SE534817C2 - Process for producing a crystalline surface layer - Google Patents

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

534 BT? US 4,911,792 discloses various intermetallic compounds for use in jewelry, such as AuAlz, AuGaz and Aulnz. The compounds are produced by powder metallurgy. AuGag and Aulnz are described as soft and thus have a low resistance to abrasion, and their use is therefore limited to unexposed parts of jewelry.

AuAlz, AuGaz and Aulnz for jewelry applications are also described in Koltz, "Metallurgy and processing of colored gold intermetallics - Part I: Properties and surface processing", Gold Bulletin, volume 43, no. 1, 2010. It is described that AuAlz is nominally 19 carat gold , while AuGag is 14 carats and Aulng about 12 carats. It is described that deviation from the stoichiometric composition leads to a rapid loss of color. The critical properties of the intermetallic compounds have been identified as brittleness and low corrosion resistance. Koltz recommends a slightly overstoichiometric composition so that a two-phase microstructure is obtained to reduce the brittleness. Koltz also describes various manufacturing techniques, all based on diffusion surface alloy processes. Auln; was produced by electroplating followed by annealing and experiments producing blue gold using surface plating were performed. Furthermore, Koltz discusses dip coating in surface metal as an alternative to producing AuGag and Aulng due to the fact that it is a simple and easy technique to use. However, the experiments could not be repeated successfully due to substandard wetting of gold with gallium and oxidation of gallium.

In view of the problems associated with the intermetallic compounds AuGag and Aulng as discussed above, it is clear that these intermetallic compounds have not yet reached their full potential in jewelry applications and that there is still a need for a suitable manufacturing process to produce these compounds. .

Summary The object of the present invention is to provide an attractive surface of colored gold on a substrate.

The object is achieved through the procedure according to the independent claim 1.

Embodiments de nied by the dependent claims.

The process according to the invention gives rise to a crystalline surface layer comprising gold and a second metal selected from the group consisting of Bi, 15 20 25 30 534 81? 3 Ga, ln and Te. The crystalline surface layer comprises more than 10 carat gold. The crystals obtained are faceted and relatively large, thus enabling a glittering appearance on the surface. The process can typically result in about 50,000-5,000,000 crystal lcmz and each crystal is faceted, giving about 200,000-20,000,000 facet lcmz. In addition, 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 consisting mainly of intermetallic crystals of AuZBi, AuGaz, Auln; or AuTez on the surface of a substrate by allowing gold to react with a second metal selected from the group consisting of Bi, Ga, ln and Te under certain conditions, followed by removal of any unreacted portion of said second metal.

The process for producing a crystalline surface layer consisting mainly of gold and a second metal selected from the group consisting of Bi, Ga, ln and Te on a substrate according to the present invention comprises providing a substrate which has a substantially oxide-free and pure surface, the surface comprising 18 to 24 carat gold, optionally coating said surface with a coating layer of said second metal, exposing said surface to a molten phase of said second metal, said molten phase optionally further comprising gold, at a temperature where there is an equilibrium between solid phase and molten phase for an intemetallic phase selected from the group consisting of Au2Bi, AuGa2, Aulng and AuTeg depending on the selected second metal, so that crystals of said internetallic phase are formed on the surface of the substrate, optionally subjecting the substrate to a heat treatment at a temperature where there is an equilibrium between solid phase and molten phase for said intermetallic phase, to be removed any excess of said second metal which has not reacted with gold from said surface so that the surface of the substrate consists mainly of crystals of said intermetallic phase. The exposure of the surface to a molten phase of the second metal can optionally be done in a non-oxidizing environment.

According to one embodiment, the exposure of the surface to a molten phase of said second metal is effected by immersing the substrate in a melt of said second metal.

Preferably, the exposure of the surface to a molten phase of the second metal is performed at a temperature of 230-365 ° C if the second metal is Bi, 50- 534 B17 440 ° C if the second metal is Ga, 165-450 ° C if the second metal is ln or 400-450 ° C if the other metal is Te.

