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The fabrication of porous coatings using laser spark atomisation
Materials Science and Engineering A 375–377 (2004) 585–588
The fabrication of porous coatings using laser spark atomisation R. Houriet∗ , H. Hofmann, Y. Houst, M. DeCock, C. Arimatea, J. Antifakos Laboratoire de Technologie des Poudres, Institut des Matériaux-EPFL, CH-1015 Lausanne, Switzerland
Abstract The laser spark atomiser (LSA) technique, LINA-SPARKTM is used to prepare porous coatings. It is shown that the crystalline phase of the target material is conserved after atomisation and film formation. Silicon forms oxide-free films under inert atmosphere (Ar), whereas it forms silicon oxide when adding 20% oxygen to Ar, and silicon nitride in pure nitrogen atmosphere. Films made out of calcium phosphate are deposited on titanium substrates. It is expected that their porosity will improve the prosthesis-bone reconstruction after chirurgical operation. Future application in the field of microsensors is also discussed. Finally, it is shown that running the LSA under low rate of particle formation, it is possible to deposit isolated Si particles onto Si substrate thus enabling microstructuration of the surface. © 2003 Elsevier B.V. All rights reserved. Keywords: Laser spark atomisation; Nanoparticles; Porous coatings; Calcium phosphate; Microstructuration
1. Introduction The laser spark atomiser (LSA) technique, LINASPARKTM , was recently developed in our laboratory. It combines the advantages of operating at atmospheric pressure and ambient temperature to produce highly uniform porous films which can be deposited at relatively high deposition rates (ca. 20 g s−1 ). The preparation of alumina nanopowders and coating starting from commercial ceramic specimen, and other nanomaterials including zirconia, tin dioxide, lithium manganate and graphite has been previously described [1]. Highly agglomerated nanopowders and homogeneous mesoporous coatings forming a web-like structure are prepared using this technique. The primary particles consist of nanocrystalline aggregates with dimensions ranging from 5 to 10 nm. In the present study, we firstly present results showing the identity of the phase between the target to the LSA deposited film using a calcium silicate sample. Results obtained with silicon target and showing the influence of the atmosphere are then presented. We then present recent results obtained with the substrate placed under vacuum. In this configuration, the particles in the aerosol are impacted on the substrate in order to improve the coating to substrate adhesion. It is
∗ Corresponding author. Tel.: +41-21-693-4908; fax: +41-21-693-3089. E-mail address: [email protected] (R. Houriet).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.090
shown that calcium phosphate coatings on titanium with adequate mechanical stability and biocompatibility can be produced. Application in the field of microsensors is also presented. By varying the experimental parameters, it is shown that microstructuration of surface can be achieved, in this case we present the deposition of single silicon particles of ca. 7–8 nm onto silicon substrate.
2. Experimental The LINA-SPARKTM atomizer has been already described [1b]. It is equipped with a Q-switched Nd-YAG laser at 1064 nm delivering 300 mJ pulses at 20 Hz. The 6 mm diameter laser beam is focused at proximity of the target and the whole system operates under argon atmosphere with a typical argon flow rate of 1 l min−1 . The 7 ns long laser pulses create plasma pulses of approximately 1 s duration. The argon plasma thus formed evaporates material from the target over a circular area of about 1.2 mm diameter with a typical removal rate of about 0.003 mm s−1 corresponding to about 20 g of sample per second. In order to avoid the formation of crater on the sample, the laser beam can be moved over the target in such a way to produce a circular sampled area of ca. 2 mm diameter. For the impacting deposition, the substrate is placed in a vacuum can on a holder that can be moved in the x and y directions. The vacuum system uses a 253 m3 h−1 Roots pump (Leybold) equipped with a Trivac B (Leybold)
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R. Houriet et al. / Materials Science and Engineering A 375–377 (2004) 585–588
forevacuum pump, a typical deposition pressure of 10−2 mbar is generally used.
