- Email: [email protected]
ToF-SIMS)
Progress in Organic Coatings 51 (2004) 163–171
Analysis of paint defects by mass spectroscopy (LAMMA®/ToF-SIMS) Ulrich Wolff, Hartmut Thomas, Michael Osterhold∗ DuPont Performance Coatings, GmbH and Co. KG, Christbusch 25, D-42285 Wuppertal, Germany Received 22 April 2004; accepted 1 July 2004 Paper presented by M. Osterhold at XXVII FATIPEC Congress, Aix-en-Provence, France, 19–21 April 2004
Abstract In this paper, two surface sensitive mass spectroscopic methods–laser microprobe mass analysis (LAMMA® )1 and time of flight secondary ion mass spectroscopy (ToF-SIMS) – and their efficiency at the analysis of paint defects are presented. For these two similar methods, the differences especially concerning the ion formation at the target surface are pointed out as well as the resulting effect on the kind of information. While LAMMA® mainly provides inorganic information about the surface composition, ToF-SIMS offers the ability to characterize organic materials and especially to distinguish between different silicone oils. The advantages of each method are shown for typical paint defects. © 2004 Elsevier B.V. All rights reserved. Keywords: Surface analysis; Coating defects; ToF-SIMS; LAMMA®
1. Introduction Surface defects at cured coating layers [1] are a huge issue in the coatings industry. Although such defects are often very small, the human eye is able to notice them. Besides surface defects might lead to a standstill or intense repair work in automotive production lines or later onto corrosion in the field, which could be the cause for very expansive complaints. The analysis of wetting faults and craters represents a specific problem, because this defects are caused by substances with a lower surface tension. These substances only can be found in the first few monolayers of the defect. Therefore, a technique is needed, which on one hand offers a high lateral resolution and on the other hand is able to characterize a surface area with respect to the chemical composition. Laser microprobe mass analysis (LAMMA® ) as well as time of flight secondary ion mass spectroscopy (ToF-SIMS) meet these requirements. The methods are similar, their abilities for the characterization and identification of paint defects, however, are different. Both techniques are surface sensitive and use time of flight ion mass spectrometers for the detection of the ions formed in a primary process. The main difference is the ∗ 1
Corresponding author. Tel.: +49202 529 2151; fax: +49 202 529 2954. E-mail address: [email protected] (M. Osterhold). LAMMA® is a trademark of the Leybold-Heraeus Company.
0300-9440/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2004.07.006
way of ion formation at the target surface and the resulting effect on the kind of information. While LAMMA® mainly provides inorganic information, ToF-SIMS has its advantages at the characterization of organic materials and offers the ability to distinguish between different silicone oils even if they are based on the same monomers. This paper will give you a general idea about both techniques and the differences between them. The main advantages of each method are presented for some paint defects typical for the coatings industry.
2. Mass spectroscopical methods Out of a variety of analytical techniques Laser microprobe mass analysis as well as time of flight secondary ion mass spectroscopy have proven their outstanding positions for the investigation of paint defects [2,3]. Both methods are surface sensitive and have a high lateral resolution. A main part of both instruments is the time of flight ion mass spectrometer (Fig. 1) [4]. Therefore, also the principle of both methods is very similar. At a defined moment, ions are formed by a pulsed primary process at the target surface. All ions of one polarity are then extracted by an electric field. As nearly all ions are single charged, the energy they get in the electric
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field and convert into kinetic energy is the same: Energy of the ions = charge × voltage = 21 (mass × square velocity) After extraction, the ions pass a field free flight tube of about 2 m length. The same energy leading to a mass dependence of velocity and the same ion formation moment due to the pulsed primary process result in a mass separation of the ions by time at the end of the drift path. An ion mirror is integrated in the flight tube to make a first-order compensation of the initial energy ions get during the primary formation process. The mirror is an electric field, produced by an electrode with the opposite polarity to that of the flight tube. An ion with a higher energy has a longer way inside the mirror, which compensates the higher velocity of such an ion. The main differences between both methods are the kind of ion formation at the target and the type of ion detection at the end of the time of flight mass spectrometer. 2.1. Ion formation on target surfaces 2.1.1. LAMMA® In laser microprobe mass analysis (see Fig. 1), a short laser pulse is used for the formation of ions at the target surface [5]. The pulse is produced by a Nd-YAG laser with a Q-switch. The frequency of the laser pulse is twice doubled, so that the wavelength changes from 1064 nm towards higher energetic light of 266 nm. The pulse is focused on the target surface by a microscope optic with a beam diameter of about 2 m. The pulse width at the target is about 15 ns. With the normally used energy of some tenth of a microjoules the pulse leads to a power density of more than 108 W/cm2 at the target. This enormous power density results in the evaporation and partial ionization of target material in a diameter of 3–5 m and a depth of some tenth of a micrometers. The evaporated target
Fig. 2. SIMS ion formation scheme.
area size depends on the energy of the laser pulse and the temperature conductivity of the target material. The ionization probability also depends on the energy of the laser pulse and on the chemical composition of the target matrix. 2.1.2. ToF-SIMS In time of flight secondary ion mass spectroscopy, a pulsed primary ion beam is used for the formation of secondary ions at the target surface [6]. The primary ion pulse mostly produced by field emission from a liquid metal ion source (Ga or Au) has a width of about 1 ns and a diameter in the range of 10 m. In Fig. 2, the scheme of secondary ion formation at the surface is shown [7]. The energy impact of each primary ion triggers a collision cascade at the target. The trajectories of the recoiled atoms are leading back to the surface, where the energy is transferred to molecules in the topmost monolayer.
