Decorative optical coatings

ELSEVIER

Thin Solid Films 253 (1994) 33 40

Decorative optical coatings Georg Reiners a, Uwe Beck a, H e r m a n n A. Jehn b aBAM (Bundesanstalt fiir Materialforsehung und-priifung), Unter den Eiehen 87, D-12200 Berlin, Germany bForschungsinstitut fiir Edelmetalle und Metallehemie (FEM), Katharinenstrafle 17, D- 73525 Sehwiibisch-Gmiind, Germany

Abstract

The paper gives a survey of current research on decorative coatings. In the first part the main deposition techniques as well as characterization techniques are discussed. The paper is restricted to decorative physically vapor deposited hard coatings with electrochemically deposited barrier coatings. The influence of stoichiometry, structure, and surface roughness on the colour of physically vapor deposited hard coatings is reviewed. In the last decades, a number of different coating deposition techniques have been developed. The demand for environmental compatibility of low cost large-scale production techniques has resulted in the development of combinations of physical vapor deposition and electroplating. Coating systems have been developed using NiPd and CuSn(Zn) instead of Ni electroplated coatings as interlayers, strongly reducing the allergy risk. New industrial applications demonstrate the increasing economic importance of decorative hard coatings.

Keywords: Nitrides; Optical properties; Physical vapour deposition; Structural properties

I. Introduction

Decorative coatings have a long tradition in jewellery, ritual objects and even objects of daily life. For hundreds of years, for example, leaf gold has been used to decorate works of art. In 1840, the first gold plating made by electrochemical deposition (ECD) was produced [1]. The function of decorative coatings is not restricted to giving a desired colour to the surface. In many cases, the coating also has to protect the substrate material against wear and/or corrosion. Normally, the substrate material takes over the "mechanical" function. By use of decorative coatings, expensive substrate materials and/or expensive production techniques (e.g. machining of hard bulk materials) can be substituted. A number of different coating deposition techniques have been developed: painting, anodizing, electroplating (ECD), diffusion coating, thermal spraying, enamel coating, chemical and physical vapor deposition (CVD and PVD). The present paper, however, is restricted to decorative hard coatings deposited by PVD techniques. The colour of PVD thin films, as many other properties, often differs from that of the bulk material because the colour is mainly determined by the band structure 0040-6090/94/$7.00 © 1994 - - Elsevier Science S.A. All rights reserved SSDI 0 0 4 0 - 6 0 9 0 ( 9 4 ) 0 4 6 5 7 - R

of the deposited layer which depends on both the chemical composition and the crystal structure of the film. The structure of PVD coatings can differ strongly from that of undisturbed bulk structure depending on the deposition technique and the substrate material. Transition metal compounds, e.g. oxides, nitrides and carbides, find increasing use as decorative coatings combining intensive colours, high wear resistance and good corrosion resistance [2-13]. The colour variation of simple compounds such as the golden TiN and dark gray TiC is limited. Oxidizing, nitriding and carburizing of binary alloys or the addition of a second metalloid, however, widen the spectrum of colours for decorative applications. A typical example is (Ti, A1)N, changing from silvery to gold and dark blue colour depending on the A1 and N content [14]. PVD coatings are often combined with ECD coatings, especially if cheap materials are used for jewellery parts, watch cases and other objects of everyday life. In these cases the electroplated layers mainly act as corrosionresistant interlayers because PVD films are not completely defect free and corrosive media can come in contact with the substrate material. In addition, surface leveling and special gloss effects can be obtained by the deposition parameters of the ECD process. Conventional electroplated barrier layers are mostly based on

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G. Reiners et al. / Thin Solid Films, 253 (1994) 33 40

Ni which are now having to be replaced because of their allergy risk. The first part of this paper deals with fundamentals of colour generation and changes in colour along with characterization techniques needed to study the correlation between color and other properties of the coatings. The second part gives a comprehensive report of current research activities and recent applications of decorative hard coatings.

