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3.1 What Is Double Glow Discharge Phenomenon?

Plasma nitriding process uses only a single pair of electrodes to realize gas element plasma for surface alloying. In order to break the limitation of plasma nitriding, “Double Glow Plasma Surface Alloying Technology” was developed in 1980 based on the discovery of “Double Glow Discharge Phenomenon” in 1978 by Prof. Zhong Xu and his research group in China. It was named as the “Xu-Tec” process as it was first patented by Prof. Zhong Xu and his colleagues in USA in 1983. Using the Xu-Tec process, all solid-state alloying elements could be evaporated, ionized, and introduced into the surfaces of base substrate materials, like iron, steel, titanium alloy, and intermetallic compound, to form new surface alloys with special properties.

The double glow discharge phenomenon is shown in Fig. 3.1. In a vacuum chamber, there are three electrodes: the grounded anode, the cathode, and the second negatively electrode. Two DC power supplies are applied to the cathode and the second negatively electrode separately. The power supplies provide an output voltage of 0–1200 V using a silicon-controlled rectifier. The vacuum chamber is first pumped to a base pressure below 0.1 Pa and back-filled with pure argon gas to a process pressure of 10–100 Pa. Under the electric field induced by two high-voltage power supplies, argon gas will be electrically broken down and ionized, so that two sets of glow discharge plasma zones are generated, one near the cathode surface and the other surrounding the second negatively electrode. This is the so-called “Double Glow Discharge Phenomenon” [1].

Fig. 3.1
figure 1

Double glow discharge phenomenon [1]

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3.2 Discovery of Double Glow Discharge

Since 1972, during a long-time study of plasma nitriding, it was thought that, in order to break the restriction of plasma nitriding for nonmetallic element application, we could apply glow discharge to realize surface alloying for solid metal elements. It was recognized that vaporizing solid metal elements is the key issue.

During the technology development experiments in Prof. Xu’s laboratory, occasionally they observed that a spark and/or local micro arc discharge emitted from the surface of the working-piece. It was also observed that there are more and more steel fine powders on the stove chassis. These observations had made one to realize that, under the glow discharge condition, the solid metal elements in the cathode electrode could be sputtered off by positive ion bombardment from plasma. The sputtered off metallic atomic species move into the glow discharge space, then deposit on the surface of the working-piece. One can employ this phenomenon to realize gasification of solid alloy elements.

Professor Xu’s team managed to set up a second cathode (as a source electrode, made of the desired alloying elements) between the anode and cathode in plasma nitriding equipment. Driven by two DC power supplies, two glow discharge (plasma) zones would be established, respectively, between the anode and cathode as well as the anode and second cathode. Ion bombardment at the second cathode makes the desired solid alloying element to be sputtered and gasified into glow discharge space. This is described as “Double Glow Discharge Phenomenon”. In this regard, they have been continuing research on double glow discharge phenomenon and its engineering applications since 1979 [2].

Shortly thereafter, the “Double Glow Plasma Surface Alloying Technology” was invented in Prof. Xu’s lab based on the “Double Glow Discharge Phenomenon”. It was discovered that the double glow plasma surface alloying technology can be applied to any solid chemical element such as nickel, chromium, tungsten, and their combination to conduct surface alloying modification. With this method, surface alloys with gradient concentration of alloying elements have been produced on the surfaces of steels, titanium alloys, intermetallic compounds, etc. [3, 4].

3.3 Double Glow Discharge Modes

For more than 30 years, many experiments have been conducted to understand the Xu-Tec process. It has been found that the “Double Glow Discharge Phenomenon” has several modes of discharging characteristics.

3.3.1 Independent Discharge Mode

When the distance between source electrode and cathode (substrate) is much larger than 2 times of the width of cathode potential drop region (2Dk) introduced in Chap. 2, the glow discharge is in independent mode. Both the DC power supplies of the cathode and source electrode can operate independently. There is no interaction between the glow discharges surrounding the cathode and source electrode. The glow discharge surrounding the cathode and source electrode would not affect each other.

3.3.2 Dependent Discharge Mode

When the cathode and source electrode are brought closer to less than 2Dk, a much higher current density and stronger discharge are initiated and sustained by DC power supplies. Two glow discharge spaces start to overlap. The luminance intensity of the glow increases abruptly as the current density increases. That is a dependent double glow hollow cathode discharge mode (to be discussed in detail in the next section). Under this condition, the glow discharge intensity of the working-piece and the source electrode can be increased by a dozen times to a hundred times. Actually, this mode cannot be completely independently operated. There seems a cross talk between two discharge zones. When the source voltage increases or decreases, the cathode current and source current are also increased or decreased together accordingly. This mode arrangement greatly enhances double glow discharge intensity.

