Metal powder surface modification method for metal forming based on MIM (Metal Injection Molding)

Abstract

The invention discloses a metal powder surface modification method for metal forming based on MIM (metal injection molding), and belongs to the technical field of metal injection molding. 316L stainless steel powder is subjected to plasma treatment to obtain activated metal powder which is rough in surface and rich in hydroxyl, the surface of the activated metal powder is coated with PNIPAM xerogel, and hollow lubricating microspheres can be loaded by utilizing rich polar groups of the gel; during degreasing, pores of the hollow lubricating microspheres provide channels for gas escape, stress concentration of the green body and generation of pores are reduced, and the cracking risk is reduced; and the gel has the phase change characteristic and is in a swelling state at room temperature, surface hydrophilic groups can enhance the wettability of the metal powder and the binder, reduce the viscosity of the feed, promote the uniform flow of the feed and reduce the injection defects, and when the temperature is higher than 32 DEG C, the gel is in a hydrophobic shrinkage state to form a compact coating layer and release internal stress at the same time, so that the metal powder is prepared. The green body cracking risk is reduced.

Description

Technical Field

[0001] This invention belongs to the field of metal injection molding technology, specifically a method for surface modification of metal powders used in MIM metal molding. Background Technology

[0002] Metal powder injection molding (MIM) is a novel powder metallurgy technology that incorporates modern plastic injection molding technology into the field of powder metallurgy. This technology involves mixing metal powder with a binder, then using injection molding to create injection preforms. Following debinding and sintering, it produces metal products with complex geometries and high precision. Due to its advantages such as high design freedom, high strength, and low cost, MIM technology is widely used in consumer electronics, automotive parts, medical devices, and hardware tools.

[0003] Currently, the main approach is to obtain nanoscale metal particles through a series of methods and reduce metal powder agglomeration under the action of surfactants. However, MIM metal powder injection molding requires mixing metal powder with binder before injection molding. Due to the influence of intermolecular forces between metal dispersion and binder, if binder and metal powder are simply mixed during MIM metal powder injection molding, the intermolecular forces between binder and metal powder will directly affect the density and strength of the injection preform. Low intermolecular forces between binder and metal powder will reduce the density and strength of the injection preform and lead to insufficient bonding strength at the interface between binder and powder. During high-temperature debinding, this weak bonding force makes it impossible for the binder to effectively fix the powder particles, resulting in particle rearrangement or detachment, thereby increasing the dimensional changes of the material.

[0004] Chinese patent announcement CN119159078B discloses surface-modified metal powder and its application in MIM metal injection molding. However, in this solution, polyolefins and polyurethanes undergo thermal decomposition during the degreasing stage to generate gas. If these gases cannot be discharged smoothly, uneven pores will form inside the green body, resulting in a reduction in the strength of the product. Summary of the Invention

[0005] The purpose of this invention is to provide a surface modification method for metal powders used in MIM metal forming. By plasma treatment of 316L stainless steel powder and coating the surface with PNIPAM dry gel, the abundant polar groups of the gel can support hollow lubricating microspheres. The hollow lubricating microspheres, as a reinforcing phase, can effectively release the gases generated by thermal decomposition during the degreasing process through their internal cavities, thus avoiding affecting the overall strength of the sintered product.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] The surface modification method for metal powders used in MIM metal forming includes the following steps:

[0008] Step 1: Plasma treatment of 316L stainless steel powder yields activated metal powder with a rough surface and rich in hydroxyl groups.

[0009] Step 2: Coat the surface of the activated metal powder with PNIPAM dry gel to obtain dry gel coated metal powder.

[0010] Step 3: Hollow lubricating microspheres are prepared by using thermally expandable microspheres as sacrificial templates.

[0011] Step 4: Load hollow lubricating microspheres onto the surface of dry gel-coated metal powder to improve the dispersibility and flowability of the dry gel-coated metal powder during MIM metal forming, thus completing the method of surface modification of metal powder for MIM metal forming.

[0012] Furthermore, the specific preparation steps for the activated metal powder in step one are as follows:

[0013] 316L stainless steel powder with a particle size of 80-100μm was placed in a PECVD (plasma chemical vapor deposition) device, and nitrogen dioxide gas was introduced at a flow rate of 50-55mL / min for 10-12min. The powder was heated to 400-500℃ and nitrogen dioxide plasma was formed under the conditions of 200-300W power and 65-68Pa pressure. The powder was bombarded with nitrogen dioxide plasma for 20-30min. The bombarded powder was then washed with deionized water 2-3 times to remove impurities and dried under vacuum at 60-80℃ for 1-2h to obtain activated metal powder.

