GB1574677A

GB1574677A - Method of coating electrically conductive components

Info

Publication number: GB1574677A
Application number: GB18044/78A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1977-06-07
Filing date: 1978-05-05
Publication date: 1980-09-10
Also published as: IT1109053B; DE2820183C3; DE2820183B2; FR2393854B1; BE867913A; JPS544246A; CH627791A5; DE2820183A1; ATA413078A; JPS5615780B2; FR2393854A1; IT7868139A0; SE7806503L; ZA782794B; NL7806125A; AT364217B

Abstract

The article (1) made of carbon steel and the chrome-iron plate (14) form cathodes arranged facing the anode (8) in the vessel (2). They are connected to electrical generators (5, 12). A dynamic pressure of the order of 3 torr is maintained in the vessel (2) using the pump (6), the atmosphere consisting of argon. Since the potential differences between the anode and the cathodes are from 450 to 500 V and 600 V, the cathodes are subjected to a bombardment with positive ions. The abnormal luminescent discharge established between the electrodes gives rise to the deposition of a layer of alloy of high chromium content on the article (1). In the second stage of the treatment, diffusion of carbon from the article (1) into the layer is produced, and this gives a chromium carbide layer. <IMAGE>

Description

(54) IMPROVEMENTS IN AND RELATING TO A METHOD OF COATING ELECTRICALLY CONDUCTIVE COMPONENTS (71) I, MICHEL GANTOIS, of French nationality of Laboratoire de Genie Metallurgique, Ecole des Mines, Parc de Saurupt, 54000 Nancy, France, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be parti- cularly described in and by the following statement: The invention relates to a method of coating a surface of an electrically conductive component with a thick, uniform layer.

Some known methods of coating metal components are physical methods in the gaseous phase, e.g. evaporation in vacuo and cathode sputtering, both methods being performed in a chamber containing a gas under reduced pressure which, after ionization, may either participate in the physical phenomena resulting in the production of the coating or may interfere with the phenomena, in which case the residual pressure in the chamber is reduced to the maximum extent.

In the case of evaporation in vacuo, in which the coating material is brought to a high temperature inside a chamber which also contains the component to be coated which is kept at a lower temperature, the residual pressure in the chamber must be between 10-4 and 10-5 Torr. The coating material is brought to a high temperature, e.g.

by ion or electron bombardment, vaporizes and is deposited and condenses on the surface of the component to be coated. This method, besides being difficult to operate, does not provide a thick coating layer in a quick and easy manner.

Cathode sputtering is a method which is fairly widely used at present, and can be used to obtain thin films (between a few Angstroms and a few microns thick) in a short time. The method can be used for depositing metals and alloys and various compounds such as oxides pr sulphides.

In the case of cathode sputtering, a cathode made of coating material is disposed inside a chamber containing a gas, usually an inert gas such as argon, at a residual pressure of the order of 10-3 Torr, and a potential difference of a few thousand volts, e.g. 5,000 volts, is produced between the cathode and the anode, e.g. the chamber itself, the component to be coated being placed a few centimeters-up to 15 to 20 cm-away from the coating material cathode.

The result is a discharge tube inside which gas is ionized and the positive ions bombard the cathode. As a result of the irnpacts between cations and the cathode, particles are torn out of the cathode and are deposited on the component to be coated, which is brought either to the anode potential or to an intermediate potential. Usually the cathode is cooled to avoid excessive heating under ion bombardment. In the case of cathode sputtering, therefore, the residual pressure in the chamber is very low, the potential difference between the anode and cathode is high and the current between the electrodes is relatively weak, of the order of a milliampere. Under these conditions, the rate of deposition of the coating layer is of the order of 10 microns per hour, i.e. the method is suitable only for producing thin films on small components.

In some cases, e.g. chromium-plating, it is necessary to produce thick, uniform coating layers, e.g. of chromium or chromium carbide, on large components. The existing methods are not satisfactory.

