JP2017155324A - Method of hardening metal surface layer by dielectric barrier discharge - Google Patents

Abstract

PROBLEM TO BE SOLVED: To uniformly harden a metal surface layer by employing a dielectric barrier discharge technique and performing a nitriding treatment on a metal surface layer under atmospheric pressure.SOLUTION: A method of hardening a metal surface layer in which a surface layer of metal to be treatment is hardened through a nitriding treatment comprises: (i) covering the metal to be treatment with an atmosphere consisting of a nitrogen gas or, as needed, a mixed gas including an inactive gas such as a hydrogen gas, a rare gas, etc.; and (ii) generating ionizing plasma of nitrogen through dielectric barrier discharge under or substantially under atmospheric pressure, and performing a nitriding treatment on a surface layer of the metal to be treated or the surface layer in a predetermined region.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method in which a plasma containing N atoms is generated by a dielectric barrier discharge under atmospheric pressure, and a surface layer of a metal such as steel, iron, aluminum, or titanium is subjected to nitriding treatment.

  One method of hardening the steel surface layer is to dissolve nitrogen atoms in the steel surface layer to form a “compound layer” made of iron nitride with a thickness of about 10 μm on the outermost layer, with a depth of 100 μm below it. To some extent, there is a nitriding treatment in which nitrogen atoms form a “diffusion layer” that is solidified and dissolved in iron crystals. Since the compound layer is responsible for corrosion resistance and seizure resistance, and the diffusion layer is responsible for wear resistance and fatigue strength, nitriding is an important technology in the automotive industry.

  As the nitriding treatment, plasma nitriding (ion nitriding) using a direct current discharge under a low pressure is widely used. According to plasma nitriding, it is possible to generate a large amount of nitrogen atoms and other radicals due to the presence of high-energy electrons to promote nitriding treatment.

  However, plasma nitriding requires an expensive vacuum vessel, and has a problem that the working time and work process increase because it is a badge process. In view of this, for example, Patent Document 1 proposes a nitriding method in which an atmospheric pressure plasma jet generated using nitrogen as a source gas is irradiated on the surface of the material to nitride the surface of the material.

  According to the nitriding method of Patent Document 1, the hardness of the material surface layer is improved, but the toughness of the material is lowered. Usually, when the surface layer of a metal material is nitrided, the hardness is improved, but the toughness is lowered and becomes brittle. However, in Patent Document 2, N atoms are diffused and dissolved in an object to be processed under atmospheric pressure to maintain the toughness. There has been proposed a surface layer hardening method such as a metal or a resin that can improve the hardness and process the diffusion process of N atoms in a continuous line.

  In the surface layer curing method of Patent Document 2, the inside of the container is brought to a positive pressure state at or near the atmospheric pressure while supplying a mixed gas of nitrogen gas and hydrogen gas into the container, and the surface of the object to be processed is a pulse arc type. Although it is a surface layer curing method using a pulsed arc type plasma jet characterized by irradiating a plasma jet, it is difficult to uniformly cure the surface layer of the object to be processed.

  In recent years, attention has been focused on a method for uniformly processing the surface layer of an object to be processed by generating plasma using dielectric barrier discharge under atmospheric pressure.

  For example, Patent Document 3 discloses: i) a step of plasma-treating a substrate; and ii) a liquid coating material containing one or more compounds selected from the group consisting of an organopolysiloxane polymer, an organopolysiloxane oligomer, a siloxane resin, and a polysilane. Coating the substrate surface by a soft lithographic printing method to form a patterned film on the substrate surface; and iii) removing the residual liquid coating material from the substrate surface, if necessary, A method of applying a patterned thin film thereon, wherein prior to step i), the spray liquid and / or the solid coating forming material is subjected to an atmospheric pressure plasma discharge and / or an ionized / excited gas stream resulting from the atmospheric pressure plasma discharge. Disclosed is a method of exposing a substrate to a spray coating forming material under atmospheric pressure conditions

  Patent Document 4 discloses a method for coating a substrate with an inorganic-organic hybrid polymer material using a dielectric barrier discharge (DBD) technique, in the following steps: a) a sample in a space between two electrodes B) controlling the atmosphere between the electrodes, c) generating a plasma discharge between the electrodes, d) plasma forming an aerosol containing a hybrid organic / inorganic cross-linked prepolymer formed through a sol-gel process. A method is disclosed comprising mixing during discharge.