[0014] Furthermore, the specific preparation steps for the dry gel coating of metal powder in step two are as follows:

[0015] N-Isopropylacrylamide and N,N'-methylenebispropylamide were added to a reaction vessel and stirred for 20-30 min at 20-25℃ and 400-500 r / min. Then sodium dodecyl sulfate and deionized water were added, and the mixture was heated to 40-50℃ and stirred for another 20-30 min. Ammonia gas was introduced at a flow rate of 10-12 min / L for 30-40 min, and the mixture was heated to 70-80℃. Ammonium persulfate was added, and the mixture was stirred for another 6-7 h under a nitrogen atmosphere. Then the activated metal powder was added to the reaction vessel and stirred for another 20-30 min. The mixture was filtered, and the filter cake was washed 2-3 times with petroleum ether and deionized water, respectively. The cake was then vacuum dried at 60-80℃ for 1-2 h, ground into a fine powder, and passed through a 200-300 mesh sieve to obtain a dry gel-coated metal powder with PNIPAM dry gel coating.

[0016] Furthermore, the ratio of N-isopropylacrylamide, N,N'-methylenebispropylamide, sodium dodecyl sulfate, deionized water, ammonium persulfate, and activated metal powder is 500-600g: 200-220g: 10-12g: 400-500mL: 4-5mL: 800-850g.

[0017] Furthermore, the specific preparation steps for the hollow lubricating microspheres in step three are as follows:

[0018] Aluminum sec-butoxide and isopropanol were added to a reaction vessel and stirred for 20-30 minutes at 20-25°C and 400-500 r / min. Then, a 10-20% nitric acid solution was added, and the mixture was heated to 60-70°C and stirred for 3-4 hours to form an alumina gel. Thermally expandable microspheres were then added, and the mixture was vacuum impregnated for 3-4 hours. The mixture was then aged at 80-90°C for 24-26 hours and vacuum dried at 60-80°C for 1-2 hours. The product was then transferred to a muffle furnace and calcined at 210-220°C for 2-3 hours in an air atmosphere, followed by calcination at 600-700°C for 2-3 hours to obtain hollow lubricated microspheres.

[0019] Furthermore, the ratio of aluminum sec-butoxide, isopropanol, nitric acid solution, and thermally expandable microspheres is 80-90 mL: 400-500 mL: 12-14 mL: 75-80 g.

[0020] Furthermore, the thermally expandable microspheres are prepared through the following steps:

[0021] Sodium chloride, colloidal silica, and deionized water were added to a reaction vessel and stirred for 30-40 minutes at 35-40℃ and 500-600 r / min to obtain an aqueous phase mixture. Then, 190-200 g of acrylonitrile, 80-85 g of methacrylonitrile, 25-30 g of isopentane, 65-70 g of isooctane, 1-2 g of ethylene glycol dimethacrylate, 1-2 g of diallyl carbonate, 9-10 g of naphthenic oil, and 1.5-2 g of azobisisobutyronitrile were added to the reaction vessel and stirred until homogeneous to obtain an oil phase mixture. The aqueous phase mixture and the oil phase mixture were added to the reactor at a mass ratio of 10:1 and stirred evenly. The mixture was dispersed at a high speed of 1500-1600 r / min for 15-20 min. Under nitrogen protection, the mixture was stirred and polymerized at a temperature of 70-80℃, a rotation speed of 80 r / min, and a pressure of 0.5-0.7 MPa for 20-22 h. The mixture was filtered, and the filter cake was washed 2-3 times with deionized water and anhydrous ethanol, respectively. The mixture was then vacuum dried at 60-80℃ for 1-2 h to obtain thermally expandable microspheres with a particle size of 1-2 μm.

[0022] Furthermore, the ratio of sodium chloride, colloidal silica, and deionized water is 80-90g: 35-40g: 800-900mL.

[0023] Furthermore, the ratio of acrylonitrile, methacrylonitrile, isopentane, isooctane, ethylene glycol dimethacrylate, diallyl carbonate, naphthenic oil, and azobisisobutyronitrile is 190-200g: 80-85g: 25-30g: 65-70g: 1-2g: 1-2g: 9-10g: 1.5-2g.

