IL101610A - Method and apparatus for carrying out surface processes - Google Patents

Description

101610/3 now D'i7iin viK>it» i nm no»yy Method and Apparatus for Carrying Out Surface Processes The Inventor is: Moisei ESTERLIS υ'ΐηηθΝ »OMU :Nin x>xn»n EL-PLASMA LTD.

C. 88972 Method and apparatus for carrying out surface processes FIELD OF THE INVENTION This invention relates to the treatment of metal surfaces such as the cleaning (for example, the removal of scale, oxidized layers, contaminants and the like) of such surfaces, the thermal treatment and the coating thereof.

BACKGROUND OF THE INVENTION The treatment of metal surfaces for the purposes of cleaning, and so as to allow for the subsequent treatment and coating, preferably by vaporization of the thus cleaned and treated surfaces, have long been known and various mechanical and/or chemical means have been proposed to carry out this cleaning.

It has been found in many cases, and in many applications, that such mechanical and/or chemical cleaning processes are either ineffective in achieving the required degree of cleaning, or involve the use of expensive and complicated equipment and may, in fact, give rise to damage to the surfaces being treated.

It has been proposed to clean the surface of a metallic object by subjecting it to a vacuum arc discharge wherein the metallic object constitutes an effective cathode. Such proposals have, for example, been made in U.S. Patent Specification No. 4,534,921, British Patent Specification No. 2086788 and in a paper by V.E. Bulat and M.Kh. Esterlis - "Removing Scale, Oxide Films and Contaminants from Metal Components by Vacuum Electric Discharge", Fizika i Khimiya Obrabotki Materialov, Vol. 21, No. 3, 1987, pp 49-53.

In this paper, it was explained that such a vacuum arc discharge takes place between the anode and discrete locations on the cathode known as cathode spots, and that these spots move at random on the surface of the cathode. The phenomenon of the production of cathode spots in vacuum arc discharges in general has been described in detail in "Vacuum Arcs: Theory and Application, J.M. Lafferty, Editor; Wiley 1980".

The characteristics of size and movement of the cathode spots has also been investigated inter alia by the present inventor, and this in the context of the voltage current characteristics of the vacuum arc discharge. Thus, it has been shown that the voltage current characteristic can consist of successive rising and descending portions, i.e. portions having positive and negative gradients. The inventor has shown that in the initial stages of the arc discharge, and with a small anode-cathode spacing, the arc extends from a single or a very limited number of cathode spots to a very limited area of the anode. Increasing the arc current is accompanied by an increase in the arc resistance and, as a result, it is accompanied by an increase in the arc voltage. In consequence, in this initial stage of the arc discharge the arc voltage current characteristic has a positive gradient.

When, however, as a consequence of further increase of arc current, the number of cathode spots increases and, as previously indicated, they move randomly around the entire cathode surface, the volume of the arc increases substantially and, as a consequence, the vapor density in the arc decreases significantly and with it the arc resistance. As a consequence, the arc voltage current characteristic passes into a negative gradient portion.

If now, however, the vapor density in the arc rises beyond a specific critical value, for example by a continued increase in the arc current and consequent increase vaporization of the cathode surface, it has been shown that the arc voltage current characteristic passes once more into the positive gradient region.

It has furthermore been shown, inter alia by the present inventor, that passage of the voltage current characteristic from the negative gradient portion to the positive gradient portion can be achieved by ensuring that the volume of the arc discharge, or its cross-sectional area, does not exceed a certain critical value seeing that this carries with it the consequence that the vapor density in the restricted arc volume is relatively high. In other words, this transition from the negative to the positive gradient for the voltage current characteristic is achieved with increasing arc current whilst maintaining the arc cross-section substantially constant or being reduced. Now in view of the fact that the action of the arc on the surface to be treated takes place invariably in the region of the cathode spot, the fact that the cathode spots move randomly over the surface to be treated must give rise to a non-uniform treatment of the surface.

An attempt to overcome these problems arising out of the random movement of the cathode spots on the cathode surface is described in European Patent Application No. 0 468 110 Al, wherein there are disclosed means for directing the movement of the cathode spots on the cathode surface in a desired direction. It is, however, repeatedly stressed in the European patent application that the arc which is generated has a voltage current characteristic having a negative gradient and it is furthermore clear that these negative gradients arise in view of the fact that the mode of applying the arc voltage between the cathode and electrode is such that the arc occupies a maximum volume with a correspondingly maximum cross-sectional diameter.

