EP1743952B1 - Process for the treatment of titanium or titanium alloy parts. - Google Patents

Description

The present invention relates to a method of surface treatment of a titanium or titanium alloy part.

Surface treatments of titanium or titanium alloy parts intended to harden the surface of these parts in order to improve their service behavior, in particular their resistance to wear or their resistance to galling or any other property related to the contact of the surface with other media or other parts, are well known. These treatments consist in particular of ionic nitriding treatments of the parts. The purpose of the ionic nitriding treatments is to form on the surface of titanium or titanium-based alloy parts, a layer highly enriched in nitrogen whose properties of use are adapted to the intended uses. In these conventional ionic nitriding treatments, the titanium part or parts are arranged in a chamber containing a nitrogen-rich gas which is excited, for example by an electric discharge, so as to form a plasma containing nitrogen ions which react with the surface of the room and enrich it with nitrogen. Each piece is maintained at a temperature adapted so that the nitrogen can penetrate by diffusion inside thereof to a depth depending on the time during which it is subjected to contact with the plasma.

This technique has a number of disadvantages, in particular because, in order to obtain uniform treatment of the surfaces of the pieces, the pieces must be kept at a distance from each other. On the other hand the support surfaces of the parts on their support are poorly enriched in nitrogen. In addition, there are significant risks of making arc shots that lead to localized mergers of parts. Finally, the parts are processed in furnaces that are not always well temperature homogeneous, it results in dispersions in the properties of the different parts obtained.

Moreover, the nitriding treatment of titanium or titanium alloy parts leads to a coloration of these, depending on the chemical nature of the layer at the extreme surface. But this coloration is irregular, especially near any discontinuity of the surface of the pieces. Irregularities result in halo or edge effects that prohibit the use of this technique for appearance parts.

Finally, when the parts comprise hollow parts such as bores, defects can be generated by a phenomenon called "hollow cathode". This phenomenon results from the formation in the hollow parts of secondary electrons generated by ion bombardment, which generates new species. The result is a chain reaction that converts kinetic energy into caloric energy and can lead to melting of the part.

In order to overcome the drawbacks of conventional ionic nitriding of titanium or titanium-based alloy parts, it has been proposed, in particular in the European patent application. EP 1 274 873 a process for treating a part, in particular a titanium or titanium-based alloy part, according to which the part or a set of parts is placed in a container closed by a lid leaving a small gap, which is itself disposed within a reaction chamber containing a gas in which reactive species are generated via, for example, a plasma or an electric discharge. The container has a slot of sufficiently small thickness to prevent ignition of the plasma inside the container, but sufficiently wide to allow the diffusion through this slot of activated species.

In addition, in this process, the gases are specifically chosen so that the activated species formed deliver to the surface of the parts to be treated, two distinct elements selected from nitrogen, carbon, oxygen and boron.

With this method, an enriched hardened layer is obtained in two distinct interstitial elements taken from nitrogen, carbon, oxygen and boron. In particular, hardened surfaces are produced by simultaneous enrichment of nitrogen and carbon.

This method has the advantage of making it possible to treat parts taken in bulk while having a very uniform treatment of the entire surface of the parts. However, it has the disadvantage of not making it possible to obtain surface layers having hardnesses as high as those obtainable by a nitriding treatment of titanium or of a titanium alloy.

In addition, the treatment of simultaneously introducing nitrogen and carbon, which is generally done using a gas mixture consisting of nitrogen and methane, has the disadvantage of generating a very large amount of soot which pollute the treatment chamber and the parts that are being treated. Tests have been made to determine if it would not be possible to obtain better processing qualities using this process using nitrogen alone, that is to say by removing the methane. But these tests showed that the results were not satisfactory. Some hardening is obtained, but this hardening is substantially less than that obtained by conventional nitriding treatments.

The object of the present invention is to remedy these drawbacks by proposing a means of hardening the surface of titanium or titanium-based alloy parts, in particular alloy of the Ti6Al4V, Ti6Al7Nb type, or alloy containing mainly niobium and a high proportion of titanium such as the Nb30Ti20W type alloy, or made of any other alloy of the same type, which allows to obtain surface hardnesses as good as those obtained by conventional ionic nitriding treatments, without the disadvantages of these treatments, while allowing uniform treatments to be obtained on the surfaces of the parts, and possibly making it possible to treat a set of loose parts without there being irregularities in the treatment of the parts, particularly in the contact zones of the parts which are in bulk.

