Method of plasma nitriding and nitrided workpiece

Abstract

The application relates to a plasma nitriding method and a nitriding workpiece. The plasma nitriding method comprises the following steps: placing a workpiece in a reaction cavity, vacuumizing, and preheating the workpiece; introducing a cleaning gas, applying a first negative bias to the workpiece, ionizing the cleaning gas into a first plasma through arc discharge of an ion source, and performing plasma cleaning on the surface of the workpiece; introducing a nitrogen-containing gas at a nitriding temperature, ionizing the nitrogen-containing gas into a second plasma containing nitrogen ions through arc discharge of the ion source; applying a second negative bias to the workpiece, making the nitrogen-containing gas on the surface of the workpiece produce glow discharge, so as to increase ionization rate and ion energy of the second plasma; and performing plasma nitriding under the condition of maintaining the arc discharge and the glow discharge. The plasma nitriding method has high nitriding efficiency, good nitriding uniformity, and obviously improved thickness and wear resistance of a nitriding layer.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of surface nitriding technology, and in particular to a method of plasma nitriding and a nitriding workpiece. BACKGROUND

[0002] Surface nitriding is a common surface treatment process for workpieces, which can form a nitriding layer or a nitrogen-rich hardening layer on the surface of the workpiece, thereby improving the surface hardness and wear resistance of the workpiece, so that the workpiece has the effect of hard surface and tough core.

[0003] The glow plasma nitriding technology is a method of using glow discharge of a rare gas at high pressure to ionize a nitrogen-containing gas to form plasma, and using plasma bombardment on the surface of the workpiece under the action of an electric field and a magnetic field to achieve surface nitriding. However, the thickness and wear resistance of the nitriding layer are not ideal. SUMMARY

[0004] Therefore, it is necessary to provide a method of plasma nitriding and a nitriding workpiece to overcome the problems of the glow plasma nitriding technology that the thickness and wear resistance of the nitriding layer are not ideal.

[0005] The above-mentioned object of the present application is achieved by the following technical solutions:

[0006] In a first aspect of the present application, a method of plasma nitriding is provided, comprising the following steps:

[0007] The workpiece is arranged in a reaction cavity, vacuum is drawn, and the workpiece is preheated;

[0008] A cleaning gas is introduced, a first negative bias is applied to the workpiece, the cleaning gas is ionized by arc discharge of an ion source to form a first plasma, and the workpiece surface is subjected to plasma cleaning;

[0009] At a nitriding temperature, a nitrogen-containing gas is introduced, and the nitrogen-containing gas is ionized by arc discharge of the ion source to form a second plasma containing nitrogen ions;

[0010] A second negative bias is applied to the workpiece to cause glow discharge of the nitrogen-containing gas on the surface of the workpiece, so as to increase the ionization rate and ion energy of the second plasma;

[0011] The plasma nitriding is carried out under the condition of maintaining the arc discharge and the glow discharge.

[0012] In one embodiment, the nitrogen-containing gas includes argon and nitrogen.

[0013] In one embodiment, the nitrogen-containing gas satisfies one or more of the following conditions:

[0014] 1) the flow rate ratio of the argon gas and the nitrogen gas is 1 : (1-3) ;

[0015] 2) the total pressure of the nitrogen-containing gas is 0.8 Pa to 1.5 Pa.

[0016] In one of the embodiments, the ion source is a hollow cathode gun.

[0017] In one of the embodiments, the hollow cathode gun satisfies one or more of the following conditions:

[0018] 1) in the step of ionizing the purge gas to form a first plasma by arc discharge of the ion source, the filament current of the hollow cathode gun is 140 A to 180 A, and the anode current is 180 A to 220 A;

[0019] 2) in the step of ionizing the nitrogen-containing gas to form a second plasma containing nitrogen ions by arc discharge of the ion source, the filament current of the hollow cathode gun is 140 A to 180 A, and the anode current is 100 A to 160 A.

[0020] In one of the embodiments, the nitriding temperature is 450 °C to 500 °C.

[0021] In one of the embodiments, the time of the plasma nitriding is 0.5 h to 2 h.

[0022] In one of the embodiments, the purge gas includes argon and hydrogen.

[0023] In one of the embodiments, the purge gas satisfies one or more of the following conditions:

[0024] 1) the flow rate ratio of the argon gas and the hydrogen gas is (2-3) : 1;

[0025] 2) the total pressure of the purge gas is 1.0 Pa to 2.0 Pa.

