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Plasma surface engineering — innovative processes and coating systems for high-quality products
Surface and Coatings Technology 112 (1999) 351–357
Plasma surface engineering — innovative processes and coating systems for high-quality products R. Suchentrunk *, H.J. Fuesser, G. Staudigl, D. Jonke, M. Meyer Daimler Chrysler Research and Technology, P.O. Box 800465, D-81663 Munich, Germany
Abstract The paper will inform about new results in research into and the application of plasma-assisted processes such as reactive magnetron sputtering, plasma CVD, plasma cleaning and activation, and plasma diffusion treatment. Typical examples from selected industrial areas will give an impression of the wide application-potential of these innovative techniques. Examples will include the deposition of wear and chemical-resistant coatings on lightweight polycarbonate glass for cars, tribological coatings on motor and gear components, plasma cleaning and activation of aerospace components, and the fabrication of high-temperatureresistant coating systems for various applications. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Coating systems; High-quality products; Plasma surface engineering
1. Plasma — a versatile tool Plasma-assisted processes enjoy an increasing application potential in industry. They are vital for microelectronics and tool fabrication, functional coatings on glass and plastic foils, the cleaning and activation of plastic components prior to painting and for decorative coatings for instance, and offer interesting advantages in other industrial areas. The main reason for the increasing interest on the part of industry lies in the fact that industrially wellestablished surface-finishing processes such as electrodeplating and painting suffer considerably from increased environmental and societal demands. Plasma processes offer advantages compared with these by avoiding air and water pollution as well as solid waste. This paper provides information on new results in research into, and the application of, plasma-assisted processes such as reactive magnetron sputtering, plasma CVD, plasma cleaning and activation, and plasma diffusion treatment. Typical examples from selected industrial areas will give an impression of the wide application-potential of these innovative techniques. Examples will include the deposition of wear and chemical-resistant coatings on lightweight polycarbonate glass * Corresponding author. Present address: Daimler Benz Forschung und Technik, Forschung und Technik F2K/0, Postfach 80 04 65, 81663 Munich, Germany.
for cars, tribological coatings on motor and gear components, plasma cleaning and activation of aerospace components, and the fabrication of high-temperatureresistant coating systems for various applications.
2. Examples of application 2.1. Coating on lightweight glass A considerable weight reduction can be achieved by replacing conventional ceramic glass by transparent plastic materials such as PC (polycarbonate) or PMMA (polymethylmethacrylate) for motor car windows, searchlight lenses or interior components of passenger trains. Weight reductions of more than 30% seem to be realistic for the complete system. However, these plastic materials suffer from a lack of mechanical and chemical surface resistance. They are not scratch-resistant, and are attacked by cleaning chemicals, petrol and other environmental effects. Protective coatings applied by plasma polymerization or plasma CVD can help to avoid damage to the plastic material. Plasma-deposited films have an extremely high degree of three-dimensional cross-linking compared to conventional polymers. This is caused by the fragmentation of the precursor gas in the plasma. The films are non-porous and have a very dense structure, making
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 83 3 - 0
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Fig. 1. Lightweight reflector casing with coated PC searchlight.
them extremely resistant to corrosion and other forms of chemical attack. The dense and pinhole-free film is an excellent diffusion barrier against liquids and gases. The adhesion to the substrate is high because of the intensive activation of the substrate surface by the reactive plasma. Gradient films can be produced by changing the process parameters during the deposition
process. The properties of the coating can be tailored so as to be very soft and ductile near the substrate and change as to be very hard and wear-resistant on top. The adhesion to the substrate can be preserved thereby, even when materials with great differences in the coefficient of thermal expansion are combined. The possibility of depositing gradient coatings offers the additional advantage of integrating functional regions for selective UV and IR absorption or reflexion. Fig. 1 shows a lightweight reflector casing for a motor car. The searchlight lenses are protected by a siliconoxide-based coating with a thickness of less than 10 mm. The coating features good resistance to wear and chemicals, but there are still unsolved problems that hinder large-scale fabrication and have to be solved in the near future. Such problems are outlined below. The deposition rate of the coating is currently not sufficiently high for economic fabrication. Optimization of the plasma equipment and the process parameters will help to solve this problem. The coating is transparent in the UV region, and therefore, the protection of the interface substrate/ coating against UV degradation cannot be guaranteed for the expected lifetime. Special protection systems are currently under investigation. An additional example of the wide application potential of plasma-deposited coatings is given in Fig. 2. As shown in the illustration, sensitive and decorative surfaces of polished brass can be protected by a colorless, chemically and mechanically stable coating. The upper part has been protected by a silicon-oxide-based coating (Silicor) and shows no signs of mechanical or corrosion attack. The part below was not coated and is considerably damaged.
