EP2566995A1

EP2566995A1 - Process for producing a crystalline surface layer

Info

Publication number: EP2566995A1
Application number: EP11777643A
Authority: EP
Inventors: Karl-Olof Axelsson Wadell; Eric Wadell
Original assignee: Individual
Current assignee: Wadell Karl-Olof Axelsson; WADELL, NINA
Priority date: 2010-05-04
Filing date: 2011-04-11
Publication date: 2013-03-13
Also published as: SE534817C2; EP2566995A4; WO2011139206A1; SE1050446A1

Abstract

A process for producing a crystalline surface layer consisting of Au₂Bi, AuGa₂, AuIn₂ or AuTe₂ is disclosed. The process comprises exposing a surface of gold to a liquid phase of Bi, Ga, In or Te at a predetermined temperature such that inter metallic crystals are formed, and removing any non-reacted Bi, Ga, In or Te from the surface. The process results in a substrate having a surface of coloured gold which gives a glittering appearance, and the substrate is thus suitable for jewellery applications.

Description

PROCESS FOR PRODUCING A CRYSTALLINE SURFACE LAYER

The present invention relates to a process for producing a crystalline surface layer essentially consisting of gold and a second metal selected from the group consisting of Bi, Ga, In and Te on a surface of the substrate. The substrate with the surface layer is intended for use in jewellery applications or other aesthetic applications, such as pieces of jewellery, watches, exclusive casings for business or credit cards and caps for exclusive perfume bottles or the like. Background

It is commonly known that conventional carat gold may have different colours and gold jewellery with such colours is commercially available. The various colours may be obtained for example by gold alloys, intermetallic compounds, surface coating or patination. Oxidation of gold alloys is probably the most commonly used technique today to produce coloured gold for jewellery

applications. The most common example of an intermetallic compound presently used in the jewellery industry is AuAI₂, which has a purple colour and is often referred to as "purple gold". In terms of composition, AuAI₂ comprises about 79% by weight of gold.

All intermetallic compounds of gold tend to be brittle and are thus difficult to work by conventional metal working processes. Therefore, jewellery of intermetallic compounds of gold is often cast directly to the desired shape.

Alternatively, the intermetallic compound is provided as a surface coating.

Corti, "Blue, black, purple! The special colours of gold", International Jewellery Symposium, St Petersburg, 2006, discusses various processes for obtaining different colours of gold. It is disclosed that the intermetallic compound AuAI₂ can be produced by vacuum melting gold and aluminium in the correct ratio and casting, by thermal spraying of molten gold-aluminium powder, by physical vapour deposition or by thermal diffusion. Corti also mentions the intermetallic compounds Auln₂, which has a blue colour, and AuGa₂, which has a bluish colour, but concludes that the use of these intermetallic compounds in commercial jewellery appears to be limited inter alia due to their relatively low hardness.

US 4,91 1 ,792 discloses various intermetallic compounds for use in jewellery, such as AuAI₂, AuGa₂ and Auln₂.The compounds are produced by powder metallurgy. AuGa2 and Auln₂ are described as soft and thus have a low resistance to abrasion, and their use is therefore limited to non-exposed parts of jewellery.

AuAI₂, AuGa₂ and Auln₂ for jewellery applications are also disclosed in Koltz, "Metallurgy and processing of coloured gold intermetallics - Part I:

Properties and surface processing", Gold Bulletin, Volume 43, No.1 , 2010. It is disclosed that AuAI₂ is nominally 19 carat gold, whereas AuGa₂ is 14 carat and Auln₂ about 12 carat. It is disclosed that deviation from the stoichiometric composition results in a quick loss of colour. The critical properties of the intermetallic compounds are identified as brittleness and low corrosion resistance. Koltz recommend a composition slightly over stoichiometric such that a two-phase microstructure is obtained in order to reduce the brittleness. Koltz also discloses different processing techniques, all based on diffusion surface alloying processes. Auln₂ was produced by electroplating followed by annealing and experiments producing blue gold using surface cladding were conducted. Furthermore, Koltz discusses liquid metal dip-coating as an alternative for producing AuGa₂ and Auln₂ due to it being a simple and easy to use technique. However, the experiments could not be repeated successfully because of poor wetting of gold by gallium and oxidation of gallium.