[0026] In one of the embodiments, one or more of the following conditions are satisfied:

[0027] 1) the pressure after the reaction chamber is evacuated is 20 mPa to 30 mPa;

[0028] 2) the temperature of the preheating is 200 °C to 400 °C;

[0029] 3) the first negative bias is 40 V to 200 V;

[0030] 4) the second negative bias is 600 V to 800 V.

[0031] In one embodiment, prior to the step of placing the workpiece in the reaction chamber, the steps of grinding, polishing, cleaning, and drying the workpiece are further included.

[0032] In a second aspect, the present invention provides a nitrided workpiece, characterized in that it is prepared by the plasma nitriding method described above.

[0033] The present invention has the following beneficial effects:

[0034] This invention utilizes an arc discharge from an ion source to emit an arc electron beam, ionizing the cleaning gas and nitrogen-containing gas respectively, thereby obtaining a first plasma and a second plasma with high ionization rate and high density. The first plasma thoroughly cleans and activates the workpiece surface, while the second plasma facilitates the generation of high-density, high-energy nitrogen ions, significantly improving the adsorption and thermal diffusion efficiency of nitrogen ions. After obtaining a high density of high-energy nitrogen ions, a glow discharge is generated in the nitrogen-containing gas on the workpiece surface, further accelerating the nitrogen ions and increasing their ion energy, while also ensuring uniform nitriding. Therefore, plasma nitriding under conditions of maintaining arc discharge and glow discharge results in high nitriding efficiency, good nitriding uniformity, and significantly improved thickness and wear resistance of the nitrided layer. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the nitriding furnace in Example 1;

[0036] Figure 2 Here is a SEM image of the nitrided workpiece in Example 1;

[0037] Figure 3 for Figure 2 Nitrogen element distribution diagram of the nitrided workpiece;

[0038] Figure 4 Metallographic cross-section of the nitrided workpiece in Comparative Example 1;

[0039] Figure 5 This is a metallographic cross-section of the nitrided workpiece in Example 1;

[0040] Figure 6 This is a metallographic cross-section of the nitrided workpiece in Example 2;

[0041] Figure 7 This is a metallographic cross-section of the nitrided workpiece in Example 3;

[0042] Figure 8 A comparison chart of the Vickers hardness of the nitrided workpieces in Examples 1-3 and Comparative Example 1;

[0043] Figure 9 A comparison chart of the friction coefficients of the nitrided workpieces in Examples 1-3 and Comparative Example 1;

[0044] Figure 10 Grinding mark topography for the workpiece in Comparative Example 1;

[0045] Figure 11 Grinding mark topography for the nitrided workpiece in Example 1;

[0046] Figure 12 Grinding mark topography for the nitrided workpiece in Example 2;

[0047] Figure 13 Grinding mark topography for the nitrided workpiece in Example 3.

[0048] Reference numerals: workpiece holder 1, hollow cathode gun 2, cathode transformer 3, cathode controller 4, anode 5, contactor 6, anode power supply 7, coil 8, biasing assembly 9, biasing power supply 10, gas inlet 11, gas outlet 12. DETAILED DESCRIPTION

[0049] In order to make the above objectives, features and advantages of the present application more clear and comprehensible, specific embodiments of the present application will be described in detail below with reference to the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present application. It will be apparent, however, to one skilled in the art that the present application can be practiced without some or all of these specific details. In other instances, well known process steps have not been described in detail in order to avoid unnecessarily obscuring the present application. Therefore, the specific embodiments disclosed below are merely illustrative, and should not be considered limiting the scope of the present application, as defined by the appended claims.

[0050] In addition, the terms "first", "second", etc. are used herein only to describe various tenninates and do not imply or imply relative importance or a number of the indicated technical features. Therefore, the features defined with "first", "second" can explicitly or implicitly include at least one of the features. In the description of the present application, the meaning of "a plurality of" is at least two, such as two, three, etc., unless otherwise specifically defined.

[0051] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present application belongs. The terms used in the specification of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0052] Surface nitriding is a thermo-chemical method of forming a nitriding layer on the surface of a workpiece by diffusion of active nitrogen atoms at a certain temperature, and the obtained nitriding layer has high hardness, high wear resistance, high fatigue resistance and excellent thermal stability. Gas nitriding is a method of obtaining active nitrogen atoms by decomposition of ammonia gas at a high temperature of 500-580 DEG C, and then the active nitrogen atoms are adsorbed and diffused into the workpiece to form a nitriding layer with different thickness on the surface of the workpiece. However, the efficiency of gas nitriding is very low, and the nitriding time is usually 30-90 hours. Moreover, the high nitriding temperature can easily cause phase transformation of the material and affect the overall performance of the workpiece.

