EP0004824B1 - Means providing detachable contact beteween two conductors and method of coating a piece of aluminium with a layer of nickel - Google Patents

Description

The field of the invention relates to the production of fixed or mobile electrical contacts on all kinds of aluminum parts such as bars, profiles of all types or pieces of equipment.

The new method applies, among other things, to the production of highly mechanically and thermally stressed contacts such as those made by plugging in and unplugging fingers from conductive bars. It also applies to the production of sliding contact systems such as those used in electric rotary machines with collector or ring. Finally, it can also be applied to the production of switches, contactors, circuit breakers or disconnectors.

The considerable development of aluminum as an electrical conductor is well known. It has replaced copper in a large number of uses mainly because of a much lower cost price. However, in most applications for which contact problems must be resolved, copper retains a technical advantage thanks to its ability to join by soft soldering and to produce contacts of low electrical resistance by mechanical tightening, at the limit without special surface preparation.

To improve the contact characteristics of conductive aluminum, it is known to coat it with a layer of tin from 4 to 20 μm thick deposited on a sub-layer of zinc or bronze.

This is how sets of aluminum busbars, coated with tin, are developed, which replace similar copper bars for the production of electricity distribution pipes in industrial premises or in the risers of large buildings. . The desired connections between these busbars and the user devices are made using fixed or removable devices, for example by means of movable contact fingers which are plugged into the busbars. The bars thus tinned are made of conductive aluminum most often containing at least 99.5% of aluminum such as A5 (AFNOR standard), or of various aluminum alloys used as conductors such as AGS / L (standard AFNOR) which is more particularly suitable for molded parts. The contact fingers are most often made of copper or copper-based alloys, such as, for example, brasses or bronzes.

In the case of devices with removable contacts, experience has shown that these installations, which were expected to be very robust, suffered from rapid aging due essentially to the degradation of the quality of the contacts between tinned aluminum bars and contact fingers. This degradation, more or less rapid depending on the characteristics of the contact parts and the intensity of the current flowing through them, results in a progressive increase in the contact resistance which leads to overheating. These overheating cause or accelerate oxidation processes and, gradually, the contacts are severely damaged, which can lead to breakdowns and production losses.

By using bare or tinned copper fingers in contact with tinned aluminum busbars, the first incidents resulting from bad contacts often occur after only six months to a year of use.

In the course of research carried out with the aim of finding a solution to these problems of electrical contact on aluminum, it has been found that one of the essential factors in the process of degradation of these contacts is the wear of the surface layers of the bearing surfaces l 'against each other, wear caused by an alternating vibratory movement of mechanical or electrical origin in connection with the frequency of the current. This vibratory movement, which results from interactions between field and alternating current produces a kind of fretting corrosion phenomenon which has the surface abrasion of the coating layers of the conductors and, in particular, of the tin layer which protects the aluminum. It is easy to understand that at least partial removal of the coating leads to the oxidation of the underlying metal. This oxidation favors, in turn, a heating of the contact zone which then oxidizes in an accelerated way. As soon as such a process is initiated, the total destruction of the contact becomes foreseeable sooner or later. The publication IEEE Transactions on Industry Applications, Volume IA-9 n ° 3, Mais-Juin 1973, pages 326-331, describes bolted electrical connections between a bare aluminum conductor or connector (unplated) and various electrical accessories provided with a coating which is preferably nickel. This coating is produced, for example, on aluminum parts electrolytically in a Watts bath on an intermediate layer of tin, copper or bronze, which allows good resistance of the deposit of Ni in particular with respect to corrosion. The tests described show that, provided that the initial clamping force is sufficient to destroy the thin films which are always present on the surface of metals, and provided that a fixed interface is obtained without relative displacements in use, it is possible to keep low contact resistance. This publication does not provide a means for obtaining a low contact resistance which is stable over time, in the case where the interface is not fixed.

