GB2108533A - Ion plating - Google Patents

Abstract

The substrate 18 is located in an enclosure containing a plasma forming gas in which a source of the coating material is to be vaporised, a plasma is generated within such a gas by creating an electrical potential between the enclosure or a separate electrode and the substrate or a grid 22 in close proximity to the substrate, the substrate surface to be coated or its immediate environment is locally heated, e.g. by the grid 22 being resistively heated, and the coating material is vapourized to produce ionized particles which are attracted to and form a vaporised deposit of the coating material on to the cathodic substrate surface. A copper mould for the continuous casting of steel may be ion plated to increase wear resistance. <IMAGE>

Description

SPECIFICATION Electron beam coating This invention relates to methods and apparatus for depositing coating materials on to substrates using ion plating processes. The invention is especially concerned with methods of ion plating continuous casting moulds and to moulds so plated. Generally, such moulds are constructed of copper.

It is known that during the continuous casting of certain grades of steel star-like cracks are sometimes formed in the surface of the cast product. These cracks are believed to be caused by copper from the mould surface pentrating the crystallisation boundary region of the product, thereby locally reducing the high temperature strength of the product.

Such cracks must be removed subsequently so leading to material losses.

To overcome this problem and to increase the wear resistance of the mould internal surface, various proposals have been made for applying a coating of a metal other than copper to the mould internal wall. These proposals have however suffered from several disadvantages, one of which is a thin coating (eg applied by electo-plating) tends to decay rapidly whilst a thicker cladding or coating tends to peel away from the mould surface.

The present invention sets out to provide an improved ion plating process having particular application to the coating of the walls of continuous casting moulds.

Ion plating is a variant of the vapour deposition process in which a coating material is deposited onto a substrate in the presence of a plasma discharge giving simultaneous ion bombardment of the substrate. The plasma is conventionally produced as a direct current argon glow discharge by applying a negative bias voltage of the order to 1 to 5kV on the substrate.

The action of the bombardment of the substrate with argon ions accelerated by the bias voltage is to eject some of the substrate surface material on impact, the process being known as sputtering. This sputtering action is used in two ways. Firstly, the substrate itself is cleaned by removal of the surface contamination to give a good base for adhesion, and then when deposition is in process the more weakly adhering material is preferentially removed leaving the remaining deposit more fully dense. In reverse, sputtering may also be used for solid phase evaporation of the depositing material.

As an alternative to the bias voltage, high frequency power may be used to create the plasma. Since ion bombardment is effected in a soft vacuum of the order of 10-2 torr with a mean free path for argon of about 0.5cm, deposition of the coating material is not confined to line of sight from the source as in high vacuum deposition, thus resulting in good throwing power, comparable with the best obtainable in electroplating. Although the process is referred to as ion plating, ionisation of the coating material itself as it travels through the plasma to the substrate appears to be of little consequence since it has been estimated that the percentage of ionisation may be as low as 0. 1%.

Several methods other than sputtering existing for providing an evaporation source. These methods include liquid phase evaporation techniques such as electron beam heating, HF induction heating, and resistance heating in an electrically conducting refractory boat such as molybdenum. This latter method is particularly convenient for low melting point materials provided that the refractory boat is not attacked by the molten coating metal.

According to the present invention in one aspect a method of ion plating a coating material on to a surface of a substrate comprises the steps of locating the substrate in an enclosure containing a plasma forming gas in which a source of the coating material is to be vaporised, generating a plasma within such a gas by creating an electrical potential between the enclosure or a separate electrode and the substrate or a grid in close proximity to the substrate, locally heating the substrate surface to be coated or its immediate environment, and vaporising the coating material to produce ionized particles which are attracted to and form a vaporised deposit of the coating material on to the cathodic substrate surface.

Evaporation of the coating material source may be effected by solid, liquid or gaseous techniques. Examples of such techniques are sputtering, electron beam heating, radio frequency heating, and resistance heating.

The plasma forming gas may consist of an inert gas such as argon. Alternatively, the gas may be formed wholly or partially of a chemically reactive gas which supplies either all of the coating material by decomposition in the plasma at the substrate surface, or reacts chemically with vaporised source material to form the coating. Examples of such chemical ion plating are the deposition of carbon or chromium from methane or chromium containing atmospheres respectively.

According to the present invention in another aspect, there is provided a substrate having a surface ion plated by the method described in the preceding three paragraphs.

According to the present invention in a further aspect, ion plating apparatus for depositing a coating material on to a surface of a substrate comprises an enclosure connected to receive a plasma generating gas, a source of the coating material to be vaporised within the enclosure, means for supporting the substrate within the enclosure, means for creating an electrical potential between the enclosure or a separate electrode and either the substrate or to a grid positioned in close proximity to the substrate surface to be coated, and means for heating the source of the coating material to vaporise the same.

The substrate may comprise one wall of a casting mould, the mould wall being supported within the enclosure with its surface to be coated so electrically connected that it is biased negatively with respect to the enclosure or the separate electrode. The temperature of the mould during plating may be controlled or varied by passing either coolant or heating fluid through internal passageways of the mould.

According to the present invention in a still further aspect, there is provided a copper mould for continuously casting steel whose internal walls are ion plated with a wear resistant material, eg chromium or molybdenum.

The invention will now be described by way of example only with reference to the accompanying diagrammaic drawings in which Fig.

1 and Fig. 2 are sections taken through first and second ion plating apparatus in accordance with the invention.

