CH687622A5 - Article based on titanium, ductile hardness and high gloss, method of making such an article and curing process and for coloring a surface of such an 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

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Description

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, exhibits a surface of intense gloss, as well as a method of manufacturing such an article, by sintering a mixture of titanium hydride powder (TiH 2) 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. ticks. 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 implement.

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 allergens 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 these steels.

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 watchmaking, which is ductile and which, while having a high hardness comparable to that of a hard metal, which does not contain any allergenic element, and which, when properly treated, 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.

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 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 table classification of elements, as well as iron, zirconium hydride, 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 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, carbo-nitrides, 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, nio-bium, 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 article in the atmosphere of the non-reactive gas.

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 above-described process, characterized in that it consists in:

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- (g) place the article in an oven,

- (h) heating the article to a determined temperature, 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, or gives the article a surface hardness of up to 3000 HV.

The invention will now be described in detail.

Titanium hydride powder (TÌH2), 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 one or more addition elements in the form of a powder or a mixture of inorganic powders preferably having a high hardness and a particle size substantially equivalent to that of the titanium hydride powder in order to obtain a mixture homogeneous.

These inorganic powders are chosen from the group comprising nitrides, carbides, carbonitrides, silicides, borides of the metallic elements of groups, 3a, 4a, 4b, 5b, 6b, 7b, and 8 of the classification table for elements, and among the set comprising 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 according to the prior art accepted 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 connection with one or a combination of the addition elements such as those mentioned above, it was possible, unexpectedly, to reach an interesting compromise between hardness , the machinability, and the porosity of the articles produced from this mixture.

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

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 120 ° and 180 ° C. depending on the nature of the binder used. Typically, with a thermal copolymer, the temperature of the mixture is of the order of 170 ° C.

The mixture in the form of a paste which is obtained is then injected, in a conventional manner, 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, shrinking which is typically of the order of 15%. The injection is preferably carried out at a temperature typically around 140 ° C.

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 then introduced into an oven in which it is gradually brought to a temperature preferably between 200 ° and 300 ° C. During this heating, the binder is gradually eliminated by evaporation and, 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 removal of the binder is complete to avoid any pollution of the part by the carbon and / or oxygen of the binder, which can 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 of the process of removing the binder from the part without deteriorating the shape of the part.

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 an appropriate acid vapor.

After the complete removal of the binder from the part and, according to a particularly important aspect of the invention, the atmosphere of the oven 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 1000 and 1400 ° C and, preferably, substantially equal to 1200 ° C to avoid getting too close to a temperature where the part would begin to deform.

This heating lasts approximately 2 to 7 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 alter the gloss of the article's surface after it has been polished. Preferably, the heating rate is between 150 ° C and 250 ° C per hour.

According to a variant of the process, sintering can also be carried out in a vacuum or in an argon atmosphere. However, the porosity of the articles obtained is of the order of 3% due to the violent release of hydrogen from titanium hydride at the time of heating which creates a number

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large pore size. 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 the latter is therefore in this case limited to applications where the aesthetic appearance plays only a small role.

Furthermore, taking into account the high reactivity of titanium at high temperature, the titanium reacts, during the heating up to the sintering temperature and subsequently during the sintering, with the addition element or elements defined above. to form new types of titanium compounds. Different types of reaction occur depending on the nature of the addition element (s) used. Four types of reaction have been noted.

The first type of reaction that has been noticed is a chemical displacement type 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 observed is a reaction of the type with addition of titanium to form a sub-stoichiometric titanium compound such as:

xTi + TiC - »TiC (i-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 a reaction for forming an alloy of the intermetallic type such as.

xTi + yW -> 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 observed is a reaction forming particles of hard materials such as particles of tungsten carbides embedded in a titanium matrix.

xTi + WC - »WC in a titanium matrix (4)

This latter type of reaction increases the hardness of the titanium article and leaves sufficient titanium on the surface for a chemical reaction with other elements during a subsequent surface treatment such as a coloring 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, that is to say the 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 is done while maintaining the article at its sintering temperature. The duration of this stage 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 300 ° C 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 one or a combination of several addition elements chosen from the group comprising nitrides , carbides, carbonitrides, silicides, borides of the metallic elements of groups, 3a, 4a, 4b, 5b, 6b, 7b, and 8 of the classification table of the elements, and among the set including iron, zirconium hydride, silicon, niobium, molybdenum, chromium, tungsten, and vanadium.

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 1200 HV 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 gloss comparable to that of a metallic piece.

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 quasi-form definitive or definitive, can be hardened and colored while retaining the 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 until it reaches a temperature typically between 600 and 1000 ° C and preferably around 800 ° C. 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.

The carbon-containing gas stream includes

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hydrocarbon gases such as methane, propane, butane etc.

With regard to the flow of nitrogen-containing gas, care will be taken to ensure that the nitrogen 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.

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

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 character 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, it is possible to obtain, by varying their respective proportion, all the shades of color between yellow and brown and in particular the color gold.

It will further be noted that this hardening and coloring process further enhances 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.

The examples which follow are preferred examples of the implementation of the method of manufacturing by sintering of a titanium-based article according to the invention.

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 obtained is then dried and then poured into a container containing a binder formed of a copolymer comprising 32% by volume of polyethylene oxide (246 g) and 4% by volume of polypropylene (26 g). This binder is heated to a temperature of about 170 ° C until a homogeneous paste is obtained.

The cooled mixture is then 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 approximately 140 ° C.

The shaped article is then introduced into an oven in which a vacuum of approximately 10 ~ 2 millibar is then made. The article is then brought to a temperature of about 200 ° C 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 200 ° C to 1200 ° C linearly in 4 hours. . Once the temperature of 1200 ° C 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 1200 ° C for about 20 minutes.

