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Experimental and theoretical investigation of the drilling of alumina ceramic using Nd:YAG pulsed laser
Optics & Laser Technology 44 (2012) 913–922
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Experimental and theoretical investigation of the drilling of alumina ceramic using Nd:YAG pulsed laser M.M. Hanon a, E. Akman b, B. Genc Oztoprak b, M. Gunes c, Z.A. Taha a, K.I. Hajim a, E. Kacar b, O. Gundogdu c, A. Demir c,n a
Institute of Laser for Postgraduate Studies, University of Baghdad, Baghdad, Iraq Kocaeli University, Laser Technologies Research and Application Center, 41275 Yenik¨ oy, Kocaeli, Turkey c Kocaeli University, Electro-Optic Systems Engineering, 41380 Umuttepe, Kocaeli, Turkey b
a r t i c l e i n f o
abstract
Article history: Received 6 August 2011 Received in revised form 31 October 2011 Accepted 8 November 2011 Available online 21 November 2011
Alumina ceramics have found wide range of applications from semiconductors, communication technologies, medical devices, automotive to aerospace industries. Processing of alumina ceramics is rather difficult due to its high degree of brittleness, hardness, low thermal diffusivity and conductivity. Rapid improvements in laser technologies in recent years make the laser among the most convenient processing tools for difficult-to-machine materials such as hardened metals, ceramics and composites. This is particularly evident as lasers have become an inexpensive and controllable alternative to conventional hole drilling methods. This paper reports theoretical and experimental results of drilling the alumina ceramic with thicknesses of 5 mm and 10.5 mm using milisecond pulsed Nd:YAG laser. Effects of the laser peak power, pulse duration, repetition rate and focal plane position have been determined using optical and Scanning Electron Microscopy (SEM) images taken from cross-sections of the drilled alumina ceramic samples. In addition to dimensional analysis of the samples, microstructural investigations have also been examined. It has been observed that, the depth of the crater can be controlled as a function of the peak power and the pulse duration for a single laser pulse application without any defect. Crater depth can be increased by increasing the number of laser pulses with some defects. In addition to experimental work, conditions have been simulated using ANYS FLUENT package providing results, which are in good agreement with the experimental results. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Laser drilling Alumina ceramic Modeling of the laser drilling
1. Introduction Alumina ceramics are used in various engineering applications such as in automobile engines, heat exchangers, electronic substrates for microwave devices and high power radio frequency electronic circuits [1]. They are also used in the fabrication of multichip modules due to the unique properties such as high electrical resistance (volume resistivity 41014 O cm), high thermal conductivity ( 28 Wm 1/K), high dielectric strength ( 10 kV/mm), and excellent thermal stability [2]. However machining of the alumina ceramic is extremely laborious and time consuming with conventional methods due to their lower order fracture toughness and thermal shock resistance [3–6]. Five different methods such as mechanical machining, chemical machining, electrical machining, radiation machining and hybrid machining, each with its own specific problems are usually used for machining of the ceramics [7].
n
Corresponding author. Tel.: þ90 262 3031061; fax: þ 90 262 3031013. E-mail addresses: [email protected], [email protected] (A. Demir).
0030-3992/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2011.11.010
Problems related to mechanical machining are surface and subsurface cracks [8], significant residual stresses [9], optimization difficulties [10] and kerf formation [11]. Problems related to chemical machining method and electrochemical machining include precision problems during production [7] and inefficiency of the process [12]. Electrical machining combines three main methods namely Electrochemical machining (ECM), Electricaldischarge machining (EDM) and Electro-chemical discharge machining (ECDM). As the electrolyte has a tendency to erode away sharp profiles, ECM is not suitable for generating sharp corners. ECDM process is inefficient because a significant portion of the total heat developed is dissipated for increasing the temperature and the corresponding material removed while machining is less [7]. Energy can be provided by an electron beam, plasma arc or by lasers in radiation machining is a noncontact machining process so, is not affected by the abrasion of the tools. When the electron beam used as energy source, the process has the drawback that the width of the machined cavity increases while machining at high speeds due to the beam defocusing effect [7]. In plasma arc machining although to obtain smaller kerf width and good surface finish components should
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closely match the size of the vacuum chamber [7]. Hybrid machining uses a combination of two or more of the above techniques for machining the ceramic. Laser drilling has rapidly become an inexpensive and controllable alternative to conventional hole drilling methods. Laser hole drilling in materials such as polyimide, ceramic, copper, nickel, brass, aluminum, borosilicate glass, quartz, rubber and composite materials offer a high accuracy, repeatability and reproducibility for the medical device industry, semiconductor manufacturing and nanotechnology support systems [3]. While laser machining is a non-contacting and abrasionless technique eliminating tool wear, machine-tool deflections, vibrations and cutting forces, it also reduces limitations to shape formation with minimal subsurface damage. Laser machining of the ceramics is therefore extensively used in the microelectronics industry for scribing and hole drilling [3]. In laser drilling application, drilling operation is realized via conversion of the photon energy into thermal energy. The hole geometry starts forming, when the material temperature exceeds the melting and/or vaporization temperature where two different mechanisms take place depending on the temperature of the material. If the laser radiation is below a certain value (typically ca. 106 W/cm2 for steels) melting occurs without vaporization. Melted material is ejected using an asist gas jet to perform the drilling process. If the laser radiation is above the threshold value, drilling operation occurs by evaporation [13]. There are lots of parameters such as pulse duration, peak power, repetition rate, focal plane position (fpp) and pulse shape involved in laser drilling and these should be controlled in order to obtain desired hole characteristics. The quality of the hole is very important for applications. The features of a good quality hole have been decribed as; taper angle, circularity, barreling and spatter formation. Taper angle defined as the angle that the wall of the drilled hole makes relative to a normal to the surface. ‘‘The difference in minimum to maximum diameter at the hole entrance is used to measure the hole circularity’’ [14]. Feret’s diameter is also used at the entrance diameter of the machined hole to define the circularity [15]. Barreling defines how parallelsided the hole is. Spatter is the resolidified material that is deposited on the workpiece surface around the hole [14]. Kacar et al. [3] examined the effects of the peak power on the hole diameter of the 10.0 mm alumina ceramic using millisecond pulsed Nd:YAG laser. They have observed that the hole diameter increases when the peak power is increased. Effects of the laser pulse duration on the hole diameters of stainless steel have been investigated by Ng and Li [16]. They have concluded that, the hole diameters are very sensitive to changes in the pulse duration, and when the pulse duration is increased, the hole diameter increases. Tunna et al. [17] have examined the effects of the laser wavelengths on the drilling of aluminum. They have stated that processing with shorter wavelengths reduces the reflectivity of the work piece to the incident laser radiation and also reduces the dimensions of the obtainable machining geometries. Low et al. [18] have reported on the characterization and analysis of the spatter deposition during the laser drilling in Nimonic 263 alloy for various laser processing parameters. Controlling of the hole geometry, elimination of the microcracks and the thermal damages and minimizing the spatter area and the recast layer are important issues [1]. When infrared lasers CO2 and Nd:YAG lasers, (10.6 mm and 1.064 mm, respectively) are applied for micromachining of the ceramic materials, a significant amount of slag remaining in the holes, on the surface and the walls of the via holes are tapered. During the micromachining with IR lasers significant amount of melting occurs, but thermal effects can be minimized using shorter wavelength or shorter pulse duration [19]. The short
