- Email: [email protected]
Ion beam-induced enhanced adhesion of gold films deposited on glass
Surface and Coatings Technology 158 – 159 (2002) 558–562
Ion beam-induced enhanced adhesion of gold films deposited on glass L. Guzman*, A. Miotello, R. Checchetto, M. Adami INFM and Department of Physics, University of Trento, 38050 Povo, Trento, Italy
Abstract The metallisation of glass for decorative andyor functional purposes is now a well-established technique. The most popular methods are electroplating or sputter deposition. In order to obtain suitable adhesion, substrate pretreatment is a substantial part of a vapour-deposited coating that generally cannot be dispensed with. The pretreatment costs can reach the same order of magnitude as those associated with the actual coating process, or can even exceed them. In this work we studied the adhesion of Au thin films on glass. The substrates were pretreated by an ion-beam-mixing step, consisting of the deposition of AuyC bilayers or CyAuyC multilayers followed by Xeq implantation. After such preparation, the specimens were further coated (using a sputtering machine) with 150-nm-thick Au or Au-alloy films. Adhesion properties of the films were examined using a scratch tester in conjunction with scanning electron microscopy. It was observed that, without the ion-beam-mixing pretreatment, the coatings were poorly adherent. Strong adhesion enhancement was observed in the pretreated samples. The key mechanism envisaged to explain this is related to the formation of mixed SiC–Au phases at the interface region. Moreover, the mechanical properties of the pure and alloyed Au films were quantified by nano-indentation, and hardness results are in good agreement with a simple rigid-sphere model of substitutional hardening. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Ion-mixed interfaces; Au coatings; Enhanced adhesion
1. Introduction Good adhesion is of prime importance in dictating the lifetime of a coated component, and a number of techniques have been developed to improve adhesion of the coating to the substrate based on this requirement. There are several factors which must be considered in trying to reach optimal adhesion performance. Particular care must be taken in considering the chemistry, as well as the elasto-plastic behaviour, of the system to be built w1,2x. The adhesion strength of a thin film is a complex property. It is principally determined by the nature of its interface with the substrate, and is therefore influenced by the nucleation process. Controlled surface roughening and the deposition of gradient interlayers can both lead to better adhesion w3x. Most coating processes use several different pretreatments before deposition of the coating material, and thus cleaning can be a multi-stage process for which many variations are possible w4x. *Corresponding author. Tel.: q39-0461-881678; fax: q39-0461881696. E-mail address: [email protected] (L. Guzman).
It is well known that the use of any sputter cleaning can considerably improve adhesion, but the mechanism for the improvement depends on the substrate material and the deposition technology. Ion etching breaks up the surface layers, allowing the creation of dangling bonds, which are linked to the subsequently deposited interlayer, and thus the adhesion is improved w5x. Several possibilities exist for in situ etching of surfaces prior to coating. The plasma plays a complex role in the pretreatment processes that take place in vacuum. Important parameters for plasma processes are the ion current density on the substrate and the kinetic energy of the impinging particles. The plasma activation of surfaces can substitute for hazardous chemicals, ensuring a safe and fast pretreatment, whereas the usual high-rate electroplating may guarantee an economic process. However, after activation, a very thin seed layer is often necessary. The Auto-glass adhesion remains a key problem, due to the difficulty of forming a direct bond between the film and substrate. Thus, gold and other noble metal coatings on glass are usually applied with the interposition of double CryNi layers after various etchingycleaning steps. For
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 3 0 6 - 7
L. Guzman et al. / Surface and Coatings Technology 158 – 159 (2002) 558–562
Al deposited on Cr layers predeposited on glass, it was found that the adhesion increased with time, in some cases by an order of magnitude over a period of 300– 400 h. This variation with time has been explained as being due to alloying at the metal–metal interface w6x. Such an ageing effect is not observed in the case of single Au coatings, since Au is generally not expected to react with oxides, because the heat of formation of Au2O3 is q39 kcalymol w7x. In order to improve the adhesion between coating and glass, several pretreatments, such as sputter cleaning or bonding layers suitable for the formation of strong interfacial phases andyor minimising interfacial stresses, are carried out. For instance, adherent oxides of the metals involved are deliberately deposited so