JP2009167483A - Metal surface modification method with the use of plasma electron gun and apparatus therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for irradiating a metal surface with an electron beam pulse, which inhibits a crater from being formed, because it has been difficult to prevent the formation of the crater that appears on a modified surface due to deep part penetrating properties and an electron range of the electron beam pulse, though an electron beam irradiation equipment using a plasma electron gun provides an excellent effect of surface modification because of simultaneously irradiating a relatively large area with a single pulse, and is used in a post-process of a grinding step or an electric discharge machining step. <P>SOLUTION: This irradiation method irradiates the metal surface with the plasma electron gun having an energy waveform controlled to such a step form that a leading portion of the electron beam pulse has a high accelerating voltage in a short period of time, and the following portion of the electron beam pulse which continuously follows the leading portion has a low accelerating voltage in a long period of time. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention is in the field of metal surface treatment using electron beam pulses, and belongs to the field of surface modification techniques such as surface cleaning, smoothing, mirroring, and amorphization, and more specifically, electron beam pulses by a plasma electron gun. It is related with the method and apparatus for improving the waveform of this and obtaining a quality improvement surface.

Conventionally, the electron beam technology is widely used for welding, drawing, and microfabrication. In PVD (Physical Vapor Deposition), except for the example used as a means for evaporating the target metal material, it can be used for direct surface modification. It was never used. In recent years, plasma electron guns have been developed, and surface modification has been carried out by electron beam pulses passing through a low piezoelectric gas separation plasma.

FIG. 4 is a schematic explanatory diagram of a surface modification apparatus using a conventionally known plasma electron gun.

In FIG. 4, a cathode 1 having a radiation area of several cm ² and an irradiated body 3 are placed facing each other in a housing 4 filled with a low-pressure ionized gas, for example, Ar, and the Ar is placed in a space between the opposing axes. An annular electrode 2 that causes glow discharge to float into gas is formed coaxially. A solenoid 5 is provided on the outside of the housing 4 so as to create a magnetic field line parallel to the axis. The exhaust pump 9, the molecular pump 10, the Ar cylinder 8, the ionization vacuum gauge 7, the switching valves 9a, 9b, 9c, and the control valve 8a are connected as shown in the figure for adjusting the atmospheric pressure and the Ar concentration in the housing. . The entire system is centrally controlled by the controller 20.

After the gas state is maintained at a specified state of 0.1 Pa or less, the solenoid 5 is excited by a power source 15 to create the magnetic lines of force, and then a high voltage pulse is applied to the annular electrode 2 by the pulse power source 12 to cause glow discharge. The opposing space is turned into plasma. Next, when the electron beam pulse emission power source 11 is activated and an acceleration voltage pulse of several tens of kV is applied between the cathode 1 and the irradiated object 3, an electron beam pulse is emitted. A solenoid current sensor 5b, a plasma current sensor 2b, and a cathode current sensor 1b are provided as shown in the figure to advance the sequence of the entire system, and transmit respective signals 5c, 2c, and 1c to the controller 20.

Since an electron beam irradiation apparatus using a plasma electron gun irradiates a relatively large area (about 30 cm ² ) simultaneously with a single pulse, an excellent modification result can be obtained, and it is used in subsequent processes of grinding and electric discharge machining. .

Electron beam used in an electronic beam irradiation apparatus using a plasma electron gun indicates the character of this technique plainly is abbreviated as LEHCEB (L ow E nergy H igh C urrent E lectron B eams). This abbreviation is to be compared only with a conventionally converged electron beam in a vacuum, and is not necessarily low energy and high current as compared with other methods such as electric discharge machining. Since the action of the electron beam on the metal surface is quantum and not just a thermal action, the phenomenon needs to be considered independently of the others.

The LEHCEB will be briefly described. An electron gun having a cathode surface and a target surface facing each other is filled with a low piezoelectric separation gas such as Ar, and glow discharge is caused by another means to generate plasma. Further, magnetic flux of magnetic force perpendicular to the facing surface is generated by another means. At this time, when a DC high voltage pulse of several tens kV (pulse width of several μs) is applied between the cathode (cathode) and the target (anode), a bundle of electron beams having substantially the same cross-sectional area as the cathode surface from the cathode to the target. A pulse is fired at the target. The energy per area of the bundled electron beam pulse reaches about 10 J / cm ² .