DE.OS German patent application 2,337,171 describes a method of producing electrical contacts on cast aluminum parts in which a plasma bonding layer of AINi alloy is deposited by plasma spraying, which is then covered with a layer of silver or copper . The exothermic reaction which occurs during the deposition of the bonding layer gives it strong adhesion and low porosity. The roughness of this layer facilitates the subsequent bonding of silver or copper. As the description shows, this process is more particularly suitable for producing contact parts of small dimensions such as those which are mounted on circuit breakers insulated with SF ₆ .

We sought more particularly the possibility of making such contacts by depositing on the aluminum conductor (s), at least in the contact area, a nickel layer, without an intermediate layer, having a high adhesion with respect to the substrate. . The removable contact device according to the invention has the technical characteristics of claim 1.

The removable contact device according to the invention makes it possible to ensure an electrical connection with low contact resistance, stable over time, between two conductors maintained in contact by elastic clamping, at least one of these conductors being made of aluminum. This or these aluminum conductors is or are coated at least in the contact zone with a layer of electrolytic nickel.

Very good adhesion of this layer is achieved by modifying the surface condition of the substrate before carrying out the electrolytic deposition by means of a pre-deposition of a thin layer of nickel by chemical means followed by redissolution of this layer.

The invention also relates to the method which makes it possible to deposit, by electrolysis, a layer of unalloyed nickel of high adhesion directly on an aluminum substrate by virtue of a preliminary treatment of the surface of this substrate which comprises a pre-deposit of a thin layer of nickel. chemically followed by redissolution. This process has the technical features of claim 5.

Aluminum parts are understood, from the point of view of the application of the method which is the subject of the invention, all non-alloy aluminum parts used as conductor such as parts in A5, or other aluminum grades unalloyed and also all parts made of aluminum-based alloys used as conductors, such as parts made of AGS / L or even AS7G.

The very specific characteristics of adhesion of the nickel deposit produced without an intermediate layer result from the surface preparation carried out on the aluminum parts before coating.

As will be seen, this surface preparation allows both a complete elimination of the oxidized layer and the obtaining of a surface having a particular appearance, observable with the electron microscope which promotes the attachment of the nickel deposit.

The quality of the electrical contact can be further improved by bringing the nickel-coated aluminum part into contact with a silver-coated part, the core of which is aluminum or copper. It is well understood that, for the application, of the method according to the invention, it is to be understood by aluminum, nickel, silver or copper, these metals in the non-alloyed state with their usual impurities, the level of which is variable according to applications and also alloys based on aluminum, nickel, silver or copper which can be used as electrical conductors.

As the tests have shown, the remarkable quality of the contact devices according to the invention is due in the first place to the effectiveness of the nickel layer which protects the aluminum and also to the particular properties of the contact couple nickel / silver. We will see that other metals can be substituted for silver, but at the cost of significantly lower performance.

One of the main difficulties which had to be overcome in order to produce the devices according to the invention was the production of a direct nickel coating on the aluminum contact parts, in the absence of any intermediate layer.

Indeed, it was found that, to have a good heat resistance, it was necessary to prohibit the deposit; intermediate layers of metals such as tin, zinc, copper or bronze which tend to diffuse the underlying metal often with the formation of embrittling intermetallic compounds. Finally, it is desirable to use an electrolytic deposition method using stable baths and of composition as simple as possible so as to coat the contact parts under the best cost conditions.

The electrolytic process for direct deposition of nickel, which also constitutes one of the objects of the invention, comprises an initial step of temporary pre-deposition of nickel, by means of a bath of determined composition, in the absence of electric current. This pre-deposition is then eliminated and then the final coating layer of nickel is deposited by electrolytic means. The microscopic examinations carried out during the various stages of this treatment showed that the combination of a pre-deposit, of nickel by chemical means followed by a redissolution of this deposit, made it possible to obtain a surface state at the both perfectly deoxidized and having a particular appearance which constitutes an exceptionally effective bonding base for the deposition of final nickel which will then be carried out.

As will be seen effectively below, the nickel coating obtained by the process thus developed exhibits exceptional adhesion in cold and hot conditions, which allows the production of particularly durable contact elements. Indeed, nickel has, compared to other aluminum covering metals, such as copper, zinc or tin, the advantage of great thermal stability. Thus, the diffusion of nickel in aluminum remains low and without drawback, even at temperatures where the tin and zinc are already melted. Nickel also has the advantage of being a much less rare metal than tin, less expensive, and the price of which is not subject to speculative variations in the magnitude of those which strike tin or copper. .