• on dispose la ou les pièces dans un conteneur qu'on dispose dans une enceinte de traitement contenant un milieu gazeux,
• on chauffe la ou les pièce(s) à une température de traitement,
• on génère au moins une espèce chimique activée par activation du milieu gazeux à l'extérieur du conteneur, la paroi du conteneur étant fermée à l'exception d'au moins un interstice dont la dimension d'ouverture empêche l'allumage du plasma à l'intérieur du conteneur mais permet le passage d'au moins une espèce activée,
• on laisse la ou les espèce(s) activée(s) au contact de la surface de la pièce ou des surfaces des pièces pendant un temps de traitement, et
• on laisse refroidir la pièce ou les pièces.
  Selon ce procédé, le milieu gazeux est constitué d'un mélange d'un gaz contenant de l'azote et de 1 % à 99 % d'au moins un gaz de dilution neutre de telle sorte que la composition chimique du milieu gazeux est constituée exclusivement d'azote et d'un ou plusieurs éléments pris parmi l'hydrogène et les éléments neutres tels que l'argon.

To this end, the subject of the invention is a process for the surface treatment of at least one titanium or titanium-based alloy part according to which:

• the piece or parts are placed in a container that is placed in a treatment chamber containing a gaseous medium,
• the piece (s) are heated to a treatment temperature,
• at least one activated chemical species is generated by activation of the gaseous medium outside the container, the wall of the container being closed with the exception of at least one gap whose opening dimension prevents the ignition of the plasma at the inside the container but allows the passage of at least one activated species,
• the activated species (or species) are left in contact with the surface of the workpiece or the surfaces of the workpieces for a processing time, and
• the room or rooms are allowed to cool.
  According to this process, the gaseous medium consists of a mixture of a nitrogen-containing gas and from 1% to 99% of at least one neutral dilution gas so that the chemical composition of the gaseous medium is constituted exclusively nitrogen and one or more elements taken from hydrogen and neutral elements such as argon.

The dilution gas is, for example, composed of hydrogen or argon or a mixture of hydrogen and argon.

The nitrogen-containing gas is, for example, molecular nitrogen or a gaseous derivative of nitrogen.

After activation, the gas contains activated species taken in particular from the ionized species N ⁺ and N ₂ ⁺ and the neutral excited species, N, N ₂ , NH and H.

The treatment temperature is adapted to allow diffusion of the nitrogen into the room and may be between 400 ° C and 1000 ° C, and preferably between 550 ° C and 850 ° C.

The temperature and nitrogen content of the treatment atmosphere can be adjusted so that the surface of the parts is constituted only a diffusion layer whose gray color is metallic close to that of the untreated alloy.

The temperature and nitrogen content of the process atmosphere can also be adjusted so that the surface of the parts is a diffusion layer and a gold-yellow combination layer.

When the gaseous medium contains hydrogen, the cooling of the part or parts, at the end of treatment, is preferably carried out under vacuum.

To generate at least one activated species, it is possible to create, in the gaseous medium, on the outside of the container, a plasma by electric discharge.

Preferably, the pressure of the gaseous medium is less than 100 mbar.

Preferably, the container comprises at least one opening closed by means providing with the edge of the opening a clearance large enough to pass at least one active species, but low enough to prevent a plasma from entering the interior of the container .

The container is for example constituted by a box comprising a wall having at least one opening closed in a non-sealed manner by one of the following means: lid placed on the upper part of the wall forming the turn of the opening, plug engaged with play in the opening and support on which the returned box rests, according to the edge of the opening.

The container may itself be pierced with numerous interstices allowing the passage of activated species but prohibiting the ignition of the plasma inside thereof.

A plurality of pieces which are arranged in at least one container can be processed simultaneously.

At least one part may comprise at least one cavity having an opening dimension of between 0.01 mm and 1 mm, the surface of which comprises a layer hardened with nitrogen.

At least one part may consist of one of the following objects: screw, nut, ancillary, orthopedic implant, valve, connecting rod, motor segment, shaft, eyewear element, golf club, turbine parts, servo parts such as a bearing, tube, gear, watch elements, valve elements, plug, metal shutter, valve, piston, cylinder, pump part (centrifugal, paddle, gear, lobe), flow regulator part, piece of pressure regulator, solenoid valve part, pressurized nuclear reactor control cluster pencil, coil, blank.

