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Laser printing of enamels on tiles
Applied Surface Science 253 (2007) 7733–7737 www.elsevier.com/locate/apsusc
Laser printing of enamels on tiles J.M. Ferna´ndez-Pradas *, J.W. Restrepo, M.A. Go´mez, P. Serra, J.L. Morenza ` ptica, Martı´ i Franque`s 1, E08028 Barcelona, Spain Universitat de Barcelona, Departament de Fı´sica Aplicada i O Available online 27 February 2007
Abstract A Nd:YAG laser beam is used as a tool to print patterns of coloured enamels on tile substrates. For this, the laser beam is scanned over a layer of raw enamel previously sprayed on the tile surface. The possibility to focus the laser energy to heat a small zone without affecting the rest of the piece presents some advantages in front of traditional furnace techniques in which the whole piece has to be heated; among them, energy saving and the possibility to apply enamels with higher melting temperatures than those of the substrate. In this work, we study the effects of laser irradiation of a green enamel, based in chromium oxide pigment and lead frit, deposited on a white tile substrate. Lines obtained with different combinations of laser beam power and scan speeds were investigated with the aim to optimize the process from the point of view of the quality of the patterns. For this purpose, the morphology of the lines and their cross-sections is studied. The results show that lines with good visual properties can be printed with the laser. The characteristics of the marked lines were found to be directly related with the accumulated energy density delivered. Moreover, there is a linear relationship between the accumulated energy density and the volume of melted material. A minimum accumulated energy density is required to melt a shallow zone of the glazed substrate to allow the adhesion of the enamelled lines. # 2007 Elsevier B.V. All rights reserved. Keywords: Laser print; Enamel; Tile
1. Introduction Laser beams are being used for printing marks on a variety of materials owing to their ability to heat, melt and evaporate the surface of any material able to absorb their radiation. However, the possibility of direct marking patterns with a desired colour is restricted to materials with the ability to produce different colours depending on the irradiation conditions [1]. This restriction can be overcome by adding a pigmented layer onto the surface to get marks with the desired colour. Ceramic tiles and glazes are often decorated with designs of coloured enamels. In traditional decoration methods, the designs are first painted on the tile surface, and the pieces are subsequently fired in a furnace at high temperature to confer a smooth glazed texture to the enamels, and to promote their adhesion to the surface. Instead of heating the entire piece in a furnace, a focused laser beam allows heating only the zone to be printed without affecting the rest of the piece, in a process which saves energy and time. Moreover, such a process would permit pigmented enamels to be applied on substrates that * Corresponding author. Tel.: +34 93 4039216; fax: +34 93 4039219. E-mail address: [email protected] (J.M. Ferna´ndez-Pradas). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.02.129
would be damaged at the required firing temperatures. Consequently, this decoration method can be extended to a great variety of material substrates. The industrial interest on this process has motivated the apparition of several patents [2– 4]. The process is very similar and should follow mechanisms similar to the laser enamelling technique proposed by Akhter et al. for tile grout sealing [5,6]. The work here presented is a study of the laser enamelling of white glazed tiles. For this purpose, a green enamel previously deposited on the tile surface was irradiated under different laser conditions, and the marks left were analysed by optical microscopy and surface profilometry. 2. Experimental The pieces used as substrates were white glazed tiles. The glaze composition, analysed by X-ray fluorescence spectroscopy (XRF), is depicted in Table 1. The raw enamel powders were prepared by mixing 5 wt% of a chromium oxide green pigment with 95 wt% of a lead frit, which compositions can be found in Table 1. Serigraphic vehicle and water was added to the mixture of pigment and frit to be able to spray a thin coating of the raw enamel onto the glazed surface of the tile. After
J.M. Ferna´ndez-Pradas et al. / Applied Surface Science 253 (2007) 7733–7737
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Table 1 XRF results (wt%) of the chemical analysis of the raw materials used in the experiments
Glaze Frit Pigment
SiO2
ZrO2
Al2O3
CaO
Na2O
K2O
MgO
PbO
Fe2O3