Fig. 1. Schematic diagram of LAMMA® 1000.
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In a central region of about 1 nm around the primary ion impact the transferred energy is high enough for desorption and fragmentation of molecular neutrals as well as ions. In the outer region (1–2 nm) the energy is just sufficient for desorption. 2.2. Type of ion detection 2.2.1. LAMMA® At the end of the time of flight ion mass spectrometer in the LAMMA® instrument, the ions are accelerated to an electrode, where they generate electrons. The resulting electron signal is afterwards multiplied by a factor of 105 –106 . The voltage of the electron signal at a condenser is saved by a transient recorder in time channels of 10 ns. The value of the voltage in each time channel is proportional to the number of ions, which have reached the detector in the corresponding time interval. With this kind of detection, a mass spectrum of one polarity with a sufficient intensity dynamic is obtained by each laser pulse. 2.2.2. ToF-SIMS In the detector of the time of flight ion mass spectrometer in the ToF-SIMS instrument also a conversion of ions to electrons takes place, followed by a multiplier. Due to the progress in the development of digital technique, however, this method uses a time to digital converter with time channels of about 100 ps. During such a short time it is only possible to save the information, whether an ion hit the detector or not. Therefore, only a certain number of secondary ions can be detected per pulse. In order not to loose information, the primary ion current is reduced to such a level, that only one secondary ion per detectable mass unit is formed. A spectrum is accumulated over a huge number of pulses. We use for instance a measurement time of 100 s at a repeat frequency of the pulses of about 5 kHz. This kind of detection, especially the small width of the time channels, offers the opportunity of a very high mass resolution. In Fig. 3, a detail of the ToF-SIMS spectrum (positive ions) of a silicon wafer surface is shown. In the spectral region near the nominal mass m/z = 56 u several peaks can be distinguished. The chemical composition of these peaks can be evaluated by calculating combinations of element isotopes with an identical deviation from the nominal mass. With such a high mass resolution (m/m about 104 ) even unknown substances can by identified [8,9]. 2.3. Comparison of methods The big advantage of LAMMA® is the very small spot size. Nevertheless each laser pulse results in a evaporation hole, which can be directly observed on the analyzed target surface. Therefore, it is possible to aim very small particles or areas inside a defect and get typical spectra of it. Due to the very rude evaporation process, however, nearly all organic molecules are completely destroyed. Therefore, the main in-
Fig. 3. Mass resolution of ToF-SIMS on a silicon wafer.
formation obtained by LAMMA® is the element composition of the analyzed target area. As a difference to other methods of element analysis like EDX2 or XPS,3 LAMMA® is only semi-quantitative, but offers a detection probability sufficient for trace analysis. The information depth of about some tenth of a micrometers is depending on the pulse energy and the target material. ToF-SIMS on the other hand has a bigger spot size and for typical measurements the primary ion beam is scanned over a target area of 100 m × 100 m. The target surface can be observed by a camera, but it is not possible to identify the beam spot on the target. Due to the scanning, however, this method offers the opportunity of imaging a surface area [10]. With this tool the position of surface defects often can be identified in the total ion image of a bigger area and the analysis limits can be adopted to it. The main advantage of ToF-SIMS is the semi-quantitative characterization and identification of the organic and inorganic composition of the topmost monolayer at the target surface [11]. The detection probability is comparable to that of LAMMA® , the surface sensitivity, however, is by a factor of 102 –103 higher.
3. Applications in the coatings industry Both mass spectroscopic methods are used for the investigation of defects at cured coating layers. For the identification of the cause the defect is measured versus the surrounding intact surface and sometimes also versus other references. LAMMA® is a powerful tool for the analysis of particle inclusions and pimples. Even if they cannot be observed microscopically, those defects are often covered by thin coating films. Therefore, a relatively large information depth is 2 3
EDX: energy dispersive X-ray analysis. XPS: X-ray photoelectron spectroscopy.