that a graded transition from metal to oxide can occur at the interface. Ion beam processes, including ion implantation and ion beam mixing, are suitable for modifying the near-surface region of materials operating at low temperatures, thus avoiding any thermal degradation of the substrate material, and, being clean vacuum techniques, they eliminate the use of aggressive and dangerous chemicals for the pretreatment. By suitable choice of the ion species, energy and fluence, the desired properties can be well engineered. However, in order to optimise the properties of a technological surface in an economic way, it may be necessary to use a combination of ion implantation with other currently used deposition techniques, such as plating or sputtering, allowing for the production of well adherent, thicker coatings, yet maintaining the advantages of an ion beam pretreatment w8x. 2. Experimental Half-masked glass surfaces were deposited with very thin (10 nm in thickness) Au and C layers in different bi- and multilayer configurations and then implanted with Xeq ions. In the first configuration (CyAuyglass), C was intended as a protective top layer against excessive sputter loss of the seeding Au layer, depending on the ion dose. The second arrangement (CyAuyCyglass) using an intermediate C sublayer was designed to induce the creation of a more strongly bonded interface region. The pure and alloyed-Au films were sputtered from a mosaic-type cathode, whereas the C films were obtained by flash evaporation from a filament. The previous samples were subjected to ion bombardment of 100-keV Xeq ions to a fluence of 1=1016 ionsycm2. The ion current density was maintained at a low value to avoid thermal degradation of the samples (power density was kept below 2 mWymm2). Auger electron spectrometry (AES) was used to investigate the depth concentration profiles of the ion-beam-mixed CyAuyCyglass samples. After ion mixing, pure and alloyed Au coatings were
559
deposited onto the previous samples in a vacuum chamber using magnetron sputter deposition. The adhesion properties of the coatings were studied using a scratch tester equipped with a 1208 Rockwell C diamond. The scratch-tested samples were observed with a scanning electron microscope. Nanohardness and Young’s modulus measurements were made from loading and unloading curves using a Nano Instruments (type II) ultra-low-load depth-sensing nanoindenter, as described in w9x. The indenter was used in constant displacement-rate mode until a total depth of 100 nm was reached. By examining the unloading curves, it was possible to determine the elastic contribution. In addition, the friction behaviour of both untreated and pretreated gold layers against the diamond stylus was monitored during the scratch tests. 3. Results and discussion 3.1. Adhesion Amongst the processing strategies employed to enhance glassymetal adhesion, attention is given to the formation of new chemical bondings between the film and substrate. Ion beam-induced phenomena may favour the formation of chemically bonded complexes, and thus benefit the mechanical performance of the coating. A suitable preparation of the substrate, for instance by deposition of gradient interlayers, can lead to better adhesion. The SEM micrograph in Fig. 1a pertains to the first type of multilayer configuration (SiO2 yAuyC) and shows film removal from deposition-only samples (lower half) as confronted with Xe ion-beam-mixed specimens (upper half), indicating that a force as low as 1 N, which is the minimum applicable force, is sufficient to remove the deposition-only Au coating. At the same load, the coatings with an ion-mixed interface showed no failure in adhesion, while even a force of 40 N was not able to completely remove these films (being incorporated into the substrate and even harder than glass itself). A similar phenomenon is depicted in Fig. 1b for the second type of configuration (SiO2 yCyAuy C), in which a certain degree of adhesion is already guaranteed in the deposition-only sample; however, this is much improved in the pretreated zone. Based on these results, a multi-step process was then proposed for the deposition of thick, well-adherent Au coatings on glass. The most outstanding characteristic of the glassyAu system is the absence of any relevant interdiffusion between the coating and substrate atoms, both before and after the implantation process. Fig. 2 shows the AES depth concentration profiles of C, Au, Si and O elements in the near-surface and ion-beammixed interface regions pertaining to a CyAuyCyglass configuration implanted with 1=1016 Xeq ycm2. It can be observed that the multilayer structure is not strongly altered, except in the interface region. The Xeq bombardment allows the formation of Si–C bonding (pref-
560
L. Guzman et al. / Surface and Coatings Technology 158 – 159 (2002) 558–562
Fig. 1. SEM micrographs of scratched specimens consisting of only Au-coated (top) and ion beam-pretreated and Au-coated (bottom) glass samples, pertaining to two different configurations: type (a) SiO2yAuyC and type (b) SiO2yCyAuyC.