The invention makes it possible to obtain a titanium or titanium alloy piece obtainable by the process according to the invention, which comprises at least one surface hardened with nitrogen, said hardened surface having a gold-yellow color. or a metallic gray color similar to that of the untreated alloy, uniform without edge effect.

The part may comprise at least one cavity having an opening dimension of between 0.01 mm and 1 mm, the internal surface of the cavity being hardened with nitrogen.

The piece is, for example, for biomedical use.

• la figure 1 est une vue schématique en élévation et en coupe d'une insfiallation de traitement permettant de mettre en oeuvre le procédé selon l'invention,
• la figure 2 est une vue en élévation et en coupe d'un boîtier ou conteneur qui peut être utilisé pour la mise en oeuvre de traitement selon l'invention.

The invention will now be described in a more precise but nonlimiting manner with reference to the appended figures in which:

• the figure 1 is a diagrammatic view in elevation and in section of a treatment installation for implementing the method according to the invention,
• the figure 2 is an elevational view in section of a housing or container that can be used for carrying out treatment according to the invention.

The treatment method according to the invention is a method in which a part or a set of parts is placed in a closed container so as to leave small gaps allowing the passage of the activated species while preventing the ignition of a plasma through these interstices that are arranged in an atmosphere constituted a gas or a mixture of gases comprising the following elements, nitrogen and one or more elements selected from hydrogen and neutral elements such as argon or more generally noble gases. It should be noted that, especially when they are relatively large, the interstices can be lit but the plasma can not extend inside the container. To perform the treatment, on the one hand, excited chemical species are created in the gas located outside the container, for example, by means of an electric discharge or by means of plasma generation processes. using microwaves or more generally electromagnetic waves. Simultaneously, the parts are heated to maintain them at a temperature which is a treatment temperature, the assembly is left for a time sufficient for the excited species to penetrate inside the container and react with the surface of the parts arranged at inside the container and form on the surface of these parts a cured layer having the desired thickness.

The gaseous atmosphere in which the active species are generated which are on the one hand ionized species such as N ⁺ and N ₂ ⁺ ions and excited neutral species such as N, N ₂ , NH and / or H, is constituted preferably a mixture of the nitrogen and dilution gas type, the dilution gas being either hydrogen or argon, or more generally a neutral gas or a mixture of all these gases. In this gaseous atmosphere, the proportions of dilution gas are between 1% and 99%.

Indeed, the inventors have found very surprisingly that, when the treatment is carried out using a gaseous atmosphere consisting of a gas containing essentially nitrogen diluted by a neutral dilution gas in a proportion ranging from from 1% to 99%, a nitriding treatment is obtained on the surface of titanium or titanium alloy parts having both hardnesses equivalent to those obtained by conventional ionic nitriding and the uniformity qualities of surface that is obtained by the treatments of the carbonitruration type made in containers.

To carry out this treatment, the parts are heated either indirectly via the surface of the container which is itself heated, for example by means of the plasma, the surface of the container then heats the pieces by radiation, or by any means. another auxiliary heating means disposed in the enclosure and / or the container.

Depending on the holding temperature of the parts and the nitrogen content of the atmosphere, cured layers of different natures are obtained. In general, when the holding temperature of the parts and / or the nitrogen concentration are relatively low, a surface layer of nitrogen diffusion in titanium or in the titanium alloy is obtained, and in this case the surface of the pieces takes a nice gray uniform color. On the other hand, when the temperature and / or the nitrogen concentration are high, a more complex layer is obtained comprising an outer layer, called a combination layer, consisting of a mixture of titanium nitride TiN-Ti ₂ N, under which a nitrogen diffusion layer in titanium or titanium alloy. In this case, the piece takes on a very characteristic yellow-gold color which, in the case of the process used, is evenly distributed over the surface of the part. The particular conditions of temperature and nitrogen concentration to obtain or not a combination layer, further depend on the nature of the alloy. The person skilled in the art knows how to determine the treatment conditions to achieve in order to obtain the result he wishes. More generally, the temperature at which the parts must be maintained is a temperature which must be sufficient to allow the diffusion of nitrogen inside the titanium alloys and this temperature is preferably between 400 ° and 1000 °, and more preferably between 550 ° and 850 °. Indeed, beyond 850 ° the parts are likely to deform by creep and below 550 °, the diffusion may be insufficient. More precisely, the temperatures must be adapted on the one hand as a function of the nature of the layer that is to be obtained on the surface and, on the other hand, according to the nature of the alloy. The skilled person knows how to choose these temperatures depending on what he wants to get. The holding times are also variable depending on the thickness of the layers that are to be obtained, and these times can be between one hour and several tens of hours or more. Finally, the gaseous atmosphere of the apparatus in which the treatment is carried out is maintained at a low pressure which makes it possible to ignite the plasma outside the container, this pressure is generally less than 100 mbar.

