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Modes of the use of high intensity plasma beams for ceramic surface modification
Surface & Coatings Technology 206 (2011) 916–919
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Modes of the use of high intensity plasma beams for ceramic surface modification M. Barlak a,b,⁎, J. Piekoszewski a,b,†, Z. Werner a,c, B. Sartowska b, M. Pisarek c,d, L. Walis b, W. Starosta b, A. Kolitsch e, R. Gröetzchel e, K. Bochenska a, C. Pochrybniak a,f a
The Andrzej Soltan Institute for Nuclear Studies, 05-400 Otwock-Swierk, Poland Institute of Nuclear Chemistry and Technology, 16 Dorodna, 03-145 Warsaw, Poland Institute of Physical Chemistry, PAS, 44/52 Kasprzaka, 01-224 Warsaw, Poland d Warsaw University of Technology, 141 Woloska, 02-507 Warsaw, Poland e Forschunzentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, PO Box 51 01 19, D-01314 Dresden, Germany f Institute of Atomic Energy, 05-400 Otwock-Swierk, Poland b c
a r t i c l e
i n f o
Available online 17 April 2011 Keywords: Wettability High intensity pulsed plasma beams Ceramic
a b s t r a c t Wetting properties of ceramic materials may be enhanced by treating them with high-intensity plasma pulses carrying a substantial fraction of metallic ions. Rod Plasma Injectors (RPI), developed originally for fusion studies, may generate such plasma pulses containing the working gas used for discharge initiation and the metal ions eroded from the discharge electrodes. We examined the plasma pulse technology and concluded that it is possible to extend the range of system parameters appropriate for wetting enhancement. We also studied the physical properties of plasma treated carbon and silicon carbide samples in an attempt to disclose the origin of wettability differences between them. We finally conclude that these differences are due to the morphology of the treated surfaces. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The technology of ceramics is a rapidly developing branch of material science. Ceramics feature low density, high mechanical stress and corrosion resistance, also at elevated temperatures [1,2]. One of the crucial problems of the use of ceramics is their joining with other materials. In most of ceramic-metal systems wetting is not observed. Over the past decades several means of inducing wetting of ceramics have been developed. Usually the preparation of ceramic surface requires an adoption of the procedure consisting of several steps. Recently we undertook attempts to simplify this procedure by using pulse plasma generators originally developed at our institute for fusion research. The modification procedure consists in treating the ceramic samples with high-intensity plasma pulses generated in a Rod Plasma Injector (RPI) type of generator [3,4]. These pulses carry sufficient energy to melt the surface layer thus allowing penetration of the layer by ions carried by plasma pulse [5]. When the pulse contains metallic ions an enhancement of the surface in metal content occurs thus
⁎ Corresponding author at: The Andrzej Soltan Institute for Nuclear Studies, 05-400 Otwock-Swierk, Poland. Tel.: +48 22 7180644; fax: +48 22 7793481. E-mail address: [email protected] (M. Barlak). † Deceased. 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.04.028
favoring its wetting properties. The effect accumulates for multiple pulses. The RPI machine is presented schematically in Fig. 1. An evacuated chamber is equipped with a set of two concentric, crown-type electrodes made of metallic rods connected to a charged capacitor bank through a spark trigger. A fast gas-injection valve is placed behind the endpoint of the electrodes. The plasma pulse generation starts with pulse gas injection through the valve. A small (1 cm2) injected portion of gas expands in the interelectrode volume. For RT conditions and for the valve– electrode distance of 5 cm, the time for a typical gas like nitrogen (average expansion speed — 500 m/s) to expand to the end of electrodes is about 100 μs. After an electronically controlled delay time τD the trigger initiates the low pressure discharge between the electrodes. At some delay τB after the pulse initiation the electromagnetic forces acting on the plasma discharge eject a plasma stream towards the sample. A sudden current break produces an LdI/dt inductive surcharge on the electrodes leading to a significant acceleration of the ejected ions. Fig. 2 shows a typical computer oscilloscope run of voltage (upper part) and current (lower part) after the injection. The delay time τB is a complicated stochastic function of several discharge parameters, primarily, but not exclusively of τD and therefore may be associated with the period of gas expansion within the interelectrode volume. When τD decreases the plasma becomes enriched in the metal ions eroding from the electrode ends whereas for long τD it contains pure
M. Barlak et al. / Surface & Coatings Technology 206 (2011) 916–919
917
Fig. 1. Scheme of RPI-15 machine.