erentially with respect to Si–Au), together with redistribution of the gold layer within a mixed Si–C– Au region. At the same time, this new interface structure constitutes a hard and chemically inert layer (against eventual oxidation), providing good interlocking with subsequently deposited Au layers. In addition, the Auger peak shape of C is indicative of the formation of a SiC phase at the interface w10x. Although ion irradiation seems to induce only small changes in the chemical composition, the SEM images shown in Fig. 1 outline significant adhesion enhancement, which can be explained in terms of the modified interfacial chemistry. Adhesion enhancement between SiO2 and Au may occur by different mechanisms from
those usually invoked for plain ion mixing. The Si–O bond breakage occurs during ion bombardment, while C (if available) substitutes for O in Si bonding. Au sputter loss during the process is prevented by the protective action of the top C film, and Au atoms are redistributed in the interface region. The O substitution by C increases with the availability of C and (given a constant C amount) with increasing Xe dose, at least up to a dose for which all C atoms are bonded to Si. The friction behaviour of the gold coatings is also changed by the pretreatment. Indeed, the friction coefficient (measured during scratch testing with loads between 2 and 8 N) is ms0.04 compared to ms0.08 for the control specimen (not pretreated).
Fig. 2. AES depth concentration profiles for C, Au, Si and O pertaining to the near-surface and interface regions of a type (b) configuration, implanted with 1=1016 Xe ionsycm2.
L. Guzman et al. / Surface and Coatings Technology 158 – 159 (2002) 558–562
561
Fig. 3. Nanohardness and Young’s modulus for pure Au, Au–Cu, Au–Ni and Au–Cu–Ni alloy thin films on glass.
Once the need for a SiC–Au buffer layer was established for optimal adhesion behaviour of gold coatings on glass, other deposition runs of more complex coatings were performed. 3.2. Influence of alloy additions on the hardness and colour of Au films In many fields of advanced technology, thin-film properties are fitted to specific requirements by the codeposition of several elements. In particular, co-sputtering allows an almost unlimited range of film compositions. Since the very beginning, decorative aesthetic purposes have been a focus of interest besides functional purposes, such as hardening. In a series of experiments, several thin, alloyed Au films were deposited by RF magnetron sputtering in an Ar atmosphere, at a pressure of 0.1 Pa and fixed voltage of 300 V, starting from mosaic-type cathodes. The film compositions were fitted according to the following experimental sputtering rates (nmymin): Au, 120; Cu, 76; and Ni, 54, as obtained by separately depositing the single elements using the same machine parameters. The nominal compositions chosen were the following: I. II. III. IV.
Au100; Au95Cu5; Au95Ni5; and Au90Cu5Ni5.
