The surface of the workpiece is coated with a thin film layer through a pvd coating machines. During the coating process, there are several coating technologies, such as evaporation coating technology, ion multi-arc coating technology, and magnetron sputtering coating technology. When using ion multi-arc coating technology, arc discharge will generally occur. When magnetron sputtering is used, glow discharge often occurs. Glow discharge is further divided into glow discharge and abnormal glow discharge. Many people do not particularly understand the principles, so they are very confused. Below, the editor of Huicheng Vacuum will introduce in detail the two discharge methods of pvd coating machines:

glow discharge
       Low-pressure gases generally produce glow discharges after ignition. When the gas pressure is about 100 Pa and the applied voltage is moderate, the discharge shows seven areas of alternating light and dark areas. The sequence from cathode to anode is divided into 7 zones.
① Aston dark zone: It is a very thin layer of dark zone in front of the cathode, which was discovered by F.W. Aston in an experiment in 1968. In this area, the electrons have just left the cathode and their flight distance is still short. The energy obtained from the electric field is not enough to excite the gas atoms, so no light is emitted.
②Cathode glow zone: immediately following the Aston dark zone. Since electrons have enough energy to excite atoms after passing through the Aston dark zone, they cause excitation in this zone. When the excited atoms return to their ground state, glow.
③Cathode Dark Zone: Also known as Crookes Dark Zone. The electrons arriving in this area have high energy, which is conducive to ionization but not excitation, so the light is weak.
④ Negative glow area: It is adjacent to the cathode dark area and has a clear boundary with the cathode dark area. The luminescence is strongest at the dividing line, then gradually weakens and turns into a dark area, which is the Faraday dark area described later. The electron energy in the negative glow region is relatively dispersed, and is rich in both low-energy electrons and high-energy electrons.
⑤ Faraday dark zone: the transition zone from the negative glow zone to the positive column zone. In this region, the electron energy is very low and no excitation or ionization occurs, so it is a dark region.

Abnormal glow discharge
       Magnetron sputtering is generally an abnormal glow discharge. The entire cathode is covered with glow. If the current drawn is increased, abnormal glow discharge will occur. At this time, the cathode position drop is very large, and the width of the position drop zone decreases. The large cathode position drop and high current density will cause sputtering of the cathode material. In discharge devices, the gettering effect of sputtering reduces the gas pressure in the device and changes its gas composition, while the conductive film formed by sputtering reduces the insulation between electrodes. The phenomenon of cathode sputtering can also be used as a means of material coating, which is sputter coating.

arc discharge
      The multi-arc ion plating used by the pvd coating machines belongs to arc discharge. If the current limiting resistance of glow discharge is reduced, the discharge current will increase and turn into arc discharge. Arc discharge is characterized by high current density and low inter-electrode voltage, and its self-sustainment relies on new electron emission mechanisms, namely thermal emission and cold emission. Thermal emission is caused by the local high temperature caused by positive ions bombarding the cathode; cold emission is caused by the presence of a local strong electric field on the cathode surface. The former is called the hot electron arc, and the latter is called the cold cathode arc. The carbon electrode arc as a strong light source is the thermoelectron arc; the mercury arc rectifier used in the power industry uses cold cathode arc.
       An important feature of arc discharge is the cathode glow point on the cathode. The bright point of the hot electron arc is generally fixed; the bright point of the cold cathode arc such as the mercury arc rectifier liquid mercury surface jumps and moves. The cathode bright spot is the source of electron emission, and its current density is as high as hundreds to thousands of A/cm2. The volt-ampere characteristics of arc discharge vary with electrode materials, gas types, and pressure. The carbon electrode arc in the atmosphere shows typical negative resistance characteristics, so a current-limiting resistor must be connected in series with the external circuit to stabilize the current. Arc discharge produces intense radiation, the intensity of which increases with gas pressure and current density. The highest temperature point in the discharge zone is about 4200K at one atmosphere, 6520K at 10 atmospheres, and 10,000K at tens or hundreds of atmospheres. Carbon electrode arc is the earliest strong light source. Various high-pressure discharge lamps, such as high-pressure mercury lamps, xenon lamps, and sodium lamps, are light sources that perform arc discharge in the tube bulb. Arc welding and arc cutting are widely used in industry; the high temperature of the arc can be used as the heat source of the electric furnace.