2. Economic aspects of decorative coatings For many products, the design and finish fix the price whereas the function is taken for granted. Colour is a constitutive element of decorative design. Examples are watches (watch cases). Even low-price watches show the exact time with high accuracy but the prices of watches differ by at least two orders of magnitude. Estimations [15] show that the world market for thin film products is about 32 billion US$ per year, decorative thin film products contributing about 0.75 billion US$ per year. Decorative parts are attacked in a very complex way by wear and corrosion simultaneously. Again watches are taken as an example. They combine all the tribological mechanisms which are typical of decorative coatings [16]. Hence, in addition to their decorative functions, PVD coatings have to fulfil a wide range of demands: good adhesion and resistance to wear and corrosion as well as mechanical and chemical compatibility with the substrates or interlayers. Additionally, reproducible deposition with respect to the decorative properties of the coated parts is a prerequisite for any successful application. The well known high sensitivity of the human eye requires sensitive characterization techniques for production quality control. Nevertheless, the most important point is the price.

3. Deposition techniques of decorative coatings In general, various techniques for the deposition of decorative coatings are used in industry. In this paper, we restrict ourselves to PVD coatings and the combination of PVD and ECD techniques for the deposition to wear resistant coatings onto metallic substrates. ECD coatings include noble metals (Au, Au alloys, Pt, Ag, etc.), metals (e.g. Cr, Ni, Cu), and some binary alloys (e.g. CuZn, CuSn, etc.). The surface roughness can be reduced by ECD coatings. The wear resistance of ECD coatings is sufficient for many applications and the corrosion resistance is high in most cases. The typical ECD coating thickness is more than 10 ~tm. The deposition rates are much higher than 1 ~tm h-l. The PVD technique has numerous possibilities for the deposition of noble and other metals, binary, ternary

and multicomponent metal alloys, and especially reactively deposited hard coatings (nitrides, carbides, carbonitrides such as TiN, (Ti, A1)N, etc.). The roughness of PVD coatings is normally the same as the roughness of the substrate or greater. The corrosion resistance of hard coatings is limited because of pin-holes in the coating, whereas the wear resistance is extremely high. The typical PVD coating thickness ranges between 1 and 5 ~tm. The deposition rates are often of the order of 1-5 ~tm h i. A more detailed comparison of ECD and PVD coatings is given in Ref. [17]. A comparison of the properties of coatings deposited by ECD and PVD has, for example, been made for gold alloys [18] or ECD hard chrome and PVD CrN [19]. Comparison of different PVD techniques, however, showed only slight differences with respect to colour and gloss [20]. Combination of these techniques widens the field of application for decorative purposes [21 23].

4. Colour Colour and gloss are the most important properties of decorative parts and hence also of decorative coatings. The visual perception of colour is well described by the CIE L*a*b* coordinates L*, a* and b* [24], which allow better quantification of colour differences as perceived. The parameters are the lightness L* (black = 0 , white = I00), the red-green value a* and the yellow-blue value b* (a*, b * = 0-100). 4. I. Colours ~[ bulk materials

Illuminated by white light a material can generate colors by dispersion (prisms), interference (filter), diffraction (grids), scattering (granules), or absorption (atomic, molecular, solids). For decorative hard coatings the generation of colour is restricted to absorption, i.e. the response of matter to the incident light. In order to obtain coloured coatings the colors of bulk materials can be considered. Some natural colours are given in Table 1 together with the substances. The composition of the Swiss gold standards, which are important for decorative applications, are given in Table 2. Additional information on nitrides, carbides, and borides can be found in Ref. [18] and Refs. [25] and [26]. Colour standards used to calibrate colour measuring equipment can be found in Ref. [27]. Bulk colours generated by absorption processes (e.g. pigments in a dispersed state) depend on the crystal structure and the chemical composition of the compound. These dependences are the reason why standard materials are unsuccessful as PVD coatings. PVD hard coatings are highly disordered microcrystalline thin films. Their electronic properties differ from the bulk values and consequently the colour differs too.