In view of the problems associated with the intermetallic compounds

AuGa₂ and Auln₂ as discussed above, it is clear that these intermetallic

compounds have not yet reached their full potential in jewellery applications and that there is still a need for a suitable manufacturing process for producing these compounds.

Summary

The object of the present invention is to achieve an attractive surface of coloured gold on a substrate.

The object is achieved by the process in accordance with independent claim 1 . Embodiments are defined by the dependent claims.

The process according to the invention results in a crystalline surface layer comprising gold and a second metal selected from the group consisting of Bi, Ga, In and Te. The crystalline surface layer comprises more than 10 carat gold. The obtained crystals are facetted and relatively large thus enabling a glittering appearance of the surface. The process may typically result in about 50 000 - 5 000 000 crystals/cm² and each crystal is facetted giving about 200 000- 20 000 000 facets/cm². Moreover, since the process results in intermetallic crystals, the surface has a higher hardness compared to if it had not been crystalline.

The process is based on the discovery that it is possible to obtain a surface layer essentially consisting of intermetallic crystals of AU2B1, AuGa2, Auln₂ or AuTe2 on the surface of a substrate by allowing gold to react with a second metal selected from the group consisting of Bi, Ga, In and Te under certain conditions, followed by removal of any non-reacted part of said second metal.

The process for producing a crystalline surface layer essentially consisting of gold and a second metal selected from the group consisting of Bi, Ga, In, and Te on a substrate according to the present invention comprises providing a substrate having a substantially oxide free and clean surface, the surface consisting of 18 to 24 carat gold, optionally coating said surface with a coating layer of said second metal, exposing said surface to a liquid phase of said second metal, said liquid phase optionally further comprising gold, at a temperature where there is equilibrium between solid and liquid phases for an intermetallic phase selected from the group consisting of AU2B1, AuGa2, Auln₂ and AuTe2 depending on the selected second metal, such that crystals of said intermetallic phase are formed on the surface of the substrate, optionally subjecting the substrate to a heat treatment at a temperature where there is equilibrium between solid and liquid phase for said intermetallic phase, removing any excess of said second metal not reacted with gold from said surface by etching, such that the surface of the substrate essentially consists of crystals of said intermetallic phase. The exposure of the surface to a liquid phase of the second metal can optionally be made in a non-oxidising environment.

According to one embodiment, the exposure of the surface to a liquid phase of said second metal is achieved by submersing the substrate in a melt of said second metal.

Preferably, the exposure of the surface to a liquid phase of the second metal is conducted at a temperature of 230-365 °C in case the second metal is Bi, 50-340 °C in case the second metal is Ga, 165-450 °C in case the second metal is In or 400-450 °C in case the second metal is Te. The heat treatment after said exposure may preferably be made at a lower temperature than the preceding step of exposing said surface to a liquid phase of said second metal. Thereby, smaller crystals are allowed to be formed in the interface between the substrate and the already formed outer surface crystals, resulting in increased bonding strength of the larger crystals to the substrate. This is especially advantageous in case of the outermost crystals being relatively large.

In accordance with one embodiment, the process further comprises a heat treatment prior to the removal of excess metal, said heat treatment being conducted at a temperature where all phases are solid. The purpose of such a heat treatment is to allow further diffusion of the second metal into the surface of the substrate thereby further improving the adhesion of the outermost crystals to the substrate.

The removal of excess of said second metal from the surface of the substrate after formation of the intermetallic crystals is performed by etching in order to avoid affecting the geometrical form of the crystals. In accordance with a preferred embodiment, the etching process step comprises contacting the substrate with a remote piece of said second metal via connecting means such that a galvanic circuit is formed in the etching solution.

The coating of the second metal on the surface of the substrate may suitably be made by electroplating or physical vapour deposition. Moreover, the thickness of the coating layer, when made by electroplating, should preferably be at least 0.5 μιτι in order to ensure that the coating is coherent and essentially free from defects. In case of physical vapour deposition, the coating layer can be considerably thinner as long as the coating layer is coherent and covers the surface.

If desired, the process may further comprise a heat treatment after the removal of excess of said second metal such that the second metal is diffused into the substrate, thus altering the composition of the crystal while essentially preserving the size thereof. This may increase the amount of gold in the crystalline surface layer and may also result in a surface layer having a higher hardness, thus making it more scratch-resistant.

