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Laser treatment of white China surface
Applied Surface Science 252 (2006) 4516–4522 www.elsevier.com/locate/apsusc
Laser treatment of white China surface K. Osvay a,*, I. Ke´pı´ro´ a, O. Berkesi b a
Department of Optics and Quantum Electronics, University of Szeged, POB. 406, Szeged H-6701, Hungary b Institute of Physical Chemistry, University of Szeged, POB. 105, Szeged H-6701, Hungary Received 3 May 2005; accepted 1 July 2005 Available online 27 October 2005
Abstract The surface of gloss fired porcelain with and without raw glaze coating was radiated by a CO2 laser working at 10.6 mm, a choice resulted from spectroscopic studies of suspensions made of China. The shine of the untreated sample was defined as the distribution of micro-droplets on the surface. The surface alterations due to laser heating were classified by the diameter of the completely melted surface, the ring of the surface at the threshold of melting, and the size of microscopic cracks. The diameter of the laser treated area was in the range of 3 mm, while the incident laser power and the duration of laser heating were varied between 1 and 10 W and 1–8 min, respectively. The different stages of surface modifications were attributed primarily to the irradiating laser power and proved to be rather insensitive to the duration of the treatment. We have found a range of parameters under which the white China surface coated with raw glaze and followed by laser induced melting exhibited very similar characteristics to the untreated porcelain. This technique seems prosperous for laser assisted reparation of small surface defects of unique China samples after the firing process. # 2005 Elsevier B.V. All rights reserved. PACS: 81.05.Je; 81.65.Ps; 81.65. b; 82.80.Ch
1. Introduction Laser surface processing of different materials has been on stage for more than two decades [1,2]. Among many others, synthesized ceramics (and glasses) have been under extensive investigations due to their outstanding electrical, mechanical and thermal characteristics. By now techniques are even commercially available for drilling and cutting them as parts for printed circuit boards, electronic components or bio-compatible prosthesis [3–5], also including their laser-assisted microproduction (e.g. Ref. [6]). However, the old and traditional technology of China making, firing, processing and painting has prevented the ancient field of pottery from the use of lasers so far. Traditional European hard white porcelains (China) are made in a long lasting process [7]. The raw goods are formed from appropriately prepared ball-clay, consisting of kaolin (China-clay), quartz and feldspar. During a so called biscuit firing process below 1000 8C, the chemistry and hence the physical properties of the clay is changed. As a result, the
* Corresponding author. Fax: +36 62 544658. E-mail address: [email protected] (K. Osvay). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.07.127
bowls, figures, etc. become hard and self-supporting and exhibit a matt and rough surface. They are then covered by raw glaze material and fired again (gloss firing), now around 1400 8C, so that the porcelain is transformed into its final form exhibiting smooth and shiny white surfaces, which resists water and many other chemicals. The material of this status is in the focus of our investigations. For the sake of completeness we mention the final step of painting and the subsequent final firing which fixes the dyes. This ultimate firing does not change the chemical and mechanical properties of the material itself. There are, however, point-like defects on the surface of the gloss fired porcelain, which originate from either technological steps or imperfect coating with the glaze material. Although the typical size of these spots ranges from 0.1 to 0.5 mm only, but they need to be re-covered by raw glaze and fired again to obtain a perfect China sample. Our aim was to find conditions under which small defects and tiny scratches of the porcelain surface could be covered and polished by laser. To do this we describe the basic processes found upon radiating a white porcelain surface by a CO2 laser with intensities enough for melting but not causing material removal. Our investigations start with the spectroscopy of the porcelain materials and include the dependence of the melted
K. Osvay et al. / Applied Surface Science 252 (2006) 4516–4522
spot size on the dose of laser irradiation. The conditions of making perfect China-shining spots are also explored. 2. Preliminary investigations All the samples in these investigations were plates with 3 mm thickness from a regular white porcelain coffee set. The surfaces were studied by an optical microscope Nikon OptiPhot100S equipped with a Nikon Coolpix885 digital camera. Microscopic defects and cracks were analyzed with lenses of magnification 10, 20, 50 and 100 and with several filters. 2.1. Structure and features of a porcelain sample The cross section of a white China sample is shown on Fig. 1a. Three layers can be clearly distinguished, which are (from left to right) the (fired) glaze of 100 mm, a transition layer of about 10 mm and the polycrystalline (fired) clay of about 2.8 mm (just part of it is shown). The secret of the porcelain-like shine of the surface originates from the micro-droplets and bubbles on the surface (Fig. 1b). The diameter of the typical bubble is 50 20 mm, while their distribution along the surface is surprisingly even. 2.2. Spectroscopy Since a white China surface exhibits a very high reflectivity in the visible and near infrared region, our first goal was to find
Fig. 1. Cross-section (a) and the surface (b) of the gloss fired porcelain.
