EP1274873A1

EP1274873A1 - Method for treating the surface of a part and resulting part

Info

Publication number: EP1274873A1
Application number: EP01927996A
Authority: EP
Inventors: Laurent Poirier; Jean-Paul Lebrun; Bernard Delmas
Original assignee: Nitruvid SA
Current assignee: Nitruvid SA
Priority date: 2000-04-19
Filing date: 2001-04-18
Publication date: 2003-01-15
Anticipated expiration: 2021-04-18
Also published as: FR2807956A1; WO2001079579A1; WO2001079579B1; FR2807956B1; ATE301734T1; US20040045636A1; ES2247100T3; EP1274873B1; US7074460B2; DE60112564T2; DE60112564D1

Abstract

The invention concerns a surface treatment method which consists in contacting a surface of the part (7) with at least an activated species. The activated species is obtained by activating a gaseous medium containing at least two of the following elements: carbon, nitrogen, boron and oxygen. Preferably, the activated species is neutral excited CN species. The activated species brings at least an interstitial element to the metal part (7) surface which is borne and maintained at a temperature enabling at least one interstitial element to be diffused into a surface layer of the metal part (7).

Description

Method of surface treatment of a part and part obtained.

The invention relates to a method for treating the surface of a part, comprising bringing into contact with a surface of the part, at least one activated chemical species, such as an activated chemical species contained in cold plasma. Methods of treating the surface of parts are known by bringing a surface of the part into contact with at least one activated chemical species contained in a cold plasma which can be generated, for example, by an electric discharge between an anode and a cathode, inside an enclosure containing a gas and generally a gas mixture at a pressure below atmospheric pressure.

The plasma contains electrons as well as activated species themselves comprising ionized species and excited neutral species, that is to say atoms or molecules of which certain electronic layers are excited under the effect of the electric discharge. As an example of the application of surface treatments using activated chemical species, for example by an electrical discharge, mention may be made of the hardening of steel parts by the introduction of interstitial elements into a surface layer of steel. The interstitials commonly used for hardening steel consist mainly of nitrogen, carbon and boron. The treatment consists in generating a plasma, for example by an electric discharge, in a gaseous medium containing the interstitial and in bringing the plasma containing activated species into contact with the surface of the part to be treated. The interstitial in the activated state is highly reactive towards the surface of the part, so that it penetrates through the surface of the part. The part is brought, during the treatment to a temperature which ensures a diffusion of the interstitial in the surface layer of the part, to a depth which depends in particular on the temperature and the duration of the treatment.

Curing treatments are carried out in this way or, more generally, treatments aimed at modifying the surface properties of the parts, in particular steel parts, by introducing and diffusing interstitials in a surface layer of the part. . In current practice, a discharge is produced between the part brought to a cathodic potential and an anode which can be constituted for example by the wall or a part of the enclosure in which the treatment is carried out. In this case, the cold plasma is produced in situ, in the vicinity of the surface of the part to be treated, by the electrical discharge inside the gaseous medium filling the treatment enclosure. The activated species, for example the ionized species or the excited neutral species, are formed close to the surface of the part with which they react to ensure a contribution of an element of interstitial type. Generally, the heating and maintaining the temperature of the room to ensure the diffusion of the interstitial is obtained by electric discharge. One can also provide additional heating and temperature maintenance means.

The plasma can also be generated, inside the enclosure, by an electromagnetic wave generator, for example a microwave generator or a radio frequency generator, these means generally requiring different pressures of the gaseous plasma-producing medium. pressures required when using an electric shock.

The plasma can also be generated in a plasma generator outside the treatment enclosure and then transferred into the enclosure enclosing the part to be treated which is heated and maintained at temperature inside the enclosure.

In the case where the interstitial used to carry out the treatment consists of nitrogen, the gas mixture in which the plasma is formed contains nitrogen or a gaseous derivative of nitrogen, these components generally being diluted by hydrogen or a mixture of hydrogen and a neutral gas such as argon or by any other non-reactive diluent mixture. An example of a gas mixture commonly used is the N ₂ + H ₂ mixture. The plasma produced in such a gas mixture generally contains ionized species such as for example N ⁺ and N ₂ ^{+ as} well as excited neutral species such as for example N, N _2l NH and H. It has generally been observed that the excited neutral species exhibit good reactivity with respect to the metallic surface subjected to the plasma and contribute effectively to the introduction of interstitials on the surface of the part. In addition, it has been observed that, in the case of a transferred plasma or "post-discharge" plasma, the transfer time of the plasma in the enclosure must be very short in order to conserve reactive species in the plasma. It has also been observed, in the case of a transferred plasma or "post-discharge", that it was difficult to obtain a homogeneous gas flow in industrial charges.