The heat treatment after said exposure can preferably be done at a lower temperature than the previous step of exposing said surface to a molten phase of said second metal. Thus, smaller crystals are allowed to form in the interface between the substrate and the crystals already formed on the outer surface, which results in increased bonding strength of the larger crystals to the substrate. This is especially advantageous in the case where the outermost crystals are relatively large. In accordance with one embodiment, the method further comprises a heat treatment before the removal of excess metal, said heat treatment being carried out at a temperature where all phases are solid. The purpose of such heat treatment is to allow further diffusion of the second metal into the surface of the substrate and thereby further improve the adhesion of the outermost crystals to the substrate.

The removal of excess second metal from the surface of the substrate after formation of the intermetallic crystals is preferably by etching to avoid affecting the geometric shape of the crystals. In accordance with a preferred embodiment, the etching method step comprises contacting the substrate with a spaced piece of said second metal via connecting means so that a galvanic circuit is formed in the etching solution. An alternative method of removing excess of the second metal is rinsing in a suitable solution, such as rinsing in hot water if the second metal is Ga or rinsing in hot oil if the second metal is ln or Bi.

The coating of the second metal on the surface of the substrate can conveniently be done by electroplating or physical gas phase deposition. In addition, the thickness of the coating layer, when done by electroplating, should preferably be at least 0.5 μm to ensure that the coating is cohesive and substantially free of defects. In the case of physical gas phase deposition, the coating layer can be considerably thinner as long as the coating layer is continuous and covers the surface.

If desired, the method may further comprise a heat treatment after removing excess of said second metal so that the second metal diffuses into the substrate, thus changing the composition of the crystal while substantially preserving its size. This can increase the amount of gold in the 20 25 30 534 81? Crystalline surface layer and can also result in a surface layer which exhibits a higher hardness, thus making it more scratch resistant.

Brief Description of the Drawings Figure 1a shows a binary phase diagram for Au and ln Figure 1b shows a binary phase diagram for Au and Ga Figure 1c shows a binary phase diagram for Au and Bi Figure 1d shows a binary phase diagram for Au and Te Figure 2 shows a photograph in SEM of an Aulnz surface obtained by an embodiment of the method according to the invention Figure 3 shows a photograph taken in SEM of an Au2Bi surface obtained by an embodiment of the method according to the invention. Detailed description The method will be described in further detail below with reference. to some preferred embodiments. These embodiments are not to be construed 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 molten phase of the second metal" is considered to include both a molten phase consisting essentially of said second metal and a molten phase consisting essentially of said second metal and gold.

Furthermore, the temperature at which there is an equilibrium between solid phase and molten phase for an intermetallic compound is intended to mean the temperature at which, for the stoichiometric composition of the intermetallic compound, a molten phase can occur. These temperatures are directly derived from the binary phase diagrams Au-ln, Au-Bi, Au-Ga and Au-Te, respectively.

The method of producing a crystalline surface layer consisting mainly 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 molten phase of said second metal, said molten phase optionally further comprising gold, at a temperature where there is equilibrium between solid phase and molten phase for an intermetallic phase of gold and the second metal, so 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 phase and molten phase for said antenna metal phase, to remove any excess of said second metal which has not reacted with gold from said surface so that the surface of substrate it consists mainly of crystals of said intermetallic phase.

The second metal is selected from the group consisting of bismuth (Bi), gallium (Ga), indium (ln) and tellurium (T e); and the resulting internal phase is thus Au2Bi, AuGag, Auln2 or AuTeg.

Bismuth, gallium, indium and tellurium all have a relatively low melting temperature and have the ability to form intermetallic compounds with gold. Furthermore, such intermetallic compounds have an attractive color, thus making them suitable for jewelry applications. In addition, these metals do not require elevated pressure to form an intermetallic phase with gold.

In accordance with the present invention, there is provided a substrate having an area of 18 to 24 carat gold. It will be readily apparent to those skilled in the art that the entire substrate can be made of gold, but it is also possible to use substrates which have a surface coating or a surface layer of 18 to 24 carat gold. The reason why the substrate has at least 18 carat gold is to ensure that there will be a sufficient amount of gold that can react with the other metal and to also avoid impurities in the crystalline surface layer. It should be noted that other gold alloying elements, other than Ga, ln, Bi and Te, in the surface of 18 to 24 carat gold, such as Ag, Cu, Ni and Pd, can affect the formation of the desired crystalline surface layer by forming undesirable intermetallic phases. Therefore, gold alloys comprising Ag, Cu, Ni and Pd should be avoided. However, there may be some 18 carat gold alloys that do not have this problem. Preferably, the surface of the substrate consists of 20 to 24 carat gold, preferably 23-24 carat gold.