[0024] Furthermore, the specific preparation steps of the metal powder for MIM metal forming in step four are as follows:

[0025] The dry gel-coated metal powder, hollow lubricated microspheres, methanol, and deionized water were added to a reaction vessel and ultrasonically dispersed for 40-60 min. The mixture was then stirred for 1-2 h at 20-25 °C and 400-500 r / min. After filtration, the filter cake was washed 2-3 times with deionized water and anhydrous ethanol, respectively, and then vacuum dried at 60-80 °C for 1-2 h to obtain metal powder for metal forming based on MIM.

[0026] Furthermore, the ratio of dry gel-coated metal powder, hollow lubricated microspheres, methanol, and deionized water is 200-300g: 10-15g: 200-300mL: 600-800mL.

[0027] The beneficial effects of this invention are:

[0028] 1. The metal powder for MIM metal forming prepared by this surface modification method can increase the overall strength and density of the blank during the MIM metal injection molding process, making it less prone to cracking, and the resulting injection molded product has a small volume change rate.

[0029] 2. The dry gel coating of metal powder of the present invention forms a three-dimensional network structure with N-isopropylacrylamide through a free radical copolymerization reaction, resulting in a dry gel covering the surface of activated metal powder. The gel surface has many polar groups, which can increase the wettability of the metal powder, promote better bonding between the metal powder and the binder during injection molding, improve the density of the sintered metal, promote uniform feed flow, and reduce injection defects. During degreasing, the pores of the hollow lubricating microspheres provide channels for gas escape, reducing stress concentration and porosity in the green body, and lowering the risk of cracking. When the temperature is below 32°C, the gel is in a swollen state, and the hydrophilic groups on the surface enhance the wettability of the metal powder and the binder, reduce the feed viscosity, promote uniform feed flow, and reduce injection defects. When the temperature is above 32°C, the gel is in a hydrophobic shrinkage state, forming a dense coating layer, while releasing internal stress and reducing the risk of green body cracking.

[0030] 3. The hollow lubricating microspheres of the present invention are generated by a sol-gel method to produce alumina sol, and then thermally expandable microspheres are impregnated in the alumina sol, using the thermally expandable microspheres as sacrificial templates. The core of the thermally expandable microspheres contains isopentane or isooctane, which vaporizes and expands when heated to generate internal pressure, breaking through the outer shell to form a hollow structure. The surface of the hollow lubricating microspheres contains hydroxyl active groups, which can form chemical bonds with gels or binders, improve the interfacial bonding strength, and reduce the risk of interfacial delamination. The spherical structure of the hollow lubricating microspheres can act as a sliding layer to reduce the coefficient of friction during friction, and can uniformly distribute the load, reduce stress concentration, and inhibit crack propagation. The closed-cell structure of the hollow lubricating microspheres can absorb the thermal stress generated during the sintering process of the material and reduce volume expansion. Detailed Implementation

[0031] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention. Embodiment 1: A method for surface modification of metal powder for MIM metal forming, comprising the following steps:

[0032] S1: Place 316L stainless steel powder with a particle size of 80-100μm in a PECVD (plasma chemical vapor deposition) device, introduce nitrogen dioxide gas at a flow rate of 50mL / min for 10min, heat to 400℃, and form nitrogen dioxide plasma under the conditions of 200W power and 65Pa pressure to bombard the metal powder for 20min. Then wash the bombarded powder twice with deionized water to remove impurities, and dry it under vacuum at 60℃ for 1h to obtain activated metal powder.

[0033] During plasma bombardment, nitrogen and oxygen ions in the nitrogen dioxide plasma collide with atoms on the surface of stainless steel powder, causing the atoms or molecules on the surface to gain energy and undergo displacement or sputtering. This collision and sputtering process will generate defects and pores on the surface of stainless steel powder, and generate hydroxyl groups on the surface.

[0034] S2: Add 500g N-isopropylacrylamide and 200g N,N'-methylenebispropylamide to a reactor and stir for 20min at 20℃ and 400r / min. Then add 10g sodium dodecyl sulfate and 400mL deionized water, heat to 40℃, and continue stirring for 20min. Purge with ammonia gas at a flow rate of 10min / L for 30min, heat to 70℃, add 4mL ammonium persulfate, and continue stirring for 6h under a nitrogen atmosphere. Then add 800g activated metal powder to the reactor and continue stirring for 20min. Filter, wash the filter cake twice with petroleum ether and deionized water respectively, dry under vacuum at 60℃ for 1h, grind to fineness, and pass through a 200-mesh sieve to obtain a dry gel-coated metal powder with PNIPAM dry gel coating.