In the case where the plasma is produced by an electrical discharge, the electrical discharge must be maintained in an abnormal luminescent discharge regime, that is to say a regime preceding a regime of arcing between the cathode and the anode . In the case where the discharge is carried out between the part constituting the cathode and a part of the treatment enclosure constituting the anode, there are considerable risks of the formation of arcs generating surface defects on the part to be treated.

In addition, the fact that the plasma is applied directly to the part can lead to differences in heating between different parts of the part or from one part to another, when treating inside the enclosure a load comprising a plurality of parts. In the case of overheating on certain parts of the parts, when they are made of stainless steel, it is possible to locally form, in the layer enriched with interstitials, precipitates, for example of nitride, which significantly degrade the resistance to corrosion of the workpiece surface.

Indeed, the temperature of treatment of the part, for example in the case of a hardening treatment with carbon or interstitial nitrogen of steel parts and more particularly of austenitic stainless steel parts, must be carefully adjusted so to perfectly control the diffusion of interstitials in the surface layer of the part.

As long as a certain temperature is not exceeded, which is for example of the order of 460 ° C. to 480 ° C., in the case of stainless steel austenitic, a solid solution of carbon and / or nitrogen is formed in the surface layer of the part in the metal matrix of the steel, over a few micrometers to a few tens of micrometers, this surface layer being extremely hard and resistant to wear and does not deteriorate the corrosion resistance of the part.

At higher temperatures, a layer of a solid solution of carbon and / or nitrogen is formed in the metal matrix which has the disadvantage of also comprising precipitates of nitrides and / or carbides which substantially degrade the resistance to corrosion of the workpiece surface.

It can be difficult to precisely adjust the temperature of a room, in all areas of the room, especially when the room is large and / or extends over a long length, in one direction (bars or tubes). It is also difficult to very precisely regulate the temperature of each of the parts of a batch of parts whose treatment is carried out simultaneously in the treatment installation.

In addition, in the case of the treatment of a batch comprising numerous parts, it is necessary to place these parts on a support which can be, for example, a cathode support of the treatment installation, prior to the processing. This installation requires providing means for supporting and positioning the parts on the cathode support, so that the parts are perfectly exposed on their surface treated with plasma which is formed in the electrical discharge. The installation of a large number of parts also requires delicate handling and a time of execution which can be long.

In the case of parts of complex shape which include for example cavities of small dimensions, it is difficult to carry out a treatment which is satisfactory in all the parts of the parts.

Likewise, it is not possible to treat pieces in a stacked state or wound strips, since the surfaces which are not exposed to the gaseous medium in which the plasma is formed are not subjected to the treatment. The methods of surface treatment of parts with activated species, generally excited neutral species and, to a lesser degree, ionized species, as they are currently used, therefore have certain limitations, although such treatments are have proven to be extremely efficient in many application cases.

The object of the invention is therefore to propose a method of surface treatment of a part consisting in bringing into contact with a surface of the part, at least one activated chemical species, the treatment being implemented so as to increase the reactivity of the activated chemical species used in such proportions that it is possible to process parts of complex shape and / or large dimensions, in large number positioned individually or in bulk, possibly inside containers, in the form rolled or stacked, with very good control of the processing temperature. For this purpose, the activated species is obtained by activation of a gaseous medium containing at least two of the elements carbon, nitrogen, boron, oxygen and it comprises at least two of the elements carbon, nitrogen, boron and oxygen.

In a particularly advantageous manner, the treatment according to the invention is carried out by carrying out the activation of a gaseous medium containing both carbon and nitrogen, for example by an electric discharge, so as to obtain an excited neutral species CN which exhibits a very high reactivity in contact with metallic or non-metallic surfaces, such as in the case of metallic surfaces, surfaces of steel parts and more particularly of stainless steel.

In order to clearly understand the invention, we will now describe, by way of example, with reference to the attached figures, the implementation of the process according to the invention for the surface hardening of parts by one to minus carbon and nitrogen interstitials. Figure 1 is a schematic view, in elevation and in section, of a treatment installation for implementing the method of the invention. Figure 2 is an elevational and sectional view of a housing or container which can be used for the implementation of the treatment method according to the invention.