It is important that the surface of the substrate is free of surface oxides and impurities. Therefore, the surface should be treated to remove any surface oxides and thoroughly cleaned before the subsequent steps in 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 preferred that the surface be relatively smooth, although this is not necessary.

As mentioned above, the process of the present invention results in crystals of an intermetallic compound selected from the group consisting of Au2Bi, AuGaz, Aulnz and AuTez. The fact that the intermetals are in the form of crystals, said crystals also being faceted, results in the surface giving a glittering appearance. The crystals should preferably be at least 5 microns to achieve the glittering appearance and can be made up to a size of at least 150 microns by the method of the invention. Preferably, the crystals should have a size of 5-100 μm, more preferably 10-100 μm.

In accordance with one embodiment of the invention, the substrate having a surface consisting of 18 to 24 carat gold is first coated with a coating layer of the second metal. The layer should be coherent. mainly free from impurities and cover the surface. For this reason, it is preferred that the coating layer have a thickness of at least 0.5 μm if electroplating is used to form the coating layer. For example, if physical gas phase 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 molten phase of the second metal completely wets the surface in the subsequent step of the process.

The coating layer can be applied by any conventional method resulting in a cohesive and substantially dense coating layer covering at least the portion 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 gas phase deposition processes, as previously mentioned. Preferably electroplating is used and the coating layer should be at least 0.5 μm, preferably 1-100 μm, more preferably 1-20 μm. In accordance with the invention, the surface of the substrate is exposed to a molten phase of the second metal. The purpose of this step is to enable the formation of stoichiometric intermetallic crystals of gold and the other metal on the surface of the substrate. Therefore, the step is carried out at a temperature where there is an equilibrium between molten phase and solid phase to form the intermetallic compound. These temperatures are directly deducible from the binary phase diagrams of gold and the other metal. These binary charts are shown in Figures 1a-1d.

For example, Figure 1a shows the binary phase diagram of gold and indium and the temperature range where there is equilibrium between solid phase and molten phase for Auln; is marked with the arrows. As also shown in the figure, there will be an equilibrium between solid phase and molten phase above about 450 ° C for the intermetallic phase Auln. Therefore, if the second metal is ln, the temperature should preferably be at most 450 ° C to ensure that only Au | n2 is formed on the surface. A similar relationship occurs if the second metal is Ga, since at about 340 ° C there is an equilibrium between solid phase and molten phase for the internal compound AuGa, as is clear from Figure 1b. However, in case the other metal is selected from Bi and Te, the whole temperature range where there is equilibrium between the solid phase and the molten phase of the intermetallic phase can be used, since these elements have only a stoichiometric intermetallic phase with gold, as clearly shown in F Figures 1c and 1d.

The suitable temperature ranges for the various other metals, as well as the intermetallic compound formed and their respective colors are shown in Table 1. The size of the crystals obtained generally increases with increasing temperature within the specified ranges. Very large crystals may be subject to substandard adhesion to the surface of the substrate. In addition, in order to avoid any problems with possible temperature fluctuations during the steps, it is generally wise not to use a temperature which is too close to the end values of the range of the widest possible temperature range. Table 1 also specifies the preferred ranges of temperature during the exposure of the surface of the substrate to the molten phase of the second metal.