3. Results and discussion 3.1. Calcium silicate (C3 S) nanoparticulate coatings Fig. 1 shows the SEM micrographs of C3 S powders deposited using the LSA set-up. Homogeneous coatings of about 20 m thickness were obtained after 20 min deposition time. These coatings consist of nanoparticulate materials piled up in a three-dimensional mesoporous network forming a web-like structure. The film microstructure looks like a fractal-like aggregates of coalesced C3 S nanocrystals. Obtaining such highly porous structure at high deposition rate (ca. 1 m min−1 ) is one of the unique features allowed by this laser ablation technique. As can be seen in Fig. 1, the mesoporous layers are defect-free, showing that the laser spark atomizer is a suitable technique to produce powder particles and defect-free particulate coatings. These films have about 95% porosity as determined by comparing the thickness measurements using SEM and micro-X-ray fluorescence spectroscopy [1b].
Fig. 2. HRTEM micrograph of Si film formed by LSA.
XRD measurements have shown that the crystalline phase is conserved, this has been also observed in the deposition of alumina, tin dioxide and lithium manganate samples, no counter-example has been encountered so far. 3.2. Silicon Given the ability of the LSA technique at preparing nanocrystalline material, we atomised a silicon wafer target which produced a coating again showing a porous nanocrystalline structure. The HRTEM micrograph showed in Fig. 2 reveals that the silicon particles are oxide-free, in contrast to the silicon particles prepared by a plasma enhanced CVD process [2]. The absence of silicon oxide was checked by infra-red measurements which show no Si–O stretch absorption in the wavenumber range from 1050 to 1100 cm−1 . No Si–O absorption could neither be detected even after leaving the sample in air for a longer period of time (3 months). In order to test the possibility for reaction bonding, we atomised the silicon wafer in controlled atmospheres, firstly in argon containing 20% oxygen, and secondly in pure nitrogen. The infra-red spectrum shown in Fig. 3 clearly shows
Fig. 1. SEM micrographs of he calcium silicate coating.
Fig. 3. Infra-red spectra of LSA films formed from a Si wafer in various atmospheres.
R. Houriet et al. / Materials Science and Engineering A 375–377 (2004) 585–588
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an intense Si–O stretch absorption at ca.1070 cm−1 for the first sample indicating the formation of silicon oxide. We could test that pure SiO2 was formed as soon as the atmosphere contained 50% oxygen. Fig. 3 also shows that under pure nitrogen atmosphere, silicon nitride is readily formed as shown by the intense Si–N absorption at ca. 905 cm−1 . However, multiple weaker absorptions can also be seen in the 1000–1150 cm−1 range indicating that oxynitrides were also formed in this process. The long-term stability test showed a tendency for the infra-red absorptions to in the latter wavelength domain, thus indicating that oxydation is taking place over longer periods of time.
4. Deposition with the substrate under vacuum (impacting) As mentioned above, these conditions provide a better control of the adhesion of the coating to the substrate. We observed that under these conditions, the porosity of the resulting coating is somewhat reduced, down to about 65%. On the other hand, a significant improvement of the adhesion was observed, e.g. the coating pass the reference “scotch-tape test”. These conditions were used in two directions. Firstly, for depositing calcium phosphate (CaOP) coatings onto titanium substrate. We believe that the reconstruction of the bone tissue can be enhanced by using our porous coatings. A typical CaOP coating is shown in Fig. 4. These coatings were found to have excellent biocompatibility, cell proliferation tests showed over 90% proliferation of these surfaces with respect to a reference (polystyrene) surface. Secondly, tin dioxide was deposited on a microchip used in the gas sensor technology, see Fig. 5. The performances of the sensor were similar to existing high density coat-
Fig. 4. Calcium phosphate coating.
ings. Experiments are in progress in order to demonstrate that the porous nature of these coatings can be an advantage with respect to the performances of such devices i.e., the porous film can act as a filter thus preventing the poisoning of the sensor by foreign particles and thus significantly reducing the drift and extending the lifetime of the device.
5. Micostructuration The microstructuration of surface can be achieved by running the LSA under low conversion conditions (large Ar flow, low repetition rate and low energy of the laser). Fig. 6 shows the AFM micrograph obtained by the deposition of single silicon particles of ca. 7–8 nm diameter onto silicon substrate.
