FR2718376A1 - Sintered titanium-based decorative article - Google Patents

Abstract

A novel Ti-based article consists of a mixt. of (a) a Ti matrix derived from titanium hydride and (b) one or more addn. elements selected from nitrides, carbides, carbonitrides, silicides and borides of gp. IIIa, IVb, Vb, VIb, VIIb and VIIIb elements (e.g. Fe, Ti, Si, Nb, Mo, Cr, W, V and Al) or their alloys, Al2O3 and ZrO2. Also claimed are (i) a process for mfg. the above article by sintering; and (ii) a method of hardening and colouring the surface of a Ti-based article made by the above process.

Description

TITANIUM-BASED ARTICLE. DUCTILE HARDNESS AND A
HIGH BRIGHTNESS. METHOD FOR MANUFACTURING SUCH
ARTICLE AND METHOD OF CURING AND COLORING
OF A SURFACE OF SUCH AN ARTICLE
The invention relates to an article based on titanium, in particular a decorative article such as a cladding piece for watchmaking, ductile having a high hardness and which, once polished, has a surface of intense gloss, as well than a process for manufacturing such an article, by sintering a mixture of titanium hydride powder (TiH2) and of particular addition elements.

The invention also relates to a method for hardening and coloring a surface of such an article.

 In general, the cladding parts for watchmaking such as cases, middle parts, bracelet links, watch dials or the like, ductile with high hardness and high gloss, are produced from austenitic stainless steels. One of the main reasons for this choice is that this type of steel can be easily stamped or stamped.

 However, these steels generally contain a large amount of nickel (approximately 10%), which constitutes a major drawback for the production of articles intended to come into contact with the skin. Indeed, nickel as well as cobalt are highly allergenic constituents capable of contaminating the wearer's skin, so that many countries have set up standards prohibiting the sale of articles in permanent contact with the skin, susceptible to such contamination. , especially by nickel.

 To overcome this drawback, manufacturers have developed watch cases, bracelets, etc. which use oxides such as aluminum oxide and zirconium oxide. These oxides do indeed respond to the problems of hardness and contamination, but nevertheless have the drawbacks of being very fragile and generally having an aspect far from the shine of a metal and of being costly to use.

 Hard articles are also known which are made from titanium carbide or tungsten carbide.

However, although having a high hardness and a shine approaching that of a metal, these articles cannot be sintered without the aid of a binder if it is desired that they retain good toughness. However, the currently known binders are allergenic so that the articles thus obtained are also likely to contaminate the skin of the carriers and therefore subject to the same restrictions of use as the articles based on austenitic steel mentioned above.

 The main object of the invention is therefore to remedy the drawbacks of the aforementioned prior art by providing a titanium-based article, such as a piece of clothing for the watch industry, which is ductile while having a high hardness comparable to that of a hard metal, which does not contain any allergenic element, and which, when treated properly, has a shine comparable to that of a metal.

 The invention also aims to provide a method of manufacturing by sintering such an article having a very low porosity.

 Another object of the invention is to provide a process for hardening and coloring the surface of such an article in order to increase the hardness of the latter up to a level equivalent to that of a ceramic and to give it an equivalent appearance to the neighbor. metallic appearance.

 To this end, the subject of the invention is an article based on titanium, characterized in that it is essentially composed of a mixture essentially comprising a titanium matrix and one or more addition elements chosen from the group comprising nitrides, carbides, carbonitrides, silicides, and borides of the metallic elements of groups 3a, 4a, 4b, 5b, 6b, 7b, and 8 of the classification table of the elements, as well as iron, hydride zirconium, silicon, niobium, molybdenum, chromium, tungsten and vanadium.

 The article according to the invention advantageously has a high hardness which varies, depending on the choice of addition elements and their quantity in the mixture, typically between 300 and 1200 HV. The article according to the invention is ductile and also has a high resistance to corrosion.