It has now been discovered that treating a cathode surface with an arc discharge having such negative gradient voltage current characteristics carries with it certain distinct disadvantages, among which are the facts that the treatment is time-consuming and uneconomical.

The present invention, on the other hand, is based on the surprising discovery that when the surface to be treated is subjected to a vacuum arc discharge having a positive voltage current gradient, an increased efficiency of surface treatment of the cathode surface can be achieved.

BRIEF SUMMARY OF THE INVENTION According to the present invention there is provided a method for the surface treatment of a metal workpiece wherein there is generated between an anode and a surface of said workpiece acting as a cathode a steady vacuum arc discharge having an arc current with a positive voltage-current gradient; said surface being simultaneously or intermittently exposed to at least one of the following two operations: the first operation being mechanical treatment by an electrically insulated body having teeth thereof in electrical contact with said surface, and the second - electromagnetic radiation.

By electromagnetically irradiating the workpiece it is ensured that the area of treatment is heated, thereby causing a certain degree of thermionic emission from the surface.

By imparting a relative movement between the workpiece and the anode it is ensured that successive restricted areas of the workpiece are treated, for example cleaned, and in this way a uniform treatment (cleaning) of the entire surface of the workpiece is achieved.

It has been discovered by the inventor that the efficiency of the method when used for suface cleaning is enhanced when it further includes the step of forming on said workpiece indentations substantially normal to a given direction of relative displacement of said workpiece with respect to said anode. Further indentations may be formed on the workpiece substan-ti ally in said given direction. The indentations serve as guiding lines for distributing the moving cathode spots of various sizes over the workpiece, which fact improves uniformity and quality of the treatment.

Where the surface treatment which has just been described in used for the preliminary cleaning of the workpiece, additional means may, as required, be provided for coating the workpiece by a vaporization technique.

The step of vaporizing includes two substeps, the first substep constituting providing on the surface of the workpiece an intermediate layer of coating ( which layer being characterized by high adhesion to the surface), and the second substep including providing the outer coating layer on said intermediate layer. The first substep includes providing, simultaneously or intermittently with the vaporizing, at least one of the following two actions: one of them being heat treating of said surface by means of said arc discharge, and the other - mechanical treatment of said surface.

The invention also involves the provision of apparatus for carrying out the method in accordance with the invention.

Apparatus for the surface treatment of a metal workpiece, according to the invention, comprises an enclosure, means for generating a vacuum in said enclosure, first means for controlled introduction of gas into said enclosure, at least one anode, second means for displaceably supporting said workpiece constituting a cathode in said enclosure, third means for applying an arc generating voltage between said anode and said cathode, fourth means for mechanical treatment of the surface of said workpiece, and fifth means for effecting a relative displacement between said workpiece and said anode in a given direction.

In accordance with one embodiment of the apparatus said fourth means comprises an electrically insulated body having elements thereof in successive electrical contact with the surface.

According to another embodiment of the invention the apparatus additionally comprises electromagnetic irradiation means for successively irradiating said surface.

For accomplishing vaporization by the described device, a vaporizing electrode can be included in the enclosure which can, for example, either be resistance heated so as to vaporize coating material for deposition on the workpiece after it has been preliminarily cleaned in the manner described above. Alternatively, additional arc discharge means can be provided for effectively heating the vaporizing electrode so as to cause its effective vaporization. The additional arc discharge means can utilize the vaporizing electrode as an auxiliary cathode and be provided with at least one auxiliary anode.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention, and to show how the same may be carried out in practice, reference will now be made to the accompanying drawings, in which: Fig. 1 is a schematic representation of a further embodiment of apparatus in accordance with the present invention, for treating an elongated moving object; Fig. 2 is a schematic representation of apparatus in accordance with the present invention, for the cleaning and coating of an object; Fig. 3 is a schematic representation of a further form of apparatus for cleaning and coating an object, utilizing arc discharges for both purposes; Fig. 4 is a schematic representation of a portion of an apparatus in accordance with the present invention, for the coating of an irregularly shaped object.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS One of the modes for generating restricted surface areas of the cathode between which and the anode the arc discharge is concentrated, is to irradiate these areas with appropriate electromagnetic radiation, such as, for example, laser radiation.

