A spiral wave plasma nitriding method

Abstract

The present invention provides a spiral wave plasma nitriding method, which uses a spiral wave plasma spray gun to inject high ionization rate plasma into a low pressure nitrogen and argon vacuum chamber of 1 to 10 Pa at room temperature, and the TC4 titanium alloy tube and the vacuum chamber wall form a negative bias voltage of about ‑50V, and the nitrogen ions N2 + and N + Under negative bias acceleration, the inner wall of the TC4 titanium alloy tube is penetrated to a depth of 30 to 50 um with a large flux density to form a dense titanium nitride (TiN) and aluminum titanium nitride (TiAlN) film, the hardness of which is about 1200 HV and the melting point is about 2900°C. The present invention provides enhanced coating protection for the inner wall of the titanium alloy workpiece and improves the service life; the titanium nitride and aluminum titanium nitride films not only have high hardness and high melting temperature, but also the dense film coating blocks the hydrogen embrittlement effect of hydrogen atoms in high-temperature fuel gas on the titanium alloy and the oxidation effect of oxidants in the fuel gas on TC4, thereby greatly improving the service life of the TC4 titanium alloy flow guide tube resistant to high-temperature fuel gas.

Description

Technical Field

[0001] The invention relates to the technical field of plating of metal materials, in particular to a spiral wave plasma nitriding method. Background Art

[0002] The main component of TC4 titanium alloy is Ti-6Al-4V, which belongs to (α+β) type titanium alloy. TC4 has a high tensile strength of about 1000MPa and a melting point of about 1630℃. It has certain advantages in some high-temperature ablation-resistant environments. However, when TC4 titanium alloy tubes face high-temperature combustion flame plumes, the titanium alloy is more susceptible to the penetration of H atoms in the combustion flame, forming hydrogen embrittlement and causing ablation damage to the tube wall. Moreover, the thermal conductivity of TC4 titanium alloy is relatively low, about 8Wm -1 K -1 When faced with a high-temperature flame plume of 3000K, its inner wall heats up rapidly, causing severe thermal ablation.

[0003] In addition, there is currently a normal pressure high temperature nitriding process, which nitrides the TC4 titanium alloy workpiece in a closed container at an atmospheric pressure and a high temperature of 820°C. However, this surface treatment process requires a long treatment time (about 20 to 40 hours), resulting in high power consumption and high processing costs. This is mainly because the static nitriding process in a closed space causes some low-energy nitrogen molecules to be adsorbed on the surface of the TC4 titanium alloy, forming a high-density nitrogen adsorption layer, which prevents high-energy nitrogen molecules from penetrating into the surface of the titanium alloy workpiece, thereby requiring the nitriding treatment time to be extended;

[0004] The improved circulating atmospheric pressure high-temperature nitrogen nitriding process is to connect two nitriding treatment container tubes in series and circulate the high-temperature nitrogen in the two containers through a high-temperature vacuum pump. The flowing nitrogen destroys the lower energy nitrogen adsorption layer adsorbed on the surface of the titanium alloy, thereby increasing the flow density of higher energy nitrogen molecules injected into the surface of the titanium alloy and shortening the formation cycle of titanium nitride and aluminum titanium nitride coatings by about 10 hours.

[0005] However, the atmospheric pressure high temperature nitriding process still has the problems of long processing time and high energy consumption. The power consumption is mainly wasted in the heat flow transfer from the atmospheric pressure to the container wall. In addition, under atmospheric pressure, most of the energy of positive ions accelerated by negative bias pressure is consumed in the collision with atmospheric particles. This also determines that the coating time is relatively long.