According to another aspect, the invention also relates to a process for manufacturing by sintering an article based on titanium, characterized in that it consists in:
- (a) provide with a mixture of a titanium hydride powder, a powder of an element or a combination of several addition elements chosen from the group comprising nitrides, carbides , carbonitrides, silicides and borides of the metallic elements of groups 3a, 4a, 4b, 5b, 6b, 7b and 8 of the classification table of the elements, as well as iron, zirconium hydride, silicon, niobium, molybdenum, chromium, tungsten and vanadium and a temporary binder,
- (b) injecting the mixture obtained into a mold to obtain an article of desired shape,
- (c) removing the binder,
- (d) heating the part under a hydrogen atmosphere to the desired sintering temperature,
- (e) replace the hydrogen atmosphere with a non-reactive atmosphere once the sintering temperature has been reached, and
- (f) cooling the article in the non-reactive atmosphere.

 Thanks to this process, sintered titanium articles are obtained which have a porosity of less than 1% and which, after polishing, have a gloss comparable to that of a metal. This process is thus particularly well suited to the production of decorative articles such as watch cases, bracelet links or the like.

According to yet another aspect, the subject of the invention is also a process for hardening and surface coloring of an article based on titanium obtained according to the process described above, characterized in that it consists of
- (g) place the article in an oven,
- (h) heating the article to a determined temperature and maintaining the article at this temperature for a determined time, and
- (i) circulating a gas flow comprising carbon and / or nitrogen around the article to form, on the surface of the article titanium carbide, titanium nitride, titanium carbonitride or a mixture of these.

 According to this process, and thanks to the formation of these new compounds on the surface, the article is given a surface hardness of up to 3000Hv. It is also possible, depending on the nature of the gas flow circulating around the article to be hardened, to color it and in particular to give it nuances of color close to those of gold.

 The invention will now be described in detail.

 Titanium hydride powder (TiH2), preferably having a high degree of purity (99.5%) and an average particle size of the order of a few microns, typically 10 microns, is mixed in a conventional manner with a or several addition elements in the form of a powder of an inorganic material preferably having a high hardness and a particle size substantially equivalent to that of the titanium hydride powder or a mixture of powders of such inorganic materials .

 These inorganic materials are chosen from the group comprising nitrides, carbides, carbonitrides, silicides and borides of the metallic elements of groups, 3a, 4a, 4b, 5b, 6b, 7b, and 8 of the classification table for elements, as well as iron, zirconium hydride, silicon, niobium, molybdenum, chromium, tungsten and vanadium.

 The titanium hydride powder typically represents 50 to 95% by volume of the mixture and preferably 80 to 90% by volume of the mixture.

 The proportion of titanium in the mixture makes it possible to influence the hardness of the article. A priori when the proportion of titanium in the mixture increases, the hardness of the article obtained from this mixture decreases.

However, it has been found that by using 80 to 90% of titanium in combination with one of the addition elements such as those mentioned above or a combination of such elements, it was possible, unexpectedly, to reach an interesting compromise. between the hardness, the machinability and the porosity of the articles produced from this mixture.

 The mixture of titanium hydride powders and of the addition element or elements thus obtained is then stirred, in a conventional manner, with a temporary binder in the form of granules, until a homogeneous mixture in the form of a paste is obtained. .

 Preferably, the binder is formed from a thermal polymer or copolymer but can also be formed from wax. This mixing is carried out at a temperature preferably between 1200 and 1800C depending on the nature of the binder used. Typically, with a thermal copolymer, the mixing temperature is around 1700C.

 The mixture in the form of a paste thus obtained is then injected into a mold having the shape of the part which it is desired to obtain, for example a watch case, with dimensions which take account of the narrowing of the part during the subsequent stages. of the process, shrinkage which is typically of the order of 15%.

The injection is preferably carried out at a temperature typically around 1400C.