Fig. 2. Polished brass protected with Silicor and unprotected.
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353
Fig. 3. Pretreatment of bumpers ( Technics Plasma).
2.2. Plasma treatment of surfaces The advantages of plasma processes for the activation of plastic materials have already led to their application on an industrial scale. Bumpers for motorcars are pre-
Fig. 5. Metallizing of plastic parts: barrier against moisture.
Fig. 4. Valve for helium tank.
treated in plasma for subsequent painting and adhesive bonding (Fig. 3). Compared to chemical activation, the plasma is less
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Fig. 6. Plasma-degreased part (Ciam/Al ).
selective. This enables pretreating of component parts consisting of completely different materials such as plastic, aluminum and steel at the same time. The valve for a helium tank shown in Fig. 4 consists of a combination of these materials. To provide an effective barrier coating against the diffusion of helium, the valve was plasma-activated and subsequently metallized by sputter deposition. The top coat was applied by electrodeposi-
Fig. 7. MoS -sputtered gear component. 2
tion on this electrically conductive, well-adhering layer. In the same way, plastic composite parts can be activated and coated with dense, tightly adherent metallic layers as a diffusion barrier against moisture (Fig. 5). Plasma treatment has become increasingly state-ofthe-art in the area of surface cleaning. In the microelectronic industry, for example, the removal of soldering fluxes has been performed for many years using plasma. Sensitive aerospace parts are treated by Swiss Air in a reactive low-pressure plasma to remove the penetrant dye residues after crack inspection. In spite of steadily progressing research work in the area of plasma processes, it will probably never be economic to degrease heavily contaminated, low-cost parts. However, for many precision-machined parts, which are only covered by a thin film of organic contaminants, plasma degreasing will prove to be an economically and environmentally safe alternative to toxic chemicals (Fig. 6). For small-area applications, as in microelectronic fabrication, the removal of photoresist is state-of-theart. As plasma processes operate integrally, it is also possible to strip the surface of a larger part with a similar expenditure of time. The plasma acts all over the surface simultaneously and not step by step as in the case of laser or ion-beam stripping. This underpins the reason to believe that paint stripping by plasma etching may be an economical and environmentally safe tool in the future, even for large-scale applications. For maintenance reasons, aircraft, for example, have to be stripped from time to time. Today, the removal of thick
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355
Fig. 8. Diamond for thermal management in microelectronic ICs.
paint films is performed world-wide by using toxic chemicals. Alternative processes using plastic particles, water jets or laser beams are under investigation but still have not reached an acceptable state of the art. Especially, for small and medium-sized aircraft components, it is likely that interest in plasma stripping will increase in the near future. 2.3. Tribo coatings on motor and gear components Tribological coatings of various compositions can be applied to motor and gear parts by plasma-assisted
deposition (reactive sputtering), depending on the application. Hard (chromium nitride) or soft (molybdenum disulfide) coating systems are possible. A gear component coated with a soft MoS film with self-lubricating 2 properties is shown in Fig. 7. 2.4. Diamond and diamond-like coatings Intensive investigations in this area have been performed over the past few years in many countries. The process technology and quality of such coatings have
Fig. 9. Door handles coated with Diacor.
