Method for performing pulsed electric arc discharges under vacuum conditions

Abstract

In the method for carrying out pulsed electric arc discharges under vacuum conditions, at least one electric arc discharge is initiated between a cathode and an anode in that a laser beam is directed from a laser radiation source onto a surface of the cathode in order to form a starting plasma, and then, after formation of a starting plasma, by means of an electric voltage with an electric current, which is applied to the cathode and the anode, which in turn are arranged at a distance from one another, an electric arc discharge is ignited between the cathode and anode, wherein the igniting of the electric arc discharge is triggered by providing a starting plasma with a focused laser beam by means of a pulse sequence, consisting of at least two laser pulses emitted consecutively from the laser radiation source.

Description

[0001] Method for performing pulsed electrical arc discharges under vacuum conditions

[0002] The invention relates to a method for performing pulsed electrical arc discharges under vacuum conditions.

[0003] A wide variety of thin layers, particularly metal and carbon layers, can be produced using the laser arc vacuum arc process. A laser radiation source creates a starting plasma, which determines the precise ignition position and timing of a pulsed high-current arc discharge of up to several kiloamperes on the surface of the material to be evaporated (cathode). The laser radiation source serves only to ignite electrical arc discharges using a briefly generated starting plasma and contributes little or nothing to the formation of a coating plasma. This plasma, which serves to form a coating, is generated almost exclusively by the electrical discharge between a cathode and an anode.

[0004] One possible embodiment of laser arc evaporation is the use of a rotating cylindrical cathode, or several segments thereof, in combination with scanning the rotating cathode surface with the laser beam. The individual laser pulses, which usually have a pulse duration of 50 - 100 ns each, are emitted synchronized with the high-current discharge, thus creating a line plasma source. By using high frequencies (e.g. 1200 Hz), with which individual laser pulses are used to ignite a starting plasma, a homogeneous plasma is created across the entire line, which can be used for coating. The laser pulses, which are directed locally onto the cathode surface, thus serve to ignite a starting plasma, which in turn is used to ignite an electrical arc discharge.To achieve the most uniform removal of cathode material from the cathode surface, the electric arc current is switched off after a specified time, allowing the plasma generated by the electric arc discharge to extinguish before the process is repeated at various locations on the cathode surface. Thus, appropriately pulsed electric arc discharges are ignited successively at various positions on the surface by appropriately directing a laser pulse each, and then extinguish again after a specified period of time.

[0005] The laser arc process is fundamentally established and is now used in industrial production, primarily for the manufacture of diamond-like, tetrahedral amorphous carbon layers. A suitable device and method are described, for example, in DE 198 50 218 CI. Such a process, and other known analogous systems, can also be used in the invention, either as is or with slight modifications.

[0006] A constant challenge is the use of comparatively expensive lasers, a high optical alignment effort and the necessary compliance with tight optical tolerance limits in order to achieve the necessary laser energy to trigger the individual high-current arc discharges.

[0007] To date, diode- or flashlamp-pumped, Q-switched Nd-YAG lasers with a wavelength of 1064 nm have typically been used, which have a laser beam quality with a beam parameter product of less than 5 mm*mrad. The individual laser pulses deliver an energy of approximately 15 mJ each. The requirements are very high because the process is very sensitive to reduced pulse energy. If the laser pulse energy drops below 10 mJ or if the laser beam quality deteriorates, the probability of a high-current discharge between the respective cathode and anode decreases rapidly. This makes stable coating operation in industrial production impossible. Furthermore, these laser types degrade within a few years, resulting in high repair costs.

[0008] It is therefore the object of the invention to provide possibilities for reducing the costs for the acquisition and long-term operation of appropriately designed systems and at the same time to ensure and, if necessary, even increase operational safety.

[0009] According to the invention, this object is achieved by a method having the features of claim 1.

[0010] Advantageous embodiments and further developments of the invention can be realized with features specified in dependent claims.

