Plasma nitriding device and plasma nitriding method

Abstract

To realize a plasma nitriding device that can be controlled so that the surface hardness of a treated metal member that has been plasma nitrided using a combination screen is improved. [Solution] The plasma nitriding device (1) is arranged near a metal workpiece (2) and includes a combination screen (37) containing multiple elements capable of forming nitrides, and a control unit (16) that controls the plasma nitriding process using the workpiece (2) and the combination screen (37) as cathodes, so that the ratio of the thickness of the nitrogen diffusion layer to the thickness of the nitride layer formed on the workpiece (2) by the plasma nitriding process is less than 1.

Description

[Technical Field]

[0001] The present invention relates to a plasma nitriding apparatus and a plasma nitriding method for performing plasma nitriding on a metal workpiece. [Background technology]

[0002] Plasma nitriding, a surface treatment method, is a technology that uses plasma formed by glow discharge in a low vacuum containing nitrogen gas to form a nitrogen diffusion layer on the surface (Patent Documents 1 and 2). The plasma nitriding method has a short treatment time, is economical, and does not use harmful substances, so it has a lower environmental impact than other nitriding methods. Furthermore, the plasma nitriding method can also be used to nitride stainless steel, which is a material that is difficult to nitride, and is therefore used industrially in a variety of fields.

[0003] Furthermore, Non-Patent Document 1 discloses that by performing plasma nitriding using a mesh screen containing chromium (Cr) and aluminum (Al), which can form nitrides, a nitride layer (Cr-Al-N layer), which is a compound of chromium, aluminum, and nitrogen (N), is formed on a nitrogen diffusion layer.

[0004] Furthermore, Non-Patent Document 2 discloses that a nitride layer (Ti-Mo-N layer), which is a compound of titanium, molybdenum, and nitrogen, is formed on a nitrogen diffusion layer by performing plasma nitriding using a mesh screen containing titanium (Ti) and molybdenum (Mo), which can form nitrides.

[0005] Furthermore, Non-Patent Document 3 discloses that by performing plasma nitriding using a screen containing titanium and aluminum that can form nitrides, a nitride layer (Ti-Al-N layer), which is a compound of titanium, aluminum, and nitrogen, is formed on a nitrogen diffusion layer.

[0006] Hereinafter, a screen containing a plurality of elements capable of forming nitrides may be referred to as a "combination screen," and the nitride layer may be referred to as a "composite nitride layer." [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-272978 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-191784 [Non-patent literature]

[0008] [Non-Patent Document 1] Kohei Taniguchi and two others, "Effect of Screen Opening Ratio on Plasma Nitriding Using Cr-Al Combination Screen," Japan Institute of Metals, 2023 Autumn (173rd) Lecture Meeting, Abstracts, Japan Institute of Metals, September 5, 2023, Lecture Number: P68 [Non-patent document 2] Ikuya Okada and two others, "Effect of Screen Wire Diameter on Plasma Nitriding Using Ti-Mo Combination Screens," Japan Institute of Metals, 2023 Autumn (173rd) Lecture Meeting, Abstracts, Japan Institute of Metals, September 5, 2023, Lecture Number: P205 [Non-patent document 3] Tatsuo Kajimoto and two others, "Two-stage plasma nitriding treatment using Ti-Al combination screen", Japan Institute of Metals, 2022 Autumn (171st) Lecture Conference Abstracts, Japan Institute of Metals, September 6, 2022, Lecture number P184 Summary of the Invention [Problem to be solved by the invention]

[0009] In the plasma nitriding treatments described in Non-Patent Documents 1 to 3, it is not yet known how to control the treatment to improve the surface hardness.

[0010] One aspect of the present invention aims to provide a plasma nitriding device or the like that can control the surface hardness of a treated metal member that has been plasma nitrided using the above-mentioned combination screen so that the surface hardness is good. [Means for solving the problem]

[0011] In order to solve the above problems, one embodiment of the plasma nitriding apparatus of the present invention is a plasma nitriding apparatus that performs plasma nitriding treatment on a metal workpiece, and includes a screen that is placed near the workpiece and contains a plurality of elements that can form nitrides, and a control unit that controls the plasma nitriding treatment that is performed using the workpiece and the screen as cathodes, and controls the plasma nitriding treatment so that the ratio of the thickness of the nitride layer to the thickness of the nitrogen diffusion layer formed on the workpiece by the plasma nitriding treatment is less than 1.

[0012] In the plasma nitriding apparatus according to the above aspect, the screen may include at least one of the following (a) to (e): (a) a linear member including at least a first element and a second element capable of forming a nitride, the first element and the second element being different from each other; (b) a plate-like member containing at least the first element and the second element; (c) a set including at least a linear member containing the first element and a linear member containing the second element; (d) a set including at least a plate-like member containing the first element and a plate-like member containing the second element; and (e) A set including at least a linear member containing the first element and a plate-like member containing the second element.

