JP2006206959A - Method for nitriding aluminum alloy - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for nitriding aluminum alloy, which can quickly form a nitrided layer at a low temperature. <P>SOLUTION: An apparatus for nitriding aluminum alloy comprises: an electron beam excitation plasma source for generating a high-density plasma of highly dissociated nitrogen; a treatment tank for storing the plasma and nitriding the aluminum alloy therein; a vacuum pumping system, a heating device, a biased-voltage application device for an article to be treated; and a source gas system. The nitriding method includes treating the aluminum alloy in the high-density plasma atmosphere of excited and highly dissociated nitrogen by an electron beam. The electron beam excitation plasma source is operated in conditions of an accelerating voltage (V) of 50 to 150 volts and an accelerating current (A) of 4 amperes or higher. A source gas of plasma is nitrogen gas; a mixture gas of nitrogen gas and hydrogen gas; a mixture gas of nitrogen gas and argon gas; or a mixture gas of nitrogen gas, hydrogen gas and argon gas. A nitriding temperature is 300 to 550°C. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for nitriding an aluminum alloy.

  Generally, as a method for nitriding a metal, there are roughly a gas nitriding method, a gas soft nitriding method, a salt bath nitriding method, a salt bath soft nitriding method, and an ion nitriding method. The gas nitriding method uses highly toxic ammonia as a raw material and diffuses highly active nitrogen generated during decomposition by heating into a metal to obtain a nitrided layer, which requires a high decomposition temperature and a long treatment time. . Steel materials act as a catalyst to promote the decomposition of ammonia, and are suitable for the method of nitriding steel materials. Recently, high pressure and high temperature methods using nitrogen gas as a raw material have been tried.

  The gas soft nitriding method is used for processing to lower materials mainly composed of carbon steel. This is a method in which ammonia gas is added to a carburizing gas or nitrogen gas atmosphere such as a pyrolysis gas of an organic solvent, and nitrogen and carbon are simultaneously penetrated and diffused to form carbonitrides on the surface. A method using a mixed gas of nitrogen, ammonia, and carbon monoxide as a raw material has also been developed.

  The salt bath nitriding method was developed for use in general structural steels, and is a method that can be processed in a relatively short time by immersing it in a heat-dissolved salt bath mainly composed of sodium and potassium cyanide salts. . The salt bath soft nitriding method uses sodium or potassium cyanate as a salt bath to diffuse nitrogen and carbon simultaneously. All of these have problems with the safety of cyanide and cyanate compounds, post-treatment wastewater, and measures to prevent pollution of exhaust gases.

In the ion nitriding method, atomic nitrogen gas having higher activity than the above method is used as a nitrogen atmosphere. The discharge (glow, arc) between the cathode and the surface of the workpiece dissociates and ionizes the nitrogen or nitrogen mixed gas injected into the treatment tank, creating a nitrogen atmosphere, and at the same time, heating the surface of the workpiece to generate atomic form A method of thermally diffusing nitrogen into the interior has become a conventional mainstream. Ion nitriding has a nitriding rate that is 2 to 2.5 times faster than gas nitriding, and has been favorably applied. However, since the treatment is performed at a high temperature, deformation of the processed material and non-uniform nitridation due to temperature spots are not possible. It was easy to get up.
Japanese Patent Laid-Open No. 61-290629

  Aluminum alloys have a lower hardness than steel and the like, and are easy to seize and wear. For this reason, various surface treatments using plating (plating), thermal spraying, and anodic oxidation have been studied for aluminum materials. Most of them form an aluminum oxide layer on the surface of an aluminum material, and there has been no satisfactory method for nitriding the aluminum material because the nitride layer formed on the surface is thin. In addition, nitriding treatment of aluminum alloy is generally performed at a high temperature of 550 ° C. to 600 ° C., and the actual use such as a problem of deformation of the processed material due to the high temperature and deterioration of mechanical properties due to thermal modification of the aluminum alloy. Therefore, a method of nitriding at a low temperature has been desired.

  In addition, in the conventional ion nitriding method, the energy of electrons responsible for dissociation of the injected gas is low, the atomic nitrogen atmosphere is dilute, and the treated material also serves as the discharge electrode, so that the atmosphere generation and the treated surface temperature are controlled. There remained a problem of being independent and unstable.

  An object of the present invention is to solve the above-mentioned problems and to provide a method for nitriding an aluminum alloy that can form a nitride layer more quickly at a low temperature.

