WO2002006044A1 - Method for adjusting contact resistance of the interface between two components of polymetallic parts - Google Patents

Abstract

The invention concerns a method which consists in subjecting a batch of polymetallic parts to a heat cycle comprising at least a phase during which the interface (4) is compressed by differential expansion of the components (2, 3) of the polymetallic parts (1) and a phase during which the contact surfaces (2', 3') of the interface (4) are contacted with an oxidising medium. The compression phase and the contact phase with the oxidising medium can be the same phase. Thus, the protrusions of the contact surfaces (2', 3') are flattened and an oxidised layer is formed on the contact surfaces (2', 3'). The invention is useful is particular for treating polymetallic coins to enable the implementation of more efficient methods for controlling the authenticity of coins.

Description

 Method for adjusting the contact resistance of the interface between two components of polymetallic parts

The invention relates to a method for adjusting the contact resistance of the interface of two components of a polymetallic coin and in particular of a polymetallic coin, in order to allow effective verification of the authenticity of the coin and thus making counterfeiting very difficult. ^"

At present, coins with complex structures are produced which are very difficult to counterfeit. To make these hard-to-forge coins, expensive alloys and heavy industrial facilities must be used. Bi-metallic parts are produced in particular which comprise an insert in the form of a circular disc made of a first metallic material and a circular crown made of a second metallic material, the 2 constituent elements of the coin being assembled by crimping.

One of the constituent elements (generally the insert) is itself produced in polymetallic form, for example by plating on either side of an inner layer of a second metallic material, of two outer layers of a third material metallic.

The machines in which automatic payment is made at least partially with coins play an important role in modern life. At present, 10% of commercial transactions are made by automatic coin-operated payment machines. It is therefore extremely important to be able to have machines and coins such that the authentic coins put into circulation by the monetary authorities are all accepted and that the electromagnetic counterfeits are all rejected.

Coin-operated machines must be able to discern the different values of the coins for which they are designed and perform operations such as exchanging or supplying money in the form of coins, in addition to the essential function of discriminating between authentic pieces and counterfeits. The performance of coin machines has been greatly improved in the last few years thanks to the progress of microprocessors and transducers. The detection systems of coin-operated machines carry out increasingly fine discrimination, which is made necessary by the efforts and skill of the counterfeiters and the demand of the public regarding the operational safety of these machines.

At the present time, efforts to improve coin machines are mainly taken care of by the manufacturers of automatic machines, while the quality of the discrimination depends in part on what may be important, in particular in the case of coins. with a complex structure and a polymetallic composition, precise analysis of the metallic materials used and methods of manufacturing coins. The problem is made all the more complex since the production of the same coin is generally entrusted to several producers or several groups of producers. The coins are characterized first and in the simplest way by their diameter, their composition and their mass. Modern coin-operated machines also measure the physical properties of parts and in particular the electromagnetic properties of these parts. Originally in coin machines, the diameter and thickness of the coins (possibly their mass) were mainly used for their identification, since there were then only a very limited number of monetary alloys.

At present, most electronic systems of coin-operated payment machines also measure a property of the metallic material of the coin, such as conductivity or magnetic permeability. It is in fact easier to counterfeit the size of a part than its color, density, conductivity, hardness or magnetic permeability which depend on the composition of the metallic material or materials of the part.

Most industrial detection systems currently used systematically include the use of inductive sensors, since these sensors do not require contact with the coin, are very safe, precise and low cost, and finally do not fear the dirt and fluids that can cover the surfaces of the coin. Usually, the inductive sensors measure the thickness, the diameter and the conductivity of the parts which roll on the wafer, on an inclined slope passing in front of coils with ferrite cores, placed symmetrically opposite the faces of the part in circulation in the machine. These sensors can be mounted in a resonant circuit. These resonant inductive sensors operate at medium frequency (a few tens to a few hundred kHz) and include a coil which creates an axial field in the ferrite core of the sensor. The response of the resonant inductive sensor depends in particular on the field lines, that is to say on the trajectories of the magnetic field inside the coin.

The induced currents which develop in the material of the part oppose the magnetic field of the sensor and have an important influence on the field lines. The effect of the induced currents is all the more intense as one sinks into the thickness of the material and it is generally considered that as long as one does not exceed a critical depth called "skin thickness δ ", the magnetic flux is not hindered by the induced currents. Beyond this depth, the magnetic flux is neutralized by the induced currents. The skin thickness δ depends in particular on the conductivity, the magnetic permeability and the frequency of the sensor supply current which is a very important parameter in a detection system. In particular, by playing on the frequency of the sensor (the higher the frequency and the smaller the skin thickness δ), one appreciably modifies the penetration depth of the electromagnetic field in the coin. This magnetic field can be optional, depending on the frequency, only attenuated by the surface material or else attenuated by several superimposed materials in the case of plated materials used for example in the insert of a coin of mo design - last.