[0053] In order to shorten the nitriding time, glow plasma nitriding technology is developed. Glow plasma nitriding is a method of generating glow plasma of thin nitrogen gas under the condition of high voltage and low current, and then the workpiece surface is impacted at high speed by the glow plasma to diffuse active nitrogen atoms into the workpiece to form a nitriding layer. The core of the method is the glow discharge of gas. However, the workpiece needs to be applied with thousands of volts of negative bias to ionize the nitrogen gas (or the mixed gas of nitrogen and argon) in the glow plasma nitriding. If the negative bias of the workpiece is reduced to 600 V, the cavity pressure needs to be maintained at tens to hundreds of pascals to provide enough gas atoms for ionization. The cavity pressure commonly used in glow plasma nitriding is 150-250 Pa, and it is difficult to realize plasma nitriding under low voltage and low pressure. In addition, the ionization rate and density of the plasma obtained by glow discharge are very small, resulting in low nitriding efficiency, and the thickness and wear resistance of the nitriding layer are not ideal. It is reported that the thickness of the nitriding layer prepared by glow plasma nitriding is only about 10 μm, and the wear resistance is also low.

[0054] Based on the above, the first aspect of the present application provides a method for plasma nitriding, comprising the following steps:

[0055] The workpiece is arranged in a reaction cavity, vacuum is drawn, and the workpiece is preheated;

[0056] A cleaning gas is introduced, a first negative bias is applied to the workpiece, the cleaning gas is ionized to form a first plasma by arc discharge of an ion source, and the workpiece surface is plasma cleaned;

[0057] A nitrogen-containing gas is introduced at a nitriding temperature, and the nitrogen-containing gas is ionized to form a second plasma containing nitrogen ions by arc discharge of the ion source;

[0058] A second negative bias is applied to the workpiece to cause glow discharge of the nitrogen-containing gas on the surface of the workpiece, so as to increase the ionization rate and ion energy of the second plasma;

[0059] Plasma nitriding is carried out under the condition of maintaining arc discharge and glow discharge.

[0060] The present application ionizes the cleaning gas and the nitrogen-containing gas by emitting an arc electron current through arc discharge of the ion source, thereby obtaining a first plasma and a second plasma with high ionization rate and high density. The first plasma can thoroughly clean and activate the surface of the workpiece, and the second plasma is conducive to obtaining high-density high-energy nitrogen ions, greatly improving the absorption and thermal diffusion efficiency of the nitrogen ions. After obtaining high-density high-energy nitrogen ions, the nitrogen-containing gas on the surface of the workpiece is subjected to glow discharge to accelerate the nitrogen ions on the surface of the workpiece again, further increasing the ion energy, and ensuring the uniformity of nitriding. Therefore, under the conditions of arc discharge and glow discharge, the nitriding efficiency is high, the nitriding uniformity is good, and the thickness and wear resistance of the nitriding layer are obviously improved.

[0061] Optionally, the material of the workpiece is cemented carbide and / or high speed steel.

[0062] The main component of the cemented carbide is tungsten carbide (WC), which includes one or more of tungsten-cobalt cemented carbide (WC+Co), tungsten-titanium-cobalt cemented carbide (WC+TiC+Co), tungsten-tantalum-cobalt cemented carbide (WC+TaC+Co), and tungsten-titanium-tantalum-cobalt cemented carbide (WC+TiC+TaC+Co). High speed steel (HSS), also known as high speed tool steel, sharp steel or white steel, is a high carbon and high alloy steel containing tungsten (W), molybdenum (Mo), chromium (Cr), cobalt (Co), vanadium (V) and other alloying elements. The carbon content is 0.70% to 1.65%, and the total content of alloying elements is 10% to 25%. According to the types and contents of alloying elements, it can be divided into tungsten-based high speed steel, tungsten-molybdenum-based high speed steel, high molybdenum-based high speed steel, vanadium-based high speed steel, and cobalt-based high speed steel.

[0063] Further optionally, the material of the workpiece is high speed steel.

[0064] High speed steel has good process performance, good hardness, strength, toughness and wear resistance, and can be used to manufacture complex thin blades and impact-resistant cutting tools, and can also be used to manufacture cold work dies, hot work dies, rolls, high temperature bearings and high temperature springs, etc.

