Surface modification structure of graphite filled porous nitriding layer and preparation method

Abstract

The invention discloses a surface modification structure of a graphite-filled porous nitriding layer and a preparation method, and relates to the technical field of metal plasma surface treatment.The preparation method comprises the steps that (1) the surface of a steel part is subjected to grinding and polishing pretreatment; (2) low-temperature plasma nitriding; (3) performing high-temperature plasma carbon-oxygen co-permeation in the same furnace; and (4) plasma-assisted chemical vapor deposition (PECVD) treatment is carried out in the same furnace so as to fill the graphite. According to the method, a recognized green plasma surface treatment technology is adopted, multi-step same-furnace treatment is carried out in an ion nitriding furnace, the cost is reduced, the process is shortened, the depth of an infiltrated layer is increased through a high-temperature diffusion mechanism, and graphite / nitride is compounded in the depth direction of a workpiece through a graphite filling mechanism; the defect that an existing graphite / nitride surface modification structure can only play a role of a single coating structure in actual abrasion due to compounding in the workpiece surface direction can be overcome.

Description

Technical Field

[0001] The present invention relates to the technical field of metal plasma surface treatment, and particularly to a surface modification structure of a graphite-filled porous nitrided layer and a preparation method thereof. Background Art

[0002] Nitriding is the most commonly used surface treatment process for key basic components of high-end equipment, such as bearings, drills, precision molds, etc., which can significantly improve the wear resistance, corrosion resistance and fatigue resistance of workpieces. Currently, the commonly used nitriding methods in industry are salt bath nitriding, gas nitriding, vacuum nitriding and ion nitriding, etc. Among them, ion nitriding has broad application prospects due to its low cost and green and pollution-free characteristics. However, the nitrided layer has a relatively high surface friction coefficient due to the nitrides formed on the surface, and its performance is not ideal in complex friction and wear environments. With the development of high-end equipment, the need for friction reduction and depth-related performance of the nitrided layer needs to be further improved.

[0003] To solve the problem of the relatively high friction coefficient of the traditional nitrided layer, Patent CN 113430485 A performs ion carburizing or carbonitriding on nitrided workpieces under the conditions of 250°C to 550°C, and a diamond-like carbon film containing a graphite structure is obtained on the surface of the nitrided layer, which can significantly reduce the friction coefficient and reduce friction loss. However, the nitrided layer obtained by this process is compounded with the graphite phase along the surface direction of the workpiece, and the diamond-like carbon film is very thin. Under some special working conditions, the friction coefficient of the workpiece surface will rise sharply after the diamond-like carbon film is worn through or peeled off. Therefore, a new type of composite nitriding surface modification structure and preparation method are needed to solve the above problems. Summary of the Invention

[0004] One of the purposes of the present invention is to provide a surface modification structure of a graphite-filled porous nitrided layer with better comprehensive surface performance, and the graphite / nitride is compounded along the depth direction of the workpiece through a graphite filling mechanism, so as to solve the defect that the existing graphite / nitride surface modification structure is compounded along the surface direction of the workpiece and can only play the role of a single coating structure in actual wear.

[0005] To solve the above technical problems, the present invention adopts the following technical solutions: A preparation method of a surface modification structure of a graphite-filled porous nitrided layer, comprising the following steps:

[0006] (1) Grind and polish the steel workpiece to make the surface roughness (Ra) of the workpiece lower than 0.5 μm, and then perform cleaning and drying to complete the pretreatment.

[0007] (2) Put the pretreated steel workpiece into an ion nitriding furnace, introduce a nitriding atmosphere, and perform low-temperature plasma nitriding at 400-500°C for 3-20 hours to obtain dense nitrided layers with different thicknesses;

[0008] (3) Introduce a carbon-oxygen co-permeation atmosphere into the same furnace, and conduct high-temperature plasma carbon-oxygen co-permeation at 600-700 °C for 0.5-2 hours to obtain a porous multi-element infiltration layer of carbon, nitrogen, and oxygen;

[0009] (4) Introduce a nitriding atmosphere and a carbon-containing atmosphere into the same furnace, and conduct plasma-assisted chemical vapor deposition (PECVD) treatment at 500-650 °C for 1-3 hours to obtain a surface modification structure with a porous nitrided layer filled with graphite.

[0010] The present invention applies the principle that nitrides with a high nitrogen content in the nitrided layer form N2 pores at high temperatures to prepare a porous nitrided layer structure.

[0011] Preferably, the steel workpiece in step (1) is one of steel types with a chromium content of less than 13%, such as pure iron, carbon steel, maraging steel, and hot work die steel.