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the absorption peaks in the wavelength regime where lasers are available, that is up to 25 mm. Although the compounds of the raw clay are more or less known, its chemistry seriously changes upon the firing processes. Synthesized ceramics are also expected to show different spectra [8,9], thus, the spectroscopic investigations of the gloss fired white porcelain as well as the raw glazing are unavoidable. To do this, first a fine grinding was made from the bulk porcelain. To analyze the raw glaze, it was dried in thin layers in a heated (658) ultrasonic chamber for a few hours. Many conventional spectroscopic methods were tried like solvent and flame analysis, suspension-making etc., but gave unsatisfactory or only qualitative results. Hence, it became necessary to use the more elaborate technique of pellets pressing to ensure the homogeneity of the samples and to make the absolute measurement possible. It is important to estimate both the penetration depth of the laser and also the absorbed energy precisely. The preparation was done according to the standard KBrpellet (potassium bromide) method. The fine-ground sample is blended to KBr to form a homogeneous, but diluted, mixture. KBr has fairly good transmission properties in the mid-IR range and its index of refraction is expected to be similar to the materials to be analyzed. So, the quality of grinding and the similarity of refractive indices reduce the amount of diffuse light. Pellet preparation took place under vacuum, in a stainless steel die, using a hydraulic press providing 10–12 t/cm2. KBr goes into molden state at such pressures, even at low temperatures. During pressing, the gaseous components were also evacuated from the die. The absorption spectra (Fig. 2) of pellets with a concentration of 0.33% material to be analyzed were recorded with a BioRad Digilab Division FTS65A/896 FT-IR Spectrometer, configured for the mid-IR region of 4000–400 cm 1 and equipped with a KBr beamsplitter and a DTGS (deuterated triglycyl-sulphate) detector. One can see considerable absorption around 7 mm, between 9 and 11 mm and also others at longer wavelengths. The negative peak around 3 mm resulted from the different water vapour content of the measuring cell upon recording the spectra of the sample and the background.
Fig. 2. Absorption spectra of gloss fired porcelain and raw glaze.
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Following from the absorption measurements, for surface treatment of white porcelain the CO2 laser was singled out because its sufficiently high power at this spectral range. Since one can expect better result if part of the carrier material is melted, not only the surface layer, the use of the laser line at 10.6 mm ensuring relatively higher penetration depth seems more preferable over the 9.6 mm line. 3. Experimental For the experiment a Synrad CO2 laser was used. The laser beam with a diameter of 3.5 mm and a divergence of 4 mrad is focused to the sample by a ZnSe lens with a focal length of 54 cm (Fig. 3). To keep the lens at the right focusing position and also to control the process, a CCD camera was used for real time monitoring the sample moved by an X–Y translator. Since the temperature on a sample was anticipated to exceed 10008 C, a special thermal insulation blanket consisting of a 5 mm thick K50 ceramic-fiber mat and a 25 mm thick promasil mat was used between the translation stage and the sample. Upon the laser treatment the power of the laser and the duration of the irradiation were changed. The laser was found to be more stable and its power more reproducible in pulse mode. The repetition rate of 7 kHz kept constant, while its mean power, measured at the place of the sample, was changed between 1 and 10 W by varying the duration of the pulses between 10 and 125 ms using a Stanford Delay Generator (DG 535). The samples were irradiated in a 1 cm 1 cm matrix, each point representing different pairs of power and duration of irradiation. Sufficient time of 5 min was left between two subsequent treatments to let the just irradiated surface to cool down to room temperature. Each point was measured by the optical microscope described above. It is worth mentioning that during this experiment more than 250 points on a dozent of plates were created and analyzed. The raw glaze layers were taken up the porcelain samples by painters’ brushs. The thickness of the layers were measured using also the microscope operating in high resolution-low focal depth mode.We have localized the micrometer sized cracks by a self-developed method similar to that used in the China industry. That is, the surface is covered with ink
Fig. 3. Experimental setup. The inset shows a typical investigation matrix on the surface of a white porcelain sample. Each dot is resulted from one treatment and looks similar to, for instance, those on Figs. 4 and 6.