[0065] In some embodiments, the nitrogen-containing gas includes argon and nitrogen.

[0066] Mixing argon and nitrogen can increase the ionization rate of nitrogen, increase the density and ion energy of nitrogen ions, and accelerate the nitriding efficiency.

[0067] In some embodiments, the nitrogen-containing gas satisfies one or more of the following conditions:

[0068] 1) The flow ratio of the argon and the nitrogen is 1:(1-3);

[0069] 2) the total pressure of the nitrogen-containing gas is 0.8 Pa to 1.5 Pa.

[0070] By adjusting the flow ratio of argon and nitrogen and keeping the total pressure in the range of 0.8 Pa to 1.5 Pa in the mixed atmosphere of argon and nitrogen, different nitriding effects can be obtained while better nitriding efficiency is achieved to meet the needs of workpieces in different processing scenarios.

[0071] Preferably, the flow ratio of argon and nitrogen is 1:3.

[0072] Preferably, the total pressure of the nitrogen-containing gas is 1 Pa.

[0073] In some embodiments, the ion source is a hollow cathode gun.

[0074] Compared with arc sources such as columnar arc sources or rectangular planar large arc sources, the hollow cathode gun is used as an auxiliary ion source to generate arc discharge in the hollow cathode gun by utilizing the hollow cathode effect, without the need for an additional cathode target and a baffle for filtering target ions, which can avoid contamination of the workpiece surface by target particles and prevent the baffle from blocking a large amount of electrons, so that sufficient electrons can be obtained at a lower current and voltage, which is conducive to reducing the time and energy consumption of nitriding treatment.

[0075] In some embodiments, the hollow cathode gun satisfies one or more of the following conditions:

[0076] 1) In the step of ionizing the cleaning gas by arc discharge of the ion source to form a first plasma, the filament current of the hollow cathode gun is 140 A to 180 A, and the anode current is 180 A to 220 A;

[0077] 2) In the step of ionizing the nitrogen-containing gas by arc discharge of the ion source to form a second plasma containing nitrogen ions, the filament current of the hollow cathode gun is 140 A to 180 A, and the anode current is 100 A to 160 A.

[0078] In some embodiments, the nitriding temperature is 450°C to 500°C.

[0079] Controlling the nitriding temperature to be 450°C to 500°C can prevent phase transformation in the workpiece and reduce the overall performance, and also avoid the reaction of alloying element chromium in the workpiece with interstitial atom nitrogen to form chromium compounds and deteriorate the corrosion resistance of the workpiece.

[0080] In some embodiments, the time of plasma nitriding is 0.5 h to 2 h.

[0081] Under the cooperation of the arc discharge and the glow discharge, the nitrogen ions in the reaction cavity, especially the nitrogen ions on the surface of the workpiece, have very high ion energy, so that the nitriding efficiency is obviously improved, thereby shortening the time of the nitriding treatment.

[0082] Preferably, the time of the plasma nitriding is 1 h.

[0083] In some embodiments, the cleaning gas comprises argon and hydrogen.

[0084] The argon has high economy and good cleaning effect, and can remove the contaminants on the surface of the workpiece through physical etching and improve the roughness and surface tension thereof; the hydrogen is a reducing reactive gas, and can remove the contaminants such as oxides or greases on the surface of the workpiece through physical etching and chemical etching, and is not easy to cause damage to the surface of the workpiece; the mixing of the argon and the hydrogen can improve the ionization rate of the hydrogen, enhance the bombardment and decomposition capacity on the contaminants, and accelerate the cleaning and activation efficiency.

[0085] In some embodiments, the cleaning gas satisfies one or more of the following conditions:

[0086] 1) the flow rate ratio of the argon to the hydrogen is (2-3):1;

[0087] 2) the total pressure of the cleaning gas is 1.0 Pa-2.0 Pa.

[0088] Preferably, the flow rate ratio of the argon to the hydrogen is 2:1.

[0089] Preferably, the total pressure of the cleaning gas is 1.5 Pa.

[0090] In some embodiments, one or more of the following conditions is satisfied:

[0091] 1) the pressure after the reaction cavity is evacuated is 20 mPa-30 mPa;

[0092] 2) the preheating temperature is 200℃-400℃;

[0093] 3) the first negative bias is 40 V-200 V;

[0094] 4) the second negative bias is 600 V-800 V.

[0095] Preferably, the pressure after the reaction cavity is evacuated is 20 mPa.