wavelengths of excimer lasers (193–351 nm) correspond to the high photon energies (3.5–6.4 eV), which are efficiently absorbed by wide band gap materials such as aluminum nitride ( 6.2 eV). As a result, some excimer laser wavelengths are more highly absorbed by AlN than infrared laser wavelengths. Excimer laser machining produces holes with minimal slag and HAZ, as expected from the short pulse lengths and shallow penetration depths [20]. However, as it is reported by Lumpp et al. [20] approximately 3850 KrF (248 nm) laser pulses are needed to etch in air through 635 mm thick ceramic substrate. Ultrashort pulsed lasers have significantly brought new opportunities for the processing of ceramics. The important advantages of ultrashort laser pulses lie in their ability to produce very high peak power intensities and supply energy into a material before thermal diffusion starts. During ultrashort pulsed laser processing, the material is ablated significantly by vaporization process. Moreover, the amount of molten material and the zone affected by residual heat are importantly reduced since pulse duration is very short. [1]. Although, infrared lasers CO2 and Nd:YAG lasers have some disadvantages such as micro cracking during drilling ceramics, the long laser pulses in millisecond order can drill thick ceramics in shorter duration than the ultrashort laser pulses. This work has been carried out as a continuation of studies on laser drilling of the alumina ceramic performed by Kacar et al. [3]. In the present study, the effects of the laser parameters such as; peak power, pulse duration, focal plane position, repetition rate and the number of pulses on the ceramic substrate have been investigated. Geometrical and microstructural properties have been evaluated using optical and SEM images. In addition to experimental studies, simulations have also been carried out using a software package Fluent v6.3.26. A comparison between the experimental and the simulations have been made to evaluate the differences in the dimensions (diameter & penetration depth) versus various peak powers and pulse durations for single laser pulse. 2. Materials and methods In this study, laser drilling of the two different thicknesses (5 mm and 10.5 mm) alumina ceramic (Al2O3) plaques have been carried out using GSI lumonics JK760TR Series Laser (Class 4) system in a CNC cabin. The JK760TR series of the laser is a Nd:YAG laser that has a 0.3–50 ms pulse length and a maximum repetition rate 500 Hz with an average power of 600 W. Laser output power is delivered via a 600 mm radius fiber optic cable to the focus head at the workstation for processing. The laser beam has been focused on to the ceramic plates using 160 mm plano convex lens (see Fig. 1a.). The minimum spot size on the plates is 0.4 mm. A rectangular pulse shape has been used as seen Fig. 1(b) in the experiments. A higher peak-pulse power enhances the increase of surface temperature and induces more laser-beam energy absorbance. The alumina ceramic used in this study is produced from Alcoa A16SG without any additives. Alumina content is over 99% and used as ballistic tiles or substrates in microelectronic industry and its surface can be tailored for fabricating microelectronic thin film circuitry. To determine the affects of the laser parameters, related starting reference parameters have first been determined as; 2 ms pulse width, 6 kW peak power, 20 kHz repetition rate and 0 focal position. These parameters have then been changed systematically as shown in Table 1. The effects of the parameters have been determined as a result of three expriments. The drilled ceramic samples have been cut using a precision vertical diamond wire saw in order to image the cross-sections. Dimensions of the samples have been determined using an optical microscope with 80X magnification.
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Fig. 1. (a) Experimental setup for laser drilling, (b) pulse shape of the JK760TR Nd:YAG laser used in the experiments.
Table 1 Laser parameters used in the experiments. Peak power
Pulse duration
Number of pulses
Repetition rate
from 5 kW to 9 kW, with 1 kW increment
from 1 ms to 6 ms with 1 ms increment
from 10 to 100 with 10 pulse from 5 Hz to 20 Hz with 5 Hz steps increments
Focal plane position from 0 to 4 mm with 1 mm steps
Table 2 Parameters belong to the alumina ceramic used in the simulations. Density (kg/cm3)
Specific heat (J/gK)
Thermal conductivity (W/ Thermal expansion coefficient mK) (10 6 1/K)
Vaporization temperature (K)
Emissivity
3.97 Volumetric resistivity (Om) 1016
0.775 Relative permittivity (K0 ) 10
40 Young’s modulu (GPa)
5.4 Shear modulus (GPa)
3250 Hardness (HV1.0)
380
158
2000
60% Fracture toughness (MPaOm) 6
Fig. 2. The temperature distribution (in Kelvin) on the sample of alumina ceramic after single laser pulse application (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.).
ANSYS FLUENT 6.3 package program has been used to simulate the laser interaction with alumina ceramic plates with the parameters used in the experiments. It is a general-purpose Computational Fluid Dynamics (CFD) code based on the finite volume method [21]. The effects of the laser peak power and the pulse duration on the drilling process have been investigated for single laser pulse application. The aim of the simulation is to predict the parameters that would allow us to have the optimum hole diameter and reduce the cost.
GAMBIT was used for mesh and geometry preparation in order to import them into the Fluent. All the remaining operations like defining material properties, setting boundary conditions, choosing model equations, controlling the time and variation are carried out in the Solver. The parameters belonging to the alumina ceramic used in simulations are given in Table 2 [22]. At the beginning (t ¼0), the workpieces are considered to be at room temperatures in the simulation. At time t40, the following boundary conditions have been applied. On the top surface a
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Gaussian heat flux input has been used and the heat loss from all the surfaces of the alumina ceramic plate is considered. The heat flux input on the top surface of the work piece is given by [23]; ! ZQ r2 ð1Þ qðrÞ ¼ 2 exp 2 prq rq where q(r) is heat flux at the top of the alumina ceramic surface, Z is energy transfer efficiency (absorptivity), rq is radius of the laser beam and r is the radial distance from the beam center. Fig. 2 shows the temperature distribution on the alumina ceramic sample after a single laser pulse. The vaporization temperature of the alumina ceramic is 3250 K [24] and the area, which is shown in red color should therefore be removed, since it reached to a temperature more than the vaporization temperature. The dimensions (width and depth) of the hole in relation to the temperature distribution on the specimen can clearly be seen in Fig. 2.
3. Results and discussion During the pulsed laser ablation process, the most important parameter that affects the quality of the application is the peak power, which is equal to the pulse energy per pulse duration. Any change in the peak power is directly relevant to the energy or heat transfered to the material. High peak power is desirable in
the laser drilling process in order to remove the material by vaporization rather than melting [25]. Simulations have been carried out before experimental studies to determine the limit of sufficient peak power and pulse duration in order to get the vaporization for single laser pulse application. The effects of the laser parameters such as peak power, pulse duration and focal position have been examined on the average hole diameter, depth of hole and the microstructure of the alumina ceramic. To determine the effects of the peak power and the pulse duration on the depth of the craters and the hole diameter, different number of laser pulses have been sent onto the 5.0 mm and 10.5 mm thickness alumina ceramic plates. The mean ablation rate has been calculated by dividing the crater depth to the number of laser pulses used for producing the crater on the plates. The average hole (or crater) diameter has been determined by measuring the maximum and minimum hole diameter. In order to determine the effect of the laser peak power on the dimensions of the hole, the laser beam has been focused onto the surface of the sample between 5 kW to 9 kW with 1 kW increments at constant pulse width of 2 ms. When the peak power of the laser beam increases, hole depths and hole diameters increases as shown in Fig. 3(a,b). The same tendency has also been obtanined in the simulations (Fig. 3a). In laser drilling process, formation of the plasma just above the material surface
Diameter in experiment Diameter in simulation Depth in experiment Depth in simulation Average depth for multishot in experiment
Fig. 3. Variation of the crater dimensions as a function of the peak power at 2 ms pulse duration with zero focal plane position. (a) Graph of the experimental and calculated results, (b) optical images of the cross-section of the alumina ceramic plates, and (c) temperature distributions simulated after single laser pulse application.