The article is then cooled linearly under the same argon atmosphere to room temperature. The cooling rate is 300 ° C. per hour and an article based on sintered titanium is thus obtained in which the titanium carbonitride has been combined with the titanium according to reaction (2) and the porosity of which is less than 1%. The hardness measured on the article is of the order of 780 HV.

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 binder and the latter is removed by decomposition in a vapor of nitric acid at 120 ° C. The result obtained with this variant is identical to that obtained with the previous example.

Example 1a

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 (TÌH2, 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 930 HV. The appearance of the article obtained is comparable to that of a polished metal part.

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 (TÌH2, 333 g). There was thus obtained an article based on sintered titanium in

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which silicon carbide was dissociated according to reaction (1) to form titanium silicide. The porosity of this article is less than 1%. The hardness measured on this article is of the order of 900 HV. The appearance of the article obtained is comparable to that of a polished metal part.

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 hydride as starting materials. titanium (TÌH2, 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 1300 HV. The appearance of the article obtained is comparable to that of a polished metal part.

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 (TÌH2, 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 around 630 HV. The appearance of the article obtained is comparable to that of a polished metal part.

Example 4

In this example, the same steps were used as those described in Example 1, starting with 10% by volume of chromium carbide powder (CrsC2, 67 g), 10% by volume of carbide powder tungsten (WC, 156 g) and 80% by volume of titanium hydride (TÌH2, 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 of the order of 720 HV.

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

Example 5

In this example, the same steps were carried out 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 (TÌH2, 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 of the order of 920 HV. The appearance of the article obtained is comparable to that of a polished metal part.

Example 6

In this example, the same steps were carried out as those described in Example 1 with 10% by volume of chromium carbide powder (Cr3C2, 156 g), 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 of the order of 770 HV. The appearance of the article obtained is comparable to that of a polished metal part.

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 (TÌH2, 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 850 HV. The appearance of the article obtained is comparable to that of a polished metal part.

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

Example 1b

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 150 ml / min. The article is then heated to a temperature of 1000 ° C. by linear heating for 3 hours. Once this temperature has been reached, a gas flow comprising 50% nitrogen and 50% argon is circulated in the oven enclosure at a flow rate of 150 ml / min. This temperature and this gas circulation are maintained for approximately 20 minutes.

The article is then cooled linearly 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- carbon and nitrogen stoichiometry.

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

Example 7b

In this example, the same steps were used as those described in Example 1b

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starting from an article based on sintered and polished titanium according to the process described above and comprising as starting materials 20% by volume of titanium carbide powder (TiC, 98 g), 50% by volume of hydride of titanium (TÌH2, 185 g) and 30% by volume of zirconium hydride (ZrH2, 168). The article obtained after hardening and coloring has a golden yellow metallic appearance close to that of gold, and has a porosity of less than 1%. The hardness measured on this article is of the order of 1800 HV.

In this case, the nitrogen reacted with the sub-stoichiometric zirconium carbide and with the sub-stoichiometric titanium carbide which result from reaction (2) to form on the surface (over a thickness of approximately 10 microns) a mixture of titanium carbonitride and of products resulting from the reaction of zirconium carbide with nitrogen.

Example 8

In this example, the same steps were carried out as those described in Example 1b with an article based on polished titanium manufactured according to the method of the invention using as starting material 5% of titanium carbide (TiC , 25 g) and 95% titanium hydride (TÌH2, 352 g). However, in this example, a gas flow comprising 2% of methane diluted in argon was used, the article being previously heated to 800 ° C. 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 of the order of 800 HV and its appearance is comparable to that of a polished metal part.

Claims (19)

Claims 1. Titanium-based article, characterized in that it is essentially composed of a mixture comprising a titanium matrix derived from titanium hydride and one or a combination of several addition elements chosen from the group comprising: nitrides, carbides, carbonitrides, silicides, borides of the metallic elements of groups, 3a, 4a, 4b, 5b, 6b, 7b, and 8 of the classification table of the elements, and among the set including iron, zirconium hydride, silicon, niobium, molybdenum, chromium, tungsten, vanadium, aluminum oxide and zirconium oxide. 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. 3. Titanium-based article according to claim 1 or 2, characterized in that the addition elements are chosen from the group comprising TiN, TiC, TiCN, WC, SiC, Cr3C2, Ti Al, CeBe, TiB2, TiSi2, C0SÌ2, AIN, NbC, MoC, TaC, zrC and ZrH2. 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. Method of manufacturing by sintering an article based on titanium, characterized in that it consists in: (a) be provided with a mixture of a titanium hydride powder, a powder of one or a combination of several addition elements chosen from the group comprising nitrides, carbides, the carbonitrides, the silicides, the borides of the metallic elements of groups, 3a, 4a, 4b, 5b, 6b, 7b, and 8 of the table of classification of the elements, and among the group comprising iron, zirconium hydride , silicon, niobium, molybdenum, chromium, tungsten, vanadium aluminum oxide and zirconium oxide 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 vacuum or a non-reactive atmosphere once the sintering temperature has been reached, and - (f) cooling the part in the atmosphere of the non-reactive gas. 6. Method according to claim 5, characterized in that step (d) consists in heating the article to a temperature between 1000 and 1400 ° C. 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), the hydrogen is supplied in the form of a continuous flow. 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 300 ° C. 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 step (c) comprises the chemical decomposition of the polymer by an acid vapor. 15. Method according to any one 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 5 10 15 20 25 30 35 40 45 50 55 60 65 7 13 CH 687 622 A5 - (h) heating the article to a determined temperature, 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 800 ° 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. 5 10 15 20 25 30 35 40 45 50 55 60 65 8