Fig. 5. Microsensor chip (left) and deposited tin dioxide layer (right).
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R. Houriet et al. / Materials Science and Engineering A 375–377 (2004) 585–588
production. We could show that varying the argon flow rate allows for a controlled change in the microstructure of the thick film. Therefore, the nature of the gas, a reduced distance of powder collection, the temperature and the laser power are the main parameters that help for controlling the size of the condensing particles and also modify the degree of agglomeration. These conditions also allow for reaction bonding with the atmospheric molecules as shown in the experiments carried out with silicon under oxygen containing atmospheres and nitrogen. More experiments have to carried out in order to gain further insight into the mechanisms underlying particle production in the LSA and also to determine the effect of the experimental parameters on the production of nanopowders and coatings and the agglomeration of nanoparticles. Acknowledgements
Fig. 6. AFM picture of Si particles deposited on Si substrate.
6. Conclusions The present results show that LSA can be an efficient means for deposition of relatively thick films and powder production directly from sintered samples and other target materials. Homogeneous coatings over large surfaces (ca. 11.5 cm2 ) are readily deposited at a rate close to 1 m min−1 . The experimental conditions, atmospheric pressure and ambient temperature, make this laser sparking technique quite a convenient means for powder and highly porous film
We thank C. Soares, B. Senior and the Electron Microscopy Center of EPFL (CIME) for the electron micrographs.
References [1] (a) R. Houriet, R. Vacassy, H. Hofmann, W. Vogel, Mater. Res. Soc. Symp. Proc. 526 (1998) 117; (b) R. Houriet, R. Vacassy, H. Hofmann, Nanostruct. Mater. 11 (1999) 1155; (c) D. Singh, R. Houriet, R. Giovannini, H. Hofmann, V. Craciun, R.K. Singh, J. Power Sources 97–98 (2001) 826; (d) R. Houriet, R. Vacassy, H. Hofmann, Y. Von Kaenel, H. Hofmeister, Carbon 39 (2001) 1421. [2] J. Dutta, H. Hofmann, R. Houriet, H. Hofmeister, C. Hollenstein, Colloids Surf. A 127 (1997) 263.
The fabrication of porous coatings using laser spark atomisation R. Houriet∗ , H. Hofmann, Y. Houst, M. DeCock, C. Arimatea, J. Antifakos Laboratoire de Technologie des Poudres, Institut des Matériaux-EPFL, CH-1015 Lausanne, Switzerland
Abstract The laser spark atomiser (LSA) technique, LINA-SPARKTM is used to prepare porous coatings. It is shown that the crystalline phase of the target material is conserved after atomisation and film formation. Silicon forms oxide-free films under inert atmosphere (Ar), whereas it forms silicon oxide when adding 20% oxygen to Ar, and silicon nitride in pure nitrogen atmosphere. Films made out of calcium phosphate are deposited on titanium substrates. It is expected that their porosity will improve the prosthesis-bone reconstruction after chirurgical operation. Future application in the field of microsensors is also discussed. Finally, it is shown that running the LSA under low rate of particle formation, it is possible to deposit isolated Si particles onto Si substrate thus enabling microstructuration of the surface. © 2003 Elsevier B.V. All rights reserved. Keywords: Laser spark atomisation; Nanoparticles; Porous coatings; Calcium phosphate; Microstructuration
1. Introduction The laser spark atomiser (LSA) technique, LINASPARKTM , was recently developed in our laboratory. It combines the advantages of operating at atmospheric pressure and ambient temperature to produce highly uniform porous films which can be deposited at relatively high deposition rates (ca. 20 g s−1 ). The preparation of alumina nanopowders and coating starting from commercial ceramic specimen, and other nanomaterials including zirconia, tin dioxide, lithium manganate and graphite has been previously described [1]. Highly agglomerated nanopowders and homogeneous mesoporous coatings forming a web-like structure are prepared using this technique. The primary particles consist of nanocrystalline aggregates with dimensions ranging from 5 to 10 nm. In the present study, we firstly present results showing the identity of the phase between the target to the LSA deposited film using a calcium silicate sample. Results obtained with silicon target and showing the influence of the atmosphere are then presented. We then present recent results obtained with the substrate placed under vacuum. In this configuration, the particles in the aerosol are impacted on the substrate in order to improve the coating to substrate adhesion. It is
∗ Corresponding author. Tel.: +41-21-693-4908; fax: +41-21-693-3089. E-mail address: [email protected] (R. Houriet).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.090
shown that calcium phosphate coatings on titanium with adequate mechanical stability and biocompatibility can be produced. Application in the field of microsensors is also presented. By varying the experimental parameters, it is shown that microstructuration of surface can be achieved, in this case we present the deposition of single silicon particles of ca. 7–8 nm onto silicon substrate.