Cr2O3
Others
57.87 20.32 –
13.12 0.27 –
11.47 – –
5.49 0.10 –
5.26 – –
3.65 – –
1.36 – –
0.42 77.29 –
0.21 – 0.14
– – 99.41
1.15 2.02 0.45
spraying, the pieces were left to dry in an oven at 100 8C. Afterwards, the thickness of the deposited layer was measured with a profilometer. A Nd:YAG laser (l = 1064 nm) operating in continuous wave mode was used for the laser printing of lines on the samples. The laser beam intensity distribution had a Gaussianlike profile in the direction perpendicular to the lines, and a profile with a small valley between two maxima in the scanning direction of the lines. The tiles were placed on a lift table in order to position their surface on the focus of the laser scan head where the beam spot has a diameter of about 300 mm. Samples were irradiated by making series of 5 mm long lines with laser beam powers of 12, 20, 25 and 30 W and scan speeds varying from 5 to 1000 mm/s. The average accumulated energy density Ed, supposing the laser intensity distribution homogeneous and rectangular, can be calculated by means of the formula: Ed ¼
P dv
3. Results and discussion During laser irradiation of the samples, the formation of a plasma plume and a thin line of smoke just over the laser spot were observed. Fig. 1 shows images of a series of lines made at 30 W laser beam power before and after washing the sample to remove the excess of raw enamel. It can be seen that some material that was affected is no longer present after the washing process. For instance, the lines marked at faster speeds have completely disappeared. This indicates that the absorption of the laser radiation principally occurs in the coating composed by frit and pigment. If the energy delivered by the laser beam is not high enough to melt the material in the interface of the glazed surface and the enamel layer to produce their adhesion, the modified enamel disappears with the washing process. Lines marked at scan speeds between 7 and 20 mm/s present good visual characteristics.
(1)
where P is the laser beam power, d the beam diameter and v is the scan speed. After irradiation, the samples where thoroughly washed with water for removing the layer of raw enamel that was not fixed to the glazed substrate. The lines were characterised by optical microscopy and surface profilometry, before and after cleaning the samples. One series of lines at 30 W was also marked through a microscope glass slide put at 1 mm above the enamel surface, in order to collect the material evaporated. This material was analysed trough energy dispersive spectroscopy (EDS) in a scanning electron microscope.
Fig. 1. Images of a series of lines marked with a laser beam power of 30 W and increasing scan speed from left to right (5, 6, 7, 8, 9, 10, 20, 40, 60, 80, 100, 120, 140, 160, 180, 200, 300, 400, 500, 600, 700, 800, 900, and 1000 mm/s): (a) before and (b) after cleaning.
Fig. 2. Images of lines obtained under different laser beam power and scan speed leading to the same accumulated energy density (see Table 2).
J.M. Ferna´ndez-Pradas et al. / Applied Surface Science 253 (2007) 7733–7737
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Fig. 3. Lines marked at (a) 71 J/cm2, (b) 1.7 102 J/cm2 and (c) l.0 l03 J/cm2. From top to bottom: surface images before cleaning, surface images after cleaning, images of cross-sections after cleaning, and transverse surface profiles (solid lines: before cleaning; dashed lines: after cleaning). Table 2 List of lines appearing in Fig. 2, marked with combinations of power and scan speed that lead to a same accumulated energy density Image
Power (W)
Scan speed (mm/s)
Ed (J/cm2)
a b c d e f g h
25 35 20 30 12 30 20 30
100 140 40 60 8 20 6 9
8.3 10 1 8.3 10 1 1.7 l02 1.7 l02 5.0 l02 5.0 l02 1.1 l03 1.1 l03
Looking at the different series, it can be seen that lines marked at faster speeds are thinner, and that series at higher powers present wider lines. When looking in detail, it can be observed that lines marked with combinations of laser power and scan speed that lead to similar Ed present similar features and morphology (Fig. 2). This suggests that the mechanisms involved in the marking process are basically determined by the Ed. The lines consist of a central zone and borders. The central zone seems to have a glassier and smoother texture than the borders. However, the borders present darker and more intense green tones. The use of higher Ed produces wider lines with wider borders as well. The lines marked at low Ed in these series present big pores in their central zone. These pores do not appear in lines marked at higher Ed.
Fig. 4. EDS spectra of (a) raw enamel material, (b) material evaporated and (c) a spherical droplet on the evaporated material.