[0035] Using N-isopropylacrylamide as a monomer and N,N'-methylenebispropylamide as a crosslinking agent, a three-dimensional network structure is formed with N-isopropylacrylamide through a free radical copolymerization reaction, resulting in a dry gel that covers the surface of activated metal powder. The surface of the activated metal powder contains hydroxyl groups, which can increase the adhesion strength of the gel, thus obtaining a dry gel coating the metal powder.

[0036] S3: Add 80g sodium chloride, 35g colloidal silica and 800mL deionized water to the reactor and stir for 30min at 35℃ and 500r / min to obtain an aqueous phase mixture. Add 190g acrylonitrile, 80g methacrylonitrile, 25g isopentane, 65g isooctane, 1g ethylene glycol dimethacrylate, 1g diallyl carbonate, 9g naphthenic oil and 1.5g azobisisobutyronitrile to the reactor and stir until homogeneous to obtain an oil phase mixture. Add the aqueous phase mixture and the oil phase mixture to the reactor at a mass ratio of 10:1 and stir until homogeneous. Disperse at 1500r / min for 15min. Under nitrogen protection, stir and polymerize for 20h at 70℃, 80r / min and 0.5MPa. Filter and wash the filter cake twice with deionized water and anhydrous ethanol, respectively. Dry under vacuum at 60℃ for 1h to obtain thermally expandable microspheres with a particle size of 1-2μm.

[0037] Sodium chloride and colloidal silica form a dispersion system. The colloidal silica reduces the interfacial tension and prevents droplet aggregation by adsorbing onto the surface of the oil phase droplets. Free radical copolymerization is induced by crosslinking agents and initiators, and the oil phase is sheared into micron-sized droplets under high-speed dispersion to obtain thermally expandable microspheres.

[0038] The water-insoluble oil phase is dispersed into small droplets in the aqueous phase under high-speed shear and the action of a dispersant. Subsequently, the monomers in the small droplets undergo polymerization under the action of an initiator and heating, encapsulating low-boiling-point alkanes to form a shell, thus obtaining thermally expandable microspheres.

[0039] S4: Add 80 mL of aluminum sec-butoxide and 400 mL of isopropanol to the reactor and stir for 20 min at 20 °C and 400 r / min. Then add 12 mL of 10% nitric acid solution, heat to 60 °C, and continue stirring for 3 h to form alumina gel. Then add 75 g of thermally expandable microspheres, vacuum impregnate for 3 h, age at 80 °C for 24 h, and vacuum dry at 60 °C for 1 h. Transfer the product to a muffle furnace and calcine at 210 °C for 2 h in air atmosphere, and then calcine at 600 °C for 2 h to obtain hollow lubricated microspheres.

[0040] Alumina sol is generated by sol-gel method, and then thermally expandable microspheres are immersed in alumina sol. The thermally expandable microspheres are used as sacrificial templates. The core of the thermally expandable microspheres contains isopentane or isooctane. When heated to 210°C, they vaporize and expand to generate internal pressure, breaking through the outer shell to form a hollow structure.

[0041] S5: Add 200g of dry gel-coated metal powder, 10g of hollow lubricating microspheres, 200mL of methanol and 600mL of deionized water to a reaction vessel, ultrasonically disperse for 40min, and stir for 1h at 20℃ and 400r / min to achieve more uniform dispersion through liquid dispersion. Filter, wash the filter cake twice with deionized water and anhydrous ethanol respectively, and vacuum dry at 60℃ for 1h to allow the hollow lubricating microspheres to adhere to the surface of the dry gel-coated metal powder, thereby improving the dispersibility and flowability of the dry gel-coated metal powder during MIM metal molding, and completing the method of surface modification of metal powder for MIM metal molding.

[0042] Example 2: A method for surface modification of metal powder for MIM metal forming, comprising the following steps:

[0043] S1: 316L stainless steel powder with a particle size of 90μm was placed in a PECVD (plasma chemical vapor deposition) device, and nitrogen dioxide gas was introduced at a flow rate of 52.5mL / min for 11min. The powder was heated to 450℃ and nitrogen dioxide plasma was formed under the conditions of 250W power and 66.5Pa pressure to bombard the metal powder for 25min. The bombarded powder was then washed twice with deionized water to remove impurities and dried under vacuum at 70℃ for 1.5h to obtain activated metal powder.