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G. Reiners et al. / Thin Solid Films, 253 (1994) 33-40

Table 1 Colours of bulk materials [16] Common name

Base

Addition

Colour

Ruby Ruby Ruby Sapphire Sapphire Sapphire Topaz

AI203 AI203 AI203 AI203 A1203 AI203 A1203 A1203 A1203 AI203 TiO 2 TiO 2 TiO 2 TiO 2 TiO 2

2"/0_3% Cr2O3

Scarlet Pink Blue Violet Blue Yellow Gold Sky blue Marine blue Blue violet White Lantern red Yellow Blue Canary

0.01%-0.5% Cr203 2%-3% V203 0.5% TiO 2 + 1.5% Fe203 + 0.1% Cr203 0.5% TiO 2 + 1.5% Fe203 0.5%- 1% NiO 0.5% NiO + 0.01%-0.05% Cr203 MgO + 0.1%-0.5% CoO MgO + 0.5%-1% CoO MgO + 1.5% CoO 0.5% Cr203 0.01% Cr203 1.5% Fe203 0.05% NiO

Table 2 Swiss gold standards (ISO 8654) ISO notation

ON 1N 2N 3N 4N 5N

Composition (atoms per 1000) Au

Ag

Cu

585 585 750 750 750 750

300-340 240-265 150-160 120-130 85-95 45-55

Rest Rest Rest Rest Rest Rest

Colour

Yellow green Pale yellow Bright yellow Yellow Ros6 Red

Absorption-flee interference layers generate colours on a highly reflecting substrate according to the length of the optical path within the layer. Consequently, the impression of colour depends on the angle of view. Multilayers may reduce this dependence. The colours of hard coatings presently produced by PVD techniques on an industrial scale are summarized in Table 3 together with typical hardness values. A more detailed collection of thin film colours prepared on a laboratory scale can be found in Ref. [31]. The colours depend additionally on the stoichiometry and the roughness of the substrate and thin film as well as the coating thickness [32].

4.2. Colours of thin films 5. Characterization techniques In general, there are two possible ways of varying the colour of thin films, i.e. changes in structure and stoichiometry which both influence the electronic structure of the deposited film resulting in changes in the selective absorption. Consequently, a shift in the optical gap results in changes in reflectivity over the visible wavelength range. TiN is an example of stoichiometric dominated colour generation [14, 28] whereas ZrN is an example of microstructure dominated colour changes [29, 30]. A very straightforward common example of the exclusive influence of structure on colour is graphite and diamond layers. In principal one has also to consider second order of magnitude effects for colour changes: surface and volume scattering. This is all the more important for semitransparent coatings (e.g. Ti inclusions in TiO2 on a highly reflecting substrate). According to the penetration depth of the incident light wave, the fraction of surface to volume scattering changes or even in-layer reflection takes place.

Research into the correlation between colour, stoichiometry and the crystal structure of decorative PVD coatings requires the typical spectrum of techniques to chatacterize, in addition to colour, the composition (glow discharge optical spectroscopy GDOS, Auger electron spectroscopy AES, electron spectroscopy for chemical analysis ESCA) structure (X-ray diffraction XRD), hardness (universal hardness test), adhesion (scratch test, bending tests) and roughness. Testing of the corrosion and wear behavior of decorative coated components requires adapted test conditions which take into account the specific tribological and corrosion conditions of the used component. Again, watches are a good example. All types of wear and corrosion attack take place. For industrial and laboratory wear and corrosion test for decorative coatings see for example Refs. [33, 34]. The optical properties are usually measured by three techniques to verify the impression of colour both

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G. Reiners et al. / Thin Solid Films, 253 (1994) 33-40

Table 3 Thin film colours from Ref. [31] Type

Compound

Colour

Hardness (Hv)"

Nitrides

TiN~ ZrN Cr2N, CrN TaN (Ti, A1)N (Ti, AI)ON (Ti, Zr)N

Golden ~ brown-yellow Yellow-green Metallic --, brown Blue-grey Gold --+dark blue Transparent --, black Golden