The generation mechanism of the electron beam pulse bundle is a high potential gradient of the electric double layer at the interface between the Ar plasma and the cathode, and suddenly accelerated ions knock out electrons from the cathode, causing an avalanche in the Ar plasma. While hitting the target. At this time, the space potential between the two electrodes is mainly the cathode drop at the front surface of the cathode, and the drop at the plasma space and the anode is small. Therefore, when the energy per area (J / cm ² ) of the electron beam pulse bundle is measured between the cathode and the target, there is almost no change.

The surface of the target is irradiated and irradiated with an electron beam pulse, and is dissolved and vaporized to remove and clean the surface oxides, melt and smooth the unevenness, and make the surface layer amorphous (amorphous).

Consider the mechanism of surface modification by the electron beam pulse in the previous stage. As an example, when a 2 μF capacitor is charged to 30 kV with a plasma electron gun and the target is irradiated with an electron beam pulse, the discharge energy is 900 J. According to the experiment, when a 6 cm diameter (area 28 cm ² ) disk is used as the cathode, the effective diameter of the electron beam is about 6 centimeters. When the energy density at each point in the cross section is measured and averaged, it is about 10 J / It is about cm ² and there is no metal modification ability outside the effective diameter. Since the effective cross-sectional area is 28 cm ² , the total energy of the electron beam pulse is 280 J, and 280 J corresponding to about 30% of the discharge energy of 900 J becomes the energy of the electron beam, and the remainder becomes the cathode loss at the time of launch. That's right. Expressing the energy that was effective in units of electron volts,
Since 1J = 6.241 × 10 ¹⁸ eV, the energy density (about 10 J / cm ² ) of the measured electron beam is expressed in units of electron volts.
10 J / cm ² = 6.241 × 10 ¹⁹ eV / cm ² -------- (Equation 1)
It becomes.

  When the electron beam is irradiated to the metal surface to be modified, the exposed part is dissolved and vaporized to be modified, but the electrons penetrating the surface atomic lattice network exchange energy in the deep part. It becomes heat and melts and vaporizes deep metal, creating a crater on the surface.

If the metal lattice spacing is about 10 ⁻⁸ cm, it is much larger than the electron diameter (10 ⁻¹³ cm), so that it penetrates without losing energy. When the energy of the penetrating electron beam is calculated according to (Equation 1) calculated in the previous stage, it is estimated to be several to several hundred keV, and this energy is melted and vaporized and ejected around a depth of several μm to form a nozzle. An amount of heat of several to several hundred keV is a small value, but the electron beam penetrating into the deep part of the lattice gap vaporizes and ejects the local part to form a nozzle hole on the surface. This phenomenon is the peculiarity of the electron beam pulse, which is further explained by the electron penetration range depth described later, but it is a mechanism different from the crater phenomenon by other machining methods such as electric discharge machining and laser machining. .

The surface modified surface by the electron beam irradiation apparatus is smoothed, mirrored, and amorphized, and the effect is recognized. However, as a defect, a large crater visually recognized with a microscope or the naked eye is several tens of μm. The diameter and depth of several μm. The location where the nozzle hole is generated is not specified, but there is almost no exception to aligning along the residual stress line of the rolling process performed when the material is produced, and the origin is strongly attributed to lattice defects. Suggests. Such deep penetration of an electron beam is advantageous when the purpose is to quench the surface of a steel material, but is a disadvantage when a smooth surface or mirror surface is required.

In conventionally known narrowed electron beam processing, electrons penetrate to the electron penetration range depth due to deep penetration, and melt and vaporize the metal to generate a crater. It is well known that the fine limit of electron beam processing cannot be made below the electron range, but the origin of the throat that appears on the modified surface by the electron beam pulse of the present invention is also explained easily by this electron range. Can do.

Regarding the electron range (R), the acceleration voltage (V) and the density (ρ) are R = 2.2 × 10 ⁻¹² V ² / ρ (cm) −−−−−−−−− (Equation 2)
For example, when 5 keV energy penetrates into the lattice gap, the so-called atomic temperature is raised to a depth of about 7 μm in steel.