[0044] S2: 550g N-isopropylacrylamide and 210g N,N'-methylenebispropylamide were added to a reaction vessel and stirred for 25min at 22.5℃ and 450r / min. Then, 11g sodium dodecyl sulfate and 450mL deionized water were added, and the mixture was heated to 45℃ and stirred for another 25min. Ammonia gas was introduced at a flow rate of 11min / L for 35min, and the mixture was heated to 75℃. 4.5mL ammonium persulfate was added, and the mixture was stirred for another 6.5h under a nitrogen atmosphere. Then, 825g activated metal powder was added to the reaction vessel and stirred for another 25min. The mixture was filtered, and the filter cake was washed twice with petroleum ether and deionized water, respectively. The cake was then vacuum dried at 70℃ for 1.5h, ground into a fine powder, and passed through a 200-300 mesh sieve to obtain a dry gel-coated metal powder with PNIPAM dry gel coating.

[0045] S3: Add 85g sodium chloride, 37.5g colloidal silica, and 850mL deionized water to a reaction vessel and stir for 35min at 37.5℃ and 550r / min to obtain an aqueous phase mixture; add 195g acrylonitrile, 82.5g methacrylonitrile, 27.5g isopentane, 67.5g isooctane, 1.5g ethylene glycol dimethacrylate, 1.5g diallyl carbonate, 9.5g naphthenic oil, and 1.75g ​​azobisisobutyronitrile to the reaction vessel and stir. The mixture was homogeneous to obtain an oil phase mixture. The aqueous phase mixture and the oil phase mixture were added to the reactor at a mass ratio of 10:1 and stirred until homogeneous. The mixture was dispersed at a high speed of 1550 r / min for 17.5 min. Under nitrogen protection, the mixture was stirred and polymerized for 21 h at a temperature of 75℃, a rotation speed of 80 r / min, and a pressure of 0.6 MPa. The mixture was filtered, and the filter cake was washed twice with deionized water and anhydrous ethanol, respectively. The cake was then dried under vacuum at 70℃ for 1.5 h to obtain thermally expandable microspheres with a particle size of 1-2 μm.

[0046] S4: Add 85 mL of aluminum sec-butoxide and 450 mL of isopropanol to the reactor and stir for 25 min at 22.5 °C and 450 r / min. Then add 13 mL of 15% nitric acid solution, heat to 65 °C, and continue stirring for 3.5 h to form an alumina gel. Then add 77.5 g of thermally expandable microspheres, vacuum impregnate for 3.5 h, age at 85 °C for 25 h, and vacuum dry at 70 °C for 1.5 h. Transfer the product to a muffle furnace and calcine at 215 °C for 2.5 h in an air atmosphere, and then calcine at 650 °C for 2.5 h to obtain hollow lubricated microspheres.

[0047] S5: 250g of dry gel-coated metal powder, 12.5g of hollow lubricating microspheres, 250mL of methanol and 700mL of deionized water were added to a reaction vessel and ultrasonically dispersed for 50min. The mixture was then stirred for 1.5h at 22.5℃ and 450r / min to achieve a more uniform dispersion through liquid dispersion. The mixture was filtered, and the filter cake was washed twice with deionized water and anhydrous ethanol, respectively. It was then vacuum dried at 70℃ for 1.5h to allow the hollow lubricating microspheres to adhere to the surface of the dry gel-coated metal powder, thereby improving the dispersibility and flowability of the dry gel-coated metal powder during MIM metal molding. This completed the method for surface modification of metal powder used in MIM metal molding.

[0048] Example 3: A method for surface modification of metal powder for MIM metal forming, comprising the following steps:

[0049] S1: 316L stainless steel powder with a particle size of 100μm was placed in a PECVD (plasma chemical vapor deposition) device, and nitrogen dioxide gas was introduced at a flow rate of 55mL / min for 12min. The powder was heated to 500℃ and nitrogen dioxide plasma was formed under the conditions of 300W power and 68Pa pressure to bombard the metal powder for 30min. The bombarded powder was then washed three times with deionized water to remove impurities and dried under vacuum at 80℃ for 2h to obtain activated metal powder.

[0050] S2: 600g N-isopropylacrylamide and 220g N,N'-methylenebispropylamide were added to a reaction vessel and stirred for 30min at 25℃ and 500r / min. Then, 12g sodium dodecyl sulfate and 500mL deionized water were added, heated to 50℃, and stirred for another 30min. Ammonia gas was introduced at a flow rate of 12min / L for 40min, and then heated to 80℃. 5mL ammonium persulfate was added, and the reaction was stirred for another 7h under a nitrogen atmosphere. Then, 850g activated metal powder was added to the reaction vessel and stirred for another 30min. The mixture was filtered, and the filter cake was washed three times with petroleum ether and deionized water, respectively. The cake was then vacuum dried at 80℃ for 2h, ground into a fine powder, and passed through a 300-mesh sieve to obtain a dry gel-coated metal powder with PNIPAM dry gel coating.

