CH705973B1 - A method of treating the surface of an object. - Google Patents

Abstract

The present invention relates to a method of surface treatment of an object having at least one conductive substrate (1). A voltage is applied between an electrode (20) and said substrate (1) of the object to initiate at least one discharge, the electrode (20) being separated from the substrate (1) by a predetermined distance (d) or occupied gap by a dielectric medium in which the discharge occurs, said voltage and the discharge time being set so that said discharge induces a breakdown in the dielectric medium and the formation of a conductive ionized plasma channel (5). An electric current from a current source is then discharged through said conducting ionized plasma channel (5), said current being set so that a surface (6) of the substrate (1) at the base of the arc electrical grounding so as to obtain a print on said substrate (1) having different chemical and / or physical and / or metallurgical properties. The invention can be applied for example to modify the stoichiometric composition of a transition metal nitride, to obtain a metallic glass or an amorphous alloy or to strengthen the tip of a ballpoint pen or the active end of a tweezers tweezers.

Description

[0001] The present invention relates to a method for treating the surface of an object.

[0002] Several electrochemical surface treatment methods are known which make it possible to very locally modify a substrate of an object.

[0003] For example, anodization is an electrolytic passivation process used to increase the natural oxide layer on the surface of a metal substrate. The name "anodization" comes from the fact that the treated part forms the anode of an electrical circuit. Anodization is achieved by passing an electric current with a voltage of between 1 and 300 V between an electrode and the substrate (anode) through an electrolytic solution. Anodization deforms the substrate in that an oxide layer is added to the base metal piece. Anodizing helps to give materials better resistance to wear, corrosion and heat. This process also allows you to change the appearance of the room (color).

Electroplating is a process for applying by means of a direct electric current, a metal deposit on the surface of an object, the metal being initially in the form of cations in a solution called electrolyte and containing other ions to conduct electricity. This technique is used either to preserve the object from oxidation, or to beautify it, or to take its imprint. It follows the principle of electrolysis for its implementation. A substrate treated by electroplating can therefore exhibit changed physical properties such as improved corrosion resistance or an improved exterior appearance.

[0005] Electroerosion is a well-known machining process which consists in removing material from a conductive substrate by using electrical discharges between an electrode and said substrate both immersed in a dielectric medium, such as deionized water , a special oil or even air. Under the action of a high electrical voltage (typically between 60 and 400V), a breakdown (phenomenon which occurs in an insulator or dielectric when the electric field is greater than what this insulator can withstand, it then forms a conductive ionized plasma channel in which an electric arc can propagate (lightning is an example) occurs in the dielectric medium creating an ionized plasma channel between the electrode and the substrate through which an electric arc propagates. At the foot of said electric arc and given the high current, a small surface of the substrate is melted and deteriorated: the volatilized particles of the substrate are then cooled by the dielectric medium to form debris which will be evacuated at the same time as the dielectric medium. A mark is thus drawn on the surface of the substrate. However, if in theory the conditions of the discharge are perfectly defined, in practice, the breakdown distance (distance between the electrode and the substrate) can vary and the physico-chemical composition of the dielectric is not always controlled, in particular at cause of volatilized substrate debris found in the dielectric.

The aim of the present invention is to improve the techniques of surface treatment of a known substrate, in particular those which implement an electric discharge between an electrode and a substrate through a dielectric to obtain a safe, easily implemented and precisely reproducible process which allows a greater variety of results to be obtained depending on the parameters and the substrate used than the known processes. An aim of the present invention is in particular the production of a process making it possible to obtain new alloys or new compounds whose physical and chemical properties are advantageous, such as nitrides, carbides or metallic glasses.

[0007] The subject of the present invention is a surface treatment method according to claim 1.

[0008] A subject of the present invention is also the application of the method according to the invention for modifying the stoichiometric composition of a transition metal nitride according to claim 9, the application of the method according to the invention for the 'obtaining metallic glass or amorphous alloy according to claim 10, applying the method according to the invention for strengthening the tip of a ballpoint pen according to claim 11 and applying the method according to the invention for strengthening of the active end of a clamp according to claim 12.

[0009] The method according to the invention therefore uses electrical discharges between a metal electrode, for example a very fine metal tip, a wire, a bar or any other suitable geometric shape and a conductive surface constituting the substrate of an object. The electrode and the substrate are separated by a distance or a gap of a few micrometers which is completely filled by a dielectric medium in which the discharge therefore takes place, which is a micro discharge. The discharges are set to create a breakdown in the dielectric medium forming a conductive ionized plasma channel between the electrode and the substrate and through which an electric current generated by a generator or other suitable means can pass. Said electric current is in turn configured to melt the substrate at the base of the electric arc on a surface having the shape of a disc without volatilizing the substrate. Indeed, surprisingly and unexpectedly, by using lower currents and shorter discharge times than those usually used for spark erosion, a greater variety of fingerprints can be obtained. In addition, with these parameters, the conditions of the discharge are better controlled even in practice.

[0010] This configuration also makes it possible to modify the metallurgical structure of the substrate in order to change its physical or chemical properties. For example, thanks to the method according to the invention, it is possible to harden a steel substrate very locally and thus make it more resistant in a very specific place.

