DE102022211045A1 - Device and method for carrying out plasma-assisted thermochemical diffusion processes on metallic components - Google Patents

Abstract

In the device for carrying out plasma-supported thermochemical diffusion processes on metallic components, at least one metallic component is arranged on a work table (1) within a reaction chamber (12) and enclosed by a steel mesh grid (6). The steel mesh grid (6) has an opening in the vertical direction above the component(s) and is connected to a negative pole of a first or second electrical direct current source (5, 10), wherein a disk-shaped element (9) is arranged above the opening, which is formed with carbon or consists entirely of carbon. The disk-shaped element (9) is connected or can be connected to a negative pole of the first or second electrical direct current source (5, 10). An internal pressure is maintained in the reaction chamber (12) which is lower than the ambient pressure and a plasma treatment can be carried out with reactive species which have been formed by means of a reaction gas fed into the reaction chamber (12) via at least one gas inlet (4).

Description

The invention relates to a device and a method for carrying out plasma-assisted thermochemical diffusion processes on metallic components.

Conventionally used metallic materials often do not meet the desired properties in relation to the intended applications. Therefore, modification of the surface properties of the components made from such metallic materials is necessary to achieve reliable performance and long service life. Among the various surface modification processes, plasma-assisted thermochemical diffusion processes have great potential due to simple process implementation, reduced costs and low environmental impact. Plasma-assisted processes are often used to improve wear and corrosion resistance, fatigue strength and surface hardness of metallic components. Various plasma-assisted diffusion treatment processes such as plasma nitriding (PN), plasma carburizing (PC) and plasma nitrocarburizing (PNC) can be implemented by introducing nitrogen and/or carbon-containing precursor gases into the respective process atmosphere.

Plasma-assisted thermochemical diffusion treatments are carried out within a vacuum reactor by creating an electrical potential between a cathodically connected work table (with the components to be treated) and the anodically connected reactor wall. Depending on the thermochemical diffusion treatment process, a gas mixture of H ₂ and N ₂ (for PN) and/or an admixture of CH ₄ (for PNC) specified within certain limits is used. If the energy of the electrons is high enough to generate ions, a glow discharge occurs that spreads evenly over the surface of the components to be treated. During the glow discharge, electrons are accelerated from the cathode to the anode, interact with the molecules of the gases introduced into the reactor and thus create an environment of electrons, positive and negative ions, neutral and energetic species. The positive ions are accelerated and collide with the negatively charged components (components to be treated) with relatively high kinetic energy. During this process, the components are covered by the negative glow line - this plasma-solid interface is called the cathode shell, where all interactions with the surface take place. The interaction between the species in the cathode shell and the surface is essential for surface modification. Such interaction causes various effects, mainly heating the components to the treatment temperature and generating the required reactive species for diffusion. In general, about 10% - 20% of the supplied plasma energy is required for activating the chemical reactions and generating reactive species, while about 80% - 90% of the plasma energy is required in the form of heat to increase the component temperature and set the desired treatment temperature.

The process control of a plasma-assisted thermochemical diffusion treatment of metallic components, i.e. the control of the process atmosphere in order to achieve the desired properties within the modified surface layer of the workpieces, is of interest from both an industrial and research perspective.

In order to achieve a defined material reaction in plasma-assisted thermochemical diffusion treatment, the following process parameters must be taken into account. These can be the treatment temperature, the treatment duration, the composition of the respective process gas, the flow rate of the process gas, the process pressure and/or the global plasma parameters (electrical voltage, plasma current density). The control of the process atmosphere depends on the selection and combination of pressure, temperature, time and the process gas composition (N ₂ +H ₂ -/+CH4). However, the aforementioned individual parameters significantly influence the plasma parameters, as there are different correlations and interactions with each other.

In order to achieve reliable process control, all of the above parameters should be separately and independently controllable. To achieve this, it is necessary that the specific plasma parameters, such as vacuum pressure, electrical voltage and plasma current density, can be defined and that the temperature of the workpieces to be treated can be controlled separately, i.e. independently of the plasma parameters.

The process control therefore depends not only on the aforementioned global treatment parameters, but also on the configuration of the reactor.

For the configuration of the reactor to ensure reliable process control, the decoupling of the functions (i) heating the components to be treated to treatment temperature and (ii) generation of the reactive species is advantageous. The first aspect is based on empirical, material-specific knowledge The second aspect depends on the configuration and the resulting technological properties of the reactor used.