Table 1 Second Temperature Range Preferred Intermetallic Color Metal Temperature Range Compound Bi 241-371 ° C 250-325 ° C Au2Bi White Insert Ga 30-339 ° C 60-280 ° C AuGaz Bluish ln 157-454 ° C 175-350 ° C Aulng Blue Tea 416-447 ° C 410-440 ° C AuTeg Green-yellow-gray-white It is essential that the metals are not oxidized, and the exposure of the surface of the substrate to the molten phase of the other metal is therefore preferably carried out in a non-oxidizing environment. . This can be conveniently accomplished by using a shielding gas atmosphere of argon or vacuum. It should be noted that in some cases this step can be performed without a shielding gas atmosphere or vacuum. One such example is 20 25 30 534 81? 9 if the molten phase is formed only at the interface between the substrate and a possible coating layer, the outermost part of the coating layer being solid and where the coating layer is so thick that any surface oxide on the coating layer will not risk affecting the formation of intermetallic crystals.

Furthermore, the exposure can preferably be made at approximately atmospheric pressure.

It will be readily apparent to those skilled in the art that an increase or decrease in pressure will change the appropriate temperatures for exposing the surface to the molten phase since the temperatures specified above 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.

Exposure of the surface of the substrate to a molten phase of the second metal can be accomplished by various steps of the process. For example, it can be accomplished by contacting the surface with a solid film, or the like, of the second metal and heating to a temperature where the film melts so that a molten 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 heat up to a temperature where the coating layer melts so that a molten phase is formed on the surface of the substrate.

However, in accordance with a preferred embodiment, the exposure occurs by immersing the substrate in a melt of the other metal. This embodiment is applicable to the cases where the second metal is selected from the group consisting of Bi, Ga and ln. If the other metal is Te, such an embodiment is not practically applicable because the melting temperature of Te is equal to the maximum temperature where there is equilibrium between solid phase and molten phase to form the intermetallic phase.

Alloying the second metal with gold reduces the liquidus temperature of the second metal if the other metal is Bi or Te. When the second metal is ln or Ga, the liquidus temperature increases as the second metal is alloyed with gold. However, a two-phase system is created in each case, where a solid phase comprising the intermetallic compound and a molten phase comprising the other metal and optionally gold are formed. Therefore, the temperature of the exposure of the surface of the substrate to the molten phase of the second metal is not necessarily a temperature which is above the melting temperature of the pure second metal. When the other metal is Ga or In, the temperature of the exposure of the surface of the molten is 534 H1? Phase above the melting temperature of the other metal. If the other metal is Bi, however, it may be both above and below the melting temperature of Bi; and in the case of Te, the exposure temperature is in fact equal to or below the melting temperature of Te.

When the surface is in contact with the molten 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 keeping the temperature within the interval where there is an equilibrium between the solid phase and the molten phase of the intermetallic compound, crystals of the antenna metallic compound will be formed.

The surface of the substrate should be in contact with the molten phase of the second metal for a sufficient period of time to form the desired amount and size of crystals of the intermetallic compound on the surface. It will be readily apparent to those skilled in the art that the appropriate time depends on the temperature selected and the selected other metal, but can be readily determined by pure routine tests. Since gold has a very high tendency to react with Bi, Ga, ln and Te, crystals of the intermetallic compounds will start to form practically immediately when the substrate is brought to the temperatures specified above.

According to an embodiment of the invention, the substrate is then heat treated at a temperature where there is an equilibrium between solid phase and molten phase for said intermetallic phase, but preferably at a lower temperature than the step at which the exposure of the molten phase of the other metal and the surface of the substrate was performed. The purpose of such 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 previous step because such large crystals can sometimes have a weak adhesion to the substrate. During such heat treatment, there will be a thin layer of molten phase of the second metal on the surface of the antenna metallic crystals, which ensures that there is a sufficient amount of the second metal to react with the gold. It is not necessary to add more of the other metal to the substrate for this heat treatment, although this can be done if desired.

During the heat treatment, the second metal will diffuse into the substrate and react with gold to form additional crystal. This 20 25 30 534 81? 11 will in turn lead to the formation of relatively small crystals in the interface between the substrate and the already formed larger crystals at the outer surface, and thus improved adhesion of the outermost crystals on the surface to the substrate.

It will be readily apparent to those skilled in the art that the heat treatment described above can be carried out in a non-oxidizing atmosphere to ensure that the metals are not oxidized during the heat treatment. Furthermore, the heat treatment can be done after the substrate has been allowed to cool to room temperature, or in the same oven as the previous step and immediately thereafter but at a lower temperature.