2. Experimental The LINA-SPARKTM atomizer has been already described [1b]. It is equipped with a Q-switched Nd-YAG laser at 1064 nm delivering 300 mJ pulses at 20 Hz. The 6 mm diameter laser beam is focused at proximity of the target and the whole system operates under argon atmosphere with a typical argon flow rate of 1 l min−1 . The 7 ns long laser pulses create plasma pulses of approximately 1 s duration. The argon plasma thus formed evaporates material from the target over a circular area of about 1.2 mm diameter with a typical removal rate of about 0.003 mm s−1 corresponding to about 20 g of sample per second. In order to avoid the formation of crater on the sample, the laser beam can be moved over the target in such a way to produce a circular sampled area of ca. 2 mm diameter. For the impacting deposition, the substrate is placed in a vacuum can on a holder that can be moved in the x and y directions. The vacuum system uses a 253 m3 h−1 Roots pump (Leybold) equipped with a Trivac B (Leybold)
586
R. Houriet et al. / Materials Science and Engineering A 375–377 (2004) 585–588
forevacuum pump, a typical deposition pressure of 10−2 mbar is generally used.
3. Results and discussion 3.1. Calcium silicate (C3 S) nanoparticulate coatings Fig. 1 shows the SEM micrographs of C3 S powders deposited using the LSA set-up. Homogeneous coatings of about 20 m thickness were obtained after 20 min deposition time. These coatings consist of nanoparticulate materials piled up in a three-dimensional mesoporous network forming a web-like structure. The film microstructure looks like a fractal-like aggregates of coalesced C3 S nanocrystals. Obtaining such highly porous structure at high deposition rate (ca. 1 m min−1 ) is one of the unique features allowed by this laser ablation technique. As can be seen in Fig. 1, the mesoporous layers are defect-free, showing that the laser spark atomizer is a suitable technique to produce powder particles and defect-free particulate coatings. These films have about 95% porosity as determined by comparing the thickness measurements using SEM and micro-X-ray fluorescence spectroscopy [1b].
Fig. 2. HRTEM micrograph of Si film formed by LSA.
XRD measurements have shown that the crystalline phase is conserved, this has been also observed in the deposition of alumina, tin dioxide and lithium manganate samples, no counter-example has been encountered so far. 3.2. Silicon Given the ability of the LSA technique at preparing nanocrystalline material, we atomised a silicon wafer target which produced a coating again showing a porous nanocrystalline structure. The HRTEM micrograph showed in Fig. 2 reveals that the silicon particles are oxide-free, in contrast to the silicon particles prepared by a plasma enhanced CVD process [2]. The absence of silicon oxide was checked by infra-red measurements which show no Si–O stretch absorption in the wavenumber range from 1050 to 1100 cm−1 . No Si–O absorption could neither be detected even after leaving the sample in air for a longer period of time (3 months). In order to test the possibility for reaction bonding, we atomised the silicon wafer in controlled atmospheres, firstly in argon containing 20% oxygen, and secondly in pure nitrogen. The infra-red spectrum shown in Fig. 3 clearly shows
Fig. 1. SEM micrographs of he calcium silicate coating.
Fig. 3. Infra-red spectra of LSA films formed from a Si wafer in various atmospheres.