According to one aspect of the invention there is provided a method of coating a surface of an electrically conductive component with a thick, uniform layer, comprising disposing said conductive component to be coated in a chamber at a distance less than 50 mm from a component made at a distance less than 50 mm from a component made of the coating material, filling said chamber with gas at a pressure between 0.1 and 15 Torr, and bringing said component to bw coated and said component made of coating material, which form two separate cathodes, to potentials such that the potential differences between an anode in said chamber, which may be said chamber, and said cathodes is between 200 and 1500 volts, coating being brought about on said conduc tiye. component by abnormal glow discharge between said anode and said cathodes.

According to another aspect of the invention, there is provided apparatus for carrying out the above described method comprising a chamber having an inlet for gas, means connected to said inlet for supplying gas to said chamber for filling said chamber with gas at a pressure between 0.1 and 15 Torr, an internal heat shield in said chamber, means for supporting a first component made of coating material in said chamber, means for separately supporting a second component to be coated in said chamber and at a distance less than 50 mm from said first component, means for supplying current to the components, and at least one electric generator provided with an arc breaking device, said generator or said generators being conected to said current supply means for bringing the components to potentials such that the potential difference between an anode in said chamber, which may be said chamber, and said components is between 200 and 1500 volts.

The invention will be more fully understood from the following description of an embodiment thereof, given by way of ex-.

ample only, with reference to the accom- panying drawing.

In the drawing: The single drawing is a view in elevation of an embodiment of a coating apparatus according to the invention, the front of the chamber being open to show the arrange ment of the various components inside the apparatus.

The application of a chromium carbide coating- on a steel plate containing 0.30% carbon will now be described, by way of example only.

A carbon steel plate 1 to be coated is disposed inside a chamber 2 at the end of a current supply support member 3 connected via an insulating duct 4 to a current generator 5. The chamber 2 is connected at one end to a pump 6 and at the other end to a gas inlet 7 connected to a source of argon. The pumping-and the supply of argon are controlled so that the dynamic- pressure inside the chamber is kept at 3 Torr. At its end; gas inlet 7 bears an anode 8. Chamber 2 is internally lined! with a heat shield 9: pre- venting heat exchange by radiation between the interior and exterior of the chamber.

A second current supply support member 10 identical to member 3 is connected to a generator 12 of the same kind as generator 5 - The end of member 10 is curved and bears a plate 14 made of iron-chromium alloy containing 36% chromium. The plate 1 to be coated and the ferrochromium plate 14 are disposed parallel and spaced apart by a distance of 12 mm.

At the beginning of the operation plates 1 and 14 are secured to the members 3 and 10 in the chamber, the chamber is evacuated by pump 6 and a stream of argon is introduced through 7, the pumping and the gas supply being adjusted so that the pressure remains at 3 Torr. Next, generators 5 and 12 are put into operation and supply electric current to plates 1 and 14, which act as two separate cathodes. The potential of plate 1 is kept at 450 to 500 V and the potential of plate 14 is kept at 600 V, relative to anode 8.

The flow of current between the anode and the cathodes results in intensive ionization of the gas in the chamber near the two cathodes, which are intensely bombarded by the resulting positive ions.

As a result of the ion bombardment, plate 1 heats up to, and is maintained constant at, a temperature of 900"C. The generator is then supplying power to plate 1 at a density of the order of 4 to 5W per cm2; In the same time, the temperature of plate 14 becomes, and is maintained constant at, 12500C, the density of power supplied to plate 14 being of the order of 30 W per cm2.

After an hour under the above conditions, a layer of a chromium alloy containing 56% chromium and 80 microns thick is observed on the surface of plate 1 facing plate 14.

Consequently, material has been transferred from plate 14 to plate 1.

During the entire coating operation, a luminescent discharge under abnormal glow discharge conditions persists between the anode and the cathodes. Are formaton is prevented by an arc cut-off device associated with each generator 5 and 12.

Thermocouples are disposed in plates 1 and 14 so as to control their temperatures, each of which can be kept at a given level by adjusting electric parameters r.elatingt to generators 5 and 12.