[0006] Therefore, a fast and efficient nitriding method is urgently needed. Summary of the invention

[0007] The invention provides a spiral wave plasma nitriding method in order to improve the efficiency of the nitriding method and reduce the energy consumption. The spiral wave plasma spray gun is used to inject high ionization rate plasma into a nitrogen argon vacuum chamber with a low pressure of 1 to 10 Pa and room temperature. The TC4 titanium alloy tube and the vacuum chamber wall form a negative bias voltage of about -50V. The nitrogen ions N 2 + and N + Under negative bias acceleration, the inner wall of the TC4 titanium alloy tube is penetrated to a depth of 30 to 50 um with a large flux density to form a dense titanium nitride (TiN) and aluminum titanium nitride (TiAlN) film, the hardness of which is about 1200 HV and the melting point is about 2900°C. The present invention provides enhanced coating protection for the inner wall of the titanium alloy workpiece and improves the service life; the titanium nitride and aluminum titanium nitride films not only have high hardness and high melting temperature, but also the dense film coating blocks the hydrogen embrittlement effect of hydrogen atoms in high-temperature fuel gas on the titanium alloy and the oxidation effect of oxidants in the fuel gas on TC4, thereby greatly improving the service life of the TC4 titanium alloy flow guide tube resistant to high-temperature fuel gas.

[0008] The present invention provides a helical wave plasma nitriding method, comprising the following steps:

[0009] S1, the working gas enters the helical wave plasma plume discharge gun, and the helical wave plasma plume is obtained under the action of the background magnetic field and electric field;

[0010] S2, the helicon plasma plume is ejected from the plasma plume discharge gun and enters the vacuum chamber to obtain a directional flow helicon plasma, and the workpiece to be processed is located at the front end of the directional flow helicon plasma;

[0011] S3. The directional flow helical wave plasma penetrates into the interior of the part to be processed through a high voltage glow discharge method and chemically reacts with the part to be processed to form a coating, and the helical wave plasma nitriding is completed.

[0012] In the helicon wave plasma nitriding method of the present invention, as a preferred embodiment, in step S1, the working gas includes nitrogen;

[0013] In step S2, the pressure of the vacuum chamber is 1-10 Pa, the vacuum chamber is at room temperature, the helicon wave plasma plume discharge gun and the workpiece to be processed are both arranged in the vacuum chamber, and the workpiece to be processed is a titanium alloy part or a titanium alloy-plated part;

[0014] In step S3, the high voltage glow discharge method is to make the chamber wall of the vacuum chamber at zero voltage and the part to be processed at negative bias voltage;

[0015] Directed helicon plasma contains ionized N 2 + and N +, N 2 + and N + The collision mean free path in the plasma sheath of the workpiece surface to be processed is greater than the plasma sheath thickness, N 2 + and N + Under the action of negative bias voltage, the material is accelerated to be injected into the inner part of the workpiece to be processed at a depth of 30 to 60 um to form titanium nitride and aluminum titanium nitride thin film coatings.

[0016] In the spiral wave plasma nitriding method of the present invention, as a preferred embodiment, in step S3, the titanium alloy is Ti-6Al-4V;

[0017]

[0018]

[0019] In the helicon wave plasma nitriding method of the present invention, as a preferred embodiment, the electron density of the helicon wave plasma plume is 10 19 ~10 20 m -3 , electron temperature 20~50eV, gas temperature 1~20eV.

[0020] The helicon wave plasma nitriding method described in the present invention, as a preferred embodiment, the helicon wave plasma plume discharge gun comprises a hollow insulating layer, a shielding layer and a DC electromagnetic coil sequentially arranged outside the insulating layer, one end of the insulating layer is a working gas inlet, and the other end is a helicon wave plasma plume outlet;

[0021] The DC electromagnetic coil forms a background magnetic field, and the electrons are confined in the background magnetic field.

[0022] In the helical wave plasma nitriding method of the present invention, as a preferred embodiment, in step S1, the cyclotron radius of the electrons in the background magnetic field is:

[0023]

[0024] Among them, m e is the mass of the electron, v ⊥e is the perpendicular magnetic field component of the electron thermal velocity, e is the charge of the electron, and B is the background magnetic field;

[0025] The insulating layer is a quartz glass tube.