[0096] Preferably, the preheating temperature is 350℃.

[0097] Preferably, the first negative bias is 100 V.

[0098] Preferably, the second negative bias is 700V.

[0099] In some embodiments, before the step of setting the workpiece in the reaction cavity, the method further comprises the steps of polishing, polishing, cleaning and drying the workpiece.

[0100] Optionally, the polishing, polishing, cleaning and drying of the workpiece comprises the following steps: polishing the surface of the workpiece to mirror surface, so that the surface roughness of the workpiece is less than 50nm; placing the workpiece in hot alkali solution for soaking to remove oil, then placing it in alcohol solution for ultrasonic cleaning, and then placing it in deionized water for soaking cleaning; drying after cleaning, and wrapping with dust-free paper for standby.

[0101] In a second aspect of the present application, a nitriding workpiece is provided, which is prepared by the above-mentioned plasma nitriding method.

[0102] The present application will be further described in detail below in conjunction with specific examples. In the following examples, the raw materials used are all commercially available products unless otherwise specified.

[0103] Example 1

[0104] The material of the workpiece of the present embodiment is M35 high-speed steel, and the specific steps are as follows:

[0105] (1) Polishing the surface of the workpiece to mirror surface, so that the surface roughness of the workpiece is less than 50nm; placing the workpiece in hot alkali solution for soaking to remove oil, then placing it in alcohol solution for ultrasonic cleaning, and then placing it in deionized water for soaking cleaning; drying after cleaning, and wrapping with dust-free paper for standby.

[0106] (2) Please refer to Figure 1 which is a structural schematic diagram of the nitriding furnace in the present embodiment. The nitriding furnace comprises a workpiece rack 1, a hollow cathode gun 2, a cathode transformer 3, a cathode controller 4, an anode 5, a contactor 6, an anode power supply 7, a coil 8, a bias assembly 9 and a bias power supply 10, etc. components, and is provided with a plurality of gas inlets 11 and gas outlets 12.

[0107] Place the workpiece on the workpiece rack 1 in the nitriding furnace, and vacuumize to a vacuum degree of 20mPa in the nitriding furnace, and start heating, and preheat the workpiece at 350℃ for 40min.

[0108] (3) argon gas is introduced at a flow rate of 300 seem until the atmosphere in the nitriding furnace is stable; the hollow cathode gun 2 is turned on, the filament current is set to 160 A, and the anode current is set to 200 A; a negative bias of 100 V is applied to the workpiece, then hydrogen gas is introduced at a flow rate of 150 seem, the argon and hydrogen are ionized to form a first plasma by the arc discharge of the hollow cathode gun 2 to emit an arc electron stream, and the first plasma is used to perform plasma cleaning on the surface of the workpiece for 90 min; after the plasma cleaning is completed, the hollow cathode gun 2 is turned off.

[0109] (4) the temperature of the workpiece is increased to a nitriding temperature of 480℃, then argon gas is introduced at a flow rate of 210 seem and nitrogen gas is introduced at a flow rate of 210 seem to make the total pressure in the nitriding furnace 1 Pa; the hollow cathode gun 2 is turned on, the filament current is set to 160 A, and the anode current is set to 140 A, the argon and nitrogen near the hollow cathode gun 2 are ionized to form a second plasma by the arc discharge of the hollow cathode gun 2 to emit an arc electron stream;

[0110] The second plasma contains argon ions, nitrogen ions, excited argon atoms, excited nitrogen atoms and electrons, and the argon ions and nitrogen ions fly to the anode 5 under the action of the negative bias of the workpiece, and energy exchange is generated by collision with the un-ionized gas, so that the ionization rate and ion energy of the second plasma gradually increase.

[0111] (5) the hollow cathode gun 2 is kept on, a negative bias of 700 V is applied to the workpiece, and the argon and nitrogen near the workpiece generate glow discharge, thereby increasing the ionization rate and ion energy of the second plasma.

[0112] (6) the arc discharge of the hollow cathode gun 2 and the glow discharge on the surface of the workpiece are kept, the temperature in the nitriding furnace is kept within 500℃, the workpiece is plasma nitrided for 1 h, and the furnace is opened after the workpiece is cooled to below 180℃ to take samples.

