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Spectroscopic analysis of the interface chemistry of ultra-thin plasma polymer films on iron
Surface & Coatings Technology 200 (2005) 100 – 103 www.elsevier.com/locate/surfcoat
Spectroscopic analysis of the interface chemistry of ultra-thin plasma polymer films on iron K. Wapner, G. Grundmeier* Department of Interface Chemistry and Surface Engineering, Max-Planck-Institute for Iron Research, Max-Planck-Str. 1, 40237 Duesseldorf, Germany Available online 17 March 2005
Abstract The bonding of organosilanes to iron surfaces was analysed on three model samples using FTIR-reflection absorption spectroscopy (IRRAS) for the layer characterisation and ToF-SIMS to characterise specific fragments that are commonly interpreted as the proof for a covalent +Si–O–Me-bonding. On an ultra-thin HMDSO plasma polymer coated specimen, characteristic fragments of a covalent bonding of organosilanes to iron oxide surfaces as discussed by other authors could be reproduced, whereas on the pure iron substrate no such peaks could be observed. On an iron surface dip-coated from a diluted polydimethylsiloxane solution, the discussed characteristic fragments of a covalent bonding could be found. This arises questions on the validity of the common interpretation, since in the case of pure physical adsorption of a non-reactive long-chain siloxane, no such bonds should be detectable. Additionally, ToF-SIMS sputter depth profiles on the HMDSO plasma polymer coated iron surface and of the iron surface covered with polydimethylsiloxane could be shown to behave qualitatively identical. D 2005 Elsevier B.V. All rights reserved. Keywords: Radio frequency plasma; Iron; Fourier transform infrared spectroscopy; Secondary ion mass spectroscopy (ToF-SIMS)
1. Introduction Recently, plasma polymer films have been developed for the corrosion protection of metals or as stable adhesion promoting films. In most cases organosilane plasma polymer films were chosen to provide strong adhesion to an oxide covered metal. Several publications considered initial states of film growth by means of in situ FT-IRAS [1–4] or in situ XPS [5]. The results indicated a special structure of the plasma polymer directly adjacent to the metal and the formation of covalent bonds depending on the pre-treatment step. However, no clear evidence of the existence of covalent bonds derived from these results. Grundmeier and Stratmann [6] showed that ultra-thin plasma polymer films can be deposited on polished iron by a simple audio-frequency-discharge. The films proved to be extremely smooth with a roughness of about 0.3– T Corresponding author Tel.: +49 211 6792 290. E-mail address: [email protected] (G. Grundmeier). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.02.075
0.4 nm measured on flame annealed gold surfaces even at a thickness of about 2–3 nm. Thus these films are ideally suited for fundamental studies of adhesion by the ToF-SIMS. In the case of organosilane adhesion promoting thin films deposited from organic solvents or water, especially ToF-SIMS was applied for the detection of covalent Si– O–Fe bonds, as other methods like mechanical joint stability testing or XPS only lead to indirect information (XPS via relative peak intensities [7] or unusual high Fe2p binding energies [8]). Kinloch, Watts et al. observed ToF-SIMS-peaks that were attributed to +Si–O–Fe [7–9] and recently also for +Si–O–Al [10,11] for iron and aluminium substrates, respectively. However, these investigations were mainly done on grit-blasted or ground surfaces and with films which are known to be laterally inhomogeneous. Often sputter depth profiles were performed to reach or to analyse the interface. Here, an initial increase of the relative ToF-SIMS-intensity of the fragment +Si–O–Me is typically discussed as uncovering the interface beneath a
K. Wapner, G. Grundmeier / Surface & Coatings Technology 200 (2005) 100–103
discrete organosilane layer and the following decrease as a reaching of the substrate (published for iron [9] and for aluminium [10] substrates). In the current work the authors focus on the application of ToF-SIMS for the study of this kind of bonding on welldefined model samples with ultra-thin organosilane films allowing a direct access to the interface without the need of sputtering overlayers.