One of the fundamental aspects of the process according to the invention is that the activated species and the cold plasma comprising these species are produced in a gaseous medium containing at least two of the elements carbon, nitrogen, boron and oxygen, that is to say say at least two elements that can constitute interstitials in the metal matrix of a part to be treated.

In fact, such gaseous media can be obtained in various ways, for implementing the method of the invention.

As an example, the media which can be used in the case of a surface treatment using the nitrogen and carbon interstitials will be indicated below.

Gaseous media containing both nitrogen and carbon: The gaseous medium may be a gaseous mixture consisting of molecular nitrogen gas N ₂ and / or a compound containing nitrogen, a compound containing carbon and optionally , at least one diluting gas such as hydrogen and / or a neutral gas. The nitrogen-containing compound may be, in addition to molecular nitrogen, a gaseous derivative of nitrogen. The carbon-containing compound can be a hydrocarbon, for example an aϋphatic or aromatic hydrocarbon, a cyclan, an alkene, an alkyne, an alkane, and in particular methane.

The mixture of nitrogen and gaseous compounds containing carbon can be diluted by hydrogen or a neutral gas such as argon. A typical mixture that can be used is the N ₂ + H ₂ + mixture

CH ₄ .

The gaseous medium containing carbon and nitrogen can also consist of a compound whose molecule contains both carbon and nitrogen and which can be obtained easily in the gaseous state. Such a compound can for example be an amine. Such a gaseous compound can be diluted by hydrogen or by a neutral gas such as argon or by any other non-reactive diluent mixture. The activation of the gaseous medium to obtain a cold plasma containing activated species and in particular activated species containing both nitrogen and carbon can be carried out in different ways which will be indicated below. Activation of the gaseous medium based on nitrogen and carbon:

The cold plasma can be generated by an electric discharge between an anode and a cathode inside an enclosure containing the gaseous medium.

The electrical discharge can be carried out between the part to be treated and a part of the treatment installation at an anode potential or, preferably, as will be explained below, between a container containing one or more parts to be treated and a part of the treatment facility.

The cold plasma can also be generated by an electromagnetic wave generator, for example a microwave generator or a radiofrequency generator.

The plasma can be generated in the processing enclosure or outside of the parts processing enclosure.

The activated species can also be generated in the gas mixture by other means.

In all cases, the pressure of the gaseous medium in which the plasma or the activated species is generated is adapted to the mode of generation of the activated species.

For example, in the case of an electrical discharge between a cathode and an anode inside the gaseous medium, the pressure of the gaseous medium, for example of a mixture N ₂ + H ₂ + CH ₄ , is lower at atmospheric pressure and for example less than 100 mbar.

In the case of the generation of a plasma by microwaves, the pressure of the gaseous medium is, for example, less than 100 mbar. In all cases, the plasma is generated under conditions such that among the activated species, that is to say the excited ionized or neutral species, there is a significant proportion of species containing both nitrogen and carbon and in particular the excited neutral species of the CN form whose reactivities are particularly high.

In the case where the gaseous medium contains, in addition to nitrogen and carbon, oxygen, the neutral excited species CNO is also obtained, also having very good reactivity.

In general, oxygen is an additive which can act as a catalyst for the formation of complex activated species containing at least two elements of interstitial type.

The gaseous medium containing nitrogen and carbon can also be generated in situ, for example inside the treatment enclosure, before or simultaneously with the formation of the activated species used in the context of the invention. .

It is possible for example to introduce, into the treatment enclosure, a gaseous mixture containing only nitrogen and possibly a dilution gas such as hydrogen and / or argon. The carbon is introduced into the enclosure, in the form of a solid carbon target, for example graphite or a solid element containing carbon. The target is subjected, inside the treatment enclosure (or in a plasma generator separate from the enclosure), to an ion beam formed from the nitrogen-based gas mixture. The target could also be bombarded by any other incident particle beam independent of the plasma formed from the gaseous medium containing nitrogen. The bombardment of the target results in a sputtering of carbon and an emission of the carbon element in the gaseous medium or the plasma formed from the gaseous medium. In all cases, energy must be communicated to the gaseous medium or to the plasma in order to obtain a combination of carbon and nitrogen in the form of activated species and in particular of excited neutral species of the CN form.

