various micro-patterns [13,17] can be fabricated because of the strong absorption of radiation in these materials. Laguarta et al. [14] proposed a space- and timecontrolled uniform CO2 laser for polishing conventional glasses with high coefficients of thermal expansion. The results show that the

y

Scanning speed V

x

Ly

Lz

Lx Fig. 2. Schematic of coordinate system used in the laser irradiation model.

technique can be applied indiscriminately to preheated samples made of different glasses with any topography and any size, the application depends only on the available laser power. Choi et al. demonstrated a CO2 laser reshaping technique for fabricating a large variety of microlensed fibers with convex, concave and conical shapes [18]. Recently, they reported on fabricating plano-convex micro-lens arrays on a fused silica glass surface through a double-step process comprising femtosecond laser machining followed by CO2 laser polishing [19,20]. They proved that the process is fast, reliable, repeatable and flexible. The application of CO2 laser polishing for smoothening micro-patterns and 498

Optics and Laser Technology 111 (2019) 497–508

1200

3

1100

2.8

1000

2.6 2.4

Heat capacity

900

2.2

800

2

700

1.8

600

1.6

500

1.4

400

Thermal conductivity

300 0

1.2

Laser beam

Thermal conductivity W/(m·K)

Heat capacity J/(Kg·K)

J. Wang, P.J. Liew

CO2 laser

Mirrors

X-Stage Y-Stage

Z

1 200 400 600 800 1000 1200 1400 1600

X

Temperature K

6 μm

Maximum feed depth μm

Workpiece dimension Lx Workpiece dimension Ly Workpiece dimension Lz Initial temperature T Density of soda-lime glass ρ

Z-Platform (Stationary)

Fig. 5. Schematic of the laser irradiation system.

Table 3 Simulation parameters for the laser irradiation model. 5W 1 mm 0.84 8907.8 1/cm 700 mm/min 300 mm/min 6 mm 8 mm 1 mm 293.15 K 2.49 g/cm3

Workpiece

Y

Fig. 3. Heat capacity [35] and thermal conductivity [36] of soda-lime glass as a function of temperature (dashed line indicates an approximation in the corresponding temperature range).

Average laser beam power P0 Laser beam radius r Absorptivity A Absorption coefficient α Scanning speed V

Focusing lens

500 mm/min 100 mm/min

4 μm 2 μm

Feed rate μm/s Fig. 6. Maximum feed depth under different feed rates with spherical Al2O3 abrasives that have different mean diameters.

micro-shapes becomes a new popular study topic [21,22]. The laser beam can be focused on a minuscule spot and can achieve an extremely high irradiance; thus, high temperatures can be obtained at a specified small area within an extremely short time. The approach has potential advantages for polishing micro-structures, including maintaining the high form accuracy of machined parts and reducing thermal alterations and damage if appropriate irradiation conditions are used. However, predicting and controlling the final surface morphology are difficult because of the complicated phenomenon involved in the laser irradiation process and the incomplete knowledge on crucial process parameters, including surface temperature and heating time

[23]. Laser processing may produce many defects, such as bulges, debris, microcracks and scorches, if the processing conditions are not suitable. Some protection processes have been proposed to reduce the temperature gradient and the heat-affected zone for crack-free glass machining, such as using poly-dimethylsiloxane protection [17]. Hence, the importance of investigating the nature of laser-induced evolution in glass materials remains as it will allow us to determine the conditions for the successful and reliable use of laser irradiation. Therefore, all types of glass materials with several chemical elements and high

Evaluation point Center of heat source

Moving direction of heat source

Fig. 4. Initial state of the laser irradiation system. 499

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Bottom surface

Bottom surface

1.0 μm

1 μm

1 μm

(a)

Bottom surface

Bottom surface

1 m

1 m

(b) Fig. 7. Cross-sections of machined surfaces: (a) using spherical Al2O3 abrasive particles 2 μm in diameter and (b) using spherical Al2O3 abrasive particles 6 μm in diameter.