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J.M. Ferna´ndez-Pradas et al. / Applied Surface Science 253 (2007) 7733–7737
Fig. 5. Volume per unit length calculated from the profiles of the lines (&) before and (&) after the cleaning step vs. the accumulated energy density.
Fig. 7. (&) Total width, (&) central zone width, (*) border width and (~) depth of lines vs. the accumulated energy density. Solid lines are fits with a linear function of the square root of Ed.
A complete series of lines marked at different scan speeds with a laser beam power of 30 W was analysed, before and after the cleaning step, by profilometry and microscopy in order to study the amount of material lost and the amount of material that was added in order to conform the line (Fig. 3). Comparing the planar view of the lines before removing the excess of material and their profile, it can be seen that the entire affected zone is in a depression with respect to the original surface. As seen during irradiation, evaporation occurred, but also densification and compaction can take place due to the effect of melting and resolidification of the powder. The mixing or dilution of this material in the
substrate can also contribute to the observed volume diminution. After cleaning the surface, the comparison between the images and the profiles shows that what is seen as borders of the lines is enamel material remaining attached to the surface of the glazed substrate. The central zone corresponds to a zone where the glazed substrate melted and was mixed with the frit and pigment of the enamel coating. The profile of some lines shows that the depression between the central zone and the borders can be even below the level of the glazed substrate surface (Fig. 3a and b). Looking at the cross-sections of the lines, it can be seen that the substrate melted zone is deeper and wider as higher is the energy density. If the energy density is very high, the effects can reach the underlying ceramic substrate, causing the formation of bubbles of gas [7] that produce discontinuities and cracks in the marked lines, as it can be seen in the lines marked at the highest energy densities shown in Fig. 1. The EDS spectra in Fig. 4 show that the evaporated material mainly presents Pb and Si, which are the components of the frit, but there is a very small content of Cr from the pigment. This indicates that the evaporated material is principally composed of elements from the frit, and thus the material remaining on the glaze is pigment enriched. Especially at low energy densities, black metal lines can be observed on the glass collecting the evaporated material, what can be concomitant with the presence of reduced lead. Some droplets of several micrometers can also be found on the evaporated material. These droplets contain Zr, which is evidence that they were ejected from the glaze substrate. The diminution in volume after irradiation and the volume of material added to the surface were evaluated by integrating the area below the line profiles before and after the cleaning step, respectively. In fact, these integrated areas
Fig. 6. Volume per unit length from the cross-section area of the melted glazed substrate vs. the accumulated energy density.
J.M. Ferna´ndez-Pradas et al. / Applied Surface Science 253 (2007) 7733–7737
correspond to volume per unit length, and thus, they were represented versus Ed in Fig. 5. The increase of the energy density provokes the increase of the lost volume. This trend seems to be stopped for Ed above 0.5 kJ/cm2 in Fig. 5. However, it has to be said that the measurements of these points at high Ed present some degree of uncertainty, due to the fact that the bottoms of the depressions are out of the range of measurement of the profilometer, and therefore, the points corresponding to the profile used to integrate the area below the zero level were manually interpolated from the data acquired after cleaning. At the same time, it is also important to note that the amount of material forming the lines does not vary significantly until Ed reaches 0.5 J/cm2, but appreciably increases beyond this value. Therefore, the halt in the increasing trend of lost volume seems to correspond with the start of the increase of the volume of material incorporated to form the lines. Indeed, if we look at the colour of the lines, no dilution of the green intensity can be appreciated, despite there is a major volume of glaze where the pigment must be diluted, what is also indicative of a greater incorporation of pigment at higher Ed. The volume per unit length of melted glazed substrate versus Ed is represented in Fig. 6. This volume per unit length was calculated by integrating the melted area in the cross-section images, which is proportional to the total volume of the lines in the glazed substrate. A linear correspondence was found between this area and Ed. A similar linear relationship was also found between the crosssections areas of laser vitrified lines marked on red clay brick pavers and Ed [8]. This finding should not be surprising, as it seems logical to think in a proportional relationship between the amount of energy delivered to the system and the volume of melted material. Fig. 7 shows how the width and the depth of the lines increase with the different Ed. The central zone usually corresponds to the 75% while borders correspond to the 25% of the total width of the lines. Following the mechanisms suggested in Restrepo et al. to the laser vitrification of red clay brick pavers [8], both width and depth dimensions fit quite well linear functions of the square root of Ed.