2400 2600 2500

Carbides

TiC TiC/WC TaC.,. SiC

Bright gray Dark gray Yellow~ brown Black

1500-2900 1400 1800

Carbonitrides

TiC,N>, ZrC.~N>,

Red golden--, violet Silver--*gold --. violet

2700

2400-2900

~Different test conditions; parameters not always mentioned in the literature.

physically and physiologically. Integrating sphere measurements (ISM), which uses standared industrial equipment, and angular dependent measurements obtained by goniospectroscopy (GS) describe the physiological colour perception due to directly and diffuse reflected parts of light by means of the CIE L * a * b * coordinates. Both systems refer to the ideal mat white surface (require a white reference) and measure the spectral reflectance factor R~[x]. The influences of the measuring system used, the white reference sample used, and whether the measurement was taken with or without gloss trap are discussed in Ref. [35]. Using polarized light in spectroscopic ellipsometry (SE) allows determination of the complex refractive index N =n(~o)+ik(~o) and the dielectric function 8 = 8)(¢o)+ i82(c0) of the layer. As this technique uses directly reflected light, a smooth surface is required for measurement [35]. There is no reference to white light needed and the geometric influences of illumination and measurement are negligible, but the visual impression is not correctly described. However, it is possible to describe optical properties more physically, to separate interference effects and to detect changes in stoichiometry. SE can be used to investigate changes in the optical constants of the deposited layers as a function of both stoichiometry (target material composition, reactive gas flow rate) and structure (film thickness, substrate temperature).

6. Current research activities

Current research is foccused on three main fields: (i) the search for new colours of PVD hard coatings, (ii) demonstration of the applicability of cheap materials used as substrates for decorative hard coatings, and (iii)

scaling-up of the optimized deposition processes to produce decorative hard coatings on large and threedimensional parts under industrial conditions. The investigations, however, cover not only these fields (new hard coatings, selection and coating of substrate materials and equipment construction) but also substrate pretreatment and cleaning procedures, interlayer and top layer selection and deposition as well as wear and corrosion tests of coating/substrate systems. In the following, standard and new films and procedures are discussed with special emphasis on coating development and system design. Scaling-up as a technical problem is not treated here in more detail. In principal, hard coatings are usually deposited by magnetron sputtering, and also by arc ion plating and combined techniques [ 13]. 6.1. Decorative hard coatings 6.1. I. Standard colours

The colour variation of simple compounds such as the golden TiN and the dark gray TiC is limited. Oxidizing, nitriding and carburizing of binary alloys or the addition of a second metalloid, however, widen the spectrum of colours for decorative applications. A typical example is (Ti,A1)N changing from silvery to gold and dark blue colour depending on the A1 and N content [ 14]. The reactive deposition of ZrN coatings with varying nitrogen contents allows tuning of the colour to match the colour of brass which is needed for decorative coatings for bathroom furnishings [36, 37]. The reactive deposition of CrN results in a coating colour quite similar to ECD hard chrome coatings which are also needed for decorative coatings for bathroom furnishings [ 19].

G. Reiners et al. / Thin Solid Films, 253 (1994) 3 3 - 4 0 Table 4 C o m p o s i t i o n o f t a r g e t m a t e r i a l s ( a t . % ) [35, 36] Base

V

Cr

AI

Y

Ti Ti

30 50

---

50 --

---

Zr

10

5

10

20

Zr Zr

20 --

10 40

20 --

---

&O

.~""

30

~

".~

~..".". . . . . .

.

"*-.

~

",~

......

-.

• 10

•

*~,

',.,

;./

sooOS°= o

.....'"

- .~:.C~,.

-10 -t,

I -2

.....

.........

I 0

I +2

I t,

1 6

I 8

I 10

ZrV 80:20

----

Zr

ZrAI 8 0 : 2 0

.....

ZrY 80:20

Zrln 80:20

Fig. 1. T h e c o l o u r o f nitrides o f Z r - b a s e d alloys as a function o f different alloying elements [38].