[0013] In the plasma nitriding apparatus according to the above aspect, the elements capable of forming nitrides may be selected from chromium, aluminum, titanium, molybdenum, vanadium, zirconium, tantalum, tungsten, niobium, silicon, and iron, and the workpiece may be selected from iron, stainless steel, carbon steel, or alloy steel.

[0014] The plasma nitriding apparatus according to the above aspect may further include a reactor that accommodates the workpiece and the screen, and a gas supply unit that supplies nitrogen gas to the reactor, wherein the plasma nitriding is performed without changing the pressure in the reactor, and the control unit may control the gas supply unit so that the pressure in the reactor is equal to or lower than an upper limit. The gas supply unit may mix multiple gases, including the nitrogen gas, at a set ratio and supply the gas to the reactor, and the control unit may set the upper limit based on the ratio. The screen may be formed into a mesh shape using linear members, and the control unit may set the upper limit based on either or both of the wire diameter of the linear members and the aperture ratio of the screen.

[0015] The plasma nitriding apparatus according to the above aspect may further include a reactor that houses the workpiece and the screen, and a gas supply unit that supplies nitrogen gas to the reactor, and the plasma nitriding process may be a process that is carried out by changing the pressure in the reactor at least once.

[0016] Another aspect of the present invention is a plasma nitriding method for performing plasma nitriding on a workpiece, which includes a control step for controlling the plasma nitriding using a screen placed near the workpiece, the screen containing multiple elements capable of forming nitrides, and the workpiece as cathodes, and controlling the plasma nitriding so that the ratio of the thickness of the nitride layer to the thickness of the nitrogen diffusion layer formed on the workpiece by the plasma nitriding is less than 1.

[0017] The plasma nitriding apparatus according to each aspect of the present invention may be realized by a computer. In this case, the control program for the plasma nitriding apparatus that causes the computer to operate as each part (software element) of the plasma nitriding apparatus and thereby realizes the plasma nitriding apparatus on a computer, and the computer-readable recording medium on which the program is recorded, also fall within the scope of the present invention. [Effects of the Invention]

[0018] According to one aspect of the present invention, it is possible to realize a plasma nitriding apparatus or the like that can control the surface hardness of a treated metal member that has been plasma nitrided using a combination screen so that the surface hardness is good. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a cross-sectional view showing an outline of a main part of a plasma nitriding apparatus according to an embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view of a surface portion of a treated member, which is a member to be treated that has undergone ASDCPN treatment in the plasma nitriding apparatus. [Figure 3] 1 is a graph showing the relationship between the ratio of the thickness of the composite nitride layer to the thickness of the nitrogen diffusion layer in the treated member and the surface hardness of the treated member. [Figure 4] FIG. 10 is a table showing the results of an example of ASDCPN processing using the plasma nitriding apparatus having the above configuration. [Figure 5] FIG. 10 is a perspective view showing an outline of a combination screen in a plasma nitriding apparatus according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, embodiments of the present invention will be described in detail. For the sake of convenience, the same reference numerals will be used to designate components having the same functions as those in the embodiments, and the description thereof will be omitted where appropriate.

[0021] [Embodiment 1] An embodiment of the present invention will be described with reference to FIGS.

[0022] FIG. 1 is a cross-sectional view of a main part of a plasma nitriding apparatus according to this embodiment. In the plasma nitriding apparatus 1 of this embodiment, a combination screen 37 (screen) containing multiple elements capable of forming nitrides is placed so as to surround a metal workpiece 2, and plasma nitriding is performed using the combination screen 37 and the workpiece 2 as cathodes. This plasma nitriding process forms, on the surface of the workpiece 2, a nitrogen diffusion layer in which nitrogen is diffused into the workpiece 2, and a composite nitride layer (nitride layer) consisting of the multiple elements and nitrogen, which is formed on the nitrogen diffusion layer. In this application, this type of plasma nitriding process is referred to as ASDCPN (Active Screen Direct Current Plasma Nitriding) processing.

[0023] As shown in FIG. 1, the plasma nitriding apparatus 1 includes a gas supply unit 11, a main body unit 12, a pump unit 13, a power supply unit 14, a temperature measurement unit 15, and a control unit 16.

[0024] (Gas supply section) The gas supply unit 11 mixes a plurality of gases, including nitrogen gas (N), at a predetermined ratio and supplies the mixed gas to the main body 12. In this embodiment, the gas supply unit 11 includes a nitrogen cylinder 21, a hydrogen cylinder 22, a flow meter 23, and a mixer 24.

[0025] The nitrogen cylinder 21 is a container that contains compressed high-pressure nitrogen gas (N2), and the hydrogen cylinder 22 is a container that contains compressed high-pressure hydrogen gas (H2). The nitrogen gas from the nitrogen cylinder 21 and the hydrogen gas from the hydrogen cylinder 22 are supplied to a mixer 24.

[0026] The flow meter 23 measures the flow rate of the nitrogen gas supplied from the nitrogen cylinder 21 to the mixer 24 and the flow rate of the hydrogen gas supplied from the hydrogen cylinder 22 to the mixer 24 .