Brief description of the drawings

Figure 1 a shows a binary phase diagram of Au and In Figure 1 b shows a binary phase diagram of Au and Ga

Figure 1 c shows a binary phase diagram of Au and Bi

Figure 1 d shows a binary phase diagram of Au and Te

Figure 2 shows a photograph taken in SEM of an Auln₂ surface obtained by an embodiment of the process according to the invention

Figure 3 shows a photograph taken in SEM of an AU2B1 surface obtained by an embodiment of the process according to the invention

Detailed description

The process will be further described in detail below with reference to some preferred embodiments. These embodiments shall not be considered as limiting the scope of the invention but the invention may be modified within the scope of the independent claims.

In the following, the term "a liquid phase of the second metal" is

considered to encompass both a liquid phase consisting essentially of said second metal and a liquid phase essentially consisting of said second metal and gold.

Furthermore, the temperature where there is equilibrium between solid and liquid phase for an intermetallic compound is intended to mean the

temperature at which, for the stoichiometric composition of the intermetallic compound, a liquid phase may be present. These temperatures are directly derivable from the binary phase diagrams Au-ln, Au-Bi, Au-Ga and Au-Te, respectively.

The process for producing a crystalline surface layer essentially consisting of gold and a second metal on a substrate according to the present invention comprises providing a substrate having a substantially oxide free and clean surface, the surface consisting of 18 to 24 carat gold, optionally coating said surface with a coating layer of said second metal, exposing said surface to a liquid phase of said second metal, said liquid phase optionally further comprising gold, at a temperature where there is equilibrium between solid and liquid phases for an intermetallic phase of gold and the second metal, such that crystals of said intermetallic phase are formed on the surface of the substrate, optionally subjecting the substrate to a heat treatment at a temperature where there is equilibrium between solid and liquid phases for said intermetallic phase, removing any excess of said second metal not reacted with gold from said surface by etching such that the surface of the substrate essentially consists of crystals of said internnetallic phase. The second metal is selected from the group consisting of bismuth (Bi), gallium (Ga), indium (In) and tellurium (Te); and the resulting intermetallic phase is thus Au₂Bi, AuGa2, Auln₂ or AuTe₂.

Bismuth, gallium, indium and tellurium all have a relatively low melting temperature and are able to form intermetallic compounds with gold. Furthermore, such intermetallic compounds have an attractive colour thus making them suitable for jewellery applications. Moreover, these metals do not require increased pressure for formation of intermetallic phase with gold.

In accordance with the present invention, a substrate is provided which has a surface of 18 to 24 carat gold. It will be readily apparent to the skilled person that the entire substrate may be made of gold, but it is also possible to use substrates having a surface coating or surface layer of 18 to 24 carat gold. The reason for the substrate having at least 18 carat gold is to ensure that there will be a sufficient amount of gold which may be reacted with the second metal and also avoid impurities in the crystalline surface layer. It should be noted that other alloying elements of gold, other than Ga, In, Bi and Te, in the surface of 18 to 24 carat gold, such as Ag, Cu, Ni and Pd, may influence the formation of the desired crystalline surface layer by forming unwanted intermetallic phases. Therefore, gold alloys comprising Ag, Cu, Ni and Pd should be avoided. There may however be certain 18 carat gold alloys which do not suffer from this problem. Preferably, the surface of the substrate consists of 20 to 24 carat gold, most preferably 23-24 carat gold.

It is important that the surface of the substrate is free from surface oxides and impurities. Therefore, the surface should be treated to remove any surface oxides and thoroughly cleaned before the subsequent steps of the process. If the surface of the substrate is not oxide free and properly cleaned, the adhesion of the crystalline surface layer may be insufficient. Furthermore, there is a risk of defects in the crystalline surface layer. In most cases, it is also preferred that the surface is relatively smooth even though this is not necessary.

As mentioned above, the process according to the present invention results in crystals of an intermetallic compound selected from the group consisting of Au₂Bi, AuGa2, Auln₂ and AuTe₂. The fact that the intermetallics are in the form of crystals, said crystals also being facetted, results in the surface giving a glittering appearance. The crystals should preferably be at least 5 μηη in order to achieve the glittering appearance and may be produced up to a size of at least 150 μιτι by means of the process according to the invention. Preferably, the crystals should have a size of 5-100 μιτι, more preferably 10-100 μιτι.