The apparatus illustrated in Fig. 1 of the drawings includes a chamber 1 connected via a gas outlet pipe 2 to a vacuum pump 3 and via a gas inlet pipe 4 to a source of plasma forming gas (eg argon) 5. A leak valve 6 is provided in the inlet pipe 4 and is operated to control the gas flow and consequently the pressure within the chamber 1.

The chamber 1 includes a cover 7, side walls 8 and a base 9. A pair of cathodes 11 pass through openings formed in the cover and protrude into the chamber with their protruding ends enveloped within earthed shields 12. The cathodes are electrically insulated from the cover by insulators 13 and are cooled by jacket lead-throughs 14. Each cathode 11 is connected to the negative terminal of a high tension power supply 15. An isolating transformer 16, powered by an alternating current power supply 17 is connected in circuit, between the power supply 15 and the cathodes 11 Supported from the cover 7 is a substrate to be plated; in the embodiment illustrated, the substrate comprises one side wall 18 of a copper mould used in the continuous casting of steel.The mould wall is supported from a holder 19 which includes a pair of conduits 20 connected between a source of coolant or heating fluid and the internal coolant passageways of the mould wall. The holder 19 is insulated from the cover 7 by insulators 21.

Clamped between and electrically connected in circuit with the ends of the cathodes 11 is a grid or mesh 22 positioned in close proximity to the wall surface to be plated.

A crucible 23 is positioned within the base 9 and contains a quantity of the material to be plated on to the mould wall. Adjacent the crucible is an electron beam gum 24 operable to direct a beam of charged particles on to the surface of the coating material to vaporise the same. In the particular embodiment described the coating material consists of chromium.

In use of the apparatus described above the chamber is pumped down to a vacuum of approximately 10-2 torr and the argon gas introduced into the chamber 1 from the source 5. An electrical potential is then created between the positively charged walls of the chamber and the negatively charged grid 22 through the cathodes 11.

The argon plasma is attracted to and through the cathodic grid 22 and initially sputters off impurities present on the wall surface 18. One sufficient cleaning has been achieved, the electron beam is operated to vaporise the chromium coating material within the crucible 23. lonised chrome particles are then attracted to and through the grid 22 to form a plating on the surface of the mould wall.

The grid 22 is resistively heated by the current passing between the cathodes 11 to heat the gas in the immediate vicinity of the mould wall surface to assist the ion plating process. Other forms of heating the grid, the gas or the mould wall surface itself, may be employed.

Fig. 2 shows the sputter ion plating apparatus in accordance with the invention in which a magnetron source is employed. Like integers to those illustrated in Fig. 1 bear the same reference numerals. In this embodiment the target cathode 25 is located adjacent to an anode 26 within a casing 27. The cathode 25 is supported above a magnet 28, cooling channels 29 being provided between the opposed surfaces of the magnet 28 and cathode 25. Typically, the voltages employed are of the order of - 10 to - 800 volts across the grid 22, - 300 to - 700 volts at the cathode 25 and 0 to + 35 volts at the anode 26.

It is to be understood that the apparatus described above with reference to Figs; 1 and 2 may be modified in many respects without departing from the salient features of the present invention.

Claims (11)

1. A method of ion plating a coating material on to a surface of a substrate comprises the steps of locating the substrate in an enclosure containing a plasma forming gas in which a source of the coating material is to be vaporised, generating a plasma within such a gas by creating an electrical potential between the enclosure or a separate electrode and the substrate or a grid in close proximity to the substrate, locally heating the substrate surface to be coated or its immediate environment, and vaporising the coating material to produce ionized particles which are attracted to and form a vaporised deposit of the coating material on to the cathodic substrate surface.

2. A method as claimed in Claim 1 wherein evaporation of the coated material is effected by means of a sputtering, electron beam heating, radio frequency heating or resistance heating technique.

3. A method as claimed Claim 1 or Claim 2 wherein the plasma forming gas is argon.

4. A method as claimed in Claim 1 or Claim 2 wherein the gas is formed wholly or partially of a chemically reactive gas which supplies either all of the coating material by decomposition in the plasma at the substrate surface.

5. Ion plating apparatus for depositing a coating material on to a surface of a substrate comprises an enclosure connected to receive a plasma generating gas, a source of the coating material to be vaporised within the enclosure, means for supporting the substrate within the enclosure, means for creating an electrical potential between the enclosure or a separate electrode and either the substrate or to a grid positioned in close proximity to the substrate surface to be coated, and means for heating the source of the coating material to vaporise the same.

6. Apparatus as claimed in Claim 5 wherein substrate comprises one wall of a casting mould, the mould wall being supported within the enclosure with its surface to be coated so electrically connected that it is biased negatively with respect to the enclosure or the separate electrode.

7. Apparatus as claimed in Claim 5 or claim 6 wherein the temperature of the mould during plating is controlled or varied by passing either coolant or heating fluid through internal passageways of the mould.

8. A copper mould for continuously casting steel whose internal walls are ion plated with a wear resistant material

9. Apparatus for depositing a coating material onto a surface of a substrate substantially as herein described with reference to Fig. 1 or Fig. 2 of the accompanying drawings.

10. A guide roll, work roll or table roll in a rolling mill whose external surface is ion plated with a wear resistant material.

11. A top zone, strand guide, straightener or run out table roll in a continuous casting machine whose external surface is ion plated with a wear resistant material.