The films, all with 150-nm nominal thickness, were deposited onto half-pretreated glass plates, as previously described. All films showed different colouration, (I) exhibiting the typical gold tonality, (II) tending to red,
(III) tending to a white tonality, and (IV) being again compensated and very close to pure gold. The adhesion of the alloy coatings, as measured by scratch testing, was found to be superior or at least equal to that encountered in the case of pure Au coatings. The structure of the films was investigated by X-ray diffraction. The alloyed films are all solid solutions with the f.c.c. structure; there was no evidence of film texture. Assuming equal Cu and Ni concentrations in specimen (IV), the CuqNi concentration was found to be 8 at.%, with reference to both bulk Au and to the pure Au film. These data imply the value rs18916 kgym3 for the mass density of the film. From the Scherrer equation, grain sizes of 10 nm for the (111) peak and 9 nm for the (222) peak were obtained. Due to the lack of texture and to the grain size, which was one order of magnitude smaller than the film thickness, elastic isotropy of the film was safely assumed. Hardness values, as a function of the alloy addition, were measured using an ultra-low-load nano-indentation technique. Results are shown in Fig. 3. Each bar is the average of six independent measurements. Important increases in the nanohardness of the alloyed samples were established. The hardening effect is evident for single Cu, as well as for single Ni addition. Furthermore, the additivity of both single hardening effects is ensured in the ternary alloy. The relative low variation of the Young’s modulus, shown in the same figure, confirms that the crystalline structure is not strongly influenced. By observing the rigid-sphere model for substitutional hardening of Au, it appears that the percentage differ-
L. Guzman et al. / Surface and Coatings Technology 158 – 159 (2002) 558–562
562
Table 1 Solid solution hardening model for gold Element
Atomic radius r (nm)
Dr (nm)
Dryr %
Hardness enhancement for 5% alloying (experimental)
Au Cu Ni Sm Be
0.146 0.128 0.124 0.181 0.112
0 0.18 0.22 0.35 0.34
0 12 15 24 23
1 2 2.3 – –
ence in the atomic radius of Au with regard to the different alloying elements correlates perfectly with the solid solution hardening obtained; see Table 1. Indeed, Ni has a higher theoretical hardening power than Cu, which is experimentally observed (Fig. 3). According to the same model, two very efficacious elements for alloy-hardening of Au would be beryllium (very small atom size) and samarium (very large atom size).
and Si across the interface and (b) the filmysubstrate interlocking connected to the Au gradient created. Following the pretreatment described, we have deposited strongly adherent gold and gold alloy coatings on glass. The adhesion of these coatings, as measured by the scratch-tester, is appreciably better for the pretreated surfaces. In conclusion, ion beam pretreatment is a very effective means to improve the deposition of decorative and functional coatings on glass. This is certainly due to the superior mechanical properties exhibited by the ion-mixed interface. Acknowledgments The authors are indebted to Ms R. Belli (University of Trento) for SEM observations. This research activity was partially supported by Ministero dell’Universita` e della Ricerca (MIUR) within the framework of COFIN 2001 projects. References
4. Summary and conclusions Interface engineering was performed before Au coating deposition on glass substrates. Xe ion bombardment of CyAuyC multilayers deposited on SiO2 provoked strong adhesion of subsequently deposited Au coatings. The ion beam pretreatment of seeding layers was able to induce mixing of the near-surface region, which allowed perfect adhesion and the integrity of subsequently deposited thicker Au coatings. The adhesion of such coatings, as confirmed by scratch tests, was appreciably better for the pre-implanted specimens. The coatings deposited reveal enhanced mechanical properties. In particular, the better scratch resistance for the Xe ionpretreated samples indicates that the adhesion enhancement is primarily due to the improvement in the fracture toughness behaviour of the interface. The chemical enhancement of the interfacial joint includes two aspects: (a) the bridging bonds between C
w1x J.E. Baglin, in: L.-H. Lee (Ed.), Fundamentals of Adhesion, Plenum, New York, 1991, pp. 363–381. w2x L. Guzman, A. Tuccio, A. Miotello, N. Laidani, L. Calliari, D.C. Kothari, Surf. Coat. Technol. 66 (1994) 458–463. w3x W. Kulisch, R. Freudenstein, A. Klett, M.F. Plass, Thin Solid Films 377y378 (2000) 170–176. w4x H.K. Pulker, Coatings on Glass, Elsevier, Amsterdam, 1984, p. 59. w5x J. Bull, P.R. Chalker, C.F. Ayres, D.S. Rickerby, Mater. Sci. Eng. A 139 (1991) 71. w6x C. Weaver, R.M. Hill, Philos. Mag. 3 (1958) 1402. w7x W.A. Weyl, in: J.E. Rutzler, R.L. Savage (Eds.), Adhesion and Adhesives—Fundamentals and Practice, Wiley, New York, 1953, p. 36. w8x K. Awazu, H. Yoshida, H. Watanabe, M. Iwaki, L. Guzman, Surf. Coat. Technol. 51 (1992) 509. w9x W. Gissler, J. Haupt, P.N. Gibson, et al., in: T.S. Sudarshan (Ed.), Surface Modification Technologies VII, The Institute of Metals, London, 1994, p. 571. w10x N. Laidani, M. Bonelli, A. Miotello, et al., J. Appl. Phys. 74 (1993) 2013–2020.