[0027] The mixer 24 mixes the nitrogen gas and the hydrogen gas at a predetermined ratio (for example, N2:H2=20:80 to 80:20) and supplies the mixed gas to the main body 12.

[0028] (Main body) The main body 12 includes a vacuum reactor 31 (reactor). The vacuum reactor 31 is a metal container that accommodates a workpiece 2 such as a stainless steel material and a combination screen 37, and causes various reactions, such as chemical reactions, on the workpiece 2 under vacuum conditions.

[0029] An air inlet 31a through which the mixed gas flows in from the gas supply unit 11 is provided at an appropriate position on the inside upper surface of the vacuum reactor 31. An exhaust port 31b for exhausting the air inside is provided at an appropriate position on the inside lower surface of the vacuum reactor 31. An opening 31c is formed at an appropriate position on the inside side surface of the vacuum reactor 31, and a glass window 32 is provided in the opening 31c.

[0030] A conductive base 34 is provided on the bottom surface of the vacuum reactor 31 via an insulating member 33. A conductive plate-like member 35 is provided on the entire upper surface of the base 34. The material of the plate-like member 35 may be, for example, SUS304. The material of the plate-like member 35 may be changed depending on the material of the member 2 to be treated. Instead of providing the plate-like member 35 on the entire upper surface of the base 34, the plate-like member 35 may be provided on the upper surface of the base 34 for each member 2 to be treated.

[0031] The member 2 to be treated is placed on the upper surface of the plate-like member 35 via a conductive adjustment table 36. This allows the member 2 to be placed in an optimal position for the ASDCPN treatment. Only one adjustment table 36 may be placed on the upper surface of the plate-like member 35, or multiple adjustment tables 36 may be placed. When multiple adjustment tables 36 and members 2 to be treated are placed, it is desirable to arrange the adjustment tables 36 and members 2 to be treated so that the distances from the combination screen 37 to each member 2 to be treated are equal.

[0032] The workpiece 2 is any metal member that can be subjected to plasma nitriding. Examples of such metal members include iron, stainless steel, carbon steel, and alloy steel. The stainless steel may be an austenitic, ferritic, or duplex stainless steel material.

[0033] A combination screen 37 is provided on the edge of the plate-like member 35. The combination screen 37 is formed in a mesh shape using one or more linear members, and is provided so as to cover the member 2 to be treated. Therefore, the mixed gas can pass through the combination screen 37 and reach the member 2 to be treated. In addition, an observation window (see FIG. 5) is formed in an appropriate position on the combination screen 37 to allow observation of the state of the member 2 to be treated.

[0034] The combination screen 37 includes a plurality of elements capable of forming nitrides, such as chromium (Cr), aluminum (Al), titanium (Ti), molybdenum (Mo), vanadium (V), zirconium (Zr), tantalum (Ta), tungsten (W), niobium (Nb), silicon (Si), or iron (Fe).

[0035] Furthermore, the combination screen 37 may include a set including at least linear members containing a first element capable of forming nitrides and linear members containing a second element capable of forming nitrides, the second element being different from the first element. For example, the combination screen 37 may include a set of linear members containing Ti (first element) capable of forming nitrides and linear members containing Al (second element) capable of forming nitrides. Needless to say, the set may further include linear members containing an element capable of forming nitrides other than the first element and the second element.

[0036] The linear member containing the first element may be composed of the first element alone or may be composed mainly of the first element (for example, 40% or more). The same applies to the linear member containing the second element.

[0037] The combination screen 37 may also include linear members containing at least the first and second elements. For example, the combination screen 37 may include linear members containing Ti and Al. Needless to say, the linear members containing the first and second elements may further include elements capable of forming nitrides other than the first and second elements. The linear members containing the first and second elements may be composed of the first and second elements, or may be composed of the first and second elements as the main components (e.g., 40% or more).

[0038] An anode member 38 is disposed on the inner wall of the vacuum reactor 31 via an insulating member (not shown). The anode member 38 is a conductive member that functions as an anode for generating an arc discharge in the vacuum reactor 31. The base 34, the plate-like member 35, the adjustment table 36, the combination screen 37, and the member to be treated 2 function as a cathode for generating the arc discharge. The vacuum reactor 31 and the anode member 38 may be integrated.

[0039] Furthermore, a thermocouple 39 is provided at an appropriate position inside the vacuum reactor 31. The thermocouple 39 measures the temperature inside the vacuum reactor 31.

[0040] (Pump section) The pump unit 13 includes a vacuum pump 41 and a vacuum gauge 42. The vacuum pump 41 exhausts air from the vacuum reactor 31 of the main body 12 through the exhaust port 31b. Examples of the vacuum pump 41 include a rotary pump, a diffusion pump, and a turbo pump. The vacuum gauge 42 measures the degree of vacuum in the vacuum reactor 31.

[0041] (Power supply part) The power supply unit 14 includes a DC power supply 43. The anode of the DC power supply 43 is electrically connected to the anode member 38 in the vacuum reactor 31, and the cathode of the DC power supply 43 is electrically connected to the base 34. The DC power supply 43 applies a voltage between the anode member 38 in the vacuum reactor 31 and the base 34. The voltage applied by the DC power supply 43 is measured by a voltmeter (not shown).