In the above method the plate 14 serving as an emitter of the coating material is heated to a high temperature (1250"C) under the reduced pressure in the chamber, so that metal from the emitter is vaporized and becomes: deposited on plate 1 which is thu; a receiver of coating- material. At the same time, the intense bombardment by the high energy-ions from the emitter results in atomi Zaflon of metal particles torn from the emitter, the particles being sprayed and driven towards the receiver, which is disposed near the emitter.

In constrast to known methods, therefore, ehromium is deposited on the receiver by evaporation under reduced pressure and simultaneously by cathode sputtering.

In the emitter material, the iron and chromium atoms are bonded by iron-iron, chromium-chromium and iron-chromium bonds. To judge from the results, i.e. the fact that the deposited chromium alloy is much richer in chromium than the emitter alloy, the chromium-chromium atom bonds are destroyed in preference to the iron bonds.

As can be seen the novelty of the above method is (a) the use of abnormal glow discharge conditions, which have hitherto been avoided in cathode sputtering owing to the risk of arc formation and (b) the use of two separate cathodes, one as the emitter and the other as the receiver.

The receiver, when heated by ion bombardment, produces a region favouring the penetration of the coating layer, which can thus adhere to the substrate.

In the above described method according to the invention, the potential differences are much lower than in the case of cathode sputtering, but the residual pressure in the chamber (of the order of a few Torr) is much greater than that used in cathode sputtering.

Consequently the current between the anode and cathodes is much greater than in the case of cathode sputtering.

After an adherent and thick chromium coating has been obtained on plate 1, the next step of the treatment begins, i.e. a diffusion treatment to produce chromium carbide from the layer of deposited chromium and the carbon in the steel.

To this end, the supply to plate 14 is cut off and the cathode potential of, and the power density of, the supply to plate 1 are kept at the same values as during the first step of the coating treatment, so that the temperature of the plate 1 remains constant at 900"C as in the preceding treatment.

These conditions are maintained from 10 to 15 hours, the plate 1 remaining at a constant temperature of 900"C. During this diffusion treatment, interaction occurs between the carbon in the steel and the chromium in the deposited layer, thus producing a layer of chromium carbide at the surface of plate I.

Plate 1 can thus be treated by diffusion without removing it from member 3, and the treatment can be performed under economic conditions with regard to energy consumption, since the losses of heat through convection and conduction are almost zero and the losses of heat. by radiation from plate 1 are limited' to a minimum by the heat shield 9: in the chamber: Consequently, the above described method can be used to obtain a thick chromium layer in a first step and a thick chromium carbide layer on the surface of a steel component in a second step, under particularly good economic conditions and with great reliability.

The invention, however, is not intended to be limited to the embodiment which has been described, but includes variants and modifications within its scope as defined by the appendent claims. For example in the- above described embodiment, each cathode is connected to a current generator for cutting off arcs and for producing an abnormal luminescent discharge without endangering the component under treatment, in which case the electric parameters of the two cathodes can be independently adjusted so as independently to regulate the temperatures of the cathodes. However, it is possible to have two different cathodes made from different materials and connected to a single generator and therefore raised to the same potential, since the transfer of material from one cathode to the other is due mainly to the fact that the constituents of one material are torn out in preference to the constituents of the other material. In this case, however, there is the disadvantage that the temperature of each cathode cannot be adjusted independently.

In the above described embodiment, the component to be coated and the emitter of coating material are both flat plates disposed parallel and side by side. However, the component to be coated can have any shape, the only requirement being that the shape of the coating material emitter should correspond to the shape of the component to be coated so that the particles of coating material traveI substantially the same distance over the entire component to be coated.

In the case where a tube is to be coated internally, for example, the- receiving cathode is of course the tube and the emitting cathode is a solid cylinder or a thick wire disposed along the tube axis. The distance between the outer surface of the internal cylinder and' the inner surface of the hollow cylinder must in all cases be less than 50 mm in order to obtain good quality coating.