• - les pièces à recouvrir, après si nécessaire un décapage, subissent un pré-dépôt sans passage de courant dans un bain fluoborique contenant du nickel. C'est une solution aqueuse contenant:
  

This direct nickel plating operation can be carried out either in continuous or discontinuous treatment on parts which will then be used for the production of all types of contacts. This operation includes the following steps:

• - the parts to be covered, after if necessary pickling, undergo a pre-deposition without current flow in a fluoboric bath containing nickel. It is an aqueous solution containing:
  

The temperature is preferably between 20 and 50 ° C.

The contact time is short: a few seconds to a few tens of seconds.

• the very weak deposit of nickel thus produced is then redissolved, for example by means of a hydrofluoric acid bath containing:
  

It only takes a few minutes of contact at a temperature between 20 and 50 ° C.

• - The parts thus prepared are then nickel-plated by a known electrolytic method. One can use, for example, the nickel-plating bath containing:
  

The current density is from 2 to 20 A / dm2.

The thickness of the Ni layer is determined according to the applications. It will most often be from about 3 to about 25 pm. Other baths can also be used. One can, in particular, use baths allowing the deposition of nickel-based alloys.

Example

Nickel-plated sections of AGS / L aluminum alloy bars 40 x 6 mm in cross section were used for making sliding contacts by skewering.

• 1. Dégraissage alcalin par une solution aqueuse à 15 g/I de DIVERSEY 708 (marque de la Société DIVERSEY FRANCE) à une température de 60°C. durée du traitement: 5 nm.
• 2. Décapage alcalin par une solution aqueuse à 50 g/I d'Aluminux (marque de DIVERSEY FRANCE) à une température de 50°C. - durée du traitement: 5 mn.
• 3. Neutralisation fluonitrique (N0₃H: 400 g/I; HF: 15 g/I), 30 secondes.
• 4. Pré-dépôt de nickel en l'absence de courant électrique au moyen d'une solution aqueuse contenant:
  

We operated as follows:

• 1. Alkaline degreasing with an aqueous solution at 15 g / I of DIVERSEY 708 (brand of the company DIVERSEY FRANCE) at a temperature of 60 ° C. duration of treatment: 5 nm.
• 2. Alkaline pickling with a 50 g / I aqueous solution of Aluminux (DIVERSEY FRANCE brand) at a temperature of 50 ° C. - duration of treatment: 5 min.
• 3. Fluonitric neutralization (NO ₃ H: 400 g / I; HF: 15 g / I), 30 seconds.
• 4. Pre-deposition of nickel in the absence of electric current by means of an aqueous solution containing:
  

The contact time was 15 seconds at 30 ° C.

durant 3 mn à environ 20°C.5. Dissolution of the nickel deposit by means of a fluonitric bath containing:

for 3 min at around 20 ° C.

_; 6. Electrolytic nickel plating using an aqueous solution containing:

The electrolysis is carried out between nickel anodes and the bars to be covered. We operate at 40 ° C. with a current density of 3 A / dm ² for 25 min. The Ni layer obtained has a thickness of approximately 15 microns.

We then compared the quality of contacts by skewering made between these bars and contact fingers having different coatings, with that of contacts made between identical bars coated with tin and similar contact fingers. The quality of the contacts was assessed by a so-called fretting corrosion test, which consists in subjecting the surfaces in contact under pressure to alternating microglissions which reproduce, to a certain extent, what occurs in reality under the action of the resulting forces. field / alternating current interactions with frequencies most often double the base frequency of this current.

• La figure 1 est une vue en élévation d'un doigt de contacta
• La figure 2 est une vue en plan d'un doigt de contact.
• La figure 3 est une vue d'un doigt de contact fixé sur une lame support et embroché dans une barre.
• La figure 4 représente un détail de la figure 3.
• La figure 5 est une vue schématique en plan du dispositif d'essai.
• La figure 6 est un schéma d'un dispositif de mesure de résistance de contact entre revêtement et substrat.
• La figure 7 est un détail en coupe de la figure 6.