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reached a sophisticated state of the art. Various methods such as reactive sputtering, arc-bonded sputtering and plasma-assisted CVD are available today. Polycrystalline diamond coatings can be produced by processes such as microwave plasma CVD, r.f. plasma CVD or supersonic arcjet plasma CVD. The application potential of diamond films is extremely high. Typical examples are as follows: $ wear-protection coatings $ coatings on tools $ low-friction coatings $ coatings on optical components $ high-temperature electronics $ high-temperature sensors $ radiation-resistant electronics $ heat sinks and heat spreaders for thermal management. Diamond offers the highest possible thermal conductivity and at the same time excellent electrical insulation properties ( Fig. 8). This makes it possible to improve the heat-sink properties of conventional designs for high-power devices by using a CVD diamond to a considerable extent. The dissipation of heat by the diamond film reduces temperature affecting the transistor by more than 100 °C. Therefore, diamond films are well suited for effective thermal management in microelectronic ICs. Diamond coatings can also be deposited in doped form. Initial investigations on the deposition of p-doped polycrystalline diamond coatings for highpower transistors have already led to promising results. The deposition of diamond-like carbon (DLC ) is described in many publications and has been state-ofthe-art for some years. One of the interesting features of DLC is the extremely low coefficient of friction, which can be used for lubricant-free bearings, for example. Through a special deposition process for DLC (DIACOR), decorative coatings with a broad variety of colors can be applied on practically all materials. They feature a high hardness, good ductility and wear resistance, guarantee corrosion protection and chemical stability, and can be used in a temperature range between −196 and +260 °C. Door handles in various colors can be seen in Fig. 9. 2.5. Plasma diffusion treatment By plasma-nitriding, gearshift levers (Fig. 10) or other motor car components can be treated on a large fabrication scale. Modification of the surface of the workpiece results in an improvement in wear resistance and service life. Especially for thin-walled structures or sintered steel parts with a residual porosity, plasma nitriding offers advantages compared with a salt bath or gas nitriding, such as low distortion, no corrosive salt residues in
Fig. 10. Large-scale plasma nitriding of gearshift levers.
pores, and no embrittlement caused by ‘‘internal coating’’. The wear resistance and service life of milling tools ( Fig. 11) can be increased by a Duplex treatment: plasma nitriding provides a stable underground for a thin PA-CVD-deposited TiN–TiC–TiCN hard coating and avoids any ‘‘egg-shell cracking’’. 2.6. Future trends and perspectives The increased application of plasma processes will have considerable benefits in different industrial sectors. This trend can be demonstrated by two typical examples. Nano-structured materials and nano-technologies assisted by plasma processing will lead to completely new microelectronic and micromechanical products, such as ductile ceramics and high-temperature supercon-
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357
exhaust gas cleaning by plasma discharges diffusion barriers in lightweight plastic tanks against petrol and moisture $ corona treatment of textiles for improved softness and dyeing properties $ environmentally benign pretreatment of plastic parts prior to painting and adhesive bonding $ plasma lamps with a high light intensity $ plasma-deposited protection coatings on lightweight searchlight lenses $ advanced tribological coatings in motor and gear sections. New processes under investigation today may help to make plasma technology a technically and environmentally safe and economical working tool. Plasma processes operating at atmospheric pressure will help to increase the application potential of plasma processes considerably. Equipment for corona treatment has already become state-of-the-art for the activation of plastic surfaces prior to painting or printing and is produced by competent suppliers. In Germany, intensive investigations are ongoing in the area of atmospheric plasma treatment and deposition, currently sponsored and funded by the German Ministry of Education, Science, Research and Development. The goal of coating three-dimensional large parts still lies in the future. However, research and industry in close contact with suppliers of sources and equipment are anxious to reach this goal. $ $
Fig. 11. Milling tools with Duplex treatment PT+PA-CVD.
ductive materials for microelectronics, flat panel displays and bioartificial systems. In the motor car industry, plasma technology will offer various technical and environmental solutions (Fig. 12) such as:
Acknowledgements
Fig. 12. Plasma technology in the automotive industry.
The following companies have contributed material for the paper presented: ANTEC GmbH., Kelkheim, Germany Technics Plasma GmbH., Kirchheim, Germany Plasma Technik Gru¨n GmbH, Siegen, Germany.