[0042] (Temperature measurement part) The temperature measurement unit 15 includes a radiation thermometer 44 and a support stand 45 that supports the radiation thermometer 44. The radiation thermometer 44 detects the intensity of electromagnetic waves emitted from the workpiece 2 through the observation window of the combination screen 37 and the glass window 32, and measures the temperature of the workpiece 2. An example of the radiation thermometer 44 is an infrared thermometer. The radiation thermometer 44 may also be a thermograph that displays the temperature distribution as a two-dimensional image.

[0043] (Control unit) The control unit 16 comprehensively controls the operations of various components of the plasma nitriding apparatus 1, and is configured by, for example, a computer including a CPU (Central Processing Unit) and a memory. The operations of the various components are controlled by causing the computer to execute a control program.

[0044] (ASDCPN processing) Next, we will explain the ASDCPN treatment (plasma nitriding treatment, plasma nitriding method) in the plasma nitriding apparatus 1 configured as described above. First, the workpiece 2 is placed in an appropriate position in the vacuum reactor 31 using the base 34 and adjustment table 36, and the radiation thermometer 44 is focused on the workpiece 2. Next, the vacuum reactor 31 is depressurized using the vacuum pump 41, and then the mixed gas is introduced into the vacuum reactor 31 and exhausted, maintaining the interior of the vacuum reactor 31 at a predetermined pressure (for example, 100 Pa to 1000 Pa, preferably 100 Pa to 200 Pa).

[0045] Next, a voltage is applied between the anode member 38 and the base 34, the plate-like member 35, the adjustment table 36, the combination screen 37, and the member to be treated 2 using the DC power supply 43, and a DC current is gradually passed to generate a glow discharge. As a result, plasma is generated in the vicinity of the member to be treated 2 and the combination screen 37 (the region indicated by the dashed line in FIG. 1).

[0046] Nitrogen ions in the plasma near the member 2 to be treated cause sputtering on the surface of the member 2 to be treated. This removes unnecessary deposition layers on the surface of the member 2 to be treated. Furthermore, the sputtering generates metal nitrides (MN) with a high nitrogen concentration, which are deposited on the member 2 to be treated. The nitrogen released in the process of decomposing the metal nitrides with a high nitrogen concentration into metal and nitrogen diffuses into the interior of the member 2 to promote nitriding of the member 2 to be treated.

[0047] Furthermore, nitrogen ions in the plasma near the combination screen 37 cause sputtering on the surface of the combination screen 37, producing complex nitrides with a high nitrogen concentration. The complex nitrides are deposited on the workpiece 2 by the gas flow in the vacuum reactor 31. Some nitrogen is released from the complex nitrides with a high nitrogen concentration deposited on the workpiece 2 and diffuses into the workpiece 2, thereby progressing the nitriding of the workpiece 2. As a result, a layer of the complex nitride (complex nitride layer) is formed on the surface of the workpiece 2, and a layer in which the nitrogen has diffused from the surface to the interior of the workpiece 2 (nitrogen diffusion layer) is also formed.

[0048] The nitriding treatment is carried out for a predetermined time (e.g., 3 to 20 hours) at a predetermined temperature (e.g., 700 to 900 K) for the workpiece 2 (treatment temperature). Thereafter, the application of the voltage is terminated, and the mixed gas is allowed to flow into the vacuum reactor 31 to cool the workpiece 2 until its temperature drops to around room temperature. Next, air is introduced into the vacuum reactor 31, and the workpiece 2 is removed.

[0049] Fig. 2 is a cross-sectional view of a surface portion of a treated member 51, which is the treated member 2 that has been subjected to the above-mentioned ASDCPN treatment. As shown in Fig. 2, in the treated member 51, a nitrogen diffusion layer 52 is formed on the surface of the treated member 2, and a composite nitride layer 53 (nitride layer) is formed on the nitrogen diffusion layer 52. In other words, the composite nitride layer 53 is formed closer to the surface of the treated member 51 than the nitrogen diffusion layer 52.

[0050] The present inventors performed the above-mentioned ASDCPN treatment on the treated member 2 using various combination screens 37, and investigated the surface hardness of the treated member 51. As a result, they found a relationship between the thickness of the nitrogen diffusion layer 52 and the thickness of the composite nitride layer 53 in the treated member 51 and the surface hardness of the treated member 51.

[0051] 3 is a graph showing the relationship between the ratio (Tn / Td) of the thickness Tn of the composite nitride layer 53 to the thickness Td of the nitrogen diffusion layer 52 in the treated member 51 and the surface hardness of the treated member 51. In the example of FIG. 3, austenitic stainless steel SUS304 was used as the treated member 2. The surface hardness is measured by micro-Vickers hardness. The micro-Vickers hardness was measured by applying a load of 0.25 N to the treated member 51 and conducting a surface hardness test.

