EP0221492A2 - Article coated with a wear-resistant film of a precious metal - Google Patents

Abstract

The object is covered with a wear-resistant and corrosion-resistant coating, said coating consisting mainly of a precious metal or a precious metal alloy, for example gold or an alloy containing gold. The coating consists of a first layer (3) of said precious metal or its alloy (4) comprising discrete inclusions of a metal compound (5) consisting for example of titanium nitride. These inclusions are co-deposited in the vapor phase with the precious metal and distributed in a substantially homogeneous manner throughout the thickness of the layer, said thickness being equal to or greater than 0.4 μm. A second layer (6) made of a precious metal or an alloy of said metal is disposed between the object and said first layer (3). The object can be a watch case, a bracelet link or a piece of jewelry.

Description

The invention relates to an object, in particular a watch case, surmounted by a wear-resistant coating and consisting mainly of a precious metal from the group of gold, silver, rhodium, palladium, platinum, iridium, osmium and ruthenium or an alloy of said precious metal, said coating consisting of a first layer of said precious metal or its alloy, said first layer comprising discrete inclusions of a compound metallic consisting of an oxide, a nitride, a carbide, a boride, a phosphide, a silicide or a fluoride, said inclusions being distributed in a substantially homogeneous manner throughout the thickness of the first layer, said first layer being applied by deposition in vapor phase (CVD or PVD), its thickness being equal to or greater than 0.4 µm.

Wear-resistant and corrosion-resistant coatings containing a precious metal, in particular gold, have already been described many times. The aim is to obtain a product with good scratch resistance while having the appearance and luster of gold.

This is the case, for example, of the object defined in application GB-A-2,000,812 where a layer of noble metal or alloy is deposited on a first layer of TiN, TaN, TaC, ZrN or VN either by evaporation and ion plating (examples 1 and 2), or by electrochemical plating followed by a diffusion heat treatment (examples 3 and 5). It is also envisaged to successively make layers of Ti, of TiN, of a mixture of TiN and of noble metal, and of gold or of a gold alloy. The coatings obtained in all these embodiments consist of superimposed layers of very different compositions, deposited one after the other. They are therefore likely to present defects in uniformity or resistance to scratching or thermal shock at the interfaces between the different layers.

It is also provided in the same British patent application (Example 4) to form, after the deposition by evaporation and ion plating of a layer of TiN, a second layer obtained by the simultaneous evaporation of two distinct sources formed respectively of Ti and Cu. The above remarks still apply, but, moreover, it will be noted that this latter method does not make it possible to obtain a layer of noble metal on the external surface of the coating.

In the object described in the aforementioned document, the surface gold layer must be thin (equal to or less than 1 μm) for the coating to produce the expected effect, namely to make scratches caused by foreign bodies not very visible. In this the coating described does not allow to reach a real plating containing enough precious metal so that said object can carry the maker's mark.

To overcome the drawbacks mentioned in the document above, patent EP-B-0038294 (= US 4,403,014) proposes a simple process, making it possible to deposit hard, metallic coatings whose composition varies gradually from the surface of the object towards the external surface, and this by means of a source subjected to a single process of evaporation or spraying. Thus, during deposition, we simultaneously increase the volume concentration of precious metal and decrease that of a metal compound (for example TiN) deposited according to opposite gradients by means of a source comprising gradients of concentration in volume of opposing precious metal and metal compound. Another embodiment of the cited invention starts from two separate sources constituted respectively by the noble metal and the metallic compound, these sources being subjected to an evaporation process at increasing, respectively decreasing, speeds.

The process which has just been described has the drawback of having to use either a composite source whose thickness must be in relation to the thickness of the deposit to be made, or a delicate adjustment of the depositing apparatus which must be able to precisely control the variation in spray speed during the process. On the other hand this process leads by definition to deposits of which the volume percentage of gold and of compound metallic is identical which does not allow to reach a gold deposit with very high title. Finally, it will be noted that the process is ill-suited to depositing plating in thick layers because it is slow and expensive.

The coatings which are the subject of the two documents cited above, although being different in their composition, have a common characteristic: they both have a layer of precious metal, for example gold, deposited on underlying layers much harder than that of precious metal, for example titanium nitride. If the surface layer is scratched, it will be only slightly deep and the scratch will be only slightly visible, and this all the more if the underlying layer is of a color close to that of the precious metal deposited on the surface.