In general, a cold plasma generated from a gaseous medium containing carbon and nitrogen, under the conditions of the invention, contains different ionized species and different neutral species which exhibit different behaviors during the implementation of the processing of the invention. For example, in the case of a gaseous medium consisting of the gas mixture N ₂ + H ₂ + CH ₄ , the plasma generated, for example by an electrical discharge, contains ionized species such as for example N ⁺ , N ₂ ⁺ , CN ⁺ , (CN) ₂ ⁺ , C ^* and excited neutral species such as N, N ₂ , NH, H, C, CN and (CN) ₂ .

The inventors have been able to show that among all these activated species, the excited neutral species containing carbon and nitrogen, and in particular the excited neutral species CN, exhibited very high reactivity, for example in the case of surface treatment of '' an austenitic stainless steel.

In fact, as will be explained later, the completely exceptional behavior of the excited neutral species CN, that is to say of an excited neutral species containing both nitrogen and carbon, allows to consider surface treatments, under conditions of implementation that have not been possible until now and on parts which could not be treated by treatment methods using ionized species.

By way of example, a description will now be given, with reference to the appended figures, of a particular implementation of the invention, for carrying out a hardening treatment of austenitic stainless steel parts.

As indicated above, in the case of the treatment of austenitic stainless steel parts, it is preferable to carry out a treatment by keeping the part or parts to be treated at a temperature below a temperature level at which precipitates of nitrides begin to appear or carbides in the metallic matrix of the surface layer of the parts enriched in nitrogen and carbon by the surface treatment.

Although in general the hardening treatment with nitrogen and carbon of austenitic stainless steel parts can be carried out at a temperature between 200 ° C and 600 ° C, to avoid the formation of precipitates, it is recommended to treat parts in a temperature range between 300 ° C and 480 ° C and preferably between 300 ° C and 460 ° C. As can be seen in FIG. 1, the treatment installation consists of an oven enclosure 1, for example produced in two parts 1a and 1b, separable from one another to charge the oven and joined together with interposition of seals, so that the oven enclosure 2 is practically gas tight, so as to prevent the entry of air into the oven, during the treatment.

The oven enclosure can be evacuated and filled with a gaseous mixture such as N ₂ + H ₂ + CH, for example by means of a discharge nozzle 3 'and a filling nozzle 3. L' enclosure 1 of the treatment oven contains a support 4 on which parts to be treated 5 can be arranged.

As will be explained later, in the case of the implementation of the method of the invention, it is advantageous to have, on the support 4, one or more non-leaktight containers containing the parts to be treated. The support 4 is connected to a cathode terminal of an electric generator 6, the second, anode terminal of which is electrically connected to the oven 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 anodic potential.

After having carried out the evacuation of the enclosure 2 of the furnace 1 and its filling with gaseous mixture N ₂ + H ₂ + CH ₄ , at a pressure of less than 100 mbar, the generator 6 is put into operation so as to create a discharge abnormal luminescent between the cathode formed by the containers 5 and the wall 1 of the treatment furnace.

A plasma is generated around the containers 5, in the luminescent discharge.

The discharge is controlled so as to produce activated species in the gas mixture and in particular the excited neutral CN species characteristic of the implementation of the process of the invention in a gas mixture containing carbon and nitrogen.

The parts are further heated and their temperature is regulated, throughout the duration of the treatment, as will be described later. During the whole treatment, the gases contained in the enclosure 2 are also renewed, continuously, to regulate the pressure inside the enclosure 2 and constantly supply the nitrogen and carbon necessary to generate the activated species used during processing.

An extremely important characteristic of the process according to the invention is obtained thanks to the exceptional reactivity of the excited neutral species containing carbon and nitrogen, and in particular of the excited neutral species CN, this excited neutral species retaining its reactivity even after passage through a space not allowing the ignition of a plasma.

In the plasma technique, it is known that a piasma cannot propagate through an interstice whose opening dimension is less than a length called the Debye length which depends in particular on the nature and the pressure of the gaseous medium. 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 ignite a plasma in a part of a room or in the interior volume of a container separated from the discharge zone in the treatment enclosure 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 both carbon and nitrogen, the surface treatment was carried out on unexposed surfaces plasma and separated from the plasma area by a gap ^" having an opening of a size that does not allow the ignition of a plasma. The inventors have been able to show that this effect was due to the reactivity quite exceptional and of activated species containing both carbon and nitrogen, and in particular of the excited neutral species CN. On parts not exposed to the plasma, nitrogen and carbon are supplied by the excited neutral CN species, outside the cold plasma field.