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needed to get information about the included material. Another often analyzed class of defects are craters and wetting faults. This kind of defects are mostly caused by substances with a lower surface tension. Therefore, the chemical information about the cause only can be found in the topmost monolayers of the defect. Due to the high surface sensitivity and the possibility to get organic information, ToF-SIMS has an outstanding position in this field of analysis. A very special problem for the coatings industry is the identification of silicone oil as the cause of a defect, because coating additives are also often based on silicones. In the next chapter, the abilities of each method for the characterization of silicone oils are shown. 3.1. Characterization of silicone oils With LAMMA® the identification of a silicone oil is very difficult, because nearly only the elemental silicon ion can be observed. Nevertheless, it is possible to find out, whether the silicon ion comes from a silicone oil or from a silicate due to their different ionization probabilities (silicone oil is ionized by a lower laser pulse energy than silicate). If the investigations are confined to coating layers without silicone based additives, even crater problems caused by a silicone oil can be identified with this method. ToF-SIMS, on the other hand offers the ability to distinguish between different silicone oils. In the spectra of negative ions, silicone oils have a characteristic fragmentation
pattern. In the mass range below 100 u (m/z) one or more fragments can be observed. In the higher mass range silicone peaks of the fragments with additional repeat units of the polymer occur. These reiterated signals can be used for the identification of the polymer repeat unit of the specific silicone oil. With this tool it is possible to distinguish silicone oils, based on different monomers. In Figs. 4 and 5 coating additives are shown, where these differences clearly can be observed. The silicone additive in Fig. 4 has a repeat unit of m/z = 172 u, whereas both silicone additives in Fig. 5 are part of the most common family of polydimethylsiloxane (PDMS) based oils. In Fig. 5, it can also be observed, that PDMS based silicone oils are able to form up to three different fragmentation series in the spectra of negative ions. The peak pattern of the individual silicone oil is depending on its end- and sidegroups, which lead to different building probabilities for the three fragmentation series. The initial fragments of the three series are at the following masses: F1: m/z = 75 u (SiO2 CH3 ) F2: m/z = 89 u (SiOC3 H9 ) F3: m/z = 91 u (SiO2 C2 H7 ) For the comparison of PDMS silicone oils the peaks of the initial fragments with one additional repeat unit (signals at the masses m/z = 149, 163 and 165 u) are the most significant. In this area, the peak intensity is very high and not influenced by signals of different based silicone oils like the
Fig. 4. Negative ion spectrum (ToF-SIMS) of a not PDMS based coating additive.
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Fig. 5. Negative ion spectra (ToF-SIMS) of two coating additives on PDMS base.
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3.2. Investigation of craters
Fig. 6. Microscope image of a crater.
additive in Fig. 4. A significant change in the ratios between the three silicone oil fragmentation series inside of a defect compared to the surrounding intact surface indicates a change in the silicone oil composition. Possible reasons for such a change are the contamination with a foreign silicone oil, or, for coatings with more than one silicone additive, the separation or enrichment of one of them. For a direct identification of the silicone oil the fragmentation pattern in the fingerprint region of the spectrum (mass range m/z up to approximately 800 u) is not significant enough. A suspicious material, however, with a silicone oil fragmentation pattern corresponding to the change in the defect can be considered for the probable cause.
3.2.1. Example 1 Fig. 6 shows the microscope image of a production line crater in a complete paint system. In the center of the defect, which is reaching down to the basecoat, a hole can be observed even in that layer. This crater was analyzed by ToFSIMS in comparison to the surrounding intact clearcoat surface. In the spectra of negative ions (presented in Fig. 7) a significant change in the fragmentation pattern of PDMS based silicone oils occurred between the crater and the intact clearcoat surface. This led to the assumption, that the crater was caused by a silicone oil on PDMS base. As the craters occurred at the same time in different basecoats, the conclusion was drawn that a contamination of a raw material might have been the cause of that problem. A data research pointed out, that a supplier exchange of a basecoat resin took place at the same time. Therefore, the correspondent resin charge of the new supplier was investigated by chromatography by a SiO2 filled glass tube. One of the detected fractions clearly showed silicone oil signals. The residue of this fraction, resolved in a droplet of hexane, was given by a micropipette onto the surface of a dried basecoat. After the solvent was evaporated a clearcoat layer was sprayed and cured. At the deposition place of the extracted silicone oil a crater occurred, which was also analyzed with ToF-SIMS. Compared with the surrounding intact clearcoat surface, this crater showed the same change in silicone oil fragmentation pattern as the line crater. 3.2.2. Example 2 In Fig. 8, the microscope image of a primer crater on a car body is shown. First LAMMA® analysis was carried out on two similar craters from the same panel. By this method it was not possible to find any indication to a
Fig. 7. Negative ion spectra (ToF-SIMS) of crater and intact clearcoat area.
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Fig. 8. Positive ion spectra (ToF-SIMS) of a primer crater in comparison to the surrounding intact surface and the corresponding microscope image.
possible crater cause. Then the crater of Fig. 8 was analyzed with ToF-SIMS and significant differences between the defect and the intact primer surface were found. In the spectra of positive ions (Fig. 8) additional signals of triglycerides and in the spectra of negative ions, the corresponding fatty acid fragments occurred. Triglycerides are often used in skin creams. Therefore, the crater for example might be caused by a fingerprint of an employee in the paint shop of the car builder. 3.2.3. Example 3 The last crater presented here shows a dark particle in the center of the defect. In the ToF-SIMS spectra of this crater no significant differences between the particle and the in-
tact clearcoat could be observed. By LAMMA® , however, it was possible to characterize the element composition of the particle. Fig. 9 shows the positive spectra of the particle in the center of the crater and of the surrounding intact clearcoat. While ToF-SIMS spectra directly show the number of detected ions (counts) versus the mass (exactly mass to charge: m/z), the intensity in the LAMMA® spectra is the voltage at the detector, which is also proportional to the number of ions per time interval. In the spectrum of the intact clearcoat surface the typical signals of an organic substance can be observed. Due to the rude evaporation process organic molecules are strongly fragmented. In the evaporation area a kind of plasma is formed, from which single charged carbon clusters arise by recombination. Most of the observed
Fig. 9. Positive ion spectra (LAMMA® ) of a dark particle in the center of a crater in comparison to the surrounding intact clearcoat.