[0051] S3: Add 90g sodium chloride, 40g colloidal silica and 900mL deionized water to the reactor and stir for 40min at 40℃ and 600r / min to obtain an aqueous phase mixture. Add 200g acrylonitrile, 85g methacrylonitrile, 30g isopentane, 70g isooctane, 2g ethylene glycol dimethacrylate, 2g diallyl carbonate, 10g naphthenic oil and 2g azobisisobutyronitrile to the reactor and stir until homogeneous to obtain an oil phase mixture. Add the aqueous phase mixture and the oil phase mixture to the reactor at a mass ratio of 10:1 and stir until homogeneous. Disperse at 1600r / min for 20min. Under nitrogen protection, stir and polymerize for 22h at 80℃, 80r / min and 0.7MPa. Filter and wash the filter cake three times with deionized water and anhydrous ethanol respectively. Dry under vacuum at 80℃ for 2h to obtain thermally expandable microspheres with a particle size of 1-2μm.

[0052] S4: Add 90 mL of aluminum sec-butoxide and 500 mL of isopropanol to the reactor and stir for 30 min at 25 °C and 500 r / min. Then add 14 mL of 20% nitric acid solution, heat to 70 °C, and continue stirring for 4 h to form alumina gel. Then add 80 g of thermally expandable microspheres, vacuum impregnate for 4 h, age at 90 °C for 26 h, and vacuum dry at 80 °C for 2 h. Transfer the product to a muffle furnace and calcine at 220 °C for 3 h in an air atmosphere, and then calcine at 700 °C for 3 h to obtain hollow lubricating microspheres.

[0053] S5: Add 300g of dry gel-coated metal powder, 15g of hollow lubricating microspheres, 300mL of methanol and 800mL of deionized water to a reaction vessel, ultrasonically disperse for 60min, and stir for 2h at 25℃ and 500r / min to achieve more uniform dispersion through liquid dispersion. Filter, wash the filter cake three times with deionized water and anhydrous ethanol respectively, and vacuum dry at 80℃ for 2h to allow the hollow lubricating microspheres to adhere to the surface of the dry gel-coated metal powder, thereby improving the dispersibility and flowability of the dry gel-coated metal powder during MIM metal molding, and completing the method of surface modification of metal powder for MIM metal molding.

[0054] Comparative Example 1: Based on Example 3, without the treatment in step S1, the activated metal powder in step S2 was replaced with 316L stainless steel powder with a particle size of 80-100μm in step S1, and the other steps remained unchanged, to prepare metal powder for MIM metal forming.

[0055] Comparative Example 2: Based on Example 3, without step S2, the dry gel-coated metal powder in step S5 was replaced with the activated metal powder in step S1, and the other steps remained unchanged, to prepare metal powder for MIM metal forming.

[0056] Comparative Example 3: Based on Example 3, the thermally expandable microspheres in step S4 were replaced with commercially available polystyrene microspheres. Hollow lubricating microspheres were obtained by using polystyrene microspheres as sacrificial templates. The remaining steps remained unchanged, and metal powder for metal forming based on MIM was prepared.

[0057] The metal powders obtained in Examples 1-3 and Comparative Examples 1-3 were subjected to performance tests. The density, strength, flexural modulus, and dimensional change rate of the injection-molded preforms were tested. Density was tested according to standard GB / T10421-2002 "Determination of Density of Sintered Metal Friction Materials". Strength was determined according to standard GB / T5160-2002 "Determination of Strength of Green Powder Metal Preforms - Transverse Fracture Method of Rectangular Press". Flexural modulus was tested using a CMT4104 universal electronic tensile testing machine manufactured by Shenzhen Xin Sansi Co., Ltd., following international standard ISO178:2001. The displacement rate of the tensile testing machine was set to 0.4 mm / min, and the span was fixed at 64 mm. The results are shown in Tables 1 and 2.

