- Email: [email protected]
Diffusion bonding from antiquity to present times
Nuclear Instruments and Methods in Physics Research B 226 (2004) 222–230 www.elsevier.com/locate/nimb
Diffusion bonding from antiquity to present times S. Mathot b
a,*
, G. Demortier
b
a CERN, Department TS/MME, CH-1211 Gene´ve 23, Switzerland FUNDP, Laboratoire dÕAnalyses par Re´actions Nucle´aires (LARN), 61 Rue de Bruxelles, B-5000 Namur, Belgium
Received 2 October 2003; received in revised form 24 May 2004
Abstract This paper describes attempts made to improve diffusion bonding procedures based on antique processes as granulation and filigree. Two modern procedures implying diffusion of cadmium and silicon respectively are reported. The results obtained with PIXE and NRA using nuclear microprobe are discussed. This paper concludes with a comparison of these bonding techniques and some other modern methods with the antique procedures. 2004 Elsevier Ltd. All rights reserved.
1. Introduction Jewellery craft has a story of seven thousand years. Gold or silver brazing is a technique almost so old, which was born with the discovery of metallurgy, about 4500–4000 BC in Mesopotamia. Brazing refers here to the use of a low melting point alloy to joint parts of a jewel. Since the discovery of the metal melting and alloying, this method has been permanently developed. In the antiquity times, another technique, referring to granulation and filigree, has been also used.
* Corresponding author. Tel.: +41 22 767 3368; fax: +41 22 767 9150. E-mail addresses: [email protected] (S. Mathot), guy. [email protected] (G. Demortier).
The earliest recorded uses of this technique come from the city of Ur (2500 BC) but the highest achievements have been obtained in well-known Etruscan jewellery (600–300 BC). There the approach is completely different. The joining process involves a local surface melting of the metals without any use of previously elaborated low melting point alloy. This was the unique method which has allowed the Etruscan metalworkers to bond in a single operation several hundreds of tiny gold granules or fine wires on a thin shaped gold foil. In recent times, this technique seems to have been lost and excepted some imitations, modern goldsmiths have no more possibility to achieve fine granules bonding as for the flamboyant Etruscan jewellery. The reasons of this lack of skill are not clear. While the description of brazing alloys is common, we have to note that only two ancient
0168-583X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.nimb.2004.06.039
S. Mathot, G. Demortier / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 222–230
‘‘recipes’’ describing this technique have been found, one in the Pliny encyclopedia (77 AD) and the other by Theophilus (1122 AD). The Italian goldsmith Cellini has also described an approach of that technique in 1568 AD but the more recent or modern jeweller manuals give no detail about granulation and filigree. Nevertheless, many scientific investigations of this technique have been performed and several descriptions of the antique granulation procedure are available. Thouvenin has given the most complete one [1]. The metallurgical aspect of the method is based on the use of copper inducing by diffusion the local formation of a low melting point gold–copper alloy. But the main difficulty of the technique lies less on the copper source (mainly in natural form of copper ores), than on the preparation of the organic binder. This binder has to act like a flux, as for brazing, and also as a ‘‘glue’’ allowing before the melting an appropriate positioning of the granules in contact with the base metal. This binder is really the ‘‘secret’’ of the antique recipe. Using pure copper powder and a common flux like natron, it is possible to produce a low melting point alloy at the surface of a gold-rich jewel. However, the boiling of the flux will completely compromise the achievement of a jewellery item composed of hundreds tiny gold granules with a perfect alignment [1]. These ‘‘chemical’’ aspects of the antique method of granulation are very important. In another way, metallurgical investigations of this technique based on the replacement of copper by other elements may be a source of new techniques leading to modern applications. We will describe two attempts made at LARN in this respect. These new methods are based on the diffusion of cadmium and silicon respectively in pure gold based alloys. The experimental approach has been influenced more by the antique techniques than by the present use of Au–Cd and Au–Si alloys. Modern experimental techniques as silicon evaporation under vacuum have been however used. The benefits of ion beam analysis methods, for these metallurgical investigations are also described. Finally, this paper concludes with a discussion of some modern diffusion bonding techniques
223
and their likenesses with the fascinating antique granulation.