Finally, when the treatment is complete, the parts must be cooled. In particular, when the atmosphere in which the treatment is carried out contains hydrogen, it is desirable to degas the hydrogen that could have been absorbed by the titanium parts during cooling, to prevent hydrogen reacts with titanium and forms titanium hydrides. In order to ensure this degassing, the parts are cooled after treatment under vacuum.

An embodiment of the treatment will now be described in more detail with reference to the Figures 1 and 2 which schematically represent equipment in which the processing of parts is carried out.

As it is visible on the figure 1 , the treatment plant is constituted by an oven chamber 1, for example made in two parts 1a and 1b, separable from each other to perform the loading of the furnace and assembled to one another with interposition joints, so that the chamber 2 of the oven is substantially gas-tight, so as to prevent the entry of air into the oven, during the treatment.

The enclosure of the furnace can be evacuated and filled with a gaseous mixture such as N ₂ + H ₂ + Ar, for example via a discharge nozzle 3 'and a filling nozzle 3.

The enclosure 1 of the treatment furnace contains a support 4 on which parts can be arranged to be treated 5.

As will be explained below, in the case of the implementation of the method of the invention, can advantageously be disposed on the support 4, one or more leaky containers containing the parts to be treated.

The support 4 is connected to a cathode terminal of an electrical generator 6 whose second anode terminal is electrically connected to the furnace enclosure 1.

The support 4 and the parts or containers 5 disposed on the support 4 are thus brought to a cathodic potential with respect to the enclosure 1 which is at an anode potential.

After carrying out the evacuation of the chamber 2 of the furnace 1 and its filling gas mixture N ₂ + H ₂ + Ar at a pressure less than 100 mbar, the generator 6 is put into operation so as to create a glow discharge abnormal between the cathode formed by the tray 4 and the containers 5 and the wall 1 of the treatment furnace.

Plasma is generated around the containers 5 in the glow discharge.

The discharge is controlled so as to produce activated species in the gaseous mixture and in particular the N, N ₂ , NH excited neutral species characteristic of the implementation of the process of the invention in a gaseous mixture containing nitrogen.

The parts are further heated and their temperature is controlled throughout the duration of treatment, as will be described later.

During the entire treatment, the gases contained in the chamber 2 are also continuously renewed in order to regulate the pressure inside the chamber 2 and to constantly supply the nitrogen necessary to generate the activated species used during the treatment. treatment.

An extremely important characteristic of the process according to the invention is obtained thanks to the exceptional reactivity with respect to titanium of the nitrogen-containing excited neutral species, and in particular the neutral excited species N, N ₂ , NH, these species. neutral excited retaining their reactivity even after passing through a space that does not allow the ignition of a plasma.

In the plasma technique, it is known that a plasma can not propagate through a gap whose opening dimension is smaller than a length called Debye length which depends in particular on the nature and pressure of the medium. gaseous plasma.

In the case of the gas mixture and the pressure mentioned above, the Debye length is of the order of a few tenths of a millimeter.

It is therefore not possible to achieve the ignition of a plasma in a part of a room or in the interior volume of a container separated from the discharge zone in the treatment chamber by an opening of a minimum dimension, for example of a thickness, less than a few tenths of a millimeter.

The inventors have observed that, extremely surprisingly, in the case of a plasma obtained from a gaseous mixture containing nitrogen and hydrogen or neutral elements, the surface treatment of parts made of titanium or titanium alloy on surfaces not exposed to plasma and separated from the plasma area by a gap having an opening of a size not allowing the ignition of a plasma.