[0026] The spiral wave plasma nitriding method described in the present invention is, as a preferred embodiment, in step S2, a vacuum unit is arranged in the vacuum chamber, and the vacuum unit evacuates air to maintain a pressure of 1 to 10 Pa in the vacuum chamber, and a spiral wave plasma plume is ejected from a plasma plume discharge gun to obtain a directional flow spiral wave plasma.

[0027] In the helicon wave plasma nitriding method described in the present invention, as a preferred embodiment, in step S1, the working gas also includes argon gas, and the argon ions are not constrained by the background magnetic field and generate secondary electrons on the inner wall of the insulating layer to activate the helicon wave plasma plume.

[0028] The spiral wave plasma nitriding method described in the present invention is, as a preferred embodiment, in step S3, the argon ions in the directional flow spiral wave plasma first eliminate the gas adsorption layer on the surface of the workpiece to be processed, thereby reducing the energy consumption of the gas adsorption layer on the injected ions, and then impact the surface of the workpiece to be processed to form titanium alloy lattice distortion.

[0029] In the helicon wave plasma nitriding method described in the present invention, as a preferred embodiment, in step S3, the temperature of the background gas in the vacuum chamber is close to room temperature.

[0030] The workpiece to be processed in the present invention may be a TC4 titanium alloy tube, or may be a titanium alloy workpiece of other shapes or a metal part plated with titanium alloy.

[0031] The present invention adopts spiral wave plasma nitriding technology to form a titanium nitride and aluminum titanium nitride film with a thickness of about 50um on the inner wall of the TC4 titanium alloy tube. The hardness of the film is about 1200HV and the melting point is about 2900℃. It provides enhanced coating protection for the inner wall of the TC4 titanium alloy guide tube and improves the service life of the TC4 titanium alloy. Titanium nitride and aluminum titanium nitride films not only have high hardness and high melting temperature, but also the dense film coating blocks the hydrogen embrittlement effect of hydrogen atoms in high-temperature fuel gas on titanium alloy and the oxidation effect of oxidants in fuel gas on TC4. Thereby greatly improving the service life of the TC4 titanium alloy guide tube resistant to high-temperature fuel gas.

[0032] The high voltage glow discharge plasma nitriding process is to apply high voltage between the titanium alloy workpiece and the container wall. The container wall is at ground potential, the titanium alloy workpiece is at a negative high voltage potential of -2000V, the distance between the container wall and the workpiece is about 10cm, and the electric field strength is about 2*10 4 V / m. The density of nitrogen in the container is about 6.7*10 21 m -3 , the gas temperature is about 1100K (827℃), and the gas pressure is about 0.001 atmosphere (101.3Pa). The elastic collision mean free path of the initial electrons in the gas is about

[0033]

[0034] where σ c =1.8×10 -19 (m 2) is the elastic collision cross section of the electron colliding with the nitrogen molecule. The electron is accelerated by the electric field within its mean free path, and the kinetic energy of the electron increases by about

[0035] Δε e =λ mfp E=8.4×10 -4 ×2×10 4 =16.8(eV)

[0036] The kinetic energy of the electron increases, making the energy of the collision between the electron and the nitrogen molecule exceed the ionization energy of the nitrogen molecule∈ i =15.56eV, causing avalanche ionization of the gas. The conductivity of the ionized gas increases, which reduces the voltage of the ionized gas, forming a plasma sheath on the surface of the workpiece being processed, generating a glow discharge on the surface of the workpiece. In the sheath, the ionized nitrogen ions N 2 + It is accelerated by the electric field and injected into the workpiece at a depth of 20 to 50um to form titanium nitride and aluminum titanium nitride thin film coatings. The glow discharge nitriding method can further reduce the electrical energy consumption of the nitriding process and reduce the cost of nitriding treatment. During the glow discharge process, nitrogen ions are accelerated by the electric field, so the injected ion flux density is much greater than the neutral high-temperature molecular flux density, and the ion flux in the higher energy range is larger, and the ion implantation depth of the titanium alloy surface is deeper.