In accordance with one embodiment of the invention, the substrate having a surface consisting of 18 to 24 carat gold is firstly coated with a coating layer of the second metal. The layer should be coherent, essentially free from impurities and cover the surface. For this reason, it is preferred that the coating layer has a thickness of at least 0.5 μιτι if electroplating is used to form the coating layer. If for example physical vapour deposition is used, a much thinner coating layer can be used as long as it covers the surface. The purpose of the coating layer is to ensure that the liquid phase of the second metal wet the surface completely in the subsequent step of the process.

The coating layer may be applied with any conventional method resulting in a coherent and substantially dense coating layer covering at least the part of the surface of the substrate on which the crystalline surface is to be formed. Examples of suitable methods for coating the substrate with said coating layer are

electroplating and physical vapour depositions processes, as previously

mentioned. Preferably, electroplating is used and the coating layer should be at least 0.5 μιτι, preferably 1 -100 μιτι, more preferably 1 -20 μιτι.

In accordance with the invention, the surface of the substrate is exposed to a liquid phase of the second metal. The purpose of this step is to allow formation of stoichiometric intermetallic crystals of gold and the second metal on the surface of the substrate. Therefore, the step is performed at a temperature where there is equilibrium of liquid and solid phases under formation of the intermetallic compound. These temperatures are directly derivable from the binary phase diagrams of gold and the second metal. These binary diagrams are shown in Figures 1 a-1 d.

For example, Figure 1 a shows the binary phase diagram of gold and indium and the temperature range where there is equilibrium between solid and liquid phase for Auln₂ is marked by the arrows. As can also be seen from the figure, there will be equilibrium between solid and liquid phase above about 450 °C for the intermetallic phase Auln. Therefore, in case the second metal is In, the temperature should preferably be maximally 450 °C in order to ensure that only Auln₂ is formed on the surface. A similar situation arises in case the second metal is Ga, since at about 340 °C there is equilibrium between solid and liquid phase for the intermetallic compound AuGa as is evident from Figure 1 b. However, in the case of the second metal being selected from Bi and Te, the entire temperature interval where there is equilibrium between the solid and liquid phase for the intermetallic phase can be used since these elements only have one

stoichiometric intermetallic phase with gold, as is evident from Figures 1 c and 1 d.

The suitable temperature ranges for the different second metals, as well as the intermetallic compound formed and their colours respectively are shown in Table 1 . The size of the crystals obtained generally increases with increasing temperature within the specified intervals. Very large crystals may suffer from poor adhesion to the surface of the substrate. Moreover, in order to avoid any problems with possible fluctuations of temperature during the steps, it is generally advisable not to use a temperature too close to the end values of the interval of the broadest possible temperature range. Table 1 also specifies the preferred ranges of the temperature during the exposure of the surface of the substrate to the liquid phase of the second metal.

Table 1

It is essential that the metals are not oxidised, and the exposure of the surface of the substrate to the liquid phase of the second metal is therefore preferably conducted in a non-oxidising environment. This may suitably be achieved by using an argon protective atmosphere or vacuum. It has to be noted that in some cases, this step may be conducted without protective atmosphere or vacuum. One such example is if the liquid phase is only formed in the interface between the substrate and a possible coating layer, the outermost part of the coating layer being solid and wherein the coating layer is so thick that a possible surface oxide on the coating layer will not risk affecting the formation of

intermetallic crystals.

Furthermore, the exposure may preferably be made at about atmospheric pressure. It will be readily apparent to the skilled person that an increased or reduced pressure will alter the suitable temperatures of the exposure of the surface to the liquid phase since the above specified temperatures are based on atmospheric pressure. The temperatures for cases where the pressure is not atmospheric should be adjusted to the corresponding temperatures according to binary phase diagrams for such pressures.

Exposing the surface of the substrate to a liquid phase of the second metal may be achieved by different process steps. For example, it may be achieve by contacting the surface with a solid foil, or the like, of the second metal and heating to a temperature where the foil is melted such that a liquid phase of the second metal is formed on the surface of the substrate. It is also possible to apply a coating layer of the second metal and heating to a temperature where the coating layer is melted such that a liquid phase is formed on the surface of the substrate.

However, in accordance with a preferred embodiment, the exposure is performed by submersing the substrate in a melt of the second metal. This embodiment is applicable to the cases when the second metal is selected from the group consisting of Bi, Ga and In. In case the second metal is Te, such an embodiment is not practically applicable since the melting temperature of Te is equal to the maximal temperature where there is equilibrium between solid and liquid phases under formation of the intermetallic phase.