To ensure reliable process control for plasma-assisted thermochemical diffusion processes of metallic components, heating to process temperature and the generation of reactive species should be efficiently separated from each other and at the same time independently controllable.

The current state of the art for plasma-assisted diffusion treatment includes four technologies. This is the conventional plasma-assisted process in the cold wall reactor ( 1a) and in the warm wall reactor ( 1b) , a plasma-assisted process with steel active grid ( 1c ) and a plasma-assisted process with carbon active grid ( 1d ) in the cold wall reactor.

In the conventional cold wall reactor using continuous direct current plasma (DC), the components to be treated are glowed with plasma and heated by the kinetic energy resulting from the ion bombardment. This process is controlled by regulating the electrical voltage and current density. A high plasma voltage (600 V - 800 V) is necessary on the components to achieve and maintain the process temperature. The generation of reactive species is inevitable and cannot be controlled independently.

Continuous DC plasma can be replaced by pulsed DC plasma, whereby the component temperature can be controlled by simply adjusting the plasma pulse characteristics (pulse length, pulse pause) without changing the electrical voltage. Negative effects such as hollow cathode or arc effects can be effectively suppressed and a uniform treatment result can be achieved on components with complex geometries. Using the plasma to heat up the metallic components requires up to four times as much electrical power as the plasma-assisted diffusion treatment itself.

A conventional warm wall reactor with a heating system inside the reactor wall and an application of pulsed DC plasma uses two independent heat sources (electrical resistance heating and plasma glow). The metallic components to be treated are heated using ohmic heating elements installed inside the reactor wall; (additional) plasma glow is not necessary. The pulsed DC plasma is used to activate the surfaces of the metallic components, to ionize the supplied process gases and to generate reactive species. The plasma is (mainly) used to generate reactive species, so the plasma voltage applied to the respective metallic components (400 V - 500 V) is lower than in the cold wall reactor. Using warm wall reactors, the metallic components are heated with a homogeneous temperature distribution using economically justifiable time and energy.

The further development of the aforementioned conventional plasma-supported processes is the active grid technology in the cold wall reactor. Here, the plasma is shifted to a metal grid (= active screen, AS) that surrounds the material to be treated. A steel-based grid is used as the active grid, which is arranged inside the vacuum reactor and is electrically connected to the direct current source for plasma generation. The plasma is primarily generated between the vacuum reactor and the active grid. The active grid exposed to plasma fulfills two functions: radiation-related temperature homogeneity on the workpiece and generation of reactive species.

The active grid plasma provides the heat required to reach the process temperature, heats the metal components to be treated by thermal radiation and ensures inertia-free temperature control. The active grid on the metal surface also interacts with the supplied (N ₂ +H ₂ -/+CH ₄ ) process gas and forms a reactive gas mixture (electrons, positive and negative ions, neutral and energetic species) as a result of the mechanism of physical sputtering. In order to direct the active species generated on the active grid onto the metal components to be treated, the material to be treated is additionally subjected to a low electrical potential (bias). As a result of the strong reduction in the plasma discharge on the material to be treated, edge effects and hollow cathode formation are reduced or avoided, sharp edges and cross-sectional transitions as well as the smallest bores and blind holes can be treated. To regulate the process temperature in the reactor, the power density supplied to the active grid is varied by means of an electrical voltage change. The use of a steel active grid allows the treatment processes PN, PC and PNC.

The substitution of the steel active grid by a carbon active grid (graphite, carbon fiber reinforced carbon - CFC) in the cold wall reactor results in improved PNC/PC processes. As a result of the interaction between the plasma discharge on the carbon surface of the carbon active grid and the N ₂ -H ₂ reaction gas, a highly reactive process gas is created, consisting of from saturated and unsaturated hydrocarbons, radicals, ammonia, CN-containing species and carbon monoxide (CO). The formation mechanism of the volatile unsaturated hydrocarbons (HCN, C2H2, CN and others) is based on the principle of chemical sputtering of plasma-activated solid carbon surfaces. Compared to a steel active grid, the generation rate of reactive species with a carbon active grid is one to two orders of magnitude higher. The use of a carbon active grid in a cold-wall reactor enables the treatment processes PC and PNC. The addition of gaseous carbon carriers (e.g. CH ₄ ) is no longer necessary for PC or PNC.