[0037] Although high voltage glow discharge can further reduce energy consumption and improve the performance of titanium nitride coating, at 0.001 atmosphere pressure and 827℃ temperature (gas density is about 6.7*10 21 m -3 ), the heat flux density of the neutral gas through the container wall is still not small, and the collision between ions and neutral gas in the plasma sheath will lose a lot of energy, so the ionization rate of the gas in the sheath is low, and the electron density is only 10 16 ~10 17 m -3 The ionization rate is only 1%. Most of the energy that causes high voltage discharge is consumed in the collision loss between nitrogen ions and nitrogen.

[0038] In order to further reduce the electrical energy consumption and cost of the nitriding process, the present invention will adopt the spiral wave plasma nitriding technology route, make full use of the high ionization rate characteristics of the spiral wave plasma, and efficiently implement the nitriding operation under the negative bias state of the workpiece under the low gas pressure of 1 to 10Pa and low gas temperature in the container through the plasma generated by the spiral wave to form high-quality titanium nitride and aluminum titanium nitride films, thereby extending the life of the titanium alloy tube under high-temperature combustion plume conditions.

[0039] The present invention utilizes the characteristics of spiral waves to efficiently form high-density plasma, and provides ionized nitrogen-argon mixed plasma to the vacuum chamber, thereby forming a plasma environment in the background gas of the vacuum chamber at low power;

[0040] The spiral wave plasma jet plume and vacuum unit exhaust maintain the air pressure in the vacuum chamber at about 10Pa, thus forming a stable directional flow in the vacuum chamber and eliminating the adsorbed gas layer on the surface of the titanium alloy;

[0041] The negative bias voltage setting of the titanium alloy workpiece allows the nitrogen ions and nitrogen ions in the plasma environment to be directly injected into the titanium alloy material at a high speed under the acceleration of the negative bias voltage to form a nitriding coating. Argon ions will not produce chemical reactions with elements such as titanium and aluminum, but only provide distortion of the titanium alloy lattice, providing more favorable penetration conditions for the nitrogen ions and nitrogen ions to penetrate deeply;

[0042] The mean free path of the collision of nitrogen ions and nitrogen ions in the plasma sheath on the surface of the workpiece is about 0.11m, which is greater than the thickness of the plasma sheath. Therefore, the accelerated nitrogen ions and nitrogen ions penetrate into the interior of the titanium alloy without collision loss. This is one of the core mechanisms of the spiral wave plasma nitriding method with low energy consumption;

[0043] In the spiral wave plasma nitriding technology, the temperature of the background gas in the vacuum chamber is near room temperature. The heat flow energy transferred from the background gas to the wall of the vacuum chamber is very small and can be ignored. The higher energy nitrogen ions and nitrogen ions are accelerated by the negative bias in the plasma sheath near the surface of the workpiece and injected into the titanium alloy without collision loss. The spiral wave plasma nitriding method will not cause thermal deformation and thermal phase change of the titanium alloy workpiece, thereby ensuring that the quality of the titanium alloy workpiece will not be adversely changed due to nitriding.

[0044] The present invention has the following advantages:

[0045] The present invention uses a spiral wave plasma to spray plasma to the background gas in the vacuum chamber to form a directional flow plasma environment for nitriding processing. The energy of nitrogen ions and nitrogen ions penetrating into the titanium alloy is directly accelerated by a negative bias voltage, and the mean free path of nitrogen ions in the vacuum chamber is greater than the thickness of the negative bias plasma sheath, and the accelerated nitrogen ions will not lose energy due to collision. This is one of the main mechanisms for the low efficiency of the spiral wave plasma vacuum nitriding method; the spiral wave plasma jet beam is also one of the methods for efficiently generating high-density plasma, which also constitutes a more economical plasma generation method. In addition, the coordinated work of the spiral wave beam and the vacuum pump forms a stable directional flow environment in the vacuum chamber, eliminating the gas adsorption layer on the surface of the titanium alloy workpiece, thereby reducing the energy consumption of the gas adsorption layer on the injected ions. In the spiral wave plasma vacuum nitriding method, the pressure of the vacuum chamber background gas is about 1 to 10Pa, and the gas temperature is about room temperature. It will not cause thermal deformation and thermal phase change of the nitrided workpiece, thereby ensuring that the quality of the nitrided workpiece remains unchanged. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 The present invention is a flow chart of a spiral wave plasma nitriding method;

[0047] Figure 2 It is a schematic diagram of the structure of a spiral wave plasma plume discharge gun in a spiral wave plasma nitriding method.