In a preferred embodiment of the method according to the invention, a reagent is brought into the zone of the discharge in order to be melted in the ionized plasma channel formed following the breakdown and to be mixed there with the substrate melted to create an imprint on the substrate which is actually a new compound or alloy formed from elements of the reagent and elements of the substrate. This reagent can be a mixture of ultrafine powders mixed with the dielectric medium, the constituent elements of the dielectric medium or of the electrode or can be brought into the zone of the discharge in the form of a thin film deposited on the substrate before the discharge.

The ionized plasma channel produced following the breakdown due to the discharge in the dielectric medium, locally reaches temperatures and pressures unachievable by means of conventional metallurgy. As a result, this configuration allows the development of new compounds, as well as the synthesis of new variants of metallurgical phases unknown to date, synthesis of the substrate and of the reagent brought into the zone of the electric discharge. This configuration also makes it possible to modify the metallurgical structure of the substrate in order to change its physical or chemical properties. The cylindrical ionized plasma channel created by the discharge and encapsulated by the surrounding dielectric medium constitutes a microscopic autoclave.

The method according to the invention can also be compared to a "writing" method since a high frequency discharge rate associated with a displacement of the electrode relative to the substrate allows the production of alloys in continuous, just like their implantation on a surface of the object according to a predefined path.

[0014] Other advantages of the present invention will become clear on reading the following description in which some preferred applications of the surface treatment process according to the invention are detailed with reference to the following figures. Figs. 1a to 1c illustrate the method according to the invention according to a first embodiment. Fig. 2a illustrates the conductive ionized plasma channel created as a result of breakdown due to electric discharge occurring in the dielectric medium between an electrode and the substrate of an object. Fig. 2b illustrates the ionized plasma channel collapsing on itself after the current between the electrode and the substrate is turned off. Figs. 3a to 3c illustrate an embodiment of the method according to the invention in which the reagent is supplied to the discharge zone in the form of ultra fine powders mixed with the dielectric medium. Figs. 4a to 4c illustrate an embodiment of the method according to the invention in which the reagent is supplied to the discharge zone in the form of at least one thin layer deposited by appropriate means on the substrate of the object to be treated. Figs. 5a to 5d illustrate an embodiment of the method according to the invention in which the reagent consists of a composite electrode which degrades progressively and constantly during the discharge. Figs. 6 and 7 illustrate an embodiment of the method according to the invention in which the substrate is a sacrificial substrate intended to be dissolved after treatment to keep only the imprint obtained by the method. Figs. 8a to 8c illustrate a ballpoint pen tip seen in section after treatment by a method according to the invention. Fig. 9 illustrates another example of application of the method according to the invention in which the object to be treated is the end of the branches of tweezers.

Several embodiments of the method according to the invention will now be described with reference to FIGS. 1a to 5d.

[0016] In general, the method of surface treatment of an object according to the invention uses electric discharges between an electrode 20 and a conductive substrate 1 of said object. Electrode 20 may be in the form of a very fine metal tip, wire, bar, or any other suitable geometric shape. In the figures, electrode 20 acts as a cathode and substrate 1 as an anode. The polarities can however be reversed.

The electrode 20 can be made of any metal, but preferably is made of a refractory metal such as tungsten, molybdenum, iridium or any other metal having a high melting point or properties thermionic emission. The high melting point ensures minimal tip wear, and the electronic thermionic emission properties ensure the efficiency of the discharge to melt the substrate, as well as the cooling of the electrode.

As illustrated in FIGS. 1a, 2a, 4a and 5a, the electrode 20 and the conductive substrate 1 are separated by a distance or gap d of a few micrometers, preferably between 1 and 200 μm. This gap d is entirely occupied by a dielectric medium 3 in which the discharge occurs between the electrode 20 and the substrate 1. The dielectric medium 3 is preferably a gel or a liquid such as mineral oil or deionized water.

[0019] A voltage of preferably between 1 and 400v is applied between the electrode 20 and the substrate 1 in the dielectric medium 3 to produce a breakdown in said dielectric medium. A conductive ionized plasma channel 5 is then formed between the electrode 20 and the substrate 1 as illustrated in FIGS. 1b, 2a, 3b, 4b, 5b. An electric current from the discharge of the capacitor, generator or other current source is discharged into the conductive ionized plasma channel 5 as illustrated in FIG. 2a. The energy thus supplied contributes to the formation of a micro-plasma confined at very high temperatures and pressures.

The mass of the plasma, surrounded by a gaseous envelope or gas bubble 51 visible in FIG. 2a, grows during the discharge which typically lasts between 10 ns and several seconds. But the radial expansion of the plasma is strongly restricted by the presence of the dielectric medium, and the energy of the discharge is concentrated in a very small volume having essentially the shape of a cylindrical channel 5. The ultra-hot plasma radiates from the energy to the surface of the electrodes causing the metal to melt. The mass of the plasma is in fact made up of molecules of the dielectric medium pulverized and dissociated by the energy of the discharge. The high temperature of the plasma inside the ionized plasma channel 5 causes melting on the substrate of a surface 6 having the shape of a disc of a few tens of micrometers in diameter, typically 0.5 to 200 µm. However, the high pressure of the plasma limits the evaporation of molten material, particularly from the substrate. This mechanism results in the formation of near-circular imprints of molten metal at the roots of the electric arc passing through the ionized plasma channel 5. The size of the molten metal disc is a direct function of the energy of the discharge.