  However, the methods of Patent Document 3 and Patent Document 4 are methods of coating the surface of a substrate (base material) with a polymer material, and are not methods of curing a metal surface layer by nitriding treatment.

  Non-Patent Document 1 discloses a method of supplying ammonia gas to a vacuum reactor and performing nitriding treatment with dielectric barrier discharge to cure the metal surface layer. Since ammonia gas is used as the gas, there are problems in terms of cost and environment.

  In recent years, a technique for nitriding a metal surface in the form of a figure or a pattern has been developed (see Non-Patent Document 2). According to this technique, it is possible to cure only a desired place with high accuracy in surface curing of a mold or the like.

  However, in order to perform nitriding patterning with an accuracy of millimeters or less, it is necessary to cover a metal surface with a mask fabricated by microfabrication so that nitrogen does not diffuse from the gas or plasma into the masked portion. Therefore, the above technique has a problem that it takes time to produce masking by fine processing, and masking is consumed.

  In order to enable nitriding patterning without masking, DBD that generates plasma in the shape of the counter electrode is a promising technique. However, in the DBD nitridation process of Non-Patent Document 1, since ammonia gas is used, nitrogen is directly supplied from the ammonia gas even to a portion where plasma is not generated. Consequently, the entire metal surface is nitrided. Therefore, patterning nitriding cannot be performed.

JP 2009-202087 A JP 2013-023769 A Japanese Patent No. 5090739 Japanese Patent No. 52297652

Li Yan, Plasma Chemistry and Plasma Processing, Vol.25 No.5 (October 2005), “A New Approach to Metal Surface Nitriding Using Dielectric Barrier Discharge at Atmospheric Pressure” Marcos et al., Surface & Coatings Technology 205 (2011), S275-S279, `` Stainless steel patterning by combination of micro-patterning and driven strain produced by plasma assisted nitriding ''

  The present invention, in view of the prior art, (a) using a dielectric barrier discharge technology, subjecting the metal surface layer to nitriding under atmospheric pressure, uniformly curing the metal surface layer, and (b) metal The object is to pattern and nitride the metal surface layer without masking the surface layer, and to provide a method for curing the metal surface layer that solves these problems.

  The present inventors diligently studied a method for solving the above problems.

  As a result, (i) the metal to be treated is wrapped in a mixed gas containing nitrogen gas and, if necessary, an inert gas such as hydrogen gas or a rare gas, and dielectric barrier discharge can be performed at or near atmospheric pressure. For example, an ionized plasma of nitrogen is generated, and the surface layer of the metal to be processed can be nitrided. (Ii) If the planar shape of the counter electrode is patterned into a required shape, the surface layer of the metal to be processed can be formed. It has been found that nitrogen treatment can be performed on the surface layer region of the counter electrode in a planar shape without masking.

  This invention was made | formed based on the said knowledge, and the summary is as follows.