2. Experimental A bell-jar type custom made reactor was used to carry out audio frequency plasma modification of the surfaces. The experimental details were published elsewhere [12]. Pure iron substrates were polished with diamond paste down to 1 Am, thoroughly cleaned with ethanol in an ultrasonic bath for 10 min and pre-treated with oxygen plasma cleaning for 10 min. This procedure leads to a carbon free surface and a freshly grown 3–5 nm thick iron oxihydroxide layer on top of the metallic iron [13]. Hexamethyldisiloxane (HMDSO) (Fluka, purity 99.5%) was plasma polymerized without further purification prior to the plasma deposition. For the deposition, the plasma was ignited for 2 s in an atmosphere containing 0.025 mbar HMDSO and 0.125 mbar argon at a flow rate of 1.5 sccm/min. Polydimethylsiloxane (PDMS) (Alfa Aesar, purity 99.95%) was deposited by dip-coating for 5 min from a 0.5 vol.% solution of cyclohexene. IRRAS data were measured by means of a BioRad FTS300 spectrometer with an 808 reflection unit using a DTGS detector at a resolution of 4 cm 1. ToF-SIMS (Trift II, Phi, USA) measurements were carried out with a gallium gun operated at 15 kV as primary ion source and with an acquisition time of 4 min. ToF-SIMS intensities shown in this paper have been normalised to the total amount of counts.
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3. Results On the polished iron reference sample, ToF-SIMS measurements have been performed to test for impurities on the used samples. In Fig. 1a, additional to the iron substrate peaks at the masses 54, 56 and 57 amu, only to strong peaks of Na+ at 23 amu and Ca+ at 40 amu could be observed in the spectra which are most presumable due to the grinding procedure of the iron substrate prior to the polishing. Almost no residues of aliphatic contaminations could be found, pointing out the excellent surface cleaning by the oxygen plasma pre-treatment even as the samples were transported in normal air to the ToF-SIMS load lock. In the ToF-SIMS detail spectra in Fig. 1b it can be seen that at mass 99.9 amu, which has been discussed by other authors as +Si–O–Fe fragment, on the reference sample only experimental noise could be observed. The IRRAS data of the sample coated with an ultra-thin HMDSO plasma polymer layer in Fig. 2a shows the typical bands of Si–O–Si at 1100 cm 1 and of Si(CH3)x at 1265 / 890 / 850 cm 1 of a HMDSO plasma polymer and indicates a very small coating thickness (absorption only about 2 10 3). In the ToF-SIMS spectra in Fig. 2b, new peaks originating from the HMDSO plasma polymer layer as like + Si at 28 amu, +SiH at 29 amu, +SiCH3 at 43 amu, + Si(CH 3 ) 3 at 73 amu, C 3 H 9 O + Si 2 at 117 amu and C5H15+Si2O at 147 amu (the HMDSO monomer without one methyl group) could be observed. In the ToF-SIMS detail spectra in Fig. 2c, a small peak could be seen at the mass 99.9 amu as it has been discussed by other groups as + SiOFe. The peak intensity increases when a short sputtering of the surface is done. Several other significant peaks which support this assignment could be classified in the spectra (not shown here in detail): H+SiOFe at 100.9 amu, C+SiOFe at 111.9 amu, CH3+SiOFe at 114.9 amu and CO2+SiFe at 127.9 amu, so the indications for the covalent bonding of organosilanes to a metal surface could be reproduced by our measurements.
Fig. 1. ToF-SIMS spectra of the polished iron substrate reference, a) ToF-SIMS spectra of the positive ions mass region 0–150 amu, b) ToF-SIMS detail spectra of the peak at 99.9 amu.
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K. Wapner, G. Grundmeier / Surface & Coatings Technology 200 (2005) 100–103
Fig. 2. FTIR-RAS and ToF-SIMS spectra of an ultra-thin HMDSO plasma polymer layer on polished iron substrate: a) FTIR-RAS of the most predominant bands of the layer, b) ToF-SIMS spectra of the positive ions mass region 0–150 amu, c) ToF-SIMS detail spectra of the peak at 99.9 amu.
To crosscheck these findings, we prepared a modified blank test: PDMS is a silicone oil with a backbone that is chemically very similar to the HMDSO structure. But, in contrast to the already discussed HMDSO plasma polymer, a thin layer of this silicon oil prepared from dip-coating in a diluted solution should apparently not be able to establish any kind of covalent bonding to the oxide covered iron substrate.