 We then proceed to the elimination of the binder contained in the shaped part. This elimination of the binder is carried out thermally. To do this, the shaped part is introduced into an oven in which it is gradually brought to a temperature preferably between 2000 and 3000C. During this heating, the binder is gradually removed by evaporation. In order not to deteriorate the shape of the part, this heating is carried out in a time typically between 2 and 9 hours and, preferably, in 8 hours. It is also important that the elimination of the binder is complete to avoid any pollution of the part by the carbon and / or oxygen of the binder, which could lead to a deterioration of the mechanical properties of the part to be manufactured.

 Preferably, the removal of the binder is carried out under vacuum or in a hydrogen atmosphere in order, on the one hand, to avoid any oxidation of the binder during its removal, and, on the other hand, to increase the speed the process of removing the binder from the part without damaging its shape.

 According to a variant of the process and in particular in the case where the binder is a thermal polymer, the latter can also be eliminated chemically, by decomposition using a vapor of an appropriate acid.

After the complete removal of the binder from the part; the furnace atmosphere is replaced by a hydrogen atmosphere (if the removal of the binder has not already been carried out in a hydrogen atmosphere). Preferably, this hydrogen atmosphere is produced in the form of a flow circulating continuously in the furnace
At the same time, the temperature of the part is gradually increased until the desired sintering temperature is reached. The sintering temperature is typically between 1,000 and 1,400 C and preferably substantially equal to 1,200 C to avoid getting too close to the temperature where the part would begin to deform.

 This heating lasts approximately 2 to 8 hours. During heating, the titanium hydride gradually releases its hydrogen. In this regard it is important (according to the method of the invention) that the heating is not too rapid so as not to cause a rapid release of the hydrogen which could cause the formation of pores within the room and thereby even alter the shine of the article's surface after polishing it. Preferably, the heating rate is between 1500C and 2500C per hour.

 According to a variant of the process, sintering can also be carried out in a vacuum or in an argon atmosphere.

However, in such a case, the porosity of the articles obtained is of the order of 3% due to the violent release of the hydrogen from the titanium hydride at the time of heating which creates a large number of pores.

These pores appear in particular on the surface of the article after polishing and scatter the incident light, which prevents perfect specular reflection of the light reaching the article. The use of such articles is therefore in this case limited to applications where the aesthetic appearance plays only a small role.

 Furthermore, since titanium has a high reactivity at high temperature, it reacts, during heating to the sintering temperature and subsequently during sintering, with the addition element or elements defined above to form new types of titanium compounds. I1 occurs different types of reaction depending on the nature of the addition element (s) used. Four types of reaction have been noted.

The first type that has been noticed is a chemical shift reaction such as
Ti + SiC <TiC + TixSiy (1)
Advantageously, the high reactivity of titanium is used at high temperature to dissociate chemically very stable materials and form new titanium compounds having a high hardness.

The second type of reaction which has been noticed is a reaction with the addition of titanium to form a sub-stoichiometric titanium compound such as
xTi + TiC o TiC (1-x) (2)
With this type of reaction, the stoichiometry of the new titanium compound can be changed over a wide range and thus the hardness of the article can be adjusted.

The third type of reaction which has been noticed is that forming an alloy of the intermetallic type such as
xTi + yW o TixWy (3)
This type of reaction makes it possible in particular to obtain compounds having very high hardnesses.

Finally, the fourth type of reaction which has been noticed is that forming particles of hard materials, for example particles of tungsten carbides, embedded in a titanium matrix such as
xTi + WC -, WC in a titanium matrix (4)
This latter type of reaction increases the hardness of the titanium article and leaves enough titanium on the surface to allow a chemical reaction with other elements during a subsequent surface treatment such as a coloring treatment and / or surface hardening.

 In this regard, it will be recalled that the hardness of the article obtained according to this process depends in particular on the nature and the quantity of the compound or compounds which result from reactions (1) to (4) between the titanium and the element (s) of 'addition.