working gas. For historical reasons we refer to the conditions of pure gas pulse as Pulse Implantation Doping (PID)—long τD and τB and to the one with eroded metal content one as Deposition by Pulse Erosion (DPE)—(short τD and τB). The DPE process in the substrate proceeds as follows. First, the plasma pulse containing the most energetic working gas and the most of pulse energy ions melts the near surface layer of the substrate. The time interval of energetic ions operation is of the order of 0.1–1.0 μs with the melt duration in microsecond scale and the melt depth of 0.1–2.0 μm. Diffusion constants of impurities in the liquid phase are higher by several orders of magnitude than those in solid phase and range usually from 1e-5 to 1e-4 cm2s−1. Thus a rapid impurity diffusion into the molten phase occurs. When the surface becomes resolidified the slower metal vapor atoms and ions eroded from electrodes reach the substrate forming a few nm thick film deposit. The subsequent pulses melt both the metallic film and the near surface layer of the substrate. In both cases PID and DPE, similar to the laser irradiation, some amount of a substrate evaporates forming a vapor plume exerting a pressure on the melted layer and hence preventing its further evaporation. In our papers on DPE process in C–Ti and SiC–Ti system published thus far [6–8] we used a fixed τB and we focused our attention mainly on basic physical conditions which must be satisfied to assure a good wettability with liquid copper. In particular we have found that: 1) area concentration of alloyed titanium should be sufficiently high, i.e. no less than 3e18 cm−2, 2) top layer to a depth of about 0.25 μm must contain at least 60 at.% of Ti atoms,
3) Ti concentration should gradually decrease with depth without a sharp step just beneath the surface. In the present work we have attempted to find a useful period of current break delay τB enabling efficient processing. In addition we examined elemental distribution of the deposited material on the ceramic surface and we analyzed the chemical state of surface with respect to its influence on the wettability. 2. Experimental Glassy carbon (Cg), pyrolytic carbon (Cp), carbon–carbon composite (C/C) and silicon carbide (SiC) were used as substrate materials. The samples were processed in the IBIS pulse plasma generator [3,4] with parameters given in Table 1, using nitrogen gas and the range of τD between 170 and 220 μs and the condenser bank voltage of 25–35 kV. The system was adjusted to vary the delay τB between 2.5 and 5 μs. Under such conditions the plasma gun generates pulses of duration about 1 μs and energy density adjusted in the range of 1–7 J cm−2. The energies of ions in the generated plasma pulse range from a few tens to thousands electron volt. The effect of system parameters was studied by applying three pulses of 2 J cm−2 at a given τB and examining the surface concentration of Ti by EDX using Brucker EDX Quantax 400 insert to Zeiss DSM 942 electron microscope. For examination of the elemental distribution of the deposited material on a ceramic surface the samples referred to as “200 Ti multi” were irradiated in succession with 20, 40, 60 and 80 pulses of energy density of 7 J cm−2, 5 J cm−2, 3 J cm−2 and 1 J cm−2, respectively. After processing the samples of “200 Ti multi” type the “sessile drop” wettability test was performed. A strip of 4 N Cu foil of 3 × 2×0.5 mm3 was placed on the sample prior to loading to a box-type vacuum furnace. The wettability test process was carried at 1150 ± 5 °C and a pressure of 5e-3 Pa for about 15 min.
Table 1 Technical parameters of RPI 15 plasma generator.
Fig. 2. Typical oscilloscope traces of voltage (upper part) and current (lower part) during PRI pulse.
Type
RPI-15
Condenser bank capacity Condenser voltage Total condenser energy Working gas Injected gas pressure Outer/inner electrode diameter Electrode length Electrode material Maximum discharge current intensity Pulse energy density Pulse duration
72 μF 25–35 kV Up to 45 kJ Nitrogen or argon 1.5 atm 125/90 mm 25.0 mm Ti Up to 300 kA 3–7 J/cm2 0.5–1 μs
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M. Barlak et al. / Surface & Coatings Technology 206 (2011) 916–919
Fig. 3. The concentration of deposited titanium, as determined by EDX, vs. current breakdown time τB for SiC substrates.