7737
4. Conclusion Green lines of a chromium oxide based enamel with good visual characteristics can be printed on white glazed tiles with a Nd:YAG laser operating in continuous wave mode. The characteristics of the lines are basically determined by the accumulated energy density of the laser beam which is determined by the combination of beam power and scan speed. A minimum accumulated energy density is required to allow the melting of the glazed substrate in order to allow the adhesion of the lines. The lines present a glassy central zone, where enamel material is mixed in the melted glazed substrate, and borders with darker tones, where enamel forms a layer attached to the glazed substrate. Evaporation of the raw enamel occurs during irradiation, basically affecting the frit components. The amount of material forming the lines does not increase significantly until the accumulated energy density reaches 500 J/cm2. There is a linear relationship between the accumulated energy density and the volume of melted glazed substrate where enamel is incorporated. The width and depth of the lines increase with the accumulated energy density similar to a linear function of its square root. References [1] A. Pe´rez del Pino, J.M. Ferna´ndez-Pradas, P. Serra, J.L. Morenza, Surf. Coat. Technol. 187 (2004) 106. [2] H. Gugger, F. Herren, M. Hofmann, A. Pugin, Laser marking of ceramic materials, glazes, glass ceramics and glasses, US Patent 4,769,310 (1988). [3] P.W. Harrison, High contrast surface marking using metal oxides, US Patent 6,313,436 (2000). [4] A. Hory, J.M. Gaillard, Method and device for marking objects with sintered mineral powders, US Patent application n. 20030012891 (2003). [5] R. Akhter, L. Li, R.E. Edwards, A.W. Gale, Appl. Surf. Sci. 208/209 (2003) 453. [6] R. Akhter, L. Li, R.E. Edwards, A.W. Gale, Appl. Surf. Sci. 208/209 (2003) 447. [7] D. Triantafyllidis, L. Li, F.H. Stott, Appl. Surf. Sci. 208/209 (2003) 458. [8] J.W. Restrepo, J.M. Ferna´ndez-Pradas, M.A. Go´mez, P. Serra, J.L. Morenza, J. Laser Appl. 18 (2006) 156.
Laser printing of enamels on tiles J.M. Ferna´ndez-Pradas *, J.W. Restrepo, M.A. Go´mez, P. Serra, J.L. Morenza ` ptica, Martı´ i Franque`s 1, E08028 Barcelona, Spain Universitat de Barcelona, Departament de Fı´sica Aplicada i O Available online 27 February 2007
Abstract A Nd:YAG laser beam is used as a tool to print patterns of coloured enamels on tile substrates. For this, the laser beam is scanned over a layer of raw enamel previously sprayed on the tile surface. The possibility to focus the laser energy to heat a small zone without affecting the rest of the piece presents some advantages in front of traditional furnace techniques in which the whole piece has to be heated; among them, energy saving and the possibility to apply enamels with higher melting temperatures than those of the substrate. In this work, we study the effects of laser irradiation of a green enamel, based in chromium oxide pigment and lead frit, deposited on a white tile substrate. Lines obtained with different combinations of laser beam power and scan speeds were investigated with the aim to optimize the process from the point of view of the quality of the patterns. For this purpose, the morphology of the lines and their cross-sections is studied. The results show that lines with good visual properties can be printed with the laser. The characteristics of the marked lines were found to be directly related with the accumulated energy density delivered. Moreover, there is a linear relationship between the accumulated energy density and the volume of melted material. A minimum accumulated energy density is required to melt a shallow zone of the glazed substrate to allow the adhesion of the enamelled lines. # 2007 Elsevier B.V. All rights reserved. Keywords: Laser print; Enamel; Tile
1. Introduction Laser beams are being used for printing marks on a variety of materials owing to their ability to heat, melt and evaporate the surface of any material able to absorb their radiation. However, the possibility of direct marking patterns with a desired colour is restricted to materials with the ability to produce different colours depending on the irradiation conditions [1]. This restriction can be overcome by adding a pigmented layer onto the surface to get marks with the desired colour. Ceramic tiles and glazes are often decorated with designs of coloured enamels. In traditional decoration methods, the designs are first painted on the tile surface, and the pieces are subsequently fired in a furnace at high temperature to confer a smooth glazed texture to the enamels, and to promote their adhesion to the surface. Instead of heating the entire piece in a furnace, a focused laser beam allows heating only the zone to be printed without affecting the rest of the piece, in a process which saves energy and time. Moreover, such a process would permit pigmented enamels to be applied on substrates that * Corresponding author. Tel.: +34 93 4039216; fax: +34 93 4039219. E-mail address: [email protected] (J.M. Ferna´ndez-Pradas). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.02.129