2. Gold bonding by diffusion of cadmium The background of this study is a request of Prof. T. Hackens, archeologist at the University of Louvain (Belgium), for an expertise by ion beam analysis of ancient gold jewellery artifacts. The problem was the presence of cadmium in solders of some jewellery considered at that time as an indication of modern origin or at least as a modern repair. One of the authors, Demortier, has proposed the use of non destructive PIXE analysis in a non vacuum geometry. This technique is effectively much more sensitive than conventional scanning electron microprobe for the analysis of Cd in ancient gold jewellery when silver is also present, as generally observed for antique materials [2]. By systematic analysis of modern and supposed antique solders, Demortier has shown that the relative proportion of Ag, Cu and Cd may be very different in both cases [3]. These observations indicate that cadmium could have been used in antique times. Moreover, the suggestion that cadmium may be a component of the antique recipe of granulation has been developed. Indeed, in PlinyÕs recipe, we find as the main component the chrysocolla (literally ‘‘Gold glue’’). This component is generally recognized since the beginning of the 20th century as malachite, a copper-rich mineral. Demortier has proposed in 1987 that chrysocolla may be in fact the greenockite, a CdS mineral [3] and not the malachite. The main arguments were first the color of the greenockite. This mineral is effectively yellow– orange as for the chrysocolla described by Pliny when the malachite is green. The second argument is the presence in the antique recipe of the ‘‘verdegris’’ a copper-rich component in addition of the chrysocolla. Then we can better understand that chrysocolla may be a copper free mineral as greenockite than malachite. Independently of this archeological study, some experiments have been performed at LARN with CdS. We have observed that from a CdS pellet
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S. Mathot, G. Demortier / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 222–230
Fig. 1. ‘‘First’’ pure gold granules, 1 mm in diameter, bonded by cadmium diffusion.
deposited on a gold foil, heating induces atomic cadmium which rapidly diffuses into the substrate [4]. This diffusion is efficient during the first minutes of the heat treatment performed at temperature ranging from 600 to 875 C. After this short treatment the formation of CdO prevents further cadmium diffusion. The composition of the Au– Cd alloy formed is dependent on the heat treatment temperature. The local cadmium content is low around 600 C but increases rapidly up to several percents above 750 C. At higher temperature, the cadmium concentration raises up to the Au– Cd liquidus and the gold substrate locally melts. These experiments have been performed in air, without any additional flux. Further on re-heating gold substrates containing a cadmium-enriched region formed by this way, we have easily bond pure gold granules by simply maintaining it in contact with the foil (Fig. 1). This bonding of granules by diffusion is also performed in air and without use of any flux.
3. Silicon diffusion in gold substrate A next step of improvement after cadmium diffusion attempts to further reduce again the bonding temperature and to avoid the use of potentially toxic cadmium has been checked.
Silicon has been chosen as a potential candidate using the property that the gold–silicon system presents a eutectic transformation at a very low temperature (363 C). The eutectic gold–silicon alloy (3 wt% or 19 at% of Si) is effectively a widely used solder, mainly for silicon devices. Our goal was to deposit a silicon film on a pure gold substrate to form locally this low temperature solder. Silicon films have been deposited under vacuum by evaporation. With gold substrates maintained at room temperature, we have observed very poor adherence of the films. However, when the substrates are maintained at a temperature above the Au–Si eutectic point, the rapid formation of a gold silicon alloy is observed along well-defined paths corresponding to the gold grain boundaries. On thin gold samples (10–20 lm) the presence of this alloy is even observed on the backside of the foils, also along the grain boundaries. These observations indicate a rapid reaction of the evaporated silicon films with the gold substrate. Fig. 2 shows typical optical metallographies of a polycrystalline gold substrate after silicon diffusion at 400 C. An alloy of eutectic shape is formed along the gold grain boundaries where the eutectic reaction seems to have been initiated. PIXE technique using the LARN ion microprobe has been used to characterize this alloy. A 2.5 MeV focused proton beam (spot of 3 to 5 lm
S. Mathot, G. Demortier / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 222–230
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Fig. 2. Typical metallographies of polycrystalline gold foil substrates showing the formation by diffusion at 400 C of a gold silicon alloy around the deposited Si film (A) or at the back side of the foils (B). Typical foil thicknesses: 12–16 lm.
in diameter) scan the polycrystalline gold sample on a narrow region situated around the deposited silicon films or at the backside of the samples. This scanning is performed by mechanical displacements of the samples (steps of 2.5 lm). K X-rays of silicon and M X-rays of gold are used as analytical signals and the absolute silicon concentration is calculated using a bulk Au–Si eutectic sample as reference material [5]. Fig. 3 shows a silicon map obtained by scanning the backside (the one opposite to the one on which silicon is evaporated) of a 14 lm thick gold foil after silicon diffusion (1 h – 400 C). This microPIXE analysis confirms that silicon is present mainly along the gold grain boundaries. The max-
imum silicon concentration is quite constant along the grain boundaries and close to the eutectic composition (19 at% Si). Towards the center of the gold grains, the silicon concentration rapidly decreases. Quantitative measurements of the silicon concentrations are however delicate to be performed by PIXE due to the important matrix effects related to the high absorption of the Si Ka X-rays lines in gold (Fig. 4). The strong overlap of the Si Ka and Au M lines cannot give accurate measurements at low silicon concentration. Further, due to the high absorption effects, the analyzed depth is low and analysis cannot be performed at depths greater than about 1–2 lm.
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Fig. 3. Silicon concentration map on area of 200 · 200 lm2 in step of 5 lm measured of the back side of a 14 lm thick gold foil after silicon diffusion at 400 C during 1 h. 64 C of grey give the silicon concentration in a range of 2–25 at%.