a buffered oxide etch (BOE) solution [hydrofluoric acid (50% concentration)/ammonium fluoride (40% concentration) = 9/100].

thermal expansion coefficients can be processed as well. In this study, we propose the use of CO2 laser heating to repair cracks on glass substrates machined by USM. An improved 3D finite element method (FEM) model based on Laguarta’s work [14] was established to investigate the change in the temperature field with time and space. Different beam scanning speeds were applied in the calculation. Temperature change around the moving laser beam can be investigated effectively. Laser irradiation experiments were conducted on both raw glass surfaces and USM machined surfaces. The experimental results of the raw glass surfaces can be explained well by the simulation results. The characteristics of irradiated USM machined surfaces under different conditions were studied. A mirror-like surface with no cracks was successfully obtained. Both the experimental and numerical results led to an enhanced understanding and control of the processes involved.

2.2. Basic crack repairing mechanism by CO2 laser irradiation In general, the absorbed CO2 laser irradiation will result in rapid temperature rise that softens the thin surface layer [24] and then decreases the material viscosity. A study on the mechanism of CO2 laser mitigation of laser damage growth in fused silica [25] has suggested that healing cracks and improving surface quality are based on this thermally induced viscosity reduction and the flow of a thin surface layer under the pressure generated by surface tension forces. The closure of cracks is possible only by heating the material to high temperatures to attain low viscosity. Table 2 lists the typical viscosity and corresponding temperature for soda-lime glass [26]. The temperature shown in the table provides a certain temperature range for the transition between liquid and solid states of soda-lime glass. Working in the appropriate region above the transition temperature Tg (530 °C), the viscosity of the glass drops drastically, thereby enabling crack healing. However, CO2 laser processing induces large temperature gradients in glass which cause thermal stress σ [27]. Thermal stress is proportional to the coefficient of thermal expansion, Young’s modulus and the temperature gradient [28]. If the induced tensile stress exceeds the fracture limit of the material, cracks will be generated at the surface. Soda-lime glass with a comparatively large thermal expansion coefficient (approximately 8.7 × 10−6 1/K) is more sensitive to thermal stress than those with small coefficients. Thus, further damage may be generated if the soda-lime glass is irradiated with high intensity power. Both crack healing and thermal-energy-induced damage growth are highly sensitive to the temperature. Therefore, to repair the cracks on

2. Materials and methods 2.1. Hole drilling experiments using ultrasonic machining In this study, blind-hole drilling on glass plates was conducted using spherical Al2O3 abrasive particles with diameters of 2, 4, and 6 μm (Showa Denko Corporation, Japan). Fig. 1 shows a schematic view and photograph of the USM experiments. Table 1 lists the detailed USM experimental conditions. All experiments were conducted under the same stop conditions, where the maximum tool feed depth and maximum machining force were 500 μm and 3 N, respectively, and the machining force was measured using the load cell shown in Fig. 1(c). Cross-sections of the machined surfaces were observed with field emission SEM (model: SM-71010, JEOL Corporation) after etching with 500

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M oving direction of laser beam

Temperature °C 800

0 1 5 10 20

700 600 500 400 300 1 mm

200 100

(a)

Time s

Temperature °C 800

0 1 5 10 20

700 600 500 400 300 200 1 mm

100

(b)

Time s

Temperature °C 800 700

0 1 5 10 20

600 500 400 300 200 1 mm

100

(c)

Time s

Temperature °C 800 700

0 1 5 10 20

600 500 400 300 1 mm

200 100

(d)

Time s

Fig. 8. Temperature distribution of soda-lime glass when the evaluation point reaches the maximum temperature (left). Temperature evolution at the evaluation point and its subsurface points of different depths (right) under scanning speeds of (a) 100, (b) 300, (c) 500 and (d) 700 mm/min.