In the case of resonant inductive sensors whose field is practically perpendicular to the plane of the faces of the part, a parameter called critical penetration depth d ^c _p is taken into account. depth being substantially equal to 3 δ. We can consider that any part of a material located more than 3 δ deep for example in a plated material constituting an insert of a coin will not be seen by the sensor and will therefore not influence the signal of the sensor. Furthermore, when a sensor is used on each side of the coin, while scrolling in the discriminating device of the coin machine, it is necessary to avoid that the magnetic fluxes generated by each of the sensors meet at the center of the part to avoid the establishment of a mutual inductance between the two sensors which will appreciably affect the measurement signal. Therefore, for detection measurements, a frequency and materials such as d ^c _p <e / 2, where e is the thickness of the part, are required. Furthermore, the more intense the induced currents, the lower δ is, and the greater the attenuation of the internal field, and therefore the lower the effective permeability μ _ma t and the magnetic flux (and therefore the sensor signal) in materials. therefore difficult to detect. The effective permeability μ m _at constitutes the mean in the thickness of the material at the working frequency of the ratio induction on excitation field. One can calculate the dynamic effective permeability μ _ma t of the part, by means of the dynamic relative permeability according to the frequency and the maximum relative permeability in direct current μ ^{rτ, ax} ' ^r _cc .

We will give below in Table 1 the results relating to the insert of a Euro coin with a thickness e = 2.33 millimeters whose inner layer of nickel has a thickness of 7% of the total thickness of the insert, ie 160 μm and the outer layers of which are constituted by cupronickel Cu ₅ Ni ₂₅ , that is to say an alloy with 75% copper and 25% nickel by mass. The results are given for frequencies of 100 and 500 kHz as it appears in the second column of the table and the third column of the table gives the skin thickness δ in μm. The fourth column of the table gives the relative permeability in direct current μ ^{max, r} _cc> the fifth column of the table gives the thickness of the layers in millimeters and the last column of the table gives the dynamic relative permeability μ ^r mat.

Only the inner layer of the insert is magnetic, the outer layers of cupronickel not being magnetic.

We see that in the case of the insert of a 1 Euro coin, the non-magnetic materials being on the surface, the penetration of the magnetic field is about 1, 2 millimeter at 500 kHz, that is to say precisely e / 2, the critical penetration depth d ^c _p being substantially equal to 3 δ. Thus the sensor is very sensitive to all the material in the half-thickness of the part, it is therefore possible to choose this frequency of 500 kHz as the measurement frequency.

A type of sensor is used such that the magnetic flux circulates mainly in the ferrite constituting the core and closes in the coin defined by the parameters μ _m at and d ^c _{p as} well as by the residual air gap d, c 'is to say the distance between the faces of the circulating part and the faces of the inductor. In this case, the inductance of the sensor L can be calculated from the reluctance Rel of the coin. The overall magnetic reluctance Rel of the coin is expressed in particular as a function of the frequency and of the parameters μ _mat and d ^c _p characterizing the electromagnetic behavior of the coin.

The inductance L of the sensor in the presence of the coin and therefore the response of the sensor are entirely linked to the properties of the coin defined by the parameters μ mat and d ^c _p . The coins of modern design which are intended to thwart the efforts of the counterfeiters and in particular the coins of 1 and 2 Euros which will be put into circulation within the framework of the use of a single European currency have a complex structure so that the reluctance of the parts and the inductance of a sensor for discriminating these parts cannot be expressed solely as a function of the parameters considered above. "

For example, the Euro coin is provided with an insert described above surrounded by a nickel silver crown of composition Cu ₇₅ Zn ₂ o Nis, the crown and the insert being assembled one on the other by following crimping a circular or more exactly cylindrical interface, consisting of the external peripheral surface 2 ′ of the insert 2 and the internal peripheral surface 3 ′ of the crown 3. The 2 Euro coin includes an insert whose internal layer consists of nickel and represents 12% of the total thickness of the insert, the outer layers being made of nickel silver Cu ₇₅ Zn ₂ o Ni 5 - Around the insert is crimped a cupronickel crown

The analysis of the magnetic behavior of complex coins such as 1 and 2 Euro coins encounters two main difficulties: 1 - The analysis of the behavior of the insert presents difficulties, since the insert is made by plating of materials whose electrical permeabilities and conductivities are very different,

2 - It is also necessary to take into account the contact resistance R _c at the interface between the crown and the insert. The currents induced by the sensor (s) in the coin pass freely through the interface when the contact resistance is practically zero. On the contrary, these induced currents are completely separated from each other in the two constituent elements of the coin if the contact resistance R _c is much greater than 0. For intermediate values of the contact resistance R _c , we obtain intermediate situations of the induced currents which lead to variable values of the inductance L of the sensor in the presence of a coin. If the value of the contact resistance R _c is not controlled in the context of the manufacture of coins, the dispersion of the inductance values L, and therefore of the response signal from the sensor, make it impossible to fulfill conditions perfect discrimination of authentic pieces compared to even coarse counterfeits.