[0052] In the graph of FIG. 3, the data represented by diamonds are data obtained when a combination screen 37 formed from metal wires containing titanium and metal wires containing molybdenum (hereinafter referred to as the "Ti-Mo screen 37") is used. The data represented by circles are data obtained when a combination screen 37 formed from metal wires containing titanium and metal wires containing aluminum (hereinafter referred to as the "Ti-Al screen 37") is used. The data represented by crosses are data obtained when a combination screen 37 formed from metal wires containing chromium and metal wires containing aluminum (hereinafter referred to as the "Cr-Al screen 37") is used.

[0053] 3, it can be seen that, regardless of the type of combination screen 37, the micro-Vickers hardness is higher when the ratio (Tn / Td) is less than 1 compared to when the ratio (Tn / Td) is 1 or greater. In other words, it can be seen that the treated member 51 in which the thickness Tn of the composite nitride layer 53 is thinner than the thickness Td of the nitrogen diffusion layer 52 has good surface hardness.

[0054] Therefore, in this embodiment, the control unit 16 controls the ASDCPN treatment so that the ratio of the thickness of the nitrogen diffusion layer 52 to the thickness of the composite nitride layer 53 formed on the ASDCPN-treated treated member 51 is less than 1. In this way, the plasma nitriding apparatus 1 configured as described above can control the treated member 51 so that the surface hardness thereof is good.

[0055] (Additional notes) In this embodiment, the combination screen 37 has a mesh shape and is provided so as to cover the workpiece 2, but is not limited to this. The combination screen 37 is disposed near the workpiece 2 and can have any shape, such as a cylindrical shape, as long as it allows the high-nitrogen-concentration composite nitride to reach the workpiece 2.

[0056] Example 1 An example of ASDCPN processing using the plasma nitriding apparatus 1 having the above configuration will be described below.

[0057] (Processed material) In this example, austenitic stainless steel SUS304 (SUS304 steel) was used as the treated member 2. A bar of the SUS304 steel was prepared, and the bar was machined into a disk shape with a diameter of 15 mm and a thickness of 5 mm. The upper surface was wet polished with #180 to #1500 grit, and finally, a mirror finish was achieved by buffing using a colloidal silica abrasive with a grain size of 0.08 μm.

[0058] (ASDCPN processing) A direct current plasma nitriding apparatus (model: JIN-1S) manufactured by Nippon Denshi Kogyo Co., Ltd. was used, and the plate-like member 35 of this embodiment, an adjustment table 36, a combination screen 37, and a member to be treated 2 were provided in the direct current plasma nitriding apparatus. The combination screen 37 was cylindrical, with dimensions of a diameter of 170 mm and a height of 200 mm.

[0059] The combination screen 37 used was the above-mentioned Ti-Mo screen with an aperture ratio of 5%, the above-mentioned Ti-Al screen with an aperture ratio of 5%, and the above-mentioned Cr-Al screen with aperture ratios of 5% and 40%. The wire diameter of the linear members (metal wires) forming the combination screen 37 was 0.5 mm. A SUS304 round bar member was installed as the adjustment table 36 so that the distance from the center of the workpiece 2 to the combination screen 37 was 30 mm.

[0060] The pressure in the vacuum reactor 31 was then evacuated to 10 Pa or less, after which the mixed gas was introduced and the pressure was kept constant in the mixed gas atmosphere. The ASDCPN treatment was performed at a treatment temperature of 500°C (approximately 773 K) for 16 hours. In the ASDCPN treatment, the gas ratio of the mixed gas was N2:H2 = 1:1 or 3:1. The pressure was 100, 150, 200, or 300 Pa.

[0061] For the purpose of elemental analysis from the surface to the depth direction, glow discharge optical emission spectrometry (GD-OES) was performed on the ASDCPN-treated treated member 51 using a Marcus-type high-frequency glow discharge optical emission surface analyzer (model: GD-Profiler2) manufactured by Horiba, Ltd. Based on the results, the thickness Td of the nitrogen diffusion layer 52 and the thickness Tn of the composite nitride layer 53 in the treated member 51 were measured. In addition, a surface hardness test was performed on the ASDCPN-treated treated member 51 using a micro Vickers hardness tester (model: MMT-7) manufactured by Matsuzawa Corporation, applying a load of 0.25 N.

[0062] Here, the distance from the surface of the treated member 51 to the position where the concentration of elements contained in the screen becomes 0 or close to 0 was defined as the thickness Tn of the composite nitride layer 53. Also, the distance from the surface of the treated member 2, which is the boundary between the composite nitride layer 53 and the treated member 2, to the position where the concentration of nitrogen becomes 0 or close to 0 was defined as the thickness Td of the nitrogen diffusion layer 52.

[0063] FIG. 4 is a diagram showing the results of this example in tabular form. In FIG. 4, the top table shows the case where a Ti-Mo screen was used, the middle table shows the case where a Ti-Al screen was used, and the bottom table shows the case where a Cr-Al screen was used. Each table shows the pressure, gas ratio, thickness Tn of the composite nitride layer 53, thickness Td of the nitrogen diffusion layer 52, thickness ratio Tn / Td, and micro Vickers hardness. In the bottom table, the first to third data from the left are data when the aperture ratio is 5%, and the fourth to sixth data from the left are data when the aperture ratio is 40%.