(1) In a method for curing a metal surface layer in which the surface layer of the metal to be treated is cured by nitriding,
(I) wrapping the metal to be treated in an atmosphere consisting of a mixed gas containing an inert gas such as a nitrogen gas or a hydrogen gas or a rare gas, if necessary,
(Ii) An ionized plasma of nitrogen is generated by dielectric barrier discharge under atmospheric pressure or near atmospheric pressure, and nitriding is applied to the surface layer of the metal to be treated. Curing method.
(2) The method for curing a metal surface layer by dielectric barrier discharge according to (1), wherein a surface layer of a predetermined region of the metal to be processed is subjected to nitriding treatment.
(3) Volume ratio of hydrogen gas to nitrogen gas in the mixed gas: hydrogen gas / nitrogen gas is 0 to 0.5, by dielectric barrier discharge as described in (1) or (2) above A method for curing the metal surface layer.
(4) The mixed gas is supplied to the processing apparatus at a flow rate of 0.1 to 100 L / min, and the temperature of the metal to be processed is maintained at room temperature to 1000 ° C., and nitriding is performed (1) The method for curing a metal surface layer by dielectric barrier discharge according to any one of (1) to (3).
(5) The method for curing a metal surface layer by dielectric barrier discharge according to any one of (1) to (4), wherein the metal to be treated is steel or an iron alloy.

  According to the present invention, a gas mixture that does not require an expensive vacuum processing apparatus, is harmless under atmospheric pressure or near atmospheric pressure, and contains an inert gas such as hydrogen gas or a rare gas, if necessary, is not harmful. Can be used to uniformly nitride the surface layer of the metal to be processed in a short time.

  Furthermore, according to the present invention, the surface layer of the metal to be processed can be partially limited with an accuracy of 100 μm or less, that is, highly precise patterning nitriding can be performed. Furthermore, according to the present invention, when the surface area of the metal to be processed is large, the area of the counter electrode is increased accordingly, and the surface layer of the metal to be processed can be uniformly nitrided in a short time. it can.

It is a figure which shows the one aspect | mode of a processing apparatus. (A) shows the upper surface aspect of a processing apparatus, (b) shows the AA cross-sectional aspect of a processing apparatus. It is a figure which shows the hardness distribution of the steel piece cross section after a nitriding process. It is a figure which shows the metal layer Vickers hardness (Hv) distribution example in the hardening part and non-hardening part of a metal surface layer. (A) shows the metal surface layer Vickers hardness (Hv) distribution of Example 2, (b) shows the metal surface layer Vickers hardness (Hv) distribution of Example 3, and (c) shows Example 4. The metal surface layer Vickers hardness (Hv) distribution is shown.

The method for curing a metal surface layer by dielectric barrier discharge of the present invention (hereinafter referred to as “the present invention curing method”) is a method for curing a metal surface layer in which a surface layer of a metal to be treated is cured by nitriding.
(I) wrapping the metal to be treated in an atmosphere consisting of a mixed gas containing an inert gas such as a nitrogen gas or a hydrogen gas or a rare gas, if necessary,
(Ii) Nitrogen ionized plasma is generated by dielectric barrier discharge under atmospheric pressure or near atmospheric pressure, and the surface layer of the metal to be processed is subjected to nitriding treatment.

  Hereinafter, the curing method of the present invention will be described.

  The atmosphere surrounding the metal to be processed is formed of nitrogen gas and, if necessary, a mixed gas containing an inert gas such as a hydrogen gas or a rare gas. Since the ionized plasma of nitrogen is formed by the dielectric barrier discharge, the atmosphere is formed using nitrogen gas as a main component. The volume% of nitrogen gas is not particularly limited to a specific range, but when hydrogen gas is mixed to remove oxygen mixed in the atmosphere, the volume ratio of hydrogen gas to nitrogen gas: hydrogen gas / nitrogen gas is 0 to 0.5 are preferred.

  The inert gas is preferably helium gas or argon gas. Helium gas and argon gas serve to promote the ionization plasma conversion of nitrogen.

  The dielectric barrier discharge may be performed in a processing apparatus in which the mixed gas is confined, or may be performed while supplying the mixed gas to the processing apparatus at a required flow rate. The flow rate of the mixed gas may be set according to the desired degree of curing of the metal surface layer, and is not particularly limited to a specific flow rate, but is 0.1 to 100 L / min in terms of maintaining the plasma in a good state. preferable. In the processing apparatus, the metal to be processed may be held in a temperature range of room temperature to 1000 ° C.