• Es ist keine Trennung von elektrischen Parametern der Plasmaentladung für die Heizung und für die Generierung reaktiver Spezies möglich. Eine hohe Plasmaleistung führt zu einer Überhitzung bei Bauteilen mit unterschiedlichem Flächen/Volumen-Verhältnis, Arcing, Kanteneffekt an Bauteiloberfläche, Hohlkathodeneffekt und hohem Energieverbrauch. Es ist keine konturentreue Behandlung, insbesondere bei Komponenten mit komplexen Geometrien und geringer Spaltgängigkeit (z.B. Sacklöcher) möglich.

A cold wall reactor with continuous DC plasma has the following disadvantages:

• It is not possible to separate the electrical parameters of the plasma discharge for heating and for the generation of reactive species. A high plasma power leads to overheating of components with different area/volume ratios, arcing, edge effects on the component surface, hollow cathode effect and high energy consumption. Contour-accurate treatment is not possible, especially for components with complex geometries and low gap clearance (e.g. blind holes).

The warm wall process with pulsed DC plasma has the disadvantage that the entire plasma discharge occurs on the surface of the respective component. In addition, the previously mentioned problems of the cold wall reactor still occur. It is not possible to separate the electrical parameters of the plasma discharge for heating and for the generation of reactive species.

Due to the technological limitation of the maximum temperature that can be achieved by wall heating, additional heat during high-temperature treatments can only be achieved by using an intensive plasma on the respective components. Even in this case, the disadvantages mentioned above still exist.

A steel active grid in a cold wall reactor has the following disadvantages. It is not possible to decouple the electrical parameters of the plasma for heating and generating reactive species. The level of plasma power at the steel active grid can be controlled during the process via the treatment temperature. Undesirable material transfer occurs from the steel active grid to the component surface and a minimum distance must be maintained between the steel active grid and the component surface.

Contamination of the steel active grid with chemical elements, e.g. C, can occur, which are released uncontrolled during subsequent processes (take treatment history into account). The service life of a steel active grid is limited due to distortion after a certain number of treatment cycles.

The use of a carbon active grid in the cold wall reactor has the disadvantage that the electrical parameters of the plasma for heating and for the generation of reactive species are not decoupled during the process. The necessary concentration of reactive species cannot be regulated and is therefore usually too high. The level of plasma power can be controlled during the process via the treatment temperature. The carbon active grid is subject to a high erosion rate both during heating and during the actual treatment process. The possible treatment processes are limited to PC and PNC; pure PN, without carbon influence, cannot be realized. In addition, contamination or carbon contamination of the reactor cannot be avoided.

The object of the invention is to enable reliable process control of the plasma-assisted thermochemical diffusion treatment of metallic components, while also achieving better effectiveness of the energy used.

According to the invention, this object is achieved with a device having the features of claim 1. Claim 9 relates to a method in which the device is used. Advantageous embodiments and further developments of the invention can be realized with features specified in dependent claims.

In the device according to the invention, at least one metal component is arranged on a work table in a reaction chamber and is enclosed by a steel mesh grid. The steel mesh grid has an opening in the vertical direction above the component(s); this opening should have at least a free cross-sectional area that corresponds approximately to the radially outer dimensions of a disk-shaped element. The steel mesh grid is connected to a negative pole of a first or second electrical direct current source.

A disk-shaped element, which is made of carbon or consists entirely of carbon, is arranged above the opening. The disk-shaped element is connected to a negative pole of a first or second electrical direct current. power source or can be connected to it. The direct current sources can be operated continuously or pulsed.

An electrical resistance heater may be arranged on or integrated into the wall of the reaction chamber and connected to a positive pole of the first or second electrical direct current source if a warm wall reactor is to be used.

An internal pressure is maintained in the reaction chamber that is lower than the ambient pressure and a plasma process can be carried out with reactive species that have been formed by means of a reaction gas fed into the reaction chamber via at least one gas inlet. The respective reaction gas can be a single gas or a suitable gas mixture, depending on which surface modification is to be achieved on components and which plasma-assisted diffusion treatment is to be carried out. For example, H ₂ can be supplied as a reaction gas in plasma carburizing, N ₂ in plasma nitriding and a mixture of H ₂ and N ₂ in plasma nitrocarburizing.