Subsequently the samples were photographed and the wetting angle was determined using a home-made computer-supported sessile drop tester. The structural and compositional state of the near-surface region of samples were characterized by: scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction at a glancing angle ω = 1° (GXRD) and X-ray photoelectron spectroscopy (XPS). Prior to the measurements all samples were ion etched with 3 keV Ar ions for 3 min.
3. Results and discussion Fig. 3 presents an example of dependence of the concentration of deposited titanium on delay τB for SiC substrates (depth of information amounts to about 0.5 μm). Runs for Cp, Cg and C/C are similar to that for SiC. Careful inspection of Fig. 3 combined with the analysis of scatter of experimental data (non cited here) reveals three regions of machine operation: region I for τB = 2.5 to 3.0 μs in which machine reproducibility is poor, region II, in which machine operates with a fair reproducibility, material deposition rate is satisfactory and does not deviate more than 50% down from the maximum value and region III featuring good reproducibility but non satisfactory deposition efficiency. It is worth noting that in Ti concentration in regions II and III increasing decreases monotonically with an increase of τB. For
Ti electrodes and N2 as the working gas the precision of τB control amounts to 0.1 μs. SEM examination of the C/C and SiC surfaces after “200-Ti-multi” process together with elemental mapping of the surface suggest occurrence of chemical system such as: Ti–N, Ti–O, Ti–C and Si–N (in SiC). According to our thermodynamical considerations of Gibbs energy balance formation of compounds in these systems is possible [5,6]. In order to get insight into details of chemical composition formed by titanium the GXRD patterns and XPS spectra were recorded on C/C and SiC samples. Fig. 4 presents our X-ray results. The pattern of the virgin sample is compared to that of the processed one. In both cases the near surface region of “200 Ti multi” processed samples exhibits the reflections corresponding to TiC and TiN phases. In addition in the case of SiC also the Si reflections can be identified. These reflections were already identified for Ti plasma pulses with argon as a working gas [5] and hence we may relate them with reaction bSiCN + (Ti) → bTiCN + bSiN (ΔG is − 541 kJ/mol at 298 K and −327 kJ/mol at 1685 K). Although we do not claim that this reaction promotes wetting (in fact little is known about silicon wetting by copper) the possibility of fast silicon reduction by titanium ions is an evident difference between carbon and carbide surfaces. In Fig. 5 a comparison of XPS spectra of the virgin and the processed C/C and SiC is shown. The characteristic hump centered at about 700 eV, seen only in SiC sample cannot be removed by ion etching and is normally attributed to oxynitride contaminations. The spectra of processed samples are generally similar to each other, apart from: – lack of silicon in the SiC sample, – high C concentration (in graphite form) in SiC sample, exceeding the 50% stoichiometric content and suggesting a dissociation of SiC at the surface. Analysis of spectra observed in “200 Ti multi” treated samples revealed the presence of the following components: Ti (metallic), C (graphite), TixOy, Ti2O3/TiOxNy, and TixNy. According to literature [9] if titanium oxides and nitrides are hypostoichiometric i.e. y/x is less than 0.5–0.65 then they are beneficial for wetting. Fig. 6 shows the photos of the results of wettability tests conducted on virgin and treated C/C and SiC samples. The observed contact angles are 83° and 33° respectively.
Fig. 4. GXRD patterns recorded on C/C and SiC samples.
M. Barlak et al. / Surface & Coatings Technology 206 (2011) 916–919
919
Fig. 5. A comparison of XPS spectra of virgin and processed C/C and SiC.
delay time τB between the moment of discharge ignition and the moment of current breakdown for Ti electrodes and N2 as the working gas, is 3.0 to 3.5 μs. 2. Chemical characterization of the processed substrate ceramics reveals no essential differences which could influence the observed variations in the degree of wettability. 3. Apart from the previously revealed factors, surface morphology seems to be a decisive one that affects the desired modification. Acknowledgements This work was financed within the funds of Ministry of Education and Science of Poland for scientific projects in the period 2008–2010 under contract No. 3942/B/T02/2008/35. Part of the experiment was performed at AIM Rossendorf within the frame of RITA project (Research Infrastructures Transnational Access Contract Number 025646). The authors are very indebted to Mr. J. Krolik, Mr. K. Gatarczyk, Mr. H. Matosek and Mr. J. Zagorski for technical assistance. Fig. 6. Photos of the results of wettability tests conducted on virgin and pulse-treated C/C and SiC samples.