  The metal to be treated is not limited to a specific metal, but when the metal to be treated is steel or an iron alloy, the curing of the present curing method is remarkably exhibited.

  Further, the curing method of the present invention can perform nitriding treatment on the surface layer of a predetermined region of the metal to be processed. This point is also a feature of the present invention.

  Nitriding by DBD occurs with nitrogen supplied from DBO plasma. That is, unlike when ammonia is used as the nitriding gas, nitrogen is not directly supplied from the atmospheric gas itself. Therefore, at the time of DBD nitriding, if the planar shape of the counter electrode is patterned to a required shape, the surface layer of the metal to be processed does not need to be masked, and the surface shape of the counter electrode is the same as that of the counter electrode. Nitrogen is supplied to the region range, so that the surface treatment can be performed on the surface layer region according to the planar shape of the counter electrode.

  Then, if the patterning of the planar shape of the counter electrode is performed with high accuracy, nitriding can be performed in a highly accurate region range on the surface layer of the metal to be processed. On the other hand, when the surface area of the metal to be processed is large, if the area of the counter electrode is increased accordingly, the surface of the metal to be processed can be uniformly nitrided in a short time.

  Here, one mode of a processing apparatus for carrying out the curing method of the present invention is shown in FIG. FIG. 1A shows an upper surface aspect of the processing apparatus, and FIG. 1B shows an AA cross-sectional aspect of the processing apparatus. A processing apparatus 1 shown in FIG. 1 is a processing apparatus that is configured to supply a mixed gas at a required flow rate, and includes a barrier discharge tube 1a and an external heater 1b that surrounds the barrier discharge tube 1a.

  An AC power source 6 is electrically connected to the external heater 1b, and the barrier discharge tube 1a is heated from the outside in order to maintain the temperature of the barrier discharge tube 1a at a temperature suitable for the formation and maintenance of plasma.

  A dielectric 2 is disposed at the center of the barrier discharge tube 1a. The dielectric 2 is not limited to a specific dielectric as long as it can promote dielectric barrier discharge, and among them, an insulating alumina is preferable.

  A metal 3 to be processed is placed on the upper part of the dielectric 2 with a predetermined interval. The metal 3 to be processed functions as one electrode when dielectric barrier discharge occurs in the barrier discharge tube 1a. The distance between the dielectric 2 and the metal 3 to be processed is appropriately adjusted and set so that the dielectric barrier discharge is appropriately generated under atmospheric pressure or near atmospheric pressure. The planar shape of the counter electrode 4 is disposed on the lower surface of the dielectric 2.

  The planar shape of the metal to be processed 3 and the counter electrode 4 is electrically connected to the AC power source 5 and generates a dielectric barrier discharge in the barrier discharge tube 1a. During this time, a required voltage is applied at a required frequency.

  The barrier discharge tube 1a is preferably a quartz tube in terms of heat resistance, but may be any material that can withstand the formation and maintenance of plasma, and is not limited to a specific material. The cross-sectional shape of the barrier discharge tube 1a is not limited to a tubular shape, and may take any cross-sectional shape. For example, the barrier discharge tube 1a may have a rectangular cross section.

  Although FIG. 1 shows the external heater 1b having a rectangular cross section, the cross sectional shape of the external heater 1b is not limited to a rectangle. A tubular cross-sectional shape may be used in accordance with the cross-sectional shape of the barrier discharge tube 1a, or another cross-sectional shape may be used.

  The barrier discharge tube 1a is supplied with a mixed gas x containing nitrogen gas and, if necessary, an inert gas such as hydrogen gas or rare gas. When a required voltage is applied at a required frequency between the planar shape of the metal to be processed 3 and the counter electrode 4 sandwiching the dielectric 2, a dielectric barrier discharge occurs and the periphery of the metal to be processed 3 is ionized. A plasma containing nitrogen is formed. The ionized nitrogen enters the surface layer of the metal 3 to be processed and nitriding proceeds, and the surface layer of the metal 3 to be processed is cured.