R. Houriet et al. / Materials Science and Engineering A 375–377 (2004) 585–588
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an intense Si–O stretch absorption at ca.1070 cm−1 for the first sample indicating the formation of silicon oxide. We could test that pure SiO2 was formed as soon as the atmosphere contained 50% oxygen. Fig. 3 also shows that under pure nitrogen atmosphere, silicon nitride is readily formed as shown by the intense Si–N absorption at ca. 905 cm−1 . However, multiple weaker absorptions can also be seen in the 1000–1150 cm−1 range indicating that oxynitrides were also formed in this process. The long-term stability test showed a tendency for the infra-red absorptions to in the latter wavelength domain, thus indicating that oxydation is taking place over longer periods of time.
4. Deposition with the substrate under vacuum (impacting) As mentioned above, these conditions provide a better control of the adhesion of the coating to the substrate. We observed that under these conditions, the porosity of the resulting coating is somewhat reduced, down to about 65%. On the other hand, a significant improvement of the adhesion was observed, e.g. the coating pass the reference “scotch-tape test”. These conditions were used in two directions. Firstly, for depositing calcium phosphate (CaOP) coatings onto titanium substrate. We believe that the reconstruction of the bone tissue can be enhanced by using our porous coatings. A typical CaOP coating is shown in Fig. 4. These coatings were found to have excellent biocompatibility, cell proliferation tests showed over 90% proliferation of these surfaces with respect to a reference (polystyrene) surface. Secondly, tin dioxide was deposited on a microchip used in the gas sensor technology, see Fig. 5. The performances of the sensor were similar to existing high density coat-
Fig. 4. Calcium phosphate coating.
ings. Experiments are in progress in order to demonstrate that the porous nature of these coatings can be an advantage with respect to the performances of such devices i.e., the porous film can act as a filter thus preventing the poisoning of the sensor by foreign particles and thus significantly reducing the drift and extending the lifetime of the device.
5. Micostructuration The microstructuration of surface can be achieved by running the LSA under low conversion conditions (large Ar flow, low repetition rate and low energy of the laser). Fig. 6 shows the AFM micrograph obtained by the deposition of single silicon particles of ca. 7–8 nm diameter onto silicon substrate.
Fig. 5. Microsensor chip (left) and deposited tin dioxide layer (right).
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R. Houriet et al. / Materials Science and Engineering A 375–377 (2004) 585–588
production. We could show that varying the argon flow rate allows for a controlled change in the microstructure of the thick film. Therefore, the nature of the gas, a reduced distance of powder collection, the temperature and the laser power are the main parameters that help for controlling the size of the condensing particles and also modify the degree of agglomeration. These conditions also allow for reaction bonding with the atmospheric molecules as shown in the experiments carried out with silicon under oxygen containing atmospheres and nitrogen. More experiments have to carried out in order to gain further insight into the mechanisms underlying particle production in the LSA and also to determine the effect of the experimental parameters on the production of nanopowders and coatings and the agglomeration of nanoparticles. Acknowledgements
Fig. 6. AFM picture of Si particles deposited on Si substrate.
6. Conclusions The present results show that LSA can be an efficient means for deposition of relatively thick films and powder production directly from sintered samples and other target materials. Homogeneous coatings over large surfaces (ca. 11.5 cm2 ) are readily deposited at a rate close to 1 m min−1 . The experimental conditions, atmospheric pressure and ambient temperature, make this laser sparking technique quite a convenient means for powder and highly porous film
We thank C. Soares, B. Senior and the Electron Microscopy Center of EPFL (CIME) for the electron micrographs.
References [1] (a) R. Houriet, R. Vacassy, H. Hofmann, W. Vogel, Mater. Res. Soc. Symp. Proc. 526 (1998) 117; (b) R. Houriet, R. Vacassy, H. Hofmann, Nanostruct. Mater. 11 (1999) 1155; (c) D. Singh, R. Houriet, R. Giovannini, H. Hofmann, V. Craciun, R.K. Singh, J. Power Sources 97–98 (2001) 826; (d) R. Houriet, R. Vacassy, H. Hofmann, Y. Von Kaenel, H. Hofmeister, Carbon 39 (2001) 1421. [2] J. Dutta, H. Hofmann, R. Houriet, H. Hofmeister, C. Hollenstein, Colloids Surf. A 127 (1997) 263.