dissolved in alcohol, which penetrates into all invisible cracks. By cleaning the surfaces with alcohol of concentration 96% the colored cracks become apparent and easily recognizable. 4. Structural alterations due to laser treatment Upon the laser treatment of a sample the intensity of the radiation decreases exponentially with penetration depth. The absorbed energy transforms to heat and will heat up the surface layer. Since our intention was not to destroy the surface, the laser beam never produced plasma, hence the plume could not reduce the absorption process [10,11]. Consequently, the absorbed energy is always proportional to the laser power itself. The temperature of the layers lying below the surface would also increase due to both heat conduction from the surface and by direct heating from the attenuated laser beam. Provided the irradiation time is long enough or the power of radiation is sufficiently high, the surface layer would melt. For an intense radiation it can even boil and discolor (Fig. 4a). The melted layer spreads on the surface – in a way, which depends on cohesion forces and surface tension – and forms an even, smooth layer. The cooling rate of the melted surface is significantly higher than that of the inner layers, once-melted but already in the process of solidification. The emerging sharp temperature gradient can be the origin of the so called microscopic cracks on
Fig. 4. Heavily melted and boiled white porcelain (a) and a just melted region (b) showing microscopic (diagonal) and macroscopic (perimetrial) cracks.
K. Osvay et al. / Applied Surface Science 252 (2006) 4516–4522
the surface of the once-melted region (Fig. 4b, diagonal cracks). Similarly, the huge temperature differences and inefficiency of heat exchange between the melted region and the untreated ones result in macroscopic cracks, visible by eye, around the melted area (Fig. 4b, perimetrial cracks). It is clear that for normal spatial laser beam profiles (e.g. flat top or Gaussian), the center of the irradiated area would melt first. Hence, the diameter of the macroscopic cracks gives a good measure of the melted area. The critical value of the necessary energy (absorbed dose– laser power irradiation time) for melting the surface depends both on the laser intensity and irradiation time. Higher intensities come together with shorter irradiation times and smaller doses to produce melting. Less intense radiation will produce melting only for considerably longer irradiation times, thus for more irradiated energy, since the absorbed heat escapes from the irradiated volume. Finally, there is a limit of the radiation intensity such that for any lower intensity the melting will not occur any more, irrespectively of the irradiation time. 4.1. Surface of a porcelain sample First the surface of the white porcelain itself was studied. The laser power and the duration of the treatment was started from 1 W and from 1 min, respectively. Keeping the duration constant, first the power was increased until structural modification (signs of melting) was appeared. No structural alterations were observed up to a power of 4 W and an exposure time of 8 min. By applying a laser power of 5 W, the surface was randomly melted in small spots. Above 6 W, however, the surface was clearly melted for each irradiation time. The melted area was solidified and crystallized similarly to the base material. Hence, its optical appearance was very similar to the untreated porcelain surface. Its shade was, however, turned to be somewhat lighter. This difference in appearance may be resulted in the age of the firing, that is, the ‘‘just (locally) fired spot’’ and the few months old base porcelain.
Fig. 5. The diameter of the perimetrial cracks for a white porcelain surface in the function of the irradiation dose. The symbols with each power series represent the irradiation time of 1, 2, 4 and 8 min.
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Fig. 5 shows the size of the perimetrial cracks, as the measure of the melted area in the function of the applied dose. The curves link irradiated spots exposed to the same laser power but different irradiation time of 1, 2, 4, and 8 min. Please note that the diameter of the cracks depends primarily on the laser power but is quite independent of the duration of the treatment. Variation of the values within one power series is attributed to local inhomogeneity of the porcelain material itself. 4.2. Porcelain sample coated with raw glaze Besides of similarities, a white porcelain sample coated with raw glaze show more phenomena at laser irradiation than the white porcelain itself, due to the different absorption coefficients and the lower melting point (1200 8C) of the raw glaze. A typical irradiated spot is shown on Fig. 6a, exhibiting all features necessary for a more quantitative description. The diameter of the inner ring (Fig. 6b), where the raw glaze was melted and subsequently vitrified, the diameter of the outer ring, where no (raw) glaze material remained in any macroscopic form, and the diameter of the perimetrial crack, characteristic to the affected area, have been measured in the function of the incident laser power and the irradiation time. Under vitrification we mean that the majority of the laser-
Fig. 6. Structural alterations of a raw-glaze-layered porcelain surface caused by laser irradiation (a) and its measures (b) diameter of the inner ring, the outer ring and the perimetrial cracks.
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Fig. 7. Laser irradiated raw glaze layered onto a white porcelain after vitrification.
melted glaze surface, observing by pure eye, shows almost indistinguishable shine from the original white porcelain surface. Under microscope, however, it can be seen that both the size and the distribution of the micro-droplets are less even (Fig. 7). If the above condition of vitrification is not satisfied but the surface was melted we call the surface an onlymelted one. On the following graphs (Figs. 8–11) the size of the symbols represents successive phases of vitrification.
Fig. 9. The diameter of the outer rings for a laser treated white porcelain covered with raw glaze of a thickness of 12 mm (a) and 19 mm (b) in the function of the irradiation dose for equal incident laser powers.