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can generally lead to shielding of the work piece surfaces from the incident beam, thereby preventing the incident beam from coupling to the material surface [26]. During the application of the single laser pulse, due to the change in the laser intensity between 2.54 MW/cm2 and 5.09 MW/cm2, the shielding effect has not been observed as seen in Fig. 3(a). The depths of the craters have increased linearly with the peak power. Nedialkov et al. [27] have reported that, plasma plume begins to shield the target at intensities above 10 GW/cm2 due to the significant material ablation. However, when 10 laser pulses have been sent to the sample with 10 Hz repetition rate, crater depth/pulse has decreased when compared to the application of single laser pulse as seen in Fig. 3(a). This circumstance can be related with the absorption of the part of laser energy by the ejected material. When Fig. 3(b) has been examined, it has been observed that all the crater shapes, which have been formed with rectangular pulse shapes for different peak powers have isosceles trapezoid shapes (please see Fig. 1b). To determine the effects of the pulse duration on the dimensions of the craters, pulse duration has been increased from 1 ms to 6 ms with 1 ms increments at the fixed peak power of 6 kW. The pulse energy was increased in parallel with the pulse width with suitable energy increments in order to keep the peak power constant. When the pulse duration has been increased, crater depth and the crater diameter have also increased as seen in Fig. 4(a). After 3 ms pulse duration the crater depth and diameter have decreased relatively. Similar results have also been obtained from the simulations.
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Furthermore, after 2 ms pulse duration, the isosceles trapezoid shapes of the craters have changed to conical shapes as seen in Fig. 4(b). During the application of single laser pulse, two distinct features have emerged; first of all no spatter has not been observed and secondly the crater shapes are very clear. It has been observed that the shapes and the dimensions of the holes are strongly dependent on the number of pulses as seen from Fig. 5(a,b), as the peak power (8 kW), pulse duration (2 ms), repetition rate (10 Hz) and the focal plane position (0) has been kept constant while changing the number of pulses for 10 times between 10–100 pulses. The penetration depths of the holes have also increased when the numbers of pulses have been increased up to 50. Craters have reached to the full hole when the number of pulses reached to more than 60. The effect of the number of pulses on the hole shapes and the diameters can easily be seen, in Fig. 5(b). Formation of a huge resolidified material on the hole entrance is evident when the number of pulses is above 10, although this does not seem to be the case when the number of pulses is 10. It seems that increasing the number of pulses will increase the penetration depth of the hole while the hole diameter and the amount of the resoldified material will be constant. As a result of high recast layer, the hole diameter is normal with limited resoldification of the material, except for 10 pulses. Laser intensity (power per unit area) is very important for the laser material processing. Intensity changes as a function of the
Diameter in experiment Diameter in simulation Depth in experiment Depth in simulation Average depth for multishot in experiment
Fig. 4. Variation of the crater dimensions as a function of the pulse durtion at 6 kW peak power with zero focal plane position. (a) Graph of the experimental and calculated results, (b) optical images of the cross-section of the alumina ceramic plates, and (c) temperature distributions simulated after single laser pulse application.
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Fig. 5. Effects of (a) number of pulses versus the dimensions of the hole and, (b) number of pulses on the hole shape and resolidified material at 8 kW peak power, 2 ms pulse duration, 10 Hz repetition rate and zero focal plane position.
1400 Entrance diameter Exist diameter
Hole diameter (µm)
1200 1000 800 600 400 200 0
Fpp=0
0
Fpp=-1
0.5
1 1.5 2 2.5 3 3.5 Focal postion under the surface (mm)
Fpp=-2
Fpp=-3
4
4.5
Fpp=-4
Fig. 6. Effects of the focal position (a) versus the dimensions of the hole, (b) on the hole shape and resolidified material on front side and (c) on the hole shape and resolidified material on back side, at 8 kW peak power, 2 ms pulse duration, and 10 Hz repetition rate.
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been kept constant. Crater depths have decreased when the repetition rate has been increased, as seen in Fig. 7(a,b). After 10 Hz repetition rate laser pulses could not reach the back side of the alumina ceramic plate with a thickness of 10.5 mm. One should especially note that spatter area around the hole entrance was larger with 20 Hz repetition rate as seen in Fig. 7(b). The major problem is the melt produced by long pulsed lasermatter interaction. Irregular and incomplete melt expulsion affects the shape of the hole, producing recast layers or resolidified ejected material and may even partially close a hole during the drilling process that is open initially as discussed in [29,30]. As seen in Figs. 3(b) and 4(b), there is no resolidified (spatter) ejected material observed at the entrance region for single pulse application, which is an exact isosceles trapezoid but inside the hole a thin layer has been observed (please see Fig. 8(a–c)). Thickness of this layer has been
Dimensions (µm)
Dimensions (µm)
spot diameter on the material surface. In general, the focal plane position of the laser beam is one of the most effective factors on the hole taper and circularity [28]. The fpp is considered zero when it is set on the workpiece surface and above or below the surface is considered positive or negative, respectively. The diameter of the entrance hole does not change much at 1 and 2 mm focal plane positions when compared to the other positions as seen in Fig. 6(a). Circularity of the hole for 1 mm (feret diameter; Ent¼1.06, Ext ¼1) focal position is better than the one with 2 mm (feret diameter; Ent¼1.09, Ext¼1.3) (see Fig. 6(b,c)). It is possible to say that 1 mm focal position is a more suitable position for drilling. The superposition of a series of pulses over the same focal area is called laser percussion drilling. When the effects of the repetition rate have been determined, all the other parameters have
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Fig. 7. Effects of the pulse repetition rate (a) versus the dimensions of the hole, (b) on the hole shape and resolidified material on front side and (c) cross-section of the holes, at 8 kW peak power, 2 ms pulse duration and 2 mm fpp.
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x 1E3 Pulses/eV 1.0
0.8
0.6
0.4
0.2
0.0
0
2
4
6
8
10 keV
12
14
16
18
Fig. 8. SEM images of (a) crater drillied with single laser pulse, (b) thin layer formed in the inner walls of the crater, (c) cracks formed on the thin layer, (d) EDX results of the thin layer, and (e) resolidified materials inside the holes of the 10.5 mm thickness alumina ceramis drillied with different numbers of the laser pulses.