Neither the GXRD nor XPS results allow us to determine unambiguously the reason why the treated SiC exhibits much better wettability than C/C composite. Therefore, we believe that this is due to the morphological features of these materials. It follows from SEM images that the surface of SiC is smoother whereas nodules are present in C/C surface.
References [1] [2] [3] [4] [5] [6]
[7]
4. Conclusions
[8]
1. For wetting creation of the ceramics surface using high intensity pulsed plasma beams in DPE mode the appropriate range of the
[9]
Y. Liang, S.P. Dutta, Technovation 21 (2001) 61. M. Rosso, J. Mat. Proc. Tech. 175 (2006) 364. J. Piekoszewski, J. Langner, Nucl. Instr. Meth. B53 (1991) 148. Z. Werner, J. Piekoszewski, W. Szymczyk, Vacuum 63 (2001) 701. Z. Werner, J. Piekoszewski, A. Barcz, R. Groetschel, F. Prokert, J. Stanisławski Szymczyk, Nucl. Inst. Meth. B 175–177 (2001) 767. M. Barlak, J. Piekoszewski, J. Stanislawski, K. Borkowska, M. Chmielewski, B. Sartowska, Z. Werner, M. Miskiewcz, J. Jagielski, W. Starosta, Fusion Eng. Des. 82 (2007) 2524. M. Barlak, J. Piekoszewski, Z. Werner, J. Stanislawski, E. Skladnik-Sadowska, K. Bochenska, M. Miskiewicz, A. Kolitsch, R. Grötzschel, W. Starosta, B. Sartowska, J. Kierzek, Surf. Coat. Tech. 203 (2009) 2536. M. Barlak, J. Piekoszewski, Z. Werner, Z. Pakiela, B. Sartowska, E. SkladnikSadowska, L. Walis, J. Kierzek, W. Starosta, A. Kolitsch, R. Gröetzchel, K. Bochenska, Vacuum 83 (2009) 81. M.G. Nicholas, Material Science and Technology, Processing of ceramics, Vol. 17B Part II, Weinheim, New York, Basel, Cambridge, Tokyo, 1996, p. 262.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Modes of the use of high intensity plasma beams for ceramic surface modification M. Barlak a,b,⁎, J. Piekoszewski a,b,†, Z. Werner a,c, B. Sartowska b, M. Pisarek c,d, L. Walis b, W. Starosta b, A. Kolitsch e, R. Gröetzchel e, K. Bochenska a, C. Pochrybniak a,f a
The Andrzej Soltan Institute for Nuclear Studies, 05-400 Otwock-Swierk, Poland Institute of Nuclear Chemistry and Technology, 16 Dorodna, 03-145 Warsaw, Poland Institute of Physical Chemistry, PAS, 44/52 Kasprzaka, 01-224 Warsaw, Poland d Warsaw University of Technology, 141 Woloska, 02-507 Warsaw, Poland e Forschunzentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, PO Box 51 01 19, D-01314 Dresden, Germany f Institute of Atomic Energy, 05-400 Otwock-Swierk, Poland b c
a r t i c l e
i n f o
Available online 17 April 2011 Keywords: Wettability High intensity pulsed plasma beams Ceramic
a b s t r a c t Wetting properties of ceramic materials may be enhanced by treating them with high-intensity plasma pulses carrying a substantial fraction of metallic ions. Rod Plasma Injectors (RPI), developed originally for fusion studies, may generate such plasma pulses containing the working gas used for discharge initiation and the metal ions eroded from the discharge electrodes. We examined the plasma pulse technology and concluded that it is possible to extend the range of system parameters appropriate for wetting enhancement. We also studied the physical properties of plasma treated carbon and silicon carbide samples in an attempt to disclose the origin of wettability differences between them. We finally conclude that these differences are due to the morphology of the treated surfaces. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The technology of ceramics is a rapidly developing branch of material science. Ceramics feature low density, high mechanical stress and corrosion resistance, also at elevated temperatures [1,2]. One of the crucial problems of the use of ceramics is their joining with other materials. In most of ceramic-metal systems wetting is not observed. Over the past decades several means of inducing wetting of ceramics have been developed. Usually the preparation of ceramic surface requires an adoption of the procedure consisting of several steps. Recently we undertook attempts to simplify this procedure by using pulse plasma generators originally developed at our institute for fusion research. The modification procedure consists in treating the ceramic samples with high-intensity plasma pulses generated in a Rod Plasma Injector (RPI) type of generator [3,4]. These pulses carry sufficient energy to melt the surface layer thus allowing penetration of the layer by ions carried by plasma pulse [5]. When the pulse contains metallic ions an enhancement of the surface in metal content occurs thus
⁎ Corresponding author at: The Andrzej Soltan Institute for Nuclear Studies, 05-400 Otwock-Swierk, Poland. Tel.: +48 22 7180644; fax: +48 22 7793481. E-mail address: [email protected] (M. Barlak). † Deceased. 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.04.028