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

These observations made it possible to implement a process for treating the surface of parts inside non-leaktight containers placed inside the treatment enclosure.

In 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, as well as 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 therefore constituted in the form of a simple cylindrical box having an attached flat cover and. placed on the end edge of the cylindrical body 5a.

The container as 5 has been used to carry out, inside the treatment chamber 2 of the oven 1, the surface treatment of parts 7 placed in bulk inside the container. The parts 7 are, for example, so-called “quick” fittings made of 316L stainless steel.

Advantageously, the body 5a and the cover 5b of the cylindrical box can be made of 316L stainless steel. The internal surface of the body 5a of the box and possibly of the cover 5b can be coated with an insulating material such as a ceramic.

It has been shown 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. However, 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 short length of the order of a hundredth of a millimeter. Instead of a container 5 comprising a solid wall or body 5a closed by a cover 5b placed on one end of the wall, one can use a container 5 comprising a wall pierced with a plurality of openings inside which engages sealing elements with a small clearance not allowing the ignition of a plasma through the openings of the wall. One can also place the container 5 made in the form of a box, for example cylindrical, in an inverted arrangement so that it rests along the edge of its opening on a support ensuring a non-sealed closure of the box. In general, the container has at least one opening closed by a closure means providing, with the edge of the opening, a play that is not zero in the mechanical sense but sufficiently large to allow the activated species to pass and sufficiently weak to prevent a plasma. to get inside the container. As can be seen in FIG. 1, one or more boxes 5 are arranged on the support 4 and brought to a cathodic potential inside the treatment enclosure. It has been ensured that the residual clearance between the cover 5b and the body 5a of the containers 5 is less than the length of Debye. In fact, different experiments were carried out with a variable clearance, between 1 hundredth and three tenths of a millimeter, between the cover 5b and the body 5a of the containers due to the roughness of the surfaces and to a pressing or clamping force. variable applied to the cover 5b.

In all cases, the opening of the gap e being much less than the length of Debye, there can be no ignition of the plasma inside the container 5, when an electric 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 hardening treatment of the parts 7 could be carried out inside the container 5. On the other hand, if hermetic tightening is carried out that of the cover 5b against the body 5a, the parts 7 are not treated.

Ionized species such as N ⁺ and N ₂ ⁺ and excited neutral species such as N, N ₂ , NH cannot be found in the active state inside containers, because of their short lifespan which does not allow their transfer between the treatment enclosure and the interior of the containers.

Ionized species such as C ⁺ and excited neutral species such as C cannot also be found in the active state inside the containers, due to their short lifespan which does not allow their transfer between the enclosure of processing and interior of containers.

The species containing carbon and nitrogen and in particular the excited neutral species CN are found in the reactive state inside the container and provide nitrogen and / or carbon to the parts 7, a interstice of a few tenths of a millimeter, for example to prohibit the ignition of the plasma while ensuring the passage of active excited neutral species.

It should be noted that, in the case of the implementation of the invention, an interval of opening dimension of the interstice allowing the treatment without contact with the plasma, for example between 0.01 and 0 , 3 mm, is not an absolute condition, certain values greater than a few tenths of a millimeter making it possible for example to prohibit the ignition of the plasma while ensuring the passage of the excited neutral species. Values less than 0.01 mm also allow treatment, but with less effectiveness.

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

The treatment of the parts 7 inside the container or containers 5 is carried out at a temperature making it possible to obtain a solid solution of at least one of the carbon and nitrogen interstitials in a surface layer of the parts, without forming carbide precipitates. and nitride in this surface layer. For this, the treatment was carried out in an atmosphere of methane and nitrogen diluted in hydrogen, at a controlled temperature around 420 ° C, that is to say at a temperature between 300 ° C and 460 ° C. The treatment was carried out for periods of between 24 hours and 48 hours, depending on the batches of parts treated. Cured layers were obtained on the parts, with a thickness of between 10 μm and 30 μm, these layers having a hardness greater than 1000 Vickers and a resistance to attack by a salt spray greater than 1000 hours.

The treatment was also carried out, inside containers 5, of parts 7 formed by austenitic stainless steel nuts, the treatment time being 18 hours and the temperature of approximately 420 ° C.

The nuts thus treated had quite remarkable anti-seizing characteristics.