6.1.2. New colours The addition of a second metal or a metalloid (Table 4) to produce ternary nitrides results in a reduction in hue [29, 35]. Fig. 1 shows the colour of nitrides of Zr-based alloys depending on different alloying elements [38]. Nevertheless only limited success allows (Zr,V)N colours to be tuned to match different Swiss gold standards [39]. In general, the lightness L* decreases with increasing flow rates for these coatings, see also Ref. [29]. The surface roughness can change not only the brilliance of a coating but also its chromatic value. This was demonstrated by depositing TiN coatings onto substrates with different well defined surface roughness [32]. Typical examples illustrate some dependences. ZrN coatings of different thickness deposited under constant conditions show a nearly constant atomic concentration ratio Zr:N of about 0.47-0.48 but remarkably different L*, a*, b* values. The change in colour of these coatings are accompanied by changes in the XRD spectra. The reason for this colour variation is therefore mainly structural. (Zr,Y)N coatings deposited with increasing nitrogen flow rates exhibit strong changes in both stoichiometry and structure in the lower flow

37

regime [29]. For higher flow rates the main effect on colour change is again structural. Comparing the effect of the addition of a second or third metal, only the addition of yttrium as a third metal (quaternary nitrides such as (Zr,Y,A1)N) broadens the spectrum of colours depending on the nitrogen flow rate (e.g. (Zr,Y,A1)N [40]). The reactive deposition of (Ti,A1)ON [41] with very low oxygen flow rates results in promising coating colours, but the coating structure, composition, and chemical binding stage are not stable. After hours or days the coating colour changes normally to metallic gray. Studies are in progress to understand the mechanisms involved. In part, additional colours are observed for high reactive gas flow rates which are caused by interference effects. Target poisoning takes place which results in a lower deposition rate. The layers are transparent or at least semitransparent. The measured ellipsometric data tan(tk) and cos(~) consequently show a modulation according to an interference effect [35]. Intensive studies on sputter deposition of ZrB 2, ZrBt~, ZrBN, LaBN coatings were reported in Refs. [42] and [43]. The first applications are expected soon because of promising results with anthracite and dark blue and black colours. A completely different approach was described in Ref. [44]. The authors tried to incorporate pigments in transparent hard coatings such as SIO2. The pigment material was evaporated onto the substrate prior to deposition of the hard coating. A very simple process comparable with CVD deposition is described in Ref. [45]. A variety of metal chlorides can be deposited for example by painting onto Ti substrates. Ti has a thin intrinsic oxide layer. The coated parts were tempered at 450-650°C in air. During this tempering process a metal oxide layer of up to 21am thickness grows on the TiO2 layer. Many different colours have been found using different noble and ignoble metal chlorides as precursors. 6.2. Decorative coating systems As already mentioned, in technical applications of decorative coatings the hard compound films are often combined with top layers to improve the optical impression or with interlayers to improve adhesion or corrosion resistance. 6.2.1. Noble metal top coatings For gold coloured consumer goods it is state of the art to deposit at least a thin gold flash ( ~ 100 nm) on top of the hard coatings to obtain the brilliance of gold alloys. Better adaptation of the colour to the different

G. Reiners et al. / Thin Solid Films, 253 (1994) 33-40

38

Swiss gold standards (see Table 2) is normally achieved using different expensive gold sputter targets. The reactive deposition of for example AuV alloys allows the deposition of coatings with different gold colours using a single target [19]. A new nickel-free gold alloy target (AuV 7.5 at.%) for the deposition of colour adjustable gold coatings by reactive sputtering is described in Ref. [46]. A wear resistance three times better than that of ECD gold coatings was found.