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Fig. 10. Positive ion spectra (LAMMA® ) of a particle on the e-coat vs. the surrounding intact surface and the corresponding microscope image.
signals therefore correspond to Cn , Cn H and Cn H2 , where n is the number of atoms. Only very stable organic compounds as phthalate for example remain undamaged. In the present example, the particle inside the crater shows additional element signals in comparison to the intact clearcoat surface. Especially the iron and calcium signals are remarkable, because these elements are surely neither part of the underlying basecoat nor of the clearcoat. The Mg-, Al- and Si-signals probably belong to a silicate, which also might be a part of the basecoat recipe. These results led to the assumption, that the crater was probably caused by a steel abrasion particle maybe in combination with a calcium containing grease or carbonate and with a silicate sticking to it.
such defects can be done by micro-ATR4 infrared spectroscopy. A simple but not trivial example is shown in Fig. 10. On the microscope image a small ball of metal can be observed on the top of an e-coat layer. Although the nature of the defect seemed to be clear the question about the origin remained. In the LAMMA® analysis of the particle iron and copper were detected in the positive ion spectrum. In comparison to that, the e-coat only showed signals of the pigments (Al-silicate and Ti-oxide) and the organic resin matrix. With this optical and mass spectroscopic information the conclusion was drawn, that the defect was a welding pearl, because the wire used for electric welding consists of iron, which is covered by a copper layer to avoid corrosion.
3.3. Investigation of particles in coatings 4. Summary The second class of defects often analyzed are particles and pimples in cured coatings. Even if particles are on the top of the coating layer, they are often covered with a film of paint or another organic substance, which is not necessarily representative of the defect. If the film is very thin it is possible to measure the defect directly with LAMMA® . Otherwise the sample has to be prepared for the analysis. One possible way to remove the covering material respectively to open the defect is a cut with a scalpel. Another method to get to a defect is a cross section preparation. A ToF-SIMS analysis after one of these sample preparations normally does not make sense, because the topmost monolayers in the defect area often consist of smeared material from other places of the sample or from an aid substance. Due to the information depth of some tenth of a micrometer, a such possible contamination does not play a substantial role in a LAMMA® analysis. Therefore, this technique is a powerful tool for the inorganic characterization of particles and pimples. The organic investigation of
ToF-SIMS and LAMMA® , two different mass spectroscopic methods with the ability of trace analysis are often used for the investigation of paint defects. Both techniques were compared and the differences especially the way of ion formation at the target surface and the type of ion detection as well as the resulting effect on the kind of information were pointed out. By some examples typical of the coatings industry the main advantages of each technique were demonstrated. The examples show that both methods provide complementary information. Due to the ability to characterize the organic composition in the topmost monolayer of a surface and especially to distinguish between different silicone oils ToF-SIMS has an outstanding position in the analysis of craters. LAMMA® on the other hand is a powerful tool for the determination of 4
ATR: attenuated total reflection.
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the inorganic composition of particles and pimples in cured coating layers, because of the very small analysis spot size and the information depth of some tenth of a micrometer. So nearly every kind of paint defect can be successfully analyzed by one of the two methods.
Acknowledgement The authors would like to thank TASCON GmbH in M¨unster (Germany), for the long-standing excellent cooperation in ToF-SIMS analyses.
References [1] T. Haase, M. Osterhold, Catalog of Defects, Sec. Ed., Herberts GmbH, Wuppertal, Germany, 1996.
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[2] N. Basener, Farbe Lack 94 (10) (1988) 821. [3] A. Benninghoven, B. Hagenhoff, E. Niehuis, Anal. Chem. 65 (1993) 630A. [4] R.J. Cotter, Anal. Chem. 64 (1992) 1027A. [5] H. Vogt, H.-J. Heinen, S. Meier, R. Wechslung, Fresenius Z. Anal. Chem. 303 (1981) 195. [6] A. Benninghoven, F.G. R¨udenauer, H.W. Werner, Secondary Ion Mass Spectrometry, Wiley, New York, 1987. [7] A. Benninghoven, Ion formation from organic solids, in: Springer Series in Chemical Physics 25, Springer, Berlin, Heidelberg, New York, 1982. [8] E. Niehuis, Proceedings of Secondary Ion Mass Spectrometry (SIMS VIII), Wiley, Chichester, 1992, p. 269. [9] Product Information of the ToF-SIMS IV Instrument, ION-TOF GmbH, M¨unster, Germany, 1997. [10] J. Schwieters, H.-G. Cramer, U. J¨urgens, E. Niehuis, H. Rulle, T. Heller, J. Zehnpfenning, A. Benninghoven, Proceedings of Secondary Ion Mass Spectrometry (SIMS VIII), Wiley, Chichester, 1992, p. 497. [11] P. Bertrand, L.-T. Weng, Mikrochim. Acta 13 (Suppl.) (1996) 167.
