Chemistry: electrical and wave energy – Processes and products – Vacuum arc discharge coating
Reexamination Certificate
1999-10-04
2003-10-21
McDonald, Rodney G. (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Vacuum arc discharge coating
C204S298410
Reexamination Certificate
active
06635156
ABSTRACT:
FIELD OF TECHNOLOGY
The present invention is directed to a method and apparatus for producing electric arc plasma and use thereof for coating a substrate. During the last decades, electric arc plasma sources have been increasingly utilised in industry for depositing on articles coatings based on metals, their alloys and compositions. They are used to coat machine parts, tools, consumer goods, etc., with wear-resistant, corrosion-resistant, decorative coatings, coatings with desirable electric and magnetic characteristics, and other coatings having special properties. Further, vacuum arc plasma sources are employed for producing ion beams used for ion implantation, in ionic accelerators as well as in rocket power units.
PRIOR ART
The process for producing plasmic flux in a vacuum electric arc source consists in the following. A high-current arc discharge is ignited in vacuum on cooled cathode made of material on the base of which a coating is to be produced. Generally, arc discharge ignition is carried out either by mechanical breaking of electric contact between the cathode and a special electrode or by use of high-voltage or laser spark. Arc discharge on the cold cathode is concentrates into cathode spots sized from several microns to hundreds microns and current densities therein of up to 10
6
-10
8
A/sq. cm. Each spot emits a metallic plasma jet in a direction approximately perpendicular to the cathode surface. In the absence of magnetic and electric fields, the cathode spots move chaotically over the cathode surface. Using electric and magnetic fields, it is possible to control spot movement retaining them on the working surface and forcing to move according to desired paths.
Each spot appropriately contributes to the plasma flux which has distribution close to cosinusoidal with the axis perpendicular to the cathode surface. Due to specific features of vacuum arc burning on the cold cathode, in particular, due to creation near the cathode of a positive volume charge., plasma ions are accelerated to energies from several units to hundreds of electron-volts in a direction perpendicular to the cathode surface.
The plasma flux produced in the electric arc source is highly ionised. lonisation level for a number of materials approaches 100%. Plasma contains a considerable quantity of two- or three-fold ionised particles. This is a substantial benefit over sources based on effects of spraying (including magnetron) and evaporation (by electronic beam, laser radiation, etc.) of material where material flows have a low ionisation level. A high ionisation level allows the flux to be controlled using electro-magnetic fields, enables to check and control the energy of atoms coming to the substrate and enhances the evaporated material reactivity in forming compounds both with reaction gas and directly with the material of the substrate to be coated.
In order to deposit coatings, the plasma flux is directed on the substrate to which generally accelerating voltage is applied to produce the desired energy of surface impinging ions. In general, the process for depositing coatings comprises two steps. In the first step, at a sufficiently high vacuum (at a pressure of 10
4
mm Hg and lower) accelerating voltage as high as 1000-1500 volts is applied to the substrate. Cathode material ions are accelerated near the substrate in the Debaye layer and bombard its surface. During ion bombardment, surface cleaning from impurities, the so-called “ionic bombardment cleaning”, takes place. After conducting “ionic cleaning”, the voltage applied to the substrate is reduced to several dozens to several hundreds volts and surface impinging particles condense on its surface forming a coating corresponding to the cathode material. In order to produce composite coatings, reaction gas is introduced in working chamber, generally, to pressures of 10
−2
-10
−4
mm Hg. In this case, it is possible to produce coatings based on cathode material/reaction gas compositions.
The problem with electric arc plasma sources is that the arc discharge generates, along with a vapour component, drops of the surface molten material, i.e. particulates. Such particulates have characteristic sizes from fractions of micron to dozens of microns. Particulates can hit the substrate to be coated forming irregularities in the coat structure and defects in form of indents and protrusions. This effect considerably decreases applications of electric arc plasma sources. The presence of particulates in the plasma flux prevents from depositing coatings on parts with high grade of surface cleanness, on sharply whetted tools, and reduces considerably operating characteristics of coatings such as wear-resistance, electric and magnetic properties.
Furthermore, electric arc sources virtually do not allow for depositing coatings on the basis of relatively fusible materials such as, for example, aluminium and others, in whose flux a great amount of particulates is present. In particular, in producing ceramic coatings based on Al203, the presence of Al particulates disturbs insulation properties of coatings. This is also the case with depositing diamond-like coatings using a carbon cathode where particulates are soot parts.
In order to reduce the number of particulates in the plasma flux, various methods are employed. The first group of methods comprises using different magnetic and electric fields near the cathode surface which permits to increase the rate of arc spots movement over the cathode surface. This results in decreasing the number of generated particulates and in reduction of their sizes. The second group of methods consists in using various plasma flux separating filters. Such devices are placed between the cathode and the substrate to be sprayed so that they pass through the vapour component of the plasma flux while blocking passage of particulates.
The first group electromagnetic methods are essentially simpler to implement as compared to plasma flux separation methods. However, they are not likely to be effective and do not eliminate particulate generation by the arc discharge. The plasma flux retains a considerable quantity of molten metal drops.
Separation methods are based on deflection of vapour ionised plasma component from the rectilinear path. To this end, the plasma flux is imposed with longitudinal magnetic field. In the spacing between two Coulomb collisions, each plasma charged particle moves along the field by a helical path. When the field is uniform, the path axial line virtually coincides with one of lines of force of the field. Electron and ion movement across lines of force of the filed proves feasible only due to Coulomb collisions. At each collision, a particle moves by the distance of the order of Larmor radius. If collisions occur rarely, the particle happens to be as though bound to lines of force of the field. Such plasma is called “magnetised”. If the plasma parameter p/&lgr;>1 (p being the medium free run distance and lambda being the medium Larmor radius), the particle can displace by a marked distance across the field only having travelled a very long path along the line of force.
The ionic and electronic plasma components have different “magnetisation”. To provide “magnetisation” of the heavy metal plasma ionic component, magnetic fields up to several dozens of kiloErsted are required. Providing such magnetic fields requires very large and complex systems. Therefore, in practice systems are used which ensure “magnetisation” of only the electronic component. Due to this electrons can freely move only along lines of force of the field while the field by itself does not markedly affect ion movement. In this case electrons will be tied to lines of force of magnetic field while ions will be retained in the same space region by electric field created by the electronic component. In addition, electron magnetisation and dramatic decrease of transverse plasma electronic component mobility allow to provide electric field perpendicular to magnetic one which results in ions drifting in the desired directio
Bashkov Valery Mikhaylovich
Dodonov Alexander Igorevich
Jacobson & Holman PLLC
McDonald Rodney G.
V.I.P.-Vacuum Ion Plasma Technologies Ltd.
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