501

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0.458 μm

Fig. 9. Expansion of the material by laser heating (scanning speed of 200 mm/min).

temperature, t is the time, k is the thermal conductivity and Q is the heat source term [29]. Laser energy is represented as a volumetric heating source with an ideal Gaussian intensity profile. In this study, the Gaussian intensity distribution is simply represented as a function of the incident laser power P, the laser beam radius r at the 1/e2 intensity point and the radial coordinate x [30,31]. Accordingly, the attenuation of laser power within the material is described by the Beer–Lambert law [32,33] and the heat source Q at the point (x, y, z) with the change of time t is finally expressed as

the machined surfaces after USM, irradiation parameters should be carefully decided to ensure the temperature increases in an optimum range. 2.3. Simulation of laser irradiation To evaluate the temperature field generated by CO2 laser irradiation in soda-lime glass, FEM was used to simulate the heat conduction process. The calculations were conducted with COMSOL Multiphysics software (COMSOL Corporation). A 3D coordinate system was constructed around the laser irradiation spot, as schematically shown in Fig. 2. The CO2 laser irradiation leads to substantial heating of a thin surface layer on glass. Heat losses into the surrounding atmosphere and vaporization of materials are neglected. We assume that the laser beam moves with a velocity V along the y direction. The governing equation of heat conduction for this problem is given by

ρCP

∂T = ∇∙ (k∇T ) + Q ∂t

Q (x , y, z , t ) = Aα

−2((x − x 0 )2 + (y − y0 − Vt )2) ⎞ 2P0 exp ⎜⎛ ⎟ exp( − αz ) 2 πr r2 ⎝ ⎠ (2)

where P0 is the incident laser beam power, A is the absorptivity of the laser power on the surface, r is the laser beam radius, α is the absorption coefficient, V is the scanning speed, (x0, y0 + Vt, 0) is the center of the laser focus spot on the surface at time t and z is the depth from the surface. Fig. 3 shows the temperature dependence of heat capacity and thermal conductivity for soda-lime glass. Variations of the two thermal properties with temperature are considered and applied to the simulation model to increase the calculation reliability. Even

(1)

where ρ is the material density, CP is the specific heat capacity, T is the 502

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2.298 μm

Fig. 10. Fracture of the material by laser heating (scanning speed of 100 mm/min).

2.4. Laser irradiation experiments for repairing ultrasonic machininginduced cracks

though the absorptivity and absorption coefficients are also functions of temperature, laser energy is mainly absorbed at the surface and the temperature distribution can be adequately determined by treating them as constants. The absorptivity and absorption coefficients for soda-lime glass at room temperature [34] were directly used in this model, which is considered sufficient to analyze and describe the temperature profiles. The values related to the simulation parameters and the material properties are detailed in Table 3. A snapshot of the initial state of the laser irradiation system is shown in Fig. 4. The boundary condition for the top surface is a combination of thermal radiation of the laser energy Q and convection heat transfer with air, whose heat transfer coefficient h is 10 W/(m2 × K). To simplify the calculation, thermal insulation boundary conditions are defined on the bottom and the side surfaces. A minimum mesh with a size of 2.4 μm was defined around the laser irradiation spot at the surface. Coarser meshes were used in the region farther away to reduce the calculation amount. Temperature evolution was analyzed using a linear time-dependent solver with a time step of 100 μs.

2.4.1. Laser apparatus A self-assembled two-axis (X and Y) control laser cutter whose parts were provided by smartDIYs Corporation was used in the crack repair experiments. Fig. 5 shows the schematic of the CO2 irradiation system. The laser source was stabilized at a wavelength of 10.6 μm and the maximum power of 40 W. The output power could be adjusted by pulse width modulation at three frequency levels, 3.9 kHz, 489 Hz and 122 Hz. The three frequency levels correspond to three stages of the output power with adjustable ranges of 40–100%, 10–40% and 0–10%, respectively. The laser beam was controlled using a CNC-driven stage to scan the workpiece that was set on a stationary flat stage. The two linear axes were controlled with a maximum travel range of 600 mm in the X direction and 440 mm in the Y direction. The beam scanning speed was programmable over 0.1–10,000 mm/min. The effective focal beam diameter at the work surface was 2–5 mm, which was determined by the distance between the focusing lens and the work surface. The 503