In fact, in order to distinguish authentic coins from counterfeits, a coin acceptance range is fixed: - when the response signal from the sensor in the presence of a coin is within this range, the coin is considered to be is authentic; when the signal is outside this range, the piece is considered to be false. In order to effectively distinguish genuine coins from counterfeit coins, the acceptance range of coins must be narrow. However, when the value of the contact resistance R _c is poorly controlled, a narrow part acceptance range leads to a rejection of a large proportion of authentic parts, which is unacceptable. To overcome this drawback, the acceptance range of the coins can be widened, but then the risk of considering genuine counterfeit coins as genuine becomes significant, which is also unacceptable.

Under these conditions, coin-operated payment machines, whatever the means of discrimination, cannot therefore be satisfactory.

In batches of polymetallic parts comprising an insert and a crown made of different metallic materials, crimped one on the other, contact resistances varying over a wide range have been measured, these variations seeming due to the forming conditions of the insert and the crown, for example by cutting, and in the crimping operation of the two constituent elements of the part one on the other. Indeed, the surfaces of the constituent elements of the part which are assembled during crimping to produce an interface, can have roughness on a microscopic scale, whatever the care taken in the manufacture of the parts, due to the fact that the manufacture must meet certain industrial performance criteria. At the time of crimping, the roughness of the contact surfaces of the components of the part make more or less good electrical contact between the two components of the part on either side of the interface. and in more or less continuous zones. This results in a dispersion of the values of Rc, even in a single production of coins. For the calculation of the inductance of the sensor, one must take into account the overall magnetic reluctance in operation of the material which can depend very sensitively on the contact resistance Rc.

On the other hand, there are no known industrial processes for manufacturing polymetallic parts, such as coins comprising constituent elements assembled by crimping, which make it possible to systematically obtain contact resistances through the interface. between the constituent parts of the pieces which lie within a very narrow gap.

Apart from the very important problems relating to the production of coins, it may be necessary to obtain parts comprising constituent elements assembled by crimping, the contact interface of which has a constant and well controlled electrical resistance, for example. for the use of these parts as a component of electrical or electromagnetic devices of various types.

The object of the invention is therefore to propose a method for adjusting the contact resistance of the interface between two components of poly-metallic parts of a batch of parts, this method making it possible to obtain on all the parts of a manufacture or possibly of manufacture from various sources, electromagnetic characteristics and in particular an overall magnetic reluctance in constant operation, when the polymetallic part passes opposite an inductive sensor. For this purpose, the batch of parts is subjected to a thermal cycle comprising at least:

- a phase during which the interface is compressed by differential expansion of the components of the polymetallic parts, and

a phase during which the contact surfaces of the interface are brought into contact with an oxidizing medium, the compression phase and the phase of contacting with an oxidizing medium possibly being the same phase, in order, d on the one hand, to crush the rough edges contact surfaces and, on the other hand, to create an oxidized layer on the contact surfaces.

In order to clearly understand the invention, a description will now be given by way of example, with reference to the appended figures, of the application of the adjustment method according to the invention to the manufacture of polymetallic coins comprising an insert. and a crown according to a first and according to a second embodiment.

Figure 1 is a top view of a coin having an insert and a crown. Figure 2 is a sectional view along 2 - 2 of Figure 1.

Figure 3 is a schematic view of an inductive coin recognition sensor.

FIG. 4 is a deformation diagram as a function of the temperature of the insert and of the crown of a polymetallic coin according to a first embodiment.

FIGS. 5A and 5B are diagrams showing the variations in the temperature of coins during treatment by the process according to the invention applied to coins according to the first embodiment, carried out respectively in a single thermal cycle and in several successive cycles.

FIG. 6 is a deformation diagram as a function of the temperature of the insert and of the crown of a polymetallic coin according to a second embodiment.

FIGS. 7A and 7B are diagrams showing the variations in the temperature of coins during a treatment by the method of the invention applied to coins according to the second embodiment, carried out respectively in a single thermal cycle and in several successive cycles.

In FIG. 1, we see a polymetallic coin 1 comprising an insert 2 and a crown 3 set one on the other according to an interface 4 of circular or cylindrical shape depending on the thickness of the insert 2 and of the crown 3. In Figure 2, there is shown in section the coin 1, made in polymetallic form. The crown 3 of the part is produced in homogeneous form in a first metallic material, and the insert 2 produced in plated form comprises an internal layer 2a in a second metallic material and two external plated layers 2b in a third metallic material. In the case of a coin with the value of one Euro, the first metallic material consists of nickel silver, that is to say an alloy containing by mass 75% of copper 5% of nickel and 20% of zinc, the second material consists of nickel and the third metallic material by cupronickel, ie an alloy containing by mass 75% of copper and 25% of nickel.