  The first problem-solving means of the present invention is to treat an aluminum alloy in a high-density, high-dissociation nitrogen atom plasma bath excited by an electron beam.

  The second problem-solving means of the present invention includes, in addition to the first problem-solving means, an electron beam-excited plasma source for generating high-density, high-dissociation nitrogen atom plasma, a treatment tank for storing plasma and performing nitriding treatment, This is to use a processing apparatus comprising an evacuation system apparatus, a heating apparatus, a workpiece bias voltage application apparatus, and a raw material gas system apparatus.

  The third problem solving means of the present invention is that, in addition to the first problem solving means, the operating conditions of the electron beam excited plasma source are an acceleration voltage of 50 to 150 volts and an acceleration current of 4 amperes or more.

  In addition to the first problem solving means, the fourth problem solving means of the present invention is that the plasma source gas is nitrogen gas, a mixed gas of nitrogen gas and hydrogen gas, a mixed gas of nitrogen gas and argon gas, or nitrogen gas. And a mixed gas of hydrogen gas and argon gas.

  The fifth problem solving means of the present invention is to perform nitriding at a nitriding temperature of 300 to 550 ° C. in addition to the first problem solving means.

  The sixth problem solving means of the present invention is to perform nitriding at a nitriding temperature of 350 to 500 ° C. in addition to the first problem solving means.

  A seventh problem-solving means of the present invention is to apply a workpiece bias voltage negatively by 30 volts or more with reference to plasma in addition to the first problem-solving means.

  The present invention uses a high-density, high-dissociation nitrogen atom plasma excited by an electron beam of about 80 eV, where the collision cross-sectional area of nitrogen molecules reaches the maximum value, and treats an aluminum alloy in a plasma bath stored in a nitriding tank. In this case, a bias voltage was applied to this, and heating control was performed independently from the outside, and it was found that low temperature nitriding of an aluminum alloy, which could not be performed conventionally, was possible.

  Nitrogen plasma excited by electron beam has a much higher plasma density than ordinary nitrogen plasma, and the degree of dissociation of nitrogen molecules is 2 to 3 orders of magnitude higher, and it is reported that there is a lot of atomic nitrogen in the atmosphere. Has been. It was found that nitrogen atoms diffuse and react with metals faster than nitrogen molecules, and nitriding is possible at a lower temperature than conventional ion nitriding. (Referred to as atom nitriding)

  Japanese Patent Application Laid-Open No. Sho 61-290629 is known as an electron beam excitation plasma source for generating nitrogen atom plasma, and is composed of a discharge region and an acceleration region for generating an electron beam. Accelerate electrons and generate an electron beam. Place the treatment in a nitriding tank, evacuate it, flow a predetermined amount of nitrogen gas, nitrogen / hydrogen mixed gas, nitrogen gas / argon mixed gas, excite nitrogen gas with the generated electron beam and change it to plasma, plasma An apparatus for performing nitriding treatment is used.

  In general, the generation of an electron beam is used for an electron gun at several kV and several hundred mA, but the conditions for generating an electron beam according to the present invention are as follows: acceleration voltage = 50 to 150 volts, low voltage, acceleration current = greater than 4 amps Current is mainly used. Argon gas is used as a discharge gas for generating an electron beam. The generated electrons are directly accelerated to become an electron beam having a high energy of 50 to 150 eV.

Nitrogen gas is excited by this electron beam, and high-density dissociation and high-density nitrogen plasma (including nitrogen atom atmosphere) can be formed. In normal glow discharge, dissociation of nitrogen molecules is difficult, and only a low-density nitrogen atmosphere can be generated. The typical nitrogen atmosphere parameters are those having a plasma density of 10 ⁹ to 10 ¹¹ cm ⁻³ , a nitrogen atom density of 10 ¹⁰ to 10 ¹² cm ⁻³ , and a plasma electron temperature of 1 to 2 eV. . It is possible to increase the output of this plasma source by examining various conditions.

  As the source gas for nitrogen atom plasma, pure nitrogen gas or a mixed gas of nitrogen and hydrogen, a mixed gas of nitrogen and argon, or a mixed gas of nitrogen gas, hydrogen gas and argon gas may be used. The conditions for obtaining the target nitrided layer, such as processing temperature and processing time, may be appropriately selected depending on the type of aluminum alloy.