The FT-IRRAS spectrum of the prepared PDMS layer shown in Fig. 3a is mostly similar to the one of the HMDSO plasma polymer layer. The absorption of this layer is again in the range of about 2 10 3, indicating that also in this case an ultra-thin layer could be deposited. Most surprisingly, the ToF-SIMS spectra of this PDMS layer (Fig. 3b and c) are also almost identical to the spectra observed for the HMDSO plasma polymer. Several changes in the
Fig. 3. FTIR-RAS and ToF-SIMS spectra of a PDMS layer adsorbed from cyclohexene solution on polished iron substrate: a) FTIR-RAS of the most predominant bands of the layer b) ToF-SIMS spectra of the positive ions mass region 0–150 amu c) ToF-SIMS detail spectra of the peak at 99.9 amu.
K. Wapner, G. Grundmeier / Surface & Coatings Technology 200 (2005) 100–103
Fig. 4. ToF-SIMS sputter depths profiles of relative area of the peak at mass 99.9 amu for the reference sample, the HMDSO plasma polymer layer and the PDMS layer on iron.
relative peak intensities could be observed, but all peaks discussed earlier could be reproduced on this blank. To support this observation, the intensities of the peaks at 99.9 amu during sputter depths profiling on all three discussed samples are plotted in Fig. 4. For the reference sample, only the detector noise is seen during the whole sputter cycle. The HMDSO plasma polymer layer behaves exactly like it has been described already in the literature (increase of relative peak intensity as uncovering the interface, and decrease when reaching the substrate [9]). This behaviour is reproduced on the blank test sample covered with a layer of PDMS, giving a slightly higher peak area and an intensity maximum later during the sputter profiling which might be due to a larger layer thickness.
4. Conclusions On an ultra-thin HMDSO plasma polymer coated specimen, characteristic fragments of a covalent bonding of organosilanes to iron oxide surfaces as discussed by other
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authors could be reproduced whereas on the pure iron substrate no such peaks could be found. Surprisingly, ToFSIMS results nearly identical to the HMDSO layer could be achieved on a modified blank sample prepared by dipcoating from a diluted silicon oil solution. Thus it has to be newly discussed in how far the ToF-SIMS analysis of interfacial regions on polymer coated metals can lead to artefacts with regard to formation of Me–O–Si fragments. To the knowledge of the authors in none of the published relevant ToF-SIMS results a cross check with just physically adsorbed organosilanes has been done. Additionally, ToFSIMS sputter depth profiles of the HMDSO coated sample and of the modified blank test sample could be shown to behave qualitatively identical. The authors do not state that there are no covalent Si–O– Me bonds. However, as ToF-SIMS has to be considered to be the most cogent method to support this theory, additional work is necessary to discriminate if the discussed peaks are artefacts that are caused by the applied analysis technique or if they really originate from an already existing structure.
References [1] V. Barranco, P. Thiemann, H.K. Yasuda, M. Stratmann, G. Grundmeier, Appl. Surf. Sci. 229 (2004) 87. [2] G. Grundmeier, P. Thiemann, J. Carpentier, N. Shirtcliffe, M. Stratmann, Thin Solid Films 446 (2004) 61. [3] C.E. Taylor, F.J. Boerio, J. Adhes. 69 (1999) 217. [4] R.H. Turner, F.J. Boerio, J. Adhes. 78 (2002) 447. [5] A. Sabata, W.J.V. Ooij, H.K. Yasuda, Surf. Interface Anal. 20 (1993) 845. [6] G. Grundmeier, M. Stratmann, Thin Solid Films 352 (1999) 119. [7] V. Subramanian, W.J.V. Ooij, Corrosion 54 (1998) 204. [8] M. Gettings, A.J. Kinloch, J. Mater. Sci. 12 (1977) 2511. [9] S.J. Davis, J.F. Watts, Int. J. Adhes. Adhes. 16 (1996) 5. [10] M.L. Abel, R.P. Digby, I.W. Fletcher, J.F. Watts, Surf. Interface Anal. 29 (2000) 115. [11] A. Rattana, J.D. Hermes, M.L. Abel, J.F. Watts, Int. J. Adhes. Adhes. 22 (2002) 205. [12] G. Grundmeier, M. Brettmann, P. Thiemann, Appl. Surf. Sci. 217 (2003) 223. [13] G. Grundmeier, M. Stratmann, Appl. Surf. Sci. 141 (1999) 43.