Optics and Laser Technology 111 (2019) 497–508

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A

Irradiated surface

USM machined surface

B

10 μm

100 nm

100 nm

Fig. 11. Irradiation results of surfaces machined by 2 μm diameter particles with a scanning speed of 300 mm/min.

stop conditions in USM experiments. The tool feed rate can reach up to 40 μm/s to complete a 500 μm feed when using 6 μm diameter spherical Al2O3 abrasive particles and up to 19 μm/s by using 4 μm diameter abrasive particles. The rate has the lowest value of 3 μm/s by using 2 μm diameter abrasive particles. The material removal efficiency dropped dramatically when very small abrasive particles were used. In addition, the number and size of cracks on the bottom surfaces when using 2 μm diameter abrasive particles decreased obviously compared with those using larger abrasive particles through SEM observation. Fig. 7 shows examples of the observed cracks with 2 and 6 μm diameter abrasive particles. The largest crack, approximately 1 μm in depth, was observed with 2 μm diameter abrasive particles, as shown in Fig. 7(a). The results indicate that the machined surface quality can be improved by using smaller particles, but complete crack removal cannot be achieved.

laser power was measured by a power meter (model FieldMaxII-To, Coherence Corporation). In the experiments, beam diameter and laser power were adjusted to 2 mm and 5 W for comparison with the simulation, and the corresponding laser fluence is 0.33 J/cm2. 2.4.2. Laser irradiation of raw surfaces To compare with the simulation results and examine the variation of soda-lime glass irradiated by laser, raw surfaces were scanned by CO2 laser without preheating. Line scanning was conducted with scanning speeds of 100–500 mm/min with an increment of 100 mm/min to find the threshold. The resulting surfaces were observed with a white light interferometer (Taylor Hobson, Talysurf CCI - lite non-contact 3D profiler). 2.4.3. Laser irradiation of machined surfaces by ultrasonic machining The possibility of healing cracks generated during USM by CO2 laser irradiation was demonstrated by the following experiments. First, holes were drilled to a depth of 100 μm on soda-lime glass by USM using spherical Al2O3 abrasive particles with average diameters of 2, 4, and 6 μm, respectively. A CO2 laser was then used to irradiate the bottom surfaces of the machined holes to repair the remaining cracks. To confirm if the subsurface cracks were repaired, the irradiated workpieces were etched with BOE solution and subsequently observed using field emission SEM.

3.2. Thermal analysis results Fig. 8 shows the simulation results. Temperature distributions of soda-lime glass when the evaluation point reaches the maximum temperature under different scanning speeds are shown in the left figures. Both the results on the glass surfaces and the cross-sections along the white dash and dot lines are given. The maximum temperature occurs at the top surface as well as the center of heating source and attenuates in the depth direction for all cases. The figures on the right-hand side of Fig. 8 show the variation of temperature distributions at the evaluation point and its subsurface points with time under different scanning speeds. The maximum temperature at the evaluation point reaches 848, 634, 531 and 468 °C when the scanning speed is 100, 300, 500 and 700 mm/min, respectively. The results show that scanning with a low

3. Results and discussion 3.1. Machining efficiency and surface quality of ultrasonic machining Fig. 6 plots the results under different feed rates when reaching the 504

Optics and Laser Technology 111 (2019) 497–508

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Irradiated surface

Periodic structures A USM machined surface

B 10 μm

1 μm

1 μm

Fig. 12. Irradiation results of surfaces machined by 2 μm diameter particles with a scanning speed of 500 mm/min.

speed leads to a large rise in temperature. By contrast, with the increase in scanning speed, the high temperature can only last for a short time and its attenuation in depth direction is accelerated. As the maximum temperature of the former three cases is above the transition temperature Tg (530 °C) of soda-lime glass, the viscosity of the material will drop drastically, thereby enabling crack healing. However, the maximum temperature under the scanning speed of 100 mm/min is higher than the softening point of 720 °C, which may yield melting and a potential expansion of the material. Therefore, to obtain a good healing result, the scanning speed should be carefully chosen from the range of 100–500 mm/min.