[0113] Example 2

[0114] The preparation method of this embodiment is basically the same as that of embodiment 1, except that the flow rate ratio of argon gas to nitrogen gas is 1:2 during the nitriding process, and the specific steps are as follows:

[0115] (1) the surface of the workpiece is polished to a mirror surface after polishing, so that the surface roughness of the workpiece is <50 nm; the workpiece is soaked in a hot alkaline solution to remove oil, then ultrasonic cleaning is performed in an alcohol solution, and then the workpiece is soaked and cleaned in deionized water; after cleaning, the workpiece is dried and wrapped with dust-free paper for standby.

[0116] (2) the workpiece is placed on the workpiece holder 1 in the nitriding furnace, vacuumized to a vacuum degree of 20 mPa in the nitriding furnace, and heated; the workpiece is preheated at 350℃ for 40 min.

[0117] (3) argon gas is introduced at a flow rate of 300 seem until the atmosphere in the nitriding furnace is stable; the hollow cathode gun 2 is turned on, the filament current is set to 160 A, and the anode current is set to 200 A; a negative bias of 100 V is applied to the workpiece, then hydrogen gas is introduced at a flow rate of 150 seem, the argon gas and the hydrogen gas are ionized to form a first plasma by emitting an arc electron stream through the arc discharge of the hollow cathode gun 2, and the workpiece surface is plasma cleaned for 90 min by using the first plasma; after the plasma cleaning is completed, the hollow cathode gun 2 is turned off.

[0118] (4) the temperature of the workpiece is increased to a nitriding temperature of 480℃, then argon gas is introduced at a flow rate of 150 seem and nitrogen gas is introduced at a flow rate of 300 seem to make the total pressure in the nitriding furnace 1 Pa; the hollow cathode gun 2 is turned on, the filament current is set to 160 A, and the anode current is set to 140 A, the argon gas and the nitrogen gas near the hollow cathode gun 2 are ionized to form a second plasma by emitting an arc electron stream through the arc discharge of the hollow cathode gun 2;

[0119] The second plasma contains argon ions, nitrogen ions, excited argon atoms, excited nitrogen atoms, and electrons, etc., and the argon ions and the nitrogen ions fly to the anode 5 under the action of the negative bias of the workpiece, and energy exchange is generated by collision with the non-ionized gas, so that the ionization rate and the ion energy of the second plasma gradually increase.

[0120] (5) the hollow cathode gun 2 is kept on, a negative bias of 700 V is applied to the workpiece, and the argon gas and the nitrogen gas near the workpiece generate glow discharge, so as to increase the ionization rate and the ion energy of the second plasma.

[0121] (6) the arc discharge of the hollow cathode gun 2 and the glow discharge on the surface of the workpiece are kept, the temperature in the nitriding furnace is kept within 500℃, the workpiece is plasma nitrided for 1 h, and the furnace is opened after the workpiece is cooled to below 180℃ to take samples.

[0122] Example 3

[0123] The preparation method of this embodiment is basically the same as that of embodiment 1, except that the flow rate ratio of argon gas to nitrogen gas is 1:3 during the nitriding process, and the specific steps are as follows:

[0124] (1) the surface of the workpiece is polished to a mirror surface with a surface roughness of <50 nm; the workpiece is immersed in a hot alkali solution to remove oil, then immersed in an alcohol solution for ultrasonic cleaning, and then immersed in deionized water for cleaning; after cleaning, the workpiece is dried and wrapped with a dust-free paper for standby.

[0125] (2) Put the workpiece on the workpiece rack 1 in the nitriding furnace, vacuumize to 20 mPa, and pre-heat the workpiece at 350℃ for 40 min.

[0126] (3) Introduce argon at a flow rate of 300 seem until the atmosphere in the nitriding furnace is stable; turn on the hollow cathode gun 2, set the filament current to 160 A and the anode current to 200 A; apply a negative bias of 100 V to the workpiece, then introduce hydrogen at a flow rate of 150 seem, and form a first plasma by the arc discharge of the hollow cathode gun 2 to ionize the argon and hydrogen, and use the first plasma to perform plasma cleaning on the surface of the workpiece for 90 min; after the plasma cleaning, turn off the hollow cathode gun 2.

[0127] (4) Increase the temperature of the workpiece to a nitriding temperature of 480℃, then introduce argon at a flow rate of 110 seem and nitrogen at a flow rate of 330 seem to make the total pressure in the nitriding furnace 1 Pa; turn on the hollow cathode gun 2, set the filament current to 160 A and the anode current to 140 A, and form a second plasma by the arc discharge of the hollow cathode gun 2 to ionize the argon and nitrogen near the hollow cathode gun 2;

[0128] The second plasma contains argon ions, nitrogen ions, excited argon atoms, excited nitrogen atoms and electrons, and the argon ions and nitrogen ions fly to the anode 5 under the action of the negative bias of the workpiece, and exchange energy with the non-ionized gas through collision, so that the ionization rate and ion energy of the second plasma gradually increase.