[0058] Table 1. Results of Injection Molded Preform Performance Tests

[0059] project Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 <![CDATA[Density (g·cm 3 )]]> 5.25 5.29 5.35 4.58 4.12 4.02 Green strength (MPa) 16.7 17.2 17.8 14.3 13.1 12.5 Flexural modulus (MPa) 2877 2885 2893 2753 2611 2598

[0060] Table 2 Results of Dimensional Change Rate of Metal Injection Finished Products

[0061] project Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 long(%) 0.35 0.33 0.31 0.41 0.46 0.48 Width(%) 0.12 0.11 0.10 0.18 0.22 0.29 high(%) 0.21 0.20 0.19 0.35 0.42 0.48

[0062] As shown in Tables 1 and 2, the metal powders prepared in Examples 1-3, when applied in the MIM metal injection molding process, resulted in significantly better density, strength, and flexural modulus of the injection preform compared to the comparative example, and a significantly lower dimensional change rate in the finished metal injection product compared to the comparative example. This indicates that the metal powders prepared by this method can increase the overall strength and density of the preform during the MIM metal injection molding process, making it less prone to cracking, and resulting in a smaller volume change rate in the injection product. In Comparative Example 1, step S1 was omitted, and the activated metal powder was replaced with 316L stainless steel powder. After plasma bombardment, nitrogen and oxygen ions in the nitrogen dioxide plasma collided with atoms on the surface of the stainless steel powder, causing the surface atoms or molecules to gain energy and undergo displacement or sputtering. This collision and sputtering process generates defects and pores on the surface of the stainless steel powder, and hydroxyl groups are generated on the surface. The generation of hydroxyl groups is beneficial for subsequent coating.

[0063] In Comparative Example 2, without step S2, the dry gel-coated metal powder was replaced with activated metal powder. Using N-isopropylacrylamide as a monomer and N,N'-methylenebispropylamide as a crosslinking agent, a three-dimensional network structure was formed through free radical copolymerization with N-isopropylacrylamide. This resulted in a dry gel covering the surface of the activated metal powder. The activated metal powder surface contains hydroxyl groups, which increase the adhesion strength of the gel. The gel surface also has many polar groups, which increase the wettability of the metal powder, promoting better bonding between the metal powder and the binder during injection molding. This improves the density of the sintered metal, promotes uniform feed flow, and reduces injection defects. During debinding, the microsphere pores provide gas escape channels and simultaneously release internal stress, reducing the risk of green body cracking.

[0064] In Comparative Example 3, the thermally expandable microspheres were replaced with commercially available polystyrene microspheres. The outer shell of the thermally expandable microspheres contains functional groups such as hydroxyl and epoxy groups, which can form hydrogen bonds or covalent bonds with alumina sol, enhancing the interfacial bonding strength. However, the surface of polystyrene microspheres is inert and requires modification with silane coupling agents to bond with ceramics, and the interface is prone to cracking. The core of the thermally expandable microspheres contains isopentane or isooctane, which vaporizes and expands when heated to 110-220℃, generating internal pressure and breaking through the outer shell to form a hollow structure. The surface of the hollow lubricating microspheres contains active hydroxyl groups, which can form chemical bonds with gels or binders, improving the interfacial bonding strength and reducing the risk of interfacial delamination.

[0065] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the invention.

Claims

1. A method for surface modification of metal powder for MIM metal forming, characterized by, The method comprises the following steps: Step one: obtaining activated metal powder with rough surface and rich in hydroxyl groups by plasma treatment on 316L stainless steel powder; Step two: coating PNIPAM xerogel on the surface of the activated metal powder to obtain xerogel-coated metal powder; Step three: preparing hollow lubricating microspheres by using thermally expandable microspheres as sacrificial templates; Step four: loading the hollow lubricating microspheres on the surface of the xerogel-coated metal powder to complete the surface modification method of metal powder for MIM metal forming.

2. The metal powder surface modification method for MIM metal forming according to claim 1, characterized by, The specific preparation steps of the activated metal powder in step one are as follows: The 316L stainless steel powder with a particle size of 80-100 mu m is placed in the PECVD, nitrogen dioxide gas is introduced at a flow rate of 50-55 mL / min for 10-12 min, heated to 400-500 DEG C, and nitrogen dioxide plasma is formed under the conditions of 200-300 W power and 65-68 Pa pressure to bombard the metal powder, the bombardment time is 20-30 min, then the bombarded powder is washed with deionized water for 2-3 times to remove impurities, and vacuum drying is performed to obtain the activated metal powder.