The inventors have been able to show that this effect was due to the reactivity with respect to the titanium or the titanium alloy which is quite exceptional and durable for the activated species comprising nitrogen and, in particular, neutral species excited with nitrogen. , N ₂ , NH.

On non-plasma exposed parts, the nitrogen supply is carried out by the neutral excited species N, N ₂ , NH.

The inventors have also been able to observe that an effect of increase in plasma activity is also obtained in the case of plasmas produced by microwaves or radiofrequency, in a gaseous medium containing nitrogen.

These observations made it possible to implement a method of surface treatment of parts inside leaky containers placed inside the treatment chamber.

On the figure 2 , there is shown a container 5 which comprises a body 5a, for example of cylindrical shape closed by a bottom, at a first end, and open at a second end, and a cover 5b constituted by a simple metal plate placed on the open end of the cylindrical body 5a of the container 5. The container 5 is constituted in the form of a simple cylindrical box having a flat lid attached and placed on the end edge of the cylindrical body 5a.

The container such as 5 has been used to produce, within the treatment chamber 2 of the furnace 1, the surface treatment of pieces 7 arranged loose within the container. The parts 7 are for example nuts or screws made of titanium alloy such as Ti6Al4V.

Advantageously, the body 5a and the lid 5b of the cylindrical box may preferably be made of titanium alloy. The inner surface of the body 5a of the box and possibly the lid 5b may be coated with an insulating material such as a ceramic.

It has been possible to show that the implementation of the method, that is to say the surface treatment of the parts 7 inside the container 5, was practically independent of the wall thickness of the box body 5a. On the other hand, the treatment of the parts 7 inside the container 5 is only possible if the clearance between the cover 5b and the upper edge of the body 5a of the housing, when the cover 5b is placed on the body 5a, is at less equal to a weak length of the order of a hundredth of a millimeter.

Instead of a container 5 having a solid wall or body 5a closed by a cover 5b placed on one end of the wall, it is possible to use a container 5 comprising a wall pierced with a plurality of openings inside which one engages shutter elements with a weak clearance not allowing the ignition of a plasma through the openings of the wall. It is also possible to place the container 5 made under the shape of a box, for example cylindrical, in a disposition turned so that it rests along the edge of its opening on a support ensuring a non-sealed closure of the box. The container may itself be pierced with numerous interstices allowing the passage of activated species but prohibiting the ignition of the plasma inside thereof.

In general, the container has at least one opening closed by a closing means forming with the edge of the opening a non-zero play in the mechanical sense but important enough to let the activated species or species and low enough to prevent a plasma to penetrate inside the container.

As it is visible on the figure 1 one or more housings 5 are arranged on the support 4 and brought to a cathodic potential inside the treatment chamber. It was ensured that the residual clearance between the lid 5b and the body 5a of the containers 5 is less than the Debye length. In fact, various experiments have been carried out with a variable clearance, comprised between one hundredth and ten tenths of a millimeter, between the lid 5b and the body 5a of the containers due to the roughness of the surfaces and to a bearing or clamping force. variable applied on the lid 5b.

In any case, ignition of the plasma inside the container 5 does not occur when an electrical discharge is produced between the containers 5 and the wall 1 of the oven.

It has been observed that up to games e of the order of a hundredth of a millimeter, the curing treatment of the parts 7 could be carried out inside the container 5. On the other hand, if a hermetic tightening of the cover 5b against the body 5a, the parts 7 are not treated.

At the titanium-titanium alloy processing temperatures, as defined above, and particularly above 550 ° C, the excited neutral species such as N, N ₂ , NH can be found at the active state inside the containers, because of their lifetime sufficient. These excited species having a very high reactivity vis-a-vis titanium or titanium alloys, can achieve the supply of nitrogen to the pieces 7. In addition, a gap of a few tenths of a millimeter allows for example to prohibit the ignition of the plasma or inside the container while ensuring the passage of active neutral excited species.

It should be noted that, in the case of the implementation of the invention, a gap size gap gap allowing the treatment without contact with the plasma, for example between 0.01 and 1 mm, is not an absolute condition, some values greater than 1 mm for example to prohibit the ignition of the plasma while ensuring the passage of neutral excited species. Especially for holes of large diameter, the pressure of the gas is a factor to take into account. Those skilled in the art can, without difficulty, for example, by a few tests, determine the operating conditions for obtaining a satisfactory result.