If desired, additional protective treatments can be performed to allow further diffusion of the second metal into the gold surface. For example, a second friend treatment may conveniently be carried out at a temperature where all phases are solid. Thus, such heat treatment preferably takes place 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 1n and below 400 ° C when the second metal is Tea.

After the crystals of the intermetallic compounds have been formed, any excess of the other metal which does not exist as an intermetallic compound, i.e. which has not reacted with gold, is removed from the surface of the substrate. The removal of excess unreacted second metal from the surface after formation of the crystals is an essential part of the process to provide an outermost surface layer of intermetallic crystals and to avoid a change in the appearance of the surface during use.

The removal of excess of said second metal is preferably done by etching so as not to destroy the geometric shape of the crystals.

The etching can be done with any etching solution that is capable of etching the other metal.

It is important to be able to control the etching process because as soon as all unreacted other metal has been etched from the surface, etching of the intermetallic crystals begins, which quickly changes the composition of the crystals and thus can almost immediately change the surface appearance of the substrate. For example, in the case where the other metal is indium, the surface of the substrate changes almost instantaneously from the clear blue color to a brownish color as soon as all unreacted indium has been etched from the surface. According to a preferred embodiment, the substrate is therefore connected to a spaced piece made of the second metal via a connecting means, such as a gold wire or the like, the spaced piece of said second metal being lowered. in the same etching solution as the substrate. Thus, a galvanic circuit is formed by the substrate and the spaced piece of the second metal in the etching solution. This ensures that as soon as the excess metal has been etched from the surface of the substrate, the etching will continue only on the spaced piece of the second metal instead of the crystalline surface of the substrate because the intermetallic compound is nobler than the other metal. The galvanic circuit thus enables a much easier control of the process because it is no longer of decisive importance when the exact etching process is stopped.

»A possible alternative method for removing excess of the second metal is rinsing in a suitable id uid, such as rinsing in hot water if the second metal is Ga or rinsing in water oil if the other metal is ln or Bi.

The substrate with the intermetallic crystals on the surface thereof can be exposed to further steps in the process if desired. For example, the substrate may be subjected to a heat treatment after the unreacted portion of the second metal has been removed from the surface. The purpose of such 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 heat treatment should be performed in a non-oxidizing atmosphere and preferably at a temperature below the melting point of at least the stoichiometric intermetallic phase. In general, the size and geometry of the crystals on the surface are substantially preserved by such heat treatment, but the composition of the crystals can be changed. In case the second metal, by way of example only, is 1n and the crystalline surface formed on the surface of the substrate thus consists mainly of Aulng, a heat treatment after the removal of excess indium on the surface of the substrate may take place to allow indium to diffuse into in unreacted gold of the substrate while gold will diffuse out towards the surface. Thus, it is possible to achieve a higher content of gold in the crystals while the geometric shape and size of the crystals are generally preserved. 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 Aulng crystals, about 250 Hv compared to about 40-45 Hv for Aulnz. Thus, an outermost surface of Auln which is very scratch resistant can be obtained and which has an excellent adhesion to the substrate. Such a surface would be very suitable, for example, for exposed parts of jewelry. The color of the Auln surface will be bluish. Such a subsequent process can also be performed in the case where the second metal is Ga, and the crystalline surface formed on the surface of the substrate thus consists mainly of AuGag, which would then result in crystals of AuGa on the surface.

Example 1 - Auln; The surface of a 24 carat gold substrate was polished and thoroughly cleaned. A thin coating of substantially pure indium was applied to the surface of the substrate by electroplating, resulting in a homogeneous 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 30 minutes. This resulted in the formation of relatively large crystals of Auln; 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 up to about 200 ° C for about 10 hours without providing any additional melt to the substrate other than the pre-existing melt resulting from the unreacted indium on the surface from the previous step. During this step, relatively small crystals of Auln were formed; below the surface layer of the relatively large crystals of Aulng. These smaller crystals thus improve the adhesion of the large crystals to the surface of the substrate. The substrate was then removed from the oven and allowed to cool to room temperature.

