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Laser cutting of high-vitrified ceramic materials:

development of a method using a Nd:YAG laser to avoid

catastrophic breakdown

J. Pascual-Cosp

a

, A.J. Ramı´rez del Valle

a

, J. Garcı´a-Fortea

a

, P.J. Sa´nchez-Soto

b,

*

a

Departamento de Ingenierıá Civil, de Materiales y Fabricacioń, E.T.S. Ingenierıá Industrial, Universidad de Ma´laga, Plasa El Ejido s/n, 29013-Ma´laga, Spain

b

Instituto de Ciencia de Materiales de Sevilla, Centro Mixto Consejo Superior de Investigaciones Cientı´ficas (C.S.I.C.) Universidad de Sevilla, c/Ame´rico Vespucio s/n, Isla de la Cartuja, 41092 Seville, Spain

Received 14 May 2001; accepted 28 September 2001

Abstract

The laser cutting of a high-vitrified ceramic materials, fine porcelain stoneware tile, was studied using a Nd:YAG laser. Several conditions of laser cutting were tested and the optimum conditions of working operation were provided. The workpiece is preheated at 1000j_{C to avoid any adverse effect of the high amount of glassy phase and silica which are present. The}

material was characterized by SEM-EDX before and after the laser cutting process to assess the variations produced and the evolution of cracks induced during the operation. The results allowed to develop a procedure to avoid the catastrophic breakdown of this material under the action of the laser. A localized thermal shock is produced due to the incidence of the cold cutting gas-jet during the process. This caused the formation of cracks whose advance was limited by the cooling of the melted material which experienced viscoplastic fluency. This material behaves like an element of crack anchorage and thus it avoids a catastrophic breakdown. The process conditions and its technological interests are discussed.D2002 Elsevier Science B.V. All rights reserved.

Keywords:Nd:YAG laser; High-vitrified ceramics; Crack; Viscoplastic behaviour; Local thermal shock; Crack anchorage

1. Introduction

The cutting of ceramics and glasses has shown several problems using diamond-saw, hydrodynamic or ultrasonic machining. Cutting is usually followed by a grinding operation to remove damaged and residually

stressed materials. All these operations are time-con-suming and expensive in the processing of a particular material shape, apart from acustic contamination and material wastes. This is especially important when complex geometries must be created. During the last few years, laser technology has been developed for the cutting of metals, alloys[1 – 6]and glasses[7,8]. Thus, intricate shapes and thick sections can be cut. Using this point source of energy, no contact is made with the material. Hence, thermal damage can be kept to a minimum and superior edge quality is obtained as

* _{Corresponding author. Tel.: 89527; fax:}

+34-9544-60165.

E-mail address:[email protected] (P.J. Sa´nchez-Soto).

www.elsevier.com/locate/matlet Materials Letters 55 (2002) 274 – 280

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compared with conventional methods. The necessary stress field, i.e., compression near the crack and tension away from it, can be created by moving the concen-trated source of heat (laser or other heat source) just ahead of the crack [7]. Moreover, the laser cutting process produces narrower kerf widths than those achievable with mechanical cutting, resulting in less material wastes[8].

The quality of the cut resulting from a laser cutting of a given material depends on several factors, mainly the properties of the focussed laser beam, the cutting speed, the gas-jet, and the local temperature of the workpiece[1 – 8]. This paper reports on a preliminary study of laser cutting of a high-vitrified ceramic material, commonly known as fine porcelain stone-ware tile[9]. The authors have developed, after several test runs, the system and processing conditions to avoid catastrophic breakdown of this ceramic material using a Nd:YAG laser. The material is characterized before and after the cutting process to assess the variations produced and the evolution of cracks induced during the operation.

2. Experimental

A high-vitrified ceramic material (porosity less than 1%), namely fine porcelain stoneware tile (‘‘gres porcellanato’’) was used in this study[9]. This mate-rial is commercially available in workpieces of 8.0 – 7.8 mm of thickness (source: Taugres ceramics, Cas-tello´n, Spain). The phase analysis of this material by X-ray diffraction shows the presence of quartz as the main crystalline phase, besides nepheline (Na2O Al2O3SiO2) and mullite (3Al2O32SiO2) both as minority crystalline phases in a glassy matrix higher than 70 vol%. This product has commercial interest and sometimes can be glazed.