Spectroscopic analysis of the interface chemistry of ultra-thin plasma polymer films on iron K. Wapner, G. Grundmeier* Department of Interface Chemistry and Surface Engineering, Max-Planck-Institute for Iron Research, Max-Planck-Str. 1, 40237 Duesseldorf, Germany Available online 17 March 2005
Abstract The bonding of organosilanes to iron surfaces was analysed on three model samples using FTIR-reflection absorption spectroscopy (IRRAS) for the layer characterisation and ToF-SIMS to characterise specific fragments that are commonly interpreted as the proof for a covalent +Si–O–Me-bonding. On an ultra-thin HMDSO plasma polymer coated specimen, characteristic fragments of a covalent bonding of organosilanes to iron oxide surfaces as discussed by other authors could be reproduced, whereas on the pure iron substrate no such peaks could be observed. On an iron surface dip-coated from a diluted polydimethylsiloxane solution, the discussed characteristic fragments of a covalent bonding could be found. This arises questions on the validity of the common interpretation, since in the case of pure physical adsorption of a non-reactive long-chain siloxane, no such bonds should be detectable. Additionally, ToF-SIMS sputter depth profiles on the HMDSO plasma polymer coated iron surface and of the iron surface covered with polydimethylsiloxane could be shown to behave qualitatively identical. D 2005 Elsevier B.V. All rights reserved. Keywords: Radio frequency plasma; Iron; Fourier transform infrared spectroscopy; Secondary ion mass spectroscopy (ToF-SIMS)
1. Introduction Recently, plasma polymer films have been developed for the corrosion protection of metals or as stable adhesion promoting films. In most cases organosilane plasma polymer films were chosen to provide strong adhesion to an oxide covered metal. Several publications considered initial states of film growth by means of in situ FT-IRAS [1–4] or in situ XPS [5]. The results indicated a special structure of the plasma polymer directly adjacent to the metal and the formation of covalent bonds depending on the pre-treatment step. However, no clear evidence of the existence of covalent bonds derived from these results. Grundmeier and Stratmann [6] showed that ultra-thin plasma polymer films can be deposited on polished iron by a simple audio-frequency-discharge. The films proved to be extremely smooth with a roughness of about 0.3– T Corresponding author Tel.: +49 211 6792 290. E-mail address: [email protected] (G. Grundmeier). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.02.075
0.4 nm measured on flame annealed gold surfaces even at a thickness of about 2–3 nm. Thus these films are ideally suited for fundamental studies of adhesion by the ToF-SIMS. In the case of organosilane adhesion promoting thin films deposited from organic solvents or water, especially ToF-SIMS was applied for the detection of covalent Si– O–Fe bonds, as other methods like mechanical joint stability testing or XPS only lead to indirect information (XPS via relative peak intensities [7] or unusual high Fe2p binding energies [8]). Kinloch, Watts et al. observed ToF-SIMS-peaks that were attributed to +Si–O–Fe [7–9] and recently also for +Si–O–Al [10,11] for iron and aluminium substrates, respectively. However, these investigations were mainly done on grit-blasted or ground surfaces and with films which are known to be laterally inhomogeneous. Often sputter depth profiles were performed to reach or to analyse the interface. Here, an initial increase of the relative ToF-SIMS-intensity of the fragment +Si–O–Me is typically discussed as uncovering the interface beneath a
K. Wapner, G. Grundmeier / Surface & Coatings Technology 200 (2005) 100–103
discrete organosilane layer and the following decrease as a reaching of the substrate (published for iron [9] and for aluminium [10] substrates). In the current work the authors focus on the application of ToF-SIMS for the study of this kind of bonding on welldefined model samples with ultra-thin organosilane films allowing a direct access to the interface without the need of sputtering overlayers.