In general, various treatments of parts made of stainless steel and chromium-rich steels, the chromium content of which is at least equal to 8% by mass, have been carried out inside containers placed in the enclosure of the furnace containing the gas mixture N ₂ + H ₂ + CH ₄ .

Before filling the chamber 2 of the oven 1, after depositing the container or containers 5 on the cathode support and closing the oven, the oven is evacuated for a sufficient time to reach a pressure below the treatment pressure. The chamber 2 of the oven 1 is then filled with an N ₂ + H ₂ + CH ₄ mixture at a pressure of less than 100 mbar.

The treatment is carried out for a period ranging from one hour to several tens of hours. The treatment makes it possible to obtain a layer hardened by at least one interstitial having a thickness of 1 μm to 500 μm, depending on the duration of the treatment.

Depending on the temperatures to which the austenitic stainless steel parts are brought, the hardened layer is a solid solution of interstitials in the metallic matrix of the steel or a solid solution containing precipitates of carbides and nitrides.

The heating limit temperature to obtain a solid solution without precipitates is of the order of 460 ° C. to 480 ° C. It should be noted that in the case of a treatment in which the plasma is obtained by electric discharge, the heating of the containers can also be obtained by electric discharge, the heating of the parts 7 inside the containers being carried out by radiation. and by conduction through the wall of the containers.

It is possible to provide additional heating, for example by electrical resistances, containers 5 and parts 7 and to regulate the heating throughout the treatment.

When the treatment is carried out at a temperature above 460 ° C., the parts may exhibit an initial sensitivity to corrosion, due to the appearance of nitrides and carbides in the solid solution. The degradation of the corrosion resistance becomes very sensitive from 480 ° C. Between 480 ° C and 600 ° C, corrosion resistance is no longer guaranteed, but the part has a very high hardness, which makes it possible to envisage certain applications of the treatment at temperatures above 480 ° C.

The additional heating of the treatment enclosure can be carried out by any means other than heating resistors.

In the case of steels or alloys other than austenitic stainless steels, for example low or high alloyed structural steels, treatments can be carried out at a temperature ranging for example up to 800 ° C.

During all of the treatment, the plasma is generated around the containers 5, but due to the small thickness e of the lid closing gap, the plasma cannot ignite inside the containers in contact with the parts. The parts are thus protected from any risk of damage by electric arcs.

A surface treatment according to the invention, carried out by activated species such as the excited neutral species CN, without contact with the plasma, therefore has numerous advantages. In particular, it is possible to treat pieces positioned individually or in bulk inside containers, pieces stacked one on the other, the surfaces in contact with pieces of the stack being subjected to the same treatment. so that the exposed surfaces, or wound coils whose interstice between successive turns allows the passage of activated species such as CN. It is also possible to carry out a surface treatment with activated species of the internal surface of very small cavities of metal parts, for example the internal surface of the injection channel of a fuel injector or of the channels of a ramp d injection of a motor vehicle,

Generally, when parts are treated with activated species generated by an electrical discharge in situ in the treatment furnace according to the method of the invention, the parts are placed inside a container making it possible to isolate them. and protect them against the risk of electric arcs. The container also makes it possible to obtain a homogenization of the temperature of the parts. The room temperature can be precisely regulated, regardless of the production of the activated species. The invention makes it possible to treat parts having very small cavities, for example channels or slots having a diameter or an opening width of between 0.01 and 0.3 mm, the inner surface of which is hardened by at least an interstitial. Such parts cannot be obtained by conventional plasma nitriding treatment methods and are therefore characteristic of the invention.

The internal surface of the container may or may not be conductive, so that the parts are polarized or not during the treatment. In certain cases, it will be possible to carry out the treatment of the parts inside containers internally coated with an insulating material, for example with a ceramic.

The high reactivity of the CN type activated species makes it possible to use the method according to the invention for treating very long parts, for example for treating the interior or exterior surface of very long tubes. The invention can be implemented in many ways, as regards the nature, composition and mode of obtaining the gaseous medium whose activation is carried out and as regards the mode of activation of the gaseous medium . The invention applies to the treatment of parts made of many materials, for example to the treatment of steels or alloys having a cubic structure with centered faces, cubic centered or tetragonal, for example austenitic, martensitic, ferritic or stainless steels. austenoferitics or any other stainless steel or not which has a chromium content greater than 8% or any low or high alloy structural steel.