Fig. 8. The diameter of the inner rings for a laser treated white porcelain covered with raw glaze of a thickness of 12 mm (a) and 19 mm (b) in the function of the irradiation dose for equal incident laser powers.
The smallest one is for the only-melted glaze while the larger symbols denote the cases when the surface of the inner ring is completely vitrified. No visible trace of damage or discoloring of the surface was observed up to a power of 4 W and irradiation time of 2 min. After 8 min exposition time at 4 W, the surface of the glaze has been slightly discolored. This presumably signifies the beginning of the melting of raw glaze in a micro-grain scale. Cracks started appearing around the irradiated surface at 5 W incident power during 1 min exposure time. At a modest laser power, but above the critical dose of 6 W and 2 min for the raw glaze layer, only the coating is melted while the surface of the porcelain sample remained solid. Under this condition a drop is formed from the melted glaze due to surface tension force between the fluid–solid surfaces. With boosting the laser power not only the raw glaze becomes hot fluid but also the surface layers of the porcelain are melted so that a fluid–fluid interface is developed. The melted glaze spreads out on the surface of the fluid porcelain forming an even, smooth film. After cooling, the size of the vitrified coating is just slightly less than the irradiated spot, most probably due to the thermal contraction of the cooling fluid glaze (Fig. 8a) If the irradiation dose is further increased, the fluid glaze starts to boil and the material steams slowly out from the
K. Osvay et al. / Applied Surface Science 252 (2006) 4516–4522
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Fig. 10. The diameter of the inner (a) and outer rings (b) for a laser treated white porcelain covered with raw glaze of a thickness of 19 mm in the function of the irradiation dose for equal irradiation durations.
Fig. 11. The ratio of the diameter of the inner and outer rings for a laser treated white porcelain covered with raw glaze of a thickness of 12 mm (a) and 19 mm (b) in the function of the irradiation dose for equal incident laser powers.
surface. Therefore the size of the cooled and glaze-covered surface is expected to be significantly smaller again than the irradiated spot. With the increase of the thickness of the layer the diameter of the inner ring slightly decreases (Fig. 8b). A slight shift in the transformation phases in the function of the irradiation dose can also be observed. The experiments have shown that the thicker raw glazing yielded more uniform vitrification. The change of the size of the outer ring (Fig. 9a) is, in accordance to the expectations, proportional to the dose applied. The dimensions of the outer ring – as expected – do not depend on the mean thickness of the raw glaze (Fig. 9b). The different behavior of the inner and outer rings can be particularly well seen on the time-parameterized graphs Fig. 10a and b, deducted from the power parameterized graphs Figs. 8b and 9b, respectively. The ratio of the inner and the outer rings is a good measure of the melted or vitrified surface compared to the treated surface. In an ideal case this ratio should be close to one. As one can see from the comparison of the two graphs (Fig. 11a and b), the ratio of the rings approaches 1 for thicker layers, but much less than 1 for thin layers for only-melted glaze. For glaze layers with significant vitrification, however, this ratio becomes 0.7–0.8, irrespective of the thickness of the layer.
The crack about the treated surface shows very similar behavior and even similar values to the uncovered porcelain. 5. Summary We have studied the surface alterations of gloss fired China samples with and without additional raw glase coating due to laser induced melting. It was shown that a perfectly shining glaze with a diameter of 1.5 mm could be produced on the surface of a white porcelain sample covered with raw glaze. The necessary irradiation dose could be varying from 9 W incident laser power for 4 min duration up to 10 W and 2 min. One can select among them by considering local technological requirements or safety measures, etc. This may give an opportunity for laser assisted reparation of small surface defects of unique China samples after the firing process. Investigations are already in progress to establish circumstances under which the microscopic cracks appearing across the treated surface could be completely eliminated. Further experiments are also planned to extend this technique for reparation of larger and deeper holes or damages. Acknowledgement The authors thank J. David, Zs. Miko and T. Tanai from the Herend Porcelain Manufactory Ltd. for providing the porcelain
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samples and for introduction to the China technology. The authors are also grateful to Dr. Tama´s Szo¨re´nyi for stimulating discussions. This work was partially supported by OTKA under grant # TS049872 and T47078. References [1] D. Ba¨uerle, Laser Processing and Chemistry, 3rd ed., Springer-Verlag, 2000. [2] I.W. Boyd, Laser Processing of Thin Films and Microstructures, SpringerVerlag, 1988.