The following figures help to understand the conditions of the test carried out:

• Figure 1 is an elevational view of a contact finger
• Figure 2 is a plan view of a contact finger.
• Figure 3 is a view of a contact finger fixed on a support blade and plugged into a bar.
• FIG. 4 represents a detail of FIG. 3.
• Figure 5 is a schematic plan view of the testing device.
• FIG. 6 is a diagram of a device for measuring contact resistance between coating and substrate.
• FIG. 7 is a detail in section of FIG. 6.

Figures 1 to 4 show an embodiment of a contact finger. We see that it consists of two elastic blades, generally called fishplates, (1) and (2) made of copper or a copper-based alloy of approximately 10 x 2 mm in cross-section, formed so as to be able to pinch elastically. a contact bar (3) of approximately 40 x 6 mm in cross section. Contact between the finger and the operating circuit is ensured in the same way by pinching a strip (4) connected to this circuit. Springs (5) and (6) mounted on the axis (7) ensure the elastic clamping of the assembly with a clamping force of approximately 1 kg. The curvature of the fishplates in the area (8) and (8 ') is such that, practically, the contact bars / fishplates is located at the end of these in (9) and (10).

Figure 5 shows the test device schematically. The aluminum bar (10) of 40 x 6 mm section is fixed at its two ends in the jaws of an alternating traction / compression machine, not shown. In this way, the bar is subjected to alternating forces along the XY axis with a frequency of 155 Hertz. These efforts are reflected at the level of contacts by alternative micro-shifts comparable to those which occur in electrical installations.

Four fingers identical to that shown in Figures 1 to 4, identified (11), (12), (13) and (14) are fixed by one end to a fixed part (15) secured to a base not shown.

The compressive tensile stress applied to the bar is ± 80 MPa.

Each test carried out consisted in subjecting each contact between finger and bar to 200,000 compression tensile cycles.

We experienced 10 different couples.

The first five pairs, which correspond to the techniques practiced in the usual way for making contacts by skewering, relate to contacts comprising an AGS / L bar covered by electrolytic deposition with a layer of tin 17 μm thick on an under layer of bronze. The copper fingers are either bare or covered with tin or nickel, or a tin-nickel alloy, or silver.

The other five pairs which implement the method according to the invention comprise an AGS / L bar covered with 15 μm of nickel, as described in the example and a second series of 5 copper fingers identical to the first series .

Each of the ten couples thus defined was tested 4 times, that is to say that for each couple, 4 identical fingers were used in contact with sections of AGS / L bars coated either with nickel or with tin. and the value of the result was determined by measuring the area of the oxidation spots formed on the surfaces of the bars after 200,000 cycles.

Table I below gives the results obtained:

These results show first of all that the tin coating does not resist fretting corrosion phenomena well. They show, consequently, that, for each group of two couples comprising a similar finger, it is always the one which comprises the nickel-plated bar which gives the best result. Finally, the combination of a silver-coated finger with a nickel-plated AGS / L bar gives a particularly remarkable and completely unexpected result.

Additional tests were carried out to assess the quality of the nickel coatings produced according to the invention.

We first studied the influence of aging at 200 ° C. on the electrical resistance of contact between the nickel layer and the substrate. For this, sections of AGS / L bar 40 x 6 mm in section, coated as described in the example with a thickness of 15 μm of nickel, were coated with an additional layer of 3 μm. silver. This latter deposition was carried out in a known manner, by electrolysis in a cyanide bath.

The assembly used to measure the contact resistance is shown in Figures 6 and 7.

Two copper fingers (16) and (17), of the same dimensions as those described at the start of the example and shown in FIGS. 1 to 4, are connected to a section of bar (18) in AGS / L of 40 × 6 mm of section coated as we have just said with 1 5 _µm of nickel + 3 µm of silver. Each of these fingers is connected at its other end with contact bars (19) and (20) which are connected to a source of direct current not shown.