favoring its wetting properties. The effect accumulates for multiple pulses. The RPI machine is presented schematically in Fig. 1. An evacuated chamber is equipped with a set of two concentric, crown-type electrodes made of metallic rods connected to a charged capacitor bank through a spark trigger. A fast gas-injection valve is placed behind the endpoint of the electrodes. The plasma pulse generation starts with pulse gas injection through the valve. A small (1 cm2) injected portion of gas expands in the interelectrode volume. For RT conditions and for the valve– electrode distance of 5 cm, the time for a typical gas like nitrogen (average expansion speed — 500 m/s) to expand to the end of electrodes is about 100 μs. After an electronically controlled delay time τD the trigger initiates the low pressure discharge between the electrodes. At some delay τB after the pulse initiation the electromagnetic forces acting on the plasma discharge eject a plasma stream towards the sample. A sudden current break produces an LdI/dt inductive surcharge on the electrodes leading to a significant acceleration of the ejected ions. Fig. 2 shows a typical computer oscilloscope run of voltage (upper part) and current (lower part) after the injection. The delay time τB is a complicated stochastic function of several discharge parameters, primarily, but not exclusively of τD and therefore may be associated with the period of gas expansion within the interelectrode volume. When τD decreases the plasma becomes enriched in the metal ions eroding from the electrode ends whereas for long τD it contains pure
M. Barlak et al. / Surface & Coatings Technology 206 (2011) 916–919
917
Fig. 1. Scheme of RPI-15 machine.
working gas. For historical reasons we refer to the conditions of pure gas pulse as Pulse Implantation Doping (PID)—long τD and τB and to the one with eroded metal content one as Deposition by Pulse Erosion (DPE)—(short τD and τB). The DPE process in the substrate proceeds as follows. First, the plasma pulse containing the most energetic working gas and the most of pulse energy ions melts the near surface layer of the substrate. The time interval of energetic ions operation is of the order of 0.1–1.0 μs with the melt duration in microsecond scale and the melt depth of 0.1–2.0 μm. Diffusion constants of impurities in the liquid phase are higher by several orders of magnitude than those in solid phase and range usually from 1e-5 to 1e-4 cm2s−1. Thus a rapid impurity diffusion into the molten phase occurs. When the surface becomes resolidified the slower metal vapor atoms and ions eroded from electrodes reach the substrate forming a few nm thick film deposit. The subsequent pulses melt both the metallic film and the near surface layer of the substrate. In both cases PID and DPE, similar to the laser irradiation, some amount of a substrate evaporates forming a vapor plume exerting a pressure on the melted layer and hence preventing its further evaporation. In our papers on DPE process in C–Ti and SiC–Ti system published thus far [6–8] we used a fixed τB and we focused our attention mainly on basic physical conditions which must be satisfied to assure a good wettability with liquid copper. In particular we have found that: 1) area concentration of alloyed titanium should be sufficiently high, i.e. no less than 3e18 cm−2, 2) top layer to a depth of about 0.25 μm must contain at least 60 at.% of Ti atoms,
3) Ti concentration should gradually decrease with depth without a sharp step just beneath the surface. In the present work we have attempted to find a useful period of current break delay τB enabling efficient processing. In addition we examined elemental distribution of the deposited material on the ceramic surface and we analyzed the chemical state of surface with respect to its influence on the wettability. 2. Experimental Glassy carbon (Cg), pyrolytic carbon (Cp), carbon–carbon composite (C/C) and silicon carbide (SiC) were used as substrate materials. The samples were processed in the IBIS pulse plasma generator [3,4] with parameters given in Table 1, using nitrogen gas and the range of τD between 170 and 220 μs and the condenser bank voltage of 25–35 kV. The system was adjusted to vary the delay τB between 2.5 and 5 μs. Under such conditions the plasma gun generates pulses of duration about 1 μs and energy density adjusted in the range of 1–7 J cm−2. The energies of ions in the generated plasma pulse range from a few tens to thousands electron volt. The effect of system parameters was studied by applying three pulses of 2 J cm−2 at a given τB and examining the surface concentration of Ti by EDX using Brucker EDX Quantax 400 insert to Zeiss DSM 942 electron microscope. For examination of the elemental distribution of the deposited material on a ceramic surface the samples referred to as “200 Ti multi” were irradiated in succession with 20, 40, 60 and 80 pulses of energy density of 7 J cm−2, 5 J cm−2, 3 J cm−2 and 1 J cm−2, respectively. After processing the samples of “200 Ti multi” type the “sessile drop” wettability test was performed. A strip of 4 N Cu foil of 3 × 2×0.5 mm3 was placed on the sample prior to loading to a box-type vacuum furnace. The wettability test process was carried at 1150 ± 5 °C and a pressure of 5e-3 Pa for about 15 min.