[3] Z. Zhang, M.F. Modest, J. Laser Appl. 10 (1998) 212. [4] C.Y. Ho, J.K. Lu, J. Mater. Proc. Technol. 140 (2003) 260. [5] F. Quintero, J. Pou, F. Lusquinos, M. Boutinguiza, R. Soto, M. PerezAmor, Rev. Sci. Instrum. 74 (2003) 4199. [6] J.K.R. Weber, J.J. Felten, P.C. Nordine, Rev. Sci. Instrum. 67 (1996) 522. [7] Herend Porcelain Manufactory Ltd. www.herend.com [8] Z. Zhang, M.F. Modest, J. Heat Transfer-trans. ASME 120 (1998) 322. [9] P.A. Atanasov, S.E. Imamova, H. Hugel, T. Abeln, J. Appl. Phys. 88 (2000) 4671. [10] K.R. Kim, D.F. Farson, J. Appl. Phys. 89 (2001) 681. [11] P.A. Atanasov, E.D. Eugenieva, N.N. Nedialkov, J. Appl. Phys. 89 (2001) 2013.
Laser treatment of white China surface K. Osvay a,*, I. Ke´pı´ro´ a, O. Berkesi b a
Department of Optics and Quantum Electronics, University of Szeged, POB. 406, Szeged H-6701, Hungary b Institute of Physical Chemistry, University of Szeged, POB. 105, Szeged H-6701, Hungary Received 3 May 2005; accepted 1 July 2005 Available online 27 October 2005
Abstract The surface of gloss fired porcelain with and without raw glaze coating was radiated by a CO2 laser working at 10.6 mm, a choice resulted from spectroscopic studies of suspensions made of China. The shine of the untreated sample was defined as the distribution of micro-droplets on the surface. The surface alterations due to laser heating were classified by the diameter of the completely melted surface, the ring of the surface at the threshold of melting, and the size of microscopic cracks. The diameter of the laser treated area was in the range of 3 mm, while the incident laser power and the duration of laser heating were varied between 1 and 10 W and 1–8 min, respectively. The different stages of surface modifications were attributed primarily to the irradiating laser power and proved to be rather insensitive to the duration of the treatment. We have found a range of parameters under which the white China surface coated with raw glaze and followed by laser induced melting exhibited very similar characteristics to the untreated porcelain. This technique seems prosperous for laser assisted reparation of small surface defects of unique China samples after the firing process. # 2005 Elsevier B.V. All rights reserved. PACS: 81.05.Je; 81.65.Ps; 81.65. b; 82.80.Ch
1. Introduction Laser surface processing of different materials has been on stage for more than two decades [1,2]. Among many others, synthesized ceramics (and glasses) have been under extensive investigations due to their outstanding electrical, mechanical and thermal characteristics. By now techniques are even commercially available for drilling and cutting them as parts for printed circuit boards, electronic components or bio-compatible prosthesis [3–5], also including their laser-assisted microproduction (e.g. Ref. [6]). However, the old and traditional technology of China making, firing, processing and painting has prevented the ancient field of pottery from the use of lasers so far. Traditional European hard white porcelains (China) are made in a long lasting process [7]. The raw goods are formed from appropriately prepared ball-clay, consisting of kaolin (China-clay), quartz and feldspar. During a so called biscuit firing process below 1000 8C, the chemistry and hence the physical properties of the clay is changed. As a result, the
* Corresponding author. Fax: +36 62 544658. E-mail address: [email protected] (K. Osvay). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.07.127
bowls, figures, etc. become hard and self-supporting and exhibit a matt and rough surface. They are then covered by raw glaze material and fired again (gloss firing), now around 1400 8C, so that the porcelain is transformed into its final form exhibiting smooth and shiny white surfaces, which resists water and many other chemicals. The material of this status is in the focus of our investigations. For the sake of completeness we mention the final step of painting and the subsequent final firing which fixes the dyes. This ultimate firing does not change the chemical and mechanical properties of the material itself. There are, however, point-like defects on the surface of the gloss fired porcelain, which originate from either technological steps or imperfect coating with the glaze material. Although the typical size of these spots ranges from 0.1 to 0.5 mm only, but they need to be re-covered by raw glaze and fired again to obtain a perfect China sample. Our aim was to find conditions under which small defects and tiny scratches of the porcelain surface could be covered and polished by laser. To do this we describe the basic processes found upon radiating a white porcelain surface by a CO2 laser with intensities enough for melting but not causing material removal. Our investigations start with the spectroscopy of the porcelain materials and include the dependence of the melted
K. Osvay et al. / Applied Surface Science 252 (2006) 4516–4522
spot size on the dose of laser irradiation. The conditions of making perfect China-shining spots are also explored. 2. Preliminary investigations All the samples in these investigations were plates with 3 mm thickness from a regular white porcelain coffee set. The surfaces were studied by an optical microscope Nikon OptiPhot100S equipped with a Nikon Coolpix885 digital camera. Microscopic defects and cracks were analyzed with lenses of magnification 10, 20, 50 and 100 and with several filters. 2.1. Structure and features of a porcelain sample The cross section of a white China sample is shown on Fig. 1a. Three layers can be clearly distinguished, which are (from left to right) the (fired) glaze of 100 mm, a transition layer of about 10 mm and the polycrystalline (fired) clay of about 2.8 mm (just part of it is shown). The secret of the porcelain-like shine of the surface originates from the micro-droplets and bubbles on the surface (Fig. 1b). The diameter of the typical bubble is 50 20 mm, while their distribution along the surface is surprisingly even. 2.2. Spectroscopy Since a white China surface exhibits a very high reflectivity in the visible and near infrared region, our first goal was to find
Fig. 1. Cross-section (a) and the surface (b) of the gloss fired porcelain.