measured as 4 mm, and the chemical composition is the same with the alumina ceramic as seen from the EDX results given in Fig. 8(d). This thin layer has significantly smoother surface compared to the original material but cracks have formed due to tensile stresses during the cooling stage as seen in Fig. 8(b,c). During the percussion drilling, formation of the thin molten material reaching the vaporization temperature of the molten material and the sudden expansion of the vapour evaporating from the surface occurs[31]. Hence, during the single pulse application due to the equilibrium between the recoil pressure and the material inside the holes being very small, part of the material has resolidified at the walls and at the bottom of the hole. However, when the number of the laser pulses has been increased, the amount of the resolidified material at the wall of the hole has increased (see Fig. 8(e)), since the recoil pressure is not sufficient for splashing the molten material inside the hole. The resolidified material can easily be removed from the hole’s wall because of the weak bonds between the resolidified material and the walls of the hole. When the geometry of the hole with the resolidified layer has been examined, it has been observed that the diameter of the hole has increased at the exit when compared to the entrance diameter. In this region, the thickness of the resolidified layer decreased due to the multiple reflections that take place in the cavity as seen in Fig. 8(e). The thickness of the resolidified layer decreases towards the exit of the crater, with the thickness being 250 mm at the x1
point and 100 mm at the x2 point as seen in Fig. 8(e). When the hole formation is completed and the crater reaches the back side of the sample and molten material exits the back side of the hole due to the increasing number of laser pulses, hence the amount of the resolidified material at bottom region of the hole decreases and the hole shape changes to conical shape (Fig. 8(e)). The wall of the hole can be divided into two colored areas as seen in Fig. 8(e). The first one is the white area, which is similar to the main color of alumina ceramic starting from the surface until 0.5 mm under the surface, and the second area is the black or some what gray color along the hole. The black colored formation is interpreted in literature as a result of insufficient oxygen and the crystalline structure of alumina transforming from a-Al2O3 to g-Al2O3 inside the hole as a result of laser drilling process [32,33]. Alumina ceramics have a high vaporization temperature and it must be heated to temperatures higher than 3700 K for the drilling process to take place. Microstructural changes have also been seen around the processed region after the laser processing since alumina has a low thermal conduction. During the experiment, three different structures have been observed apart from the main material: a thin layer, a resolidified material inside the hole and a recast layer at the entrance region of the hole. As illustrated in Fig. 9, the grain size of the alumina ceramic inside the wall of the hole under the thin layer increases with increasing average power, since the heat-input increases at higher powers.
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Fig. 9. Microstructure of the alumina ceramic (a) without processing, (b) drilled with laser at 240 W average power, 6 kW peak power, 4 ms pulse duration, 10 Hz repetition rate and zero fpp, and (c) drilled with laser at 360 W average power, 6 kW peak power, 6 ms pulse duration, 10 Hz repetition rate and zero fpp.
Fig. 10. SEM images of the (a) hole, (b) recast layer formed entrance of the hole, (c) microstructure of the recast layer, (d) SEM image of the thin layer and (e) microstructure of the thin layer inside the hole. Laser parameters used in the drilling of this sample are 5 kW peak power, 80 W Average power, 2 ms pulse duration and 10 Hz repetition rate for a,b,c and 9 kW peak power and 190 W average power, 2 ms pulse duration and 10 Hz repetition rate for d and e.
When the number of the laser pulses was increased to more than 10 during the drilling of the 10.5 mm thick ceramic, a huge recast layer on the hole entrance has been observed as shown in Figs. 5(b) and 10(a). Unlike the untreated material, the microstructure of the recast layer consists of columnar grains with a diameter of nearly 5.7 mm and their direction is perpendicular to the hole wall as shown in Fig. 10(b,c). In addition to the columnar grains, alumina grains have also been observed between the columnar grains (see Fig. 10(c)). When the thin layer had been examined, a similar microstructure was seen consisting of columnar grains as seen in Fig. 10(d,e). Perpendicular orientation of these structures to the wall of the hole causes the microcracks (Fig. 10(d)). This structure
could have been formed due to the interaction of the nitrogen with alumina leading to the formation of AIN structures [34].
4. Conclusions The drilling of the alumina ceramics for different thicknesses (5 and 10.5 mm) has been employed using a pulsed Nd:YAG laser to determine the effect of the various laser parameters such as laser peak power, pulse duration, repetition rate, number of pulses and focal plane position on the dimensions and microsturcture of the holes. The results in general show that it is possible to control the depth of the laser drilled holes by precisely
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controlling the laser parameters. The crater depths and geometry can be controlled for single laser pulse applications by changing the pulse energy and the pulse duration. However, when the number of the pulses is increased, the crater depth also increases due to the insufficient recoil pressure inside the cavity resulting in the vaporized material being condenced again. The recast layer also increases inside and around the hole entrance of the alumina ceramic samples. In addition, due to the interaction of the laser beam with the ejected material during the multi-pulse application, the crater depth/pulse decreases when compared to the single laser pulse application. In all drilling processes for all the different combinations of the laser parameters, a layer forms inside the hole and the thickness of these layers varies depending on the numbers of the laser pulses. The EDX results show that, chemical composition of the treated alumina ceramic is identical with the untreated alumina ceramic. Columnar grains are formed in layers directed towards the hole/crater walls. This orientation accelerates the cracking due to stress of the columnar structures during the cooling stage.
Acknowledgments This work was supported by Kocaeli University Research Fund (project number 2004/133) and the Ministry of Higher Education of Iraq. References [1] Wang XC, Zheng HY, Chu PL, Tan JL, Teh KM, Liu Bryden T, et al. Femtosecond laser drilling of alumina ceramic substrates. Applied Physics A. 2010;101: 271–8. [2] Perrie W, Rushton A, Gill M, Fox P, O’Neill W. Characterization of ultrafast micro-structuring of alumina (Al2O3). Proceedings of SPIE 2005;5714:43–52. [3] Kacar E, Mutlu M, Akman E, Demir A, Candan L, Canel T, et al. Characterization of the drilling alumina ceramic using Nd:YAG pulsed laser. Journal of Materials Processing Technology 2009;209:2008–14. [4] Harimkar SP, Dahotre NB. Effect of laser fluence on surface microstructure of alumina ceramic. Advances in Applied Ceramics 2006;105:304–8. [5] Triantafyllidis D, Li L, Stott FH. Surface treatment of alumina-based ceramics using combined laser sources. Applied Surface Science 2002;186:140–4. [6] Abdul Majeeda M, Vijayaraghavana L, Malhotrab SK, Krishnamoorthya R. Characterization and machining of alumina ceramic reinforced with lanthanum phosphate. Journal of Materials Processing Technology 2009;209: 2499–507. [7] Samant AN, Dahotre NB. Laser machining of structural ceramics—A review. Journal of the European Ceramic Society 2009;29:969–93. [8] Kirchner HP. Damage penetration at elongated machining grooves in hotpressed Si3N4. Journal of the American Ceramic Society 1984;67(2):127–32. [9] Zhang B, Zheng XL, Tokura H, Yoshikawa M. Grinding induced damage in ceramics. Journal of Materials Processing Technology 2003;132:353–64. [10] Choi JP, Jeon BH, Kim BH. Chemical-assisted ultrasonic machining of glass. Journal of Materials Processing Technology 2007;191(1–3):153–6. [11] Gudimetla P, Wang J, Wong W. Kerf formation analysis in the abrasive waterjet cutting of industrial ceramics. Journal of Materials Processing Technology 2002;128(1–3):123–9. [12] Chak SK, Rao PV. Trepanning of Al2O3 by electro-chemical discharge machining (ECDM) process using abrasive electrode with pulsed DC supply. International Journal of Machine Tools and Manufacture 2007;47(14):2061–70.