Table 1 Technical parameters of RPI 15 plasma generator.
Fig. 2. Typical oscilloscope traces of voltage (upper part) and current (lower part) during PRI pulse.
Type
RPI-15
Condenser bank capacity Condenser voltage Total condenser energy Working gas Injected gas pressure Outer/inner electrode diameter Electrode length Electrode material Maximum discharge current intensity Pulse energy density Pulse duration
72 μF 25–35 kV Up to 45 kJ Nitrogen or argon 1.5 atm 125/90 mm 25.0 mm Ti Up to 300 kA 3–7 J/cm2 0.5–1 μs
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M. Barlak et al. / Surface & Coatings Technology 206 (2011) 916–919
Fig. 3. The concentration of deposited titanium, as determined by EDX, vs. current breakdown time τB for SiC substrates.
Subsequently the samples were photographed and the wetting angle was determined using a home-made computer-supported sessile drop tester. The structural and compositional state of the near-surface region of samples were characterized by: scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction at a glancing angle ω = 1° (GXRD) and X-ray photoelectron spectroscopy (XPS). Prior to the measurements all samples were ion etched with 3 keV Ar ions for 3 min.
3. Results and discussion Fig. 3 presents an example of dependence of the concentration of deposited titanium on delay τB for SiC substrates (depth of information amounts to about 0.5 μm). Runs for Cp, Cg and C/C are similar to that for SiC. Careful inspection of Fig. 3 combined with the analysis of scatter of experimental data (non cited here) reveals three regions of machine operation: region I for τB = 2.5 to 3.0 μs in which machine reproducibility is poor, region II, in which machine operates with a fair reproducibility, material deposition rate is satisfactory and does not deviate more than 50% down from the maximum value and region III featuring good reproducibility but non satisfactory deposition efficiency. It is worth noting that in Ti concentration in regions II and III increasing decreases monotonically with an increase of τB. For
Ti electrodes and N2 as the working gas the precision of τB control amounts to 0.1 μs. SEM examination of the C/C and SiC surfaces after “200-Ti-multi” process together with elemental mapping of the surface suggest occurrence of chemical system such as: Ti–N, Ti–O, Ti–C and Si–N (in SiC). According to our thermodynamical considerations of Gibbs energy balance formation of compounds in these systems is possible [5,6]. In order to get insight into details of chemical composition formed by titanium the GXRD patterns and XPS spectra were recorded on C/C and SiC samples. Fig. 4 presents our X-ray results. The pattern of the virgin sample is compared to that of the processed one. In both cases the near surface region of “200 Ti multi” processed samples exhibits the reflections corresponding to TiC and TiN phases. In addition in the case of SiC also the Si reflections can be identified. These reflections were already identified for Ti plasma pulses with argon as a working gas [5] and hence we may relate them with reaction bSiCN + (Ti) → bTiCN + bSiN (ΔG is − 541 kJ/mol at 298 K and −327 kJ/mol at 1685 K). Although we do not claim that this reaction promotes wetting (in fact little is known about silicon wetting by copper) the possibility of fast silicon reduction by titanium ions is an evident difference between carbon and carbide surfaces. In Fig. 5 a comparison of XPS spectra of the virgin and the processed C/C and SiC is shown. The characteristic hump centered at about 700 eV, seen only in SiC sample cannot be removed by ion etching and is normally attributed to oxynitride contaminations. The spectra of processed samples are generally similar to each other, apart from: – lack of silicon in the SiC sample, – high C concentration (in graphite form) in SiC sample, exceeding the 50% stoichiometric content and suggesting a dissociation of SiC at the surface. Analysis of spectra observed in “200 Ti multi” treated samples revealed the presence of the following components: Ti (metallic), C (graphite), TixOy, Ti2O3/TiOxNy, and TixNy. According to literature [9] if titanium oxides and nitrides are hypostoichiometric i.e. y/x is less than 0.5–0.65 then they are beneficial for wetting. Fig. 6 shows the photos of the results of wettability tests conducted on virgin and treated C/C and SiC samples. The observed contact angles are 83° and 33° respectively.