Analysis of paint defects by mass spectroscopy (LAMMA®/ToF-SIMS) Ulrich Wolff, Hartmut Thomas, Michael Osterhold∗ DuPont Performance Coatings, GmbH and Co. KG, Christbusch 25, D-42285 Wuppertal, Germany Received 22 April 2004; accepted 1 July 2004 Paper presented by M. Osterhold at XXVII FATIPEC Congress, Aix-en-Provence, France, 19–21 April 2004
Abstract In this paper, two surface sensitive mass spectroscopic methods–laser microprobe mass analysis (LAMMA® )1 and time of flight secondary ion mass spectroscopy (ToF-SIMS) – and their efficiency at the analysis of paint defects are presented. For these two similar methods, the differences especially concerning the ion formation at the target surface are pointed out as well as the resulting effect on the kind of information. While LAMMA® mainly provides inorganic information about the surface composition, ToF-SIMS offers the ability to characterize organic materials and especially to distinguish between different silicone oils. The advantages of each method are shown for typical paint defects. © 2004 Elsevier B.V. All rights reserved. Keywords: Surface analysis; Coating defects; ToF-SIMS; LAMMA®
1. Introduction Surface defects at cured coating layers [1] are a huge issue in the coatings industry. Although such defects are often very small, the human eye is able to notice them. Besides surface defects might lead to a standstill or intense repair work in automotive production lines or later onto corrosion in the field, which could be the cause for very expansive complaints. The analysis of wetting faults and craters represents a specific problem, because this defects are caused by substances with a lower surface tension. These substances only can be found in the first few monolayers of the defect. Therefore, a technique is needed, which on one hand offers a high lateral resolution and on the other hand is able to characterize a surface area with respect to the chemical composition. Laser microprobe mass analysis (LAMMA® ) as well as time of flight secondary ion mass spectroscopy (ToF-SIMS) meet these requirements. The methods are similar, their abilities for the characterization and identification of paint defects, however, are different. Both techniques are surface sensitive and use time of flight ion mass spectrometers for the detection of the ions formed in a primary process. The main difference is the ∗ 1
Corresponding author. Tel.: +49202 529 2151; fax: +49 202 529 2954. E-mail address: [email protected] (M. Osterhold). LAMMA® is a trademark of the Leybold-Heraeus Company.
0300-9440/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2004.07.006
way of ion formation at the target surface and the resulting effect on the kind of information. While LAMMA® mainly provides inorganic information, ToF-SIMS has its advantages at the characterization of organic materials and offers the ability to distinguish between different silicone oils even if they are based on the same monomers. This paper will give you a general idea about both techniques and the differences between them. The main advantages of each method are presented for some paint defects typical for the coatings industry.
2. Mass spectroscopical methods Out of a variety of analytical techniques Laser microprobe mass analysis as well as time of flight secondary ion mass spectroscopy have proven their outstanding positions for the investigation of paint defects [2,3]. Both methods are surface sensitive and have a high lateral resolution. A main part of both instruments is the time of flight ion mass spectrometer (Fig. 1) [4]. Therefore, also the principle of both methods is very similar. At a defined moment, ions are formed by a pulsed primary process at the target surface. All ions of one polarity are then extracted by an electric field. As nearly all ions are single charged, the energy they get in the electric
164
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field and convert into kinetic energy is the same: Energy of the ions = charge × voltage = 21 (mass × square velocity) After extraction, the ions pass a field free flight tube of about 2 m length. The same energy leading to a mass dependence of velocity and the same ion formation moment due to the pulsed primary process result in a mass separation of the ions by time at the end of the drift path. An ion mirror is integrated in the flight tube to make a first-order compensation of the initial energy ions get during the primary formation process. The mirror is an electric field, produced by an electrode with the opposite polarity to that of the flight tube. An ion with a higher energy has a longer way inside the mirror, which compensates the higher velocity of such an ion. The main differences between both methods are the kind of ion formation at the target and the type of ion detection at the end of the time of flight mass spectrometer. 2.1. Ion formation on target surfaces 2.1.1. LAMMA® In laser microprobe mass analysis (see Fig. 1), a short laser pulse is used for the formation of ions at the target surface [5]. The pulse is produced by a Nd-YAG laser with a Q-switch. The frequency of the laser pulse is twice doubled, so that the wavelength changes from 1064 nm towards higher energetic light of 266 nm. The pulse is focused on the target surface by a microscope optic with a beam diameter of about 2 m. The pulse width at the target is about 15 ns. With the normally used energy of some tenth of a microjoules the pulse leads to a power density of more than 108 W/cm2 at the target. This enormous power density results in the evaporation and partial ionization of target material in a diameter of 3–5 m and a depth of some tenth of a micrometers. The evaporated target
Fig. 2. SIMS ion formation scheme.
area size depends on the energy of the laser pulse and the temperature conductivity of the target material. The ionization probability also depends on the energy of the laser pulse and on the chemical composition of the target matrix. 2.1.2. ToF-SIMS In time of flight secondary ion mass spectroscopy, a pulsed primary ion beam is used for the formation of secondary ions at the target surface [6]. The primary ion pulse mostly produced by field emission from a liquid metal ion source (Ga or Au) has a width of about 1 ns and a diameter in the range of 10 m. In Fig. 2, the scheme of secondary ion formation at the surface is shown [7]. The energy impact of each primary ion triggers a collision cascade at the target. The trajectories of the recoiled atoms are leading back to the surface, where the energy is transferred to molecules in the topmost monolayer.