Ion beam-induced enhanced adhesion of gold films deposited on glass L. Guzman*, A. Miotello, R. Checchetto, M. Adami INFM and Department of Physics, University of Trento, 38050 Povo, Trento, Italy
Abstract The metallisation of glass for decorative andyor functional purposes is now a well-established technique. The most popular methods are electroplating or sputter deposition. In order to obtain suitable adhesion, substrate pretreatment is a substantial part of a vapour-deposited coating that generally cannot be dispensed with. The pretreatment costs can reach the same order of magnitude as those associated with the actual coating process, or can even exceed them. In this work we studied the adhesion of Au thin films on glass. The substrates were pretreated by an ion-beam-mixing step, consisting of the deposition of AuyC bilayers or CyAuyC multilayers followed by Xeq implantation. After such preparation, the specimens were further coated (using a sputtering machine) with 150-nm-thick Au or Au-alloy films. Adhesion properties of the films were examined using a scratch tester in conjunction with scanning electron microscopy. It was observed that, without the ion-beam-mixing pretreatment, the coatings were poorly adherent. Strong adhesion enhancement was observed in the pretreated samples. The key mechanism envisaged to explain this is related to the formation of mixed SiC–Au phases at the interface region. Moreover, the mechanical properties of the pure and alloyed Au films were quantified by nano-indentation, and hardness results are in good agreement with a simple rigid-sphere model of substitutional hardening. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Ion-mixed interfaces; Au coatings; Enhanced adhesion
1. Introduction Good adhesion is of prime importance in dictating the lifetime of a coated component, and a number of techniques have been developed to improve adhesion of the coating to the substrate based on this requirement. There are several factors which must be considered in trying to reach optimal adhesion performance. Particular care must be taken in considering the chemistry, as well as the elasto-plastic behaviour, of the system to be built w1,2x. The adhesion strength of a thin film is a complex property. It is principally determined by the nature of its interface with the substrate, and is therefore influenced by the nucleation process. Controlled surface roughening and the deposition of gradient interlayers can both lead to better adhesion w3x. Most coating processes use several different pretreatments before deposition of the coating material, and thus cleaning can be a multi-stage process for which many variations are possible w4x. *Corresponding author. Tel.: q39-0461-881678; fax: q39-0461881696. E-mail address: [email protected] (L. Guzman).