Advantageously, at least two of these different plasma-assisted thermochemical diffusion processes can be carried out alternately by appropriately supplying a reaction gas or reaction gas mixture.

A disk-shaped element can be made of pure carbon, e.g. graphite. However, a composite material such as fiber-reinforced carbon, in particular carbon fiber-reinforced carbon, can also be used.

The disc-shaped element can be arranged at a distance from the opening of the steel mesh grid and can be electrically insulated from the steel mesh grid and the wall of the reaction chamber.

However, it is also possible to change the distance between the disc-shaped element and the vertically upper front edge of the steel mesh grid using an adjustment mechanism in order to influence the respective diffusion process. The distance can also be changed in such a way that the distance is changed from a predetermined maximum distance to a direct contact with the vertically upper front edge of the steel mesh grid, thereby achieving an electrically conductive connection between the disc-shaped element and the steel mesh grid. When touched, an electrical current can flow from a direct current source via the steel mesh grid to the disc-shaped element.

Preferably, the distance should be continuously adjustable.

By varying the distance, one can also influence the activation of the species used for surface modification.

The disc-shaped element should be interchangeable and preferably disc-shaped elements with different dimensions should be usable. The dimensions can also influence the respective diffusion process in order to positively influence the result, since the disc-shaped element can also function as a carbon source. For example, a disc-shaped element with a certain surface area can be used, which can, for example, provide the amount of carbon required for the respective plasma-supported thermo-chemical diffusion process.

The at least one metal component should be arranged on the work table in an electrically isolated manner and not connected to any electrical power source. A negative BIAS voltage, which is commonly used, is not required.

The wall of the reaction chamber can be connected to earth potential or to a positive pole of a direct current electrical source.

When the process is carried out, certain types of process-relevant species are generated depending on the type of plasma-assisted thermochemical diffusion process used (plasma carburizing, plasma nitrocarburizing, plasma nitriding). The type of process-relevant species can be controlled by applying or changing the electrical current flow to the disk-shaped element and/or the reaction gas used.

When carrying out the process for plasma carburizing and/or plasma nitrocarburizing, the concentration of the process-relevant species can be controlled or regulated, whereby the following control variables can be used for control: As a function of the plasma power (electrical voltage) applied to the disk-shaped element, as a function of the distance between the disk-shaped element and the steel mesh grid, as a function of the size/surface of the disk-shaped element. Depending on the specific configuration of active hybrid systems, one or more of the three control variables mentioned can be applied.

When carrying out the plasma nitriding process, an electric current flow to the disc-shaped element is not activated, so that the only control parameter is the plasma power (electrical voltage) on the steel mesh grid.

A plasma-assisted thermochemical diffusion process with the supply of the corresponding reaction gas can be carried out at a constant temperature and constant pressure in the reaction chamber.

At least one component made from a high-alloy steel, a low-alloy steel or an unalloyed carbon steel can be treated with the corresponding plasma-assisted thermo-chemical diffusion process. A high-alloy steel can be understood as an alloy in which at least one alloying element exceeds an average content of 5% by mass, and a low-alloy steel can be understood as an alloy in which a maximum of 0.5% by mass of carbon and/or a maximum of 5% by mass of at least one other alloying element is/are contained.

In the invention, technical modifications can be made in the configuration within a reaction chamber or reactor in order to realize a decoupling of the heating of the components to treatment temperature from the generation of the reactive species required for the diffusion treatment and at the same time to enable new flexible process parameter variations (degrees of freedom).

The new concept of an active hybrid system is based on steel-based and C-based plasma-exposed surfaces within the reaction chamber. This allows a defined surface modification of the respective components to be effectively and efficiently controlled during a plasma-supported thermochemical diffusion process. The device according to the invention has a preferably cylindrical cage, which is designed in the form of a steel mesh grid and is arranged around a work table and the components to be treated arranged thereon in the reaction chamber. A solid carbon disk is arranged at a predeterminable distance above the steel mesh grid and the work table with the help of electrical insulation or completely electrically separated.