A Rofin – Sinar Nd:YAG laser, model RSY-1000 P was used in the present work. It is a solid-state pulsed laser especially indicated for processing industrial materials. This device works at a wavelength of 1.06

Am and a nominal power output of 1200 W. The shield gas was a mixture (industrial compressed air) of nitro-gen (99.99%) and oxynitro-gen (99.99%) in a proportion 80/ 20 (vol%). The gas flow was not coaxial to the nozzle laser. The gas pressure was maintained constant at a value of 20 bar (cold cutting gas-jet). The device is on a

worktable and controlled using an automatic system Siemens CNC, Model SINUMERIK-810N. The work-piece is introduced in a furnace, located outside the CNC table used with the laser cutter, and preheated at 1000j_{C. Then, the workpiece is positioned at 0.2 mm} of the laser beam for test runs.

Fig. 1. SEM micrographs of the high-vitrified ceramic material at (a) low and (b) high magnifications.

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The dilatometric study of the high-vitrified mate-rial was conducted using a Bahr dilatometer at a heating rate of 10j_{C/min and 950} j_{C as maximum} temperature. The material was examined by scanning electron microscopy (SEM) using a JEOL system, model JSM-6400 at 120 – 140 mA and 20 kV, using 30 kV exceptionally for photographic recording. The SEM is equipped with a Energy Dispersive X-ray (EDX) Analyzer Link system for chemical analysis.

3. Results and discussion

Fig. 1 shows two SEM micrographs of the high-vitrified ceramic material. The high content of glassy phase with some residual closed porosity can be observed at low magnification(Fig. 1a). The average chemical composition of the glassy matrix determined by EDX is (in wt.%): SiO270.73, Al2O319.46, Fe2O3 0.80, CaO 0.38, K2O 1.33, and Na2O 7.31. At higher magnification(Fig. 1b), the characteristic morphology of mullite as tabular crystals and quartz particles can sometimes be observed.

We tested several conditions of laser cutting. The maximum pulse energy was 120 J and the working frequency was from 0 to 500 Hz, with a phase retard within the two Nd lamps of 0.2 ms. The laser can be used in pulsed beam mode, using a range of pulse frequencies with programmed energies between 0.3 and 20 ms, profiles of 20 sections and using focalized lens. The frequency was changed between 176 and 240 Hz and then the power output changed between 940 and 970 W and the cutting speed 0.10 – 0.20 m/ min. After several previous experimental runs on the workpieces (8.0 mm thickness), we found an optimum long focal length of 120 mm, frequency of 240 Hz at a

power output of 945 W. To select these conditions, we worked with a single sector which represents a spatial energy distribution of 20% and a pulse length of 0.9 ms. In summary,Table 1shows some selected results corresponding to several runs of the workpieces. It was necessary to reach a compromise between the

Table 1

Selected results of laser-cutting runs of workpieces of the high-vitrified material

Run number P (W) f (Hz) tp (ms) ST (mm) V (m/min) 1 950 240 0.6 8.0 0.10 2 945 193.5 1.1 8.0 0.10 3 945 176.4 0.8 7.8 0.20 4 970 176.4 0.6 8.0 0.15 5 945 240 0.9 8.0 0.10 Symbols meaning: P = laser beam power; f = pulse frequency; tp = pulse length; ST = substrate thickness; V = cutting speed. Gas pressure = 20 bar in all run experiences.

Fig. 2. Dilatometric curve of the high-vitrified ceramic material.

Fig. 3. Cross-sectional SEM micrograph of a workpiece after laser cutting located 5 mm from the cutting zone (a) and the corresponding EDX spectrum (b).

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frequency and the laser beam power to achieve laser cuts at adequate cutting speeds. The gas pressure was maintained constant at a value of 20 bar (cold cutting gas-jet). It was found that the use of a gas mixture N2/ O2at that pressure was also adequate for removing molten materials produced in the cutting process without any adverse effects.

Fig. 2allows to observe the dimensional change at 573j_{C of the high-vitrified material, which is} associ-ated with the presence of quartz. There are some other important inversion changes of silica phases (tridimite and cristobalite), as reported in the literature after heating and/or cooling[10,11]. The last change (tem-perature 1470j_{C) is associated with cristobalite, with} final melting of silica at 1713 j_{C. The laser cutting} produces a local temperature higher than this value[2 –

4]. Then, to avoid the volumetric changes of quartz and cristobalite, resultant in different linear expansion – contraction rates, the workpiece must be preheated in a furnace at 1000 j_{C. The local stress due to the} presence and expansion of these SiO2phases is not observed. This treatment also favoured a relatively high thermal level and thus the alkaline aluminosilicate phases can be more fluid. Furthermore, the thermal gradient in the workpiece can be accurately low, with thermal stresses less appreciated and without cracking. Consequently, the damage was minimal and this also ensured a good finishing after laser cutting.