[0011] In the method according to the invention, at least one electrical arc discharge is initiated between a cathode and an anode, which are arranged at a distance from one another, by directing a (typically focused) laser beam from a laser radiation source onto a surface of the cathode for ignition and to form a starting plasma and then, after formation of a starting plasma, by means of an electrical voltage with an electrical current which is applied to the cathode and the anode, an electrical arc discharge is ignited between the cathode and the anode, wherein the ignition of the electrical arc discharge is triggered by providing a starting plasma with a focused laser beam by means of a pulse sequence emitted by the laser radiation source, consisting of at least two laser pulses emitted one after the other.

[0012] With the help of the starting plasma, the electrical resistance between cathode and anode can be reduced, which is used to ignite an electrical arc discharge.

[0013] The electrical voltage applied to the cathode and anode and the corresponding electrical current flow required for the formation of an electrical arc discharge can thus be reduced by means of the starting plasma on the surface of the cathode, and the arc discharge can be ignited. Theoretically, an electrical voltage could be permanently applied to the cathode and anode that is sufficient to generate an electrical arc discharge on the surface of the cathode with the existing starting plasma. By sufficiently reducing the electrical power provided by the electrical power source, for example by discharging an electrical capacitor, or by temporarily reducing the electrical voltage at the electrical power source, the electrical arc discharge can be extinguished.

[0014] Before igniting a new electrical arc discharge, which should preferably occur at a different position at a distance, a pulse sequence can be emitted from the laser radiation source onto the cathode surface again to generate a starting plasma with which the ignition of another electrical arc discharge at this position can be achieved. When carrying out the method, a pulse sequence with at least two, preferably at least three, particularly preferably at least five, but also more, laser pulses emitted one after the other is directed onto a position on the surface of the cathode to form a starting plasma.

[0015] The laser radiation source can be operated in such a way that an energy of less than 10 mJ, preferably less than 7.5 mJ, is maintained for each individual laser pulse.

[0016] Advantageously, the total energy of the laser pulses during the initial steps, i.e., preferably the steps for igniting the electric arc discharge, can be maintained at least 10 mJ, preferably at least 15 mJ. The laser beam quality should have a beam parameter product of less than 1 mm*mrad.

[0017] The laser radiation source used can be operated at a frequency of at least 10 kHz up to more than 1000 kHz, preferably greater than 50 kHz.

[0018] For the individual laser pulses when carrying out the first steps, a pulse length of at least 2 ns to a maximum of 500 ns should be maintained, with pauses between individual laser pulses of between 100 ns and 10 ps being observed.

[0019] Advantageously, a gas, solid-state or dye laser, in particular a solid-state laser designed as a fiber laser, can be used as the laser radiation source.

[0020] After the thus generated electrical arc discharge has been extinguished, another pulse sequence consisting of at least two successively emitted laser pulses can be directed at another spaced-apart position on the surface of the cathode to ignite another electrical arc discharge using another starting plasma. This can be repeated several times. This ensures that a uniform removal of cathode material is achieved on the surface of the cathode, which in turn leads to stable, long-term operation of a corresponding coating process. The invention advantageously uses a cylindrical cathode, which preferably rotates during the process. This makes it possible to achieve a uniform application of material over a correspondingly large surface area of ​​the cathode.

[0021] The method can be used to form a coating on the surface of substrates as a result of the ions of the cathode material released in the plasma by the electrical arc discharges, which coating is particularly advantageously formed with diamond-like tetrahedral amorphous carbon.

[0022] Thus, with a cathode consisting of graphite or predominantly graphite, a coating of graphite-like or diamond-like amorphous carbon or with a cathode consisting of a metal or a mixture of metals, a coating of metal, metal mixtures or by adding a reactive gas made of nitridic or oxidic material or with a cathode consisting of a sulfidic material, e.g. molybdenum disulfide or tungsten disulfide, a coating of this sulfidic material can be formed on substrate surfaces.

[0023] For the electrical arc discharges, electrical currents of at least 200 A up to a maximum of 5 kA and electrical voltages of at least 20 V can be used. The individual electrical arc discharges should be operated for a time between 100 ps and 2000 ps. In an exemplary design, the arc currents are 1600 A and the arc discharge pulse duration is 330 ps.