It is well known that the use of any sputter cleaning can considerably improve adhesion, but the mechanism for the improvement depends on the substrate material and the deposition technology. Ion etching breaks up the surface layers, allowing the creation of dangling bonds, which are linked to the subsequently deposited interlayer, and thus the adhesion is improved w5x. Several possibilities exist for in situ etching of surfaces prior to coating. The plasma plays a complex role in the pretreatment processes that take place in vacuum. Important parameters for plasma processes are the ion current density on the substrate and the kinetic energy of the impinging particles. The plasma activation of surfaces can substitute for hazardous chemicals, ensuring a safe and fast pretreatment, whereas the usual high-rate electroplating may guarantee an economic process. However, after activation, a very thin seed layer is often necessary. The Auto-glass adhesion remains a key problem, due to the difficulty of forming a direct bond between the film and substrate. Thus, gold and other noble metal coatings on glass are usually applied with the interposition of double CryNi layers after various etchingycleaning steps. For
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 3 0 6 - 7
L. Guzman et al. / Surface and Coatings Technology 158 – 159 (2002) 558–562
Al deposited on Cr layers predeposited on glass, it was found that the adhesion increased with time, in some cases by an order of magnitude over a period of 300– 400 h. This variation with time has been explained as being due to alloying at the metal–metal interface w6x. Such an ageing effect is not observed in the case of single Au coatings, since Au is generally not expected to react with oxides, because the heat of formation of Au2O3 is q39 kcalymol w7x. In order to improve the adhesion between coating and glass, several pretreatments, such as sputter cleaning or bonding layers suitable for the formation of strong interfacial phases andyor minimising interfacial stresses, are carried out. For instance, adherent oxides of the metals involved are deliberately deposited so that a graded transition from metal to oxide can occur at the interface. Ion beam processes, including ion implantation and ion beam mixing, are suitable for modifying the near-surface region of materials operating at low temperatures, thus avoiding any thermal degradation of the substrate material, and, being clean vacuum techniques, they eliminate the use of aggressive and dangerous chemicals for the pretreatment. By suitable choice of the ion species, energy and fluence, the desired properties can be well engineered. However, in order to optimise the properties of a technological surface in an economic way, it may be necessary to use a combination of ion implantation with other currently used deposition techniques, such as plating or sputtering, allowing for the production of well adherent, thicker coatings, yet maintaining the advantages of an ion beam pretreatment w8x. 2. Experimental Half-masked glass surfaces were deposited with very thin (10 nm in thickness) Au and C layers in different bi- and multilayer configurations and then implanted with Xeq ions. In the first configuration (CyAuyglass), C was intended as a protective top layer against excessive sputter loss of the seeding Au layer, depending on the ion dose. The second arrangement (CyAuyCyglass) using an intermediate C sublayer was designed to induce the creation of a more strongly bonded interface region. The pure and alloyed-Au films were sputtered from a mosaic-type cathode, whereas the C films were obtained by flash evaporation from a filament. The previous samples were subjected to ion bombardment of 100-keV Xeq ions to a fluence of 1=1016 ionsycm2. The ion current density was maintained at a low value to avoid thermal degradation of the samples (power density was kept below 2 mWymm2). Auger electron spectrometry (AES) was used to investigate the depth concentration profiles of the ion-beam-mixed CyAuyCyglass samples. After ion mixing, pure and alloyed Au coatings were
559
deposited onto the previous samples in a vacuum chamber using magnetron sputter deposition. The adhesion properties of the coatings were studied using a scratch tester equipped with a 1208 Rockwell C diamond. The scratch-tested samples were observed with a scanning electron microscope. Nanohardness and Young’s modulus measurements were made from loading and unloading curves using a Nano Instruments (type II) ultra-low-load depth-sensing nanoindenter, as described in w9x. The indenter was used in constant displacement-rate mode until a total depth of 100 nm was reached. By examining the unloading curves, it was possible to determine the elastic contribution. In addition, the friction behaviour of both untreated and pretreated gold layers against the diamond stylus was monitored during the scratch tests. 