For the measurements of silicon concentration in depth, proton spectroscopy from (d, p) nuclear reactions has been used. The 28Si(d, p)29Si is highly exoenergetic (Q = +6.249 MeV). However, the cross sections are low as generally observed in
nuclear reaction analysis. Working with a microbeam, the experimental arrangement must be adapted to take advantage of the maximum of the cross section. For the 28Si(d, p0)29Si reaction (29Si nuclei produced in their fundamental energy level), the cross section is maximum for protons emitted at 0 relative to the incident beam direction. The cross section for Ed = 2.75 MeV varies from 10 mb/st to 1 mb/st when the emitted angle varies from 0 to 35. An experimental arrangement with a proton detector and a gold absorber inserted into the incident beam has been developed [6]. During the analysis of a thin gold sample (12–16 lm) prepared by silicon diffusion, the gold absorber is used to completely stop the deuteron particles passing through the sample under analysis (Fig. 5). On the other hand, the high-energy protons produced by the exoenergetic (d, p) nuclear reactions on silicon nuclei diffused into the sample can pass through the sample and through the absorber and be detected in the surface barrier detector. The angular straggling of the incident deuterons is small and the analyzed surface may be considered equal to the beam size (3–5 lm). Another advantage of this method is the fact that the gold substrates give no contribution to the proton spectra: the Coulomb barrier of gold nuclei prevents any (d, p) reaction on this high Z nucleus [7]. Fig. 6 shows a typical proton spectrum obtained during the analysis of a 16 lm thick gold foil prepared by silicon diffusion. The microbeam
Fig. 4. (A) PIXE spectra of an Au–Si eutectic alloy. The energy resolution of the Si(Li) detector used (150 eV) cannot allow a complete separation of the Au M and Si K X-ray lines. (B) Absorption curves for the X-ray lines: Si K in Au (1), Au M in Au (2), Au M in Si (3), Au Lb in Au (4), Si K in Si (5) and Au Lb in Si (6). The important matrix effects are illustrated by the large difference between the curves 1 and 5.
S. Mathot, G. Demortier / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 222–230
Fig. 5. Schematic of the experimental arrangement for (d, p) nuclear reaction analysis at H = 0.
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The energy straggling effect leads to an uncertainty in the depth determination of the silicon profile of about 1.5 lm when protons are produced at the surface of the sample and up to 2.5 lm for deeper analyzed layers. The p0 proton group is the most useful signal for quantitative depth profiling. The cross sections have been measured at 0 by using a pure silicon sample (75 lm thick). 3-D analyses have been performed by scanning the deuteron beam on a narrow region at the surface of the samples and by counting for each step the protons emitted in different regions of the p0 proton group. Fig. 7 gives an enlarged view of the p0 protons group corresponding to the previous spectrum. The protons analyzed in the region 1 of this group correspond to the protons produced by nuclear reactions with silicon nuclei present at the surface of the gold sample. The protons analyzed in the lower energy regions correspond to protons produced in depth of the sample, by nuclear reaction with silicon nuclei induced by deuteron particles having loss one part of their incident energy. The depths of the analyzed layers corresponding to the selected regions in the p0 proton group can be easily calculated. Also, the silicon concentrations corresponding to each analyzed layer are calculated using our measured cross sections. A scanning in the three directions (x, y and depth) is performed when the deuteron microbeam scan step by step a narrow region at the surface of a gold sample and by counting for each step the protons in the different regions of the p0 proton group. Fig. 8 gives a typical 3-D map of a 16 lm
Fig. 6. Typical proton spectrum obtained on a gold foil after silicon diffusion (Edeuteron = 2.75 MeV).
is focused on a gold grain boundary. All of the proton peaks corresponding to different excited states of the residual 29Si nuclei (p0, p1, . . .) are broadened due to the presence of silicon in the depth of the sample. In addition, this broadening is due to the energy straggling of both the incident deuterons in the sample and to a lower attempt to the slowing down of the emitted protons in the sample and in the absorber.
Fig. 7. Selected regions of the p0 proton group used for depth profile of the silicon concentration.
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Fig. 8. 3-D analysis of silicon in a gold foil after diffusion during 1 h at 400 C. Each map gives the silicon concentration at different depths in the sample.
thick gold foil after silicon diffusion during 1 h at 400 C. The set of maps shows clearly the diffusion of silicon in depth along well-defined paths corresponding to the gold grain boundaries. Knowing that gold and silicon form a eutectic alloy of fixed composition, the decrease of the silicon concentration in depth can be also regarded as a variation of the width of the regions where the eutectic alloy is formed along the gold grain boundaries. These regions are rapidly narrower than the deuteron beam size leading to the measurement of lower concentrations. By imposing that the silicon concentration would be constant and equal to the eutectic composition (as only this actual alloy of eutectic composition could rapidly migrate as a liquid at a temperature close to 363), the measured silicon concentration depth profiles can be converted in width profiles of the silicon-rich regions. For the analysis of a 16 lm thick sample, the calculated width profile decrease rapidly with the depth (Vshape profile, Fig. 9(A)). On a thinner sample, 12 lm thick, where the overall thickness of the foil
Fig. 9. Calculated profiles of the width of the region of constant (eutectic) concentration formed along the gold grain boundaries. (A) V-shape profile obtained for deuteron microbeam focused on the front side of a 16 lm gold sample, analyzed area close to the deposited silicon film. (B) U-shape profile for deuteron microbeam focused on the rear side of a 12 lm gold foil. For the both analysis, the samples were prepared by diffusion of silicon during 1 h at 400 C and, for the calculations, the width of the first layer has been arbitrary fixed to 5 lm.