[0024] The individual laser pulses emitted by the corresponding laser radiation source can be deflected using known reflective elements arranged in the beam path of the laser pulses so that they impinge on the cathode surface at the respective, changing positions and generate a starting plasma there to ignite an electrical arc discharge. The inventive approach achieves stable ignition of the electrical arc discharges between a cathode and an anode, which can be used to coat substrates, with significantly lower requirements for the laser beam sources used than is the case with conventional single-pulse ignition. An inexpensive standard laser can be used. The use of a fiber laser, which is very robust and can be operated with long-term stability, is particularly advantageous.A significant change in beam properties over its lifetime is not to be expected with this type of laser beam. Integration requires no changes to the scanner system. Furthermore, this type of laser beam offers significant power reserves and can be operated well below its maximum power, which has a positive effect on an extended maximum operating time.

[0025] The invention can be used in laser arc coating systems, particularly for the deposition of diamond-like carbon coatings of the ta-C type, especially for friction reduction and wear protection on automotive sliding components and tools. Further applications, including large-area coatings of fuel cell and electrolyzer components, are also possible in the future. Metallic coatings and arc coatings with reactive gas, i.e., metal nitrides and metal oxides, are also possible using the invention.

[0026] The invention will also be better explained with reference to the embodiments shown in the figures.

[0027] Showing:

[0028] Figure 1 is a diagram showing the temporal progression of the energies of the four individual laser pulses used to generate the starting plasma, each with a laser pulse power of 4 mJ, and the electric current flow in A, of an electric arc discharge that can be generated in this way, with the ignition phase also being shown enlarged and

[0029] Figure 2 shows another correspondingly analogous diagram with a different pulse sequence of almost 50 laser pulses emitted one after the other to generate a starting plasma.

Claims

Patent claims 1. A method for carrying out pulsed electrical arc discharges under vacuum conditions, in which at least one electrical arc discharge is initiated between a cathode and an anode by directing a laser beam from a laser radiation source onto a surface of the cathode to form a starting plasma and then, after forming a starting plasma, by means of an electrical voltage with an electrical current which is applied to the cathode and the anode, which are arranged at a distance from one another, the electrical arc discharge is ignited between the cathode and the anode, wherein the ignition of the electrical arc discharge is triggered by providing a starting plasma with a focused laser beam by means of a pulse sequence consisting of at least two laser pulses emitted one after the other by the laser radiation source.

2. Method according to claim 1, characterized in that the laser radiation source is operated in such a way that an energy of less than 10 mJ is maintained for each individual laser pulse.

3. Method according to one of the preceding claims, characterized in that a sum of the energies of the laser beam pulses which are used in each of the first steps is maintained at at least 10 mJ, preferably at least 15 mJ.

4. Method according to one of the preceding claims, characterized in that the laser radiation source is operated at a frequency of at least 10 kHz up to 10 MHz, preferably from 750 kHz to 4.5 MHz.

5. Method according to one of the preceding claims, characterized in that for the individual laser pulses when carrying out the first steps, a pulse length of at least 2 ns to a maximum of 500 ns is maintained, with pauses between the individual laser pulses of between 100 ns and 10 ps being maintained.

6. Method according to one of the preceding claims, characterized in that after the electrical arc discharge has been extinguished, a further pulse sequence consisting of at least two successively emitted laser pulses is directed at another position of the surface of the cathode arranged at a distance in order to ignite a further electrical arc discharge by means of a further starting plasma.

7. Method according to one of the preceding claims, characterized in that a solid-state laser, a gas laser or a dye laser, in particular a solid-state laser designed as a fiber laser, is used as the laser radiation source.

8. Method according to one of the preceding claims, characterized in that a coating is formed on the surface of substrates using ions of the cathode material released as a result of the electrical arc discharges in the plasma.

9. Method according to the preceding claim, characterized in that with a cathode consisting of graphite or predominantly of graphite, a coating of graphite-like or diamond-like amorphous carbon on substrate surfaces or with a cathode consisting of a metal or a mixture of metals, a coating of metal, metal mixtures or by adding a reactive gas of nitridic or oxidic material on substrate surfaces or with a cathode consisting of a sulfidic material, in particular molybdenum disulfide or tungsten disulfide, a coating of this sulfidic material is formed on substrate surfaces.

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