  If necessary, the treatment can be carried out while removing the passive film on the surface by negatively biasing the treatment object and applying ion bombardment from the plasma atmosphere. In the case of an aluminum alloy, since an aluminum oxide layer that prevents the passage of nitrogen atoms exists on the surface, a workpiece bias voltage of −30 V or more is applied to the plasma. At that time, the independence of the plasma generation and the processed object is maintained, and no discharge is generated, so that the processed object is not exposed to an abnormally high temperature. Regarding the heating of the processed material, either external heating or internal heating is possible. However, when a high-purity nitrogen atmosphere is required, external heating that can suppress generation of impurities is preferable.

  The heating temperature of the processed product made of an aluminum alloy is 300 ° C to 550 ° C, preferably 350 ° C to 500 ° C. Nitriding hardly occurs at 300 ° C or lower, and a crystalline phase change occurs at 550 ° C or higher at which the melting point of the aluminum alloy is reached. Subject to thermal denaturation. Regarding the setting of the heating temperature, it is important to select a temperature at which the aluminum alloy does not undergo thermal modification. Time can be taken to achieve the desired thickness of the nitride layer. The cooling method is not particularly limited, but cooling with gas is safe and preferable.

  The nitriding method of the present invention uses high-density, high-dissociation nitrogen plasma, and its action is significantly different from that of normal molecular plasma. The compound layer is easy to control, the nitriding rate is faster than that of normal plasma treatment, and can be treated at a particularly low temperature, so there is no surface nitridation, and there is no surface cracking or peeling. Nitridation can be performed uniformly regardless of the shape, and nitriding can be performed if plasma enters even in a narrow gap. The arrangement of the processed material is simple and does not require a complicated arrangement such as ion nitriding or radical nitriding. It can also be processed with an insulating material.

  The present invention makes it possible to provide a nitride layer directly on the surface of an aluminum alloy more quickly at a low temperature by using high-density, high-dissociation nitrogen plasma. Moreover, a nitride layer with good adhesion can be obtained, and an aluminum nitride layer having a thickness of about 40 μm can be obtained relatively easily. In particular, it is an optimal method that can be performed at a low temperature for nitriding a metal material that is susceptible to thermal history.

An embodiment of the present invention will be described below with reference to the drawings.
FIG. 1 is a block diagram of a nitriding apparatus. The plasma source used in the nitriding apparatus of the present embodiment includes discharge regions 1 and 2 (first discharge region 1 and second discharge region 2) composed of a cathode K and anodes S1 and S2, and an acceleration region having an acceleration electrode T. 3 is connected to the nitriding tank 4. Argon gas is supplied to the discharge regions 1 and 2 as a discharge source gas. Nitrogen gas, or nitrogen gas + hydrogen gas, or nitrogen gas + argon gas, or mixed gas of nitrogen gas + hydrogen gas + argon gas is supplied to the nitriding treatment tank 4 from the processing gas inlet 4a. A high density and high dissociation nitrogen plasma is obtained.

  Further, a vacuum exhaust system line for keeping the discharge regions 1, 2, the acceleration region 3, and the processing tank 4 in a vacuum is provided at the exhaust port 4 b of the processing tank 4. The processing tank 4 is made of a quartz cylindrical body, and the processing object A is heated by a radial heater (seeds heater) 5 covering the outer periphery thereof. A thermocouple 6a is installed on the surface of the processed object A, and the temperature of the surface of the processed object A is measured by the temperature measuring device 6 connected thereto. The processing table 7 on which the processing object A is placed is a ladder made of quartz glass.

  A cathode electrode K having a filament F is provided in the discharge region 1, and an inert gas such as argon gas (hereinafter represented by argon gas) is supplied from an inert gas inlet 1a through a flow rate control device. Is done. The discharge region 2 is connected to the discharge region 1 on the one hand by a sub-anode electrode (anode) S1 having a small hole in the center, and on the other hand through a main anode electrode (anode) S2 having a small hole in the center. It is separated from the electron acceleration chamber 3. The electron acceleration chamber 3 is adjacent to the nitriding treatment tank 4 with an acceleration electrode T having a small hole in the center.

  11 is a filament power source, 12 is a discharge power source, 13 is an electron acceleration power source (voltage Va, current Ia) connected between the main anode S2 and the acceleration electrode T, and 14 is a bias power source applied to the workpiece A. is there. Reference numeral 15 denotes a variable resistor for adjusting the voltage to the sub-anode S1.

  Next, the operating state will be described. Electrons are emitted from the cathode electrode K heated by the filament F, and discharge is generated and maintained between the cathode electrode K and the main anode electrode S2 in the discharge chambers 1 and 2, and argon plasma is generated in the discharge regions 1 and 2. Is generated. A part of the electrons in the argon plasma in the discharge region 2 are extracted by the acceleration electrode T and accelerated in the electron acceleration chamber 3 to become an electron beam. The electron beam enters the processing tank 4 and collides with gas molecules in the tank, ionizing the molecules and generating plasma.