The contact between the ends of each of the fishplates such as (21) and (22) and the bar (18) is ensured by means of silver contact pads (23) and (24) whose flat and parallel bearing faces have the shape of a 3 x 3 mm square. One of the faces of each of these pads is brazed to the splint and the other is in abutment on the surface of the bar section. The thickness of these pellets is approximately 1 mm and the clamping pressure of the fishplates of the order of 1 kg. ensures good contact between each pellet and the silver surface of the bar on which it comes to rest.

The distance D between the fingers (16) and (17) at their contact with the bar (18) via the silver pellets is 50 mm between axes.

A recording voltmeter (V) is connected to the ends of the fishplates at the contact pads. It measures the evolution of voltage as a function of time. The intensity of the direct current is fixed once and for all at a constant value of 25 amperes. We therefore see that we can deduce from a simple voltage measurement an overall electrical resistance 'R "which is the sum of the contact resistances between the two fingers and the bar plus the resistances for current flow through the bar section located between the two fingers. As for the contact resistance between silver layer and nickel layer, it is practically negligible. It was noted, following calibration tests that, thanks to a good adjustment of the contacts between arqent pellets and silver surface of the bar, the initial value of R is only slightly higher than the sum of the two contact resistances between the nickel layer and the substrate which are in series. time therefore reflects a corresponding change in this contact resistance. This change was studied during a 1000 h test, during which the assembly described was maintained in an enclosure at 200 ° C. in a dry air atmosphere. .

The following results are each the average of 10 different tests:

It can be seen that after a slight initial increase, the contact resistance practically no longer varies as a function of time. It should be noted that, by comparison, tin coatings on aluminum are poorly resistant to stays of a few hundred hours at 200 ° C., phenomena of tin diffusion in the intermediate sublayer and then in the underlying aluminum occurs quickly.

Bending test at 90 ° were carried out on bar sections in AGS / L of 40 x 6 mm section covered 15 _j um Ni, standard ASTM B 571, before and after aging 1000 hours at 200 ° vs. No delamination of the nickel layer was observed in the bending zone. Folding up to 180 ° did not result in further delamination.

Other sections of the same bars underwent salt spray exposure tests for periods of 100 to 400 h. according to standard NF 41002. During these tests, no separation of the nickel layer was observed either.

Was also coated with a layer 15 _j um nickel platelets AGS / L of 64 x 72 x 2 mm following the method described in Example. These plates were then heated locally to a temperature close to the melting point of aluminum in 6 to 7 min. We have managed to obtain on each wafer, on a surface of the order of 1 or 2 cm ² , an initial melting of the aluminum. After cooling, it was observed that the nickel layer had retained all of its adhesion properties.

Finally, the bars thus coated with nickel are susceptible to brazing treatments at relatively high temperature, without separation of the nickel layer. This is how it is possible to connect by brazing using a Cd / Ag alloy with 95% Cd and 5% Ag bars made of AGS / L coated with nickel by the method described in the example. . This brazing alloy has a melting point between 340 ° and 395 ° C. It is also possible, using the same alloy, to braze bars made of AGS / L coated with nickel by the method described in the example with other metals, such as copper, silver or copper alloys or silver using conventional fluxes for the implementation of such solders. This shows the great advantage of this nickel coating compared to the usual tin-based coatings for which only tin or tin-lead solders can be produced with relatively poor mechanical characteristics.

For certain particular applications, in order to further improve the quality of the contacts, it is possible to deposit on the nickel layer, which, according to the invention, covers an aluminum substrate, a thin layer of silver. Such a deposition will be carried out for example by electrolysis in a silver cyanide bath, as has been said above.