would be damaged at the required firing temperatures. Consequently, this decoration method can be extended to a great variety of material substrates. The industrial interest on this process has motivated the apparition of several patents [2– 4]. The process is very similar and should follow mechanisms similar to the laser enamelling technique proposed by Akhter et al. for tile grout sealing [5,6]. The work here presented is a study of the laser enamelling of white glazed tiles. For this purpose, a green enamel previously deposited on the tile surface was irradiated under different laser conditions, and the marks left were analysed by optical microscopy and surface profilometry. 2. Experimental The pieces used as substrates were white glazed tiles. The glaze composition, analysed by X-ray fluorescence spectroscopy (XRF), is depicted in Table 1. The raw enamel powders were prepared by mixing 5 wt% of a chromium oxide green pigment with 95 wt% of a lead frit, which compositions can be found in Table 1. Serigraphic vehicle and water was added to the mixture of pigment and frit to be able to spray a thin coating of the raw enamel onto the glazed surface of the tile. After
J.M. Ferna´ndez-Pradas et al. / Applied Surface Science 253 (2007) 7733–7737
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Table 1 XRF results (wt%) of the chemical analysis of the raw materials used in the experiments
Glaze Frit Pigment
SiO2
ZrO2
Al2O3
CaO
Na2O
K2O
MgO
PbO
Fe2O3
Cr2O3
Others
57.87 20.32 –
13.12 0.27 –
11.47 – –
5.49 0.10 –
5.26 – –
3.65 – –
1.36 – –
0.42 77.29 –
0.21 – 0.14
– – 99.41
1.15 2.02 0.45
spraying, the pieces were left to dry in an oven at 100 8C. Afterwards, the thickness of the deposited layer was measured with a profilometer. A Nd:YAG laser (l = 1064 nm) operating in continuous wave mode was used for the laser printing of lines on the samples. The laser beam intensity distribution had a Gaussianlike profile in the direction perpendicular to the lines, and a profile with a small valley between two maxima in the scanning direction of the lines. The tiles were placed on a lift table in order to position their surface on the focus of the laser scan head where the beam spot has a diameter of about 300 mm. Samples were irradiated by making series of 5 mm long lines with laser beam powers of 12, 20, 25 and 30 W and scan speeds varying from 5 to 1000 mm/s. The average accumulated energy density Ed, supposing the laser intensity distribution homogeneous and rectangular, can be calculated by means of the formula: Ed ¼
P dv
3. Results and discussion During laser irradiation of the samples, the formation of a plasma plume and a thin line of smoke just over the laser spot were observed. Fig. 1 shows images of a series of lines made at 30 W laser beam power before and after washing the sample to remove the excess of raw enamel. It can be seen that some material that was affected is no longer present after the washing process. For instance, the lines marked at faster speeds have completely disappeared. This indicates that the absorption of the laser radiation principally occurs in the coating composed by frit and pigment. If the energy delivered by the laser beam is not high enough to melt the material in the interface of the glazed surface and the enamel layer to produce their adhesion, the modified enamel disappears with the washing process. Lines marked at scan speeds between 7 and 20 mm/s present good visual characteristics.
(1)
where P is the laser beam power, d the beam diameter and v is the scan speed. After irradiation, the samples where thoroughly washed with water for removing the layer of raw enamel that was not fixed to the glazed substrate. The lines were characterised by optical microscopy and surface profilometry, before and after cleaning the samples. One series of lines at 30 W was also marked through a microscope glass slide put at 1 mm above the enamel surface, in order to collect the material evaporated. This material was analysed trough energy dispersive spectroscopy (EDS) in a scanning electron microscope.
