CH627791A5 - Process for covering the surface of an electrically conductive article, plant for making use of the process and application of the process - Google Patents

Description

The object of the invention is to propose a method of covering the surface of an electrically conductive part with a thick and homogeneous layer of coating which can be easily applied on an industrial scale to large parts and under economic conditions, the coating layers obtained

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on the other hand, be strongly hung on the part to be covered to prevent chipping.

For this purpose, the invention firstly relates to a method of covering the surface of an electrically conductive part with a homogeneous coating layer, characterized in that the conductive part to be covered is arranged inside an enclosure filled with a gas at a pressure between 0.1 and 15 Torr, at a distance of less than 50 mm from an element made of coating material, the element to be covered and the element made of coating material constituting two separate cathodes, that the cathodes are placed at potentials such that there are potential differences between 200 and 1500 V between anode and cathodes and that the coating is carried out by an abnormal discharge established between the anode and the cathodes.

The invention also relates to an installation for implementing the recovery process, characterized in that it comprises an enclosure connected to pumping means and internally lined with a heat shield, two current inlets for the support and the current supply of the part to be covered and of the element of coating material, these current inlets being connected to at least one electric generator equipped with arc cutting devices.

Finally, the invention also relates to an application of the method.

We will now describe a coating operation carried out according to the method of the invention, this operation being carried out in the installation shown schematically and by way of example in the attached figure.

The single figure shows the coating installation, in elevation, the enclosure being open in its front part to show the arrangement of the various elements inside the installation.

It is desired to carry out a coating of chromium carbide on a steel plate containing 0.30% carbon.

For this, we have this plate 1 of carbon steel inside an enclosure 2 at the end of a current inlet 3 connected via an insulating passage 4 to a current generator 5 The enclosure is connected, on the one hand, to a pump 6 and, on the other hand, to a gas inlet 7 connected to a source of argon. The pumping and arrival of argon are regulated so that the dynamic pressure inside the enclosure is maintained at 3 Torr. The end of the inert gas inlet 7 carries, on the other hand, the anode 8. The interior of the enclosure is, on the other hand, doubled by a heat shield 9 making it possible to avoid exchanges by radiation between the interior and exterior of the enclosure.

A second current inlet 10 identical to the inlet 3 is, on the other hand, connected to a generator 12 of the same type as the generator 5 and carries at its bent end a plate 14 of iron / chromium alloy with 36% chromium . The plate to be covered 1 and the ferrochrome plate 14 arranged one opposite the other in parallel are spaced at a distance of 12 mm.

At the start of the operation, after having placed the plate to be covered and the ferrochrome plate on the current arrivals inside the enclosure, a vacuum is created in the enclosure using the pump 6 and sent a stream of argon through the gas inlet 7, the pumping and the gas inlet then being adjusted so that the pressure is established at 3 Torr. We then put into operation the generators 5 and 12 which supply electric current to the plates 1 and 14 which will play the role of two separate cathodes, the potential of the plate to be covered 1 being maintained at a value of 450 to 500 V and the potential of the plate 14 of iron / chromium alloy being maintained at a value of 600 V relative to the anode 8.

When the current is established between the anode and the cathodes, there occurs an intense ionization of the gas of the enclosure in the vicinity of the two cathodes which are bombarded intensely by the positive ions created.

Plate 1 heats up under the effect of ion bombardment and its temperature is established at 900 ° C.

627791

The generator then supplies a power density to this plate of the order of 4 to 5 V / cm2.

The temperature of the plate 14 is established at the same time at a temperature of 125 ° C and the power density supplied to this plate 14 is of the order of 30 W / cm2.

After 1 hour of maintenance under the conditions described above, we observe on the face of the plate 1 opposite the plate 14 the deposition of a layer of a chromium alloy with 56% chromium over a thickness from 80 (d ..

There has therefore been a transfer of material between the plate 14 playing the role of emitter and the plate 1 playing the role of receiver.

During the entire coating operation, there is established, between the anode and the cathodes, a luminescent discharge under abnormal conditions, the formation of arcs being avoided by means of an arc cutting device associated with each of the generators. 5 and 12.