After the power is cut off, the conductive ionized plasma channel 5 collapses as illustrated in FIG. 2b.

[0022] The temperature and pressure levels reached in the ionized plasma channel 5 are extreme, of the order of 5000 to 40,000 K for the temperature and from 10 bar to 1 Mbar for the pressure. However, said channel 5 is microscopic and the substrate 1 and the surrounding dielectric medium 3 are essentially at room temperature. Thus, the cooling (quenching) rates from the moment the power is turned off are gigantic, typically on the order of a million degrees K per second.

Thus, the molten substrate on its surface 6 cools extremely quickly and an imprint 7 is obtained. If, for example, the substrate 1 is the surface of a steel object, it is thus possible with the method according to the invention to perform a very local hardening on said object and to obtain very locally a very hard steel, more harder than the rest of the substrate. It is therefore possible to modify very locally the physical properties (hardness in the case of the steel object) and chemical (transformation from austenite to martensite in the case of the steel object).

In a preferred embodiment of the process according to the invention, a reagent 40 is brought into the zone of the discharge to react with the substrate 1 in the ionized plasma channel 5. In fact, the reagent 40 is found trapped in the conductive ionized plasma channel 5 created by the discharge and is melted or even ionized by the plasma radiation (FIG. 2a). Due to the extremely high temperature and pressure inside the ionized plasma channel 5, chemical and / or metallurgical reactions become possible between the different components of the ionized plasma channel 5 - element originating from reagent 40, element originating from l electrode 20 or dielectric medium 3 - and the substrate 1.

The conductive ionized plasma channel 5 created by the discharge is used in the method according to this embodiment as an autoclave (or a microreactor) in which physical and chemical reactions take place making it possible to obtain an imprint 72 on the substrate 1 which is an alloy between the elements of the reagent 40 and of the substrate 1 and whose physical and chemical properties differ from those of the rest of the substrate 1. In particular, given the extreme temperatures and pressure reached in the ionized plasma channel 5, the chemical, metallurgical and physical reactions which occur there are unattainable by other known means and make it possible to develop novel compounds or alloys. Several examples will be detailed below.

[0026] The electrical parameters of the discharge such as the shape of the current pulse are used to control the temperature and pressure inside the ionized plasma channel 5 and the duration of the reaction. The reactive mixture thus created in the autoclave formed by the ionized plasma channel 5 can be implanted on the substrate 1 to modify the physical and chemical properties of the substrate 1 for a functional purpose (hardness, corrosion resistance, etc.) or decorative (color, appearance ...).

[0027] In a preferred embodiment of the invention illustrated in FIGS. 3a to 3c, the reagent 40 consists of powders or nano-powders 41 of various types and mixed with the dielectric medium 3. These fine to ultra-fine powders 41 have a diameter substantially less than the distance d between the electrode 20 and the substrate. 1, preferably between 1 and 10,000 nanometers and even more preferably between 1 and 10 nanometers. Thus, these powders 41 are trapped in the conductive ionized plasma channel 5 created by the discharge and are melted or even ionized by the radiation of the plasma (FIG. 2a). In the ionized plasma channel 5, chemical and metallurgical reactions between the powders 41 and the substrate 1 become possible, in particular thanks to the extremely high temperature and pressure inside said ionized plasma channel 5. The mixture thus created in the ionized plasma channel 5 can be implanted in the substrate 1 in the form of an imprint 71.

A homogeneous distribution of the nano-powders 41 in the dielectric medium 3 is obtained for example by adding a surfactant or surfactant in the liquid, or dielectric gel, such as a suitable soap. The molecules of this agent organize themselves on the surface of the nanoparticles, cancel out Van der Waals force-type attractions, and prevent their coalescence or flocculation, stabilizing the composite dielectric medium.

[0029] As a variant, the reagent 40 consists of certain constituent elements of the dielectric medium 3. For example, the dielectric medium 3 can be a mineral oil naturally containing silicon. Said silicon particles will be melted or ionized in the ionized plasma channel 5 created following the electric discharge between the electrode 20 and the substrate 1 through the dielectric 3 and chemical and / or metallurgical reactions will occur between said molten elements. and substrate 1, said reactive mixture being implanted in said substrate 1 to form an imprint 7.

[0030] In yet another preferred embodiment of the invention illustrated in FIGS. 4a to 4c, the reagent 43 consists of at least one thin layer 43a of a suitable conductive material deposited by any suitable means on the substrate 1 in the zone of the discharge between the electrode 20 and the substrate 1. Figs. 4a to 4c illustrate an example in which three thin layers 43a, 43b, 43c have been successively deposited on the substrate 1 by a known technique. The high temperature inside the ionized plasma channel 5 causes the successive melting of each thin layer 43a, 43b, 43c and of the substrate 1 and the mixing of the various elements composing them to obtain an imprint at the base of the electric arc. 73 consisting of a new compound or a new alloy combination of constituent elements of the three thin layers 43a, 43b, 43c and of the substrate 1.