The invention also applies to other steels and to non-ferrous materials such as titanium, aluminum and their alloys, or to nickel and / or cobalt alloys.

In the case of austenitic stainless steels, a homogeneous solid solution of carbon and or nitrogen is produced in the metal alloy, according to the conditions, in particular thermal conditions, for conducting the process, the carbon and nitrogen content being greater than 3 atom% in the hardened surface layer, this content even reaching 50 atom%. In general, it is preferable that this content is between 3 atom% and 30 atom% to obtain good resistance to corrosion and good hardening of steels.

The treatment according to the invention can be applied to very many parts and in particular to any mechanical part subjected to wear in a corrosive medium. For example, the invention can be applied advantageously for the production of materials used in the food industry, the chemical industry, the steel industry, the nuclear industry or the automotive industry. , or even used in the marine environment or in biomedical applications.

The invention has particularly advantageous applications in the case of austenitic steels having to resist scratches, for example stainless steel dishes, these dishes being able to be treated on the starting plate, before stamping, or even in the stamped state and in a stacked arrangement in the processing enclosure.

The parts or objects treated by the process of the invention remain perfectly shiny and retain a very beautiful appearance after treatment. In the case of austenitic stainless steels to keep the shiny appearance of parts or objects treated, it is however necessary to carry out the treatment at a temperature at most equal to 480 ° C.

The invention can be applied advantageously to blades of common martensitic stainless steel objects such as knives or scalpels.

The treatment can be applied to thin sheets in the unrolled state or even wound up in the form of coils.

The invention applies to orthopedic implants.

The invention also applies to valves, motor vehicle fuel injectors, engine segments which can be treated in a stacked state and to turbine parts subjected to pitting corrosion. The invention applies to any part such as a valve, a plug, a metallic valve, a piston, a cylinder, a pump part (centrifugal, vane, gear, lobe), flow regulator part, regulator part. pressure, solenoid valve part.

The invention can be applied to crayons. control clusters for pressurized water nuclear reactors.

The treatment can be carried out on a strip or on a metal blank implemented after treatment. The treatment can be carried out on pieces arranged in a unitary manner in a container or arranged in bulk, in piles or in coils.

The surface treatment carried out on the metal part by the complex activated species or may be, instead of hardening by interstitials, any other treatment aimed at modifying at least one surface property of the metal part by interaction of the or species activated with a surface layer of the part. The surface treatment according to the invention can be carried out even on a passivated surface.

The treatment according to the invention can be used for the surface treatment of non-metallic parts, for example of ceramic, glass, rubber or polymer plastic parts, the surface properties of which are modified by the action of excited neutral species. such as CN.

The treatment according to the invention can use one or more complex activated species comprising two or more than two elements. elements among nitrogen, carbon, boron and oxygen. The cured layer of the parts may have one or more interstitials such as carbon and nitrogen.

Claims

CLAIMS 1.- Method of surface treatment of a part (7) consisting in bringing into contact with a surface of the part (7), at least one activated chemical species, characterized in that the activated chemical species is obtained by activation of a gaseous medium containing at least two of the elements carbon, nitrogen, boron, oxygen and that it comprises at least two of the elements carbon, nitrogen, boron, oxygen.

2. A treatment method according to claim 1, characterized in that the gaseous medium mainly contains carbon and nitrogen.

3.- Method according to claim 2, characterized in that the nitrogen is present in the gaseous medium in one of the following forms: molecular nitrogen N ₂ , gaseous derivatives of nitrogen.

4.- Method according to any one of claims 2 and 3, characterized in that the carbon is present in the gas mixture in at least one of the following forms: aliphatic hydrocarbon, aromatic hydrocarbon, cyclan, alkene, alkyne, alkane.

5.- Method according to claim 2, characterized in that carbon and nitrogen are present in the gaseous medium in the form of a compound, the molecule of which contains both the carbon element and the nitrogen element, such as an amino.

6.- Method according to claim 2, characterized in that the gaseous medium containing carbon and nitrogen is obtained by bombarding a carbon target, for example graphite, by a beam of particles, in the presence of a nitrogen containing gas.

7.- Method according to any one of claims 1 to 6, characterized in that the gaseous medium contains a dilution gas taken from hydrogen and / or a neutral gas.

8.- Method according to claim 7, characterized in that the gaseous medium is a mixture of N ₂ + H ₂ + CH ₄ .