Thermocouples are placed in the receiver 1 and the emitter 14 so as to control their temperature which can be fixed at a certain level by adjusting the electrical parameters relating to the generators 5 and 12.

During the abnormal discharge established between the anode and the cathodes, there is an intense ionization of the gas circulating in the enclosure in the vicinity of the cathodes and an intensive bombardment of the two cathodes by the positive ions created.

The heating of the transmitter to a high temperature (1250 ° C) under the reduced pressure prevailing in the enclosure produces a vaporization of the metal of the transmitter which is deposited on the receiver 1. At the same time, the intensive bombardment by high energy ions from the emitter causes the atomization of metal particles torn from the emitter, these particles being projected and entrained on the receiver placed near the emitter.

Unlike the previously known methods, it is therefore both by evaporation under reduced pressure and by sputtering that the chromium deposit occurs on the receptor.

In the emitter material, the iron and chromium atoms are linked by iron / iron bond, by chromium / chromium bond and by iron / chromium bond, and it seems, from the results obtained, the chromium alloy deposited being much richer in chromium than the alloy of the emitter, that the atomic bonds chromium / chromium are destroyed in preference to the bonds with iron.

We see that the originality of the process lies in the use of the abnormal discharge regime, which until now has been avoided using sputtering because of the risk of creating arcs and in the use of two separate cathodes. one constituting the transmitter, the other the receiver.

On the other hand, heating of the receiver under the effect of ion bombardment creates a zone favorable to the penetration of the coating layer for its attachment to the substrate.

It should be noted that, in the process described, the potential differences used are much lower than those used in the case of sputtering but that, on the other hand, the residual pressure in the enclosure, of the order of a few torrs , is much higher than that used in sputtering.

As a result, the intensity of the current between the anode and the cathodes is much higher than that recorded in the case of sputtering.

Once the adherent and thick chromium coating is obtained on the plate 1, we pass to a second phase of the treatment, consisting of a diffusion treatment for the production of chromium carbide from the deposited chromium layer and the carbon. contained by steel.

For this purpose, the cathodic potential and the power density of receiver 1 are maintained at values identical to what they were during the first phase of the coating treatment, this receiver having a temperature which stabilizes at 900 ° as in the previous treatment. The supply to the transmitter 14 is cut off and the supply conditions for the receiver are maintained for a period of 10 to 15 hours, this receiver remaining at a constant temperature of 900 °. During this diffusion processing, a

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interaction between the carbon of the steel and the chromium of the deposited layer, which has the effect of creating a layer of chromium carbide on the surface of the part 1.

It should be noted that the treatment by diffusion of the receiver 1 can be carried out without removing the part 1 from its support 3 and that this treatment can be carried out under economic conditions as regards the consumption of energy, since the heat losses by convection and conduction are almost zero and that the heat losses by radiation from the plate 1 are limited to a minimum thanks to the heat shield 9 disposed inside the enclosure.

The method described therefore makes it possible to obtain, firstly, a thick layer of chromium and, secondly, a layer of chromium carbide on the surface of a piece of steel, under particularly economical conditions and with high reliability.

In the embodiment which has just been described, each of the two cathodes was connected to a current generator allowing the cutting of arcs and the production of an abnormal luminescent discharge without risk for the parts to be treated and, in this In this case, the independent regulation of the electrical parameters of the two cathodes enabled independent regulation of the temperatures of the emitter and the receiver. However, it is possible to have two separate cathodes by the fact that they are made of different materials connected to the same generator and therefore brought to the same potential, the transport of material from one cathode to the other being essentially due the fact that the tearing of the constituents of one of the materials is preferably carried out over the tearing of the constituents of the other material. In this case, however, there is the disadvantage of not being able to independently adjust the temperatures of each of the two cathodes.

In the embodiment which has just been described, the part to be covered and the emitter of coating material were two flat plates arranged in parallel opposite; however, it is possible to use parts of any shape, the only requirement being that there is a correspondence between the shape of the emitter and the shape of the receiver so that the path of the particles of coating materials is substantially identical for the entire room to be covered.