  By continuously supplying a mixed gas x containing nitrogen gas and, if necessary, an inert gas such as hydrogen gas or a rare gas, to the barrier discharge tube 1a, and discharging the surplus mixed gas y from the other end, Nitriding of the surface layer of the metal to be processed 3 proceeds uniformly in the entire surface layer, and the surface layer of the metal 3 to be processed is uniformly cured.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Example 1
Steel pieces (metals to be processed) having a surface of 15 mm × 15 mm and a thickness of 4 mm were arranged on the upper part of the dielectric (alumina) at an interval of about 1 mm in the processing apparatus shown in FIG.

  Gas a (nitrogen: 45% by volume, hydrogen: 5% by volume, balance: mixed gas of argon), gas b (mixed gas of nitrogen: 90% by volume, hydrogen: 10% by volume), gas c (nitrogen: 100% by volume) ) Is fed to the barrier discharge tube, a voltage of 5.7 kV is applied between the steel (metal to be treated) and the electrode at a frequency of 12.8 kHz, and the treatment temperature is kept at 530 ° C. Barrier discharge occurred under atmospheric pressure and continued for 2 hours, and the surface layer of the steel slab was subjected to nitriding treatment.

  After the nitriding treatment, the steel piece was divided at the center, and the hardness of the cross section of the steel piece was measured by a Vickers hardness test (load 10 g). FIG. 2 shows the hardness distribution of the steel piece cross section after nitriding. It turns out that it has hardened from the surface of a steel piece to about 70 micrometers in depth.

(Example 2)
As in Example 1, a steel piece (surface to be processed) having a surface of 15 mm × 15 mm and a thickness of 4 mm is formed on the top of the dielectric (alumina) of the processing apparatus shown in FIG. Metal).

  The atmosphere gas surrounding the metal to be treated is a mixed gas of nitrogen: 45% by volume, hydrogen: 5% by volume, and the balance: argon. This mixed gas is supplied to the barrier discharge tube, a voltage of 5.7 kV is applied between the metal to be processed and the electrode at a frequency of 12.8 kHz, the processing temperature is maintained at 530 ° C., and dielectric barrier discharge is performed. Was generated under atmospheric pressure and continued for 2 hours, and a patterning nitriding treatment was applied to a predetermined region of the surface layer of the steel slab.

  FIG. 3A shows a metal surface Vickers hardness (Hv) distribution in a hardened portion and a non-hardened portion of the metal surface. It can be seen that the hardened portion and the non-hardened portion can be clearly distinguished on the metal surface, and the metal surface layer is subjected to patterning nitriding treatment.

(Example 3)
As in Example 1, a steel piece (surface to be processed) having a surface of 15 mm × 15 mm and a thickness of 4 mm is formed on the top of the dielectric (alumina) of the processing apparatus shown in FIG. Metal).

  The atmosphere gas surrounding the metal to be treated is a mixed gas of 95% by volume of nitrogen and 5% by volume of hydrogen. This mixed gas is supplied to the barrier discharge tube, a voltage of 5.7 kV is applied between the metal to be processed and the electrode at a frequency of 12.8 kHz, the processing temperature is maintained at 530 ° C., and dielectric barrier discharge is performed. Was generated under atmospheric pressure and continued for 2 hours, and a patterning nitriding treatment was applied to a predetermined region of the surface layer of the steel slab.

  FIG. 3 (b) shows the Vickers hardness (Hv) distribution of the metal sample surface in the hardened part and the non-hardened part on the surface of the metal sample. It can be seen that the hardened portion and the non-hardened portion can be clearly distinguished on the metal surface, and the metal surface layer is subjected to patterning nitriding treatment.