[0064] 4, it can be seen that, for any of the Ti-Mo screen, Ti-Al screen, and Cr-Al screen, the micro-Vickers hardness is greater (harder) when the thickness ratio Tn / Td is less than 1 compared to when the thickness ratio Tn / Td is 1 or greater. This is true whether the opening ratio is 5% or 40%. It was also confirmed that not only was the micro-Vickers hardness improved, but the wear resistance was also improved, and the adhesion between the SUS304 steel material (the treated member 2) and the composite nitride layer 53 was also improved.

[0065] 4, it can be seen that the thickness Td of the nitrogen diffusion layer 52 decreases as the pressure in the vacuum reactor 31 increases. When the thickness Td of the nitrogen diffusion layer 52 decreases, the thickness ratio Tn / Td increases. Therefore, it is desirable that the control unit 16 sets an upper limit for the pressure to make the thickness ratio Tn / Td less than 1, and controls the gas supply unit 11 so that the pressure is equal to or less than the upper limit.

[0066] 4, it can be seen that even if the pressure is the same, the thicknesses of the composite nitride layer 53 and the nitrogen diffusion layer 52 vary depending on the gas ratio. Therefore, the control unit 16 may set the upper limit of the pressure based on the gas ratio (proportion) set in the gas supply unit 11.

[0067] For example, referring to Figure 4, when the gas ratio of the mixed gas is N2:H2 = 1:1, if the pressure is set to 200 Pa or less, the thickness ratio Tn / Td will be less than 1. Also, when the gas ratio of the mixed gas is N2:H2 = 3:1, if the pressure is set to 100 Pa or less, the thickness ratio Tn / Td will be less than 1.

[0068] Furthermore, Non-Patent Document 1 discloses that the thickness of composite nitride layer 53 varies depending on the aperture ratio of combination screen 37. Furthermore, Non-Patent Document 2 discloses that the thickness of composite nitride layer 53 varies depending on the wire diameter of the metal wires that make up combination screen 37. Therefore, control unit 16 may set the upper limit of the pressure based on either or both of the wire diameter of the linear members that make up combination screen 37 and the aperture ratio of combination screen 37.

[0069] (Additional notes) This embodiment is a so-called one-stage nitriding process in which the ASDCPN process is performed while maintaining a constant pressure inside the vacuum reactor 31. On the other hand, Non-Patent Document 3 discloses a so-called two-stage nitriding process in which the pressure is changed once during the ASDCPN process. Even in the case of this two-stage nitriding process, it has been confirmed that when the thickness ratio Tn / Td is less than 1, the micro Vickers hardness is greater (harder) than when the thickness ratio Tn / Td is 1 or greater. The same applies when the pressure is changed two or more times during the ASDCPN process.

[0070] [Embodiment 2] Another embodiment of the present invention will be described with reference to Fig. 5. The plasma nitriding apparatus of this embodiment differs from the plasma nitriding apparatus shown in Figs. 1 to 3 in terms of the combination screen, but the other configurations are similar.

[0071] Fig. 5 is a perspective view showing an outline of a combination screen 61 used in the plasma nitriding apparatus 1 of this embodiment. As shown in Fig. 5, the combination screen 61 of this embodiment is a cylindrically formed plate-like member containing multiple elements capable of forming nitrides. As such, the combination screen may have any shape, such as a mesh shape or a cylindrical shape, as long as it contains multiple elements capable of forming nitrides.

[0072] The combination screen 61 may be formed into a cylindrical shape using one plate-like member, or may be formed into a cylindrical shape using multiple plate-like members. In this way, the combination screen 61 may include at least a plate-like member.

[0073] The plate-shaped member may include at least a first element and a second element that form a nitride, the first element and the second element being different from each other. For example, the plate-shaped member may include Ti (first element) and Al (second element) that can form a nitride. Needless to say, the plate-shaped member may further include an element that can form a nitride other than the first element and the second element. Furthermore, a plate-shaped member that includes the first element and the second element may be composed of the first element and the second element, or may be composed of the first element and the second element as the main component (e.g., 40% or more).

[0074] The combination screen 61 is disposed on the edge of the plate-like member 35. The composition ratio of the above-described multiple elements contained in the combination screen 61 of this embodiment may be the same as or different from the composition ratio of the multiple elements contained in the combination screen 37 shown in FIG.

[0075] Furthermore, an observation window 62 for observing the state of the member to be treated 2 is formed at an appropriate position on the combination screen 61. As described above, the observation window 62 is used to measure the temperature of the member to be treated 2. Therefore, the mixed gas from the gas supply unit 11 flows into the combination screen 61 through the upper opening 61a and the observation window 62 of the combination screen 61 and then flows out from the interior.