Fig. 1. Images of a series of lines marked with a laser beam power of 30 W and increasing scan speed from left to right (5, 6, 7, 8, 9, 10, 20, 40, 60, 80, 100, 120, 140, 160, 180, 200, 300, 400, 500, 600, 700, 800, 900, and 1000 mm/s): (a) before and (b) after cleaning.
Fig. 2. Images of lines obtained under different laser beam power and scan speed leading to the same accumulated energy density (see Table 2).
J.M. Ferna´ndez-Pradas et al. / Applied Surface Science 253 (2007) 7733–7737
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Fig. 3. Lines marked at (a) 71 J/cm2, (b) 1.7 102 J/cm2 and (c) l.0 l03 J/cm2. From top to bottom: surface images before cleaning, surface images after cleaning, images of cross-sections after cleaning, and transverse surface profiles (solid lines: before cleaning; dashed lines: after cleaning). Table 2 List of lines appearing in Fig. 2, marked with combinations of power and scan speed that lead to a same accumulated energy density Image
Power (W)
Scan speed (mm/s)
Ed (J/cm2)
a b c d e f g h
25 35 20 30 12 30 20 30
100 140 40 60 8 20 6 9
8.3 10 1 8.3 10 1 1.7 l02 1.7 l02 5.0 l02 5.0 l02 1.1 l03 1.1 l03
Looking at the different series, it can be seen that lines marked at faster speeds are thinner, and that series at higher powers present wider lines. When looking in detail, it can be observed that lines marked with combinations of laser power and scan speed that lead to similar Ed present similar features and morphology (Fig. 2). This suggests that the mechanisms involved in the marking process are basically determined by the Ed. The lines consist of a central zone and borders. The central zone seems to have a glassier and smoother texture than the borders. However, the borders present darker and more intense green tones. The use of higher Ed produces wider lines with wider borders as well. The lines marked at low Ed in these series present big pores in their central zone. These pores do not appear in lines marked at higher Ed.
Fig. 4. EDS spectra of (a) raw enamel material, (b) material evaporated and (c) a spherical droplet on the evaporated material.
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J.M. Ferna´ndez-Pradas et al. / Applied Surface Science 253 (2007) 7733–7737
Fig. 5. Volume per unit length calculated from the profiles of the lines (&) before and (&) after the cleaning step vs. the accumulated energy density.
Fig. 7. (&) Total width, (&) central zone width, (*) border width and (~) depth of lines vs. the accumulated energy density. Solid lines are fits with a linear function of the square root of Ed.
A complete series of lines marked at different scan speeds with a laser beam power of 30 W was analysed, before and after the cleaning step, by profilometry and microscopy in order to study the amount of material lost and the amount of material that was added in order to conform the line (Fig. 3). Comparing the planar view of the lines before removing the excess of material and their profile, it can be seen that the entire affected zone is in a depression with respect to the original surface. As seen during irradiation, evaporation occurred, but also densification and compaction can take place due to the effect of melting and resolidification of the powder. The mixing or dilution of this material in the
substrate can also contribute to the observed volume diminution. After cleaning the surface, the comparison between the images and the profiles shows that what is seen as borders of the lines is enamel material remaining attached to the surface of the glazed substrate. The central zone corresponds to a zone where the glazed substrate melted and was mixed with the frit and pigment of the enamel coating. The profile of some lines shows that the depression between the central zone and the borders can be even below the level of the glazed substrate surface (Fig. 3a and b). Looking at the cross-sections of the lines, it can be seen that the substrate melted zone is deeper and wider as higher is the energy density. If the energy density is very high, the effects can reach the underlying ceramic substrate, causing the formation of bubbles of gas [7] that produce discontinuities and cracks in the marked lines, as it can be seen in the lines marked at the highest energy densities shown in Fig. 1. The EDS spectra in Fig. 4 show that the evaporated material mainly presents Pb and Si, which are the components of the frit, but there is a very small content of Cr from the pigment. This indicates that the evaporated material is principally composed of elements from the frit, and thus the material remaining on the glaze is pigment enriched. Especially at low energy densities, black metal lines can be observed on the glass collecting the evaporated material, what can be concomitant with the presence of reduced lead. Some droplets of several micrometers can also be found on the evaporated material. These droplets contain Zr, which is evidence that they were ejected from the glaze substrate. The diminution in volume after irradiation and the volume of material added to the surface were evaluated by integrating the area below the line profiles before and after the cleaning step, respectively. In fact, these integrated areas
Fig. 6. Volume per unit length from the cross-section area of the melted glazed substrate vs. the accumulated energy density.