2. Experimental A bell-jar type custom made reactor was used to carry out audio frequency plasma modification of the surfaces. The experimental details were published elsewhere [12]. Pure iron substrates were polished with diamond paste down to 1 Am, thoroughly cleaned with ethanol in an ultrasonic bath for 10 min and pre-treated with oxygen plasma cleaning for 10 min. This procedure leads to a carbon free surface and a freshly grown 3–5 nm thick iron oxihydroxide layer on top of the metallic iron [13]. Hexamethyldisiloxane (HMDSO) (Fluka, purity 99.5%) was plasma polymerized without further purification prior to the plasma deposition. For the deposition, the plasma was ignited for 2 s in an atmosphere containing 0.025 mbar HMDSO and 0.125 mbar argon at a flow rate of 1.5 sccm/min. Polydimethylsiloxane (PDMS) (Alfa Aesar, purity 99.95%) was deposited by dip-coating for 5 min from a 0.5 vol.% solution of cyclohexene. IRRAS data were measured by means of a BioRad FTS300 spectrometer with an 808 reflection unit using a DTGS detector at a resolution of 4 cm 1. ToF-SIMS (Trift II, Phi, USA) measurements were carried out with a gallium gun operated at 15 kV as primary ion source and with an acquisition time of 4 min. ToF-SIMS intensities shown in this paper have been normalised to the total amount of counts.
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3. Results On the polished iron reference sample, ToF-SIMS measurements have been performed to test for impurities on the used samples. In Fig. 1a, additional to the iron substrate peaks at the masses 54, 56 and 57 amu, only to strong peaks of Na+ at 23 amu and Ca+ at 40 amu could be observed in the spectra which are most presumable due to the grinding procedure of the iron substrate prior to the polishing. Almost no residues of aliphatic contaminations could be found, pointing out the excellent surface cleaning by the oxygen plasma pre-treatment even as the samples were transported in normal air to the ToF-SIMS load lock. In the ToF-SIMS detail spectra in Fig. 1b it can be seen that at mass 99.9 amu, which has been discussed by other authors as +Si–O–Fe fragment, on the reference sample only experimental noise could be observed. The IRRAS data of the sample coated with an ultra-thin HMDSO plasma polymer layer in Fig. 2a shows the typical bands of Si–O–Si at 1100 cm 1 and of Si(CH3)x at 1265 / 890 / 850 cm 1 of a HMDSO plasma polymer and indicates a very small coating thickness (absorption only about 2 10 3). In the ToF-SIMS spectra in Fig. 2b, new peaks originating from the HMDSO plasma polymer layer as like + Si at 28 amu, +SiH at 29 amu, +SiCH3 at 43 amu, + Si(CH 3 ) 3 at 73 amu, C 3 H 9 O + Si 2 at 117 amu and C5H15+Si2O at 147 amu (the HMDSO monomer without one methyl group) could be observed. In the ToF-SIMS detail spectra in Fig. 2c, a small peak could be seen at the mass 99.9 amu as it has been discussed by other groups as + SiOFe. The peak intensity increases when a short sputtering of the surface is done. Several other significant peaks which support this assignment could be classified in the spectra (not shown here in detail): H+SiOFe at 100.9 amu, C+SiOFe at 111.9 amu, CH3+SiOFe at 114.9 amu and CO2+SiFe at 127.9 amu, so the indications for the covalent bonding of organosilanes to a metal surface could be reproduced by our measurements.
Fig. 1. ToF-SIMS spectra of the polished iron substrate reference, a) ToF-SIMS spectra of the positive ions mass region 0–150 amu, b) ToF-SIMS detail spectra of the peak at 99.9 amu.
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Fig. 2. FTIR-RAS and ToF-SIMS spectra of an ultra-thin HMDSO plasma polymer layer on polished iron substrate: a) FTIR-RAS of the most predominant bands of the layer, b) ToF-SIMS spectra of the positive ions mass region 0–150 amu, c) ToF-SIMS detail spectra of the peak at 99.9 amu.
To crosscheck these findings, we prepared a modified blank test: PDMS is a silicone oil with a backbone that is chemically very similar to the HMDSO structure. But, in contrast to the already discussed HMDSO plasma polymer, a thin layer of this silicon oil prepared from dip-coating in a diluted solution should apparently not be able to establish any kind of covalent bonding to the oxide covered iron substrate.