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

To remove excess indium on the surface of the substrate, the substrate was etched with an HCl solution. To ensure that the etching solution did not change the properties of the crystals of Auln; a piece of indium was connected to the substrate via a gold wire to create a galvanic circuit. Thus, when the indium present on the surface of the substrate had been etched, the spaced piece of indium was further etched instead of the crystals of Aulnz on the surface of the substrate.

Figure 2 shows a photograph taken in SEM of the surface of the substrate after the indium had been etched from the surface. It is clear from the att guren that faceted crystals with a size in the order of tens of micrometers were formed. The faceted crystals resulted in a glittering appearance on the surface and the color 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 the formation of relatively large crystals of Au2Bi on the surface of the substrate. The substrate was then removed from the melt and allowed to cool to room temperature.

To remove excess bismuth on the surface of the substrate, the substrate was etched with an HNO 3 solution. To ensure that the etching solution did not change the properties of the crystals of Au2Bi, a piece of bismuth was connected to the substrate via a gold wire to create a galvanic circuit. Thus, when the bismuth present on the surface of the substrate had been etched, the spaced piece of Bi was further etched instead of the crystals of Au2Bi on the surface of the substrate.

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

Example 3 - AuGag A surface of a 24 carat gold substrate was polished and thoroughly cleaned.

The substrate was dipped in molten gallium at about 40 ° C to provide a coating layer and allowed to cool to room temperature. Thereafter, the substrate was heated to 225 ° C for 60 minutes so that a molten phase of gallium was present on the surface. This resulted in the formation of crystals of AuGag on the surface of the substrate. The substrate was then allowed to cool to room temperature.

To remove excess gallium on the surface of the substrate, the substrate was etched with an HCl solution. To ensure that the etching solution does not 53 !! 81? Changing the properties of the crystals of AuGaz, a piece of gallium was connected to the substrate via a gold wire to create a galvanic circuit. Thus, when the gallium present on the surface of the substrate had been etched, the spaced piece of gallium was further etched instead of the crystals of AuGag on the surface of the substrate.

The color of the surface was bluish.

Claims (9)

5 20 25 30 534 B17 1 6 PATENT REQUIREMENTS A method of producing a crystalline surface layer consisting essentially of gold and a second metal selected from the group consisting of Bi, Ga, ln and Te on a substrate, the method comprising the steps of providing a substrate having a substantially oxide-free and pure 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 molten phase of said second metal, said molten phase optionally further comprising gold, at a temperature at which there is an equilibrium between solid phase and molten phase for an intermetallic phase selected from the group consisting of Au2Bi, AuGaz, Auln; and AuTeg depending on the selected second metal, so 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 phase and molten phase for said intermetallic phase - removing any excess of said intermetallic phase second metal which has not reacted with gold from said surface so that the surface of the substrate consists mainly of crystals of said intermetallic phase, the removal of excess of said second metal being effected by etching. A method according to claim 1, in which the exposure of the surface to a molten phase of said second metal is effected by immersing the substrate in a melt of said second metal. A method according to claim 1 or 2, wherein the temperature during the exposure of said surface to a molten phase of said second metal is 230-365 ° C if the second metal is Bi, 50-350 ° C if the second metal is Ga, or 165-450 ° C if the other metal is ln. 10 20 25 534 81? 17 The method of claim 1, wherein the second metal is Te and the temperature during the exposure of said surface to a molten phase of said second metal is 400-450 ° C. A method according to any one of the preceding claims, in which said heat treatment is carried out at a lower temperature than the previous step of exposing said surface to a molten phase of said second metal. A method according to any one of the preceding claims, wherein the method further comprises a heat treatment prior to the removal of excess metal, said heat treatment being performed at a temperature where all phases are solid. The method of claim 1, wherein said etching comprises contacting the substrate with a spaced piece of said second metal via connecting means so that a galvanic circuit is formed in the etching solution. A method according to any one of the preceding claims, in which the coating of said second metal is carried out by electroplating or physical gas phase deposition. A method according to any one of the preceding claims, further comprising a heat treatment after removing excess of said second metal so that the second metal diffuses into the substrate, thus changing the composition of the crystal while substantially preserving its size and shape.