[0012] Preferably, the nitriding atmosphere in step (2) is a mixed gas composed of N2 and H2 containing 40%-60% (v / v) N2, and the nitriding pressure is 300-800 Pa, forming a nitrided layer with a high nitrogen content mainly composed of ε-Fe3N phase or ε-Fe3(N,C) phase.

[0013] The present invention applies the mutual transformation relationship between ε-Fe3N nitride and θ-Fe3C carbide, and by strictly controlling the carbon-oxygen co-permeation atmosphere, pressure, treatment temperature, and treatment time, the formation rate of N2 pores is controlled by forming carbides at high temperatures.

[0014] Preferably, the carbon-oxygen co-permeation atmosphere in step (3) is a mixed gas composed of CO2 and H2 containing 50%-70% (v / v) CO2, and the carbon-oxygen co-permeation pressure is 700-1300 Pa, forming a carbon, nitrogen, and oxygen porous infiltration layer mainly composed of θ-Fe3C phase + ε-Fe3(N,C) phase.

[0015] The present invention applies the plasma-assisted chemical vapor deposition (PECVD) effect, and by controlling the PECVD treatment temperature, atmosphere, pressure, and time, the internal stress of the carbon film is regulated to form a graphite phase capable of filling pores.

[0016] Preferably, the nitriding atmosphere and the carbon-containing atmosphere in step (4) are mixed gases composed of N2, CO2, and H2, where the N2 content is 10%-40%, the CO2 content is 30%-60%, and the H2 content is 10%-50%. The PECVD treatment pressure is 700-1300 Pa. Finally, a new nitrided layer structure mainly composed of graphite phase + γ′-Fe4(N,C) phase and with the graphite phase and γ′-Fe4(N,C) phase compounded along the depth direction of the workpiece is formed. The porosity range of the nitrided layer is 20%-40%.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0018] 1. Through the graphite filling mechanism, the present invention enables the composite of graphite / nitride along the depth direction of the workpiece. In actual wear, graphite and nitride can act synergistically, thereby synchronously improving the wear resistance and friction reduction effect. It can solve the drawback that the existing graphite / nitride surface modification structure, due to the composite along the surface direction of the workpiece, can only play the role of a single coating structure in actual wear.

[0019] 2. Compared with the prior art methods of preparing composite coatings of nitride phase and graphite phase by gas nitriding, salt bath nitriding, magnetron sputtering, arc ion plating, etc., the present invention prepares the composite coating through multi-step in-furnace treatment in an ion nitriding furnace, which not only achieves environmental protection but also reduces costs and shortens the process.

[0020] 3. The present invention improves the depth of the nitrided layer through the high-temperature diffusion mechanism. At the same time, through the mutual transformation of nitride and carbide, the porosity of the surface nitrided layer is effectively controlled. While increasing the depth of the nitrided layer, the surface hardness is maintained, and it has good load-bearing performance. Description of the Drawings

[0021] Figure 1 XRD detection diagrams for Examples 1-2 and Comparative Examples 1-2;

[0022] Figure 2 Cross-sectional backscattered electron imaging diagram for Example 1;

[0023] Figure 3 Cross-sectional hardness detection diagrams for Examples 1-2 and Comparative Examples 1-2;

[0024] Figure 4 Friction coefficient diagrams for Examples 1-2 and Comparative Examples 1-2;

[0025] Figure 5 Surface indentation method toughness detection diagrams for Examples 1-2 and Comparative Examples 1-2. Detailed Embodiments

[0026] For the convenience of understanding by those skilled in the art, the present invention will be further described below in conjunction with examples and drawings. The content mentioned in the embodiments does not limit the present invention.

[0027] Example 1

[0028] A preparation method for a surface modification structure of a graphite-filled porous nitrided layer, comprising the following steps:

[0029] (1) Grind and polish the steel workpiece to make the surface roughness (Ra) of the workpiece lower than 0.5 μm, and then perform cleaning and drying to complete the pretreatment.

[0030] (2) Put the pre-treated steel parts into an ion nitriding furnace for low-temperature plasma nitriding. The nitriding temperature is 500 °C, the nitriding time is 4 hours, the nitriding atmosphere is a mixed gas composed of N2 and H2, the volume ratio of N2 to H2 is about 1:1, the working pressure in the furnace is 700 Pa, and the basic bias voltage is 550 V. The obtained nitrided layer mainly consists of ε-Fe3N phase, containing a small amount of γ′-Fe4N phase.