[0048] Reference numerals:

[0049] 1. Helical wave plasma plume discharge gun; 11. Insulation layer; 12. Shielding layer; 13. DC electromagnetic coil. DETAILED DESCRIPTION

[0050] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0051] Example 1

[0052] like Figure 1 As shown, a spiral wave plasma nitriding method comprises the following steps:

[0053] S1, the working gas enters the helical wave plasma plume discharge gun 1, and a helical wave plasma plume is obtained under the action of the background magnetic field and electric field;

[0054] Working gases include nitrogen and argon;

[0055] The electron density of the helicon plasma plume is 10 19 ~10 20 m -3, electron temperature 20 ~ 50eV, gas temperature 1 ~ 20eV;

[0056] like Figure 2 As shown, the helicon wave plasma plume discharge gun 1 comprises a hollow insulating layer 11 and a shielding layer 12 and a DC electromagnetic coil 13 which are sequentially arranged outside the insulating layer 11. One end of the insulating layer 11 is a working gas inlet, and the other end is a helicon wave plasma plume outlet.

[0057] The DC electromagnetic coil 13 forms a background magnetic field, and the electrons are confined in the background magnetic field;

[0058] The cyclotron radius of an electron in the background magnetic field is:

[0059]

[0060] Among them, m e is the mass of the electron, v ⊥e is the perpendicular magnetic field component of the electron thermal velocity, e is the charge of the electron, and B is the background magnetic field;

[0061] The insulating layer 11 is a quartz glass tube;

[0062] The argon ions are not confined by the background magnetic field and generate secondary electrons on the inner wall of the insulating layer 11 to activate the helicon plasma plume;

[0063] S2, the helicon plasma plume is ejected from the plasma plume discharge gun 1 and enters the vacuum chamber to obtain a directional flow helicon plasma, and the workpiece to be processed is located at the front end of the directional flow helicon plasma;

[0064] The pressure of the vacuum chamber is 1-10 Pa, the vacuum chamber is at room temperature, the helicon wave plasma plume discharge gun 1 and the workpiece to be processed are both arranged in the vacuum chamber, and the workpiece to be processed is a titanium alloy part or a titanium alloy-plated part;

[0065] A vacuum unit is arranged in the vacuum chamber, and the vacuum unit evacuates air to maintain a pressure of 1-10 Pa in the vacuum chamber, and a spiral wave plasma is obtained after the spiral wave plasma plume is ejected from the plasma plume discharge gun 1 to obtain a directional flow spiral wave plasma;

[0066] S3, the directional flow helical wave plasma penetrates into the interior of the part to be processed by a high voltage glow discharge method and chemically reacts with the part to be processed to form a coating, and the helical wave plasma nitriding is completed;

[0067] The high voltage glow discharge method is to make the wall of the vacuum chamber zero voltage and the workpiece to be processed negative bias voltage, and the negative bias voltage means that the voltage relative to the vacuum chamber wall is negative;

[0068] Directed helicon plasma contains ionized N 2+ and N + , N 2 + and N + The collision mean free path in the plasma sheath of the workpiece surface to be processed is greater than the plasma sheath thickness, N 2 + and N + Under the action of negative bias, the injection is accelerated to form titanium nitride and aluminum titanium nitride thin film coatings at a depth of 30 to 60 μm inside the part to be processed;

[0069] Titanium alloy is Ti-6Al-4V;

[0070]

[0071]

[0072] The argon ions in the directional flow helicon wave plasma first eliminate the gas adsorption layer on the surface of the workpiece to be processed, thereby reducing the energy consumption of the gas adsorption layer on the injected ions, and then collide with the surface of the workpiece to be processed to form the titanium alloy lattice distortion;

[0073] The temperature of the background gas in the vacuum chamber is close to room temperature.