Finally, in a last preferred embodiment illustrated in FIGS. 5a to 5d, the reagent 40 is formed by the electrode 21 which in this variant is a composite electrode illustrated in detail in FIG. 5d. The electrode 21 is for example a cylinder composed of several layers 44a, 44b, 44c, 44d forming part of the reagent 44. These layers can be constituted by any suitable material and in particular metals or also be a mixture between a metal and powders. extremely fine various.

In this variant, the shape and energy of the electric discharge between the electrode 21 and the substrate 1 are adjusted to induce a progressive and constant wear of the electrode 21. The progressive and constant wear of the electrode 21 brings the reagents 44a, 44b, 44c, 44d of the component into the ionized plasma channel 5 where they are melted or ionized due to the temperature and the extreme pressure prevailing in the channel 5 and mixed with the substrate 1. An imprint 74 is thus obtained on the substrate 1, the composition of which comprises elements of said substrate 1 and elements of each of the layers 44a, 44b, 44c, 44d of the electrode 21.

[0033] As will be described in detail in the examples, the method of surface treatment of an object according to the invention allows a large number of results to be obtained, fulfilling various objectives. Indeed, it is possible by the method described to modify very locally the substrate 1 of the object to give it certain physical (hardness, corrosion resistance ...) or chemical (colors ...) properties. In addition, the number of possible results for fingerprint 7 increases further if reagent 40 is added to the landfill area. It is even possible to use the process to create new alloys unattainable by conventional metallurgical techniques.

In addition, the morphology of the imprints 7 obtained by the implementation of the method according to the invention, which are in fact craters with edges that can form protuberances, depends on the electrical energy of the discharges (current, duration pulse). It is therefore also possible to modify the texture of the substrate of an object, for example to make it easier to grip the object (see application 5 below).

[0035] It is also possible from a substrate 1 to produce and recover the alloys produced by the process according to the invention and in particular by the embodiment using a reagent described above. To do this, the method is carried out on an object serving as a "sacrificial" metal support 10 which can be removed or dissolved by chemical or other means. Once an imprint 7 has been obtained on said metal support 10, said support is dissolved and the alloy or compound which forms the imprint 7 can be recovered either in the form of powder (by suitable filtration means, FIG. 6), or in the form of a part 77 with a certain geometric structure close to 2D (fig. 7). The sacrificial substrate may consist of an aluminum foil to be dissolved after treatment with a suitable acid, for example.

[0036] Several applications of the method according to the invention will now be described by way of example.

Application 1

This first application case relates to obtaining new variants in the stoichiometric composition of transition metal nitrides (TiN, ZrN, MoN, TaN, HfN, VN, NbN, CrN, WN), also known as the name of refractory nitrides. In addition to their very high melting points (2000 ° C – 4000 ° C), the technical importance of these compounds stems from their extreme hardness as well as their resistance to corrosion. Indeed, because of their very high hardness, these materials are used for cutting tools and as anti-abrasive coating. They are also great heat shields because of their very high melting point.

[0038] In addition, because of their electronic properties, these materials are used in certain circuits. Some nitrides are also superconductors. The fields of use are thus multiple.

[0039] The properties of these materials depend crucially on the defects in their crystal structure. In fact, these compounds do not form completely stoichiometric phases (for example, for titanium nitride, TiNx, with x being able to vary in a wide range between 0.5 and 1.1 approximately), and their properties depend on the proportion between metal and non- metal as well as the concentration of defects or vacancies in their crystal structure. These metallurgical phases exist over a wide range of composition, with a fairly high possible defect rate (up to 50%) at the non-metal site. The various known synthesis techniques give rise to defect structures of a different nature leading to different physical properties.

It has been noted that the process according to the invention makes it possible to obtain new stoichiometric compositions of metal nitride and in particular compositions which are unachievable by conventional synthetic means.

To implement the method according to the invention in the present application, a composite dielectric medium is produced containing the nitride in the form of a very fine powder in suspension (for example titanium nitride TiN available commercially and produced by by conventional means, mixed in powder form with a mineral oil of the hexadecane or paraffin type which constitutes the dielectric medium). This initial nitride powder has an initial stoichiometric composition TiNxidonne with xientre 0.5 and 1.1. Using this composite dielectric medium in the process described above, the titanium nitride TiNxis in powder form is subjected in the ionized plasma channel 5 to ultra-high temperatures and pressures, which changes its structure and gives rise to high temperatures and pressures. new stoichiometric variants of the compound, some of which cannot be produced with metallurgical or synthetic means known today. The reaction induced by the electric discharge between the electrode 20 and a suitable substrate 1 such as for example an aluminum or silver wafer is therefore as follows:

xf being a new stoichiometry different from the initial stoichiometry xi, and potentially unattainable by conventional synthetic means.

[0042] In addition, the extremely high temperature and pressure inside the cylindrical ionized plasma channel 5 created by the discharge and the gigantic cooling rates which result from it induce a particular crystallization (by rapid quenching) of the compound, this which promotes the production of structures with crystalline defects not known to date.

The nitrides and their new phases thus obtained are distinguished by their electronic properties (technical applications) and by their visual appearance (decorative applications). In particular: The titanium nitride TiN can thus be made ferromagnetic at room temperature. The color of the titanium nitride TiN can be adjusted according to the power of the discharges (yellow, black, blue or even other colors), the color depending on the nitrogen content of the titanium nitride resulting from the reaction.