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the absorption peaks in the wavelength regime where lasers are available, that is up to 25 mm. Although the compounds of the raw clay are more or less known, its chemistry seriously changes upon the firing processes. Synthesized ceramics are also expected to show different spectra [8,9], thus, the spectroscopic investigations of the gloss fired white porcelain as well as the raw glazing are unavoidable. To do this, first a fine grinding was made from the bulk porcelain. To analyze the raw glaze, it was dried in thin layers in a heated (658) ultrasonic chamber for a few hours. Many conventional spectroscopic methods were tried like solvent and flame analysis, suspension-making etc., but gave unsatisfactory or only qualitative results. Hence, it became necessary to use the more elaborate technique of pellets pressing to ensure the homogeneity of the samples and to make the absolute measurement possible. It is important to estimate both the penetration depth of the laser and also the absorbed energy precisely. The preparation was done according to the standard KBrpellet (potassium bromide) method. The fine-ground sample is blended to KBr to form a homogeneous, but diluted, mixture. KBr has fairly good transmission properties in the mid-IR range and its index of refraction is expected to be similar to the materials to be analyzed. So, the quality of grinding and the similarity of refractive indices reduce the amount of diffuse light. Pellet preparation took place under vacuum, in a stainless steel die, using a hydraulic press providing 10–12 t/cm2. KBr goes into molden state at such pressures, even at low temperatures. During pressing, the gaseous components were also evacuated from the die. The absorption spectra (Fig. 2) of pellets with a concentration of 0.33% material to be analyzed were recorded with a BioRad Digilab Division FTS65A/896 FT-IR Spectrometer, configured for the mid-IR region of 4000–400 cm 1 and equipped with a KBr beamsplitter and a DTGS (deuterated triglycyl-sulphate) detector. One can see considerable absorption around 7 mm, between 9 and 11 mm and also others at longer wavelengths. The negative peak around 3 mm resulted from the different water vapour content of the measuring cell upon recording the spectra of the sample and the background.
Fig. 2. Absorption spectra of gloss fired porcelain and raw glaze.
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Following from the absorption measurements, for surface treatment of white porcelain the CO2 laser was singled out because its sufficiently high power at this spectral range. Since one can expect better result if part of the carrier material is melted, not only the surface layer, the use of the laser line at 10.6 mm ensuring relatively higher penetration depth seems more preferable over the 9.6 mm line. 3. Experimental For the experiment a Synrad CO2 laser was used. The laser beam with a diameter of 3.5 mm and a divergence of 4 mrad is focused to the sample by a ZnSe lens with a focal length of 54 cm (Fig. 3). To keep the lens at the right focusing position and also to control the process, a CCD camera was used for real time monitoring the sample moved by an X–Y translator. Since the temperature on a sample was anticipated to exceed 10008 C, a special thermal insulation blanket consisting of a 5 mm thick K50 ceramic-fiber mat and a 25 mm thick promasil mat was used between the translation stage and the sample. Upon the laser treatment the power of the laser and the duration of the irradiation were changed. The laser was found to be more stable and its power more reproducible in pulse mode. The repetition rate of 7 kHz kept constant, while its mean power, measured at the place of the sample, was changed between 1 and 10 W by varying the duration of the pulses between 10 and 125 ms using a Stanford Delay Generator (DG 535). The samples were irradiated in a 1 cm 1 cm matrix, each point representing different pairs of power and duration of irradiation. Sufficient time of 5 min was left between two subsequent treatments to let the just irradiated surface to cool down to room temperature. Each point was measured by the optical microscope described above. It is worth mentioning that during this experiment more than 250 points on a dozent of plates were created and analyzed. The raw glaze layers were taken up the porcelain samples by painters’ brushs. The thickness of the layers were measured using also the microscope operating in high resolution-low focal depth mode.We have localized the micrometer sized cracks by a self-developed method similar to that used in the China industry. That is, the surface is covered with ink
Fig. 3. Experimental setup. The inset shows a typical investigation matrix on the surface of a white porcelain sample. Each dot is resulted from one treatment and looks similar to, for instance, those on Figs. 4 and 6.