[0076] (Additional notes) In this embodiment, the combination screen 61 includes one plate-shaped member, but may include two or more plate-shaped members. Specifically, the combination screen 61 may include a set including at least a plate-shaped member containing a first element capable of forming nitrides and a plate-shaped member containing a second element capable of forming nitrides, the second element being different from the first element. For example, the combination screen 61 may include a set including a plate-shaped member containing Ti (first element) capable of forming nitrides and a plate-shaped member containing Al (second element) capable of forming nitrides. Needless to say, the set may further include a plate-shaped member containing an element capable of forming nitrides other than the first element and the second element.

[0077] The plate-shaped member containing the first element may be composed entirely of the first element, or may be composed mainly of the first element (for example, 40% or more). The same applies to the plate-shaped member containing the second element.

[0078] Example 2 An example of ASDCPN treatment using the plasma nitriding apparatus 1 having the above configuration will be described below. The member 2 to be treated is the same as in Example 1.

[0079] (ASDCPN processing) A direct current plasma nitriding apparatus (model: JIN-1S) manufactured by Nippon Denshi Kogyo Co., Ltd. was used, and the plate-like member 35 of this embodiment, an adjustment table 36, a combination screen 61, and a treated member 2 were provided in the direct current plasma nitriding apparatus. The combination screen 61 was a cylindrical alloy plate with a thickness of 0.8 mm. The alloy constituting the alloy plate was a Ti-Mo alloy, and more specifically, an alloy based on Ti and containing 16% Mo by mass.

[0080] The dimensions of the combination screen 61 were 170 mm in diameter and 200 mm in height, similar to the combination screen 37 of Example 1. A SUS304 round bar member was installed as an adjustment table 36 so that the distance from the center of the workpiece 2 to the combination screen 61 was 30 mm.

[0081] The vacuum reactor 31 was then evacuated to a pressure of 10 Pa or less, and the mixed gas was then introduced, and the pressure in the mixed gas atmosphere was adjusted to 100 Pa. The ASDCPN treatment was performed at a treatment temperature of 600°C (approximately 873 K) for 16 hours. In the ASDCPN treatment, the gas ratio of the mixed gas was N2:H2 = 1:1.

[0082] For the treated member 51 subjected to the ASDCPN treatment, the thickness Td of the nitrogen diffusion layer 52 and the thickness Tn of the composite nitride layer 53 were measured in the same manner as in Example 1. As a result, the thickness Td of the nitrogen diffusion layer 52 was 21.2 μm, and the thickness Tn of the composite nitride layer 53 was 2.8 μm. Therefore, the thickness ratio Tn / Td was approximately 0.132, which was less than 1. Furthermore, a surface hardness test was performed on the treated member 51 by applying a load of 0.25 N in the same manner as in Example 1. As a result, the micro Vickers hardness was 1540 HV, indicating good surface hardness.

[0083] (Additional notes) The combination screen may be a combination of a part of the combination screen 37 shown in FIG. 1 and a part of the combination screen 61 shown in FIG.

[0084] Specifically, the combination screen may include a set including at least a linear member containing a first element capable of forming a nitride and a plate-like member containing a second element capable of forming a nitride, the second element being different from the first element. For example, the combination screen may include a set including a linear member containing Ti (first element) capable of forming a nitride and a plate-like member containing Al (second element) capable of forming a nitride. Needless to say, the set may further include a linear member containing an element capable of forming a nitride other than the first and second elements, and the set may further include a plate-like member containing the element.

[0085] A linear member containing a first element may be composed of the first element or may be composed mainly of the first element (e.g., 40% or more), and a plate-like member containing a second element may be composed of the second element or may be composed mainly of the second element (e.g., 40% or more).

[0086] From the above, the combination screen may be provided with at least one of the following (a) to (e): (a) a linear member including at least a first element and a second element capable of forming a nitride, the first element (e.g., Ti) and the second element (e.g., Al) being different from each other; (b) a plate-like member containing at least the first element and the second element; (c) a set including at least a linear member containing the first element and a linear member containing the second element; (d) a set including at least a plate-like member containing the first element and a plate-like member containing the second element; and (e) A set including at least a linear member containing the first element and a plate-like member containing the second element.

[0087] It goes without saying that the combination screen may include two or more of the above (a) to (e).

[0088] Furthermore, it goes without saying that the combination screen may comprise the linear members of (a) above and linear members containing the first element, or the linear members of (a) above and linear members containing the second element, or the linear members of (a) above and linear members containing an element capable of forming a nitride other than the first element and the second element. The same applies when the linear members of (a) above are replaced with the plate-like members of (b) above.

[0089] Furthermore, it goes without saying that the combination screen may comprise the linear members of (a) above and a plate-like member containing the first element, or the linear members of (a) above and a plate-like member containing the second element, or the linear members of (a) above and a plate-like member containing an element capable of forming a nitride other than the first element and the second element. The same applies when the linear members of (a) above are replaced with the plate-like members of (b) above.