[13] Salonitis K, Stournaras A, Tsoukantas G, Stavropoulos P, Chryssolouris G. A theoretical and experimental investigation on limitations of pulsed laser drilling. Journal of Materials Processing Technology 2007;183:96–103. [14] Masmiati N, Philip PK. Investigations on laser percussion drilling of some thermoplastic polymers. Journal of Materials Processing Technology 2007;185(1–3):198–203. [15] Ghoreishi M. Statistical analysis of repeatability in laser percussion drilling. International Journal of Advanced Manufacturing Technology 2006;29(1–2): 70–8. [16] Ng GKL, Li L. The effect of laser peak power and pulse width on the hole geometry repeatability in laser percussion drilling. Optics & Laser Technology 2001;3(6):393–402. [17] Tunna L, O’Neill W, Khan A, Sutcliffe C. Analysis of laser micro drilled holes through aluminium for micro-manufacturing applications. Optic and Lasers in Engineering 2005;43(9):937–50. [18] Low DKY, Li L, Byrd PJ. The effects of process parameters on spatter deposition in laser percussion drilling. Optics & Laser Technology 2000;32(5): 347–54. [19] Mendes M, Oliveira V, Vilar R, Beinhorn F, Ihlemann J, Conde O. XeCl laser ablation of Al2 O3–TiC ceramics. Applied Surface Science 2000;154– 155:29–34. [20] Lumpp JK, Allen SD. Excimer laser machining and metallization of vias in aluminum nitride. IEEE Transactions on Components, Packaging, and Manufacturing Technology, Part B 1997;20:241–6. [21] S Daftardar, Laser assisted friction stir welding: finite volume method and metaheuristic optimization, MSc thesis in Industrial Engineering, B.E, Maharashtra Institute of Technology, India. 2009 p. 18–27. [22] Auerkari P. Mechanical and physical properties of engineering alumina ceramics. Espoo: Technical research center of Finland; 1996. [23] Shihab A, Building a simulation model for the prediction of temperature distribution in pulsed laser spot welding of dissimilar low carbon steel 1020 to aluminum alloy 6061. InternationalConference on Advances in Materials and Processing Technologies. 2010. p. 24–27. [24] Raymond R. Adipic Acid. Handbook of Pharmaceutical Excipients 2009:11–2. [25] Bandyopadhyay S, Sarin Sundar JK, Sundararajan G, Joshi SV. Geometrical features and metallurgical characteristics of Nd:YAG laser drilled holes in thick IN718 and Ti–6Al–4V sheets. Journal of Materials Processing Technology 2002;127:83–95. [26] Cristoforetti G, Lorenzetti G, Benedetti PA, Tognoni E, Legnaioli S, Palleschi V. Effect of laser parameters on plasma shielding in single and double pulse configurations during the ablation of an aluminium target. Journal of Physics D: Applied Physics 2009;42:225207–14. ˇ ˇ [27] Nedialkov N, Sawczak M, Jendrzejewski R, Atanasov P, Martin M, Sliwi nski G. Analysis of surface and material modifications caused by laser drilling of AlN ceramics. Applied Surface Science 2007;254:893–7. [28] Ghoreishi M, Low DKY, Li L. Comparative statistical analysis of hole taper and circularity in laser percussion drilling. International Journal of Machine Tools and Manufacture 2002;42:985–95. [29] Abeln T, Radtke J, Dausinger F, High precision drilling with short pulsed solid state lasers. in: Proceedings of the International Congress on Applications of Lasers & Electro-Optics 1999. Laser Institute of America, Orlando, FL, USA. p. 195–203. [30] Govorkov SV, Scaggs M, Theoharidis H, High resolution microfabrication of hard materials with diode-pumped solid state (DPSS) UV laser (paper M603), in: Proceedings of the International Congress on Applications of Lasers & Electro-Optics 2001. Jacksonville, FL, USA. [31] Verhoeven K, Modelling Laser Percussion Drilling, PhD, Eindhoven University Press. 2004. p. 11–13. [32] Solodkii EN, Solodkii NF. Reasons for tinting of corundum ceramics (A Review). Glass and Ceramics 2001;58:11–2. [33] Smirnov SV, Vereshchagin VI, Dvoretskaya LV. Laser-chemical activation of alumina ceramics surface. Glass and Ceramics 2005;62:11–2. [34] Yilbas BS, Karatas C, Arif AFM, AbdulAleem BJ. Laser control melting of alumina surfaces and thermal stress analysis. Optics & Laser Technology 2011;43:858–65.
Contents lists available at SciVerse ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Experimental and theoretical investigation of the drilling of alumina ceramic using Nd:YAG pulsed laser M.M. Hanon a, E. Akman b, B. Genc Oztoprak b, M. Gunes c, Z.A. Taha a, K.I. Hajim a, E. Kacar b, O. Gundogdu c, A. Demir c,n a
Institute of Laser for Postgraduate Studies, University of Baghdad, Baghdad, Iraq Kocaeli University, Laser Technologies Research and Application Center, 41275 Yenik¨ oy, Kocaeli, Turkey c Kocaeli University, Electro-Optic Systems Engineering, 41380 Umuttepe, Kocaeli, Turkey b
a r t i c l e i n f o
abstract
Article history: Received 6 August 2011 Received in revised form 31 October 2011 Accepted 8 November 2011 Available online 21 November 2011
Alumina ceramics have found wide range of applications from semiconductors, communication technologies, medical devices, automotive to aerospace industries. Processing of alumina ceramics is rather difficult due to its high degree of brittleness, hardness, low thermal diffusivity and conductivity. Rapid improvements in laser technologies in recent years make the laser among the most convenient processing tools for difficult-to-machine materials such as hardened metals, ceramics and composites. This is particularly evident as lasers have become an inexpensive and controllable alternative to conventional hole drilling methods. This paper reports theoretical and experimental results of drilling the alumina ceramic with thicknesses of 5 mm and 10.5 mm using milisecond pulsed Nd:YAG laser. Effects of the laser peak power, pulse duration, repetition rate and focal plane position have been determined using optical and Scanning Electron Microscopy (SEM) images taken from cross-sections of the drilled alumina ceramic samples. In addition to dimensional analysis of the samples, microstructural investigations have also been examined. It has been observed that, the depth of the crater can be controlled as a function of the peak power and the pulse duration for a single laser pulse application without any defect. Crater depth can be increased by increasing the number of laser pulses with some defects. In addition to experimental work, conditions have been simulated using ANYS FLUENT package providing results, which are in good agreement with the experimental results. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Laser drilling Alumina ceramic Modeling of the laser drilling
1. Introduction Alumina ceramics are used in various engineering applications such as in automobile engines, heat exchangers, electronic substrates for microwave devices and high power radio frequency electronic circuits [1]. They are also used in the fabrication of multichip modules due to the unique properties such as high electrical resistance (volume resistivity 41014 O cm), high thermal conductivity ( 28 Wm 1/K), high dielectric strength ( 10 kV/mm), and excellent thermal stability [2]. However machining of the alumina ceramic is extremely laborious and time consuming with conventional methods due to their lower order fracture toughness and thermal shock resistance [3–6]. Five different methods such as mechanical machining, chemical machining, electrical machining, radiation machining and hybrid machining, each with its own specific problems are usually used for machining of the ceramics [7].
n
Corresponding author. Tel.: þ90 262 3031061; fax: þ 90 262 3031013. E-mail addresses: [email protected], [email protected] (A. Demir).