Plasma surface engineering — innovative processes and coating systems for high-quality products R. Suchentrunk *, H.J. Fuesser, G. Staudigl, D. Jonke, M. Meyer Daimler Chrysler Research and Technology, P.O. Box 800465, D-81663 Munich, Germany
Abstract The paper will inform about new results in research into and the application of plasma-assisted processes such as reactive magnetron sputtering, plasma CVD, plasma cleaning and activation, and plasma diffusion treatment. Typical examples from selected industrial areas will give an impression of the wide application-potential of these innovative techniques. Examples will include the deposition of wear and chemical-resistant coatings on lightweight polycarbonate glass for cars, tribological coatings on motor and gear components, plasma cleaning and activation of aerospace components, and the fabrication of high-temperatureresistant coating systems for various applications. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Coating systems; High-quality products; Plasma surface engineering
1. Plasma — a versatile tool Plasma-assisted processes enjoy an increasing application potential in industry. They are vital for microelectronics and tool fabrication, functional coatings on glass and plastic foils, the cleaning and activation of plastic components prior to painting and for decorative coatings for instance, and offer interesting advantages in other industrial areas. The main reason for the increasing interest on the part of industry lies in the fact that industrially wellestablished surface-finishing processes such as electrodeplating and painting suffer considerably from increased environmental and societal demands. Plasma processes offer advantages compared with these by avoiding air and water pollution as well as solid waste. This paper provides information on new results in research into, and the application of, plasma-assisted processes such as reactive magnetron sputtering, plasma CVD, plasma cleaning and activation, and plasma diffusion treatment. Typical examples from selected industrial areas will give an impression of the wide application-potential of these innovative techniques. Examples will include the deposition of wear and chemical-resistant coatings on lightweight polycarbonate glass * Corresponding author. Present address: Daimler Benz Forschung und Technik, Forschung und Technik F2K/0, Postfach 80 04 65, 81663 Munich, Germany.
for cars, tribological coatings on motor and gear components, plasma cleaning and activation of aerospace components, and the fabrication of high-temperatureresistant coating systems for various applications.
2. Examples of application 2.1. Coating on lightweight glass A considerable weight reduction can be achieved by replacing conventional ceramic glass by transparent plastic materials such as PC (polycarbonate) or PMMA (polymethylmethacrylate) for motor car windows, searchlight lenses or interior components of passenger trains. Weight reductions of more than 30% seem to be realistic for the complete system. However, these plastic materials suffer from a lack of mechanical and chemical surface resistance. They are not scratch-resistant, and are attacked by cleaning chemicals, petrol and other environmental effects. Protective coatings applied by plasma polymerization or plasma CVD can help to avoid damage to the plastic material. Plasma-deposited films have an extremely high degree of three-dimensional cross-linking compared to conventional polymers. This is caused by the fragmentation of the precursor gas in the plasma. The films are non-porous and have a very dense structure, making
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 83 3 - 0
352
R. Suchentrunk et al. / Surface and Coatings Technology 112 (1999) 351–357
Fig. 1. Lightweight reflector casing with coated PC searchlight.
them extremely resistant to corrosion and other forms of chemical attack. The dense and pinhole-free film is an excellent diffusion barrier against liquids and gases. The adhesion to the substrate is high because of the intensive activation of the substrate surface by the reactive plasma. Gradient films can be produced by changing the process parameters during the deposition
process. The properties of the coating can be tailored so as to be very soft and ductile near the substrate and change as to be very hard and wear-resistant on top. The adhesion to the substrate can be preserved thereby, even when materials with great differences in the coefficient of thermal expansion are combined. The possibility of depositing gradient coatings offers the additional advantage of integrating functional regions for selective UV and IR absorption or reflexion. Fig. 1 shows a lightweight reflector casing for a motor car. The searchlight lenses are protected by a siliconoxide-based coating with a thickness of less than 10 mm. The coating features good resistance to wear and chemicals, but there are still unsolved problems that hinder large-scale fabrication and have to be solved in the near future. Such problems are outlined below. The deposition rate of the coating is currently not sufficiently high for economic fabrication. Optimization of the plasma equipment and the process parameters will help to solve this problem. The coating is transparent in the UV region, and therefore, the protection of the interface substrate/ coating against UV degradation cannot be guaranteed for the expected lifetime. Special protection systems are currently under investigation. An additional example of the wide application potential of plasma-deposited coatings is given in Fig. 2. As shown in the illustration, sensitive and decorative surfaces of polished brass can be protected by a colorless, chemically and mechanically stable coating. The upper part has been protected by a silicon-oxide-based coating (Silicor) and shows no signs of mechanical or corrosion attack. The part below was not coated and is considerably damaged.