[0129] (5) Keep the hollow cathode gun 2 on, and apply a negative bias of 700 V to the workpiece to make the argon and nitrogen near the workpiece produce glow discharge, thereby increasing the ionization rate and ion energy of the second plasma.

[0130] (6) Keep the arc discharge of the hollow cathode gun 2 and the glow discharge on the surface of the workpiece, keep the temperature in the nitriding furnace within 500℃, and perform plasma nitriding on the workpiece for 1 h, then cool the furnace to below 180℃ and open the furnace to take samples.

[0131] Comparative Example 1

[0132] The comparative example is a blank control group, i.e., the M25 high-speed steel workpiece is not subjected to plasma nitriding treatment.

[0133] Test Example

[0134] The nitriding workpieces of Examples 1-3 and Comparative Example 1 are tested as follows:

[0135] (1) Elemental analysis: Elemental analysis was performed using a Zeiss Gemini Sigma 300 scanning electron microscope (SEM) and an Oxford energy dispersive spectroscopy probe. The results are as follows: Figures 2-3 As shown. Among them, Figure 2 This is a SEM image of the nitrided workpiece in Example 1. Figure 3 for Figure 2 The nitrogen element distribution diagram of the nitrided workpiece. (From...) Figures 2-3 It can be seen that the N element signal on the surface of the nitrided workpiece in Example 1 is strong, indicating that the plasma nitriding treatment has achieved a good nitriding effect.

[0136] (2) Metallographic Analysis: Metallographic sections were prepared from the nitrided workpieces in Examples 1-3 and Comparative Example 2 according to the ASTM E3-11 Standard Guide for Metallographic Specimen Preparation. The sections were observed using a Leica DMI8 C metallographic microscope at 1000x magnification. The results are as follows: Figures 4-7 As shown.

[0137] Figure 4 The image shown is a metallographic section of the workpiece in Comparative Example 1. The section has no boundary lines and no nitriding layer has been formed. Figure 5 The image shown is a metallographic section of the nitrided workpiece in Example 1, with a nitrided layer depth of approximately 26 μm. Figure 6 The image shown is a metallographic section of the nitrided workpiece in Example 2, with a nitrided layer depth of approximately 32 μm. Figure 7 The image shown is a metallographic section of the nitrided workpiece in Example 3, with a nitrided layer depth of approximately 34 μm.

[0138] Depend on Figures 4-7 As can be seen, the plasma nitriding method described in Examples 1-3 forms a nitrided layer with a depth of 26μm to 34μm on the surface of the nitrided workpiece. This depth is significantly greater than that of the nitrided layer prepared by traditional glow discharge plasma nitriding, resulting in better nitriding efficiency. The depth of the nitrided layer increases with the increase of nitrogen content in the nitriding atmosphere; therefore, the nitrogen content in the nitriding atmosphere can be adjusted according to different processing requirements. Furthermore, the nitriding process does not require high voltages of thousands of volts and high pressures of tens to hundreds of Pascals; plasma nitriding conditions are milder and energy consumption is lower.

[0139] (3) Hardness, coefficient of friction, and wear track morphology: The Vickers hardness of the nitrided workpieces in Examples 1-3 and Comparative Example 1 at different depths was tested according to GB / T 4343-1991, "Metal Fiber Vickers Hardness Test Method". The results are as follows: Figure 8 As shown; according to the "Standard Test Method for Wear Testing Using a Needle Tester" (ASTM G99-17), the above-mentioned nitrided workpiece was subjected to a friction and wear test using a Lanzhou Huayi MS-T3001 friction and wear testing machine. The friction coefficient test results are as follows.Figure 9 As shown in the figure, the wear track morphology result of the nitrided workpiece of Example 1 is as shown in the figure. Figures 10-13 As shown in the figure.

[0140] As shown in the figure, the Vickers hardness of the nitrided workpiece of Example 1 is obviously greater than that of the nitrided workpiece of Comparative Example 1 in the range of 0-25 μm. Figure 8 As shown in the figure, the Vickers hardness of the nitrided workpiece of Example 1 is obviously greater than that of the nitrided workpiece of Comparative Example 1 in the range of 0-25 μm. Figure 9 As shown in the figure, the Vickers hardness of the nitrided workpiece of Example 1 is obviously greater than that of the nitrided workpiece of Comparative Example 1 in the range of 0-25 μm.

