UA53365C2 - A mechanism for the vacuum-plasma treatment of articles - Google Patents

Abstract

A mechanism for the vacuum-plasma treatment of articles contains working chamber with anode, chamber with cathode, direct current power supply connected thereto at least through one opening. The mechanism has also additional chamber with additional anode, connected with the additional direct current power supply, being connected with a working chamber at least through one opening. As the anode, the articles holder is chosen, and the cathode is made in the form of vacuum-arc discharge cathode.

Description

The invention is related to vacuum-plasma processing of materials and can be used mainly in the process of chemical-thermal processing. It is advisable to use the invention for complex surface treatment, which includes chemical and thermal treatment of the surface of the product followed by application of a strengthening coating.

Carrying out the process of chemical and thermal treatment by methods of vacuum-plasma technology includes two necessary factors: heating the product and creating a gas plasma, which helps to activate the process.

The device and method of vacuum-plasma processing of products in the plasma of a two-stage vacuum-arc discharge are known (US patent Mo 5.503.725, IPC C23С14/34, 1996). The device has a working chamber with a cathode and an anode placed in it, which is the holder of the product. An opaque screen is placed between them, permeable to plasma electrons. The device has a direct current power source. When implementing the method, the product, which is in the gas plasma, is the anode of the discharge, and is subjected to electron bombardment. The screen (the "chevron" or "blind" type is more often used) prevents metal plasma generated by the cathode of a vacuum-arc discharge from hitting the product (due to condensation on it of metal ions that spread along straight paths). Electrons of the gas plasma heat the product to the working temperature and at the same time activate the gas environment. As a result, in the process of chemical and thermal treatment, unwanted etching of the surface of the product is excluded.

However, if large-sized products are processed in this device, a large discharge current must be created. At the same time, the uniformity of the electron flow over the entire surface of the product is not ensured, because the movement of electrons is affected by the magnetic field created by the discharge current. Therefore, the product heats up unevenly. This determines the low efficiency of chemical and thermal treatment.

The well-known device, which was chosen as a prototype, can be used for vacuum-plasma processing of products (A.

Apaegz apa M. Kiinp Spagasiegigayop oi a Iom-epegdu sopvigpisiea-riazta 5o0-igse," Veum. si. Ipvigyt. 69 (1998), 1340 - 1343).

The device is a gas discharge plasma source and contains a working chamber with an anode placed in it. The camera with the cathode placed in it is connected to the working chamber at least through one hole in the form of a nozzle. The device has a direct current power source connected by poles to the cathode and anode. Such a plasma source is called a compressed plasma source (CRP). The cathode in it has a cavity with a channel for supplying working gas. During processing, the working gas is supplied to the chamber through it. Pumping of the CP5 source takes place from the side of the working chamber. As a result, a pressure difference is formed between the chamber and the working chamber, the value of which is determined by the gas conductivity of the hole, the amount of gas supplied, and the pumping speed of the pump. The working gas from the chamber enters the working chamber in the form of a supersonic jet. When voltage is applied to the electrodes of the plasma source, a glow discharge occurs in the cathode cavity. The pressure parameters of the working gas in the cavity are selected under the condition of the stable existence of the glow discharge. In the region of the hole, a double electric layer is formed in the plasma, in which the voltage drop can reach the ZOV. Electrons with this energy are able to ionize the gas jet coming out of the nozzle. The average energy of gas ions formed by the source is - 5 eV.

CP5 plasma sources are used in the production of film coatings of gallium arsenide and oxide films on glass and plastics. Typical electrical parameters for SPO: voltage between cathode and anode 300...5008, discharge current 10...200mA, full ion saturation current from 0.1 to 1mA. By changing the geometry of the nozzle, the pumping speed, and the gas flow, it is possible to change the pressure in the working chamber in a wide range from 103 to 100 Pa.

However, a source with such parameters cannot provide effective chemical-thermal treatment of materials in large volumes, because the magnitude of the discharge current and, accordingly, the power of the source are small. In addition, the energy of gas ions was low, which is not enough for the dissociation of gas molecules. Therefore, the necessary heating of the products and activation of the gas environment is not provided.

The basis of the invention is the task of creating such a device for vacuum-plasma processing of products, which, in comparison with the device chosen as a prototype, will allow to increase the efficiency of chemical-thermal processing of products.

The task is solved in a device that contains a working chamber with an anode, connected to it, at least through one hole, a chamber with a cathode, a DC power source. According to the invention, the device has an additional chamber with an additional anode connected to an additional source of direct current, which is connected to the working chamber through at least one hole, the product holder is selected as the anode, and the cathode is made in the form of a vacuum-arc discharge cathode.