The method according to the invention and described above can also be applied in a similar way to adjust the stoichiometric composition of transition metal carbides, compounds with properties similar to nitrides. These carbides and nitrides are indeed very similar in their crystallographic structures, carbon and nitrogen being similar in size, electronic structure and electronegativity.

Application 2

Another application of the method according to the present invention is the synthesis of metallic glasses or amorphous alloys. By re-solidifying, a molten metal or alloy reaches a state of equilibrium in the lowest possible energy state, namely the crystal lattice of the metal or alloy in question. Metallic glasses, discovered in 1960, are alloys devoid of a crystalline structure. They have very interesting physical properties for many applications: • High resistance to corrosion, • High mechanical resistance, • Superior hardness, • Unusual elastic properties (for elastic energy storage).

Metallic glasses are obtained by ultra-rapid thermal toughening processes from the simultaneous melting of metals with atoms of non-similar size or structure. At the time of re-solidification, the atoms moving through the liquid do not have time to arrange themselves in a "standard" crystal structure before solidification.

[0047] The key point for obtaining metallic glasses is the development of a mixture of molten metals at high temperature. This mixture must contain certain specific types of elements. At a given moment, the mixture of molten metals is cooled at very high speed. The dissimilarities between atoms create a "confusing effect", and if the rate of cooling is high enough, a metallic glass is formed.

The method according to the invention, and in particular the embodiment in which the reagent 40 is brought into the zone of the discharge in the form of ultrafine powders 41 mixed with the dielectric medium 3 (Fig. 2a to 3c), is particularly suitable for the synthesis of metallic glasses due to the following three characteristics:

1) It is possible to locally create a mixture of several molten metals, depending on the composition of the mixture of nanopowders 41 initial.

2) The re-solidification of this molten mixture takes place at gigantic cooling rates, the heat source (the ionized plasma channel 5) being microscopic and the mass of the substrate being at room temperature.

[0051] 3) The mixture of molten metals is found encapsulated, confined by the pressure inside the ionized plasma channel 5. The losses by sublimation or evaporation of certain components of the mixture are greatly limited.

[0052] Combinations of typical elements leading to the formation of metallic glasses are as follows: Au-Si Pd-Cu-Si Pd-Fe-P Au-Pb-Sb La-Al-Ni Zr-AI-Ni-Cu Zr-Ti-Cu-Ni-Be, and many others.

In practice, for the implementation of the method, a composite dielectric medium is prepared (for example based on liquid paraffin) containing a mixture of nano-powders comprising the elements suitable for the formation of metallic glass, for example a mixture containing Zr, Al, Ni and Cu. After choosing a suitable substrate, for example a steel wafer, electric discharges are produced through the composite dielectric medium between an electrode and the substrate as shown in Figs. 2a, 2b and 3b. Said discharges are caused by the application of a voltage between 1 V and 1200 V. Once the discharge is established, a current generator sends an electric current through the conductive ionized plasma channel 5 formed as a result of the breakdown due to the discharge for a while discharge. The current is between 100 mA and 1000 A, the duration of the discharge being between 10ns and several seconds. The intensity of the current as well as the duration of the discharge make it possible to adjust the parameters of the metallurgical reaction, parameters which will be optimized according to the mixture of powders to be reacted. It is thus possible to obtain on the substrate an imprint 7 which in this application is a metallic glass obtained by melting and mixing the powders included in the dielectric medium.

[0054] In a variant of this method, one or more components of the metallic glass may be supplied by the electrode or by the substrate. For example, for the synthesis of an Au - Si metallic glass, one can use a gold substrate or a gold alloy and a composite dielectric containing a fine silicon powder.

[0055] The technique can thus be used for direct writing with metallic glass on an ordinary alloy substrate, to fabricate a region having high corrosion resistance or for an application which requires a particular physical property such as higher hardness. .

Application 3

During the implementation of the method according to the invention, the temperature inside the ionized plasma channel 5 causes the melting of the substrate 1 and of the reagent 40 (powders, constituent elements of the electrode or of the medium dielectric or elements deposited in a thin film on the substrate 1). The discharges used are in fact very fast thermal "flashes" (lasting a few microseconds) and repetitive.

[0057] In this application of the method according to the invention, the temperature inside the ionized plasma channel 5 is used to initiate very local exothermic reactions. These reactions can last well beyond the duration of the discharge or the current passing through the ionized plasma channel 5 and well beyond the collapse of said channel once the current is cut off and act as a very localized annealing. along the path of the electrode. New textures, colors and visual renderings can thus be obtained.

[0058] The electrode - gap - substrate configuration allows a wide variety of microscopic and nanoscopic powders to be inserted at the site of the discharge. In this process, mixtures known as "thermites" are inserted (a thermite is a pyrotechnic mixture of a powdered metal and a metal oxide that generates an exothermic oxidation-reduction reaction). The most famous thermite is a mixture of iron oxide and aluminum. The combustion of such a pyrotechnic mixture causes very intense heat releases which can reach 4000 ° C: Fe2O3 + 2Al → 2Fe + Al2O3 + Heat

There is a very large number of metal - oxide mixtures which can be used for this type of reaction, in particular "nano-thermites" (the oxidant and the reducing agent are no longer in the form of powders, but of nano- particles or nano-powders).