S. Mathot, G. Demortier / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 222–230
can be analyzed, we deduce a minimum of the width in the center of the foil (U-shape profile, Fig. 9(B)). These calculations are in good agreement with the dissolution of the gold grain boundaries by the liquid eutectic alloy leading to the accumulation of this alloy on the surfaces.
4. Gold bonding by silicon diffusion Our analyses have confirmed the mechanism of the formation of a low melting point eutectic Au– Si alloy during the deposition at temperature slightly above the eutectic point of a silicon film on a gold substrate. This alloy is formed along the grain boundaries and rapidly diffuses in depth. With treated gold foils to give small grain sizes and with annealing process, we have manufactured by silicon diffusion foils having a useful layer of low melting point alloy at their surface. Pure gold
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pieces, like granulations, maintained in contact with these substrates have been soldered by reheating the assemblies only for a few minutes at 400 C. These soldering have been performed in the air and also without flux (Fig. 10). This bonding process is characterized by the fact that the solder is formed by diffusion in one of the two pieces to be bonded. The soldering is then achieved by re-diffusion of this solder alloy.
5. Modern diffusion bonding Comparing this silicon diffusion joining process with the antique granulation technique, we note that in both cases the joint is achieved by local formation of a liquid phase. For granulations, this liquid phase is formed by diffusion into the both pieces to be brazed. With silicon diffusion, the liquid phase is formed first in one of the pieces.
Fig. 10. (A) Schematic of the silicon diffusion bonding procedure: 1: Silicon deposition and diffusion. 2: Etching of the silicon excess. 3: Diffusion bonding of pure gold pieces on the prepared gold substrate. (B) SEM micrography of a joint failure between two foils. The adherence is observed mainly along the grain boundaries of the gold substrate.
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Preparation, temperature, atmosphere (flux) are also well different. Moreover the antique technique has been a significant source of inspiration for us. Granulations cannot be compared to modern diffusion bonding processes which are, by definition, associated to solid state methods performed with very good surface finishing and limited pressure (3–10 MPa) or with higher pressure (100– 200 MPa) like in hot isostatic pressing. But granulation is very close to a modern joining method called TLP (Transient Liquid Phase) bonding or Diffusion Brazing. For this process, a dissimilar layer is inserted between the pieces. Upon heating, solid state diffusion leads to the formation of a new phase (generally eutectic) liquid at the bonding temperature. This technique is used mainly for aluminum alloys as well as for titanium, cobalt or nickel alloys used in aerospace industry. TLP bonding procedure is also associated to an extensive post heat treatment leading to efficient solid state diffusion of the filler elements. This treatment induces generally a complete vanishing of the original joint. When properly performed, this technique is very advantageous for assembled parts having to serve at elevated temperature as often requested in aerospace industry. Granulation was not performed with so stringent conditions. However the principle of copper diffusion has authorized ancient goldsmiths to post-braze reheating and multi-step procedures helping certainly for the assembly of complex jewellery. Also in present times, many pseudo-Diffusion Brazing processes are performed without complete post diffusion. For example, copper can be brazed by using a pure silver layer. Above 779 C, silver and copper interdiffusion leads to the formation of a liquid phase forming after cooling an efficient brazed joint.
6. Summary Starting with archeological interrogations, two diffusion bonding procedures of pure gold samples
have been investigated. PIXE and NRA measurements performed by using a microbeam have been used to characterize the gold alloys formed. The first procedure results in the solid state diffusion of cadmium in gold from CdS pellets. The Au–Cd alloy is formed homogenously at the surface of the gold samples. The second procedure involves the diffusion of silicon from a pure silicon film deposited on a gold foil. The diffusion mechanism corresponds in a eutectic reaction between gold and silicon along the gold grain boundaries. Likeness between the antique granulation technique and these procedures as well as some modern diffusion bonding processes have been approached. For practical reasons, these modern techniques are not used in jewellery manufacture. Developments in this field could be however interesting having regard to the likenesses mentioned and the perfect results obtained with the granulation technique.
Acknowledgments This paper gives a few examples of interesting researches managed at LARN by Demortier. One of the authors would like to express in this paper heartfelt thanks for his supervision and teaching. Acknowledgments are also addressed to the LARN staff and colleagues for their permanent help during these works.