  During this time, the temperature of the processed material A is raised by the heater 5. While maintaining the surface temperature at a predetermined temperature and adjusting the electron acceleration voltage Va, the electron acceleration current Ia, and the pressure in the layer, nitriding is performed by introducing a processing gas (such as nitrogen gas and hydrogen gas).

  Thus, in the nitriding method of the present invention, a processing gas (reactive gas containing nitrogen gas, hydrogen gas, and argon gas) is supplied to the processing tank, and an electron beam extracted from the discharge plasma by the acceleration electrode is supplied to the nitriding chamber. Then, the nitrogen gas layer or the nitrogen diffusion layer is formed on the surface by converting the reaction gas into plasma by an electron beam and allowing it to act on the processed material.

A sample A of a disc-shaped aluminum alloy (A5083) of 2 cmφ × 0.2 cmt was placed on the processing table 7 and installed in the processing tank 4. A vacuum was applied to control the inside of the treatment tank so as to maintain a vacuum of 10 ⁻⁴ Pascal. Argon gas was flowed through the treatment tank at 70 sccm to 0.5 Pascal, the acceleration voltage Va was set to 100 V, the acceleration current Ia was set to 4.5 A, and an electron beam was generated. A bias VB of −200 V was applied to the sample, and sputtering treatment was performed at 470 ° C. for 1 hour.

  Next, 35 sccm of argon gas and 35 sccm of nitrogen gas were supplied into the treatment tank to generate nitrogen atom plasma, and the treatment tank was filled with plasma. While applying a sample bias of −200 V, the temperature in the vicinity of the processed product was adjusted and heated to 470 ° C. with the heater 5 serving as an external heating furnace, and nitriding was performed for 3 hours. After the nitriding treatment, the heater 5 was opened and cooled, and the treated product was taken out after reaching room temperature.

  The appearance, surface hardness (Hv), and thickness of the nitrided layer of the processed product taken out were examined. It was observed that the surface had a matte black color, was not cracked, and was nitrided. The thickness of the nitride layer was about 45 μm.

A sample of a disc-shaped aluminum alloy (A5052) having a diameter of 2 cmφ × 0.2 cmt was placed on a processing table and placed in a processing tank. A vacuum was applied to control the inside of the treatment tank so as to maintain a vacuum of 10 ⁻⁴ Pascal. Argon gas was flowed through the treatment tank at 70 sccm to 0.5 Pascal, the acceleration voltage Va was set to 100 V, the acceleration current Ia was set to 4.5 A, and an electron beam was generated. A bias VB of −200 V was applied to the sample, and sputtering was performed at 500 ° C. for 1 hour.

  Next, 35 sccm of argon gas and 35 sccm of nitrogen gas were supplied into the treatment tank to generate nitrogen atom plasma, and the treatment tank was filled with plasma. While applying a sample bias of −200 V, the heater 5 was adjusted and heated so that the temperature in the vicinity of the treatment became 500 ° C., and nitriding was performed for 0.5 hour. After the nitriding treatment, the heater 5 was opened and cooled, and the treated product was taken out after reaching room temperature.

  The appearance, surface hardness (Hv), and thickness of the nitrided layer of the processed product taken out were examined. The surface was matte black and uniform nitriding without cracking was observed. The thickness of the nitride layer was about 5 μm.

As a result of observing the cross section of the treated product with a transmission electron microscope, it was confirmed that the surface layer was an aggregate of columnar crystals having a length of about 5 μm and was wurtzite type aluminum nitride from the X-ray revolving pattern. Moreover, it has confirmed that the intermediate | middle layer existed between the base material layer and the aluminum nitride layer. As a result of HVTEM-EDX analysis, this intermediate layer was found to be a MgAl ₂ O ₄ layer by elemental analysis and recovery. It was speculated that metals other than aluminum contained in the aluminum alloy were concentrated as reactants in the intermediate layer, and this intermediate layer contributed to improving the adhesion between the aluminum nitride and the substrate. .
[Comparative Example]

  The sample was replaced with pure aluminum (A-1080), and the treatment was performed under the same conditions as in Example 3. A black aluminum nitride layer was formed on the surface of the treated sample, but partial chipping or peeling was observed, and uniform surface treatment was not possible. However, high purity aluminum nitride may be obtained.