Fig. 2. Polished brass protected with Silicor and unprotected.
R. Suchentrunk et al. / Surface and Coatings Technology 112 (1999) 351–357
353
Fig. 3. Pretreatment of bumpers ( Technics Plasma).
2.2. Plasma treatment of surfaces The advantages of plasma processes for the activation of plastic materials have already led to their application on an industrial scale. Bumpers for motorcars are pre-
Fig. 5. Metallizing of plastic parts: barrier against moisture.
Fig. 4. Valve for helium tank.
treated in plasma for subsequent painting and adhesive bonding (Fig. 3). Compared to chemical activation, the plasma is less
354
R. Suchentrunk et al. / Surface and Coatings Technology 112 (1999) 351–357
Fig. 6. Plasma-degreased part (Ciam/Al ).
selective. This enables pretreating of component parts consisting of completely different materials such as plastic, aluminum and steel at the same time. The valve for a helium tank shown in Fig. 4 consists of a combination of these materials. To provide an effective barrier coating against the diffusion of helium, the valve was plasma-activated and subsequently metallized by sputter deposition. The top coat was applied by electrodeposi-
Fig. 7. MoS -sputtered gear component. 2
tion on this electrically conductive, well-adhering layer. In the same way, plastic composite parts can be activated and coated with dense, tightly adherent metallic layers as a diffusion barrier against moisture (Fig. 5). Plasma treatment has become increasingly state-ofthe-art in the area of surface cleaning. In the microelectronic industry, for example, the removal of soldering fluxes has been performed for many years using plasma. Sensitive aerospace parts are treated by Swiss Air in a reactive low-pressure plasma to remove the penetrant dye residues after crack inspection. In spite of steadily progressing research work in the area of plasma processes, it will probably never be economic to degrease heavily contaminated, low-cost parts. However, for many precision-machined parts, which are only covered by a thin film of organic contaminants, plasma degreasing will prove to be an economically and environmentally safe alternative to toxic chemicals (Fig. 6). For small-area applications, as in microelectronic fabrication, the removal of photoresist is state-of-theart. As plasma processes operate integrally, it is also possible to strip the surface of a larger part with a similar expenditure of time. The plasma acts all over the surface simultaneously and not step by step as in the case of laser or ion-beam stripping. This underpins the reason to believe that paint stripping by plasma etching may be an economical and environmentally safe tool in the future, even for large-scale applications. For maintenance reasons, aircraft, for example, have to be stripped from time to time. Today, the removal of thick
R. Suchentrunk et al. / Surface and Coatings Technology 112 (1999) 351–357
355
Fig. 8. Diamond for thermal management in microelectronic ICs.
paint films is performed world-wide by using toxic chemicals. Alternative processes using plastic particles, water jets or laser beams are under investigation but still have not reached an acceptable state of the art. Especially, for small and medium-sized aircraft components, it is likely that interest in plasma stripping will increase in the near future. 2.3. Tribo coatings on motor and gear components Tribological coatings of various compositions can be applied to motor and gear parts by plasma-assisted
deposition (reactive sputtering), depending on the application. Hard (chromium nitride) or soft (molybdenum disulfide) coating systems are possible. A gear component coated with a soft MoS film with self-lubricating 2 properties is shown in Fig. 7. 2.4. Diamond and diamond-like coatings Intensive investigations in this area have been performed over the past few years in many countries. The process technology and quality of such coatings have
Fig. 9. Door handles coated with Diacor.