dissolved in alcohol, which penetrates into all invisible cracks. By cleaning the surfaces with alcohol of concentration 96% the colored cracks become apparent and easily recognizable. 4. Structural alterations due to laser treatment Upon the laser treatment of a sample the intensity of the radiation decreases exponentially with penetration depth. The absorbed energy transforms to heat and will heat up the surface layer. Since our intention was not to destroy the surface, the laser beam never produced plasma, hence the plume could not reduce the absorption process [10,11]. Consequently, the absorbed energy is always proportional to the laser power itself. The temperature of the layers lying below the surface would also increase due to both heat conduction from the surface and by direct heating from the attenuated laser beam. Provided the irradiation time is long enough or the power of radiation is sufficiently high, the surface layer would melt. For an intense radiation it can even boil and discolor (Fig. 4a). The melted layer spreads on the surface – in a way, which depends on cohesion forces and surface tension – and forms an even, smooth layer. The cooling rate of the melted surface is significantly higher than that of the inner layers, once-melted but already in the process of solidification. The emerging sharp temperature gradient can be the origin of the so called microscopic cracks on
Fig. 4. Heavily melted and boiled white porcelain (a) and a just melted region (b) showing microscopic (diagonal) and macroscopic (perimetrial) cracks.
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the surface of the once-melted region (Fig. 4b, diagonal cracks). Similarly, the huge temperature differences and inefficiency of heat exchange between the melted region and the untreated ones result in macroscopic cracks, visible by eye, around the melted area (Fig. 4b, perimetrial cracks). It is clear that for normal spatial laser beam profiles (e.g. flat top or Gaussian), the center of the irradiated area would melt first. Hence, the diameter of the macroscopic cracks gives a good measure of the melted area. The critical value of the necessary energy (absorbed dose– laser power irradiation time) for melting the surface depends both on the laser intensity and irradiation time. Higher intensities come together with shorter irradiation times and smaller doses to produce melting. Less intense radiation will produce melting only for considerably longer irradiation times, thus for more irradiated energy, since the absorbed heat escapes from the irradiated volume. Finally, there is a limit of the radiation intensity such that for any lower intensity the melting will not occur any more, irrespectively of the irradiation time. 4.1. Surface of a porcelain sample First the surface of the white porcelain itself was studied. The laser power and the duration of the treatment was started from 1 W and from 1 min, respectively. Keeping the duration constant, first the power was increased until structural modification (signs of melting) was appeared. No structural alterations were observed up to a power of 4 W and an exposure time of 8 min. By applying a laser power of 5 W, the surface was randomly melted in small spots. Above 6 W, however, the surface was clearly melted for each irradiation time. The melted area was solidified and crystallized similarly to the base material. Hence, its optical appearance was very similar to the untreated porcelain surface. Its shade was, however, turned to be somewhat lighter. This difference in appearance may be resulted in the age of the firing, that is, the ‘‘just (locally) fired spot’’ and the few months old base porcelain.
Fig. 5. The diameter of the perimetrial cracks for a white porcelain surface in the function of the irradiation dose. The symbols with each power series represent the irradiation time of 1, 2, 4 and 8 min.
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Fig. 5 shows the size of the perimetrial cracks, as the measure of the melted area in the function of the applied dose. The curves link irradiated spots exposed to the same laser power but different irradiation time of 1, 2, 4, and 8 min. Please note that the diameter of the cracks depends primarily on the laser power but is quite independent of the duration of the treatment. Variation of the values within one power series is attributed to local inhomogeneity of the porcelain material itself. 4.2. Porcelain sample coated with raw glaze Besides of similarities, a white porcelain sample coated with raw glaze show more phenomena at laser irradiation than the white porcelain itself, due to the different absorption coefficients and the lower melting point (1200 8C) of the raw glaze. A typical irradiated spot is shown on Fig. 6a, exhibiting all features necessary for a more quantitative description. The diameter of the inner ring (Fig. 6b), where the raw glaze was melted and subsequently vitrified, the diameter of the outer ring, where no (raw) glaze material remained in any macroscopic form, and the diameter of the perimetrial crack, characteristic to the affected area, have been measured in the function of the incident laser power and the irradiation time. Under vitrification we mean that the majority of the laser-
Fig. 6. Structural alterations of a raw-glaze-layered porcelain surface caused by laser irradiation (a) and its measures (b) diameter of the inner ring, the outer ring and the perimetrial cracks.
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Fig. 7. Laser irradiated raw glaze layered onto a white porcelain after vitrification.
melted glaze surface, observing by pure eye, shows almost indistinguishable shine from the original white porcelain surface. Under microscope, however, it can be seen that both the size and the distribution of the micro-droplets are less even (Fig. 7). If the above condition of vitrification is not satisfied but the surface was melted we call the surface an onlymelted one. On the following graphs (Figs. 8–11) the size of the symbols represents successive phases of vitrification.