3.3.3 Pulse Discharge Mode

The pulse discharge mode is developed using the controllable DC pulse power suppliers. The pulse power supply is mainly used for the working-piece only, though it can be used for both the working-piece and the source cathode. Pulsed discharge can be also used in the hollow cathode discharge mode. In this case the electric potential and output power can be adjusted flexibly by altering the duty cycle, and temperature adjustment on the working-piece surface is separated from other processing parameters. Another advantage of using pulsed power is its function to suppress the development of arc discharge. As a result, the discharge is more homogeneous than that of conventional DC power supply and improves the stability of surface alloying process and quality of surface alloy. A pulsed DC power supply is strongly recommended for the Xu-Tec process.

3.3.4 Other Discharge Mode

The use of high-frequency microwave discharge power supply can increase the ionization rate and can also be applied to nonconductive target materials.

In some cases, we can also use one DC power supply for both the workpiece surface and the source electrode to realize plasma surface alloying. In this case, it is easy to operate with a DC adjustable power supply, but the composition of the surface alloy is difficult to control.

It should be stressed here that, for all the above discharge modes, the cathode potential can be higher, equal, and lower than the source electrode potential. In most cases, in order to increase sputtering rate and alloying elements supply, the source electrode potential should be lower than that of cathode potential.

3.4 Double Glow Hollow Cathode Discharge (DG-HCD)

3.4.1 Hollow Cathode Discharge (HCD)

As discussed in Sect. 3.3.2, the hollow cathode discharge (HCD) is a special dependent double glow discharge mode. The typical hollow cathode discharge device is shown in Fig. 3.2. The system includes an anode, a cylinder cathode, and a DC power supply in a vacuum chamber.

Fig. 3.2
figure 2

General diagram of a hollow cathode discharge device

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When the inner diameter of the cylinder cathode (D) is larger or the internal gas pressure is relatively high, the cathode glow ignites from the cathode inner surface. The negative glow will appear inside the cylinder, the Faraday dark area and the positive column region appear in the center of the cylinder without HCD. If the inner diameter of the cathode is reduced, the Faraday dark area and the positive column region are also reduced. When the inner diameter of the cylinder cathode is close or less than two times of the width of cathode potential drop region (2Dk) and larger than one Dk, HCD phenomenon occurs, and the Faraday dark zone and the positive column region disappeared completely in the center of the cylinder. At this time, the cathode current density and the negative glow intensity of the area will be greatly enhanced. The occurrence of HCD is due to the oscillation of electrons inside the cylinder. The electrons from one point A at the inner wall of the cylinder are accelerated and moved to another opposite point B. But the electrons will be rejected by the electric field of point B. In this way, the electrons will be bouncing back and forth between A and B many times, which greatly increases the chances of the electron collision with atoms and greatly causes the neutral atoms excited and ionized. As a result, the glow discharge will be much stronger. The feature of the hollow cathode discharge is that the luminance intensity of the glow and the discharge current density increase simultaneously and abruptly.

Hollow cathode discharge (HCD) has been widely used in the fields of spectral analysis, vacuum coating, surface treatment, gas laser, etc. If the hollow cathode is made into a micro structure (submillimeter), it can be used in the high pressure, and the HCD can be also used in the ultraviolet light source, the plasma display, and other fields.

3.4.2 Concept of DG-HCD

The hollow cathode discharge described above is only a special type of discharge with a single cathode. The Double Glow Hollow Cathode Discharge (DG-HCD) is formed by two sets of cathodes (i.e., the working-piece and the source) driven with two different electric potentials [5]. Experimental apparatus for the formation of a hollow cathode discharge in double glow discharge is shown in Fig. 3.3.

Fig. 3.3
figure 3

Experimental setup for DG-HCD of double glow discharge. 1 and 5 power supplies, 2 and 3 two cathodes, 4 argon gas inlet [1]

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The experimental device of DG-HCD is set in a sealed vacuum container, including the first cathode (2) and the second cathode (3, the source electrode), two power supplies (1 and 5). Two cathodes, made of a low-carbon steel plate with size of 100 × 100 × 4  mm, are placed in parallel with a relative distance adjustable in the range of 10–l00 mm. The supply output voltage can range between 0 and 1000 V. The working discharge gas is industrial-grade pure argon and the working gas pressure ranges between 10 and 100 Pa.