In the case of a 2 Euro coin, the first metallic material consists of cupronickel Cu ₇ 5 Ni ₂ 5, the second material consists of nickel and the third material of nickel silver Cu ₇ 5 Ni ₅ Zn ₂ o -

In the case of a Euro coin, the internal layer of nickel 2a represents 7% of the total thickness of the insert 2 and in the case of the 2 Euro coin, the internal layer of nickel 2a insert 2 represents 12% of the total thickness of the insert.

For the production of coins, the ring 3 of annular shape is cut from a blank of homogeneous composition _. constituted by the first material, and the insert in the form of a circular disc in a blank of plated material having the same thickness as the blank in which the crown is cut and comprising an internal layer of the second material on the faces of which are plated two external layers of the third material, and the crown and the insert are assembled by crimping along the interface 4.

The interface 4 is constituted by the respective contact surfaces 2 'and 3' of the insert 2 and of the crown 3 attached to one another. The contact surface 2 'of the insert is its external peripheral surface and the contact surface 3' of the crown is its internal peripheral surface. In Figure 2, there is shown in a greatly exaggerated manner, asperities which may be present on the surfaces 2 'and 3' opposite the insert and the crown, after crimping. These roughness, depending on the crimping conditions of the two parts, can provide perfect electrical contact between the insert 2 and the crown 3, along the entire length of the interface 4 or, on the contrary, introduce a significant electrical resistance between the contact surfaces of the two constituent elements of the part, along the interface 4.

In the first case, the contact resistance R _c of the interface is substantially zero (R _c = 0); in the second case, the resistance R _c can have a non-zero and variable value in a wide range ranging for example from the 10th of mΩ to values much greater than mΩ. To design a method for standardizing the response of an inductive sensor to the passage of a bimetallic coin as described above, the inventors have carried out numerous measurements of electrical resistance on numerous coins. coin from different lots from various manufacturers and correlatively, the measurement of a parameter reflecting the electromagnetic behavior of the coin in an inductive sensor as shown in FIG. 3. The inductive sensor in resonant circuit generally designated by the reference 5 comprises two inductors 5a and 5b between which the part 1 is passed over which an authenticity check is carried out by electromagnetic measurements.

Each of the inductors 5a and 5b comprises a coil 6 and a ferrite core 7 disposed inside the coil 6. The coils 6 are supplied with alternating electric current at medium frequency, and preferably with current at a frequency of 500 kHz by a unit 8 for power supply and measurement processing.

The coin 1 passes between the inductors 5a and 5b, so that its two faces are directed towards the internal faces opposite one another of the inductors 5a and 5b, with a very small air gap d and equal on both sides of the coin 1 which has a thickness e.

Field lines 10 are also shown, ie the lines of circulation of the magnetic field which form around the coils 6 and close in the material of the coin 1. In general, the signal of response of the sensor 5 depends on the inductance L of the sensor in the presence of the coin 1. As was explained above, when the coin 1 is a polymetallic coin bearing an insert and a crown crimped on each other according to an interface, the reluctance of the coin on which the inductance L of the sensor depends is generally variable with the value of the contact resistance between the insert and the coin crown, due to the different behavior of induced currents depending on contact resistance. In order to design a process for standardizing the electromagnetic characteristics of polymetallic coins and in particular of coins comprising an insert and a crimped crown, the inventors carried out resistance measurements on very many coins of different lots from various coin manufacturers. . The measurements were made on the coins, at high frequencies between 100 kHz and 1 MHz. At these frequencies, the skin thickness δ, defined above, is small, and in the case of coins in which measurements are made at the indicated frequencies, this skin thickness δ is much less than the thickness e of the coin. . We can calculate the inductance of a sensor as a function of the frequency, permeability and conductivity of a homogeneous material and deduce an equivalent conductivity from a polymetallic coin leading to the same inductance. say the same response signal from the sensor. This equivalent conductivity is expressed as a function of the overall magnetic reluctance in operation of the material.

Thus any variation of the material of the part or of the contact resistance R _c depending on the interface of the part, resulting in a variation of the overall magnetic reluctance R _ma t can be reduced to a variation of equivalent conductivity, when the frequency is high. Inductance measurements are therefore carried out on the parts by means of resonance frequency measurements, which makes it possible to obtain the equivalent conductivity and therefore to quickly characterize the parts with regard to their physical characteristics such as the conductivity of the components. different materials of the room. In the frequency ranges for which δ is much less than the thickness e of the part, these frequencies being used for conductivity measurements, the sensor signal is mainly affected by the conductivities of the materials and their geometric characteristics (thickness and characteristics of the interface between the crown and the insert) and little or not at all by the intrinsic permeability of the materials.

The measurements carried out by the inventors on a very large number of parts made it possible to show that for a contact resistance comprised between approximately 0.1 and approximately 1 mΩ, the induced currents and therefore the inductance and the response of the sensor strongly depend on the contact resistance while for a contact resistance R _c greater than 1 mΩ, the induced currents are compartmentalized in the insert and the crown and that above this value, any dispersion of the values of the contact resistance does not influence the distribution of the induced currents and therefore the inductance and the response of the sensor. This finding results from contact resistance measurements and the signal from an inductive sensor on a very large number of coins.