In the case of the internal covering of a tube for example, the receiving cathode will obviously be constituted by the tube, the emitting cathode being a solid cylinder or a wire of large section arranged along the axis of the tube.

The distance between the external surface of the internal cylinder and the internal surface of the hollow cylinder must in any case be less than 50 mm to obtain a good quality coating. In the case where we would like to directly obtain a coating of chromium carbide without going through the second phase of the process in which we do, spread the coating inside the steel, by maintaining the receiver at high temperature, as a gas filling the enclosure, a mixture of inert gas and a gas containing carbon, for example methane, can be used. In this case, a carefully proportioned mixture of argon and methane will be brought in via piping 7, the ionization of which in the vicinity of the cathodes will cause a release of the carbon from the methane which will thus be able to react with the chromium of the emitter during the bombardment. of the receiver for the constitution of a layer of chromium carbide directly on the surface of the steel. In the case where the enclosure gas is not consumed, that is to say in the case of the use of an inert gas which does not participate in the chemical constitution of the coating, it is possible theoretically to use a gas under static pressure without gas inlet and without pumping during the coating operation. However, it seems preferable to use the method described in the exemplary embodiment, since this technique allows better control of the leaks and of the cleanliness of the gas circulating in the enclosure.

The details which have been given concerning the cathode potentials of the transmitter and the receiver and the distance between the transmitter and the receiver, in the case of chromization, constitute optimal values in the application described; however, it is possible to use slightly different values while obtaining satisfactory conditions for carrying out the process and obtaining a coating of good quality.

Thus, in the case of the covering of a steel containing 0.30 to 0.35% of carbon by a layer of chromium obtained from an iron / chromium alloy emitter, in an atmosphere of inert gas , the pressure of the inert gas can be maintained between 1 and 5 Torr, 30 the cathode potential of the receiver between 450 and 500 V and the cathode potential of the transmitter between 550 and 650 V, and finally the distance between emitter and receiver, c '' i.e. between their active surface, between 8 and 15 mm.

In the case of direct covering with chromium carbide, the gas contained in the enclosure being constituted by a mixture of inert gas and a carbon compound such as methane, the various parameters mentioned above may be maintained in identical intervals.

Finally, the process described applies not only to the production of coatings of chromium or chromium carbide, but also to the production of other metallic or non-metallic coatings such as coatings of titanium, titanium carbide, boron , tantalum or vanadium.

In all cases, it has been seen that it is advantageous to maintain the potentials at values such that there exist between anode and cathodes potentials between 300 and 800 V in the context of the process described where the residual pressure in the enclosure is of the order of a few torrs.

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1 sheet of drawings

Claims (13)