Where it is desired to obtain a chromium carbide coating directly without need for the second step in the method (where the coating is made to diffuse into the the steel by maintaining the receiver at a high tem- perature) the chamber can be filled with a mixture of inert gas and a carbon-containing gas such as methane. In this case, a carefully proportioned mixture of argon and methane is supplied through pipe 7 and ionizes near the cathode thus releasing carbon from the methane so that it can react with the chromium of the emitter during bombardment of the reservoir, thus farming a layer of chromium carbide directly on the surface of the steel component. In the case where the gas in the chamber is not used, i.e.

is inert and does not form part of the chemical make-up of the coating, it is theoretically possible to use a gas under static pressure without supplying gas and without pumping during the coating operation. However, it appears preferable to use the method described above, since this method gives better control of leaks and of the purity of the gas flowing in the chamber.

The figures given for the cathode potentials of the two cathodes and the distance between the cathodes, in the case of chromiumplating, are optimum values in the application described. Even if the values used are slightly different, however, the process conditions are still satisfactory and a good quality coating is obtained.

For example, in the case where a component of steel containing 0.30 to 0.35 % carbon is to be coated with a layer of chromium obtained from an iron-chromium alloy emitter in an inert gas atmosphere, the pressure of inert gas can be kept between 1 and 5 Torr, the cathode potential of the receiver between 450 and 500 V, the cathode potential of the emitter between 550 and 650 V and the distance between emitter and receiver, i.e. between their active surfaces, between 8 and 15 mm.

In the case of direct coatings with chromium carbide, the gas in the chamber will be a mixture of inert gas and a carbon compound such as methane and the various aforementioned parameters can be kept within identical intervals.

The method according to the invention can be used not only to obtain chromium or chromium carbide coatings but also to obtain other metal or non-metallic coatings, e.g. of titanium, titanium carbide, boron, tantalum or vanadium.

It will be seen that in all cases it is advantageous to maintain the potentials at values such that the potentials between anode and cathodes are between 300 and 800 V, where the residual pressure in the chamber is of the order of a few Torr.

There is thus provided a method of coating a surface of an electrically conductive component with a thick, uniform layer which can easily be applied to an industrial scale to large components and under good economic conditions. In addition, the resulting coatings adhere firmly to the component so that they do not flake off.

WHAT WE CLAIM IS: 1. A method of coating a surface of an electrically conductive component with a thick, uniform layer, comprising disposing said conductive component to be coated in a chamber at a distance less than 50 mm from a component made of the coating material, filling said chamber with gas at a pressure between 0.1 and 15 Torr, and bringing said component to be coated and said component made of coating material, which form two separate cathodes, to potentials such that the potential differences between an anode in said chamber, which may be said chamber, and said cathodes is between 200 and 1500 volts, coating being brought about on said conductive component by abnormal glow discharge between said anode and said cathodes.

2. A method according to claim 1, wherein said potential differences are between 300 and 800 volts.

3. A method according to either claim 1 or claim 2, including independently varying said potentials to which said cathodes are brought to independently regulate the temperatures of said cathodes.

4. A method according to either claim 1 or claim 2, wherein said cathodes are made of different materials, and including bringing said cathodes to the same potential.

5. A method according to any one of the preceding claims, including causing said gas filling said chamber to flow therethrough, the pressure of said gas being a dynamic pressure measured at a place in said gas flow.

6. A method according to any one of the preceding claims, wherein said gas filling said chamber is argon.

7. A method according to any one of claims 1 to 5, wherein said gas filling said chamber has a chemically active constituent.

8. A method of coating a surface of a steel component containing 0.30 to 0.35% carbon with a thick, uniform layer of chromium carbide, comprising placing said steel component to be coated in a chamber between 8 and 15 mm from a component made of an alloy containing iron and chromium, filling said chamber with a gas at a pressure between 1 and 5 Torr, bringing said steel component to be coated and said iron-chromium alloy component, which form two separate cathodes, to potentials such that the potential difference between an anode in said chamber, which may be said chamber itself, and said steel component is between 450 and 500 volts, and the potential difference between said anode and said iron-chromium alloy component is between 550 and 650 volts, a coating having a high chromium content being deposited on said steel component as a result of an abnormal glow discharge between said anode and said cathodes, and finally maintaining said steel component at a high temperature by keep-- ing it at the same potential as in the first stage and cutting off the supply of current to said iron-chromium alloy component.