When the DC source is turned on, an arc discharge is excited between the cathode and the anode (the holder with the products).

The difference between a vacuum-arc discharge cathode and a glow discharge cathode is that, regardless of the area of the cathode, it can provide almost any discharge current (the magnitude of the latter is determined only by the thermophysical properties of the cathode material, its cooling conditions, and power source parameters). In addition, the cathodic potential drop in an arc discharge is ten times smaller than in an incandescent one, which allows efficient use of the power source. The hole that connects the working chamber with the chamber with the cathode contributes to an increase in the voltage drop in the area of the hole, which leads to an increase in the speed of electrons in the plasma of the arc discharge and, as a result, to an increase in the heating power of the processed product.

When an additional source of direct current is turned on between the plasma of the vacuum-arc discharge of the working chamber and the additional anode, an additional vacuum-arc discharge is excited. Plasma ions, due to the opening connecting the additional and working chambers, are accelerated towards the holder with the products, ensuring more effective dissociation of working gas molecules than electrons. An increase in the number of atoms of the working gas accelerates the process of formation of chemical compounds of the mentioned atoms with the metals of the product, and also promotes active diffusion into the depths of the metal. These circumstances lead to an increase in the efficiency of chemical and thermal treatment.

Figure 1 shows the schematic diagram of the proposed device for one-time processing of products with electrons and ions of a gas plasma. Figure 2 shows a diagram of an improved version of the device. Fig. 3 shows a graph of the dependence of microhardness on the depth of the nitrided layer, obtained for cutting plates made of PbM5 steel, for two modes of operation: during processing only with electrons (curve 1); during treatment with electrons and ions (curve 2).

The device contains a working chamber 1, in which a product holder 2, which is an anode, is installed. The working chamber 1 is connected to the chamber 3, in which the cathode 4 of the vacuum-arc discharge is placed. The working chamber 1 is connected to the chamber C through an opening 5 of a small diameter. Cathode 4 and anode 2 (product holder) are connected to source 6 of direct current. The additional chamber 7 is connected to the working chamber 1 through the opening 8. The device can have a second additional source 11 of direct current (Fig. 2), which is included between the cathode 4 (negative pole) and the chamber 3.

We will consider the operation of the device using the example of chemical and thermal treatment (nitriding).

Chambers 1, Z and 7 are pumped to a low pressure (for example, up to 1.3 x 103Pa) with high-vacuum pumps (not shown in the drawing), and then working gas is injected into them using a working gas supply system (not shown in the drawing) to a pressure of 0.1...1Pa. When the DC source 6 is turned on between the cathode 4 and the holder 2 with the products placed on it, an arc discharge is excited. The emission of electrons, which ionize the gas in the working chamber 1, occurs from the plasma surface in a small hole 5 (several millimeters). A voltage drop is created in its zone, which leads to an increase in the speed of electrons. In the other part of the cavity of the working chamber 1, which is adjacent to the opening 5, the voltage drop is insignificant (as a rule, the electric field strength is no more than 0.5V/cm). Therefore, practically the entire voltage of the power source 6 is applied to the zone of the hole 5 and determines the energy of the accelerated electrons. With a discharge current of 100...150A and a power source voltage of 150...3008, the heating power can reach several tens of kilowatts, which allows you to heat relatively large masses of products in a short time.

In order to carry out the process of chemical and thermal treatment (for example, nitriding), it is very important that the heated surface of the processed product is surrounded by a gas atmosphere, which is enriched with particles of atomic nitrogen, which can penetrate into the crystal lattice of the nitriding material, forming a solid solution of nitrogen in the material. The presence of atomic nitrogen accelerates the formation of chemical compounds of nitrogen with the metal and promotes the diffusion of nitrogen into the surface of the material. When electrons collide with a molecule, the dissociation of the latter is insignificant due to the small mass of the electron. Effective dissociation of gas molecules can be ensured only by accelerated heavy particles (gas ions). To carry out an effective process of dissociation of gas molecules into atomic particles, the proposed device has a gas ion accelerator, which is created by a closed cavity of an additional chamber 7, an additional anode 9, a working chamber 1 and an opening 8 between the working chamber 1 and the chamber 3. When the power source 10 is turned on, additional anode 9 has a positive potential relative to anode 2, and therefore relative to the plasma that fills the working chamber 1. Therefore, a vacuum arc discharge is excited between the plasma and anode 9. The plasma of this discharge is compressed by hole 8 and therefore a voltage drop occurs in the area of hole 8. The magnitude of the voltage drop on the accelerating gap can be at the level of tens or hundreds of volts. The voltage of the electric field in the area of the hole 8 is directed so that the gas ions are accelerated in the direction of the product placed on the holder 2.