A first observation by the inventors is also related to the fact that a contact resistance between the insert and the crown of a coin greater than 1mΩ makes it possible to overcome the influence of the contact resistance on the signal d '' an inductive discrimination sensor. Of course, to avoid any influence of the contact resistance on the sensor signal, it is desirable that all the coins have a contact resistance R _c much greater than 1mΩ.

The invention therefore consists in providing a method for setting the contact resistance of all the coins in a batch of coins at a value greater than a determined limit and in the case of coins as described, at a limit of 1 mΩ, The method of the invention is based on the production of an insulating oxide layer at the interface between the insert and the crown of the parts combined with a plastic deformation of the interfacial protuberances, so that the resistance contact is greater than the limit, for each of the parts subject to treatment. The treatment according to the invention is carried out on parts of which the insert and the crown have been previously assembled by crimping, an imperative condition is that the treatment must be carried out so as to avoid ter all modifications of colors and more generally of appearance of the parts.

The treatment of the parts allowing the oxidation of the interface until obtaining a contact resistance higher than the fixed limit will be described in two different cases.

In example 1 which corresponds to the case of 1 Euro coins, the coin insert has a lower coefficient of expansion than the crown. In example 2 which corresponds to the case of 2 Euro coins, the insert has a higher coefficient of expansion than the crown. Example 1. The treatment is carried out on 1 Euro coins, the crown of which is made of nickel silver and the insert of a plated material comprising an inner layer of nickel and two outer layers plated with cupronickel. In FIG. 4, the expansions λ of the insert and of the crown (for example in a diametrical direction) have been represented as a function of the temperature; the expansion λ j of the insert is represented by the line 11 of slope α i (coefficient of expansion of the insert) and the expansion λ _c of the crown is represented by the line 12 having the slope α _c which is the coefficient of nickel silver expansion.

The origin O corresponds to an ambient temperature, for example 20 ° C, at which the crown and the insert are supposed to nest perfectly one inside the other, without compression or expansion of one on the other ; the interface air gap is δ = Rλ _c - Rλj (R radius of the insert).

Note in particular that Δλ = λj - λ _c = 0 at room temperature, which corresponds to the origin 0. When the temperature of the rooms is lowered below 20 ° C, the insert (line 11) is contracts less than the crown (right 12). This results in compression of the interface 4 between the insert 2 and the crown 3.

The compression of the interface by temperature reduction below 20 ° C makes it possible to compress and possibly crush the roughness of the surfaces 2 'and 3'.

When the parts are brought to a temperature above 20 ° C., the expansion of the insert 2 represented by the line 11 is less than the expansion of the crown 3 represented by the line 12, so that the interface 4 enters the contact surfaces 2 'and 3' of the insert and the crown expands. There is thus obtained an opening of the interface which can be used in the implementation of the method according to the invention for penetrating an oxidizing medium between the surfaces 2 ′ and 3 ′ in contact with the asperities and thus oxidizing the contact surface.

The principle of the method of treatment by oxidation of the contact surfaces of the constituent elements of the parts is based on an opening of the interface between the parts, this opening being however limited to avoid the dismantling of the parts, and on the bringing into contact of a oxidizing medium with the facing surfaces of the interface at a temperature or during a temperature transition allowing the oxidation of the materials of the insert and of the crown. This operation can be preceded by a contraction of the interface intended to compress the roughness of the contact surfaces on either side of the interface. In FIG. 5A, a first embodiment of a treatment according to the invention is shown in the case of coins of 1 Euro, that is to say coins in which the insert has a coefficient of thermal expansion less than the crown.

Because the insert has a lower coefficient of expansion than that of the crown:

- if the part is cooled, the interface contracts,

- if the room is heated, the interface expands.

The processing comprises two compulsory phases which will be designated as phase 2 and phase 3, these phases can themselves be preceded by a phase 1, generally optional.

Phase 2 consists of cooling to a first temperature T ₃ <-40 ° C and preferably <-100 ° C to obtain compression of the interface resulting from a contraction of the crown greater than the contraction of the insert. Phase 3 consists in throwing the cold parts into an oxidizing or amphoteric liquid such as water, acetone, alcohol or weakly acidic aqueous solutions whose temperature T ₄ is between 25 ° C and 75 ° C ( preferably between 35 ° C and 65 ° C). The transfer time between the time when the parts are at the first temperature T ₃ and the moment when they are immersed in the oxidizing or amphoteric liquid at the second temperature T ₄ must be short enough so that the parts do not have time to warm up and therefore that the interface doesn't have time to decompress. This time should preferably be less than 5 seconds and more preferably less than 2 seconds.

Under these conditions, and due to the thermal inertia of the parts when they are immersed in the liquid, the crown (located outside the part) is subjected, in a transient manner, to heating and therefore to a stronger expansion than that of the insert, thus opening the interface and thereby penetrating the oxidizing liquid. Phase 3 will be designated as the oxidative differential expansion phase.