Fig. 1. Schematic diagram of LAMMA® 1000.
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In a central region of about 1 nm around the primary ion impact the transferred energy is high enough for desorption and fragmentation of molecular neutrals as well as ions. In the outer region (1–2 nm) the energy is just sufficient for desorption. 2.2. Type of ion detection 2.2.1. LAMMA® At the end of the time of flight ion mass spectrometer in the LAMMA® instrument, the ions are accelerated to an electrode, where they generate electrons. The resulting electron signal is afterwards multiplied by a factor of 105 –106 . The voltage of the electron signal at a condenser is saved by a transient recorder in time channels of 10 ns. The value of the voltage in each time channel is proportional to the number of ions, which have reached the detector in the corresponding time interval. With this kind of detection, a mass spectrum of one polarity with a sufficient intensity dynamic is obtained by each laser pulse. 2.2.2. ToF-SIMS In the detector of the time of flight ion mass spectrometer in the ToF-SIMS instrument also a conversion of ions to electrons takes place, followed by a multiplier. Due to the progress in the development of digital technique, however, this method uses a time to digital converter with time channels of about 100 ps. During such a short time it is only possible to save the information, whether an ion hit the detector or not. Therefore, only a certain number of secondary ions can be detected per pulse. In order not to loose information, the primary ion current is reduced to such a level, that only one secondary ion per detectable mass unit is formed. A spectrum is accumulated over a huge number of pulses. We use for instance a measurement time of 100 s at a repeat frequency of the pulses of about 5 kHz. This kind of detection, especially the small width of the time channels, offers the opportunity of a very high mass resolution. In Fig. 3, a detail of the ToF-SIMS spectrum (positive ions) of a silicon wafer surface is shown. In the spectral region near the nominal mass m/z = 56 u several peaks can be distinguished. The chemical composition of these peaks can be evaluated by calculating combinations of element isotopes with an identical deviation from the nominal mass. With such a high mass resolution (m/m about 104 ) even unknown substances can by identified [8,9]. 2.3. Comparison of methods The big advantage of LAMMA® is the very small spot size. Nevertheless each laser pulse results in a evaporation hole, which can be directly observed on the analyzed target surface. Therefore, it is possible to aim very small particles or areas inside a defect and get typical spectra of it. Due to the very rude evaporation process, however, nearly all organic molecules are completely destroyed. Therefore, the main in-
Fig. 3. Mass resolution of ToF-SIMS on a silicon wafer.
formation obtained by LAMMA® is the element composition of the analyzed target area. As a difference to other methods of element analysis like EDX2 or XPS,3 LAMMA® is only semi-quantitative, but offers a detection probability sufficient for trace analysis. The information depth of about some tenth of a micrometers is depending on the pulse energy and the target material. ToF-SIMS on the other hand has a bigger spot size and for typical measurements the primary ion beam is scanned over a target area of 100 m × 100 m. The target surface can be observed by a camera, but it is not possible to identify the beam spot on the target. Due to the scanning, however, this method offers the opportunity of imaging a surface area [10]. With this tool the position of surface defects often can be identified in the total ion image of a bigger area and the analysis limits can be adopted to it. The main advantage of ToF-SIMS is the semi-quantitative characterization and identification of the organic and inorganic composition of the topmost monolayer at the target surface [11]. The detection probability is comparable to that of LAMMA® , the surface sensitivity, however, is by a factor of 102 –103 higher.
3. Applications in the coatings industry Both mass spectroscopic methods are used for the investigation of defects at cured coating layers. For the identification of the cause the defect is measured versus the surrounding intact surface and sometimes also versus other references. LAMMA® is a powerful tool for the analysis of particle inclusions and pimples. Even if they cannot be observed microscopically, those defects are often covered by thin coating films. Therefore, a relatively large information depth is 2 3
EDX: energy dispersive X-ray analysis. XPS: X-ray photoelectron spectroscopy.