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R. Suchentrunk et al. / Surface and Coatings Technology 112 (1999) 351–357
reached a sophisticated state of the art. Various methods such as reactive sputtering, arc-bonded sputtering and plasma-assisted CVD are available today. Polycrystalline diamond coatings can be produced by processes such as microwave plasma CVD, r.f. plasma CVD or supersonic arcjet plasma CVD. The application potential of diamond films is extremely high. Typical examples are as follows: $ wear-protection coatings $ coatings on tools $ low-friction coatings $ coatings on optical components $ high-temperature electronics $ high-temperature sensors $ radiation-resistant electronics $ heat sinks and heat spreaders for thermal management. Diamond offers the highest possible thermal conductivity and at the same time excellent electrical insulation properties ( Fig. 8). This makes it possible to improve the heat-sink properties of conventional designs for high-power devices by using a CVD diamond to a considerable extent. The dissipation of heat by the diamond film reduces temperature affecting the transistor by more than 100 °C. Therefore, diamond films are well suited for effective thermal management in microelectronic ICs. Diamond coatings can also be deposited in doped form. Initial investigations on the deposition of p-doped polycrystalline diamond coatings for highpower transistors have already led to promising results. The deposition of diamond-like carbon (DLC ) is described in many publications and has been state-ofthe-art for some years. One of the interesting features of DLC is the extremely low coefficient of friction, which can be used for lubricant-free bearings, for example. Through a special deposition process for DLC (DIACOR), decorative coatings with a broad variety of colors can be applied on practically all materials. They feature a high hardness, good ductility and wear resistance, guarantee corrosion protection and chemical stability, and can be used in a temperature range between −196 and +260 °C. Door handles in various colors can be seen in Fig. 9. 2.5. Plasma diffusion treatment By plasma-nitriding, gearshift levers (Fig. 10) or other motor car components can be treated on a large fabrication scale. Modification of the surface of the workpiece results in an improvement in wear resistance and service life. Especially for thin-walled structures or sintered steel parts with a residual porosity, plasma nitriding offers advantages compared with a salt bath or gas nitriding, such as low distortion, no corrosive salt residues in
Fig. 10. Large-scale plasma nitriding of gearshift levers.
pores, and no embrittlement caused by ‘‘internal coating’’. The wear resistance and service life of milling tools ( Fig. 11) can be increased by a Duplex treatment: plasma nitriding provides a stable underground for a thin PA-CVD-deposited TiN–TiC–TiCN hard coating and avoids any ‘‘egg-shell cracking’’. 2.6. Future trends and perspectives The increased application of plasma processes will have considerable benefits in different industrial sectors. This trend can be demonstrated by two typical examples. Nano-structured materials and nano-technologies assisted by plasma processing will lead to completely new microelectronic and micromechanical products, such as ductile ceramics and high-temperature supercon-
R. Suchentrunk et al. / Surface and Coatings Technology 112 (1999) 351–357
357
exhaust gas cleaning by plasma discharges diffusion barriers in lightweight plastic tanks against petrol and moisture $ corona treatment of textiles for improved softness and dyeing properties $ environmentally benign pretreatment of plastic parts prior to painting and adhesive bonding $ plasma lamps with a high light intensity $ plasma-deposited protection coatings on lightweight searchlight lenses $ advanced tribological coatings in motor and gear sections. New processes under investigation today may help to make plasma technology a technically and environmentally safe and economical working tool. Plasma processes operating at atmospheric pressure will help to increase the application potential of plasma processes considerably. Equipment for corona treatment has already become state-of-the-art for the activation of plastic surfaces prior to painting or printing and is produced by competent suppliers. In Germany, intensive investigations are ongoing in the area of atmospheric plasma treatment and deposition, currently sponsored and funded by the German Ministry of Education, Science, Research and Development. The goal of coating three-dimensional large parts still lies in the future. However, research and industry in close contact with suppliers of sources and equipment are anxious to reach this goal. $ $
Fig. 11. Milling tools with Duplex treatment PT+PA-CVD.
ductive materials for microelectronics, flat panel displays and bioartificial systems. In the motor car industry, plasma technology will offer various technical and environmental solutions (Fig. 12) such as:
Acknowledgements
Fig. 12. Plasma technology in the automotive industry.
The following companies have contributed material for the paper presented: ANTEC GmbH., Kelkheim, Germany Technics Plasma GmbH., Kirchheim, Germany Plasma Technik Gru¨n GmbH, Siegen, Germany.