The process of modifying a metal surface by an electron beam irradiation apparatus using a plasma electron gun usually achieves its purpose by irradiating several to several tens of electron beam pulses. In the course of this process (number of irradiations), the surface undergoes processes of cleaning, smoothing, and focusing.

[Cleaning process]
The surface of the sample is covered with an oxide film, and foreign matter is sinking in the valleys of the irregularities. This is a process in which foreign matter, grinding, buffing, lapping abrasive grains, etc. that cannot be removed by a method such as ultrasonic cleaning are removed. These are removed by the first 1-2 exposures. In this process, the surface is cleaned and activated. When the surface roughness is measured, the surface roughness increases conversely.

[Smooth process]
Smoothing progresses by melting the top of the uneven surface roughness. Smoothness is a microscopic range and does not mean that large unevenness is made uniform. Mirroring and amorphization proceed while leaving the original large irregularities. During this process, the crater becomes visible. Some craters disappear and others grow with each irradiation. The surface properties obtained in this process are glossy and strong in rust resistance.

[Focusing process]
It is modified so deeply that it loses its original shape on the surface, and the metal crystal becomes coarse and loses its strength.

Japanese Patent No. 3202244 JP 2003-111778 A JP 2004-1086 A JP 2007-5011 A

The electron beam irradiation apparatus converts an inert ionized gas such as Ar into plasma in a container and emits an electron beam pulse from a cathode toward a target. For this purpose, the plasma conditions, the lines of magnetic force in the container, and the high voltage pulse applied to the cathode must satisfy appropriate conditions. If any one of the conditions is not appropriate, the electron beam pulse is not fired and ends up in a simple air discharge. In particular, the condition for ensuring the generation of the electron beam pulse is that a double layer having a sufficiently high potential gradient is formed at the boundary between the cathode front surface and the plasma before the DC high voltage pulse is applied.

Since there are restrictions as in the previous stage before the emission of the electron beam pulse, it is difficult to obtain a free choice of waveform and energy. For example, even in the case where only the pulse width is taken, in the circuit in which a plurality of elements in the high voltage DC circuit are made variable, the selection and cost of the changeover switch element are limited. Further, regarding the magnitude of the pulse energy, there is a circumstance that the degree of freedom is small because there is a restriction on the lower limit of the pulse voltage applied to the cathode. Therefore, in the conventional surface modification apparatus, the highest priority is to be able to emit an electron beam pulse, and there is no room for considering the pulse waveform and pulse energy.

The measures for the mouth opening described in the previous paragraphs [0012]-[0017] suggest that the nature of the electron beam pulse must be selected according to the characteristics of the irradiated metal. The characteristics include pulse energy, pulse time width, pulse waveform, and the like. In addition, the character must be within a range that allows generation of an electron beam pulse.

In the present invention, the cause of the occurrence of the above-mentioned phlegm is that the electron penetrates to the electron penetration range depth due to the deep penetration of the electron beam, dissolves the metal, and vaporizes and generates the crater. Therefore, it is proposed to change the waveform of the electron beam pulse according to the phenomenon, but of course, the conditions under which the electron beam can be reliably emitted are also considered.

According to a first aspect of the present invention, the cathode surface and the target irradiation surface are made to face each other along the axis of the electron gun, and the low piezoelectric separation gas in the space surrounded by the both surfaces is converted into plasma, and the magnetic force lines parallel to the axis And a metal surface modification method in which a direct current high voltage pulse is applied to the cathode and an electron beam pulse is emitted toward the target surface through the plasma.
The electron beam pulse is an electron beam pulse having a stepped energy waveform in which the leading portion of the electron beam pulse has a high acceleration voltage and a short time width, and the subsequent succeeding portion has a low acceleration voltage and a long time width. The present invention seeks to solve the above-mentioned problems by providing a metal surface modification method.