0030-3992/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2011.11.010
Problems related to mechanical machining are surface and subsurface cracks [8], significant residual stresses [9], optimization difficulties [10] and kerf formation [11]. Problems related to chemical machining method and electrochemical machining include precision problems during production [7] and inefficiency of the process [12]. Electrical machining combines three main methods namely Electrochemical machining (ECM), Electricaldischarge machining (EDM) and Electro-chemical discharge machining (ECDM). As the electrolyte has a tendency to erode away sharp profiles, ECM is not suitable for generating sharp corners. ECDM process is inefficient because a significant portion of the total heat developed is dissipated for increasing the temperature and the corresponding material removed while machining is less [7]. Energy can be provided by an electron beam, plasma arc or by lasers in radiation machining is a noncontact machining process so, is not affected by the abrasion of the tools. When the electron beam used as energy source, the process has the drawback that the width of the machined cavity increases while machining at high speeds due to the beam defocusing effect [7]. In plasma arc machining although to obtain smaller kerf width and good surface finish components should
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closely match the size of the vacuum chamber [7]. Hybrid machining uses a combination of two or more of the above techniques for machining the ceramic. Laser drilling has rapidly become an inexpensive and controllable alternative to conventional hole drilling methods. Laser hole drilling in materials such as polyimide, ceramic, copper, nickel, brass, aluminum, borosilicate glass, quartz, rubber and composite materials offer a high accuracy, repeatability and reproducibility for the medical device industry, semiconductor manufacturing and nanotechnology support systems [3]. While laser machining is a non-contacting and abrasionless technique eliminating tool wear, machine-tool deflections, vibrations and cutting forces, it also reduces limitations to shape formation with minimal subsurface damage. Laser machining of the ceramics is therefore extensively used in the microelectronics industry for scribing and hole drilling [3]. In laser drilling application, drilling operation is realized via conversion of the photon energy into thermal energy. The hole geometry starts forming, when the material temperature exceeds the melting and/or vaporization temperature where two different mechanisms take place depending on the temperature of the material. If the laser radiation is below a certain value (typically ca. 106 W/cm2 for steels) melting occurs without vaporization. Melted material is ejected using an asist gas jet to perform the drilling process. If the laser radiation is above the threshold value, drilling operation occurs by evaporation [13]. There are lots of parameters such as pulse duration, peak power, repetition rate, focal plane position (fpp) and pulse shape involved in laser drilling and these should be controlled in order to obtain desired hole characteristics. The quality of the hole is very important for applications. The features of a good quality hole have been decribed as; taper angle, circularity, barreling and spatter formation. Taper angle defined as the angle that the wall of the drilled hole makes relative to a normal to the surface. ‘‘The difference in minimum to maximum diameter at the hole entrance is used to measure the hole circularity’’ [14]. Feret’s diameter is also used at the entrance diameter of the machined hole to define the circularity [15]. Barreling defines how parallelsided the hole is. Spatter is the resolidified material that is deposited on the workpiece surface around the hole [14]. Kacar et al. [3] examined the effects of the peak power on the hole diameter of the 10.0 mm alumina ceramic using millisecond pulsed Nd:YAG laser. They have observed that the hole diameter increases when the peak power is increased. Effects of the laser pulse duration on the hole diameters of stainless steel have been investigated by Ng and Li [16]. They have concluded that, the hole diameters are very sensitive to changes in the pulse duration, and when the pulse duration is increased, the hole diameter increases. Tunna et al. [17] have examined the effects of the laser wavelengths on the drilling of aluminum. They have stated that processing with shorter wavelengths reduces the reflectivity of the work piece to the incident laser radiation and also reduces the dimensions of the obtainable machining geometries. Low et al. [18] have reported on the characterization and analysis of the spatter deposition during the laser drilling in Nimonic 263 alloy for various laser processing parameters. Controlling of the hole geometry, elimination of the microcracks and the thermal damages and minimizing the spatter area and the recast layer are important issues [1]. When infrared lasers CO2 and Nd:YAG lasers, (10.6 mm and 1.064 mm, respectively) are applied for micromachining of the ceramic materials, a significant amount of slag remaining in the holes, on the surface and the walls of the via holes are tapered. During the micromachining with IR lasers significant amount of melting occurs, but thermal effects can be minimized using shorter wavelength or shorter pulse duration [19]. The short
wavelengths of excimer lasers (193–351 nm) correspond to the high photon energies (3.5–6.4 eV), which are efficiently absorbed by wide band gap materials such as aluminum nitride ( 6.2 eV). As a result, some excimer laser wavelengths are more highly absorbed by AlN than infrared laser wavelengths. Excimer laser machining produces holes with minimal slag and HAZ, as expected from the short pulse lengths and shallow penetration depths [20]. However, as it is reported by Lumpp et al. [20] approximately 3850 KrF (248 nm) laser pulses are needed to etch in air through 635 mm thick ceramic substrate. Ultrashort pulsed lasers have significantly brought new opportunities for the processing of ceramics. The important advantages of ultrashort laser pulses lie in their ability to produce very high peak power intensities and supply energy into a material before thermal diffusion starts. During ultrashort pulsed laser processing, the material is ablated significantly by vaporization process. Moreover, the amount of molten material and the zone affected by residual heat are importantly reduced since pulse duration is very short. [1]. Although, infrared lasers CO2 and Nd:YAG lasers have some disadvantages such as micro cracking during drilling ceramics, the long laser pulses in millisecond order can drill thick ceramics in shorter duration than the ultrashort laser pulses. This work has been carried out as a continuation of studies on laser drilling of the alumina ceramic performed by Kacar et al. [3]. In the present study, the effects of the laser parameters such as; peak power, pulse duration, focal plane position, repetition rate and the number of pulses on the ceramic substrate have been investigated. Geometrical and microstructural properties have been evaluated using optical and SEM images. In addition to experimental studies, simulations have also been carried out using a software package Fluent v6.3.26. A comparison between the experimental and the simulations have been made to evaluate the differences in the dimensions (diameter & penetration depth) versus various peak powers and pulse durations for single laser pulse. 2. Materials and methods In this study, laser drilling of the two different thicknesses (5 mm and 10.5 mm) alumina ceramic (Al2O3) plaques have been carried out using GSI lumonics JK760TR Series Laser (Class 4) system in a CNC cabin. The JK760TR series of the laser is a Nd:YAG laser that has a 0.3–50 ms pulse length and a maximum repetition rate 500 Hz with an average power of 600 W. Laser output power is delivered via a 600 mm radius fiber optic cable to the focus head at the workstation for processing. The laser beam has been focused on to the ceramic plates using 160 mm plano convex lens (see Fig. 1a.). The minimum spot size on the plates is 0.4 mm. A rectangular pulse shape has been used as seen Fig. 1(b) in the experiments. A higher peak-pulse power enhances the increase of surface temperature and induces more laser-beam energy absorbance. The alumina ceramic used in this study is produced from Alcoa A16SG without any additives. Alumina content is over 99% and used as ballistic tiles or substrates in microelectronic industry and its surface can be tailored for fabricating microelectronic thin film circuitry. To determine the affects of the laser parameters, related starting reference parameters have first been determined as; 2 ms pulse width, 6 kW peak power, 20 kHz repetition rate and 0 focal position. These parameters have then been changed systematically as shown in Table 1. The effects of the parameters have been determined as a result of three expriments. The drilled ceramic samples have been cut using a precision vertical diamond wire saw in order to image the cross-sections. Dimensions of the samples have been determined using an optical microscope with 80X magnification.
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Fig. 1. (a) Experimental setup for laser drilling, (b) pulse shape of the JK760TR Nd:YAG laser used in the experiments.
Table 1 Laser parameters used in the experiments. Peak power
Pulse duration
Number of pulses
Repetition rate
from 5 kW to 9 kW, with 1 kW increment
from 1 ms to 6 ms with 1 ms increment
from 10 to 100 with 10 pulse from 5 Hz to 20 Hz with 5 Hz steps increments
Focal plane position from 0 to 4 mm with 1 mm steps
Table 2 Parameters belong to the alumina ceramic used in the simulations. Density (kg/cm3)
Specific heat (J/gK)
Thermal conductivity (W/ Thermal expansion coefficient mK) (10 6 1/K)
Vaporization temperature (K)
Emissivity
3.97 Volumetric resistivity (Om) 1016
0.775 Relative permittivity (K0 ) 10
40 Young’s modulu (GPa)
5.4 Shear modulus (GPa)
3250 Hardness (HV1.0)
380
158
2000
60% Fracture toughness (MPaOm) 6
Fig. 2. The temperature distribution (in Kelvin) on the sample of alumina ceramic after single laser pulse application (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.).