Alloying of the second metal with gold reduces the liquidus temperature of the second metal in case the second metal is Bi or Te. When the second metal is In or Ga, the liquidus temperature increases when the second metal is alloyed with gold. However, a two phase is created for all cases, wherein a solid phase comprising the intermetallic compound and a liquid phase comprising the second metal and optionally gold are formed. Therefore, the temperature of the exposure of the surface of the substrate to the liquid phase of the second metal is not necessarily a temperature above the melting temperature of the pure second metal. When the second metal is Ga or In, the temperature of the exposure of the surface to the liquid phase is above the melting temperature of the second metal. However, in case the second metal is Bi, it may be both over and under the melting temperature of Bi; and in the case of Te, the temperature of the exposure is in fact equal to or below the melting temperature of Te.

When the surface is in contact with the liquid phase of the second metal, the second metal will diffuse into the surface consisting of 18 to 24 gold and react with the gold. By the fact that the temperature is kept within the interval where there is equilibrium between solid and liquid phase for the intermetallic compound, crystals of the intermetallic compound will be formed.

The surface of the substrate should be in contact with the liquid phase of the second metal for a sufficient period of time for formation of the desired amount and size of crystals of the intermetallic compound on the surface. It will be readily apparent to the skilled person that the appropriate time depends on the

temperature selected and the selected second metal, but can easily be determined by mere routine tests. Since gold has a very high tendency of reacting with Bi, Ga, In and Te, crystals of the intermetallic compounds will start to form practically immediately when the substrate is brought to the above specified temperatures.

According to one embodiment of the invention, the substrate is thereafter heat treated at a temperature where there is equilibrium between solid and liquid phase for said intermetallic phase, but preferably at a lower temperature than the step at which the exposure of the liquid phase of the second metal and the surface of the substrate was conducted. The purpose of such a heat treatment is to improve the bonding of the obtained crystals to the surface of the substrate. Such a heat treatment is especially advantageous when relatively large crystals were formed in the preceding step as such large crystals sometimes may have a low adhesion to the substrate. During such a heat treatment, there will be a thin layer of liquid phase of the second metal on the surface of the intermetallic crystals, ensuring that there is a sufficient amount of second metal to react with the gold. It is not necessary to add more of the second metal to the substrate for this heat treatment, even though this may be done if desired.

During the heat treatment, the second metal will diffuse into the substrate and react with gold under formation of more crystal. This will in turn lead to formation of relatively small crystals in the interface of the substrate and the already formed larger crystals at the outer surface, and thus improved adhesion of the outermost crystals of the surface to the substrate.

It will be readily apparent to the skilled person that the above described heat treatment can be performed in a non-oxidising atmosphere in order to ensure that the metals are not oxidised during the heat treatment. Furthermore, the heat treatment can be made after the substrate has been allowed to cool to room temperature, or in the same furnace as the preceding step and directly thereafter but at a lower temperature.

If desired, further heat treatments may be performed in order to allow further diffusion of the second metal into the gold surface. For example, a second heat treatment may suitably be performed at a temperature where all phases are solid. Thus, such a heat treatment is suitably performed at a temperature below 230 °C when the second metal is Bi, below 50 °C when the second metal is Ga, below 156 °C when the second metal is In, and below 400 °C when the second metal is Te.

After the crystals of the intermetallic compounds have been formed, any excess of the second metal which are not present as intermetallic compound, i.e. which has not reacted with gold, is removed from the surface of the substrate. The removal of excess non-reacted second metal from the surface after formation of the crystals is an essential part of the process in order to achieve an outermost surface layer of intermetallic crystals and avoid any alteration of the appearance of the surface during use. The removal of excess of said second metal is made by etching in order not to destroy the geometrical form of the crystals. The etching may be performed by any etching solution that is able to etch the second metal.

It is important to be able to control the etching process since as soon as all of the non-reacted second metal has been etched from the surface, etching of the intermetallic crystals starts which quickly alters the composition of the crystals and may thus almost immediately alter the surface appearance of the substrate. By way of example, in the case the second metal is indium, the surface of the substrate almost instantaneously changes from the clear blue colour to a brownish colour as soon as all of the non-reacted indium has been etched from the surface.