3. Results and discussion 3.1. Adhesion Amongst the processing strategies employed to enhance glassymetal adhesion, attention is given to the formation of new chemical bondings between the film and substrate. Ion beam-induced phenomena may favour the formation of chemically bonded complexes, and thus benefit the mechanical performance of the coating. A suitable preparation of the substrate, for instance by deposition of gradient interlayers, can lead to better adhesion. The SEM micrograph in Fig. 1a pertains to the first type of multilayer configuration (SiO2 yAuyC) and shows film removal from deposition-only samples (lower half) as confronted with Xe ion-beam-mixed specimens (upper half), indicating that a force as low as 1 N, which is the minimum applicable force, is sufficient to remove the deposition-only Au coating. At the same load, the coatings with an ion-mixed interface showed no failure in adhesion, while even a force of 40 N was not able to completely remove these films (being incorporated into the substrate and even harder than glass itself). A similar phenomenon is depicted in Fig. 1b for the second type of configuration (SiO2 yCyAuy C), in which a certain degree of adhesion is already guaranteed in the deposition-only sample; however, this is much improved in the pretreated zone. Based on these results, a multi-step process was then proposed for the deposition of thick, well-adherent Au coatings on glass. The most outstanding characteristic of the glassyAu system is the absence of any relevant interdiffusion between the coating and substrate atoms, both before and after the implantation process. Fig. 2 shows the AES depth concentration profiles of C, Au, Si and O elements in the near-surface and ion-beammixed interface regions pertaining to a CyAuyCyglass configuration implanted with 1=1016 Xeq ycm2. It can be observed that the multilayer structure is not strongly altered, except in the interface region. The Xeq bombardment allows the formation of Si–C bonding (pref-
560
L. Guzman et al. / Surface and Coatings Technology 158 – 159 (2002) 558–562
Fig. 1. SEM micrographs of scratched specimens consisting of only Au-coated (top) and ion beam-pretreated and Au-coated (bottom) glass samples, pertaining to two different configurations: type (a) SiO2yAuyC and type (b) SiO2yCyAuyC.
erentially with respect to Si–Au), together with redistribution of the gold layer within a mixed Si–C– Au region. At the same time, this new interface structure constitutes a hard and chemically inert layer (against eventual oxidation), providing good interlocking with subsequently deposited Au layers. In addition, the Auger peak shape of C is indicative of the formation of a SiC phase at the interface w10x. Although ion irradiation seems to induce only small changes in the chemical composition, the SEM images shown in Fig. 1 outline significant adhesion enhancement, which can be explained in terms of the modified interfacial chemistry. Adhesion enhancement between SiO2 and Au may occur by different mechanisms from
those usually invoked for plain ion mixing. The Si–O bond breakage occurs during ion bombardment, while C (if available) substitutes for O in Si bonding. Au sputter loss during the process is prevented by the protective action of the top C film, and Au atoms are redistributed in the interface region. The O substitution by C increases with the availability of C and (given a constant C amount) with increasing Xe dose, at least up to a dose for which all C atoms are bonded to Si. The friction behaviour of the gold coatings is also changed by the pretreatment. Indeed, the friction coefficient (measured during scratch testing with loads between 2 and 8 N) is ms0.04 compared to ms0.08 for the control specimen (not pretreated).
Fig. 2. AES depth concentration profiles for C, Au, Si and O pertaining to the near-surface and interface regions of a type (b) configuration, implanted with 1=1016 Xe ionsycm2.
L. Guzman et al. / Surface and Coatings Technology 158 – 159 (2002) 558–562
561
Fig. 3. Nanohardness and Young’s modulus for pure Au, Au–Cu, Au–Ni and Au–Cu–Ni alloy thin films on glass.
Once the need for a SiC–Au buffer layer was established for optimal adhesion behaviour of gold coatings on glass, other deposition runs of more complex coatings were performed. 3.2. Influence of alloy additions on the hardness and colour of Au films In many fields of advanced technology, thin-film properties are fitted to specific requirements by the codeposition of several elements. In particular, co-sputtering allows an almost unlimited range of film compositions. Since the very beginning, decorative aesthetic purposes have been a focus of interest besides functional purposes, such as hardening. In a series of experiments, several thin, alloyed Au films were deposited by RF magnetron sputtering in an Ar atmosphere, at a pressure of 0.1 Pa and fixed voltage of 300 V, starting from mosaic-type cathodes. The film compositions were fitted according to the following experimental sputtering rates (nmymin): Au, 120; Cu, 76; and Ni, 54, as obtained by separately depositing the single elements using the same machine parameters. The nominal compositions chosen were the following: I. II. III. IV.