6.2.2. Interlayer systems ECD, standard or allergy-free Nickel or nickel-containing alloys have so far been state of the art for ECD deposition. However, nickel causes considerable problems with respect to allergy risk. A nickel proof test according to a Danish standard of 1988 [47] or a German standard in artificial sweat solution shows that nickel can penetrate gold layers as they are removed from the surface over time. However, even with a PVD hard coating on top of nickel the problem is not solved because of pin-holes. An improvement in quality can be achieved by the introduction of an NiPd layer [30]. However, this is not suitable for applications which are in direct contact with human skin. A standard layer system on brass consists of a 7 tam thick electroplated nickel layer on a brass substrate with a 0.4 lam thick PVD coating on top. An advanced system was patented in Germany [21, 22]. On top of the first ECD nickel layer a second ECD NiPd coating (1-1.5 ~tm thick) was deposited. Coated with a decorative PVD layer, such systems protect brass and aluminum alloys much better against corrosion attack. Fig. 2 shows such an advanced layer system [35] in 3 '~ angled cross-sections on a brass substrate. Fig. 3 shows the depth profile of this coating system measured by GDOS. Applying the same system to substrates made from die cast zinc one finds severe problems with pores beneath the coating of the cast. Only high quality die cast substrates can fulfil the demands for decorative hard coatings.

lS58 Geatzl

20~1

Fig. 2. 3° angled cross-section of the coating system TiN/Ti/CuSn on a brass substrate.

I0

JT~ I 6

~2 0

Pb

~\

10

20

, , 30 ~0 50 sputter time [see]

J

~ 60

', BO

70

, 90

I 100

~-

Fig. 3. Depth profile of a coating system TiN/Ti/CuSn on a brass substrate measured by GDOS.

As ECD CuSn and CuSn(Zn) layers have no known allergy risk, they can be used as a substitute for nickelcontaining barrier layers. The adhesion of these layers to the substrate is good, while the adhesion of PVD coatings deposited directly onto CuSn or CuSn(Zn) has yet to be optimized. The corrosion behavior of P V D - E C D coating systems has been studied by electrochemical methods aswell as short-term technical test (see e.g. Refs. [33] and [48]). Such studies are of high importance for coating system development.

6.2.3. Interlayer systems (PVD) A new barrier coating (NiFeCrPB, analog to Metglass "' 2826A) deposited by magnetron sputtering is described in Ref. [46]. A modified sputter deposited titanium barrier layer was tested in Ref. [49]. 6.3. Substrate selection and pretreatment The suitability of machinable brass alloy (CuZn39 Pb2) and an aluminum alloy (A1MgSi0.5) as substrate materials for decorative hard coatings was successfully demonstrated in Ref. [30]. The use of ignoble substrate materials requires the combination of an ECD layer for corrosion protection with a PVD decorative thin film. Brass substrates were cleaned using a five- to seven-step cleaning procedure without fluorinated or chlorinated hydrocarbons, for some applications preceded by grinding and polishing of the substrate surface. Most of the multicomponent coatings discussed in Section 6.1.2 were deposited onto polished SUS 304 stainless steel sheets. In the past fluorinated or chlorinated hydrocarbons were frequently used in cleaning and drying processes prior to deposition. Now it has become normal on an industrial as well as on a laboratory scale to use CFCfree processes. Detailed information on the development of pretreatment and drying processes without using fluorinated or chlorinated hydrocarbons is given in Ref. [50].

G. Reiners et al. / Thin Solid Films, 253 (1994) 33-40

Advances in three-dimensional decorative coatings were reported in Ref. [50]. Larger parts can be coated using a new on-line production coater equipped with new magnetron sputter sources.

7. Applications of decorative hard coatings Scanning the literature reporting decorative coatings, more than 100 articles have been published in the last 5 years. In the following some of the major present and promising future applications are summarized. The most important applications in the field of consumer goods are still eye-glass frames, eye glasses, writing utensils, pens, watch cases and bands, lighters, custom jewellery, and cuttlery [51-53]. A few reports also mentioned bicoloured coatings [54] which may be too expensive for most applications. An increasing number of papers reports on new applications for fittings and furnishings (e.g. Ref. [34]), which demonstrates the progress of industrial coating technologies for three-dimensional parts [13, 50] and successful corrosion resistance by interlayers. Architectural glass coating is mainly done to obtain special optical functions such as IR reflection. Many of these coatings have intrinsic colours. Therefore the demand for homogeneity and reproducibility of the window colour is very high and requires sophisticated process quality control. During the last few years an increasing number of papers has dealt with decorative coatings on steel sheets for possible applications in architecture [55-63].