material is not obvious and almost no change can be observed on the work surface. It is believed that a very thin layer of material (softening/ viscosity reduction) may flow under surface tension even though no outstanding change in the surface profile occurs. These results show that the simulation model is adequate for understanding the major features of the surface temperature, which is high and long-lasting with a low scanning speed. Therefore, based on the simulation and experimental results, the scanning speed should be controlled to be higher than 100 mm/min under the irradiation power of 5 W for avoiding thermal damage and under 500 mm/min for repairing the microcracks and obtaining a smoothed surface.

3.3. Laser irradiation results of raw surfaces

3.4. Laser irradiation results of machined surfaces by ultrasonic machining

When the scanning speed was 200 mm/min, a protrusion of 0.458 μm in height was generated as shown in Fig. 9. In consideration of the simulation results, the maximum surface temperature at this scanning velocity would be close to the softening point of 720 °C, which may lead to material melting and expansion/swelling. However, when the scanning speed was decreased to 100 mm/min, a fracture can be found, as shown in Fig. 10. The morphology of the fracture is a continuous extrusion instead of an inside crack. The simulation results indicate that the highest temperature at 100 mm/min is approximately 848 °C, a swelling may occur under the high temperature, which may further fracture due to excessive tensile stress. Given that no preheating and cooling treatments were conducted on the glass samples, thermal stress induced by a drastic rise-and-fall of the temperature may exceed the fracture limit and lead to material failure. Thus, an extrusion fracture may be generated, as shown in Fig. 10. By contrast, when the scanning speed is 300, 400, and 500 mm/min, the expansion of the

First, surfaces machined by 2 μm diameter abrasive particles in USM were investigated. The irradiated results with a scanning speed of 300 mm/min are shown in Fig. 11. The upper micrograph was captured at low magnification, and obvious differences could be found between the laser-irradiated surface and USM machined surface. The dynamics of the generated temperature rise allowed the surface layer to flow and resulted in a polished appearance of the treated area. Magnified area A exhibits the close view of the surface after CO2 laser irradiation, thereby confirming that no cracks were left on the machined surface. Given that the irradiated surface was etched with BOE solution, we can believe that there are no subsurface cracks remained, and thus verified the possibility of healing cracks using laser irradiation. Simulation results with the same scanning speed in Fig. 8(b) also showed that the transition depth due to temperature rise was sufficiently large to heal cracks. Surface roughness was measured with a non-contact laser probe profilometer (Model: NH-3SP; MitakaKouki Co. Ltd., Japan). The 505

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100 nm

100 nm

(a)

1 μm

1 μm (b)

Fig. 13. Irradiation results of surfaces machined by 4 μm diameter particles: (a) scanning speed of 300 mm/min and (b) scanning speed of 500 mm/min.

reached in the deepest crack depth surface layer is sufficiently high and long-lasting. The satisfaction of both conditions can smoothen the treated surface, thus, scanning speed has a considerable effect on the repairing result. When the scanning speed was increased to 500 mm/ min, dimple-like structures were found, as shown in Fig. 13(b). Similar results were observed on the irradiated surfaces originally machined by 6 μm diameter abrasive particles, as shown in Fig. 14. The results imply that when the scanning speed is 500 mm/min, the irradiation is insufficient to make the discontinuities of USM machined surface layer to be homogenized.