ANSYS FLUENT 6.3 package program has been used to simulate the laser interaction with alumina ceramic plates with the parameters used in the experiments. It is a general-purpose Computational Fluid Dynamics (CFD) code based on the finite volume method [21]. The effects of the laser peak power and the pulse duration on the drilling process have been investigated for single laser pulse application. The aim of the simulation is to predict the parameters that would allow us to have the optimum hole diameter and reduce the cost.
GAMBIT was used for mesh and geometry preparation in order to import them into the Fluent. All the remaining operations like defining material properties, setting boundary conditions, choosing model equations, controlling the time and variation are carried out in the Solver. The parameters belonging to the alumina ceramic used in simulations are given in Table 2 [22]. At the beginning (t ¼0), the workpieces are considered to be at room temperatures in the simulation. At time t40, the following boundary conditions have been applied. On the top surface a
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Gaussian heat flux input has been used and the heat loss from all the surfaces of the alumina ceramic plate is considered. The heat flux input on the top surface of the work piece is given by [23]; ! ZQ r2 ð1Þ qðrÞ ¼ 2 exp 2 prq rq where q(r) is heat flux at the top of the alumina ceramic surface, Z is energy transfer efficiency (absorptivity), rq is radius of the laser beam and r is the radial distance from the beam center. Fig. 2 shows the temperature distribution on the alumina ceramic sample after a single laser pulse. The vaporization temperature of the alumina ceramic is 3250 K [24] and the area, which is shown in red color should therefore be removed, since it reached to a temperature more than the vaporization temperature. The dimensions (width and depth) of the hole in relation to the temperature distribution on the specimen can clearly be seen in Fig. 2.
3. Results and discussion During the pulsed laser ablation process, the most important parameter that affects the quality of the application is the peak power, which is equal to the pulse energy per pulse duration. Any change in the peak power is directly relevant to the energy or heat transfered to the material. High peak power is desirable in
the laser drilling process in order to remove the material by vaporization rather than melting [25]. Simulations have been carried out before experimental studies to determine the limit of sufficient peak power and pulse duration in order to get the vaporization for single laser pulse application. The effects of the laser parameters such as peak power, pulse duration and focal position have been examined on the average hole diameter, depth of hole and the microstructure of the alumina ceramic. To determine the effects of the peak power and the pulse duration on the depth of the craters and the hole diameter, different number of laser pulses have been sent onto the 5.0 mm and 10.5 mm thickness alumina ceramic plates. The mean ablation rate has been calculated by dividing the crater depth to the number of laser pulses used for producing the crater on the plates. The average hole (or crater) diameter has been determined by measuring the maximum and minimum hole diameter. In order to determine the effect of the laser peak power on the dimensions of the hole, the laser beam has been focused onto the surface of the sample between 5 kW to 9 kW with 1 kW increments at constant pulse width of 2 ms. When the peak power of the laser beam increases, hole depths and hole diameters increases as shown in Fig. 3(a,b). The same tendency has also been obtanined in the simulations (Fig. 3a). In laser drilling process, formation of the plasma just above the material surface
Diameter in experiment Diameter in simulation Depth in experiment Depth in simulation Average depth for multishot in experiment
Fig. 3. Variation of the crater dimensions as a function of the peak power at 2 ms pulse duration with zero focal plane position. (a) Graph of the experimental and calculated results, (b) optical images of the cross-section of the alumina ceramic plates, and (c) temperature distributions simulated after single laser pulse application.
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can generally lead to shielding of the work piece surfaces from the incident beam, thereby preventing the incident beam from coupling to the material surface [26]. During the application of the single laser pulse, due to the change in the laser intensity between 2.54 MW/cm2 and 5.09 MW/cm2, the shielding effect has not been observed as seen in Fig. 3(a). The depths of the craters have increased linearly with the peak power. Nedialkov et al. [27] have reported that, plasma plume begins to shield the target at intensities above 10 GW/cm2 due to the significant material ablation. However, when 10 laser pulses have been sent to the sample with 10 Hz repetition rate, crater depth/pulse has decreased when compared to the application of single laser pulse as seen in Fig. 3(a). This circumstance can be related with the absorption of the part of laser energy by the ejected material. When Fig. 3(b) has been examined, it has been observed that all the crater shapes, which have been formed with rectangular pulse shapes for different peak powers have isosceles trapezoid shapes (please see Fig. 1b). To determine the effects of the pulse duration on the dimensions of the craters, pulse duration has been increased from 1 ms to 6 ms with 1 ms increments at the fixed peak power of 6 kW. The pulse energy was increased in parallel with the pulse width with suitable energy increments in order to keep the peak power constant. When the pulse duration has been increased, crater depth and the crater diameter have also increased as seen in Fig. 4(a). After 3 ms pulse duration the crater depth and diameter have decreased relatively. Similar results have also been obtained from the simulations.
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Furthermore, after 2 ms pulse duration, the isosceles trapezoid shapes of the craters have changed to conical shapes as seen in Fig. 4(b). During the application of single laser pulse, two distinct features have emerged; first of all no spatter has not been observed and secondly the crater shapes are very clear. It has been observed that the shapes and the dimensions of the holes are strongly dependent on the number of pulses as seen from Fig. 5(a,b), as the peak power (8 kW), pulse duration (2 ms), repetition rate (10 Hz) and the focal plane position (0) has been kept constant while changing the number of pulses for 10 times between 10–100 pulses. The penetration depths of the holes have also increased when the numbers of pulses have been increased up to 50. Craters have reached to the full hole when the number of pulses reached to more than 60. The effect of the number of pulses on the hole shapes and the diameters can easily be seen, in Fig. 5(b). Formation of a huge resolidified material on the hole entrance is evident when the number of pulses is above 10, although this does not seem to be the case when the number of pulses is 10. It seems that increasing the number of pulses will increase the penetration depth of the hole while the hole diameter and the amount of the resoldified material will be constant. As a result of high recast layer, the hole diameter is normal with limited resoldification of the material, except for 10 pulses. Laser intensity (power per unit area) is very important for the laser material processing. Intensity changes as a function of the
Diameter in experiment Diameter in simulation Depth in experiment Depth in simulation Average depth for multishot in experiment
Fig. 4. Variation of the crater dimensions as a function of the pulse durtion at 6 kW peak power with zero focal plane position. (a) Graph of the experimental and calculated results, (b) optical images of the cross-section of the alumina ceramic plates, and (c) temperature distributions simulated after single laser pulse application.
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Fig. 5. Effects of (a) number of pulses versus the dimensions of the hole and, (b) number of pulses on the hole shape and resolidified material at 8 kW peak power, 2 ms pulse duration, 10 Hz repetition rate and zero focal plane position.
1400 Entrance diameter Exist diameter
Hole diameter (µm)
1200 1000 800 600 400 200 0
Fpp=0
0
Fpp=-1
0.5
1 1.5 2 2.5 3 3.5 Focal postion under the surface (mm)
Fpp=-2
Fpp=-3
4
4.5
Fpp=-4
Fig. 6. Effects of the focal position (a) versus the dimensions of the hole, (b) on the hole shape and resolidified material on front side and (c) on the hole shape and resolidified material on back side, at 8 kW peak power, 2 ms pulse duration, and 10 Hz repetition rate.