8. Advanced coating systems Future applications as for example hot plates and microwave barriers require thermal resistant decorative coatings on glass. The work until now has not been successful because of delamination of the coatings at higher temperatures. Thin film systems for so-called smart windows (automobile windshields, architectural glass) with adjustable reflectance and transmission coefficients (e.g. electrochromic systems) also have intrinsic colours. For large-area application similar demands on the homogeneity and reproducibility of the window colour also requires sophisticated process quality and control.

9. Conclusions Many coatings can act as decorative hard coatings. This paper has reviewed optimized standard decorative PVD coatings as well as coating systems with advanced

39

corrosion resistance and new colours. Coating systems have been developed with NiPd and CuSn(Zn) instead of Ni electroplated coatings acting as interlayers, thus strongly reducing the allergy risk. The colour of PVD hard coatings is influenced by stoichiometry, structure and surface roughness. New industrial applications demonstrate the increasing economic importance of decorative hard coatings. Acknowledgement A part of the reviewed work (Refs. 8, 11, 12, 18, 23, 29, 30, 32, 33, 35, 36, 39, 41, 48, 50) were supported by the Federal Ministry of Research and Technology (BMFT # 13N5834-37). In this joint project "Decorative PVD coatings--Basic Properties and Performance", Leybold AG (Hanau, Germany), METALEUROP Coating Technology GmbH, (Hohenlockstedt, Germany), BAM (Berlin, Germany), and FEM (Schw/ibischGmiind, Germany) are working together. The authors thank many coworkers for their assistance and DEMETRON GmbH (Schw/ibisch-Gmfind, Germany) for cooperation. References [1] F. Simon, 5th Leybold Syrup. on Decorative Coatings, 28 30 April, 1993 Steinheim Leybold, Hanau, Germany. [2] A. J. Perry, J. Vae. Sci. Technol A, 4 (6) (1986) 2670. [3] A. J. Perry, M. Georgson and C. G. Ribbing, J. Vae. Sci. Technol. A, 4 (1986) 2674. [4] Y. Fukui, T. Miono and T. Kittaka, Curr. Adv. Mater. Process., 2 (5) (1989) 1638-1639. [5] H. Randhawa, Surf. Coat. Teehnol., 36 (1988) 829-836. [6] G. Hakansson, J. E. Sundgren, D. Mclntyre, J. E. Greene and W. D. Mtinz, Thin Solid Films, 153 (1987) 55. [7] I. Musil, S. Kodlec, J. Vyskocil and L. Valvoda, Thin Solid Films, 167(1988) 107 119. [8] U. Kopacz and S. Schulz, Asia Pacific lnterfinish 90, Singapore, 19-22 November 1990, Australasian Institute of Metal Finishing, Parkville, Victoria, 1990, pp. 8.1 8.37. [9] P. Seserko, U. Kopacz and S. Schulz, Galvanotechnik, 80 (12) (1989) 4274 4277. [10] R. Riedl, Galvano-Organo Trait. Surf., 617 (June-July 1991) 699 704. [11] U. Kopacz and R. Riedl, Z. Metallkd., 83 (7) (1992) 492-499. [12] S. Schulz, Diinne Schicten, 3 (1) (March 1992) 7-11. [13] W. D. Miinz, Annu. Tech. Conf., Society of Vacuum Coaters, Society of Vacuum Coaters, Albuqerque, NM, 1993, pp. 411418. [14] H. A. Jehn, S. Hofmann and V.-E. Rfickborn, J. Vac. Sci. Technol. A, 4 (6) (1986) 701. [15] Programmevaluation Diinnschichttechnologien, August 1993 (VDI-Technologeizentrum Physikalische Technologien, Diisseldorf, im Auftrag des BMFT). [16] R. Riedl, Moderne Verfahren zur dekorativen Oberfliichenveredelung, 27-28 September 1993, Diisseldorf, VDI-TZ VDI-W, Bl-19. [17] S. Grainger, Engineering Coatings Design and Application, Woodhead, Cambridge, 1993.

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