Fig. 4. GXRD patterns recorded on C/C and SiC samples.
M. Barlak et al. / Surface & Coatings Technology 206 (2011) 916–919
919
Fig. 5. A comparison of XPS spectra of virgin and processed C/C and SiC.
delay time τB between the moment of discharge ignition and the moment of current breakdown for Ti electrodes and N2 as the working gas, is 3.0 to 3.5 μs. 2. Chemical characterization of the processed substrate ceramics reveals no essential differences which could influence the observed variations in the degree of wettability. 3. Apart from the previously revealed factors, surface morphology seems to be a decisive one that affects the desired modification. Acknowledgements This work was financed within the funds of Ministry of Education and Science of Poland for scientific projects in the period 2008–2010 under contract No. 3942/B/T02/2008/35. Part of the experiment was performed at AIM Rossendorf within the frame of RITA project (Research Infrastructures Transnational Access Contract Number 025646). The authors are very indebted to Mr. J. Krolik, Mr. K. Gatarczyk, Mr. H. Matosek and Mr. J. Zagorski for technical assistance. Fig. 6. Photos of the results of wettability tests conducted on virgin and pulse-treated C/C and SiC samples.
Neither the GXRD nor XPS results allow us to determine unambiguously the reason why the treated SiC exhibits much better wettability than C/C composite. Therefore, we believe that this is due to the morphological features of these materials. It follows from SEM images that the surface of SiC is smoother whereas nodules are present in C/C surface.
References [1] [2] [3] [4] [5] [6]
[7]
4. Conclusions
[8]
1. For wetting creation of the ceramics surface using high intensity pulsed plasma beams in DPE mode the appropriate range of the
[9]
Y. Liang, S.P. Dutta, Technovation 21 (2001) 61. M. Rosso, J. Mat. Proc. Tech. 175 (2006) 364. J. Piekoszewski, J. Langner, Nucl. Instr. Meth. B53 (1991) 148. Z. Werner, J. Piekoszewski, W. Szymczyk, Vacuum 63 (2001) 701. Z. Werner, J. Piekoszewski, A. Barcz, R. Groetschel, F. Prokert, J. Stanisławski Szymczyk, Nucl. Inst. Meth. B 175–177 (2001) 767. M. Barlak, J. Piekoszewski, J. Stanislawski, K. Borkowska, M. Chmielewski, B. Sartowska, Z. Werner, M. Miskiewcz, J. Jagielski, W. Starosta, Fusion Eng. Des. 82 (2007) 2524. M. Barlak, J. Piekoszewski, Z. Werner, J. Stanislawski, E. Skladnik-Sadowska, K. Bochenska, M. Miskiewicz, A. Kolitsch, R. Grötzschel, W. Starosta, B. Sartowska, J. Kierzek, Surf. Coat. Tech. 203 (2009) 2536. M. Barlak, J. Piekoszewski, Z. Werner, Z. Pakiela, B. Sartowska, E. SkladnikSadowska, L. Walis, J. Kierzek, W. Starosta, A. Kolitsch, R. Gröetzchel, K. Bochenska, Vacuum 83 (2009) 81. M.G. Nicholas, Material Science and Technology, Processing of ceramics, Vol. 17B Part II, Weinheim, New York, Basel, Cambridge, Tokyo, 1996, p. 262.