Values less than 0.01 mm also allow treatment but with less efficiency.

On the figure 2 , there is shown a nozzle 8 of a container 5 which can be connected to a means of evacuation of the gas mixture to the outside of the treatment chamber 2 of the furnace. This promotes the introduction of the gaseous mixture containing neutral species activated inside the containers 5, when such a mode of evacuation through the containers is used.

The treatment of the parts 7 inside the container 5 is carried out at a temperature which makes it possible to obtain a diffusion of nitrogen inside the part and the formation of a layer of nitrogen diffusion in the alloy of titanium and possibly at the extreme surface of the piece a so-called combination layer consisting of titanium nitride.

For this, the treatment is carried out at a temperature which is preferably between 400 ° C and 1000 ° C, and preferably between 550 ° C and 850 ° C, with a treatment time which is adapted to obtain a diffusion of nitrogen to a sufficient depth. This treatment time may be for example between 1 hour and 24 hours, or more depending on the nature of the parts and the thickness of the layer that is desired.

The treatment temperature is preferably greater than 550 ° C, so as to obtain a sufficiently rapid diffusion of nitrogen. It is preferably less than 850 ° C, because beyond 850 ° C, the parts are likely to flow under their own weight, because of the high temperature.

Moreover, the temperature and the gaseous mixture may be adjusted according to whether it is desired to obtain a surface having no combination layer or, conversely, comprising a combination layer.

At a sufficiently low temperature and with a gas mixture low in nitrogen (for example at a temperature of 550 ° C. and with a nitrogen percentage of 15%) no combination layer is formed, and the extreme surface of the the piece is titanium or a titanium alloy with just a high concentration of nitrogen. Such a surface is metallic gray color close to that of the untreated alloy.

On the other hand, when the temperature is higher and the gaseous mixture is richer in nitrogen (for example at the temperature of 750 ° C. and with a percentage of nitrogen of 90%), an extreme layer is formed at the extreme surface. consisting of a TiN-Ti mixture ₂ N, consisting of extremely hard titanium nitride over a small thickness. Such a layer is extended inside the room by a diffusion layer. When forming a combination layer on the surface of the piece, the piece takes on a beautiful golden yellow color. Of course, the temperatures of 550 ° C. and 750 ° C. as well as the gas nitrogen contents are given for information only and may also depend on the exact nature of the alloy.

The hardened layers that can be obtained are layers whose thickness may be between 1 micron and 200 microns, depending on both the duration of the treatment and the temperature of the treatment.

When the surface of the part consists of a diffusion layer containing nitrogen in contents which can be between 3 atoms% and 50 atoms%, the hardness of the layer obtained at the extreme surface is of the order of 600 Vickers, more generally is between 500 and 700 Vickers (HV01) measured under a load of 100 grams. The hardness decreases regularly when the depth below the surface increases to reach the normal hardness of the alloy considered.

On the other hand, when the treatment is carried out under the conditions such that an extreme surface combination layer is formed, the thickness of which is between 2 and 4 μm, in general the hardness at the extreme surface is between 600 Vickers. and 1000 Vickers (HV01) measured under a load of 100 grams. The greater hardness corresponding to the combination layer. Under this layer, the hardness corresponds to that of a diffusion layer. This hardness decreases regularly as the depth below the surface increases. Note that the intrinsic value of the combination layer measured under a load less than 100 g is in turn greater than 1000 Vickers and reaches for example, about 3500 Vickers under 1 gr.

To carry out this treatment, and in particular to carry out the heating, it is possible to heat the parts, for example by means of heating resistors arranged in the treatment chamber. The heating may also be effected by indirect heating means in which the walls of the enclosure containing the parts are heated by the externally generated plasma to create the active species. In this case, it is the radiation of the wall of the enclosure that heats the parts that one wishes to treat. Of course, it is possible to combine the heating means, that is to say, on the one hand to provide a heating by the walls of the enclosure, themselves heated by the plasma, and a supplementary heating produced, for example, by electrical resistors.