Colliding with neutral gas molecules, ions are highly likely to dissociate them, which leads to activation of the chemical-thermal treatment process.

Thus, thanks to hole 5, the product is effectively heated by an accelerated electron flow, and thanks to hole 8, the product is treated with gas atoms, which significantly intensifies the chemical-thermal treatment process.

During the operation of the device for vacuum-plasma processing of products, the current of the vacuum-arc discharge between the cathode 4 and the anode 2 cannot be lower than the minimum current (Imin.) of the stable existence of the arc discharge on the cathode 4. The magnitude of this current depends on the material of the cathode and the design of the cathode assembly ( as a rule, this current is several tens of amperes).

If the temperature of the product is maintained at a current value less than Imin., then the discharge does not exist, which reduces the efficiency of the process. To eliminate such a situation, the device provides a source 11 (fig. 2) of direct current, which is connected to the cathode 4 (negative pole) and the chamber 3. Thus, the additional anode for the cathode 4 is the inner walls of the chamber 3. With such a design device, the minimum current for stable burning of the arc discharge is not provided by the current between the cathode 4 and the anode 2, but by the current between the cathode 4 and the inner walls of the chamber 3, which is maintained by the power source 11. The temperature of the product at the required level is maintained by the voltage regulation of the power source 6. Otherwise the operation of the device shown in Fig. 2 is similar to the operation of the device shown in Fig. 1

Example 1. Titanium cathode 4 (diameter 100 mm) was stirred in chamber C. Working chamber 1 is connected to chamber C by means of five holes with a diameter of 10 mm, and working chamber 1 is connected to an additional chamber 7 by means of five holes with a diameter of 8 mm. We used a source of 11 direct currents with a voltage of 40V and a power of bkW. Power sources 6 and 10 had a voltage of 2208 and a power of 10 kW. Plates made of RBeM5 tool steel, size 8 x 20 x 20 mm, were placed on a massive cylinder (total weight - 8 kg) with a diameter of 250 mm and a height of 250 mm. The cylinder was placed on the holder of 2 products. The cylinder was rotated at a speed of 30 rpm. The temperature was measured using an optical pyrometer. A mixture of argon and nitrogen was fed into the chamber. The working pressure was maintained at the level of 0.7 Pa (at a partial pressure of argon of 0.2 Pa). The products were heated to a temperature of 500"C with the power source 11 turned on, with a discharge current between the cathode 4 and anode 2 (cylinder) of the ZOA and a voltage of 2008. The heating time was 8 minutes. After the products reached the working temperature, the current was reduced by adjusting the power source 6 .

Nitriding of the plates was carried out without turning on the ion accelerator (power source 10 was turned off). The time of the chemical and thermal treatment process - ЗОхв. After processing, the microhardness of the nitrided layer was determined depending on the depth. The graph of this dependence is shown in Fig. 3 (curve 1). Practically, this process in its physical essence is close to the process that takes place in the device chosen as a prototype.

Example 2. All the same parameters as in example 1 were maintained, but after reaching the operating temperature (5007), the accelerator of working gas ions was included. With the help of the power source 10, the discharge between the cathode 4 and the anode 9 was maintained at a voltage of 1008 at the source 10 and a current of З0A. The time of the chemical and thermal treatment process - ЗОхв.

After processing, the microhardness of the nitrided layer was determined depending on the depth. The graph of this dependence is shown in Fig. 3 (curve 2).

As can be seen from the graph, the use of a gas ion accelerator leads to an increase in the thickness of the nitrided layer by approximately 4095 at the same time of the process (ZОхв.)

Thus, in comparison with the device chosen as a prototype, the proposed device for vacuum-plasma treatment of products allows to simultaneously effectively heat the products with accelerated electrons and activate the gas environment with accelerated ions, which indicates an increase in the efficiency of chemical-thermal treatment.

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Claims (1)

A device for vacuum-plasma treatment of products, which includes a working chamber with an anode connected to it through at least one opening, a chamber with a cathode, a source of direct current, characterized by the fact that it has an additional chamber with an additional anode connected to the additional source direct current, which is connected to the working chamber through at least one hole, the product holder is selected as the anode, and the cathode is made in the form of a vacuum-arc discharge cathode.