Typical examples of oxidant - reducing mixtures (thermites) are as follows: Al + Fe3O4 Al + MoO3 Al + AgO Al + Bi2O3 Al + WO3 B + Fe3O4 Be + SiO2 Hf + CuO The + CuO Zr + CuO and many others.

In practice, for the implementation of the method, a composite dielectric is prepared (for example based on liquid paraffin) containing as reagent a mixture of thermites (at least one thermite). After choosing a suitable substrate, for example a steel wafer, electrical discharges are produced through the composite dielectric between an electrode 20 and the substrate 1 (Figs. 3a to 3c). Said discharges are caused by the application of a voltage between 1 V and 1200 V. Once the discharge has been established, a current generator sends an electric current through the conductive ionized plasma channel 5 thus created for a certain period of time. dump. The current is between 100 mA and 1000 A, the duration of the discharge being between 10ns and several seconds. The intensity of the current as well as the duration of the discharge make it possible to adjust the parameters to start the exothermic reaction on the path of the electrode. These parameters may vary depending on the mixture of thermites to be reacted. The discharge allows to start an exothermic reaction due to the combustion of thermites in the ionized plasma channel 5, an exothermic reaction which continues beyond the discharge time, the current cut and the collapse of the ionized plasma channel. . The process according to the invention thus causes a local chemical reaction which at least leads to the formation of a new oxide on the substrate.

[0062] The originality of this variant lies in the use of the discharge to start a very localized exothermic reaction, while using the composite dielectric to deliver the fuel mixtures to the reaction zone.

Another variant consists in using the durable thermal release, but very local of the reaction to explore the formation of new compounds by a “combinatorial” approach known under the name of “Single Sample Concept” (Jürg Hulliger and Muhammad Aslam Awan , Chem. Eur. J. 2004, 10, 4964–4702). In this variant, the process according to the invention is used to explore the synthesis of new compounds in parallel, by preparing a composite dielectric containing as reactant a very large number N of elements capable of reacting together.

Application 4

A fourth example of application of the method according to the present invention is that of strengthening the tips of the ballpoint pen.

The ball 8 of a ballpoint pen generally made of tungsten (diameter approximately 1mm) is crimped in a housing 91 made in a part 92 of steel as illustrated in FIGS. 8a to 8c. The thin steel walls 11 at the end of the housing 91 constitute the weak point of the device. These walls 11 easily give way under the pressure of writing which leads to unwanted and excessive flow of the ink contained in the cartridge (not shown). This is the major flaw, unresolved until today, of ballpoint pen tips.

[0066] By using the method according to the invention, the steel of the walls 11 can be quenched and locally hardened on a thin ring 78 around the crimping. The ink cartridge thus produced has a much better behavior in terms of ink leaks.

For the implementation of the method according to the invention, electrical discharges are caused between the walls 11 (substrate) of the crimping of the ball 8 and an electrode 22 (symbolized by the sign - in the figures), for example the edge of a cavity made in a copper coin. The space between the walls 11 and the electrode 22 is filled with a dielectric medium 3 such as liquid paraffin for example. The discharges cause intense but very local heating of the walls 11 of the crimping, causing by quenching a consequent hardening of the steel on a ring 78 as illustrated in Figs. 8a and 8b.

Another variant consists in using a composite dielectric medium containing a suitable reagent in powder form such as, for example, a hardener such as titanium nitride. Following the repeated application of discharges on the walls 11 of the crimping according to the method according to the invention, a surface alloy is encrusted which reinforces the mechanical properties of said walls 11.

Application 5

[0069] The method according to the invention can also be used to treat fine and precise mechanical instruments such as tweezers (used in cosmetics, watchmaking, microelectronics, in the medical field, etc.). This last example relates to the application of the method according to the invention on the end 101 of the branches of tweezers 102 (or any other similar tool) with the dual aim of reinforcing the mechanical properties of a fine structure (for example made of steel) , but also greatly improve the gripping properties of the object. This second effect is obtained by obtaining a texture with controlled roughness on the active surface of the tool, this textured surface itself being a surface alloy of optimum hardness for the application. This application case is illustrated in fig. 9 in the case of cosmetic tweezers intended for hair removal.

The localized treatment is carried out here in a similar way to that of the ballpoint pen tips, by causing electric discharges between the ends 101 of the branches of the tool and an electrode (not shown), the distance of breakdown or gap being filled with an appropriate dielectric medium (comprising a reagent in powder form or not). An alternative is also that of moving a wire electrode along the path to be treated, in a working mode similar to direct writing. The use of electric discharges also offers the option of being able to control the texture or roughness of the surface via the parameters of the generator (current, energy, pulse duration), with the aim of bringing functional improvements to the surface. mechanical instrument.