An alternative, or additional mode for ensuring that the arc is confined to a region between a restricted surface area and an anode, is to position, adjacent the restricted surface area, an electrically insulated body having teeth thereof in successive electrical contact with the restricted area. The actual contact of these teeth with the restricted area of the workpiece and the movement of the workpiece with respect to these teeth gives rise to a degree of sparking which, in turn, results in the confining of the arc discharge to the restricted surface area in which sparking takes place. Additionally, the teeth are effective in trapping the cathode spots.

The use of such an electrically insulated body is schematically strated in Fig. 1 of the drawings. As seen in this figure, an elongated ject 57, such as a wire or the like, is subjected to vacuum arc discharge aning. For this purpose, the object 57 is longitudinally displaced in good ctrical contact with a plurality of conductive rollers 58 which are, in their n, connected to the negative pole of a voltage source. The object 57 sses through a slotted, rotatable tubular screen 59 before passing through an annular anode 60 which, in its turn, is connected to the positive pole of ] the voltage source. The anode 60 is provided with tubular screening 61 so as to prevent the generation of arc discharges in undesired portions of the , workpiece 57 and the anode 60.

Juxtaposed with respect to the workpiece 57 and in close proximity with the anode 60 is an insulated body 62 from which depend electrically conductive teeth 63 which are in electrical contact with the object 57.

In operation, the contact of the teeth 63 with the moving object 57 traps the cathode spots and gives rise to sparking and, as a consequence, the sparking region becomes a restricted surface area between which and the anode 60 a relatively confined arc discharge is set up. The restricted area is effectively cleaned by the arc discharge. At the same time, the teeth 63 bearing on the object 57 and the continued displacement of the object 57 with respect to the teeth 63 give rise to a mechanical cleaning action which effectively supplements or completes the cleaning action effected by the arc discharge.

The teeth can, as indicated, be formed of an electrically conductive material or, alternatively, the teeth can be formed of an insulating material which can be rendered conductive by the deposition thereon of electrically conductive material. Preferably, the teeth are spaced apart by an amount (e.g. 0.5 - 5 mm) such as to trap the cathode spots and therefore to confine the arc.

Where the teeth are designed essentially for confining the arc and/or surface cleaning, they can be formed of any suitable electrically conductive, relatively hard, material. Where, however, and as to be explained in greater detail below, the teeth are designed for use for arc confining and/or the mechanical treatment of a coated surface, then the elements should be formed of a softer material such as, for example, the coating material or the material of the workpiece. The materials for the teeth can be chosen in accordance with the purpose of treatment.

A still further means for ensuring the confining of the arc to a restricted surface area is to form fine indentations on the surface prior to or during the arc treatment and the presence of those indentations serve to restrict the movement of the cathode spots. These indentations can be formed initially along the longitudinal extent of the workpiece for the guiding of the cathode spots and normally thereto for the concentration of the cathode spots.

The teeth can be employed in creating the indentations normal to the direction of treatment of the object whilst carrying out their other functions. These indentations serve to concentrate the LS cathode spots.

As seen in Fig. 2, an enclosure 81 is coupled to a vacuum pump 82, thereby ensuring the creation of a vacuum in the enclosure having a low gas pressure capable of sustaining a vacuum arc discharge. An elongated object 83 whose surface is to be cleaned and coated is supported, by means not shown, within the enclosure 81 and is provided with means (not shown) for the translational displacement thereof along its longitudinal axis and for the rotation thereof about its longitudinal axis. The elongated object 83 is coaxial with a central bore of an annular anode 84 provided with suitable screening 85. The object 83 which constitutes an effective cathode is electrically connected at contact 86 to the negative pole of a voltage source 88. The positive pole of the voltage source 88 is connected, on the one hand, via a resistor 89 to the anode 84 and, on the other hand, and in parallel, via a resistor 91 to an auxiliary trigger electrode 92. Juxtaposed with respect to the object 83 is an insulated metal body 93 in which is located a replaceable unit 94 from which depend conductive teeth 95. The body 93 is associated with drive means 96 by means of which the body can be displaced along the surface of the object 83.

Located within the enclosure 81 and directed towards the object 83 is a source 97 of electromagnetic radiation, for example a laser source, from which a beam of electromagnetic radiation can be directed onto a restricted surface area of the object 83 adjacent the anode 84.