[0074] Example 2

[0075] like Figure 1 As shown, a spiral wave plasma nitriding method includes the following three steps:

[0076] 1. A helical wave plasma spray gun 1 prepares a helical wave plasma plume;

[0077] The principle is as follows: Helicon wave is a plasma wave excited and propagated in a finite radius cylindrical magnetized plasma. The helicon wave propagates in the cylindrical magnetized plasma and has a component k parallel to the background magnetic field. || .

[0078] Current research results show that the attenuation mechanism of helicon waves in plasma is mainly Landau damping and Trevelpiece-Gloud (TG) damping mechanism. Helicon wave plasma can produce electron density 10 19 ~10 20 m -3 , the electron temperature is about 20~50eV, and the ion and neutral gas temperature is about 1~20eV.

[0079] The cyclotron radius of the electron in the background magnetic field is approximately

[0080]

[0081] Among them, me is the mass of the electron, v ⊥e is the perpendicular magnetic field component of the electron thermal velocity, e is the charge of the electron, and B is the background magnetic field. Assume that in this invention, the temperature of the electron is about 30eV, the background magnetic field is about 500Gs,

[0082]

[0083] Argon and nitrogen mixed gas (1:1 molecular number) is used as the ionizing medium. The temperature of argon ions and nitrogen ions is 5eV, and the cyclotron radius of argon ions is

[0084]

[0085] Nitrogen ion N 2 + Radius of gyration

[0086]

[0087] In a 500Gs background magnetic field, electrons are confined by the background magnetic field, while argon ions cannot be confined by the magnetic field and can collide with the quartz tube wall to generate secondary electrons. The plasma electron density in the quartz tube is about 10 19 m -3 , while the temperature of argon ions, nitrogen ions and nitrogen ions and neutral argon atoms, nitrogen molecules and nitrogen atoms is about 5eV. The length of the quartz glass tube is about 0.2m, which is full of plasma. The average density of plasma electrons is about 5*10 19 m -3 , the electron temperature is about 30 eV, the temperature of ions and neutral gas is about 5 eV, and the density of ions is also 5*10 19 m -3 , while the density of neutral atoms (molecules) is about 5*10 19 m -3 , the ionization rate is about 50%. The directional speed of the plasma at the quartz tube outlet is about 10kms -1 , the cross-sectional area of ​​the quartz glass tube

[0088]

[0089] Directed atomic (molecular) flow of plasma plume

[0090]

[0091] 60 seconds requires 9.42*10 20 Argon atoms, the volume of the gas at standard atmospheric pressure is about

[0092]

[0093] The gas intake volume is about 175 sccm (175 ml of standard gas per minute).

[0094] The structure of the spiral wave plasma discharge tube is as follows Figure 1 As shown: the length of the quartz glass tube 11 is about 300 mm and the diameter is 10 mm.

[0095] The background magnetic field constrains the electrons of the spiral wave plasma and prevents the radial heat flow of the electrons. Therefore, the heat flux density of the electrons to the wall of the quartz glass tube is greatly reduced, and the heat flux density transferred by argon atoms and argon ions, nitrogen ions and nitrogen molecules, nitrogen atoms and nitrogen ions is limited by the background magnetic field and is not enough to melt the quartz glass. The spiral wave plasma is ejected into a vacuum chamber with a pressure of 10Pa, and the spiral wave plume is ejected from the quartz glass tube and flows to a place with a weaker magnetic field (ignoring the magnetic pressure). The radial pressure of the plume is:

[0096] P t =P oi +P e =n oi kT oi +n e kT e

[0097] Where n oi is the sum of the densities of all ions, atoms, and molecules in the plume, so n oi =10 20 (m -3 ), here we ignore the process of nitrogen molecules dissociating into nitrogen atoms. e =5×10 19 (m -3 ), kT e =30eV, kT oi = 5 eV, so the radial pressure of the unexpanded plume is about

[0098] P t =n oi kT oi +n e kT e =(10 20 ×5+5×10 19 ×30)×1.6×10 -19

[0099] =320(Pa)

[0100] Therefore, the spiral wave plume is ejected into the vacuum chamber gas at a pressure of 10Pa, and the plume rapidly expands to a density close to the vacuum chamber ambient pressure in the low magnetic field region. -4 The number of atoms and molecules ejected into the vacuum chamber in one second is only 7.85×10 15, which is much smaller than the number of nitrogen-argon mixed gas molecules at low temperature (400K) and low pressure (10Pa) in a 0.2m diameter cylinder after the plume expands. The density of the nitrogen-argon mixed gas in the vacuum chamber is about

[0101]

[0102] Where D o =0.2(m) is the diameter of the spiral wave plume in the vacuum chamber after expansion, and L=1(m) is the length of the spiral wave plume in the vacuum chamber.

[0103]

[0104] The number of high-temperature ions and atoms (molecules) ejected by the expanded plume is about 7.85*10 15 , which is much smaller than the number of low-temperature gas atoms (molecules) in the expanding plume, which is about 1.8*10 19 The collision of plume particles with the gas atoms in the vacuum chamber will not increase the temperature of the gas in the vacuum chamber much, and the gas in the vacuum chamber also transfers heat to the outside through the vacuum chamber wall. Therefore, the low-temperature gas in the vacuum chamber basically maintains a temperature of 400K, and the temperature of the outer wall of the vacuum chamber is still near room temperature (about 30℃).

[0105] 2. Nitriding of titanium alloy workpieces with negative bias

[0106] In the vacuum chamber, nitrogen ions The cross section for collisions with background gas atoms (molecules) is approximately σ c+ =5×10 -19 (m 2 ), the density of the background gas particles in the vacuum chamber is about 1.8*10 19 m -3 The mean free path of elastic collision of nitrogen ions is about

[0107]

[0108] The thickness of the plasma sheath near the negatively biased nitriding workpiece is less than the mean free path of the nitrogen ion collision, so the energy lost by the nitrogen ions in the sheath due to collision can be ignored. After being accelerated by the -20V bias voltage, the nitrogen ions are basically injected into the titanium alloy material with a kinetic energy of 20eV. A nitriding layer with a depth of 30 to 60um can be formed. Since the background gas particle density is about 2*10 19 m -3 The nitrogen ions efficiently generated by the spiral wave plasma have almost no kinetic energy loss in the negative bias workpiece plasma sheath, so its energy utilization rate of nitriding is higher than some other nitriding methods, such as the atmospheric pressure high temperature static nitrogen nitriding method, the atmospheric pressure high temperature circulating nitrogen nitriding method, and the low pressure glow discharge nitriding method.

[0109] 3. Titanium alloy nitriding surface coating performance

[0110] The main component of TC4 titanium alloy is Ti-6Al-4V. Through spiral wave plasma vacuum nitriding, a titanium nitride (TiN) and aluminum titanium nitride (TiAlN) coating with a thickness of about 50um and a dense structure can be formed on the surface of the titanium alloy. The surface hardness of the titanium nitride (TiN) and aluminum titanium nitride (TiAlN) coatings is about HV1200, and the melting point is about 2900℃. This greatly improves the surface hardness and melting point of the TC4 titanium alloy material (the hardness of TC4 titanium alloy is about HV330, and the melting point is about 1660℃). In particular, due to the formation of dense titanium nitride and aluminum titanium nitride coatings, high-temperature (2700℃) combustion gases (mainly CO) are prevented from entering the surface. 2 , H 2 O, NOx, O 2 , H, etc.) on the oxidation and hydrogen embrittlement of titanium alloys at high temperature. The internal hardness of the TC4 titanium alloy flow guide tube is about HV330. After the spiral wave plasma nitriding treatment, the hardness gradient of the surface nitriding layer changes gently, thereby increasing the service life of the TC4 titanium alloy high-temperature gas flow guide tube by about 100 times. The bonding force between the coating and the base material is much greater than the bonding force of physical adhesion, which can reach 350MPa. The bonding force of physical adhesion is about 40MPa.