The optical, electrical, mechanical and corrosion resistance properties of a gold film comprising silica particles (Si0 ₂ ) and deposited by a vapor phase process were discussed in the review "Thin Solid Films" vol. 39 (1976) p. 165 to 174, Elsevier Sequoia, Lausanne, Taylor et al: "Properties of metal - dielectric codeposited films". The article in question first analyzes the optical properties of such a layer. In particular, the spectral reflectivity of pure gold is compared to that of a composite film of gold and Si0 ₂ . Very briefly the article mentions that the scratch resistance of the compound Au-Si0 ₂ is very good in the whole range of composition examined, said resistance being much better than that of a film of pure gold. This remark, purely qualitative, is not supplemented by any quantitative data. The article in question gives no teaching on the modifications of the optical and mechanical properties due to an inhomogeneous structure (columnar growth at low density, for example) which could appear for film thicknesses of the order of 0.4 vm or more.

Document JP-A-59 185774 describes a plating of a gold alloy deposited in the vapor phase and further comprising hardening agents in the form of nitrides. Thanks to this contribution, the hardness and the corrosion resistance of the plating are improved.

Document JP-A-60 114567 proposes the use of a precious metal codeposited with a transparent oxide to form the coating of an object which then has better resistance to wear and scratches. The use of gold is proposed in conjunction with Si0 ₂ deposited by methods such as vacuum deposition. The method can be applied to watch cases and makes it possible to reduce the gold content to one fifth of a conventional coating while increasing the abrasion resistance.

Finally, the document JP-A-60 67 654 describes a coating of an object formed of gold or of a gold alloy and of titanium nitride by evaporation under vacuum, for example. This improves the abrasion and scratch resistance of objects such as watches or spectacle frames.

In the last four documents cited above, the emphasis is on the thinness of the deposited layer, hence saving precious material, the concern being to provide a layer which is above all resistant to wear. There is no mention of thick layers of precious metal intended to carry the goldsmith's punch or also of thick layers to be colored while being resistant to wear.

It is the main object of the present invention to provide a coating comprising a second layer of a precious metal or of an alloy of said precious metal, second layer disposed between the object and the first layer, said first layer being deposited. according to the teaching of the state of the art revealed by the documents cited above.

• - la figure 1 est une coupe dans l'objet selon l'art antérieur,
• - la figure 2 est une coupe dans l'objet de l'invention,
• - la figure 3 est un diagramme présentant des mesures de réflectivité exécutées sur les objets obtenus par l'invention, et
• - la figure 4 est un diagramme rendant compte de mesures de résistance à l'usure des objets en question.

The invention will be better understood now with the aid of the description which follows and of the drawings which illustrate it by way of example and in which

• FIG. 1 is a section through the object according to the prior art,
• FIG. 2 is a section through the subject of the invention,
• FIG. 3 is a diagram showing reflectivity measurements carried out on the objects obtained by the invention, and
• - Figure 4 is a diagram showing measures of resistance to wear of the objects in question.

Figure 1 is a partial sectional view of the object according to the prior art. Object 1 can be a watch case, a link making up a bracelet or any other item of jewelry. he includes a box body or substrate 2 which can be made of noble metal, common metal (stainless steel, brass) or plastic. The substrate 2 is surmounted by a coating 3 called the first layer. This first layer consists mainly of a precious metal from the group of gold, silver, rhodium, palladium, platinum, iridium, osmium and ruthenium or an alloy of said precious metal, represented by an oblique hatch 4. This layer 3 also includes discrete inclusions of a metallic compound constituted by an oxide, a nitride, a carbide, a boride, a phosphide, a silicide or a fluoride, represented by points 5.

The term "discrete" means that these are inclusions, that is to say aggregates composed of a large number of molecules without chemical bonds with the precious metal. These inclusions are distributed in a substantially homogeneous manner throughout the thickness of the layer considered. This layer is obtained by vapor deposition, for example by CVD or PVD methods well known in the state of the art. These techniques are described in detail in the documents cited above and there is therefore no need to return to them here.

However, special precautions must be taken if one wishes to obtain a deposit whose appearance is that of the precious metal or one of its alloys. As the layer to be deposited is relatively thick, special care will be taken to cool the object to be covered throughout the process. This will avoid the low density columnar growths which have been mentioned above. The deposit is applied in a single operation, either from a composite source, or from two sources, one containing the metal, the other the metallic compound.

Thanks to the vapor deposition, the fine inclusions of metallic compound (of the order of 100 A) are distributed very evenly in the layer. This would not be the case if this layer was galvanically deposited from a bath containing metallic compounds in the form of powders whose dimensions would necessarily be much larger (of the order of 1 μm). In the latter case the distribution of inclusions in the layer would be less homogeneous because the dimension of these inclusions is of the same order than the thickness of the layer. This would result in a decrease in resistance to wear.