In fact, electric discharges cause circular impressions on the surface of the metal. The morphology of the imprints (craters with edges that can form protuberances) depends on the electrical energy of the discharges (current, duration of the pulse). In addition to the addition of a hard alloy on the active surface of the tweezers, we can therefore provide a texture that will improve the grip of the object by the instrument. In the case of cosmetic tweezers 102, the grip of a hair will be considerably improved. The same goes for fine wires in the case of tweezers for microelectronics or watchmaking. As shown in fig. 9, the method even allows different textures to be produced on the two ends of the tweezers, for a secure grip on the object to be apprehended. Thus, in the case shown schematically in FIG. 9, the texture on the upper arm is obtained by using more energetic discharges than for obtaining the texture on the lower arm.

[0072] It is understood that the embodiments described above are in no way limiting and that they can accommodate any desirable modifications within the frame as defined by claim 1.

As regards the type of discharge, it may be the discharge of a capacitor, or the current may also be supplied by an ad-hoc generator or any other source of current after breakdown of the dielectric medium . The duration of the discharge, the amplitude of the current as well as the shape of the pulse are optimized for the implantation of the ions contained in the dielectric medium.

The configuration illustrated in the figures with a cathode electrode and an anode substrate is particularly favorable. The anode very quickly undergoes electronic bombardment at the very beginning of the discharge, which causes the metal to melt. The cathode would not be reached by positive ions, much heavier and less mobile, until much later and in the worst case. On the other hand, since the cathode emits electrons, the diameter of the plasma on the cathode is also much smaller. However, the reverse is also possible: the electrode can be the anode and the substrate can be the cathode. Likewise, one could provide for the deposition of materials from the tip to the substrate.

A high frequency spark rate allows the continuous production of alloys, as does their implantation on the substrate according to a predefined path. Thus, once the arc is established, the electrode can be moved without interrupting the current like a micro torch. In this case, the arc can be maintained for several seconds or even longer. Another obvious application example of the invention is therefore that of writing on a substrate of an electronic circuit. Indeed, by choosing the substrate and the electrical medium or the appropriate electrode, it is possible to use to obtain on the substrate a meander in a particular alloy, for example a superconducting alloy.

Electrically insulating gels or pastes such as petroleum jelly, hydrocarbon-based greases or vacuum grease can be used as dielectric. Very high viscosity liquids, such as silicone oils and polydimethyl siloxane (PDMS), can also be used for this purpose.

The viscosity of this gel or this paste is chosen so that the dielectric medium does not flow, even placed on an inclined or vertical surface. The use of a gel or dielectric paste provides in particular the following advantages: The solution obtained by intimately mixing the gel or paste with the various powders and nanopowders is stable and uniform, there is no sedimentation or agglomeration between the constituents which can no longer move freely in the dielectric medium. In the case of liquid dielectric, powders can settle or move uncontrollably on the surface of the part to be marked, which can affect the uniformity of the chemical marking. An important advantage for the user is the use of the gel for the treatment of complex surfaces, vertical, inclined or otherwise, where a liquid which may start to flow is not adequate. The gel or paste does not evaporate. Once applied, the thickness remains constant which is comfortable for the user. In addition, since it does not spread out, different gels or pastes containing different powders can be placed simultaneously very closely without formation of mixtures between the gels. This allows multiple treatment on small surfaces. Finally, the configuration with a semi-solid dielectric allows the addition of a large number of different chemical elements. An emulsion is much more tolerant than a liquid in producing a homogeneous mixture of several constituents. Thus, we can provide for the inclusion of elements, such as sulfur, selenium or tellurium, to reduce the surface tension of the molten metal and obtain better adhesion of the impression. Appropriate elements, such as silicon or phosphorus, can be added to the dielectric, which may promote the formation of an amorphous alloy or metallic glass, which is much harder than an ordinary alloy. It is also possible to include powders based on rare earths, such as europium or praseodymium. All of these additions can be done simultaneously in the gel or paste, in order to obtain an optimal mixture for a given application.

[0078] In addition, a liquid, gel or paste offers a high molecular concentration, suitable for the creation of dense plasma. At the same time, the liquid or gelatinous medium allows the electrode to remain mobile on the surface to be treated.

As liquid or gel, we can use: mineral oils or fats, deionized water or petroleum jelly, or any other dielectric liquid or gel having the dielectric properties required to achieve breakdown. dielectric liquids or gels, the molecules of which contain as constituent elements the element or elements that one wishes to implant in the surface of the part to be marked. For example, if you want to implant silicon, you will use silicone oil or silicone grease. In general, however, any other liquid or gel of suitable molecular composition can be used, up to very long molecules such as those in liquid crystals. The choice of molecular composition ensures the presence of the desired ions for implantation in the plasma of the landfill. These same liquids or gels containing powders in suspension (emulsions).

The use of a liquid or semi-solid dielectric makes it possible to add, in addition to the elements intended to be implanted in the substrate, other additives which may themselves promote the breakdown of the dielectric under the effect of the electric field applied, an equally important phenomenon for this application. Indeed, the inclusion of metallic or semiconductor microparticles of a dielectric significantly reduces the resistance to breakdown under the effect of an electric field. But the effect is all the more pronounced if the inclusions are in the form of fibers with a very large length / diameter ratio.