Therefore, in accordance with a preferred embodiment, the substrate is connected to a remote piece made of the second metal via a connection means, such as a gold wire or the like, the remote piece of said second metal being submersed in the same etching solution as the substrate. Thus, a galvanic circuit is formed by the substrate and the remote piece of the second metal in the etching solution. Thereby is ensured that as soon as the excess of the second metal has been etched from the surface of the substrate, the etching will continue only on the remote piece of the second metal instead of the crystalline surface of the substrate since the intermetallic compound is nobler than the second metal. The galvanic circuit thus enables much easier process control since it is no longer critical exactly when the etching process is stopped.

The substrate with the intermetallic crystals on the surface thereof may be exposed to further process steps if desired. For example, the substrate may be subjected to a heat treatment after the non-reacted part of the second metal has been removed from the surface. The purpose of such a heat treatment may be to allow said second metal to diffuse from the intermetallic crystals into the substrate, thereby increasing the gold content of the crystals. Such a heat treatment should be conducted in a non-oxidising atmosphere and preferably at a temperature below the melting point of at least the stoichiometric intermetallic phase.

Generally, the size and the geometry of the crystals on the surface are

substantially preserved by such a heat treatment, but the composition of the crystals may be altered.

By way of example only, in the case of the second metal being In and the crystalline surface formed on the surface of the substrate thus essentially consisting of Auln₂, a heat treatment after the removal of excess indium on the surface of the substrate may be performed in order to allow indium to diffuse into non-reacted gold of the substrate whereas gold will diffuse out towards the surface. Thereby it is possible to achieve a higher content of gold in the crystals while generally preserving the geometrical form and the size of the crystals. In fact, it is possible to obtain crystals of Auln on the surface of the substrate by means of such a heat treatment. The Auln crystals are more stable and have a considerably higher hardness than Auln₂ crystals, about 250 Hv compared to about 40-45 Hv for Auln₂. Thus, an outermost surface of Auln which is highly scratch resistant may be obtained and which has an excellent adhesion to the substrate. Such a surface would be highly suitable for example for exposed parts of jewellery. The colour of the Auln surface will be bluish. Such a subsequent process may also be conducted in case of the second metal being Ga, and the crystalline surface formed on the surface of the substrate thus essentially consisting of AuGa2, which would then result in crystals of AuGa on the surface.

Example 1 - Auln₂

The surface of a 24 carat gold substrate was polished and thoroughly cleaned. A thin coating of essentially pure indium was coated on the surface of the substrate by electroplating resulting in a homogenous coating layer of indium.

The coated substrate was then placed in an indium melt in an argon atmosphere at atmospheric pressure and at a temperature of about 275 °C for about 30 minutes. This resulted in formation of relatively large crystals of Auln₂ on the surface of the substrate. The substrate was then removed from the melt and allowed to cool to room temperature.

Thereafter, the substrate was heated to about 200 °C for about 10 hours without any additional melt being provided to the substrate than the already present melt resulting from the not reacted indium on the surface from the preceding step. During this step, relatively small crystals of Auln₂ were formed under the surface layer of the relatively large crystals of Auln₂. These smaller crystals thus improve the adherence of the large crystals to the surface of the substrate. The substrate was then removed from the furnace and allowed to cool down to room temperature.

A heat treatment at about 150 °C for about 10 hours to allow further diffusion of indium into the substrate and further improve the adherence of the crystals to the surface of the substrate was performed.

In order to remove the excess indium on the surface of the substrate, the substrate was etched with a HCI-solution. To ensure that the etching solution did not alter the properties of the crystals of Auln₂, a piece of indium was connected to the substrate through a gold wire in order to create a galvanic circuit. Thus, when the indium present on the surface of the substrate had been etched, the remote piece of indium was further etched instead of the crystals of Auln₂ on the surface of the substrate.

Figure 2 shows a photograph taken in SEM of the surface of the substrate after indium had been etched from the surface. It is clear from the figure that facetted crystals with a size in the order of tens of micrometers were formed. The facetted crystals resulted in a glittering appearance of the surface and the colour was blue.

Example 2 - Au₂Bi

A surface of a 24 carat gold substrate was polished and thoroughly cleaned. The substrate was then placed in a bismuth melt, without any prior coating of the surface, in an argon atmosphere at atmospheric pressure and at a temperature of about 300 °C for about 60 minutes. This resulted in formation of relatively large crystals of AU2B1 on the surface of the substrate. The substrate was then removed from the melt and allowed to cool to room temperature.