[0031] (3) Carry out carbon-oxygen co-permeation in the same furnace. The co-permeation temperature is 650 °C, the co-permeation time is 2 hours, the carbon-oxygen co-permeation atmosphere is a mixed gas composed of CO2 and H2, the volume ratio of CO2 to H2 is about 3:2, the working pressure in the furnace is 1200 Pa, and the basic bias voltage is 600 V. A porous carbonitrogen-oxygen multi-permeation layer with a porosity of about 24% is obtained. The main components of the permeation layer are θ-Fe3C phase + ε-Fe3(N,C) phase.

[0032] (4) Carry out plasma-assisted chemical vapor deposition (PECVD) in the same furnace. The deposition temperature is 550 °C, the deposition time is 3 hours, the deposition atmosphere is a mixed gas composed of N2, CO2, and H2, the volume ratio of N2, CO2, and H2 is 3:4:3, the working pressure in the furnace is 900 Pa, and the basic bias voltage is 550 V. A surface modification structure of a graphite-filled porous nitrided layer is obtained. The main component of the surface layer is γ′-Fe4(N,C) phase, as Figure 1 shown. It can be seen from the scanning image that the pores in the nitrided layer are filled with graphite phase, as Figure 2 shown. The porosity is about 28%.

[0033] The hardness of the permeation layer of the parts processed in steps (1) to (4) in this example is detected. The detection results are as Figure 3 shown. It can be seen that the hardness of the near-surface layer of the permeation layer prepared in this example can reach 700 HV.

[0034] The surface friction coefficient of the permeation layer of the parts processed in steps (1) to (4) in this example is detected under heavy load conditions. The results are as Figure 4 shown. The surface friction coefficient of the permeation layer is lower than 0.2.

[0035] The surface toughness of the permeation layer of the parts processed in steps (1) to (4) in this example is detected. The results are as Figure 5 shown. It can be seen that there are no cracks near the surface indentation of the part specimen processed by the method described in this example, and the surface toughness is good.

[0036] Example 2

[0037] A preparation method for the surface modification structure of a graphite-filled porous nitrided layer, comprising the following steps:

[0038] (1) Grind and polish the steel workpiece to make the surface roughness (Ra) of the workpiece less than 0.5 μm, then carry out cleaning and drying to complete the pretreatment.

[0039] (2) Put the pretreated steel workpiece into an ion nitriding furnace for low-temperature plasma nitriding. The nitriding temperature is 450 °C, the nitriding time is 10 hours, the nitriding atmosphere is a mixed gas composed of N2 and H2, the volume ratio of N2 to H2 is about 1:1, the working pressure in the furnace is 750 Pa, and the basic bias voltage is 580 V; the obtained nitrided layer composition is mainly ε-Fe3N phase, containing a small amount of γ′-Fe4N phase.

[0040] (3) Carry out carbon-oxygen co-permeation in the same furnace. The co-permeation temperature is 700 °C, the co-permeation time is 1 hour, the carbon-oxygen co-permeation atmosphere is a mixed gas composed of CO2 and H2, the volume ratio of CO2 to H2 is about 3:2, the working pressure in the furnace is 1200 Pa, and the basic bias voltage is 600 V, to obtain a porous carbonitrogen-oxygen multi-permeated layer with a porosity of about 34%. The main components of the permeated layer are θ-Fe3C phase + ε-Fe3(N,C) phase.

[0041] (4) Carry out plasma-assisted chemical vapor deposition (PECVD) in the same furnace. The deposition temperature is 520 °C, the deposition time is 3 hours, the deposition atmosphere is a mixed gas composed of N2, CO2, and H2, the volume ratio of N2, CO2, and H2 is 2:5:3, the working pressure in the furnace is 1000 Pa, and the basic bias voltage is 560 V, to obtain a surface modification structure of a graphite-filled porous nitrided layer. The main component of the surface layer is γ′-Fe4(N,C) phase, and there is a part of α-Fe phase, as Figure 1 shown. The porosity is about 40%.

[0042] Carry out hardness detection on the nitrided layer of the parts processed in steps (1) to (4) in this embodiment. The detection results are as Figure 3 shown. It can be seen that the hardness of the near-surface layer of the nitrided layer prepared in this embodiment can reach 670 HV.

[0043] Carry out friction coefficient detection on the surface of the nitrided layer of the parts processed in steps (1) to (4) in this embodiment under heavy load conditions. The results are as Figure 4 shown. The friction coefficient of the nitrided layer surface is less than 0.2.