[0081] In the context of this application, it is essential to obtain a breakdown of the dielectric when an electrical voltage is applied to the electrode. This breakdown occurs anyway if you get the electrode close enough to the substrate, even in an undoped dielectric. However, for some applications, it is preferable to stay a certain distance from the surface in terms of micrometers, either to decrease the risk of the tip sticking or soldering to the surface, or to increase the mass of the tip. plasma or quantity of material melted by the discharge. The key point is that the filiform inclusions must not interfere with the chemical dosage carried out on the dielectric, i.e. have a negligible mass. Such inclusions are for example known under the names of “nanotubes” or “nanorods”. Initially discovered for the element carbon, nanorods and nanotubes now exist for a wide range of elements such as Cu, Mo, Ag, Pb or compounds such as oxides WO3, MoO3. In the case of carbon nanotubes, their dimensions can reach a few nanometers in diameter and several tens of micrometers in length. Length-to-diameter ratios of the order of 100,000 are common and can even reach 500,000. Such particles greatly promote dielectric breakdown at comfortable distances from the surface. The use of carbon nanotubes of a determined length is therefore also a means of adjusting the breakdown distance. A composite dielectric containing the doping elements as well as nanotubes and / or nanorods therefore constitutes a very suitable composite dielectric medium for the use of the present invention.

[0082] The dielectric medium could also be a gas, such as air, or a gas with fine particles in suspension, for example acetylene with silicon which would be integrated into the impression.

In particular, instead of assigning a single discharge to a predetermined location, two or more discharges can be performed at the same location or at offset locations on the substrate.

[0084] The surface treatment method according to the invention has many advantages.

In particular, the gap d can be adjusted to the nearest nanometer by piezoelectric actuators to obtain reproducible fingerprints, unlike spark erosion where discharges occur randomly with great variability of the gap.

[0086] In addition, the method according to the invention adapts to the physical properties of the part to be treated, such as the melting point, thermal conductivity etc., this adaptation is done via the energy of the discharge; the surface finish or the roughness of the part by adjusting the diameter of the cavity; the chemical composition of the part to be marked via the choice of dielectric medium and / or reagent.

Compared to traditional spark erosion, this process is distinguished by the following facts: a) The discharge conditions are perfectly defined, in particular the breakdown distance (gap) and the physicochemical composition of the dielectric in the gap. b) The vicinity of the discharge is reproducible. The discharges will in principle be identical and reproducible. On the other hand, in an electroerosion gap, we are in the presence of a boiling liquid (bubbles, debris, chaotic movement of the liquid). Also the frictional forces of the liquid vitrified by the pressure on the surface of the electrodes are variable from one discharge to another. In this case, the geometry used for the discharge is always the same.

Claims (12)

A method of surface treatment of an object having at least one conductive substrate (1) characterized in that A voltage is applied between an electrode (20) and said substrate (1) of the object to initiate at least one discharge, the electrode (20) being separated from the substrate (1) by a predetermined distance (d) or gap between 1 and 400 μm occupied by a dielectric medium (3) in which the discharge occurs, said voltage being between 1 and 400 V and the discharge time being between 1 and 800 microseconds so that said discharge induces a breakdown in the dielectric medium (3) and the formation of a conductive ionized plasma channel (5), An electric current from a current source is discharged through said conductive ionized plasma channel (5), said current whose intensity is between 100 mA and 1000 A being parameterized so that a surface (6) substrate (1) at the base of the electric arc melts so as to obtain an imprint (7) on said substrate (1) having different chemical and / or physical and / or metallurgical properties. Surface treatment method according to claim 1, characterized in that a reagent (40) is fed into the discharge zone so that reagent elements (40) are trapped and melted in the ionized plasma channel. and are implanted in the substrate (1) in the molten surface (6) at the base of the electric arc. 3. Method according to claim 2, characterized in that the reagent (40) comprises constituent elements of the dielectric medium (3). 4. surface treatment method according to one of claims 2 or 3, characterized in that said reagent (40) comprises particles in the form of powders (41) whose diameter is less than the predetermined distance or gap (d ) and which are mixed with the dielectric medium (3). 5. Surface treatment method according to one of claims 2 to 4, characterized in that the reagent (40) comprises at least one element initially forming part of the electrode (20), the parameters of the discharge being chosen to cause progressive and constant wear of said electrode at each discharge. 6. Surface treatment method according to one of claims 2 to 5, characterized in that the reagent (40) comprises a thin layer (43a) of a conductive material deposited on the substrate (1) before the implementation. process. 7. Surface treatment method according to one of the preceding claims, characterized in that the dielectric medium (3) consists of a liquid, a gel or a dielectric paste. 8. Surface treatment method according to one of the preceding claims, characterized in that the electrode (20) is made of refractory metal having a melting point equal to or greater than that of iridium and / or properties thermionic emission. 9. Application of the process according to one of claims 2 to 8 for modifying the stoichiometric composition of a transition metal nitride, characterized in that the reagent (40) comprises the transition metal nitride in an initial stoichiometric composition . 10. Application of the method according to one of claims 2 to 8 to obtain a metal glass or amorphous alloy, characterized in that the reagent (40) comprises the elements used in the composition of the metal glass. 11. Application of the method according to one of claims 2 to 8 for reinforcing the metal tip of a ball pen, said tip comprising a housing in which is crimped a ball. 12. Application of the method according to one of claims 2 to 8 for reinforcing the active end of a clamp.