9.- Method according to any one of claims 1 to 8, characterized in that one generates, in the gaseous medium, a plasma by one of the following methods: electric discharge, electromagnetic wave such as microwave or radio frequency, in a processing enclosure (1) outside a container (5) containing the part to be treated (7).

10.- Method according to any one of claims 1 to 9, characterized in that the gaseous medium contains, after activation, ionized species such as N ⁺ and N ₂ ⁺ , C ⁺ , CN ⁺ , (CN ₂ ) ⁺ and excited neutral species such as N, N ₂ , NH, C, H, CN and (CN) ₂ .

11.- Method according to any one of claims 1 to 10, characterized in that an excited neutral species is formed such as CN which contains at least two of the elements N, C, O and B and which remains reactive vis- vis-à-vis the surface of the part (7), after passing through an interstice whose opening dimension (e) prevents ignition of plasma through the interstice.

12.- Method according to claim 11, characterized in that the gap has an opening dimension between 0.01 and 0.3 mm.

13.- Method according to any one of claims 11 and 12, characterized in that the part (7) is arranged inside a container (5) whose wall is closed, with the exception of at least one gap whose opening dimension (e) prevents the ignition of a plasma inside the container (5) but allows the passage of the activated species.

14.- Method according to claim 3, characterized in that the part (7) is arranged inside a container (5) having at least one opening closed by a means providing with the edge of the opening a play not zero in the mechanical sense but sufficiently large to allow the passage of at least one reactive species and sufficiently weak to prevent a plasma from entering the interior of the container (5).

15.- Method according to claim 14, characterized in that the container (5) is produced in the form of a box comprising a wall (5a) having at least one opening closed in a leaktight manner by one of the means following: cover (5b) placed on an upper part of the wall around the opening, sealing means engaged with play in the opening, support on which the inverted box rests, along the edge of the opening.

16.- Method according to any one of claims 11 to 15, characterized in that the part (7) is placed with a plurality of parts (7) in at least one container (5).

17.- Method according to any one of claims 13 to 16, characterized in that the container (5) is arranged in a treatment chamber (2) containing the gaseous medium, in which a plasma is produced by electric discharge.

18.- Method according to claim 17, characterized in that one carries out an evacuation of the gaseous medium towards the outside of the treatment chamber (2), from inside the container (5).

19.- Method according to any one of claims 17 and 18, characterized in that the pressure of the gaseous medium is less than 100 mbar.

20.- Method according to any one of claims 1 to 19, characterized by the fact that the part (7) is metallic and that the metallic part (7) is worn and maintained at a temperature allowing the diffusion of at least one interstitial element taken from one of the elements carbon, nitrogen, boron and oxygen brought to the surface of the metal part (7) by the at least one activated species, inside a layer surface of the metal part (7).

21- A method according to claim 20, characterized in that it carries and maintains the metal part (7) which is austenitic stainless steel at a temperature between 200 ° C and 600 ° C and preferably between 300 ° C and 460 ° C.

22.- Method according to any one of claims 1 to 21, characterized by ⁽ e that the metal part is made of one of the following materials: low or high alloyed structural steel, austenitic, martensitic, ferritic or austeno stainless steel - ferritic, steel with a chromium content of more than 8% by mass, nickel-based alloy, cobalt-based alloy, aluminum, aluminum alloy, titanium, titanium alloy.

23.- Part obtained by a process according to any one of claims 1 to 22.

24.- Metal part according to claim 21, characterized in that it comprises at least one cavity with an opening dimension of between 0.01 mm and 0.3 mm having a surface layer hardened by at least one interstitial consisting of at least one of the elements carbon, nitrogen, boron and oxygen.

25.- Metal part according to claim 24 constituting an injector or a fuel injection rail of a motor vehicle.

26.- Metal part according to claim 23 constituting one of the following objects: stainless steel dish, knife blade, scalpel blade, thin sheet metal, sheet metal coil, orthopedic implant, valve, motor segment, turbine part, tube with external or internal surface hardened by interstitials, valve, bushel, metal shutter, valve, piston, cylinder, pump part (centrifugal, vane, gear, lobe), flow regulator part, regulator part pressure, solenoid valve part, pressurized water nuclear reactor control rod pencil.

27.- Metal part according to any one of claims 21 and 24, characterized in that it has a shiny appearance after treatment.

28.- Part according to any one of claims 1 to 19, characterized in that it is made of a non-metallic material such as glass, ceramic, rubber, polymeric plastic material.