[0044] Carry out toughness detection on the surface of the nitrided layer of the parts processed in steps (1) to (4) in this embodiment. The results are as Figure 5 shown. It can be seen that there are no cracks near the surface indentation of the part specimen processed by the method described in this embodiment, and the surface toughness is good.

[0045] Comparative Example 1

[0046] A preparation method for the surface modification structure of a graphite-filled porous nitrided layer, comprising the following steps:

[0047] (1) Grind and polish the steel workpiece to make the surface roughness (Ra) of the workpiece lower than 0.5 μm, and then perform cleaning and drying to complete the pretreatment.

[0048] (2) Put the pretreated steel workpiece into an ion nitriding furnace for low-temperature plasma nitriding. The nitriding temperature is 500 °C, the nitriding time is 4 hours, the nitriding atmosphere is a mixed gas composed of N2 and H2, the volume ratio of N2 to H2 is about 1:1, the working pressure in the furnace is 700 Pa, and the basic bias voltage is 550 V; the obtained nitrided layer composition is mainly ε-Fe3N phase, containing a small amount of γ′-Fe4N phase.

[0049] (3) Carry out carbon-oxygen co-permeation in the same furnace. The co-permeation temperature is 750 °C, the co-permeation time is 2 hours, the carbon-oxygen co-permeation atmosphere is a mixed gas composed of CO2 and H2, the volume ratio of CO2 to H2 is about 3:2, the working pressure in the furnace is 1200 Pa, and the basic bias voltage is 600 V, to obtain a porous carbonitrogen oxygen multi-permeated layer with a porosity of about 44%. The main components of the permeated layer are θ-Fe3C phase + ε-Fe3(N,C) phase.

[0050] (4) Carry out plasma-assisted chemical vapor deposition (PECVD) in the same furnace. The deposition temperature is 550 °C, the deposition time is 3 hours, the deposition atmosphere is a mixed gas composed of N2, CO2, and H2, the volume ratio of N2, CO2, and H2 is 5:3:2, the working pressure in the furnace is 900 Pa, and the basic bias voltage is 550 V, to obtain the surface modification structure of the graphite-filled porous nitrided layer. The main component of the surface layer is γ′-Fe4(N,C) phase, and there is a small amount of ε-Fe3(N,C) phase, as Figure 1 shown. The porosity is about 52%.

[0051] Perform nitrided layer hardness detection on the parts processed in steps (1) to (4) of this embodiment. The detection results are as Figure 3 shown. It can be seen that the highest hardness of the nitrided layer near the surface layer prepared in this embodiment is 600 HV, and the hardness of the outermost surface layer is lower than 370 HV.

[0052] Perform nitrided layer surface friction coefficient detection on the parts processed in steps (1) to (4) of this embodiment under heavy load conditions. The results are as Figure 4 shown. The surface friction coefficient of the nitrided layer is about 0.4.

[0053] Perform nitrided layer surface toughness detection on the parts processed in steps (1) to (4) of this embodiment. The results are as Figure 5 shown. It can be seen that there are cracks near the surface indentation of the part specimen processed by the method described in this embodiment, and the surface layer toughness is poor.

[0054] Comparative Example 2

[0055] A preparation method for the surface modification structure of a graphite-filled porous nitrided layer, comprising the following steps:

[0056] (1) Grind and polish the steel workpiece to make the surface roughness (Ra) of the workpiece lower than 0.5 μm, and then carry out cleaning and drying to complete the pretreatment.

[0057] (2) Put the pretreated steel workpiece into an ion nitriding furnace for low-temperature plasma nitriding. The nitriding temperature is 450 °C, the nitriding time is 10 hours, the nitriding atmosphere is a mixed gas composed of N2 and H2, the volume ratio of N2 to H2 is about 1:1, the working pressure in the furnace is 750 Pa, and the basic bias voltage is 580 V; the obtained nitrided layer mainly consists of ε-Fe3N phase and contains a small amount of γ′-Fe4N phase.

[0058] (3) Carry out carbon and oxygen co-permeation in the same furnace. The co-permeation temperature is 680 °C, the co-permeation time is 1 hour, the carbon and oxygen co-permeation atmosphere is a mixed gas composed of CO2 and H2, the volume ratio of CO2 to H2 is about 3:2, the working pressure in the furnace is 1200 Pa, and the basic bias voltage is 600 V, to obtain a porous carbonitrogen oxygen multi-permeated layer with a porosity of about 30%. The main components of the permeated layer are θ-Fe3C phase + ε-Fe3(N,C) phase.