In such an active hybrid system, the plasma-exposed solid carbon disk as a disk-shaped element generates concentrations of process-relevant reactive species for the diffusion treatment in a controllable form depending on the applied plasma power or plasma power density. The plasma-exposed, preferably hollow-cylindrical steel mesh grid serves to set the temperature without inertia and to further activate the reactive species generated on the plasma-activated solid carbon surface.

The active hybrid system is a new technological development for plasma-assisted thermochemical diffusion processes and can be used in both cold-wall and warm-wall reactors.

The invention offers flexible approaches for the use of the active hybrid system in reactors/vacuum chambers with regard to the energy management for the solid carbon disk as a disk-shaped element as well as the steel mesh grid and enables new degrees of freedom of the plasma and process parameters.

In industrial-scale reaction chambers, problems arise due to the limited lifetime of the generated reactive species. These particles may no longer be active when they reach the surface of the components to be treated, which can lead to unevenly treated components. Therefore, the use of plasma on the steel mesh can further activate the generated reactive species from the solid carbon disk and improve their reactivity at the component surface.

The plasma generated on the steel mesh grid can react without inertia to temperature inhomogeneities within the reaction chamber, guarantees temperature homogeneity and enables optimal and rapid temperature control.

The new technological invention/development can be used both for high-temperature treatment with temperatures up to 590 °C and especially for low-temperature treatment with temperatures < 400 °C.

In warm wall reactors, the desired treatment temperature is mainly achieved by the wall heating and the smaller part of the heat can be provided by plasma on the steel mesh grid for the final control of the treatment temperature. The reactive species generated at the carbon solid source can be further activated by the plasma on the steel mesh grid and thereby improve the reactivity at the component surface.

In cold wall reactors, the desired treatment temperature is mainly achieved by heating by plasma on the steel mesh grid. In addition, the plasma on the steel mesh grid can promote the generation of the active species further activated by the plasma on the solid carbon disk.

With the invention, both PN, PC and PNC processes can be realized.

Reaction chambers for plasma-assisted thermochemical diffusion treatment have the following components: reaction chamber with a pump system to maintain a gas pressure of approx. 10 Pa - 1000 Pa (approx. 0.1-10 mbar), gas supply for introducing reactive gases, electrical power supply for generating a glow discharge, if necessary an electrical power source for a separate wall heater, a work table and process control unit.

The invention will be explained in more detail below by way of example.

• 1a-d Beispiele für Vorrichtungen nach dem Stand der Technik;
• 2 ein Beispiel einer erfindungsgemäßen Vorrichtung;
• 3 ein weiteres Beispiel einer erfindungsgemäßen Vorrichtung;
• 4 ein drittes Beispiel einer erfindungsgemäßen Vorrichtung;
• 5 ein viertes Beispiel einer erfindungsgemäßen Vorrichtung und
• 6 ein fünftes Beispiel einer erfindungsgemäßen Vorrichtung.

Showing:

• 1a -d examples of devices according to the state of the art;
• 2 an example of a device according to the invention;
• 3 another example of a device according to the invention;
• 4 a third example of a device according to the invention;
• 5 a fourth example of a device according to the invention and
• 6 a fifth example of a device according to the invention.

In the 1a -d shows examples as they are known from the state of the art. 1a an example in which a cooling system 3a is integrated into the wall of the reaction chamber 12. A plasma is generated by means of the first electrical direct current source 5, which is connected to the work table 1 with the negative pole. The anode required for plasma generation is formed by the wall of the reaction chamber 12.

1b shows a warm wall reactor in which electrical resistance heating elements form a resistance heater 3 and are present on the wall of the reaction chamber 12. Otherwise, the structure corresponds to that according to 1a .

When in 1c The example shown is again a cold wall reactor with integrated cooling 3a in the wall. In addition, a cage 6a, which is made of a steel grid, is present in the reaction chamber 12 around the work table 1 and the components to be treated, which are arranged on a support frame 2. The cage 6a completely encloses the components on three sides. The work table 1 is connected to the fourth side. The cage 6a is connected to a separate second electrical power source 10. It serves both to heat the components and to generate the plasma with which reactive species are produced from a reaction gas that can be introduced via a gas inlet 4. The first direct current source 5 is again connected to the work table 1.

1d shows again a cold wall reactor in which cage 6b is formed with a carbon grid, which is again separately connected to the second electrical direct current source 10 and otherwise to the 1c example shown.