In order to remove the excess bismuth on the surface of the substrate, the substrate was etched with a HNO3-solution. To ensure that the etching solution did not alter the properties of the crystals of AU2B1, a piece of bismuth was connected to the substrate through a gold wire in order to create a galvanic circuit. Thus, when the bismuth present on the surface of the substrate had been etched, the remote piece of Bi was further etched instead of the crystals of AU2B1 on the surface of the substrate.

Figure 3 shows a photograph taken in SEM of the surface of the substrate after the non-reacted bismuth had been removed. The crystals obtained had sharp corners and the surface layer was found to have a hardness in the order of about 200 Hv and should therefore be fairly scratch resistant. The colour of the surface was white-pink.

Example 3 - AuGa₂

A surface of a 24 carat gold substrate was polished and thoroughly cleaned. The substrate was dipped in molten gallium at about 40 °C in order to achieve a coating layer and allowed to cool to room temperature. Thereafter, the substrate was heated to 225 °C during 60 minutes such that a liquid phase of gallium was present on the surface. This resulted in formation of crystals of AuGa2 on the surface of the substrate. The substrate was then allowed to cool to room temperature.

In order to remove the excess gallium on the surface of the substrate, the substrate was etched with a HCI-solution. To ensure that the etching solution did not alter the properties of the crystals of AuGa2, a piece of gallium was connected to the substrate through a gold wire in order to create a galvanic circuit. Thus, when the galliunn present on the surface of the substrate had been etched, the remote piece of gallium was further etched instead of the crystals of AuGa2 on the surface of the substrate.

The colour of the surface was bluish.

Claims

1 . Process for producing a crystalline surface layer essentially consisting of gold and a second metal selected from the group consisting of Bi, Ga, In, and Te on a substrate, the process comprising the steps of

- providing a substrate having a substantially oxide free and clean surface, the surface consisting of 18 to 24 carat gold

- optionally coating said surface with a coating layer of said second metal

- exposing said surface to a liquid phase of said second metal, said liquid phase optionally further comprising gold, at a temperature where there is equilibrium between solid and liquid phases for an intermetallic phase selected from the group consisting of AU2B1, AuGa2, Auln₂ and AuTe2 depending on the selected second metal, such that crystals of said intermetallic phase are formed on the surface of the substrate

- optionally subjecting the substrate to a heat treatment at a temperature where there is equilibrium between solid and liquid phase for said intermetallic phase

- removing any excess of said second metal not reacted with gold from said surface by etching such that the surface of the substrate essentially consists of crystals of said intermetallic phase.

2. Process according to claim 1 wherein the exposure of the surface to a liquid phase of said second metal is achieved by submersing the substrate in a melt of said second metal.

3. Process according to claim 1 or 2 wherein the temperature during the

exposure of said surface to a liquid phase of said second metal is 230-365 °C in case the second metal is Bi, 50-350 °C in case the second metal is Ga, or 165-450 °C in case the second metal is In.

4. Process according to claim 1 wherein the second metal is Te and the

temperature during the exposure of said surface to a liquid phase of said second metal is 400-450 °C. Process according to any of the preceding claims wherein said heat treatment is performed at a lower temperature than the preceding step of exposing said surface to a liquid phase of said second metal.

Process according to any of the preceding claims wherein the process further comprises a heat treatment prior to the removal of excess metal, said heat treatment being conducted at a temperature where all phases are solid.

Process according to any of the preceding claims wherein said etching comprises contacting the substrate with a remote piece of said second metal via connecting means such that a galvanic circuit is formed in the etching solution.

Process according to any of the preceding claims wherein the coating of said second metal is performed by electroplating or physical vapour deposition.

Process according to any of the preceding claims further comprising a heat treatment after the removal of excess of said second metal such that the second metal is diffused into the substrate, thus altering the composition of the crystal while essentially preserving the size and the shape thereof.

Substrate comprising a crystalline surface layer essentially consisting of crystals of an intermetallic compound selected from the group consisting of AuBi₂, AuGa2, Auln₂, AuTe2, AuGa and Auln and wherein said crystals are facetted and have an average size of 5-150 μιτι.