[0111] The processing time of this embodiment is about 1 hour, and the power consumption is about 5 kW.

[0112] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A helical wave plasma nitriding method, characterized in that: The following steps are involved: S1, the working gas enters the helical wave plasma plume discharge gun (1), and a helical wave plasma plume is obtained under the action of the background magnetic field and electric field; The helicon wave plasma plume discharge gun (1) comprises a hollow insulating layer (11), a shielding layer (12) and a DC electromagnetic coil (13) which are sequentially arranged outside the insulating layer (11); one end of the insulating layer (11) is a working gas inlet, and the other end is a helicon wave plasma plume outlet; The DC electromagnetic coil (13) forms the background magnetic field, and the electrons are confined in the background magnetic field; The working gas includes nitrogen and argon, the argon ions are not constrained by the background magnetic field and generate secondary electrons on the inner wall of the insulating layer (11) to activate the helicon wave plasma plume, and the helicon wave plasma plume is an ionized nitrogen and argon mixed plasma; S2, the helicon plasma plume is ejected from the helicon plasma plume discharge gun (1) and enters a vacuum chamber to obtain a directional flow helicon plasma, wherein the workpiece to be processed is located at the front end of the directional flow helicon plasma; The pressure of the vacuum chamber is 1-10 Pa, the vacuum chamber is at room temperature, the helicon wave plasma plume discharge gun (1) and the workpiece to be processed are both arranged in the vacuum chamber, and the workpiece to be processed is a titanium alloy part or a titanium alloy-plated part; The electron density of the helicon plasma plume is 10 19 ~10 20 m -3 , electron temperature 20~50eV, gas temperature 1~20eV; S3, the directional flow helicon wave plasma penetrates into the interior of the workpiece to be processed by a high voltage glow discharge method and chemically reacts with the workpiece to be processed to form a coating, and the background gas temperature in the vacuum chamber is close to room temperature; The high voltage glow discharge method is to make the chamber wall of the vacuum chamber at zero voltage and the workpiece to be processed at negative bias voltage; The directional flow helicon wave plasma includes ionized N2 + and N + , N2 + and N + The collision mean free path in the plasma sheath of the surface of the workpiece to be processed is greater than the plasma sheath thickness, N2 + and N + Under the action of negative bias, the injection is accelerated into the workpiece to be processed to a depth of 30 to 60 μm to form a titanium nitride and aluminum titanium nitride thin film coating, and the spiral wave plasma nitriding is completed; Titanium alloy is Ti-6Al-4V; ； ； The argon ions in the directional flow helicon wave plasma first eliminate the gas adsorption layer on the surface of the workpiece to be processed, thereby reducing the energy consumption of the gas adsorption layer on the injected ions, and then collide with the surface of the workpiece to be processed to form titanium alloy lattice distortion.

2. A helicon wave plasma nitriding method according to claim 1, characterized in that: In step S1, the cyclotron radius of the electron in the background magnetic field is: ； in, is the mass of the electron, is the perpendicular magnetic field component of the electron thermal velocity, is the charge of the electron, is the background magnetic field; The insulating layer (11) is a quartz glass tube.

3. A helicon wave plasma nitriding method according to claim 1, characterized in that: In step S2, a vacuum unit is arranged in the vacuum chamber, and the vacuum unit evacuates air to maintain a pressure of 1-10 Pa in the vacuum chamber. The helicon wave plasma plume is ejected from the helicon wave plasma plume discharge gun (1) to obtain the directional flow helicon wave plasma.

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