The treatment that is carried out on titanium or titanium alloy parts, has the advantage of giving these parts properties of interesting uses. By way of example, treatment was carried out inside a container of pieces made of Ti6Al4V type alloy screws at a temperature of about 750 ° C. for about 18 hours. The screws thus treated had anti-seizing characteristics quite remarkable. By way of example also, the batch processing of titanium alloy parts has been carried out at a temperature of the order of 750 ° C., for a duration of the order of ten hours. Diffusion layers have thus been obtained on parts with a thickness of between 20 μm and 100 μm, having a hardness greater than 600 Vickers (HV01) measured under a load of 100 grams and characteristics in terms of friction and wear very strongly. improved compared to untreated parts.

In the above examples, the pieces were in containers such as the container that has been described, placed in a chamber into which a mixture of nitrogen and hydrogen or nitrogen, hydrogen and hydrogen had been introduced. Argon, at a pressure less than 100 mbar, and in which active species had been generated by creating a plasma by electric charges.

The method which has just been described has the advantage of making it possible to process a set of parts without generating plasma or electric arcs in the vicinity of the surface of the parts. This avoids damaging the surface of the parts.

This treatment which has many advantages since it avoids the direct contact of the parts with the plasma, makes it possible to treat parts arranged in a unitary or bulk manner inside the container, parts stacked one on the other, in this case, the surfaces in contact with the parts of the stack are subjected to the treatment in the same way as the apparent surfaces, or coiled coils whose interstice between successive turns allow the passage of the activated species.

The treatment also makes it possible to carry out a surface treatment with activated species of nitrogen inside cavities of very small dimensions, and for example internal surfaces of an injection channel of a fuel injector or channels of an injection ramp of a motor vehicle.

In addition, the method makes it possible to treat, in good conditions, parts having cavities or slots the dimensions of which would make it possible, under the conditions of conventional ionic nitriding treatment, to ignite the plasma. Indeed, because of the presence of the container, in the case of the invention, a plasma can not light in the cavities, and therefore there is no risk of damaging the surface of these cavities, unlike the case of classical ionic nitriding.

The inner surface of the container may be conductive or otherwise non-electrically conductive so that the parts are either polarized or unpolarized during processing. For example, the treatment of container parts coated internally with an insulating material, for example with a ceramic, can be carried out.

The reactivity of the activated species which makes it possible to treat cavities of small dimensions makes it possible to treat parts of very great length which have bores in the very long axis. These parts can be for example tubes.

In general, the invention can be applied to very diverse parts and in particular to any mechanical part subjected to wear in a corrosive medium. For example, the invention can advantageously be used to produce materials used in the biomedical industry, the aviation industry, the watch industry, the nuclear industry and the automotive industry. the food industry, the leisure and competition sports industry, the chemical industry or parts used in the marine environment.

The invention has particularly interesting applications in the context of titanium alloys subjected to frictional stresses, or moderately high contact pressures and having to resist scratching, wear or seizing

Without being limiting, the invention can be advantageously applied to screws or fasteners, for example used in the biomedical industry or in the aeronautical industry. In particular in the biomedical industry, the invention can be applied to any orthopedic implant element.

The invention is applicable to valves, motor vehicle fuel injectors, motor segments that can be stacked or impeller corrosion-prone turbine parts. The invention is applicable to all parts such as valves, plugs, metal shutters, valves, pistons, cylinders, pump parts, (centrifugal, paddle, gear, lobe), flow control parts, regulator part pressure valves, solenoid valve parts, servo parts such as bearings.

The treatment can be carried out on a band which can be rolled up or on a metal blank implemented after treatment.

It will be noted that the surface treatment according to the invention can be carried out for another purpose than to ensure hardening. In general, any treatment aimed at modifying at least one property of the surface of the part by interactions with the activated species can be envisaged. In particular, the surface treatment according to the invention can be carried out even on a passivated surface.

Finally, the treatment according to the invention is a treatment in which only nitrogen is introduced into the rooms. It is therefore made with a gas mixture whose chemical composition consists exclusively of nitrogen and neutral elements. However, to carry out the treatment, it is possible to use industrial gases which contain impurities. These impurities, which may especially contain carbon, oxygen and boron, must be in an amount small enough to have no significant effect on the treatment. It is possible in particular to use industrial gases of conventional purity, for example industrial nitrogen with a purity greater than or equal to 99.8% by volume, industrial hydrogen with a purity greater than or equal to 99.8% by volume, industrial argon of purity greater than or equal to at 99.99% by volume.