J.M. Ferna´ndez-Pradas et al. / Applied Surface Science 253 (2007) 7733–7737
correspond to volume per unit length, and thus, they were represented versus Ed in Fig. 5. The increase of the energy density provokes the increase of the lost volume. This trend seems to be stopped for Ed above 0.5 kJ/cm2 in Fig. 5. However, it has to be said that the measurements of these points at high Ed present some degree of uncertainty, due to the fact that the bottoms of the depressions are out of the range of measurement of the profilometer, and therefore, the points corresponding to the profile used to integrate the area below the zero level were manually interpolated from the data acquired after cleaning. At the same time, it is also important to note that the amount of material forming the lines does not vary significantly until Ed reaches 0.5 J/cm2, but appreciably increases beyond this value. Therefore, the halt in the increasing trend of lost volume seems to correspond with the start of the increase of the volume of material incorporated to form the lines. Indeed, if we look at the colour of the lines, no dilution of the green intensity can be appreciated, despite there is a major volume of glaze where the pigment must be diluted, what is also indicative of a greater incorporation of pigment at higher Ed. The volume per unit length of melted glazed substrate versus Ed is represented in Fig. 6. This volume per unit length was calculated by integrating the melted area in the cross-section images, which is proportional to the total volume of the lines in the glazed substrate. A linear correspondence was found between this area and Ed. A similar linear relationship was also found between the crosssections areas of laser vitrified lines marked on red clay brick pavers and Ed [8]. This finding should not be surprising, as it seems logical to think in a proportional relationship between the amount of energy delivered to the system and the volume of melted material. Fig. 7 shows how the width and the depth of the lines increase with the different Ed. The central zone usually corresponds to the 75% while borders correspond to the 25% of the total width of the lines. Following the mechanisms suggested in Restrepo et al. to the laser vitrification of red clay brick pavers [8], both width and depth dimensions fit quite well linear functions of the square root of Ed.
7737
4. Conclusion Green lines of a chromium oxide based enamel with good visual characteristics can be printed on white glazed tiles with a Nd:YAG laser operating in continuous wave mode. The characteristics of the lines are basically determined by the accumulated energy density of the laser beam which is determined by the combination of beam power and scan speed. A minimum accumulated energy density is required to allow the melting of the glazed substrate in order to allow the adhesion of the lines. The lines present a glassy central zone, where enamel material is mixed in the melted glazed substrate, and borders with darker tones, where enamel forms a layer attached to the glazed substrate. Evaporation of the raw enamel occurs during irradiation, basically affecting the frit components. The amount of material forming the lines does not increase significantly until the accumulated energy density reaches 500 J/cm2. There is a linear relationship between the accumulated energy density and the volume of melted glazed substrate where enamel is incorporated. The width and depth of the lines increase with the accumulated energy density similar to a linear function of its square root. References [1] A. Pe´rez del Pino, J.M. Ferna´ndez-Pradas, P. Serra, J.L. Morenza, Surf. Coat. Technol. 187 (2004) 106. [2] H. Gugger, F. Herren, M. Hofmann, A. Pugin, Laser marking of ceramic materials, glazes, glass ceramics and glasses, US Patent 4,769,310 (1988). [3] P.W. Harrison, High contrast surface marking using metal oxides, US Patent 6,313,436 (2000). [4] A. Hory, J.M. Gaillard, Method and device for marking objects with sintered mineral powders, US Patent application n. 20030012891 (2003). [5] R. Akhter, L. Li, R.E. Edwards, A.W. Gale, Appl. Surf. Sci. 208/209 (2003) 453. [6] R. Akhter, L. Li, R.E. Edwards, A.W. Gale, Appl. Surf. Sci. 208/209 (2003) 447. [7] D. Triantafyllidis, L. Li, F.H. Stott, Appl. Surf. Sci. 208/209 (2003) 458. [8] J.W. Restrepo, J.M. Ferna´ndez-Pradas, M.A. Go´mez, P. Serra, J.L. Morenza, J. Laser Appl. 18 (2006) 156.