If we refer again to the section presented in FIG. 1 which presents the prior art, the coating comprises only a single first layer of precious metal and inclusions of metallic compound. If it is desired to produce an imitation of plating only, this layer will remain thin and its thickness will be between 0.5 and 1.5 μm for example. In addition to the metallic compound, this layer will include precious metal in the pure state or in the alloy state.

However, in the event that one wishes to place on the market a plated object authorized to carry the maker's mark, this layer will have to be much thicker to obey the various national legislations. To take the example of the legislation currently in force in Switzerland, the veneer must at least have a thickness of 8 pm and titrate at least 9 carats (C) to be authorized to carry the hallmark of the master. By using a single first layer 8 μm thick containing pure precious metal and deposited as indicated, the titer will be very high if the coating discussed above containing 6% by volume of TiN (more than 23 C) is used. ). To lower the title and therefore the price of the object, one could naturally increase the proportion of metallic compound, but one would then risk obtaining a reflectivity no longer corresponding to the color standard that one wishes to obtain, like this will be discussed below in connection with Figure 3.

To remedy this drawback, it is possible to use a precious metal alloy instead of pure precious metal, an alloy with a title equal to or greater than that required by legislation. One could for example choose a 14 C gold alloy containing in particular 14 parts of gold, 8 1/4 parts of silver and 1 3/4 part of copper and include TiN or Si0 ₂ , alloy and compounds therein. metallic being co-deposited in the vapor phase.

The fact remains that a single layer deposited according to the prior art is very expensive if it is desired to obtain thick layers because the vapor deposition is very slow and therefore expensive in time. We therefore come now to describe the object of the invention and we will therefore refer to FIG. 2.

Directly applied to the substrate 2 there is a second layer 6 consisting of a precious metal or an alloy of said precious metal, for example galvanically deposited. Above this second layer 6 is a first layer 3 composed of a precious metal or a precious metal alloy 4 and inclusions of metallic compound 5 defined during the description of FIG. 1. The second layer 6 can be pure precious metal if you want a high-grade coating or an alloy at 9, 14, 18 C depending on the title you want to reach.

This second embodiment has the advantage of being the cheapest since most of the coating is galvanically deposited, a very common process well known to specialists.

Spectral reflectivity and wear resistance measurements were carried out on wafers covered with the coating according to the invention, the thickness of the first layer 3 of which was at least equal to 4000 A (= 0.4 vm).

FIG. 3 reports spectral reflectivity measurements carried out on several samples using a spectrophotometer. We find the wavelength of light in nm (nanometer) on the abscissa and the ratio of the energy reflected to the total incident energy in percent on the ordinate. Curve A is that of fine gold (or pure gold), curves B and C are those of standard colors defined respectively by symbols 1N14 and 2N18 of the standards of the Swiss watch industry (NIHS) and curve M is that of gold with inclusions according to the invention. Curve M is obtained with inclusions of titanium nitride (TiN) in fine gold, these inclusions representing by volume 5 to 7% of the total volume occupied by the first layer. This curve shows that the reflectivity of the first layer is close to that of the standard colors of layers commonly used in the watch industry (curves B and C) and which are generally obtained by galvanic means. The layer studied here is naturally only an example and it is clear that by varying the percentage of inclusions and / or by replacing TiN with another metallic compound, it will be possible to obtain very different reflectivities and hence the colors related to other standards than those considered above.

An interesting particular case is that for which the amount of inclusions is such that the reflectivity of the first layer is substantially the same as the reflectivity of the metal in the pure state. It has been found, for example, that inclusions of silica (Si0 ₂ ) at less than 10% by volume in fine gold lead to a reflectivity curve which is almost identical to that of fine gold.

FIG. 4 is a diagram reporting measurements on the wear resistance of the coated articles according to the invention compared to the wear resistance of a galvanically coated object. In this example, it is a gold coating with inclusions at 6% by volume of TiN. The time in hours during which the samples are subjected to the wear test was plotted on the abscissa and the mass of material removed in milligrams (mg) on the ordinate. The samples to be tested are put in a smoothing machine by sliding with ceramic sticks, water and a wetting agent. The abrasive mass and the samples oscillate in several directions and undergo a smoothing pressure which is pulsed. The mass loss of the samples is determined as a function of the treatment time. Such a smoothing machine was described in communication no. 18 entitled: "The wear of watch cases in galvanic gold plated" from the 57th congress of the Swiss Chronometry Society, Montreux 22 and 23 October 1982.