Fig. 9. The diameter of the outer rings for a laser treated white porcelain covered with raw glaze of a thickness of 12 mm (a) and 19 mm (b) in the function of the irradiation dose for equal incident laser powers.
Fig. 8. The diameter of the inner rings for a laser treated white porcelain covered with raw glaze of a thickness of 12 mm (a) and 19 mm (b) in the function of the irradiation dose for equal incident laser powers.
The smallest one is for the only-melted glaze while the larger symbols denote the cases when the surface of the inner ring is completely vitrified. No visible trace of damage or discoloring of the surface was observed up to a power of 4 W and irradiation time of 2 min. After 8 min exposition time at 4 W, the surface of the glaze has been slightly discolored. This presumably signifies the beginning of the melting of raw glaze in a micro-grain scale. Cracks started appearing around the irradiated surface at 5 W incident power during 1 min exposure time. At a modest laser power, but above the critical dose of 6 W and 2 min for the raw glaze layer, only the coating is melted while the surface of the porcelain sample remained solid. Under this condition a drop is formed from the melted glaze due to surface tension force between the fluid–solid surfaces. With boosting the laser power not only the raw glaze becomes hot fluid but also the surface layers of the porcelain are melted so that a fluid–fluid interface is developed. The melted glaze spreads out on the surface of the fluid porcelain forming an even, smooth film. After cooling, the size of the vitrified coating is just slightly less than the irradiated spot, most probably due to the thermal contraction of the cooling fluid glaze (Fig. 8a) If the irradiation dose is further increased, the fluid glaze starts to boil and the material steams slowly out from the
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Fig. 10. The diameter of the inner (a) and outer rings (b) for a laser treated white porcelain covered with raw glaze of a thickness of 19 mm in the function of the irradiation dose for equal irradiation durations.
Fig. 11. The ratio of the diameter of the inner and outer rings for a laser treated white porcelain covered with raw glaze of a thickness of 12 mm (a) and 19 mm (b) in the function of the irradiation dose for equal incident laser powers.
surface. Therefore the size of the cooled and glaze-covered surface is expected to be significantly smaller again than the irradiated spot. With the increase of the thickness of the layer the diameter of the inner ring slightly decreases (Fig. 8b). A slight shift in the transformation phases in the function of the irradiation dose can also be observed. The experiments have shown that the thicker raw glazing yielded more uniform vitrification. The change of the size of the outer ring (Fig. 9a) is, in accordance to the expectations, proportional to the dose applied. The dimensions of the outer ring – as expected – do not depend on the mean thickness of the raw glaze (Fig. 9b). The different behavior of the inner and outer rings can be particularly well seen on the time-parameterized graphs Fig. 10a and b, deducted from the power parameterized graphs Figs. 8b and 9b, respectively. The ratio of the inner and the outer rings is a good measure of the melted or vitrified surface compared to the treated surface. In an ideal case this ratio should be close to one. As one can see from the comparison of the two graphs (Fig. 11a and b), the ratio of the rings approaches 1 for thicker layers, but much less than 1 for thin layers for only-melted glaze. For glaze layers with significant vitrification, however, this ratio becomes 0.7–0.8, irrespective of the thickness of the layer.
The crack about the treated surface shows very similar behavior and even similar values to the uncovered porcelain. 5. Summary We have studied the surface alterations of gloss fired China samples with and without additional raw glase coating due to laser induced melting. It was shown that a perfectly shining glaze with a diameter of 1.5 mm could be produced on the surface of a white porcelain sample covered with raw glaze. The necessary irradiation dose could be varying from 9 W incident laser power for 4 min duration up to 10 W and 2 min. One can select among them by considering local technological requirements or safety measures, etc. This may give an opportunity for laser assisted reparation of small surface defects of unique China samples after the firing process. Investigations are already in progress to establish circumstances under which the microscopic cracks appearing across the treated surface could be completely eliminated. Further experiments are also planned to extend this technique for reparation of larger and deeper holes or damages. Acknowledgement The authors thank J. David, Zs. Miko and T. Tanai from the Herend Porcelain Manufactory Ltd. for providing the porcelain
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samples and for introduction to the China technology. The authors are also grateful to Dr. Tama´s Szo¨re´nyi for stimulating discussions. This work was partially supported by OTKA under grant # TS049872 and T47078. References [1] D. Ba¨uerle, Laser Processing and Chemistry, 3rd ed., Springer-Verlag, 2000. [2] I.W. Boyd, Laser Processing of Thin Films and Microstructures, SpringerVerlag, 1988.
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