According to a second aspect of the present invention, the cathode surface and the target irradiation surface are made to face each other along the axis of the electron gun, and the low piezoelectric separation gas in the space surrounded by the both surfaces is turned into plasma, and the magnetic force lines parallel to the axis And an apparatus comprising a power source that applies a direct-current high-voltage pulse to a cathode and emits an electron beam pulse toward the target surface through the plasma,
The power source is a circuit (A) including a capacitor C1 for applying a pulse voltage P1 having a high acceleration voltage and a short time width, and a closing switch S1,
A capacitor C2 for applying a pulse voltage P2 having a long time width with a low acceleration voltage and a circuit (B) comprising a closing switch S2,
As a result of the continuous application of the short pulse voltage P1 and the long pulse voltage P2 to the cathode, the circuit (A) generates an electron beam pulse Q1, and the circuit (B) continuously generates an electron beam pulse. An object of the present invention is to provide a metal surface reforming apparatus characterized in that Q2 is generated and the metal surface is reformed by an electron beam pulse Q12 having a stepped energy waveform.

  According to a third aspect of the present invention, a current sensor is inserted into the circuit (A), and the circuit (B) is detected via a signal obtained by detecting the pulse current of the circuit (A) due to the closing of the closing switch S1. It is proposed to ensure the cooperative operation of the circuit (A) and the circuit (B) by closing the circuit closing switch S2.

Considering the mechanism of the hole generation from the phenomenon of electron range, there is no doubt that the waveform of the electron beam pulse is a major factor. As shown in the equation (2), the acceleration voltage is a determinant of the electron range. However, if the applied voltage is lowered or the pulse width is reduced to reduce the energy, the nozzle is certainly suppressed, but the reforming efficiency is extremely lowered, or the electron beam pulse cannot be emitted. turn into. Therefore, the idea of changing the waveform of the electron beam pulse appears, but because the high voltage is handled and the emission conditions of the electron beam pulse itself are subject to a number of constraints, the waveform of the waveform can be adjusted with a single power source. It was considered difficult to deform.

In order to keep the emission probability of the electron beam pulse high and to suppress the generation of the nozzle, on the other hand, the contradictory conditions must be satisfied at the same time. One is application of a high voltage pulse and the other is application of a low voltage pulse. As a method for satisfying the contradictory conditions as described above, “a waveform in which a high-voltage short-width pulse is placed as a start pulse and a low-pressure long-width pulse is subsequently placed as an action pulse”, that is, an irregular pulse is applied to the cathode.

In a conventionally known electron beam irradiation apparatus, a high-voltage DC pulse applied to the cathode discharges the charge charged in one capacitor. In order to stably generate the electron beam pulse, the high voltage pulse applied to the cathode cannot be lowered below a certain limit. On the other hand, increasing the applied voltage makes it easy to generate a throat and increases the depth and diameter of the throat.
Therefore, in the present invention, a two-step pulse waveform is proposed as an unusual pulse different from the conventional one, and the surface modification is performed by a method of “a waveform in which a high-pressure short-width pulse is placed as a start pulse and a low-pressure long-width pulse is subsequently placed as an action pulse”. The occurrence of shed is suppressed by executing.

By using the electron beam pulse of the present invention, it is possible to suppress the formation of a mouth opening on the metal surface to be modified, and even if the mouth opening occurs, the diameter is small and the depth is shallow.

FIG. 1 is an explanatory diagram showing a specific configuration of the present invention. The cathode 1 and the target 3 to be processed are disposed inside the housing 4 so as to face each other, and the interior of the housing 4 is evacuated by a vacuum system 20 including an exhaust pump and a molecular pump. The gas is sucked and kept at about 0.1 Pa.

At this time, when the solenoid 5 provided on the outside of the housing 4 is excited by the power source 15, an axial magnetic field line is created in the space between the cathode 1 and the target 3 and the plasma is confined and held in the space. Ready to do.
An annular reflective discharge electrode 2 is provided to turn low-pressure Ar gas into plasma. When a DC high-pressure pulse is applied between the annular electrode and the housing 4 from an independent pulse generation power source 12, glow discharge occurs. The indoor Ar gas is excited and turned into plasma. Since the magnetic field lines in the axial direction are formed in the space between the cathode 1 and the target 3 to be processed, the plasma is confined and held in the space.

When prepared as described above, an electron beam pulse is generated by the power supply 90 that applies a DC high-voltage pulse between the cathode 1 and the target 3 to be processed. In the modification method and apparatus of the present invention, the electron beam pulse is improved to obtain a good surface.