[0059] (4) Carry out plasma-assisted chemical vapor deposition (PECVD) in the same furnace. The deposition temperature is 580 °C, the deposition time is 3 hours, the deposition atmosphere is a mixed gas composed of N2, CO2, and H2, the volume ratio of N2, CO2, and H2 is 1:3:1, the working pressure in the furnace is 1000 Pa, and the basic bias voltage is 560 V, to obtain the surface modification structure of the graphite-filled porous nitrided layer. The main component of the surface layer is θ-Fe3C phase, as Figure 1 shown. The porosity is about 38%.

[0060] Carry out nitrided layer hardness detection on the parts processed in steps (1) to (4) of this example. The detection results are as Figure 3 shown. It can be seen that the hardness of the near-surface layer of the nitrided layer prepared in this example can reach 660 HV.

[0061] Carry out nitrided layer surface friction coefficient detection on the parts processed in steps (1) to (4) of this example under heavy load conditions. The results are as Figure 4 shown. The surface friction coefficient of the nitrided layer is about 0.35.

[0062] Carry out nitrided layer surface toughness detection on the parts processed in steps (1) to (4) of this example. The results are as Figure 3As shown, it can be seen that there are cracks near the indentation on the surface of the part specimen processed by the method described in this embodiment, and the surface layer has poor toughness.

[0063] Example 3 - 10

[0064] The difference between Example 3 - 10 and Example 1 lies only in the specific parameters of the preparation process, and the parameter differences are shown in Table 1 below. At the same time, the phase composition and main performance parameters of the finally prepared graphite-filled porous nitrided layer in Example 3 - 10 are attached to Table 1.

[0065] Table 1 Parameter values in each example and comparative example, as well as the phase composition and main performance parameters of the prepared products

[0066]

[0067]

[0068] In order to enable those of ordinary skill in the art to more conveniently understand the improvements of the present invention over the prior art, some of the drawings and descriptions of the present invention have been simplified, and the above embodiments are preferred implementation schemes of the present invention. In addition, the present invention can also be implemented in other ways, and any obvious replacement without departing from the concept of the present technical solution is within the protection scope of the present invention.

Claims

1. A preparation method of a surface modification structure of a graphite-filled porous nitrided layer, characterized in that It includes the following steps: (1) Grind and polish the steel workpiece to make the surface roughness of the workpiece lower than 0.5 μm, and then carry out cleaning and drying to complete the pretreatment; (2) Put the pretreated steel workpiece into an ion nitriding furnace, introduce a nitriding atmosphere, and carry out low-temperature plasma nitriding at 400-500 °C for 3-20 hours to obtain a dense nitrided layer with different thicknesses; (3) Introduce a carbon-oxygen co-nitriding atmosphere into the same furnace, and carry out high-temperature plasma carbon-oxygen co-nitriding at 600-700 °C for 0.5-2 hours to obtain a porous carbonitrogenoxygen multi-nitrided layer; (4) Introduce a nitriding atmosphere and a carbon-containing atmosphere into the same furnace, and carry out plasma-assisted chemical vapor deposition treatment at 500-650 °C for 1-3 hours to obtain a surface modification structure with a graphite-filled porous nitrided layer.

2. The preparation method of the surface modification structure of the graphite-filled porous nitrided layer according to claim 1, characterized in that: In step (1), the steel workpiece is one of pure iron, carbon steel, maraging steel, and hot work die steel, and the chromium content of the steel workpiece is below 13%.

3. The preparation method of the surface modification structure of the graphite-filled porous nitrided layer according to claim 1, characterized in that: In step (2), the nitriding atmosphere is a mixed gas composed of N2 and H2, which contains 40% - 60% by volume of N2, and the nitriding pressure is 300 - 800 Pa.

4. The preparation method of the surface modification structure of the graphite-filled porous nitrided layer according to claim 1, characterized in that: In step (3), the carbon-oxygen co-nitriding atmosphere is a mixed gas composed of CO2 and H2, which contains 50% - 70% by volume of CO2, and the carbon-oxygen co-nitriding pressure is 700 - 1300 Pa.

5. The preparation method of the surface modification structure of the graphite-filled porous nitrided layer according to claim 1, characterized in that: In step (4), the nitriding atmosphere and the carbon-containing atmosphere are a mixed gas composed of N2, CO2, and H2, where the N2 content is 10% - 40%, the CO2 content is 30% - 60%, and the H2 content is 10% - 50%. The pressure of the plasma-assisted chemical vapor deposition treatment is 700 - 1300 Pa.

6. A surface modification structure of a graphite-filled porous nitrided layer, characterized in that: It is prepared by using the preparation method described in any one of claims 1-5.

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