[0090] [Software implementation example] The functions of the plasma nitriding apparatus 1 (hereinafter referred to as the "apparatus") can be realized by a program that causes a computer to function as the apparatus, and a program that causes a computer to function as each control block of the apparatus (particularly each part included in the control unit 16).

[0091] In this case, the device includes a computer having at least one control device (e.g., a processor) and at least one storage device (e.g., a memory) as hardware for executing the program. The control device and storage device execute the program, thereby realizing the functions described in each of the above embodiments.

[0092] The program may be non-transitory and may be recorded on one or more computer-readable recording media. The recording media may or may not be included in the device. In the latter case, the program may be supplied to the device via any wired or wireless transmission medium.

[0093] Furthermore, some or all of the functions of the control blocks can be realized by logic circuits. For example, an integrated circuit in which a logic circuit that functions as each of the control blocks is formed is also included in the scope of the present invention. In addition, the functions of the control blocks can also be realized by, for example, a quantum computer.

[0094] Furthermore, each process described in each of the above embodiments may be executed by AI (Artificial Intelligence). In this case, the AI ​​may run on the control device or on another device (for example, an edge computer or a cloud server).

[0095] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. [Explanation of symbols]

[0096] 1. Plasma nitriding equipment 2. Treated material 11 Gas supply section 12 Main body 13 Pump section 14 Power supply section 15 Temperature measurement part 16 Control Unit 21 Nitrogen Cylinder 22 Hydrogen Cylinder 23 Flow meter 24 Mixer 31 Vacuum reactor (reactor) 31a Air supply port 31b Exhaust port 31c opening 32 Glass Window 33 Insulating materials 34 Foundation 35 Plate-shaped members 36 Adjustment stand 37 Combination Screen (Screen) 38 Anode member 39 Thermocouple 41 Vacuum pump 42 Vacuum gauge 43 DC power supply 44 Radiation thermometer 45 Support stand 51 Treated materials 52 Nitrogen diffusion layer 53 Composite nitride layer (nitride layer) 61 Combination Screen (Screen) 61a Upper opening 62 Observation window

Claims

1. A plasma nitriding apparatus for performing plasma nitriding on a metal workpiece, a screen disposed near the workpiece, the screen including a plurality of elements capable of forming nitrides; a control unit that controls a plasma nitriding process performed using the workpiece and the screen as cathodes, and controls the plasma nitriding process so that a ratio of the thickness of a nitrogen diffusion layer and a nitride layer formed on the workpiece by the plasma nitriding process to the thickness of the nitride layer is less than 1.

2. The screen is (a) a linear member including at least a first element and a second element capable of forming a nitride, the first element and the second element being different from each other; (b) a plate-like member containing at least the first element and the second element; (c) a set including at least a linear member containing the first element and a linear member containing the second element; (d) a set including at least a plate-like member containing the first element and a plate-like member containing the second element; and (e) a set including at least a linear member containing the first element and a plate-like member containing the second element; The plasma nitriding apparatus of claim 1 , comprising at least one of:

3. 2. The plasma nitriding apparatus according to claim 1, wherein the plurality of elements capable of forming nitrides are selected from the group consisting of chromium, aluminum, titanium, molybdenum, vanadium, zirconium, tantalum, tungsten, niobium, silicon, and iron.

4. 2. The plasma nitriding apparatus according to claim 1, wherein the workpiece is selected from the group consisting of iron, stainless steel, carbon steel, and alloy steel.

5. a reactor that accommodates the workpiece and the screen; a gas supply unit that supplies nitrogen gas to the reaction furnace; the plasma nitriding treatment is a treatment performed without changing the pressure inside the reactor, The plasma nitriding apparatus according to claim 1 , wherein the control unit controls the gas supply unit so that the pressure in the reaction furnace is equal to or lower than an upper limit value.

6. the gas supply unit mixes a plurality of gases including the nitrogen gas at a set ratio and supplies the gas to the reaction furnace; The plasma nitriding apparatus according to claim 5 , wherein the control unit sets the upper limit value based on the ratio.

7. The screen is formed in a mesh shape using linear members, 6. The plasma nitriding apparatus according to claim 5, wherein the control unit sets the upper limit based on either or both of a wire diameter of the linear member and an aperture ratio of the screen.

8. a reactor that accommodates the workpiece and the screen; a gas supply unit that supplies nitrogen gas to the reaction furnace; 2. The plasma nitriding apparatus according to claim 1, wherein the plasma nitriding process is performed by changing the pressure inside the reactor at least once.

9. A plasma nitriding method for performing plasma nitriding on a metal workpiece, comprising: A plasma nitriding method including a control step of controlling a plasma nitriding process using a screen placed near the treated member, the screen containing a plurality of elements capable of forming nitrides, and the treated member as cathodes, in which the plasma nitriding process is controlled so that the ratio of the thickness of the nitride layer to the thickness of the nitrogen diffusion layer formed on the treated member by the plasma nitriding process is less than 1.

10. 2. A control program for causing a computer to function as the plasma nitriding apparatus according to claim 1, the control program causing the computer to function as the control unit.

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