FIG. 3 is a time chart of each element until generation of an electron beam pulse in the method and apparatus for solving the problems relating to the electron beam surface modification. The horizontal axis represents time, and the vertical axis represents current (i), voltage (V), and energy (eV) by each element.
FIG. 4A shows an excitation period by a solenoid for giving a magnetic flux in the electron gun.

FIG. 2B shows an operation period of the power source 12 for generating glow discharge from the annular electrode 2 for generating and maintaining plasma in the electron gun.

FIG. 7C shows the timing and voltage when the circuit (A) is applied with the start pulse voltage P1 to the cathode 1 by the closing of the closing switch S1. The start pulse voltage P1 needs to be an acceleration voltage sufficient to cause an electron avalanche in the plasma in the electron gun, and a specific numerical value is an applied voltage of 30 kV or more. The pulse width of the start pulse voltage P1 is preferably short in order to maintain the quality of the surface modification, and a specific numerical value is 3 μs or less.

FIG. 4D is an output signal of a current sensor inserted in the circuit (A), a signal output in response to a current (electron beam pulse from the cathode 1) generated by the closing of the closing switch S1, and an operation time. In consideration of the delay, the closing switch S2 of the circuit (B) is operated.

FIG. 5E shows the timing and voltage when the circuit (B) applies the action pulse voltage P2 to the cathode 1 by the closing of the closing switch S2. The working pulse voltage P2 should be an accelerating voltage that supplies sufficient electron beam energy for the surface modification of the irradiated object 3, and is naturally selected to be lower than the pulse voltage P1, and specifically, 10 kV is desirable. The pulse width of the working pulse voltage P2 is desirably as long as possible in order to increase the efficiency of surface modification. However, the tail may be physically terminated as the charge of the capacitor C2 decreases. However, it is desirable to last for 5 μs or longer.
An inductance 98 is inserted into the circuit (B) for shaping the working pulse voltage P2.

FIG. (F) shows an acceleration voltage pulse waveform P12 applied to the cathode 1, in which P1 and P2 are connected. The operation timing of the switch S2 is taken to ensure that P1 and P2 are continuously connected.

FIGS. (G) and (h) show the energy of the electron beam emitted from the electron gun cathode 1, and the electron beam energy Q1 in FIG. (G) corresponds to P1, and the electron beam in FIG. (H). Energy Q2 corresponds to P2.

FIG. 6 (i) shows the resulting energy waveform Q12 of the electron beam, which is a combination of Q1 and Q2.

FIG. 2 is an explanatory diagram of the power supply 90 for generating the abnormal pulse in the previous stage. The power supply 90 includes a circuit A, a circuit B, and an operation circuit (not shown).
The electron beam pulse (P12) explains the waveform of the irregularly shaped pulse, and is a step-shaped irregularly shaped pulse waveform composed of the leading portion (P1) and the action portion (P2).

The circuit A is a circuit for forming the head portion (P1) described with reference to FIG. 3C, and is composed of the capacitor C1 charged by the DC power source 91 and the charging resistor 92, the closing switch S1, the irradiated body 3, and the cathode 1. Circuit.
The circuit B is a circuit for producing the action part (P2) described in FIG. 3D, and is composed of the DC power source 95, the capacitor C2 charged by the resistor 96, the closing switch S2, the irradiated body 3, and the cathode 1. Circuit.

In order to generate the cathode acceleration voltage pulse (P12) of FIG. 3 (f), the high-voltage pulse generation power supply 90 output pulse applied to the cathode 1 is a step-shaped variant consisting of a leading portion (P1) and an action portion (P2). It is a pulse voltage waveform,
The first part (P1) with a high acceleration voltage and a short pulse width is created by circuit A, and the active part (P2) with a long pulse width and a low acceleration voltage is created by circuit B. Then, when the circuit B is closed, an electron beam waveform (Q12) having a stepped irregular pulse waveform can be obtained.

Diodes 93 and 97 are inserted in circuits A and B, respectively, to suppress interference and backflow. The circuit A is provided with a current sensor 94, and the switch S2 of the circuit B is closed via the sensor output. If necessary, an inductance 98 is inserted into the circuit B to extend the pulse width.