References [1] [2] [3] [4]
A. Thouvenin, Rev. Arch. de lÕEst et Centre-Est 24 (1978). G. Demortier, Gold Bull. 17 (1) (1984) 27. G. Demortier, Archaeometry 29 (2) (1987) 29. D. Decroupet, S. Mathot, G. Demortier, J. Mater. Sci. Lett. 8 (1989) 849. [5] S. Mathot, G. Demortier, Nucl. Instr. and Meth. B 49 (1990) 505. [6] S. Mathot, G. Demortier, Nucl. Instr. and Meth. B 50 (1990) 52. [7] G. Demortier, S. Mathot, Nucl. Instr. and Meth. B 77 (1993) 312.
Diffusion bonding from antiquity to present times S. Mathot b
a,*
, G. Demortier
b
a CERN, Department TS/MME, CH-1211 Gene´ve 23, Switzerland FUNDP, Laboratoire dÕAnalyses par Re´actions Nucle´aires (LARN), 61 Rue de Bruxelles, B-5000 Namur, Belgium
Received 2 October 2003; received in revised form 24 May 2004
Abstract This paper describes attempts made to improve diffusion bonding procedures based on antique processes as granulation and filigree. Two modern procedures implying diffusion of cadmium and silicon respectively are reported. The results obtained with PIXE and NRA using nuclear microprobe are discussed. This paper concludes with a comparison of these bonding techniques and some other modern methods with the antique procedures. 2004 Elsevier Ltd. All rights reserved.
1. Introduction Jewellery craft has a story of seven thousand years. Gold or silver brazing is a technique almost so old, which was born with the discovery of metallurgy, about 4500–4000 BC in Mesopotamia. Brazing refers here to the use of a low melting point alloy to joint parts of a jewel. Since the discovery of the metal melting and alloying, this method has been permanently developed. In the antiquity times, another technique, referring to granulation and filigree, has been also used.
* Corresponding author. Tel.: +41 22 767 3368; fax: +41 22 767 9150. E-mail addresses: [email protected] (S. Mathot), guy. [email protected] (G. Demortier).
The earliest recorded uses of this technique come from the city of Ur (2500 BC) but the highest achievements have been obtained in well-known Etruscan jewellery (600–300 BC). There the approach is completely different. The joining process involves a local surface melting of the metals without any use of previously elaborated low melting point alloy. This was the unique method which has allowed the Etruscan metalworkers to bond in a single operation several hundreds of tiny gold granules or fine wires on a thin shaped gold foil. In recent times, this technique seems to have been lost and excepted some imitations, modern goldsmiths have no more possibility to achieve fine granules bonding as for the flamboyant Etruscan jewellery. The reasons of this lack of skill are not clear. While the description of brazing alloys is common, we have to note that only two ancient
0168-583X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.nimb.2004.06.039
S. Mathot, G. Demortier / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 222–230
‘‘recipes’’ describing this technique have been found, one in the Pliny encyclopedia (77 AD) and the other by Theophilus (1122 AD). The Italian goldsmith Cellini has also described an approach of that technique in 1568 AD but the more recent or modern jeweller manuals give no detail about granulation and filigree. Nevertheless, many scientific investigations of this technique have been performed and several descriptions of the antique granulation procedure are available. Thouvenin has given the most complete one [1]. The metallurgical aspect of the method is based on the use of copper inducing by diffusion the local formation of a low melting point gold–copper alloy. But the main difficulty of the technique lies less on the copper source (mainly in natural form of copper ores), than on the preparation of the organic binder. This binder has to act like a flux, as for brazing, and also as a ‘‘glue’’ allowing before the melting an appropriate positioning of the granules in contact with the base metal. This binder is really the ‘‘secret’’ of the antique recipe. Using pure copper powder and a common flux like natron, it is possible to produce a low melting point alloy at the surface of a gold-rich jewel. However, the boiling of the flux will completely compromise the achievement of a jewellery item composed of hundreds tiny gold granules with a perfect alignment [1]. These ‘‘chemical’’ aspects of the antique method of granulation are very important. In another way, metallurgical investigations of this technique based on the replacement of copper by other elements may be a source of new techniques leading to modern applications. We will describe two attempts made at LARN in this respect. These new methods are based on the diffusion of cadmium and silicon respectively in pure gold based alloys. The experimental approach has been influenced more by the antique techniques than by the present use of Au–Cd and Au–Si alloys. Modern experimental techniques as silicon evaporation under vacuum have been however used. The benefits of ion beam analysis methods, for these metallurgical investigations are also described. Finally, this paper concludes with a discussion of some modern diffusion bonding techniques
223
and their likenesses with the fascinating antique granulation.