9; A method of coating a surface of a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. a layer of chromium carbide directly on the surface of the steel component. In the case where the gas in the chamber is not used, i.e. is inert and does not form part of the chemical make-up of the coating, it is theoretically possible to use a gas under static pressure without supplying gas and without pumping during the coating operation. However, it appears preferable to use the method described above, since this method gives better control of leaks and of the purity of the gas flowing in the chamber. The figures given for the cathode potentials of the two cathodes and the distance between the cathodes, in the case of chromiumplating, are optimum values in the application described. Even if the values used are slightly different, however, the process conditions are still satisfactory and a good quality coating is obtained. For example, in the case where a component of steel containing 0.30 to 0.35 % carbon is to be coated with a layer of chromium obtained from an iron-chromium alloy emitter in an inert gas atmosphere, the pressure of inert gas can be kept between 1 and 5 Torr, the cathode potential of the receiver between 450 and 500 V, the cathode potential of the emitter between 550 and 650 V and the distance between emitter and receiver, i.e. between their active surfaces, between 8 and 15 mm. In the case of direct coatings with chromium carbide, the gas in the chamber will be a mixture of inert gas and a carbon compound such as methane and the various aforementioned parameters can be kept within identical intervals. The method according to the invention can be used not only to obtain chromium or chromium carbide coatings but also to obtain other metal or non-metallic coatings, e.g. of titanium, titanium carbide, boron, tantalum or vanadium. It will be seen that in all cases it is advantageous to maintain the potentials at values such that the potentials between anode and cathodes are between 300 and 800 V, where the residual pressure in the chamber is of the order of a few Torr. There is thus provided a method of coating a surface of an electrically conductive component with a thick, uniform layer which can easily be applied to an industrial scale to large components and under good economic conditions. In addition, the resulting coatings adhere firmly to the component so that they do not flake off. WHAT WE CLAIM IS:

1. A method of coating a surface of an electrically conductive component with a thick, uniform layer, comprising disposing said conductive component to be coated in a chamber at a distance less than 50 mm from a component made of the coating material, filling said chamber with gas at a pressure between 0.1 and 15 Torr, and bringing said component to be coated and said component made of coating material, which form two separate cathodes, to potentials such that the potential differences between an anode in said chamber, which may be said chamber, and said cathodes is between 200 and 1500 volts, coating being brought about on said conductive component by abnormal glow discharge between said anode and said cathodes.

9; A method of coating a surface of a

steel component with a thick, uniform layer of chromium carbide, comprising placing said steel component to be coated in a chamber between 8 and 15 mm from a component made of iron-chromium alloy, filling said chamber with a mixture of inert gas and a gaseous carbon compound at a pressure between 1 and 5 Torr, bringing said component to be coated and said coating material component, which constitute two separate cathodes, to potentials such that the potential difference between an anode in said chamber, which may be said chamber, and said steel component is between 450 and 500 volts, and the potential difference between said anode and said coatingmaterial component is between 550 and 650 volts, a chromium carbide coating being produced on said steel component by an abnormal glow discharge between said anode and said cathodes.

10. A method according to any one of the preceding claims, wherein said component to be coated and said coating-material component have complementary shapes such that there is a substantially constant distance between the bombarded surface of said coating material component and the surface of said component which is to be coated.

11. Apparatus for carrying out a method according to claim 1, comprising a chamber having an inlet for gas, means connected to said inlet for supplying gas to said chamber for filling said chamber with gas at a pressure between 0.1 and 15 Torr, an internal heat shield in said chamber, means for supporting a first component made of coating material in said chamber, means for separately supporting a second component to be coated in said chamber and at a distance less than 50 mm from said first component, means for supplying current to the components, and at least one electric generator provided with an arc breaking device, said generator or said generators being connected to said current supply means for bringing the components to potentials such that the potential difference between an anode in said chamber, which may be said chamber, and said components is between 200 and 1500 volts.

12. A method of coating a surface of an electrically conductive component substantially as herein described with reference to the accompanying drawing.

13. Apparatus for coating a surface of an electrically conductive component substantially as herein described with reference to the accompanying drawing.