627791 1. Method for covering the surface of an electrically conductive part with a homogeneous coating layer, characterized in that the conductive part to be covered (1) is arranged inside an enclosure (2) filled with a gas at a pressure between 0.1 and 15 Torr, at a distance of less than 50 mm from an element made of covering material (14), the element to be covered (1) and the element made of coating (14) constituting two distinct cathodes, that the cathodes are put at potentials such that there are between anode (8) and cathodes (1,14) potential differences between 200 and 1500 V and that one realizes coating with an abnormal discharge established between the anode (8) and the cathodes (1,14). 2. Method according to claim 1, characterized in that the cathodes (1,14) are set to potentials which can vary independently of one another so as to independently regulate the temperature of each of the two cathodes (1, 14). 2 3. Method according to claim 1, characterized in that the two cathodes (1,14) are set to the same potential but that they are distinct by the material constituting them. 4. Method according to one of claims 1 to 3, characterized in that the gas is circulated in the enclosure and that the dynamic pressure of the gas is measured at a point in the gas circuit. 5. Method according to one of claims 1 to 4, characterized in that the gas filling the enclosure (2) is argon. 6. Method according to one of claims 1 to 4, characterized in that the gas filling the enclosure (2) comprises a chemically active constituent. 7. Method according to one of the preceding claims, characterized in that the part to be covered (1) and the element of coating material (14) have complementary shapes so that the distance between the surface undergoing bombardment of the transmitter and the surface to be covered by the receiver is constant. 8. Method according to claim 1, characterized in that establishes between anode and cathodes potential differences between 300 and 800 V. 9. Installation for implementing the covering method according to claim 1, characterized in that it comprises an enclosure (2) connected to pumping means (6) and internally lined with a heat shield (9) , two current inlets (3.10) for the support and the current supply of the part to be covered (1) and of the element made of coating material (14), these current inlets being connected to at least one electric generator (5,12) equipped with arc cutting devices. 10. Application of the method according to claim 1, to cover the surface of a steel part with a homogeneous layer of chromium carbide, characterized in that the steel part (1) to be covered is arranged inside of an enclosure (2) filled with a gas at a pressure between 1 and 5 Torr, at a distance between 8 and 15 mm from an element (14) made of an alloy comprising iron and chromium, that an anode (8) is provided, that the steel part (1) to be covered and the iron / chromium alloy element (14) constituting two separate cathodes (1,14) are placed at potentials such that '' there are between anode and cathodes potential differences between 450 and 500 V with regard to the piece of steel to be covered (1), and between 550 and 650 V with regard to the iron / chromium alloy (14 ) and that a coating is deposited by an abnormal discharge established between the anode (8) and the cathodes (1,14), said coating containing mainly chromium. 11. Application according to claim 10, characterized in that the steel part is made of a steel containing 0.30 to 0.35% of carbon and that the thickness of the layer of chromium carbide reaches approximately 80 [». 12. Application according to claim 10 or 11, characterized in that said steel part (1) is maintained at high temperature by maintaining it at the same potential as in the first phase and by cutting off the supply to the cathode (14 ) consisting of the coating material of iron / chromium alloy. 13. Application according to claim 10, characterized in that the gas which fills the enclosure (2) is a mixture of inert gas and a gaseous carbon compound. Among the processes for coating metal parts, physical processes in the gas phase are known, such as vacuum evaporation and sputtering, which are both implemented in an enclosure containing a gas under pressure. reduced which can participate, after ionization, in the physical phenomena leading to the production of the coating or on the contrary to hinder the course of these physical phenomena. In the latter case, the residual pressure in the enclosure is reduced to the maximum. In the case of evaporation under vacuum, where the coating material is brought to high temperature inside an enclosure also containing the part to be covered maintained at a lower temperature, the residual pressure in the enclosure must be between 10 "4and 10-5 Torr. The coating material brought to high temperature, for example by ion bombardment or by electronic bombardment, vaporizes and is deposited on the surface of the part to be covered on which it condenses. Besides its delicate implementation, this process does not allow thick layers of coating to be obtained quickly and easily. Cathode sputtering, which is a process currently used quite commonly, makes it possible to obtain in a short time the deposit of thin films (with a thickness ranging from a few angstroms to a few microns). This process allows the deposition of metals and alloys as well as various compounds such as oxides or sulfides. In the case of sputtering, a cathode made of material is placed inside a chamber containing a gas, generally an inert gas such as argon, at a residual pressure of the order of 10 "3 Torr. of coating and a potential difference of a few thousand volts, for example 5000 V, is established between this cathode and an anode which may be the enclosure itself, the part to be covered being placed at a distance of a few centimeters from the cathode in covering material, this distance being up to 15 to 20 cm. A discharge tube has thus been created inside which the gas is ionized, the positive ions bombarding the cathode. Under the effect of shocks between the cations and the cathode, there is a tearing of particles from the cathode, these particles thus released coming to be fixed on the part to be covered which is placed either at the anode potential or at an intermediate potential. Generally the cathode is cooled in order to avoid excessive heating due to the effect of ion bombardment. In the case of sputtering, the residual pressure in the enclosure is therefore quite low, the potential difference between anode and cathode high and the intensity of the current between the electrodes relatively small, of the order of 1 mA. Under these conditions, the deposition rate of the coating layer is of the order of 10 (x / h, and this method is therefore only applicable for the production of thin films on parts of reduced dimensions. In certain cases, as in that of chromization, it is necessary to produce coating layers (of chromium or chromium carbide) which are thick and homogeneous on parts of large dimensions, and the methods known up to now do not give all satisfaction.