Magnetic field - assisted ion plating arc source and its discharge characteristics are analyzed
The structure, working principle, movement of arc spot and discharge characteristics of several magnetic field controlled arc ion plating sources are analyzed. The target structure and magnetic field configuration of different magnetic field assisted controlled arc sources are compared. The development of arc ion plating arc source controlled by magnetic field is prospected.
Improving the processing quality and service life of tools and moulds is a subject that people are always exploring. Arc ion plating technology is a kind of surface modification technology of die materials, which has the advantages of high ionization rate, low temperature deposition, good film quality and fast deposition rate, etc., which are not available in other coating methods. However, the existence of large particles caused by arc discharge limits the further application of die coating technology, which has become the main topic of the development of arc ion plating technology.
Ion plating arc source is the source of arc plasma discharge and is the key component of ion plating technology. The arc source used in arc ion plating is the cold cathode arc source, the behavior of the arc in this arc source is controlled by many fast moving and highly bright cathode spots on the cathode surface. In the process of developing and perfecting arc ion plating technology, effective control of arc cathode spot movement is very important, because it determines the stability of arc discharge, effective utilization of cathode target, removal of large particles, improvement of film quality and many other key problems. At home and abroad, the research focus is mainly on the arc source design of magnetic field control.Due to the physical characteristics of vacuum arc, the applied electromagnetic field is an effective method to control the movement of arc spots. At present, all the magnetic field designs consider the formation of certain magnetic field configuration on the target surface, use the acute Angle rule to limit the movement path of arc spots, and use the transverse component to improve the movement speed of arc spots.
The ideal magnetic field design is embodied as follows: on the one hand, enlarge the area and intensity of the transverse component of the magnetic field as far as possible; on the other hand, control and limit the movement of arc spots to the greatest extent. Due to the long duration of tool plating film, the performance of the film requires high, the industrial application of ion plating arc source should have the following characteristics :(1) stable discharge, not often out of arc; (2) arc spot movement constraints are reasonable, do not run arc; (3) high target material utilization rate; (4) exquisite arc spot, small discharge power density, small particles; (5) the plasma density and ionization rate are high, and the plasma flux transported to the workpiece is sufficient.
In view of the present popular several tools used in coating field auxiliary source of arc ion plating, in this paper, the comparative analysis of different auxiliary magnetic field controlled arc source target structure, the magnetic field configuration and the generation mechanism, discussed the different configuration of the magnetic field of arc spot movement, the effect of discharge and the advantages and disadvantages of the resulting coating process, etc., to control the magnetic field of the development of the electric arc ion plating arc source is discussed.
1. Circular small arc source
The ion plating arc source which is first used in tool plating is a typical small arc source introduced from Russia, which has the advantages of simple structure, convenient installation, target combination with different components at any location, easy to realize multi-layer coating preparation and so on. The target of the arc source is generally about 60~160 mm in diameter and 20~40 mm in thickness. As compared with other cathode arc sources the size is much smaller, so it is generally called small arc source. Small arc sources usually adopt electromagnetic or pneumatic mechanical arc ignition mode. When working, the arc needle is pulled back to contact the cathode target material for arc ignition, and the arc ignition needle circuit is cut off after triggering the arc ignition, and arc discharge is maintained by the arc power source.
The control magnetic field of the arc source usually comes from the permanent magnet placed behind the target. Magnets can be cylindrical, circular, or ring-cylindrical in shape. Using the magnetic field generated by permanent magnet on target surface can restrain arc spot movement, improve discharge stability and avoid arc running. The target magnetic field intensity is generally 1~5 mT to maintain steady discharge of small arc source. With the advance of permanent magnet, the intensity of electromagnetic field in front of the target increases, generating radial driving force and circumferential force on the electron, so that the arc spot rotates circumferentially on the surface and radially outward from the center of the circle.Increasing the magnetic field intensity can increase the velocity and radius of arc spot movement, as shown in FIG. 1(a). However, the permanent magnet installed behind the traditional small arc source target is usually soaked in cooling water, which is easy to demagnetize after a long soak, and the magnet needs to be constantly replaced. Moreover, the magnetic field strength is not easy to adjust, so the magnetic field intensity of the target can only be adjusted by moving the permanent magnet behind the target back and forth.
At present, many companies have improved the arc source, mainly including: adopting indirect water-cooling channel, placing permanent magnet outside the water-cooling channel, avoiding demagnification and providing larger magnetic field design space, which is conducive to the design of composite magnetic field of electromagnetic coupled permanent magnetic field, and promoting the development of ion plating arc source, as shown in figure 1(c).For example, balchas USES a target with a diameter of 160 mm, which is indirectly water-cooled. The back of the target provides a variety of magnetic circuit designs, and there are various matching magnetic field configurations for the control and improvement of arc spot discharge. A number of domestic companies have gradually developed the great arc with a diameter of about 150 mm, which generally adopts the indirect water-cooling structure and the magnetic field structure is also a variety of modes, providing more solutions for coating uniformity, refining large particles and deposition of large thickness film layer.
FIG. 1 improved small arc source structure and arc spot discharge
In view of the current popular magnetic field assisted ion plating arc sources used in tool plating, the cathode and magnetic field configuration of different magnetic field assisted controlled arc sources were analyzed and compared, and the influences on the arc spot movement, discharge and coating process were also analyzed.
(1) circular small arc source has the advantages of simple structure, convenient installation, target material combination with different components at any location, and easy multi-component coating preparation. The control magnetic field of traditional circular small arc source generally comes from the permanent magnet placed behind the target. At present, the development of circular arc source adopts indirect water-cooling channel and target structure with large diameter (150 mm), and the magnetic field configuration is multi-mode, providing more solutions for uniform coating, fine large particles and deposition of large thickness film layer.
(2) Rectangular plane large arc source and rotary cylindrical arc source can improve the uniformity of coating, reduce the discharge power of arc spot and reduce large particles, which can be used to prepare fine film layer as well as the base of tools and decorative coatings. However, its disadvantages are that the target is single, difficult to prepare multiple coating, at the same time, the target utilization rate is low, magnetic field design has certain skills, unreasonable structure is easy to cause arc running, discharge instability and other problems, not conducive to long-term tool coating.
(3) The mechanical rotating magnetron arc source can form a variety of rotating magnetic fields with adjustable speed on the target surface, but it needs to add complicated mechanical control mechanism. Electromagnetic rotating magnetic-controlled arc source makes use of the comprehensive effect of transverse magnetic field strength and rotation frequency to realize the strong dispersive arc state distributed throughout the target surface on the cold cathode target material, which is conducive to the uniform heating of the whole target surface and the significant decrease of current density.
(4) The multi-mode alternating coupling magnetic field assisted arc ion plating arc source can form a dynamic arch coupling magnetic field by using the axisymmetric divergent magnetic field pointing to the edge of the target and the focusing guiding magnetic field forming an acute Angle to the center of the target, so as to control the movement of arc spots, improve the discharge state of arc spots and reduce particle emission. The stable transmission of plasma under the guidance of focused magnetic field can enhance the particle collision probability, ionization rate and ion density of plasma.
(5) Electromagnetic rotating magnetic-controlled arc source and multi-mode alternating coupling field-assisted arc ion plating arc source are two new types of dynamic magnetic field controlled arc plating arc source.