 Once the sintering temperature has been reached and the hydrogen in the part has been released for the most part, the atmosphere of the furnace, ie hydrogen, is again replaced by a non-reactive atmosphere such as an atmosphere of argon or helium, or vacuum. Argon will be preferred. The replacement of hydrogen by the non-reactive atmosphere or the vacuum is done while maintaining the article at its sintering temperature. The duration of this replacement is typically between 5 and 80 minutes and is preferably about 20 minutes.

 The part is then cooled to ambient temperature in said non-reactive atmosphere at a cooling rate of the order of 3000C per hour. During this cooling, the part slowly releases the rest of its hydrogen which is eliminated gradually.

 The sintered article obtained by the process which has just been described is composed of a mixture comprising a titanium matrix derived from titanium hydride and the element or elements of addition used during step (a ) described above.

 This titanium-based article has a remarkably low porosity, typically from 2% to 0.1%.

 In addition, this article has a hardness which can typically vary from 300 to 1200HV depending on the amount and type of addition element (s) added.

 The article may then be subjected to a specular polishing of its surface in order to obtain a decorative article such as a watch case, a bracelet link, a dial or the like, having a surface of a polish and a shiny comparable to that of a metal part.

 As mentioned above, according to another aspect of the invention, the articles obtained by the process described above can be subjected to a hardening and coloring process.

 This hardening and coloring process can be carried out before or after polishing the article.

Also, for example for reasons of convenience, any polishing and / or machining operation of the article can be carried out before the implementation of this hardening and coloring process so that the article, in its form almost definitive or definitive, can be hardened and colored while retaining its shine and its shape.

 To harden and color the article, if already already machined in its final and / or polished form, the article is placed in an oven and heated to a temperature typically between 600 and 10000C and preferably around 8000C.

Once this temperature has been reached, a flow of gas comprising carbon and / or nitrogen is circulated around the article to be treated for a period of the order of 10 to 30 minutes, the article being kept at temperature to which it was previously heated.

 As the carbon-containing gas, a hydrocarbon gas such as methane, propane, butane, etc. can be used.

 When nitrogen is used, care should be taken to ensure that it is as pure as possible and in particular contains only minimum quantities of water and oxygen.

 To obtain a satisfactory result, the gas stream which comprises carbon and / or nitrogen is preferably diluted in an inert gas such as argon or helium.

 Where appropriate, the proportion of nitrogen in the gas flow is typically between 50 and 100% and preferably of the order of 95.

 Likewise, the proportion of the gas containing carbon in the gas flow is typically between 2% and 20% and preferably of the order of 5%.

 In the case of a mixture of nitrogen and a carbon-containing gas, the above-mentioned proportions for each of the gases must be respected.

 The depth to which the surface is cured depends on the temperature of the article and the reaction time of the article with carbon and / or nitrogen from the gas flow circulating around it.

 It will be noted however that in the case where the article has been previously polished, the latter will be heated to a temperature such that the reactions for the formation of carbides, nitrides and carbonitrides are sufficiently slow to obtain a hard surface which retains its appearance polished.

 By using a gas stream containing only carbon, an article having a metallic appearance is obtained.

 On the other hand, if a gas flow containing only nitrogen is used, an article having a colored appearance is obtained.

 By using a mixture of carbon and nitrogen in the gas flow, one can obtain, by varying their respective proportions, all the nuances of colors between yellow and brown and in particular the color gold.

 It will further be noted that this hardening and coloring process reinforces the corrosion resistance properties of the article. This reinforcement results from the properties of the elements formed in the surface layer which are similar to those of ceramics.

 The increase in the hardness of the article depends on several factors and in particular on the reaction of the compound or compounds which result from reactions (1) to (4) with the gas flow which circulates in the oven.

 A few examples of the implementation of the sintering process for manufacturing a titanium-based article according to the invention will be described below.