2 shows a first example of a device according to the invention. In this case, electrical resistance heating elements are integrated as electrical resistance heating 3 in the wall of the reaction chamber 12 and are connected with a positive pole to the first and second electrical direct current sources 5 and 10.

A support frame 2 is arranged on the work table 1, on which components (not shown) that are to be treated are arranged. The support frame 2 with the components is enclosed by a hollow cylindrical steel mesh grid 6. The steel mesh grid 6 is open on the vertically upward facing front side and is electrically connected to a negative pole of the first electrical power source 5.

Work table 1 and support frame 2 are separated from each other by an electrical insulator 8.

A disk-shaped element 9 made of carbon is arranged above the opening of the steel mesh grid 6 and is fastened to the wall of the reaction chamber 12 by means of ceramic electrically insulating elements 7 and 11. The disk-shaped element 9 and the steel mesh grid 6 are arranged at a distance from one another and are electrically separated from one another.

Reaction gas can be fed into the reaction chamber 12 in a metered form via the gas inlet 4 when the plasma-assisted diffusion process is to be carried out. The disk-shaped element 9 is electrically connected to a negative pole of the second direct current source 10. A positive pole of the second electrical current source 10 is connected to the resistance heater. An electrical current flows to the disk-shaped element 9 only when a sufficient temperature for the respective diffusion process has been reached on the components. The heating This is mainly done by means of the electrical resistance heating 3. It can be supported by glow discharge and/or generation of plasma on the steel mesh grid 6.

With the plasma from the steel mesh 6, the reactive species activated by the plasma on the disk-shaped element 9 can be further activated, whereby a uniform modification on the surfaces of the respective components can be achieved.

No negative BIAS voltage needs to be applied to the components.

The second in 3 The example shown differs from the first example only in that only a first electrical direct current source 5 with negative poles is connected both to the steel mesh grid 6 and to the disk-shaped element 9.

By means of a suitable control or regulation, a sequential generation of plasma can also be achieved with the disk-shaped element 9, in which the required component temperature has been reached by means of the electrical resistance heating 3, possibly supported by a plasma that can be generated at the steel mesh grid 6.

When in 4 In the third example shown, there are again a first and a second direct current source 5 and 10, which are connected with their negative poles to the steel mesh grid 6 and the disk-shaped element 9. Positive poles are connected to the electrical resistance heater 3.

The disk-shaped element 9 is placed on the opening of the steel mesh grid 6 in such a way that an electrically insulating element 7 is arranged between the steel machine grid 6 and the disk-shaped element 9, so that there is no electrically conductive connection. Here too, the plasma formation on the disk-shaped element 9 and steel mesh grid 6 can be regulated or controlled individually, so that it is possible to initially only generate plasma on the steel mesh grid 6. Only after a minimum temperature has been reached on the components can the disk-shaped element 9 be switched on and plasma generated there too in order to activate the reactive species. The plasma generated on the steel mesh grid 6 can then be used to transfer further energy to the excited species.

This in 5 The example shown corresponds in many respects to the third example. Only the second electrical direct current source 10 has been omitted and the steel mesh grid 6 and the disc-shaped element 9 are connected to the negative poles of the first direct current source 5 and can be connected in the same way as in the second example. 3 example shown.

This in 6 The fifth example shown again uses only a first direct current source 5. This is electrically connected with its negative pole to the steel mesh grid 6. The disk-shaped element 9 is arranged above the opening of the steel mesh grid 6 and can be moved in a translational manner, as indicated by the double arrow, using a preferably motor-operated adjusting mechanism 13. The distance between the disk-shaped element 9 and the vertical upper front edge of the steel mesh grid 6 at its opening can preferably be varied continuously.

The adjusting mechanism 13 is attached to the steel mesh grid 6 with an electrically insulating element 7, so that if a distance is maintained between the disk-shaped element 9 and the upper front edge of the steel mesh grid 6, there is no electrically conductive connection between these two elements. In this way, a plasma can only be generated on the steel mesh grid 6, which can also be used for additional heating of the components.

However, the disk-shaped element 9 can be moved towards the steel mesh grid 6 using the adjustment mechanism 13 until both elements touch and are then electrically connected to one another. In this position, plasma can also be generated on the disk-shaped element 9 and the respective species can be activated in the process. Additional activation of the species can be achieved with the plasma from the steel mesh grid 6. By appropriately designing the radially outer edge of the disk-shaped element 9, at least one gap can be left open at the opening of the steel mesh grid 6.