The study of the microstructure produced after laser cutting was performed by SEM-EDX.Fig. 3shows a cross-sectional SEM micrograph of a workpiece after laser cutting (5 mm from the cutting zone) and the

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corresponding EDX spectrum. A zone of crack growth along the workpiece could be detected as well as the formation of some crystalline structures. The progres-sion of the laser cutting produced the fluency of melted aluminosilicate containing elements such as sodium, potassium and calcium. This can be deduced fromFig. 4, where the EDX dot-map image of Si, Al, O and Na is shown. Taking into account the information of Na2O – SiO2– Al2O3phase diagram[12,13], the formation of first liquid phase is at 732j_{C. In the present case, the} presence of minority elements, such as Ca and K, will lower this temperature, resulting in melted liquids of different viscosities. The laser cutting produced a strong compression and induced stresses in the work-piece, resulting in cracks which progressed along the same path. The cracks find this liquid and their propagation was not stopped. It can also induce the crystallization of aluminosilicates after slow cooling, because there is enough time for an ionic structural arrangement, and thus, some small crystals could be observed inFig. 3.

Fig. 5shows a cross-sectional SEM-EDX examina-tion of a zone where a crack is found very close (approximately 1 mm) to the thermally affected zone of laser cutting. A zone of anchorage produced by the action of viscoplastic fluency of melted aluminosili-cates could be observed, which impeded the crack propagation. This region was at temperatures estimated as high as 1100 – 1200 j_{C and the presence of Ca} produced a liquid phase of higher viscosity, as com-pared with alkaline aluminosilicates alone[13].Fig. 6 shows the EDX dot-map images of Si, Al, O, Na and Ca of this zone examined by SEM (Fig. 5). There is a thermal shock located on the workpiece in the laser cutting zone thermally affected. The thermal shock is due to the incidence of the cold cutting gas-jet during the process. The gas flow was not coaxial to the nozzle laser and it influences a surface zone larger than the cutting zone. This causes the formation of cracks whose advance is limited by the cooling of the material, with composition deduced fromFig. 6, which experi-enced viscoplastic fluency. This material behaves like an element of crack anchorage. The zone appears completely vitrified after the laser cutting process (Fig. 5) and it is composed of aluminosilicates (Fig. 6). This phenomenon has technological interests from two points of views: (a) it is a system which contributes with important stresses to the closing of cracks, and (b)

the interest of further study to achieve a basic knowl-edge of the fluency/temperature behaviour in these materials.

4. Summary and conclusions

In the present paper, the laser cutting of a high-vitrified ceramic material (fine porcelain stoneware tile) was studied using a Nd:YAG laser. Several con-ditions of laser cutting were tested and the optimum conditions of working operation were provided. It was necessary to previously heat the workpiece at 1000j_C to avoid any adverse phase change effect associated with the presence of a high amount of glassy phase and SiO2. A located thermal shock is produced due to the incidence of the cold cutting gas-jet during the process. This caused the formation of cracks whose advance

Fig. 5. Cross-sectional SEM examination of a zone where a crack is found at approximately 1 mm of the thermal affected zone of laser cutting (a) and the corresponding EDX spectrum (b).

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was limited by the cooling of the material that expe-rienced viscoplastic fluency, mainly melted alumino-silicates. This material behaves like an element of crack anchorage and thus the catastrophic breakdown is avoided. The present results allowed to develop a procedure to avoid this adverse effect under the action of the laser.

Finally, it should be noted that the precedent findings can be extended to ceramics of very low porosity (less than 1%), or sintered in the presence of the glassy phase[14], resulting in high-vitrified mate-rials with technological applications.

References

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[3] N. Rajendram, M.B. Pate, in: S.L. Ream (Ed.), Proc. 6th Int. Congress on Applications of Lasers and Electroptics, 1987, p. 129, San Diego, California.

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[6] G. Chryssolouris, J. Bredt, S. Kordas, E. Wilson, J. Eng. Ind. 2 (1998) 110.

[7] Yu.V. Khlopkov, Steklo Keram. 7 – 8 (1994) 33. [8] I. Black, Am. Ceram. Soc. Bull. 77 (1998) 53.

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[10] J. Hlavac, The Technology of Glass and Ceramics: An Intro-duction, Elsevier, New York, 1983.

[11] J.M. Ferna´ndez Navarro, ‘‘El Vidrio’’, Ed. C.S.I.C. and Fun-dacio´n Centro Nacional del Vidrio, 2nd edn., Madrid (1991). [12] E.F. Osborn, A. Muan, Phase Diagrams of Oxide Systems for Ceramists, The American Ceramic Society, USA, 1960, Fig. 501, System Na2O – Al2O3– SiO2.

[13] M.O. Bjorn, D. Virgo, F.A. Seifert, Am. Mineral. 70 (1985) 88.

[14] R.M. German, Liquid Phase Sintering, Plenum Press, New York, 1985.