In this specification, the plasma electron gun is defined as follows. An electron gun portion of an electron beam irradiation apparatus, and a cathode having a radiation surface of several cm ² or more and a target (object to be processed) are coaxially arranged in a container filled with an inert ionized gas such as Ar. The electron gun is provided with a hollow annular electrode for converting the ionized gas into plasma, and is provided with a solenoid for generating a magnetic force line parallel to the axis in the container.

After maintaining the atmospheric pressure of the ionized gas at a specified atmospheric pressure of 0.1 Pa or less and preparing a line of magnetic force, when the glow discharge is generated between the annular electrode and the container, the ionized gas is turned into plasma. At that time, when a DC high voltage pulse of several kV or more is applied between the cathode and the target, the target is irradiated with an electron beam pulse through the ionized gas plasma from the cathode emission surface. The cross-sectional area of the electron beam pulse is several cm ² or more, and the energy per area is a value of several J / cm ² per pulse.

The energy per area of the electron beam pulse is measured by irradiating the surface of the sample metal with the electron beam pulse and calculating the amount of heat from the temperature rise.
At an intermediate position between the cathode and the target (if the plasma density and magnetic flux do not change), the energy per area is a constant value.

The generation mechanism of the electron beam pulse is considered as follows. In FIG. 2, when electric charge is stored in the capacitor, an electric double layer is formed between the front surface of the cathode and the plasma, so that the capacitor is on standby with a very large potential gradient. When a switch (for example, a gas switch) is closed at an appropriate time, the electric charge is emitted as an electron beam according to the acceleration voltage, so there is a large cathode loss, but there is little loss in the plasma space and the anode. This is the reason why the energy per area is constant in the plasma space.

In the plasma electron gun, since the electric double layer in front of the cathode is an element of the electron beam emission mechanism, it is desirable that the initial acceleration voltage is high. On the other hand, it is desirable that the acceleration voltage is low in order to suppress the opening of the target metal surface.

Explanatory drawing which shows the specific structure of this invention. Explanatory drawing of the power supply 90 for making the abnormal pulse of this invention. The wave form diagram explaining the action | operation sequence of the apparatus of FIG. The schematic explanatory drawing of the surface modification apparatus using the plasma electron gun of a prior art example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, Cathode 2, Ring electrode 3, To-be-irradiated object, to-be-processed object 4, Housing 5, Solenoid 8, Ar cylinder 12, Glow discharge pulse power supply 15, Solenoid power supply 90, Electron beam pulse emission power supply 20, Controller

Claims (3)

The cathode surface and the target irradiation surface are made to face each other along the axis of the electron gun, the means for converting the low piezoelectric gas separation in the space surrounded by the two surfaces into plasma, the means for creating magnetic force lines parallel to the axis, and the direct current to the cathode In the metal surface modification method of applying a high-pressure pulse and emitting an electron beam pulse toward the target surface through the plasma,
The electron beam pulse is an electron beam pulse having a stepped energy waveform in which the leading portion of the electron beam pulse has a high acceleration voltage and a short time width, and the subsequent succeeding portion has a low acceleration voltage and a long time width. Metal surface modification method. The cathode surface and the target irradiation surface are made to face each other along the axis of the electron gun, the means for converting the low piezoelectric gas separation in the space surrounded by the two surfaces into plasma, the means for creating magnetic force lines parallel to the axis, and the direct current to the cathode In an apparatus comprising a power source that applies a high-pressure pulse to emit an electron beam pulse toward a target surface through the plasma,
The power source is a circuit (A) including a capacitor C1 for applying a pulse voltage P1 having a high acceleration voltage and a short time width, and a closing switch S1,
A capacitor C2 for applying a pulse voltage P2 having a long time width with a low acceleration voltage and a circuit (B) comprising a closing switch S2,
As a result of the continuous application of the short pulse voltage P1 and the long pulse voltage P2 to the cathode, the circuit (A) generates an electron beam pulse Q1, and the circuit (B) continuously generates an electron beam pulse. A metal surface reforming apparatus characterized in that Q2 is generated and the metal surface is reformed by an electron beam pulse Q12 having a stepped energy waveform.   A current sensor is inserted into the circuit (A), and the closing switch S2 of the circuit (B) is closed via a signal that detects a pulse current of the circuit (A) due to closing of the closing switch S1. The metal surface modification apparatus according to claim 2.

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