Example 1
10% by volume of titanium carbonitride powder (TiCN, 51 g) and 90% by volume of titanium hydride (TiH2, 333 g) are poured into a conventional mixer containing a solvent, for example cyclohexane.

 These elements are mixed for about 30 minutes at room temperature. The homogeneous mixture thus obtained is then dried and then poured into a container containing a binder in the form of a copolymer comprising 32% by volume of polyethylene oxide (246 g) and 4% by volume of polypropylene (26 g). This binder and this mixture are heated to a temperature of around 1700C until a homogeneous paste is obtained.

 Next, the cooled dough is granulated. The granules obtained are then introduced into an injection press and injected into a mold, for example having the shape of a watch case, at a temperature of around 1400C.

 The article conformed by this invention is then introduced into an oven in which a vacuum of about 10-2 millibar is then created. The article is then brought to a temperature of about 2000C by linear heating in 8 hours.

 The article is then sintered. To do this, the vacuum of the oven is replaced by a hydrogen atmosphere in the form of a hydrogen flow having a flow rate of 150 ml / min, and the part is brought from 2000C to 1'2000C linearly in 4 hours. Once the temperature of 1'2000C is reached, the hydrogen atmosphere is replaced by an argon atmosphere in the form of an argon flow having a flow rate of 150 ml / min, and the temperature is maintained at 1 ' 2000C for approximately 20 minutes.

 The article is then cooled linearly under the same argon atmosphere to room temperature.

The cooling rate is 3000 ° C. per hour and an article based on sintered titanium is thus obtained in which the titanium carbonitride has been combined with the titanium by a reaction of the type of reaction (2) above and whose porosity is less than 1%. The hardness measured on the article is of the order of 780HV.

 The sintered article is finally subjected to electropolishing to obtain a watch case having an intense shiny appearance comparable to that of a piece of polished metal.

 In a variant of the above example, polyacetal is used as a binder and the latter is removed by decomposition in a vapor of nitric acid at 1200 C. The result obtained with this variant is identical to that obtained with the previous example .

Example the
In this example, the same steps were used as those described in Example 1 with 20% by volume of titanium carbonitride powder (TiCN, 102 g) and 80% by volume of hydride as starting material. titanium (TiH2, 296 g). There was thus obtained an article based on sintered titanium whose porosity is less than 1%. The hardness measured on this article is around 930HV. The appearance of the article obtained is comparable to that of a piece of polished metal.

Example 2
In this example, the same steps were used as those described in Example 1 with 10% by volume of silicon carbide powder (SiC, 32 g) and 90% by volume of hydride as starting materials. titanium (TiH2, 333 g). There was thus obtained an article based on sintered titanium in which the silicon carbide was dissociated in a reaction similar to the reaction (1) above to form titanium silicide. The porosity of this article is less than 1%. The hardness measured on this article is around 900HV. The appearance of the article obtained is comparable to that of a piece of polished metal.

Example 2a
In this example, the same steps were used as those described in Example 1, with 20% by volume of silicon carbide powder (SiC, 64 g) and 80% by volume of titanium hydride as starting materials ( TiH2, 296 g). There was thus obtained an article based on sintered titanium whose porosity is less than 1%. The hardness measured on this article is around 1300HV. The appearance of the article obtained is comparable to that of a piece of polished metal.

Example 3
In this example, the same steps were used as those described in Example 1 with 10% by volume of tungsten carbide powder (wC, 156 g) and 90% by volume of hydride as starting materials. titanium (TiH2, 333 g). There was thus obtained an article based on sintered titanium in which the tungsten carbide is embedded in a titanium matrix and whose porosity is less than 1% The hardness measured on this article is of the order of 630Hv. The appearance of the article obtained is comparable to that of a piece of polished metal.