2. Gold bonding by diffusion of cadmium The background of this study is a request of Prof. T. Hackens, archeologist at the University of Louvain (Belgium), for an expertise by ion beam analysis of ancient gold jewellery artifacts. The problem was the presence of cadmium in solders of some jewellery considered at that time as an indication of modern origin or at least as a modern repair. One of the authors, Demortier, has proposed the use of non destructive PIXE analysis in a non vacuum geometry. This technique is effectively much more sensitive than conventional scanning electron microprobe for the analysis of Cd in ancient gold jewellery when silver is also present, as generally observed for antique materials [2]. By systematic analysis of modern and supposed antique solders, Demortier has shown that the relative proportion of Ag, Cu and Cd may be very different in both cases [3]. These observations indicate that cadmium could have been used in antique times. Moreover, the suggestion that cadmium may be a component of the antique recipe of granulation has been developed. Indeed, in PlinyÕs recipe, we find as the main component the chrysocolla (literally ‘‘Gold glue’’). This component is generally recognized since the beginning of the 20th century as malachite, a copper-rich mineral. Demortier has proposed in 1987 that chrysocolla may be in fact the greenockite, a CdS mineral [3] and not the malachite. The main arguments were first the color of the greenockite. This mineral is effectively yellow– orange as for the chrysocolla described by Pliny when the malachite is green. The second argument is the presence in the antique recipe of the ‘‘verdegris’’ a copper-rich component in addition of the chrysocolla. Then we can better understand that chrysocolla may be a copper free mineral as greenockite than malachite. Independently of this archeological study, some experiments have been performed at LARN with CdS. We have observed that from a CdS pellet
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Fig. 1. ‘‘First’’ pure gold granules, 1 mm in diameter, bonded by cadmium diffusion.
deposited on a gold foil, heating induces atomic cadmium which rapidly diffuses into the substrate [4]. This diffusion is efficient during the first minutes of the heat treatment performed at temperature ranging from 600 to 875 C. After this short treatment the formation of CdO prevents further cadmium diffusion. The composition of the Au– Cd alloy formed is dependent on the heat treatment temperature. The local cadmium content is low around 600 C but increases rapidly up to several percents above 750 C. At higher temperature, the cadmium concentration raises up to the Au– Cd liquidus and the gold substrate locally melts. These experiments have been performed in air, without any additional flux. Further on re-heating gold substrates containing a cadmium-enriched region formed by this way, we have easily bond pure gold granules by simply maintaining it in contact with the foil (Fig. 1). This bonding of granules by diffusion is also performed in air and without use of any flux.
3. Silicon diffusion in gold substrate A next step of improvement after cadmium diffusion attempts to further reduce again the bonding temperature and to avoid the use of potentially toxic cadmium has been checked.
Silicon has been chosen as a potential candidate using the property that the gold–silicon system presents a eutectic transformation at a very low temperature (363 C). The eutectic gold–silicon alloy (3 wt% or 19 at% of Si) is effectively a widely used solder, mainly for silicon devices. Our goal was to deposit a silicon film on a pure gold substrate to form locally this low temperature solder. Silicon films have been deposited under vacuum by evaporation. With gold substrates maintained at room temperature, we have observed very poor adherence of the films. However, when the substrates are maintained at a temperature above the Au–Si eutectic point, the rapid formation of a gold silicon alloy is observed along well-defined paths corresponding to the gold grain boundaries. On thin gold samples (10–20 lm) the presence of this alloy is even observed on the backside of the foils, also along the grain boundaries. These observations indicate a rapid reaction of the evaporated silicon films with the gold substrate. Fig. 2 shows typical optical metallographies of a polycrystalline gold substrate after silicon diffusion at 400 C. An alloy of eutectic shape is formed along the gold grain boundaries where the eutectic reaction seems to have been initiated. PIXE technique using the LARN ion microprobe has been used to characterize this alloy. A 2.5 MeV focused proton beam (spot of 3 to 5 lm
S. Mathot, G. Demortier / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 222–230
225
Fig. 2. Typical metallographies of polycrystalline gold foil substrates showing the formation by diffusion at 400 C of a gold silicon alloy around the deposited Si film (A) or at the back side of the foils (B). Typical foil thicknesses: 12–16 lm.
in diameter) scan the polycrystalline gold sample on a narrow region situated around the deposited silicon films or at the backside of the samples. This scanning is performed by mechanical displacements of the samples (steps of 2.5 lm). K X-rays of silicon and M X-rays of gold are used as analytical signals and the absolute silicon concentration is calculated using a bulk Au–Si eutectic sample as reference material [5]. Fig. 3 shows a silicon map obtained by scanning the backside (the one opposite to the one on which silicon is evaporated) of a 14 lm thick gold foil after silicon diffusion (1 h – 400 C). This microPIXE analysis confirms that silicon is present mainly along the gold grain boundaries. The max-
imum silicon concentration is quite constant along the grain boundaries and close to the eutectic composition (19 at% Si). Towards the center of the gold grains, the silicon concentration rapidly decreases. Quantitative measurements of the silicon concentrations are however delicate to be performed by PIXE due to the important matrix effects related to the high absorption of the Si Ka X-rays lines in gold (Fig. 4). The strong overlap of the Si Ka and Au M lines cannot give accurate measurements at low silicon concentration. Further, due to the high absorption effects, the analyzed depth is low and analysis cannot be performed at depths greater than about 1–2 lm.