The FT-IRRAS spectrum of the prepared PDMS layer shown in Fig. 3a is mostly similar to the one of the HMDSO plasma polymer layer. The absorption of this layer is again in the range of about 2 10 3, indicating that also in this case an ultra-thin layer could be deposited. Most surprisingly, the ToF-SIMS spectra of this PDMS layer (Fig. 3b and c) are also almost identical to the spectra observed for the HMDSO plasma polymer. Several changes in the
Fig. 3. FTIR-RAS and ToF-SIMS spectra of a PDMS layer adsorbed from cyclohexene solution on polished iron substrate: a) FTIR-RAS of the most predominant bands of the layer b) ToF-SIMS spectra of the positive ions mass region 0–150 amu c) ToF-SIMS detail spectra of the peak at 99.9 amu.
K. Wapner, G. Grundmeier / Surface & Coatings Technology 200 (2005) 100–103
Fig. 4. ToF-SIMS sputter depths profiles of relative area of the peak at mass 99.9 amu for the reference sample, the HMDSO plasma polymer layer and the PDMS layer on iron.
relative peak intensities could be observed, but all peaks discussed earlier could be reproduced on this blank. To support this observation, the intensities of the peaks at 99.9 amu during sputter depths profiling on all three discussed samples are plotted in Fig. 4. For the reference sample, only the detector noise is seen during the whole sputter cycle. The HMDSO plasma polymer layer behaves exactly like it has been described already in the literature (increase of relative peak intensity as uncovering the interface, and decrease when reaching the substrate [9]). This behaviour is reproduced on the blank test sample covered with a layer of PDMS, giving a slightly higher peak area and an intensity maximum later during the sputter profiling which might be due to a larger layer thickness.
4. Conclusions On an ultra-thin HMDSO plasma polymer coated specimen, characteristic fragments of a covalent bonding of organosilanes to iron oxide surfaces as discussed by other
103
authors could be reproduced whereas on the pure iron substrate no such peaks could be found. Surprisingly, ToFSIMS results nearly identical to the HMDSO layer could be achieved on a modified blank sample prepared by dipcoating from a diluted silicon oil solution. Thus it has to be newly discussed in how far the ToF-SIMS analysis of interfacial regions on polymer coated metals can lead to artefacts with regard to formation of Me–O–Si fragments. To the knowledge of the authors in none of the published relevant ToF-SIMS results a cross check with just physically adsorbed organosilanes has been done. Additionally, ToFSIMS sputter depth profiles of the HMDSO coated sample and of the modified blank test sample could be shown to behave qualitatively identical. The authors do not state that there are no covalent Si–O– Me bonds. However, as ToF-SIMS has to be considered to be the most cogent method to support this theory, additional work is necessary to discriminate if the discussed peaks are artefacts that are caused by the applied analysis technique or if they really originate from an already existing structure.
References [1] V. Barranco, P. Thiemann, H.K. Yasuda, M. Stratmann, G. Grundmeier, Appl. Surf. Sci. 229 (2004) 87. [2] G. Grundmeier, P. Thiemann, J. Carpentier, N. Shirtcliffe, M. Stratmann, Thin Solid Films 446 (2004) 61. [3] C.E. Taylor, F.J. Boerio, J. Adhes. 69 (1999) 217. [4] R.H. Turner, F.J. Boerio, J. Adhes. 78 (2002) 447. [5] A. Sabata, W.J.V. Ooij, H.K. Yasuda, Surf. Interface Anal. 20 (1993) 845. [6] G. Grundmeier, M. Stratmann, Thin Solid Films 352 (1999) 119. [7] V. Subramanian, W.J.V. Ooij, Corrosion 54 (1998) 204. [8] M. Gettings, A.J. Kinloch, J. Mater. Sci. 12 (1977) 2511. [9] S.J. Davis, J.F. Watts, Int. J. Adhes. Adhes. 16 (1996) 5. [10] M.L. Abel, R.P. Digby, I.W. Fletcher, J.F. Watts, Surf. Interface Anal. 29 (2000) 115. [11] A. Rattana, J.D. Hermes, M.L. Abel, J.F. Watts, Int. J. Adhes. Adhes. 22 (2002) 205. [12] G. Grundmeier, M. Brettmann, P. Thiemann, Appl. Surf. Sci. 217 (2003) 223. [13] G. Grundmeier, M. Stratmann, Appl. Surf. Sci. 141 (1999) 43.