average roughness (Ra) of the laser-irradiated USM machined surface and the USM machined surface were 0.006 and 0.303 μm, respectively. The roughness is comparable with former reported results regarding CO2 laser polishing of conventional glass while the glass was preheated before laser irradiation in the previous work [14]. After laser irradiation, the surface quality was improved considerably. The laser repairing process is quite efficient and effective compared with common polishing technology, such as hand, chemical and electrochemical polishing. Fig. 12 shows the results when a scanning speed of 500 mm/min was used, periodic micro-structures generated on the surface after laser irradiation. The magnification views show randomly distributed dimple structures in the laser irradiation area. Although a CO2 laser is commonly used to fabricate dome-shaped bumps and crater structures on silicate glasses [37,38], direct formation of the periodic structures is rarely reported. Fine scaled periodic structures can offer additional freedom to create new functions or combinations of functions, such as optical retroreflective and antireflective structures [39,40]. The generated periodic micro-structures in the current study are considered potentially useful. At the present stage, even though the exact formation mechanism is not yet well known, it remains considered a result of the combined influences of USM and laser irradiation, which can be further studied for practical applications. The irradiated results of surfaces machined by 4 μm diameter abrasive particles are illustrated in Fig. 13. The cracks generated by micro-USM could be repaired and a smooth surface was obtained, as shown on the left side of Fig. 13(a), when the scanning speed was 300 mm/min. However, because the crack depth increased when larger abrasive particles were used, some deeper cracks were not completely healed and they remained on the work surface, as shown on the right side of Fig. 13(a). The simulation results indicate that the temperature is depth-decreasing and, consequently, the material flow time is too short to close deep cracks. Therefore, the effectiveness of the proposed laser irradiation method will be ensured only when the temperature

4. Conclusion In this study, a laser irradiation technology was proposed for repairing microcracks generated during USM. The crack size was first decreased to less than 1 μm using fine abrasive particles in micro-USM. Then, these cracks were healed by CO2 laser irradiation with 5 W power under 300 mm/min scanning speed. Therefore, a mirror-like surface with no subsurface cracks was successfully obtained. The crack healing was very sensitive to temperature. When the temperature increase was drastic, thermal energy-induced damage growth and material expansion occur, which may degenerate the surface quality and form accuracy. However, if the temperature was not increased to the transition temperature of the material, crack healing cannot be achieved. A numerical simulation model was developed to investigate temperature evolution by laser irradiation. Comparisons of the numerical results with the experimental results allowed us to determine the conditions for the successful and reliable use of the laser smoothing technique. In addition, periodic dimple-like micro-structures were observed on the surfaces after being irradiated by the CO2 laser with 5 W power under 500 mm/min scanning speed. These structures are considered a result of the mixed influences of USM and laser irradiation. The combination of micro-USM and laser irradiation presents the potential for fabricating 506

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1 μm

100 nm (a)

1 μm

10 μm (b)

Fig. 14. Irradiation results of surfaces machined by 6 μm diameter particles: (a) scanning speed of 300 mm/min and (b) scanning speed of 500 mm/min.

various micro-shapes and structures on hard and brittle materials with high efficiency, high form precision and high surface quality.

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Acknowledgements The author gratefully acknowledges the support from Professor Tsunemoto Kuriyagawa, Associate Professor Masayoshi Mizutani, and Assistant Professor Keita Shimada at Tohoku University in Japan for their kind suggestions of my research work. The author would also like to thank Taga Electric Corporation in Japan with respect to the manufacture of USM tool, and smartDIYs Corporation for the provision of laser cutter parts for crack repair in the experiments. Declaration of Conflicting Interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Funding This work is supported by the Fundamental Research Funds for the Central Universities of China [Grant no. 3132018259] and National Natural Science Foundation of China [Grant no. 51805067]. References [1] T.B. Thoe, D.K. Aspinwall, M.L. Wise, Review on ultrasonic machining, Int. J. Mach. Tool Manuf. 38 (4) (1998) 239–255. [2] D.K. Baek, T.J. Ko, S.H. Yang, Enhancement of surface quality in ultrasonic machining of glass using a sacrificing coating, J. Mater. Process. Technol. 213 (4) (2013) 553–559. [3] K.P. Rajurkar, Z.Y. Wang, A. Kuppattan, Micro removal of ceramic material (Al2O3)

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