Located within the enclosure 81 directly below the object 83 is a vaporizer 98 coupled to a voltage source 99 via a switch 100 and on which is located a vaporizable element 101 constituted by the material to be vaporized and to form the coating on the object 83. Vaporization of the element 101 takes place as a result of the resistive heating of the vaporizer 98 upon closing the switch 100.

In operation, the object 83 is longitudinally displaced into the vicinity of the anode 84 and a restricted surface area thereof is irradiated from the radiation source 97, thereby heating this surface area and giving rise to a limited vaporization thereof and/or thermionic emission therefrom. The vaporized layer becomes ionized under the influence of the radiation. The generation of ions in the region of the restricted surface area and/or the thermionic emission serve to attract the arc discharge thereto. The use of the electromagnetic beam of radiation for this purpose is only necessary in the initial stages of the production of the arc discharge, after which the radiation can be switched off. The arc is maintained in the restricted surface area and in subsequent restricted surface areas, by virtue of the fact that the conductive teeth of the insulating body are located in contact with these areas, thereby ensuring that the arc discharge continues to be directed to the relevant restricted areas.

The elongated object 83 can thereupon be cleaned by being subjected along its entire length and periphery to the effect of the arc discharge, which successively treats successive restricted surface areas thereof. The cleaning treatment by the arc discharge can be supplemented or completed by mechanical cleaning by the teeth 95. It will be realized that, as in the previously described cases, the main treatment is effected by the arc discharge acting on the successive restricted areas, and this by virtue of the LS cathode spots active in these areas. Auxiliary preliminary treatment is effected by virtue of the fact that the small cathode spots are directed towards the contact 86, thereby providing preliminary cleaning of the object which is then completed by means of the main arc discharge.

The provision of the additional mechanical cleaning is of particular importance in view of the following circumstances. The movement of relatively small cathode spots towards the contact 86 creates elongated clean paths having widths which compare with the mean widths of the small cathode spots and lie in the range of 0.3 to 1 mm. Bounding these paths are effective raised ridges of contaminants, scale or the like and the existence of these ridges tends to impede the effective cleaning of the surface by the following relatively large cathode spots. In point of fact, the presence of these ridges can give rise, upon the passage of the large spots, to non-uniform, undesirable fusing of the paths already cleaned by the SF cathode spots. If, however, these regions, which have already been subjected to the preliminary cleaning action of the small cathode spots, are subjected to mechanical cleaning by the teeth which can easily remove them, then the subsequent passage thereover of the large cathode spots is effective in uniformly cleaning the surface without any undue danger of fusion.

Furthermore, by ensuring that the teeth 95 of the insulated body 93 are spaced from each other by between about 0.5 - 5 mm, these elements can serve as virtual traps for the cathode spots which are caused to discharge between the elements, and in this way the mechanical cleaning of the object is accompanied by an improved electrical cleaning thereof.

With the completion of the cleaning of the object 83, the latter can be subjected to thermal treatment by the action of the same arc discharge, this thermal treatment resulting in the strengthening of the surface of the object 83 and controlling its surface characteristics.

Simultaneously with, or intermittently with, the thermal treatment of the object 83, the latter can now be coated by the resistive heating of the vaporizer 98 (by closing the switch 100), as a result of which the vapor source 101 is vaporized and vapor is deposited on the object 83 which is rotated about its longitudinal axis. Where the thermal treatment is carried out simultaneously with the vaporization, it is possible to produce a high quality coating layer, characterized by an increased adhesion of the coating layer to the metal surface.

The coating of the object 83 can be carried out simultaneously or intermittently with a mechanical treatment of the coating by means of suitable teeth. For this purpose, the teeth should either be formed of the same material as the coating or of the same material as the object to be coated.

It will be appreciated that where relatively thick coatings are to be deposited on the object 83, the process described above, namely thermal treatment and deposition of initial coatings, followed by subsequent thermal treatment and deposition of coatings, can be repeated sufficient times to produce the required thickness of coating.

During the thermal treatment of the object, it is important to ensure that the object does not reach a temperature higher than the annealing temperature thereof, and for this purpose the object can be coupled to a thermocouple, which can be employed to control the degree of heating. Furthermore, during the coating of the object, the application of the coating can be controlled by the interposition, as desired, between the vaporizer and the object, of a removable screen.