The examples shown and described here are warm wall reactors that are mainly heated using the electrical resistance heater 3. The heat of the plasma generated by the steel mesh grid 6 then contributes to a smaller part of the heating.

In the examples, it would also be possible to dispense with the electrical resistance heating 3 and operate a cold wall reactor accordingly. The heating of the components would then be achieved solely by means of the plasma generated by the steel mesh grid 6. The steel mesh grid 6 and the first or second direct current source 5 or 10 would only have to be dimensioned and operated accordingly.

In the figures, like elements are identified by the same reference numerals. A plasma-assisted thermo-chemical diffusion treatment process is usually carried out in several steps such as heating, sputtering, diffusion treatment and cooling. The step of diffusion treatment at a specific time and temperature is considered to be the most important step of the entire process. Depending on the chemical composition of the component to be treated and the final goal of the treatment, the temperature during the diffusion treatment can be varied. Therefore, depending on the temperature applied during the diffusion treatment, two key technologies can be used: low-temperature and high-temperature technology. Low-temperature technology in the range of 400 °C - 460 °C is mainly used for high-alloy steels, such as stainless steels. Here, the main goal of the treatment is the formation of a modified layer (expanded phase) close to the surface of the treated component as a result of N and/or C diffusion.

For example, when performing a PNC diffusion treatment of an AISI 316L component using a low-temperature process (440 °C), a device with two direct current sources 5 and 10 can be used.

The at least one component is arranged on a work table within the reaction chamber 12 and is electrically insulated therefrom.

An internal pressure of 0.1 mbar is set in the reaction chamber 12 in order to carry out a leak test.

The heating phase is carried out using an electrical resistance heater 3 arranged on the wall of the reaction chamber 12 in order to reach a temperature of 380 °C, which can be measured using thermocouples on the support frame 2. 30 l/h of H ₂ at a pressure of 1.5 mbar are introduced into the reaction chamber 12 via the reaction gas supply 4. A temperature of approximately 450 °C is reached at the electrical resistance heater 3 on the wall.

After reaching the temperature of 380 °C, at an H ₂ flow of 30 l/h and an Ar flow of 3 l/h at 1 mbar, an electrical voltage of 380 V - 450 V is applied to the steel mesh grid 6 with an effective surface of about 4 m ² , whereby a negative glow discharge occurs on the steel mesh grid 6. In this case, the support frame 2 is kept at a sliding electrical potential by means of a ceramic insulating element 8. In this step, the measured temperature on the support frame 2 is increased to 420 °C due to the plasma radiation, while the temperature of the electrical resistance heater 3 on the wall is slightly reduced from 450 °C to 410 °C. Therefore, the final temperature on the loading rack 2 is controlled by using both the wall heater and the plasma. Such a condition results in a plasma current of about 12 A - 16 A and a plasma power of about 1.8 kW - 2.5 kW. It is worth noting that in this step, the second DC electric source 10 connected to the disk-shaped element 9 as a solid carbon source is turned off.

The condition described above is maintained for 30 minutes at a constant temperature of 420 °C in order to clean and activate the surface of the components to be treated.

The PNC treatment step (diffusion of N and C into the surface of the components) is initiated under the following conditions: desired reaction gas mixture N ₂ :H ₂ , pressure of 2 mbar - 3 mbar, 400 V - 450 V and 30 A - 35 A at the first electrical direct current source 5, a plasma power of 3.5 kW - 4.5 kW and final treatment temperature of 440 °C. In addition, the carbon is supplied by chemical sputtering to the disk-shaped element 9 as a carbon source made of carbon-based materials (such as CFC or graphite). The disk-shaped element 9 is attached to the top of the reaction chamber 12 with the aid of holding elements 11 and electrically insulating element 7 made of a ceramic with an effective surface area of 0.2 m ² , which is connected to the feedthrough of the second electrical direct current source 10. An electrical voltage of 350 V - 400 V is applied to the disk-shaped element 9, resulting in an electrical current flow of about 4 A - 6 A and a plasma power of about 0.5 kW - 0.6 kW. The concentrations of the process-relevant molecular species produced are high enough to form a uniform and thick diffusion layer obtained by plasma nitrocarburizing near the surface of the treated components. In this step, the temperature of the electrical resistance heating element 3 on the wall is slightly reduced from 410 °C to 380 °C due to the higher contribution of the plasma heating.