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Fig. 3. Silicon concentration map on area of 200 · 200 lm2 in step of 5 lm measured of the back side of a 14 lm thick gold foil after silicon diffusion at 400 C during 1 h. 64 C of grey give the silicon concentration in a range of 2–25 at%.
For the measurements of silicon concentration in depth, proton spectroscopy from (d, p) nuclear reactions has been used. The 28Si(d, p)29Si is highly exoenergetic (Q = +6.249 MeV). However, the cross sections are low as generally observed in
nuclear reaction analysis. Working with a microbeam, the experimental arrangement must be adapted to take advantage of the maximum of the cross section. For the 28Si(d, p0)29Si reaction (29Si nuclei produced in their fundamental energy level), the cross section is maximum for protons emitted at 0 relative to the incident beam direction. The cross section for Ed = 2.75 MeV varies from 10 mb/st to 1 mb/st when the emitted angle varies from 0 to 35. An experimental arrangement with a proton detector and a gold absorber inserted into the incident beam has been developed [6]. During the analysis of a thin gold sample (12–16 lm) prepared by silicon diffusion, the gold absorber is used to completely stop the deuteron particles passing through the sample under analysis (Fig. 5). On the other hand, the high-energy protons produced by the exoenergetic (d, p) nuclear reactions on silicon nuclei diffused into the sample can pass through the sample and through the absorber and be detected in the surface barrier detector. The angular straggling of the incident deuterons is small and the analyzed surface may be considered equal to the beam size (3–5 lm). Another advantage of this method is the fact that the gold substrates give no contribution to the proton spectra: the Coulomb barrier of gold nuclei prevents any (d, p) reaction on this high Z nucleus [7]. Fig. 6 shows a typical proton spectrum obtained during the analysis of a 16 lm thick gold foil prepared by silicon diffusion. The microbeam
Fig. 4. (A) PIXE spectra of an Au–Si eutectic alloy. The energy resolution of the Si(Li) detector used (150 eV) cannot allow a complete separation of the Au M and Si K X-ray lines. (B) Absorption curves for the X-ray lines: Si K in Au (1), Au M in Au (2), Au M in Si (3), Au Lb in Au (4), Si K in Si (5) and Au Lb in Si (6). The important matrix effects are illustrated by the large difference between the curves 1 and 5.
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Fig. 5. Schematic of the experimental arrangement for (d, p) nuclear reaction analysis at H = 0.
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The energy straggling effect leads to an uncertainty in the depth determination of the silicon profile of about 1.5 lm when protons are produced at the surface of the sample and up to 2.5 lm for deeper analyzed layers. The p0 proton group is the most useful signal for quantitative depth profiling. The cross sections have been measured at 0 by using a pure silicon sample (75 lm thick). 3-D analyses have been performed by scanning the deuteron beam on a narrow region at the surface of the samples and by counting for each step the protons emitted in different regions of the p0 proton group. Fig. 7 gives an enlarged view of the p0 protons group corresponding to the previous spectrum. The protons analyzed in the region 1 of this group correspond to the protons produced by nuclear reactions with silicon nuclei present at the surface of the gold sample. The protons analyzed in the lower energy regions correspond to protons produced in depth of the sample, by nuclear reaction with silicon nuclei induced by deuteron particles having loss one part of their incident energy. The depths of the analyzed layers corresponding to the selected regions in the p0 proton group can be easily calculated. Also, the silicon concentrations corresponding to each analyzed layer are calculated using our measured cross sections. A scanning in the three directions (x, y and depth) is performed when the deuteron microbeam scan step by step a narrow region at the surface of a gold sample and by counting for each step the protons in the different regions of the p0 proton group. Fig. 8 gives a typical 3-D map of a 16 lm
Fig. 6. Typical proton spectrum obtained on a gold foil after silicon diffusion (Edeuteron = 2.75 MeV).
is focused on a gold grain boundary. All of the proton peaks corresponding to different excited states of the residual 29Si nuclei (p0, p1, . . .) are broadened due to the presence of silicon in the depth of the sample. In addition, this broadening is due to the energy straggling of both the incident deuterons in the sample and to a lower attempt to the slowing down of the emitted protons in the sample and in the absorber.
Fig. 7. Selected regions of the p0 proton group used for depth profile of the silicon concentration.
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Fig. 8. 3-D analysis of silicon in a gold foil after diffusion during 1 h at 400 C. Each map gives the silicon concentration at different depths in the sample.