The method which has just been described operates by smoothing or rubbing the surface to be examined. It differs in this from a simple hardness measurement with a microdurometer, because we realized that it is not always correct to correlate a hardness measurement and a determination of the resistance to wear. In the case of the present invention, the coating does not have a much higher hardness in the sense that it is usually understood, but rather an excellent resistance to wear if this is measured by simulating the phenomenon of wear. as it occurs naturally.

In FIG. 4, curve A 'represents the mass loss of a sample covered with galvanic fine gold and curve M' represents the mass loss of a sample covered with the coating according to the invention, the mass losses being measured according to the method indicated above under identical conditions. We notice that the loss of mass therefore the wear is about twice as low for the sample covered according to the invention this characteristic remaining substantially constant throughout the time that the test lasted (240 hours). In the work entitled "The clothing of the watch" by JP Renaud, LSRH, Neuchâtel June 3 and 17, 1975, it has been shown that the reduction in thickness of a layer of galvanic gold alloy grading 18 carats around 2 pm per year for a watch case to wear. This reduction will only be 1 pm for the same period if the watch case is coated with the layer defined by the invention, which represents a remarkable improvement.

Decorative coatings generally include a so-called color finishing phase. This important operation generally consists in depositing a layer of gold with a thickness of between 0.5 and 1.5 μm for 23.5 C. It aims mainly at two goals: that of checking the final color first ( aspect) of the plating, because the underlying plating composed of gold, silver, copper, cadmium can have very variable colors depending on the deposition conditions (different deposition rate of the metals constituting the alloy); that of preventing the color changes in the underlying plating (tarnishing) that would occur over time, changes that are all the more important the lower the titer.

In the coating according to the invention, the galvanic coloring phase is replaced by a phase pursuing the same goals but deposited in the vapor phase. In this case, however, as shown above, the wear resistance will be at least doubled, which will lead to a plating of higher quality and better reliability, thanks to the inclusions of metallic compound deposited at the same time as the precious metal.

To repeat what has been said about the Swiss legislation on the control of trade in precious metals, the coating deposited according to the invention must have a total thickness of at least 8 µm and titrate at least 9 carats. This coating could comprise a second layer of gold alloy with a thickness equal to or greater than 7 μm, second layer on which is deposited a first layer of thickness reaching at least 1 μm obtained in the vapor phase and containing gold and nitride inclusions of titanium occupying a volume between 5% and 7% of the total volume occupied by said first layer.

It is clear that other thicknesses and / or other proportions could be chosen without departing from the scope of the present invention. One could for example adopt the criteria resulting from the recommendations of the International Organization for Standardization (ISO).

The coating according to the invention also has a corrosion resistance which is comparable to that of precious metal without inclusions. For this, tests have been carried out according to conventional methods such as, for example, exposure to salt spray and periodic soaking in artificial sea water or artificial sweat.

Claims (6)

1. Object, in particular watch case, surmounted by a wear-resistant coating and consisting mainly of a precious metal from the group of gold, silver, rhodium, palladium, platinum, iridium , osmium and ruthenium or an alloy of said precious metal, said coating consisting of a first layer (3) of said precious metal or its alloy (4), said first layer comprising discrete inclusions of a compound metallic (5) consisting of an oxide, a nitride, a carbide, a boride, a phosphide, a silicide or a fluoride, said inclusions being distributed in a substantially homogeneous manner throughout the thickness of the first layer, said first layer being applied by vapor deposition (CVD or PVD), its thickness being equal to or greater than 0.4 µm, characterized in that said coating comprises a second layer (6) consisting of a precious metal or an alloy of said metal precious, and placed between the object and said first re layer (3). 2. Object according to claim 1, characterized in that said second layer is a galvanic deposit. 3. Object according to claim 1, characterized in that the amount of inclusions contained in said first layer is such that its spectral reflectivity is substantially the same as the spectral reflectivity of the precious metal or one of its alloys. 4. Object according to claim 1, characterized in that the total thickness of the coating and its precious metal title are such that said object is authorized to wear the maker's mark according to the legislation in force in the country where said object is put on the market. 5. Object according to claim 1, characterized in that the total thickness of the coating is equal to or greater than 8 µm, that the second layer (6) is a gold alloy of at least 9 carats and that the first layer (3) comprises gold and inclusions of titanium nitride. 6. Object according to claim 5, characterized in that the thickness of the second layer (6) is equal to or less than 7 pm, that the thickness of the first layer (3) is equal to or greater than 1 pm and that the titanium nitride inclusions occupy a volume between 5 and 7% of the total volume occupied by said first layer.

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