For the wetting of the surfaces to take place under good conditions, it is preferable that the liquid is adapted to easily wet the materials of which the parts are made. The ease of wetting can be improved, if necessary, by adding a surfactant to the liquid. It is also necessary that the liquid comprises oxidizing groups capable of forming, for example, surface hydroxide layers of the metals constituting the parts (Cu, Ni, Zn). Groups containing oxygen or halides (Cl, Br, I, ...) are particularly recommended, provided that their pH remains moderate (pH> 2 and preferably> 4).

The oxidized surface compounds, formed during the thermal transient between the initial temperature T ₃ and the second temperature T lead to the formation of an insulating layer between the two interfaces which increases the contact resistance.

However, the entire surface of the parts is in contact with the oxidizing or amphoteric liquid and therefore oxidizes. This oxidation, if it is too important, deteriorates the visual aspect of the parts which is an important element of recognition. It is to avoid this deterioration in the surface appearance of the parts that the liquids used have a limited oxidizing power, and that in particular concentrated solutions of nitrates, sulphates or halides, which have very acidic pH, will be avoided. At the end of phase 3, the parts are removed from the oxidizing or amphoteric liquid and brought back to an ordinary ambient temperature, for example 20 ° C.

The treatment comprising phases 2 and 3 may be preceded by a phase 1 of oxidative expansion of the interface intended both to break the metal contacts crown insert and to oxidize the interface and in particular the roughness of the surfaces in contact.

To achieve this oxidative expansion of the interface, the parts are brought to a third temperature T ₂ which must be sufficiently high for the expansion of the interface to be significant, but not too much so that the thermodynamic conditions of formation of a stable oxide.

Preferably, the third temperature T ₂ must be between 80 ° C and 160 ° C, and better still, between 100 ° C and 140 ° C. This heating is carried out in a moderately oxidizing atmosphere so as not to deteriorate the visual appearance of the parts. This heating can be annealing under primary vacuum, annealing under protective gas of low purity, annealing under hydrogen with a controlled dew point greater than -50 ° C and preferably greater than -30 ° C, or even annealing at the air. The treatment includes a rise to the third temperature T ₂ , a plateau at the third temperature T ₂ and a return to ambient temperature Ti. The temperature rise can take place quickly, as represented by curve 13a in FIG. 5A (charging in a hot oven) or slowly, as represented by curve 13b (charging in a cold oven). It is preferable that the rate of temperature rise of the parts is greater than 2 ° C / s and better still greater than 5 ° C / s.

The duration of the plateau at the third temperature T ₂ can be adjusted as desired between a few seconds and several hours.

Return to room temperature can be obtained by natural cooling.

Processing of documents may include:

- either a single cycle of phases 2 and 3 of contraction and differential oxidative expansion; - or a single cycle of phases 2 and 3 of contraction and differential oxidative expansion preceded by a phase 1 of oxidative expansion.

- Or, as shown in FIG. 5B, a plurality of cycles of phases 2 and 3 of oxidative differential contraction and expansion, separated by phases 1 of oxidative expansion.

The first group of phases 2 and 3 of contraction and differential oxidative dilation can possibly be preceded by a phase 1 of oxidative dilation. By proceeding in this way, the effects of the successive phases are cumulated and a contact resistance R _c much greater than 1mΩ can be obtained.

It should be noted that the treatment should not end with an oxidative dilation phase, since this, in general, deteriorates the effect of the previous phases.

Example 2: A batch of 2 Euro coins is processed by a process according to the invention.

As indicated above, the 2 Euro coins include an insert made of a plated material comprising an internal layer of nickel and two external layers of nickel silver Cu ₇₅ Ni 5 Zn 20 and a crown of cupronickel Cu ₇₅ Ni ₂₅ . In the case of such parts, the insert has a coefficient of expansion α, - higher than the coefficient of expansion of the crown α _c .

As it appears in FIG. 6 which represents the thermal expansion or contraction λ of the insert (λ i) and of the crown (λ _c ) as a function of the temperature, the origin 0 corresponds to the ambient temperature of 20 ° C for which the insert and the crown are supposed to fit perfectly into each other and are kept in localized contact by the roughness of their contact surfaces.

When the temperature is raised above 20 ° C, the expansion λ _c of the crown represented by the line 16 of slope α _c is less than the deformation λi of the insert represented by the line 17 of slope, -. The contact surfaces of the crown and the insert compress each other which produces a crushing of the interfacial protuberances.