3. The method for surface modification of a metal powder for MIM metal forming according to claim 1, characterized in that, The specific preparation steps of the xerogel-coated metal powder in step two are as follows: N-isopropyl acrylamide and N,N'-methylene bispropylamide are added to the reaction kettle, stirred at 20-25 DEG C and 400-500 r / min for 20-30 min, then sodium dodecyl sulfate and deionized water are added, heated to 40-50 DEG C, and continue to stir for 20-30 min, introduce ammonia gas at a flow rate of 10-12 min / L for 30-40 min, heat to 70-80 DEG C, add ammonium persulfate, continue to stir under nitrogen atmosphere for 6-7 h, then add the activated metal powder to the reaction kettle, continue to stir for 20-30 min, filter, wash, vacuum dry, grind and refine, pass through a 200-300 mesh sieve, and obtain the xerogel-coated metal powder coated with PNIPAM xerogel on the surface.

4. The method for surface modification of a metal powder for MIM metal forming according to claim 3, characterized in that, The amount ratio of N-isopropyl acrylamide, N,N'-methylene bispropylamide, sodium dodecyl sulfate, deionized water, ammonium persulfate and activated metal powder is 500-600 g:200-220 g:10-12 g:400-500 mL:4-5 mL:800-850 g.

5. The method for surface modification of a metal powder for MIM metal forming according to claim 1, characterized in that, The specific preparation steps of the hollow lubricating microspheres in step three are as follows: Aluminum sec-butoxide and isopropyl alcohol are added to the reaction kettle, stirred at 20-25 DEG C and 400-500 r / min for 20-30 min, then 10-20 wt% nitric acid solution is added, heated to 60-70 DEG C, and continue to stir for 3-4 h to form aluminum oxide gel, then add thermally expandable microspheres, vacuum impregnate for 3-4 h, heat to 80-90 DEG C and age for 24-26 h, vacuum dry, transfer the product to a muffle furnace, calcine at 210-220 DEG C under air atmosphere for 2-3 h, then calcine at 600-700 DEG C for 2-3 h to obtain the hollow lubricating microspheres.

6. The metal powder surface modification method for MIM metal forming according to claim 5, wherein The amount ratio of the sec-butyl alcohol aluminum, isopropyl alcohol, nitric acid solution and heat-expandable microspheres is 80-90 mL:400-500 mL:12-14 mL:75-80 g.

7. The method for surface modification of a metal powder for MIM metal forming according to claim 5, wherein The heat-expandable microspheres are prepared by the following steps: The sodium chloride, colloidal silicon dioxide and deionized water are added into a reaction kettle, stirred at 35-40 ℃ and 500-600 r / min for 30-40 min to obtain an aqueous phase mixture; 190-200 g of acrylonitrile, 80-85 g of methacrylonitrile, 25-30 g of isopentane, 65-70 g of isooctane, 1-2 g of ethylene glycol dimethacrylate, 1-2 g of diallyl carbonate, 9-10 g of naphthenic oil and 1.5-2 g of azobisisobutyronitrile are added into the reaction kettle and stirred uniformly to obtain an oil phase mixture; the aqueous phase mixture and the oil phase mixture are added into the reaction kettle and stirred uniformly at a mass ratio of 10:1, and high-speed dispersion is carried out at 1500-1600 r / min for 15-20 min; under the protection of nitrogen, stirring polymerization is carried out at a temperature of 70-80 ℃, a rotation speed of 80 r / min and a pressure of 0.5-0.7 MPa for 20-22 h, and then filtration, washing and vacuum drying are carried out to obtain heat-expandable microspheres with a particle size of 1-2 μm.

8. The method for surface modification of a metal powder for MIM metal forming according to claim 7, characterized in that, The amount ratio of the sodium chloride, colloidal silicon dioxide and deionized water is 80-90 g:35-40 g:800-900 mL; the amount ratio of acrylonitrile, methacrylonitrile, isopentane, isooctane, ethylene glycol dimethacrylate, diallyl carbonate, naphthenic oil and azobisisobutyronitrile is 190-200 g:80-85 g:25-30 g:65-70 g:1-2 g:1-2 g:9-10 g:1.5-2 g.

9. The method for surface modification of a metal powder for MIM metal forming according to claim 1, wherein The specific preparation steps of the metal powder for MIM metal forming in step four are as follows: The xerogel-coated metal powder, hollow lubricating microspheres, methanol and deionized water are added into a reaction kettle, ultrasonic dispersion is carried out for 40-60 min, stirring is carried out at 20-25 ℃ and 400-500 r / min for 1-2 h, and then filtration, washing and vacuum drying are carried out to obtain the metal powder for MIM metal forming.

10. The method for surface modification of a metal powder for MIM metal forming according to claim 9, characterized in that, The amount ratio of the xerogel-coated metal powder, hollow lubricating microspheres, methanol and deionized water is 200-300 g:10-15 g:200-300 mL:600-800 mL.