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needed to get information about the included material. Another often analyzed class of defects are craters and wetting faults. This kind of defects are mostly caused by substances with a lower surface tension. Therefore, the chemical information about the cause only can be found in the topmost monolayers of the defect. Due to the high surface sensitivity and the possibility to get organic information, ToF-SIMS has an outstanding position in this field of analysis. A very special problem for the coatings industry is the identification of silicone oil as the cause of a defect, because coating additives are also often based on silicones. In the next chapter, the abilities of each method for the characterization of silicone oils are shown. 3.1. Characterization of silicone oils With LAMMA® the identification of a silicone oil is very difficult, because nearly only the elemental silicon ion can be observed. Nevertheless, it is possible to find out, whether the silicon ion comes from a silicone oil or from a silicate due to their different ionization probabilities (silicone oil is ionized by a lower laser pulse energy than silicate). If the investigations are confined to coating layers without silicone based additives, even crater problems caused by a silicone oil can be identified with this method. ToF-SIMS, on the other hand offers the ability to distinguish between different silicone oils. In the spectra of negative ions, silicone oils have a characteristic fragmentation
pattern. In the mass range below 100 u (m/z) one or more fragments can be observed. In the higher mass range silicone peaks of the fragments with additional repeat units of the polymer occur. These reiterated signals can be used for the identification of the polymer repeat unit of the specific silicone oil. With this tool it is possible to distinguish silicone oils, based on different monomers. In Figs. 4 and 5 coating additives are shown, where these differences clearly can be observed. The silicone additive in Fig. 4 has a repeat unit of m/z = 172 u, whereas both silicone additives in Fig. 5 are part of the most common family of polydimethylsiloxane (PDMS) based oils. In Fig. 5, it can also be observed, that PDMS based silicone oils are able to form up to three different fragmentation series in the spectra of negative ions. The peak pattern of the individual silicone oil is depending on its end- and sidegroups, which lead to different building probabilities for the three fragmentation series. The initial fragments of the three series are at the following masses: F1: m/z = 75 u (SiO2 CH3 ) F2: m/z = 89 u (SiOC3 H9 ) F3: m/z = 91 u (SiO2 C2 H7 ) For the comparison of PDMS silicone oils the peaks of the initial fragments with one additional repeat unit (signals at the masses m/z = 149, 163 and 165 u) are the most significant. In this area, the peak intensity is very high and not influenced by signals of different based silicone oils like the
Fig. 4. Negative ion spectrum (ToF-SIMS) of a not PDMS based coating additive.
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Fig. 5. Negative ion spectra (ToF-SIMS) of two coating additives on PDMS base.
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3.2. Investigation of craters
Fig. 6. Microscope image of a crater.
additive in Fig. 4. A significant change in the ratios between the three silicone oil fragmentation series inside of a defect compared to the surrounding intact surface indicates a change in the silicone oil composition. Possible reasons for such a change are the contamination with a foreign silicone oil, or, for coatings with more than one silicone additive, the separation or enrichment of one of them. For a direct identification of the silicone oil the fragmentation pattern in the fingerprint region of the spectrum (mass range m/z up to approximately 800 u) is not significant enough. A suspicious material, however, with a silicone oil fragmentation pattern corresponding to the change in the defect can be considered for the probable cause.
3.2.1. Example 1 Fig. 6 shows the microscope image of a production line crater in a complete paint system. In the center of the defect, which is reaching down to the basecoat, a hole can be observed even in that layer. This crater was analyzed by ToFSIMS in comparison to the surrounding intact clearcoat surface. In the spectra of negative ions (presented in Fig. 7) a significant change in the fragmentation pattern of PDMS based silicone oils occurred between the crater and the intact clearcoat surface. This led to the assumption, that the crater was caused by a silicone oil on PDMS base. As the craters occurred at the same time in different basecoats, the conclusion was drawn that a contamination of a raw material might have been the cause of that problem. A data research pointed out, that a supplier exchange of a basecoat resin took place at the same time. Therefore, the correspondent resin charge of the new supplier was investigated by chromatography by a SiO2 filled glass tube. One of the detected fractions clearly showed silicone oil signals. The residue of this fraction, resolved in a droplet of hexane, was given by a micropipette onto the surface of a dried basecoat. After the solvent was evaporated a clearcoat layer was sprayed and cured. At the deposition place of the extracted silicone oil a crater occurred, which was also analyzed with ToF-SIMS. Compared with the surrounding intact clearcoat surface, this crater showed the same change in silicone oil fragmentation pattern as the line crater. 3.2.2. Example 2 In Fig. 8, the microscope image of a primer crater on a car body is shown. First LAMMA® analysis was carried out on two similar craters from the same panel. By this method it was not possible to find any indication to a
Fig. 7. Negative ion spectra (ToF-SIMS) of crater and intact clearcoat area.
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Fig. 8. Positive ion spectra (ToF-SIMS) of a primer crater in comparison to the surrounding intact surface and the corresponding microscope image.
possible crater cause. Then the crater of Fig. 8 was analyzed with ToF-SIMS and significant differences between the defect and the intact primer surface were found. In the spectra of positive ions (Fig. 8) additional signals of triglycerides and in the spectra of negative ions, the corresponding fatty acid fragments occurred. Triglycerides are often used in skin creams. Therefore, the crater for example might be caused by a fingerprint of an employee in the paint shop of the car builder. 3.2.3. Example 3 The last crater presented here shows a dark particle in the center of the defect. In the ToF-SIMS spectra of this crater no significant differences between the particle and the in-
tact clearcoat could be observed. By LAMMA® , however, it was possible to characterize the element composition of the particle. Fig. 9 shows the positive spectra of the particle in the center of the crater and of the surrounding intact clearcoat. While ToF-SIMS spectra directly show the number of detected ions (counts) versus the mass (exactly mass to charge: m/z), the intensity in the LAMMA® spectra is the voltage at the detector, which is also proportional to the number of ions per time interval. In the spectrum of the intact clearcoat surface the typical signals of an organic substance can be observed. Due to the rude evaporation process organic molecules are strongly fragmented. In the evaporation area a kind of plasma is formed, from which single charged carbon clusters arise by recombination. Most of the observed
Fig. 9. Positive ion spectra (LAMMA® ) of a dark particle in the center of a crater in comparison to the surrounding intact clearcoat.