Whilst in the embodiment described above with reference to Fig. 2 of the drawings vaporization takes place as a result of the electrical resistive heating of the vaporizer 98, in accordance with a further embodiment of the present invention vaporization is effected by subjecting the vaporizing element to a vacuum arc discharge. Such an embodiment is schematically shown in Fig. 3 of the drawings.

As seen in this figure, an object 105 is coupled to a voltage source (not shown). The right-hand end of the object 105 is enclosed within a screen 106 which serves to limit the electric discharge and is provided with slots (not shown). In addition, however, to the provision of an anode 107 between which and the object 105 the arc discharge is created for the purpose of cleaning, in the present embodiment an additional annular anode 108 is provided which is juxtaposed with respect to a vaporizing element 109 located within a vaporizer 110 and provided with an electrical conductor 111 which is coupled to a negative pole of a voltage source 112, the positive pole of which is electrically connected to the anode 108. In this way, the vaporizing element 109 constitutes a cathode and when a discharge is initiated (originally with an auxiliary trigger electrode 113) between the anode 108 and a restricted surface area of the vaporizing element 109 opposite the conductor 111, vaporization takes place with the resultant coating of the object 105.

Fig. 4 illustrates the coating by vaporization of an irregularly-shaped object 120 such as, for example, a ratchet. For this purpose, two vaporizers 121, 122 are positioned below the object 120 and are directed towards the object 120 in respectively oppositely-angled directions so that their vaporizing beams overlap and coat the object 120 in opposite directions. In this way it is ensured that the object 120 is uniformly coated, despite its irregular shape.

It will be understood that in all the embodiments in accordance with the invention the arc discharges are maintained in a stable regime for as long as is necessary.

Claims (13)

101610/3 - 15 - I CLAIM:

1. A method for the surface treatment of a metal workpiece wherein there is generated between an anode and a surface of said workpiece acting as a cathode a steady vacuum arc discharge having an arc current with a positive voltage-current gradient; said surface being additionally exposed to at least one of the following two operations: the first operation being mechanical treatment by an electrically insulated body having teeth thereof in electrical contact with said surface, and the second - electromagnetic radiation.

2. A method according to Claim 1, wherein a relative movement is established between said workpiece and said anode in a given direction.

3. A method according to Claims 1 or 2, and furthermore including the step of forming on said workpiece indentations substantially normal to a given direction of relative displacement of said workpiece with respect to said anode.

4. A method according to Claim 3, and furthermore including the step of forming on said workpiece further indentations substantially in said given direction.

5. A method according to Claim 1, wherein said treatment is for the cleaning of the surface of the workpiece.

6. A method according to Claim 5, and furthermore including the step of vaporizing a coating material on to said surface.

7. A method according to Claim 6, wherein said step of vaporizing includes two substeps, the first substep constituting providing on said surface an intermediate layer of coating (characterized by high adhesion to the surface), and the second substep including providing the outer coating layer on said intermediate layer; said first substep includes providing, simultaneously or intermittently with said vaporizing, at least one of the following two actions: one of them being heat treating of said surface by 101610/3 - 16 - means of said arc discharge, and the other - mechanical treatment of said surface.

8. Apparatus for the surface treatment of a metal workpiece comprising an enclosure; means for generating a vacuum in said enclosure; first means for controlled introduction of gas into said enclosure; at least one anode; second means for displaceably supporting said workpiece constituting a cathode in said enclosure; third means for applying an arc generating voltage between,.said..anode and said cathode; fourth means for mechanical treatment of the surface of said workpiece; and fifth means for effecting a relative displacement between said workpiece and said anode in a , given direction.

9. Apparatus according to Claim 8, further comprising electromagnetic irradiation means for successively irradiating said surface.

10. Apparatus according to Claim 8, wherein said fourth means comprises an electrically insulated body having elements thereof in successive electrical contact with said surface.

11. Apparatus according to any one of the preceding claims, wherein there is furthermore provided a vaporizing electrode juxtaposed with respect to said workpiece; sixth means for heating said vaporizing electrode so as to give rise to the vaporization thereof.

12. Apparatus according to Claim 11, wherein said sixth means comprises electric resistance heating means.

13. Apparatus according to Claim 11, wherein said sixth means comprises an additional arc discharge means utilizing said vaporizing electrode as an auxiliary cathode and provided with at least one auxiliary anode. For the Applicants DR. REINHOLD COHN AND PARTNERS