The DG-HCD is similar to the HCD. When two power supplies (1 and 5) are switched on, the glow discharges are, respectively, generated along the surface of cathodes 2 and 3. At first, the negative glow region of the two cathodes is well-defined and shown by the curve 2 in Fig. 3.4, and the mutual does not intersect. Then, with the decrease of the argon pressure, the thickness of the negative glow region increases. When two negative glow regions are overlapped with each other in the space between two cathodes, the glow brightness is significantly enhanced. The brightness of the curves 1 and 3 are shown in Fig. 3.4. If further the pressure is reduced or the two cathode voltages are increased, the two cathode glow regions mutually overlap and cross together, the brightness of the glow discharge and two cathodes’ current density will increase sharply, shown as Curve 4. This is the Double Glow Hollow Cathode Discharge (DG-HCD). Since the discharge potentials of the two cathodes are not equal, this phenomenon is referred as the unequal potential hollow cathode discharge.

Fig. 3.4
figure 4

Intensity of double glow discharge. 1 and 3 intensity on cathodes 2 and 3, 4 total glow discharge intensity where two glow discharge zones overlap [1]

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3.4.3 Current Amplification Effect of DG-HCD

The current amplification effect of the double glow hollow cathode discharge is shown in Fig. 3.5. It is shown that when the cathode voltage Uc is increasing to 400 V, then both the main cathode current Ic and the source electrode current Is will sharply increase.

Fig. 3.5
figure 5

Current amplification effect of the unequal DG-HCD [1]

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3.5 Advent of Double Glow Plasma Surface Alloying/Metallurgy (Xu-Tec Process)

3.5.1 First Experimental Device

In our first experiment in 1978, we used a modified vacuum glass bell jar coating machine and a mercury vapor high-voltage DC power supply. The structure of the device is shown in Fig. 3.6. In the bell jar (5) as a vacuum chamber, it encloses components including an anode (6), cathode (8) for placing the working-piece, another cathode (7) as a source electrode of the tungsten wire for providing tungsten, which is connected to a power supply (12) through sliding resistor (11). At that time, we only had one DC power supply. In order to give the power to the two cathodes, respectively, we used a shunt resistance (11) to be a voltage supply for supporting the source power potential. The voltage is transferred to the source by an intermediate active tap on the shunt resistance device. Anode (6) and cathode (8) are, respectively, connected to the ends of the DC power supply. The potential difference between the anode and the cathode is the same as the potential difference between the two ends of the DC power supply.

Fig. 3.6
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Schematic diagram of double glow plasma surface alloying experiment setup unit [1]

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When the DC power is switched on, the double glow discharge phenomenon appears. Both the specimen on the working table and the source electrode are surrounded by glow discharges. The alloying elements in the electrode are sputtered out by ion bombardment with Ar plasma, then travel toward the working-piece and deposit on its surface. The alloy elements adsorbed on the surface of the working-piece will diffuse into the subsurface of the workpiece. As a result, a deep alloy layer is formed on the surface of the working-piece. The other main effect of the cathode glow is the working-piece sample heating by ion bombardment; the required high temperature provides a driving force for the diffusion of alloying elements.

3.5.2 First Microstructure of Tungsten Surface Alloy

The first microstructure of tungsten alloy layer generated by double glow plasma alloying technology is shown in Fig. 3.7. The tungsten content in the top surface alloying layer is about 10%, with the thickness of 70 μm. The success of surface alloying with tungsten of high atomic weight and high melting point indicates that this technology could be applied to all solid-state alloying elements. Afterwards, we had applied nickel, chromium, aluminum, titanium, and other metal elements successfully to diffuse into the surface of steel materials, forming a wide variety of alloy layers.

Fig. 3.7
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Microstructure and indentations of W-alloyed layer generated by Xu-Tec process [1]

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3.5.3 Other Considerations

In the previous experimental design, we not only use ion bombardment and plasma sputtering to generate alloy element vapor source, but also adopt a tungsten filament resistance heating coil to enhance the supply of alloying elements. After this surface alloying process, the experimental testing result shows that the cross section of sample has a very clear white bright tungsten alloy layer.

At the beginning, the electric potential used on the working-piece cathode is lower than that of the source electrode. Later on, we considered that the alloy elements are sputtered out, and most of them are neutral atoms rather than positive ions. In order to strengthen the ion bombardment and increase the supply of the alloy elements, we have changed the source electrode potential to be lower than the working-piece. In 1984, we introduced the hollow cathode discharge into the double glow plasma surface alloying technology, which further strengthened the glow discharge and the source sputtering rate.

In addition to the Xu-Tec process, we have also applied the double glow discharge phenomenon to invent a series of new innovative technologies, such as

  • Arc plasma added double glow surface alloying technology,

  • Double glow plasma brazing technology,

  • Double glow plasma sintering technology,

  • Double glow plasma nanopowder technology,

  • Double glow plasma thin diamond film technology,

  • Double glow plasma sputter cleaning technology,

  • Double glow plus high-frequency plasma surface alloying technology,

  • Double glow plasma chemistry.

All of these technologies mentioned above will be explained in detail in Chap. 13.