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Fig. 10. Positive ion spectra (LAMMA® ) of a particle on the e-coat vs. the surrounding intact surface and the corresponding microscope image.
signals therefore correspond to Cn , Cn H and Cn H2 , where n is the number of atoms. Only very stable organic compounds as phthalate for example remain undamaged. In the present example, the particle inside the crater shows additional element signals in comparison to the intact clearcoat surface. Especially the iron and calcium signals are remarkable, because these elements are surely neither part of the underlying basecoat nor of the clearcoat. The Mg-, Al- and Si-signals probably belong to a silicate, which also might be a part of the basecoat recipe. These results led to the assumption, that the crater was probably caused by a steel abrasion particle maybe in combination with a calcium containing grease or carbonate and with a silicate sticking to it.
such defects can be done by micro-ATR4 infrared spectroscopy. A simple but not trivial example is shown in Fig. 10. On the microscope image a small ball of metal can be observed on the top of an e-coat layer. Although the nature of the defect seemed to be clear the question about the origin remained. In the LAMMA® analysis of the particle iron and copper were detected in the positive ion spectrum. In comparison to that, the e-coat only showed signals of the pigments (Al-silicate and Ti-oxide) and the organic resin matrix. With this optical and mass spectroscopic information the conclusion was drawn, that the defect was a welding pearl, because the wire used for electric welding consists of iron, which is covered by a copper layer to avoid corrosion.
3.3. Investigation of particles in coatings 4. Summary The second class of defects often analyzed are particles and pimples in cured coatings. Even if particles are on the top of the coating layer, they are often covered with a film of paint or another organic substance, which is not necessarily representative of the defect. If the film is very thin it is possible to measure the defect directly with LAMMA® . Otherwise the sample has to be prepared for the analysis. One possible way to remove the covering material respectively to open the defect is a cut with a scalpel. Another method to get to a defect is a cross section preparation. A ToF-SIMS analysis after one of these sample preparations normally does not make sense, because the topmost monolayers in the defect area often consist of smeared material from other places of the sample or from an aid substance. Due to the information depth of some tenth of a micrometer, a such possible contamination does not play a substantial role in a LAMMA® analysis. Therefore, this technique is a powerful tool for the inorganic characterization of particles and pimples. The organic investigation of
ToF-SIMS and LAMMA® , two different mass spectroscopic methods with the ability of trace analysis are often used for the investigation of paint defects. Both techniques were compared and the differences especially the way of ion formation at the target surface and the type of ion detection as well as the resulting effect on the kind of information were pointed out. By some examples typical of the coatings industry the main advantages of each technique were demonstrated. The examples show that both methods provide complementary information. Due to the ability to characterize the organic composition in the topmost monolayer of a surface and especially to distinguish between different silicone oils ToF-SIMS has an outstanding position in the analysis of craters. LAMMA® on the other hand is a powerful tool for the determination of 4
ATR: attenuated total reflection.
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the inorganic composition of particles and pimples in cured coating layers, because of the very small analysis spot size and the information depth of some tenth of a micrometer. So nearly every kind of paint defect can be successfully analyzed by one of the two methods.
Acknowledgement The authors would like to thank TASCON GmbH in M¨unster (Germany), for the long-standing excellent cooperation in ToF-SIMS analyses.
References [1] T. Haase, M. Osterhold, Catalog of Defects, Sec. Ed., Herberts GmbH, Wuppertal, Germany, 1996.
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[2] N. Basener, Farbe Lack 94 (10) (1988) 821. [3] A. Benninghoven, B. Hagenhoff, E. Niehuis, Anal. Chem. 65 (1993) 630A. [4] R.J. Cotter, Anal. Chem. 64 (1992) 1027A. [5] H. Vogt, H.-J. Heinen, S. Meier, R. Wechslung, Fresenius Z. Anal. Chem. 303 (1981) 195. [6] A. Benninghoven, F.G. R¨udenauer, H.W. Werner, Secondary Ion Mass Spectrometry, Wiley, New York, 1987. [7] A. Benninghoven, Ion formation from organic solids, in: Springer Series in Chemical Physics 25, Springer, Berlin, Heidelberg, New York, 1982. [8] E. Niehuis, Proceedings of Secondary Ion Mass Spectrometry (SIMS VIII), Wiley, Chichester, 1992, p. 269. [9] Product Information of the ToF-SIMS IV Instrument, ION-TOF GmbH, M¨unster, Germany, 1997. [10] J. Schwieters, H.-G. Cramer, U. J¨urgens, E. Niehuis, H. Rulle, T. Heller, J. Zehnpfenning, A. Benninghoven, Proceedings of Secondary Ion Mass Spectrometry (SIMS VIII), Wiley, Chichester, 1992, p. 497. [11] P. Bertrand, L.-T. Weng, Mikrochim. Acta 13 (Suppl.) (1996) 167.