When the temperature is lowered below 20 ° C, the contraction of the crown represented by the line 16 is less than the contraction of the insert represented by the line 17. In this case, the contact surfaces of the crown and the insert move away from each other, which produces an opening of the interface and a rupture of the connections. localized metal insert / crown. As in the previous case, the heat treatment consists of one or more cycles which can include three phases:

Phase 1 is a cooling of the batch of parts from room temperature T ₀ to a very low first temperature Ti, which can be lower than -100 ° C and which is intended to break any local metallic connections between the insert and the crown by expansion of the interface; cooling can be obtained, for example, by immersing the batch of parts in liquid nitrogen. When the heat treatment comprises a single cycle, or when the heat treatment comprises several successive cycles and the cycle considered is the first in the series, this phase is optional. On the other hand, when the cycle is preceded by at least one previous cycle, this phase is compulsory.

Phase 2 consists of heating the batch of parts to a second temperature T ₂ in an oven whose atmosphere is controlled. The atmosphere must consist of an inert protective gas such as argon or helium or a reducing gas such as hydrogen. In all cases, the atmosphere of the furnace must contain an oxidizing agent and, for example, be a little humid so as to be slightly oxidizing.

The oxidizing power of the atmosphere, the maximum temperature reached T ₂ and the residence time of the parts in the oven must be sufficient for the contact resistance to be increased but not too much so that the visual appearance of the parts is not appreciably modified by a significant oxidation of their surface.

The residence time in the oven corresponds to the sum of the temperature rise time from the oven temperature to the temperature T ₂ and the time to maintain the temperature T ₂ or around this temperature.

The temperature rise can be very rapid (heating rate up to several thousand degrees per hour) or very slow (speed can be a few degrees per hour) or intermediate between these two speeds, the temperature rise time can vary between a few seconds and a few hours.

In any case, the residence time in the oven must be a few hours. The holding time at the second temperature T _∑ can vary between a few seconds (preferably more than 5 seconds) and a few hours (preferably, less than 3 hours) depending on whether the temperature rise is slow or rapid. For the treatment to be effective, the dew point of the furnace atmosphere must preferably be greater than -60 ° C, and better still greater than -50 ° C; the second temperature T ₂ must be greater than 350 ° C, and better still, greater than 370 ° C; the holding time must be greater than 5 min.

In order that the treatment does not deteriorate the visual appearance of the parts, the dew point of the furnace atmosphere must be less than 0 ° C and preferably less than -10 ° C; the second temperature T must be less than 450 ° C, and better still, less than 430 ° C; the residence time in the oven depends on the temperature and the dew point of the oven atmosphere. The higher the temperature and the higher the dew point, the shorter the residence time in the oven. Those skilled in the art can easily determine these conditions, for example by a few tests on a few parts.

For example, for a dew point equal to -20 ° C and a second temperature T2, the residence time in the oven must be less than ≈ 3 hours. Phase 3 is a return of the batch of parts to room temperature.

This return must not take place in an oxidizing atmosphere but in an atmosphere of neutral or reducing gas, to avoid deteriorating the visual appearance of the parts. From this point of view, it is preferable that the atmosphere is the same or comparable to that of the oven. In addition, it is preferable that the cooling takes place at a cooling rate greater than 200 ° C. per hour.

As indicated above, the heat treatment can comprise either a single cycle consisting of at least one phase 2 and one phase 3 and optionally a phase 1, as shown in FIG. 7A, or several successive cycles, as shown in FIG. 7B. When the treatment comprises several cycles, all cycles, except the first, must imperatively include a phase 1. In this case, phase 3 and phase 1 which follows it can be linked directly so that when it comes out of the oven , the batch of parts can be cooled directly to the first temperature Ti corresponding to phase 1.

In all cases, the treatment according to the invention makes it possible to fix, for each of the pieces of a batch of pieces, very securely, the contact resistance at a level higher than the limit determined in a first phase of the process, to obtain a practically constant response from an inductive sensor to the passage of each of the parts in a batch of parts. The contact resistance across the interface between the inserts and the crowns of the parts obtained at the end of the treatment can be much greater than 1 mΩ. More generally, in the case of treatment of coins different from those which have been described or in the case of electromagnetic polymetallic coins or components, the limit of contact resistance which is defined prior to the implementation of the treatment by resistance measurements in parallel to recording the signal from an inductive sensor on a very large number of parts, can be different from 1 mΩ.

We have thus obtained, in the case of coins as described above, by the implementation of the processing according to the invention, success rates on batches of coins, greater than 90%, that is to say say that less than 10% of the pieces in the batch have an insufficient contact resistance resulting in a response signal in an inductive sensor not allowing recognition of the coin.

The invention is not strictly limited to the embodiments which have been described. Thus, the contact resistance limit targeted by the treatment can be different from 1 mΩ, depending on the nature of the polymetallic parts and the different phases of the treatment according to example 1 or example 2 can be carried out. in different ways depending on the instal- industrial lations available for processing. The parameters defining the different phases (speeds, durations, temperatures) must however meet the conditions indicated above.

In the case of the treatment according to example 2, as in the case of the treatment according to example 1, phase 1 is only necessary to increase the effect of phases 2 and 3 during one or more treatment cycles. In addition, phase 1 is essential so that the effect of the treatment can be increased by at least two successive cycles which must successively include phases 1, 2 and 3. The invention applies not only to the parts of currency but also in the case of polymetallic components or coins used in any recognition or identification machine or in electromagnetic devices of any type.