M.M. Hanon et al. / Optics & Laser Technology 44 (2012) 913–922
been kept constant. Crater depths have decreased when the repetition rate has been increased, as seen in Fig. 7(a,b). After 10 Hz repetition rate laser pulses could not reach the back side of the alumina ceramic plate with a thickness of 10.5 mm. One should especially note that spatter area around the hole entrance was larger with 20 Hz repetition rate as seen in Fig. 7(b). The major problem is the melt produced by long pulsed lasermatter interaction. Irregular and incomplete melt expulsion affects the shape of the hole, producing recast layers or resolidified ejected material and may even partially close a hole during the drilling process that is open initially as discussed in [29,30]. As seen in Figs. 3(b) and 4(b), there is no resolidified (spatter) ejected material observed at the entrance region for single pulse application, which is an exact isosceles trapezoid but inside the hole a thin layer has been observed (please see Fig. 8(a–c)). Thickness of this layer has been
Dimensions (µm)
Dimensions (µm)
spot diameter on the material surface. In general, the focal plane position of the laser beam is one of the most effective factors on the hole taper and circularity [28]. The fpp is considered zero when it is set on the workpiece surface and above or below the surface is considered positive or negative, respectively. The diameter of the entrance hole does not change much at 1 and 2 mm focal plane positions when compared to the other positions as seen in Fig. 6(a). Circularity of the hole for 1 mm (feret diameter; Ent¼1.06, Ext ¼1) focal position is better than the one with 2 mm (feret diameter; Ent¼1.09, Ext¼1.3) (see Fig. 6(b,c)). It is possible to say that 1 mm focal position is a more suitable position for drilling. The superposition of a series of pulses over the same focal area is called laser percussion drilling. When the effects of the repetition rate have been determined, all the other parameters have
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Fig. 7. Effects of the pulse repetition rate (a) versus the dimensions of the hole, (b) on the hole shape and resolidified material on front side and (c) cross-section of the holes, at 8 kW peak power, 2 ms pulse duration and 2 mm fpp.
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x 1E3 Pulses/eV 1.0
0.8
0.6
0.4
0.2
0.0
0
2
4
6
8
10 keV
12
14
16
18
Fig. 8. SEM images of (a) crater drillied with single laser pulse, (b) thin layer formed in the inner walls of the crater, (c) cracks formed on the thin layer, (d) EDX results of the thin layer, and (e) resolidified materials inside the holes of the 10.5 mm thickness alumina ceramis drillied with different numbers of the laser pulses.
measured as 4 mm, and the chemical composition is the same with the alumina ceramic as seen from the EDX results given in Fig. 8(d). This thin layer has significantly smoother surface compared to the original material but cracks have formed due to tensile stresses during the cooling stage as seen in Fig. 8(b,c). During the percussion drilling, formation of the thin molten material reaching the vaporization temperature of the molten material and the sudden expansion of the vapour evaporating from the surface occurs[31]. Hence, during the single pulse application due to the equilibrium between the recoil pressure and the material inside the holes being very small, part of the material has resolidified at the walls and at the bottom of the hole. However, when the number of the laser pulses has been increased, the amount of the resolidified material at the wall of the hole has increased (see Fig. 8(e)), since the recoil pressure is not sufficient for splashing the molten material inside the hole. The resolidified material can easily be removed from the hole’s wall because of the weak bonds between the resolidified material and the walls of the hole. When the geometry of the hole with the resolidified layer has been examined, it has been observed that the diameter of the hole has increased at the exit when compared to the entrance diameter. In this region, the thickness of the resolidified layer decreased due to the multiple reflections that take place in the cavity as seen in Fig. 8(e). The thickness of the resolidified layer decreases towards the exit of the crater, with the thickness being 250 mm at the x1
point and 100 mm at the x2 point as seen in Fig. 8(e). When the hole formation is completed and the crater reaches the back side of the sample and molten material exits the back side of the hole due to the increasing number of laser pulses, hence the amount of the resolidified material at bottom region of the hole decreases and the hole shape changes to conical shape (Fig. 8(e)). The wall of the hole can be divided into two colored areas as seen in Fig. 8(e). The first one is the white area, which is similar to the main color of alumina ceramic starting from the surface until 0.5 mm under the surface, and the second area is the black or some what gray color along the hole. The black colored formation is interpreted in literature as a result of insufficient oxygen and the crystalline structure of alumina transforming from a-Al2O3 to g-Al2O3 inside the hole as a result of laser drilling process [32,33]. Alumina ceramics have a high vaporization temperature and it must be heated to temperatures higher than 3700 K for the drilling process to take place. Microstructural changes have also been seen around the processed region after the laser processing since alumina has a low thermal conduction. During the experiment, three different structures have been observed apart from the main material: a thin layer, a resolidified material inside the hole and a recast layer at the entrance region of the hole. As illustrated in Fig. 9, the grain size of the alumina ceramic inside the wall of the hole under the thin layer increases with increasing average power, since the heat-input increases at higher powers.
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Fig. 9. Microstructure of the alumina ceramic (a) without processing, (b) drilled with laser at 240 W average power, 6 kW peak power, 4 ms pulse duration, 10 Hz repetition rate and zero fpp, and (c) drilled with laser at 360 W average power, 6 kW peak power, 6 ms pulse duration, 10 Hz repetition rate and zero fpp.
Fig. 10. SEM images of the (a) hole, (b) recast layer formed entrance of the hole, (c) microstructure of the recast layer, (d) SEM image of the thin layer and (e) microstructure of the thin layer inside the hole. Laser parameters used in the drilling of this sample are 5 kW peak power, 80 W Average power, 2 ms pulse duration and 10 Hz repetition rate for a,b,c and 9 kW peak power and 190 W average power, 2 ms pulse duration and 10 Hz repetition rate for d and e.
When the number of the laser pulses was increased to more than 10 during the drilling of the 10.5 mm thick ceramic, a huge recast layer on the hole entrance has been observed as shown in Figs. 5(b) and 10(a). Unlike the untreated material, the microstructure of the recast layer consists of columnar grains with a diameter of nearly 5.7 mm and their direction is perpendicular to the hole wall as shown in Fig. 10(b,c). In addition to the columnar grains, alumina grains have also been observed between the columnar grains (see Fig. 10(c)). When the thin layer had been examined, a similar microstructure was seen consisting of columnar grains as seen in Fig. 10(d,e). Perpendicular orientation of these structures to the wall of the hole causes the microcracks (Fig. 10(d)). This structure
could have been formed due to the interaction of the nitrogen with alumina leading to the formation of AIN structures [34].
4. Conclusions The drilling of the alumina ceramics for different thicknesses (5 and 10.5 mm) has been employed using a pulsed Nd:YAG laser to determine the effect of the various laser parameters such as laser peak power, pulse duration, repetition rate, number of pulses and focal plane position on the dimensions and microsturcture of the holes. The results in general show that it is possible to control the depth of the laser drilled holes by precisely
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controlling the laser parameters. The crater depths and geometry can be controlled for single laser pulse applications by changing the pulse energy and the pulse duration. However, when the number of the pulses is increased, the crater depth also increases due to the insufficient recoil pressure inside the cavity resulting in the vaporized material being condenced again. The recast layer also increases inside and around the hole entrance of the alumina ceramic samples. In addition, due to the interaction of the laser beam with the ejected material during the multi-pulse application, the crater depth/pulse decreases when compared to the single laser pulse application. In all drilling processes for all the different combinations of the laser parameters, a layer forms inside the hole and the thickness of these layers varies depending on the numbers of the laser pulses. The EDX results show that, chemical composition of the treated alumina ceramic is identical with the untreated alumina ceramic. Columnar grains are formed in layers directed towards the hole/crater walls. This orientation accelerates the cracking due to stress of the columnar structures during the cooling stage.
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