A pulley made of an aluminum alloy (A5052) having a diameter of about 30 mmφ, a length of 15 mm, and a shaft hole of about 6 mmφ was placed on the treatment table and installed in the treatment tank. A vacuum was applied to control the inside of the treatment tank so as to maintain a vacuum of 10 ⁻⁴ Pascal. Argon gas was flowed through the treatment tank at 70 sccm to 0.5 Pascal, the acceleration voltage Va was set to 100 V, the acceleration current Ia was set to 4.5 A, and an electron beam was generated. A bias VB of −200 V was applied to the sample, and sputtering was performed at 500 ° C. for 1 hour.

  Next, 62.5 sccm of argon gas and 7.5 sccm of nitrogen gas were allowed to flow into the treatment tank to generate nitrogen atom plasma, and the treatment tank was filled with plasma. The sample temperature was set to 460 ° C. by external heating (heater 5), and nitriding was performed for 0.5 hour. After the nitriding treatment, the external heating furnace was opened and cooled, and the treated product was taken out after reaching room temperature.

  The appearance, surface hardness (Hv), and thickness of the nitrided layer of the processed product taken out were examined. It was observed that both pulleys had a matte black surface, no peeling cracks, were uniformly nitrided over the entire surface, and were processed even inside the shaft hole.

  The present invention is not limited to the examples and embodiments described above, and includes various modifications without departing from the scope and scope of the claims.

  Aluminum is the second most used metal material after iron. For example, 1000 series alloys (pure aluminum) are reflectors, decorations, conductive materials, containers, printed materials, and 2000 series alloys are 3000 series alloys such as aircraft, hydraulic parts, pistons, machine parts, structural materials, etc. For aluminum cans, piping materials, daily necessities, building materials, copying machine drums, etc. 4000 series alloys for filler metal, brazing materials, and 5000 series alloys for can lids, pressure vessels, structural materials for vehicles and ships, automobile bodies 6000 series alloys are for sashes, railings, automobiles and motorcycle frames, 7000 series alloys are aircraft structural materials, single vehicle structural parts, sports equipment, molds, etc. 8000 series alloys are aircraft structural materials, foil materials, buildings, etc. Used for outer walls, decorative panels, bearings, powder metallurgy, etc. In addition, it is also widely used in aluminum alloy castings and aluminum alloy die castings. A strong nitride layer can be directly formed on these surfaces to impart wear resistance, seizure resistance, insulation and heat transfer.

It is a block diagram of the nitriding apparatus used for the method of the present invention.

Explanation of symbols

A Processed material K Cathode electrode F Filament S1 Sub anode S2 Main anode T Acceleration electrode 1 Discharge area 1a Inert gas inlet 2 Discharge area 3 Acceleration area 4 Treatment layer 4a Treatment gas inlet 5 Heater 6 Temperature measuring instrument 6a Thermocouple 7 Treatment stand 11 Filament Power Supply 12 Discharge Power Supply 13 Electron Acceleration Power Supply (Voltage Va, Current Ia)
14 Bias power supply VB
15 Variable resistor

Claims (7)

A method of nitriding an aluminum alloy in which the aluminum alloy is treated in a high-density, high-dissociation nitrogen atom plasma bath excited by an electron beam. Electron beam-excited plasma source for generating high-density, high-dissociation nitrogen atom plasma, treatment tank for storing plasma and performing nitriding treatment, vacuum evacuation system device, heating device, workpiece bias voltage application device, source gas system device 2. A method for nitriding an aluminum alloy according to claim 1, wherein a processing apparatus comprising: 2. The method of nitriding an aluminum alloy according to claim 1, wherein the operating conditions of the electron beam excited plasma source are an acceleration voltage of 50 to 150 volts and an acceleration current of 4 amperes or more. The plasma source gas is nitrogen gas, a mixed gas of nitrogen gas and hydrogen gas, a mixed gas of nitrogen gas and argon gas, or a mixed gas of nitrogen gas, hydrogen gas and argon gas, 2. The method for nitriding an aluminum alloy according to 1. The method for nitriding an aluminum alloy according to claim 1, wherein nitriding is performed at a nitriding temperature of 300 to 550 ° C. The method for nitriding an aluminum alloy according to claim 1, wherein nitriding is performed at a nitriding temperature of 350 to 500 ° C. 2. The method for nitriding an aluminum alloy according to claim 1, wherein the workpiece bias voltage is negatively applied 30 volts or more with reference to plasma.

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