Claims

CLAIMS 1.- Method for adjusting the contact resistance of the interface (4) of two components (2, 3) of polymetallic parts (1) of a batch of parts, characterized in that the batch of parts with a thermal cycle comprising at least: a phase during which the interface (4) is compressed by differential expansion of the components (2, 3) of the polymetallic parts (1), and - a phase during which the contact surfaces (2 ', 3') of the interface (4) are brought into contact with an oxidizing medium, the compression phase and the phase of bringing into contact with an oxidizing medium being able be the same phase, in order, on the one hand, to crush the roughness of the contact surfaces (2 ', 3') and, on the other hand, to create an oxidized layer on the contact surfaces (2 \ 3 ' ). 2. - Method according to claim 1 characterized in that the cycle further comprises, a prior phase of expansion of the interface (4) by differential expansion. 3. - Method according to claim 1 or 2, characterized in that several successive cycles are carried out, at least the second cycle and the following cycles comprising a prior phase of expansion of the interface (4). 4. - Process for processing coins (1) comprising an insert (2) in the form of a circular disc and a crown (3) of annular shape made of different metallic materials, crimped one on the other along the periphery external of the insert (2) and the internal periphery of the crown (3), the coefficient of thermal expansion ai of the insert (2) being less than the coefficient of thermal expansion α _c of the crown (3), characterized by the fact that at least one coin processing cycle is carried out during which each of the coins (1) of a batch of coins is brought to a first temperature (T3) below - 40 ° C and preferably less than - 100 ° C, then each of the coins (1) of the batch of coins is thrown at the first temperature (T3), in an oxidizing or amphoteric liquid at a second temperature (T4) of between 25 ° C and 75 ° C and preferably between 35 ° C and 65 ° C, the time required for plo nger the parts at the first temperature (T3) in the oxidizing liquid at the second temperature (T4) being less than 5 seconds and preferably less than 2 seconds, finally, the parts are removed from the oxidizing or amphoteric liquid and brought back to an ordinary ambient temperature, for example 20 ° C. 5. - Method according to claim 4, characterized in that the oxidizing or amphoteric liquid is at least one of the following liquids: water, acetone, alcohol, weakly acidic aqueous solution whose pH is greater than or equal to 2 or preferably greater than or equal to 4. 6.- Method according to any one of claims 4 to 5, characterized in that the oxidizing or amphoteric liquid contains, a surfactant. 7. - Method according to any one of claims 5 and 6, characterized in that the coins processing cycle (1) includes an initial temperature rise phase of each of the coins (1) of the batch of coins , from an initial temperature (Ti, T ₃ ) which can be an ambient temperature of 20 ° C or a temperature (T ₃ ) lower than ambient temperature, up to a third temperature (T ₂ ) between 80 ° C and 160 ° C and preferably between 100 ° C and 140 ° C, and maintaining the batch of parts at the third temperature (T ₂ ), for a period of between a few seconds and several hours, in an annealing atmosphere among the following atmospheres: primary vacuum, air, protective gas of low purity, hydrogen with controlled dew point greater than -50 ° C and preferably greater than -30 ° C. 8. - Process for processing coins (1) comprising an insert (2) having the shape of a circular disc and a crown (3) made of different metallic materials, crimped one on the other along the periphery outer (2 ') of the insert ((2) and the inner periphery (3') of the crown (3), the insert (2) of each of the pieces in a batch of pieces (1) having a coefficient of expansion α * greater than the coefficient of expansion α _c of the crown of the coin (1) characterized in that at least one coin processing cycle is carried out during which the coins are brought to a second temperature ( T ₂ ) between 350 and 450 ° C and preferably between 370 and 430 ° C, and each of the parts (1) of the batch of parts is kept at the second temperature (T ₂ ), inside an oven , in an atmosphere of protected gas tor or reducing agent containing an oxidizing agent and the parts are cooled to an ambient temperature (To) lower than the second temperature (T ₂ ) in an atmosphere of protective or reducing gas. 9. A treatment method according to claim 8, characterized in that the furnace atmosphere has a dew point greater than -60 ° C, and preferably greater than -50 ° C and less than 0 ° C and preferably less than - 10 ° C. 10.- A method of treatment according to any one of claims 8 and 9, characterized in that the parts of the batch of parts are cooled from the second temperature (T ₂ ) to ambient temperature (T ₀ ) to a speed greater than 200 ° C per hour. 11. - Method according to any one of claims 8 to 10, characterized in that the treatment cycle comprises an initial phase during which each of the parts (1) of the batch of parts is cooled to a first temperature (Ti ) below ambient temperature (To) and preferably below -100 ° C. 12.- Method according to any one of claims 7 and 11, characterized in that several successive cycles of treatment of the batch of parts are carried out, at least the second cycle and the following cycles comprising an initial phase of expansion. of the interface.