[0141] Figure 10 The wear track morphology of the workpiece in Comparative Example 1 is shown in the figure, and the wear track width is about 375 μm, and the wear is very obvious. Figure 11 The wear track morphology of the nitrided workpiece in Example 1 is shown in the figure, and the wear track width is about 318.1 μm; Figure 12 The wear track morphology of the nitrided workpiece in Example 2 is shown in the figure, and the wear track width is about 306.3 μm; Figure 13 The wear track morphology of the nitrided workpiece in Example 3 is shown in the figure, and the wear track width is about 230.8 μm. Compared with Comparative Example 1, the wear track widths of Examples 1-3 are obviously reduced, the sample surface deformation is small, the un-worn matrix can still be seen inside the wear track, and the wear resistance is well improved.

[0142] The technical features of the above-described embodiments can be combined arbitrarily, and in order to make the description simple, all possible combinations of the technical features in the above-described embodiments are not described, however, as long as the combinations of the technical features do not exist contradictory, it should be considered that it is within the scope of the present application.

[0143] The above-described embodiments only express several embodiments of the present application, and the description is more specific and detailed, but it should not be understood as a limitation on the scope of the patent. It should be pointed out that for ordinary skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are within the scope of the present application. Therefore, the protection scope of the present application patent should be subject to the appended claims, and the description and drawings can be used to explain the content of the claims.

Claims

1. A method for plasma nitriding, characterized in that, Includes the following steps: The workpiece is placed in the reaction chamber, a vacuum is drawn, and the workpiece is preheated. A cleaning gas is introduced, a first negative bias voltage is applied to the workpiece, and the cleaning gas is ionized by the arc discharge of the ion source to form a first plasma, which is then used to perform plasma cleaning on the surface of the workpiece. At the nitriding temperature, a nitrogen-containing gas is introduced, and the nitrogen-containing gas is ionized by the arc discharge of the ion source to form a second plasma containing nitrogen ions; A second negative bias voltage is applied to the workpiece to cause the nitrogen-containing gas on the surface of the workpiece to generate glow discharge, thereby increasing the ionization rate and ion energy of the second plasma; Plasma nitriding was performed while maintaining arc discharge and glow discharge conditions. The ion source is a hollow cathode gun; In the step of ionizing the cleaning gas to form the first plasma through arc discharge of the ion source, the filament current of the hollow cathode gun is 140A~180A and the anode current is 180A~220A. In the step of ionizing the nitrogen-containing gas to form a second plasma containing nitrogen ions through the arc discharge of the ion source, the filament current of the hollow cathode gun is 140A~180A and the anode current is 100A~160A. The nitrogen-containing gas includes argon and nitrogen; the flow rate ratio of the argon to the nitrogen is 1:

3.

2. The plasma nitriding method as described in claim 1, characterized in that, The nitrogen-containing gas meets the following conditions: The total pressure of the nitrogen-containing gas is 0.8 Pa to 1.5 Pa.

3. The plasma nitriding method as described in claim 1 or 2, characterized in that, The nitriding temperature is 450℃~500℃.

4. The plasma nitriding method as described in claim 3, characterized in that, The plasma nitriding time is 0.5h to 2h.

5. The plasma nitriding method as described in claim 1 or 2, characterized in that, The cleaning gas includes argon and hydrogen.

6. The plasma nitriding method as described in claim 5, characterized in that, The cleaning gas meets one or more of the following conditions: 1) The flow rate ratio of the argon gas to the hydrogen gas is (2~3):1; 2) The total pressure of the cleaning gas is 1.0 Pa to 2.0 Pa.

7. The plasma nitriding method as described in claim 1 or 2, characterized in that, One or more of the following conditions must be met: 1) The pressure in the reaction chamber after evacuation is 20mPa~30mPa; 2) The preheating temperature is 200℃~400℃; 3) The first negative bias voltage is 40V~200V; 4) The second negative bias voltage is 600V~800V.

8. The plasma nitriding method as described in claim 1 or 2, characterized in that, Before the step of placing the workpiece in the reaction chamber, the process also includes grinding, polishing, cleaning, and drying the workpiece.

9. A nitrided workpiece, characterized in that, It is prepared by plasma nitriding as described in any one of claims 1 to 8.

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