Example 4
As in Example 1, a steel piece (surface to be processed) having a surface of 15 mm × 15 mm and a thickness of 4 mm is formed on the top of the dielectric (alumina) of the processing apparatus shown in FIG. Metal).
The atmosphere gas surrounding the metal to be treated is a simple gas of nitrogen: 100% by volume. This single gas is supplied to the barrier discharge tube, a voltage of 5.7 kV is applied between the metal to be processed and the electrode at a frequency of 12.8 kHz, the processing temperature is maintained at 530 ° C., and dielectric barrier discharge is performed. Was generated under atmospheric pressure and continued for 2 hours, and a patterning nitriding treatment was applied to a surface layer of a predetermined region on the surface of the steel piece.

  FIG. 3 (c) shows the Vickers hardness (Hv) distribution of the metal sample surface in the hardened and non-hardened parts of the metal sample surface. It can be seen that the hardened portion and the non-hardened portion can be clearly distinguished on the metal surface, and the metal surface layer is subjected to patterning nitriding treatment.

  According to the Vickers hardness (Hv) distribution of the hardened part and the non-hardened part on the metal surface in Examples 2 to 4, the distribution is not greatly affected by the type of the atmospheric gas, and the hardened part and the non-hardened part are not affected. It can be seen that the boundary can be classified with an accuracy of about 100 μm or 100 μm or less. Therefore, in the curing method of the present invention, it is possible to realize an accurate and highly accurate patterning nitriding treatment on the surface layer of the predetermined region of the metal to be processed as in the shape of the planar shape of the counter electrode.

  As described above, according to the present invention, an expensive vacuum processing apparatus is not required, and it is harmless under atmospheric pressure or near atmospheric pressure. Using a mixed gas containing a gas, the surface layer of the metal to be processed can be uniformly nitrided in a short time.

  Furthermore, according to the present invention, the surface layer of the metal to be processed can be partially limited with an accuracy of 100 μm or less, that is, highly precise patterning nitriding can be performed. Furthermore, according to the present invention, when the surface area of the metal to be processed is large, the area of the counter electrode is increased accordingly, and the surface layer of the metal to be processed can be uniformly nitrided in a short time. it can.

  Therefore, the present invention has high applicability in the machine component manufacturing industry, the mold manufacturing industry, the medical device manufacturing industry, and the like.

DESCRIPTION OF SYMBOLS 1 Processing apparatus 1a Barrier discharge tube 1b External heater 2 Dielectric material 3 Metal to be processed 4 Electrode 5, 6 AC power source x, y Mixed gas

Claims (5)

In the method for curing the metal surface layer, the surface layer of the metal to be treated is cured by nitriding,
(I) wrapping the metal to be treated in an atmosphere consisting of a mixed gas containing an inert gas such as a nitrogen gas or a hydrogen gas or a rare gas, if necessary,
(Ii) An ionized plasma of nitrogen is generated by dielectric barrier discharge under atmospheric pressure or near atmospheric pressure, and nitriding is applied to the surface layer of the metal to be treated. Curing method.   2. The method for curing a metal surface layer by dielectric barrier discharge according to claim 1, wherein a surface layer in a predetermined region of the surface of the metal to be treated is subjected to nitriding treatment.   The volume ratio of hydrogen gas to nitrogen gas in the mixed gas: hydrogen gas / nitrogen gas is 0 to 0.5, The method for curing a metal surface layer by dielectric barrier discharge according to claim 1 or 2.   The mixed gas is supplied to a processing apparatus at a flow rate of 0.1 to 100 L / min, and the temperature of the metal to be processed is maintained at room temperature to 1000 ° C to perform nitriding treatment. The metal surface hardening method by the dielectric barrier discharge of any one of Claims 1.   The method for curing a metal surface layer by dielectric barrier discharge according to any one of claims 1 to 4, wherein the metal to be treated is steel or an iron alloy.

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