Tests have been carried out to study the abrasion behavior of electrical contacts, one of the two contact elements of which is made of aluminum coated with nickel and then silver, the other element being a copper finger, comprising pellets of silver contact brazed at the ends of the fishplates, identical to the one shown in figure 7. This finger is plugged in as in the case which has just been described on a section of AGS / L bar 40 x 6 mm in section, covered 15 µm of nickel plus 3 µM of silver as described above. A known device makes it possible to subject the section of bar at the frequency of 3,600 cycles / h of alternating displacements in its plane, of approximately 6 mm of amplitude, the finger and, consequently, the silver contact pads remaining fixed. . The contact surface of these pellets with the bar is the same as in the previous example, ie 9 mm ² each. The clamping force of the two fishplates against each other is also the same, i.e. 1 kg.

A stabilized supply of known type connected, on the one hand to the finger and on the other hand to the bar section, delivers a constant alternating current of 250 A which passes through the contact between bar and finger. This current raises the bar temperature by about 75 ° C. above the ambient (about 100 ° C. for an ambient temperature of 25 ° C.)

A series of contact wear tests was performed. During each of these tests, the bar / finger contact underwent 15 to 20,000 cycles.

The contact wear resulting from cycling was followed, both by loss of mass and by variation in voltage drop.

Table II below shows the total mass losses in mmg resulting from the wear of the contacts between the bar and the finger. The voltage drop at these contacts is also given. It was measured between the ends of the fishplates at the contact pads and the bar in the immediate vicinity of the contact area.

These results show a very low wear of the contacts, in spite of severe friction conditions, and a reduction in the drops in contact voltage possibly due to a kind of polishing of the contact surfaces.

This deposit of silver therefore makes it possible to combine the remarkable qualities of a layer of nickel with high adhesion deposited directly on an aluminum substrate with the well-known qualities of silver for making electrical contacts.

It can be seen that the new method which is the subject of the invention makes it possible to produce contact devices which have very remarkable characteristics of resistance to mechanical and thermal stresses which make them suitable for use in working conditions. the most rigorous.

These contact devices can be the subject of very numerous variants which do not depart from the field of the invention.

It is possible, in particular, to produce for certain applications devices the two contact elements of which are made of aluminum coated with nickel by the method according to the invention. It is also possible to coat at least one of the aluminum contact elements with a layer of silver deposited on a layer of nickel.

It is also possible to produce, by means of the devices according to the invention, all kinds of apparatus making it possible to establish or interrupt electrical connections and, in particular, sockets and switches for domestic or professional use, and also certain types. of contactors or circuit breakers.

It is also possible to produce long conductors coated with nickel from one end to the other in order to be able to make static contacts at any point.

Finally, it is possible to envisage devices according to the invention comprising contact parts made of nickel-plated aluminum on which a contact plate made of copper, or silver, or a contact alloy or pseudo-alloy, resistant to the impact of short-lived electric arcs and also abrasion. It is then possible to use such devices for establishing and breaking under load of circuits, and also for sliding contacts such as those made on manifolds and rings.

Claims (6)

1. A device for disengageable contact between two conductors (3 and 1-2 or 18 and 21-22) wherein the contact between the two conductors is maintained by a resilient gripping action and wherein at least one of the two conductors is of aluminium, characterised in that the conductor or conductors comprising aluminium is or are coated, at least in the contact region, by an electrolytic nickel layer, the surface condition of the substrate having been modified beforehand by a preliminary deposit of a thin layer of nickel which is produced chemically by means of an aqueous solution containing:

followed by redissolution of that layer.

2. A device according to claim 1 characterised in that the nickel-coated aluminium conductor is in contact with a conductor coated with silver.

3. A device according to claim 1 characterised in that the nickel-coated aluminium conductor is coated with silver at least in the contact region.

4. A device according to one of claims 1 to 3 characterised in that the contact between the two conductors is a sliding contact.

5. A process for coating a component of aluminium with a layer of non-alloyed nickel in direct contact with the substrate, wherein the nickel is deposited by electrolysis, characterised in that a preliminary deposit of a thin layer of nickel is produced chemically, by placing the aluminium component to be coated with an aqueous solution containing:

said preliminary deposit then being removed before the definitive protective layer is deposited.

6. A process according to claim 5 characterised in that the deposit of the definitive protective layer of nickel is produced by electrolysis from an aqueous solution containing nickel sulphamate, boric acid and nickel chloride.

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