Example 4
In this example, the same steps were used as those described in Example 1 with 10% by volume of chromium carbide powder (Cr3C2, 67 g), 10% by volume of carbide powder as starting materials. tungsten (WC, 156 g) and 80% by volume of titanium hydride (TiH2, 296 g). There was thus obtained an article based on sintered titanium whose porosity is less than 1%. The hardness measured on this article is around 720HV.

 The appearance of the article obtained is comparable to that of a piece of polished metal.

Example 5
In this example, the same steps were used as those described in Example 1 with 10% by volume of chromium carbide powder (Cr3C2, 67 g), 10% by volume of carbide powder as starting materials. of titanium (TiC, 49 g) and 80% by volume of titanium hydride (TiH2, 296 g). There was thus obtained an article based on sintered titanium whose porosity is less than 1%. The hardness measured on this article is around 920HV.

The appearance of the article obtained is comparable to that of a piece of polished metal.

Example 6
In this example, the same steps were used as those described in Example 1, with 10% by volume of chromium carbide powder (Cr3C2, 156 g) and 10% by volume of carbide powder as starting materials. of titanium (TiC, 49 g) 50% by volume of titanium hydride (TiH2, 185 g) and 30% by volume of zirconium hydride (ZrH2, 168). There was thus obtained an article based on sintered titanium whose porosity is less than 1%. The hardness measured on this article is around 770HV. The appearance of the article obtained is comparable to that of a piece of polished metal.

Example 7
In this example, the same steps were used as those described in Example 1 with 20% by volume of titanium carbide powder (TiC, 98 g), 50% by volume of hydride as starting materials. titanium (TiH2, 185 g) and 30% by volume of zirconium hydride (ZrH2, 168>. An article based on sintered titanium was thus obtained whose porosity is less than 1%. The hardness measured on this article is 850HV The appearance of the article obtained is comparable to that of a piece of polished metal.

 The examples which follow now are preferred examples of the implementation of the process for hardening and coloring of a titanium-based article according to the invention.

Example lb
In this example, we started with the article based on sintered and polished titanium described in example 1. The article is placed in an oven in which a vacuum is made of approximately 10-2 millibar or containing a flow of with a flow rate of 150 ml / min. The article is then heated to a temperature of 10000C by linear heating in 3 hours. Once this temperature has been reached, a gas flow is circulated in the oven enclosure comprising 50% nitrogen and 50% argon by volume at a flow rate of 150 ml / min. This temperature and this gas circulation are maintained for approximately 20 minutes.

 The article is then gradually cooled to room temperature.

 During this hardening and coloring process, the nitrogen in the gas flow reacts with the sub-stoichiometric titanium carbonitride resulting from reaction (2) to give on the surface a compound comprising titanium carbonitride with a new sub-stoichiometry of the carbon and nitrogen.

 One then obtains an article based on titanium having, over a thickness of approximately 10 microns, a hardness of the order of 2800HV and a golden yellow metallic appearance close to that of gold.

Exemole 7b
In this example, the same steps were used as those described in example 1b, starting from an article based on sintered and polished titanium according to the method described above and which comprises as starting materials 20% by volume. of titanium carbide powder (TiC, 98 g), 50% by volume of titanium hydride (TiH2, 185 g of polished titanium base manufactured according to the process of the invention using as starting material 5% of carbide titanium (TiC, 25 g) and 95% titanium hydride (TiH2, 352 g), however, in this example, a gas stream comprising 2% methane diluted in argon was used, the article being previously heated to 8O00C.

In this case, the sub-stoichiometric titanium carbide which results from reaction (2) reacted with the carbon of the gas flow to form stoichiometric titanium carbide. The hardness measured on this article is around 800HV and its appearance is comparable to that of a piece of polished metal.