Claims (15)

Device for carrying out plasma-assisted thermochemical diffusion processes on metallic components, wherein at least one metallic component is arranged on a work table (1) within a reaction chamber (12) and enclosed by a steel mesh grid (6) which Steel mesh grid (6) has an opening in the vertical direction above the component(s) and is connected to a negative pole of a first or second electrical direct current source (5, 10), wherein a disk-shaped element (9) is arranged above the opening, which is formed with carbon or consists entirely of carbon, wherein the disk-shaped element (9) is connected or can be connected to a negative pole of the first or second electrical direct current source (5, 10) and an internal pressure is maintained in the reaction chamber (12) which is lower than the ambient pressure and a plasma treatment with reactive species which have been formed by means of a reaction gas fed into the reaction chamber (12) via at least one gas inlet (4) can be carried out. Device according to Claim 1 , characterized in that the disc-shaped element (9) is arranged at a distance from the opening of the steel mesh grid (6) and is electrically insulated (7, 11) from the steel mesh grid (6) and the wall of the reaction chamber (12). Device according to Claim 1 or 2 , characterized in that the distance of the disc-shaped element (9) to the vertically upper front edge of the steel mesh grid (6) can be changed by means of an adjusting mechanism (13). Device according to the preceding claim, characterized in that the distance is preferably continuously variable starting from a predetermined maximum distance up to a direct contact of the vertically upper front edge of the steel mesh grid (6), at which an electrically conductive connection between the disk-shaped element (9) and the steel mesh grid (6) is achieved. Device according to one of the preceding claims, characterized in that the disc-shaped element (9) is exchangeable and preferably disc-shaped elements (9) with different dimensions can be used. Device according to one of the preceding claims, characterized in that the at least one metallic component on the work table (1) is arranged electrically insulated from the work table (1) and is not connected to any electrical power source. Device according to one of the preceding claims, characterized in that the wall of the reaction chamber (12) is connected to earth potential or a positive pole of an electrical direct current source. Device according to one of the preceding claims, characterized in that an electrical resistance heater (3) is arranged on the wall of the reaction chamber (12) or is integrated therein and is connected to a positive pole of the first or second electrical direct current source (5, 10). Method for carrying out plasma-assisted thermochemical diffusion processes on metallic components with a device according to one of the preceding claims, characterized in that an electric current flows to the disk-shaped element (9) only when a plasma-assisted thermochemical diffusion process is to be carried out and a temperature sufficient for this purpose has been reached by means of the electrical resistance heater (3) within the reaction chamber (12). Method according to the preceding claim, characterized in that the electric current flow to the disk-shaped element (9) is regulated and/or its distance from the vertically upper front edge of the steel mesh grid (6) is adjusted depending on the process gas required for the diffusion treatment. Method according to one of the preceding claims, characterized in that plasma carburizing, plasma nitriding or plasma nitrocarburizing is carried out in the reaction chamber (12) as a plasma-assisted thermochemical diffusion process and in the case of plasma carburizing H ₂ , in the case of plasma nitriding N ₂ and in the case of plasma nitrocarburizing a mixture of H ₂ and N ₂ is supplied as reaction gas. Method according to one of the preceding claims, characterized in that a plasma-assisted thermochemical diffusion process is carried out with the supply of the corresponding reaction gas at a constant temperature and constant pressure in the reaction chamber (12). Method according to one of the preceding claims, characterized in that the plasma-assisted thermochemical diffusion process is carried out with alternating supply of different reaction gas compositions with a supply of N ₂ for plasma nitriding, with a supply of H ₂ for plasma carburizing and a mixture of N ₂ and H ₂ for plasma nitrocarburizing with an electric current flow to the disk-shaped element (9). Method according to one of the preceding claims, characterized in that When carrying out a plasma-assisted thermochemical diffusion process, the type of process-relevant species is controlled or regulated by influencing the electric current flow to the disk-shaped element (9). Method according to one of the preceding claims, characterized in that at least one component made of a high-alloy steel, a low-alloy steel or an unalloyed carbon steel is treated with the corresponding plasma-assisted thermochemical diffusion process.

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