Au100; Au95Cu5; Au95Ni5; and Au90Cu5Ni5.
The films, all with 150-nm nominal thickness, were deposited onto half-pretreated glass plates, as previously described. All films showed different colouration, (I) exhibiting the typical gold tonality, (II) tending to red,
(III) tending to a white tonality, and (IV) being again compensated and very close to pure gold. The adhesion of the alloy coatings, as measured by scratch testing, was found to be superior or at least equal to that encountered in the case of pure Au coatings. The structure of the films was investigated by X-ray diffraction. The alloyed films are all solid solutions with the f.c.c. structure; there was no evidence of film texture. Assuming equal Cu and Ni concentrations in specimen (IV), the CuqNi concentration was found to be 8 at.%, with reference to both bulk Au and to the pure Au film. These data imply the value rs18916 kgym3 for the mass density of the film. From the Scherrer equation, grain sizes of 10 nm for the (111) peak and 9 nm for the (222) peak were obtained. Due to the lack of texture and to the grain size, which was one order of magnitude smaller than the film thickness, elastic isotropy of the film was safely assumed. Hardness values, as a function of the alloy addition, were measured using an ultra-low-load nano-indentation technique. Results are shown in Fig. 3. Each bar is the average of six independent measurements. Important increases in the nanohardness of the alloyed samples were established. The hardening effect is evident for single Cu, as well as for single Ni addition. Furthermore, the additivity of both single hardening effects is ensured in the ternary alloy. The relative low variation of the Young’s modulus, shown in the same figure, confirms that the crystalline structure is not strongly influenced. By observing the rigid-sphere model for substitutional hardening of Au, it appears that the percentage differ-
L. Guzman et al. / Surface and Coatings Technology 158 – 159 (2002) 558–562
562
Table 1 Solid solution hardening model for gold Element
Atomic radius r (nm)
Dr (nm)
Dryr %
Hardness enhancement for 5% alloying (experimental)
Au Cu Ni Sm Be
0.146 0.128 0.124 0.181 0.112
0 0.18 0.22 0.35 0.34
0 12 15 24 23
1 2 2.3 – –
ence in the atomic radius of Au with regard to the different alloying elements correlates perfectly with the solid solution hardening obtained; see Table 1. Indeed, Ni has a higher theoretical hardening power than Cu, which is experimentally observed (Fig. 3). According to the same model, two very efficacious elements for alloy-hardening of Au would be beryllium (very small atom size) and samarium (very large atom size).
and Si across the interface and (b) the filmysubstrate interlocking connected to the Au gradient created. Following the pretreatment described, we have deposited strongly adherent gold and gold alloy coatings on glass. The adhesion of these coatings, as measured by the scratch-tester, is appreciably better for the pretreated surfaces. In conclusion, ion beam pretreatment is a very effective means to improve the deposition of decorative and functional coatings on glass. This is certainly due to the superior mechanical properties exhibited by the ion-mixed interface. Acknowledgments The authors are indebted to Ms R. Belli (University of Trento) for SEM observations. This research activity was partially supported by Ministero dell’Universita` e della Ricerca (MIUR) within the framework of COFIN 2001 projects. References
4. Summary and conclusions Interface engineering was performed before Au coating deposition on glass substrates. Xe ion bombardment of CyAuyC multilayers deposited on SiO2 provoked strong adhesion of subsequently deposited Au coatings. The ion beam pretreatment of seeding layers was able to induce mixing of the near-surface region, which allowed perfect adhesion and the integrity of subsequently deposited thicker Au coatings. The adhesion of such coatings, as confirmed by scratch tests, was appreciably better for the pre-implanted specimens. The coatings deposited reveal enhanced mechanical properties. In particular, the better scratch resistance for the Xe ionpretreated samples indicates that the adhesion enhancement is primarily due to the improvement in the fracture toughness behaviour of the interface. The chemical enhancement of the interfacial joint includes two aspects: (a) the bridging bonds between C
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