Claims (19)

 1. Titanium-based article, characterized in that it is essentially composed of a mixture comprising a titanium matrix derived from titanium hydride and an element or a combination of several addition elements chosen from the group comprising nitrides, carbides, carbonitrides, silicides and borides of the metallic elements of groups, 3a, 4a, 4b, 5b, 6b, 7b, and 8 of the classification table of the elements, as well as iron, hydride zirconium, silicon, niobium, molybdenum, chromium, tungsten and vanadium.  2. Titanium-based article according to claim 1, characterized in that the titanium matrix represents 50 to 95% and preferably 80 to 90% by volume of said mixture. TiCN, WC, SiC, Cr3C2, TiAl, Ces6, TiB2, TiSi2, CoSi2, AlN, NbC, MoC, TaC, ZrC and ZrH2.  3 Titanium-based article according to claim 1 or 2, characterized in that the addition elements are chosen from the group comprising TiN, TiC,  4. Titanium-based article according to claim 3, characterized in that it comprises 5 to 10% by volume of one or more of the addition elements chosen from TiCN, SiC and WC.  5. A method of manufacturing by sintering an article based on titanium, characterized in that it consists of  - (a) provide with a mixture of a temporary binder, a powder of titanium hydride, and a powder of an element or a combination of several addition elements chosen from set comprising the nitrides, carbides, carbonitrides, silicides and borides of the metallic elements of groups, 3a, 4a, 4b, 5b, 6b, 7b, and 8 of the classification table of the elements, as well as iron, l '' zirconium hydride, silicon, niobium, molybdenum, chromium, tungsten, vanadium, aluminum oxide and zirconium oxide,  - (b) injecting the mixture obtained into a mold to obtain an article of desired shape,  - (c) removing the binder,  - (d) heating the part under a hydrogen atmosphere to the desired sintering temperature,  - (e) replace the hydrogen atmosphere with a vacuum or a non-reactive atmosphere once the sintering temperature has been reached, and  - (f) cooling the room in the non-reactive atmosphere.  6. Method according to claim 5, characterized in that step (d) comprises heating the article to a temperature between 1'000 and 1'4000C,  7. Method according to claim 6, characterized in that step (d) is carried out for a time between 4 and 8 hours.  8. Method according to one of claims 5 to 7, characterized in that during step (d). hydrogen is supplied as a continuous stream.  9. Method according to one of claims 5 to 8, characterized in that during step (e), the article is maintained at said sintering temperature for a time between 5 and 80 minutes.  10. Method according to one of claims 5 to 9, characterized in that in steps (e) and (f) the non-reactive atmosphere comprises argon or helium  11. Method according to one of claims 5 to 10, characterized in that step (c) is carried out chemically and / or thermally.  12. Method according to claim 11, characterized in that step (c) is carried out under a non-reactive atmosphere or under vacuum at a temperature below 3000C.  13. Method according to claim 11, characterized in that step (c) is carried out for a time between 2 and 9 hours.  14. Method according to claim 11, characterized in that the binder is a thermal polymer and in that the step (c) comprises the chemical decomposition of the thermal polymer by an acid vapor.  15. A method according to any of claims 5 to 14, characterized in that it comprises an additional step during which the article is subjected to specular polishing.  16. A method of hardening and coloring on the surface of a titanium-based article obtained according to the method defined by one of claims 5 to 14, characterized in that it consists of  - (g) place the article in an oven,  - (h) heating the article to a determined temperature and maintaining the article at this temperature for a determined time, and  (i) circulating a gas flow comprising carbon and / or nitrogen around the article to form, on the surface of the article titanium carbide, titanium nitride, titanium carbonitride, or a mixture of these.  17. A method of hardening and surface coloring according to claim 16, characterized in that during step (h) the article is brought to a temperature between 600 and 1000 C and preferably to a temperature of 8000 and in that this temperature is maintained for 10 to 30 minutes.  18. A method of hardening and surface coloring according to claim 16 or 17, characterized in that the gas stream comprising carbon comprises methane and propane.  19. A method of hardening and coloring on the surface according to one of claims 16 to 18, characterized in that it is applied to an article based on polished titanium obtained according to the method defined in claim 15.