thick gold foil after silicon diffusion during 1 h at 400 C. The set of maps shows clearly the diffusion of silicon in depth along well-defined paths corresponding to the gold grain boundaries. Knowing that gold and silicon form a eutectic alloy of fixed composition, the decrease of the silicon concentration in depth can be also regarded as a variation of the width of the regions where the eutectic alloy is formed along the gold grain boundaries. These regions are rapidly narrower than the deuteron beam size leading to the measurement of lower concentrations. By imposing that the silicon concentration would be constant and equal to the eutectic composition (as only this actual alloy of eutectic composition could rapidly migrate as a liquid at a temperature close to 363), the measured silicon concentration depth profiles can be converted in width profiles of the silicon-rich regions. For the analysis of a 16 lm thick sample, the calculated width profile decrease rapidly with the depth (Vshape profile, Fig. 9(A)). On a thinner sample, 12 lm thick, where the overall thickness of the foil
Fig. 9. Calculated profiles of the width of the region of constant (eutectic) concentration formed along the gold grain boundaries. (A) V-shape profile obtained for deuteron microbeam focused on the front side of a 16 lm gold sample, analyzed area close to the deposited silicon film. (B) U-shape profile for deuteron microbeam focused on the rear side of a 12 lm gold foil. For the both analysis, the samples were prepared by diffusion of silicon during 1 h at 400 C and, for the calculations, the width of the first layer has been arbitrary fixed to 5 lm.
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can be analyzed, we deduce a minimum of the width in the center of the foil (U-shape profile, Fig. 9(B)). These calculations are in good agreement with the dissolution of the gold grain boundaries by the liquid eutectic alloy leading to the accumulation of this alloy on the surfaces.
4. Gold bonding by silicon diffusion Our analyses have confirmed the mechanism of the formation of a low melting point eutectic Au– Si alloy during the deposition at temperature slightly above the eutectic point of a silicon film on a gold substrate. This alloy is formed along the grain boundaries and rapidly diffuses in depth. With treated gold foils to give small grain sizes and with annealing process, we have manufactured by silicon diffusion foils having a useful layer of low melting point alloy at their surface. Pure gold
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pieces, like granulations, maintained in contact with these substrates have been soldered by reheating the assemblies only for a few minutes at 400 C. These soldering have been performed in the air and also without flux (Fig. 10). This bonding process is characterized by the fact that the solder is formed by diffusion in one of the two pieces to be bonded. The soldering is then achieved by re-diffusion of this solder alloy.
5. Modern diffusion bonding Comparing this silicon diffusion joining process with the antique granulation technique, we note that in both cases the joint is achieved by local formation of a liquid phase. For granulations, this liquid phase is formed by diffusion into the both pieces to be brazed. With silicon diffusion, the liquid phase is formed first in one of the pieces.
Fig. 10. (A) Schematic of the silicon diffusion bonding procedure: 1: Silicon deposition and diffusion. 2: Etching of the silicon excess. 3: Diffusion bonding of pure gold pieces on the prepared gold substrate. (B) SEM micrography of a joint failure between two foils. The adherence is observed mainly along the grain boundaries of the gold substrate.
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Preparation, temperature, atmosphere (flux) are also well different. Moreover the antique technique has been a significant source of inspiration for us. Granulations cannot be compared to modern diffusion bonding processes which are, by definition, associated to solid state methods performed with very good surface finishing and limited pressure (3–10 MPa) or with higher pressure (100– 200 MPa) like in hot isostatic pressing. But granulation is very close to a modern joining method called TLP (Transient Liquid Phase) bonding or Diffusion Brazing. For this process, a dissimilar layer is inserted between the pieces. Upon heating, solid state diffusion leads to the formation of a new phase (generally eutectic) liquid at the bonding temperature. This technique is used mainly for aluminum alloys as well as for titanium, cobalt or nickel alloys used in aerospace industry. TLP bonding procedure is also associated to an extensive post heat treatment leading to efficient solid state diffusion of the filler elements. This treatment induces generally a complete vanishing of the original joint. When properly performed, this technique is very advantageous for assembled parts having to serve at elevated temperature as often requested in aerospace industry. Granulation was not performed with so stringent conditions. However the principle of copper diffusion has authorized ancient goldsmiths to post-braze reheating and multi-step procedures helping certainly for the assembly of complex jewellery. Also in present times, many pseudo-Diffusion Brazing processes are performed without complete post diffusion. For example, copper can be brazed by using a pure silver layer. Above 779 C, silver and copper interdiffusion leads to the formation of a liquid phase forming after cooling an efficient brazed joint.
6. Summary Starting with archeological interrogations, two diffusion bonding procedures of pure gold samples
have been investigated. PIXE and NRA measurements performed by using a microbeam have been used to characterize the gold alloys formed. The first procedure results in the solid state diffusion of cadmium in gold from CdS pellets. The Au–Cd alloy is formed homogenously at the surface of the gold samples. The second procedure involves the diffusion of silicon from a pure silicon film deposited on a gold foil. The diffusion mechanism corresponds in a eutectic reaction between gold and silicon along the gold grain boundaries. Likeness between the antique granulation technique and these procedures as well as some modern diffusion bonding processes have been approached. For practical reasons, these modern techniques are not used in jewellery manufacture. Developments in this field could be however interesting having regard to the likenesses mentioned and the perfect results obtained with the granulation technique.
Acknowledgments This paper gives a few examples of interesting researches managed at LARN by Demortier. One of the authors would like to express in this paper heartfelt thanks for his supervision and teaching. Acknowledgments are also addressed to the LARN staff and colleagues for their permanent help during these works.
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