Claims (17)

1. Method of surface-treating at least one part made of titanium or titanium-based alloy, according to which:

   - the part or parts is/are placed in a container, which is placed in a treatment enclosure containing a gaseous medium,

   - the part or parts is/are heated to a treatment temperature,

   - at least one chemical species activated by activation of the gaseous medium to the outside of the container is produced, the wall of the container being closed except for at least one gap, the dimension of the opening of which prevents ignition of the plasma inside the container but which allows at least one activated species to pass through,

   - the activated species is/are allowed to remain in contact with the surface of the part or the surfaces of the parts for a treatment time, and

   - the part or parts is/are allowed to cool,

   characterised in that the gaseous medium is composed of a mixture of a gas comprising nitrogen and from 1 % to 99 % of at least one neutral diluent gas such that the chemical composition of the gaseous medium is composed exclusively of nitrogen and one or more elements selected from hydrogen and noble gases such as argon.
2. Method according to claim 1, characterised in that the diluent gas is composed of hydrogen or argon or a mixture of hydrogen and argon.
3. Method according to claim 1 or claim 2, characterised in that the gas comprising the nitrogen is molecular nitrogen or a gaseous nitrogen derivative.
4. Method according to any one of clams 1 to 3, characterised in that, after activation, the gas contains activated species selected from the ionised species N⁺ and N₂ ⁺ and the excited neutral species N, N₂, NH and H.
5. Method according to any one of claims 1 to 4, characterised in that the treatment temperature is adapted to allow diffusion of nitrogen into the part.
6. Method according to claim 5, characterised in that the treatment temperature is from 400°C to 1000°C, and preferably from 550°C to 850°C.
7. Method according to claim 5 or claim 6, characterised in that the treatment temperature and the nitrogen content of the treatment atmosphere are so adjusted that the surface of the part or parts is, after treatment, composed of a diffusion layer of a grey colour.
8. Method according to claim 5 or claim 6, characterised in that the treatment temperature and the nitrogen content of the treatment atmosphere are so adjusted that the surface of the part or parts has, after treatment, a diffusion layer and a combination layer of a golden-yellow colour.
9. Method according to any one of claims 1 to 8, characterised in that, when the gaseous medium comprises hydrogen, cooling of the part or parts at the end of treatment is carried out in vacuo.
10. Method according to any one of claims 1 to 9, characterised in that, for producing at least one activated species, a plasma is created, in the gaseous medium to the outside of the container, by electric discharge.
11. Method according to claim 10, characterised in that the pressure of the gaseous medium is less than 100 mbar.
12. Method according to any one of claims 1 to 11, characterised in that the container has at least one opening closed by a means which provides an amount of play with the edge of the opening that is sufficiently large to allow at least one active species to pass through but sufficiently small to prevent a plasma from entering inside the container.
13. Method according to claim 12, characterised in that the container is composed of a box comprising a wall having at least one opening which is closed, in a manner that does not provide a tight seal, by one of the following means: a cover placed over the top of the wall forming the surround of the opening, a stopper inserted in the opening with play, a support on which the upturned box rests, depending on the edge of the opening, or a container comprising a plurality of gaps of a size adapted to prevent ignition of the plasma inside the container whilst allowing activated species to pass through.
14. Method according to any one of claims 1 to 13, characterised in that a plurality of parts which have been placed in at least one container are treated simultaneously.
15. Method according to any one of claims 1 to 14, characterised in that at least one part has at least one cavity which has an opening dimension of from 0.01 mm to 1 mm and the surface of which has a layer hardened by nitrogen.
16. Method according to any one of the preceding claims, characterised in that at least one part forms one of the following articles: screw, nut, ancillary instrument, orthopaedic implant, valve, connecting rod, piston ring for engines, turbine parts, servo parts such as a bearing, shaft, spindle, golf club, tube, gearing, watch parts, valve parts, valve ball or plug, metal shut-off member, tap, piston, cylinder, part of a pump (centrifugal, vane, gear, lobe pump), part of a flow regulator, part of a pressure regulator, part of a solenoid valve, coil, blank.
17. Method according to any one of claims 1 to 16, characterised in that the part or parts obtained has/have at least one surface hardened by nitrogen and having a golden-yellow colour or a metallic grey colour similar to that of the alloy, uniform and without an edge effect.

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