A method for nano-crystallization of the surface of refractory metal tungsten

Abstract

The present invention belongs to the technical field of material surface treatment, and specifically relates to a method for nano-forming the surface of refractory metal tungsten. The present invention adopts a method of mechanical polishing combined with vacuum heat treatment to form a fine nanocrystalline layer on the surface of tungsten with a coarse-grained matrix. The process is simple, time-consuming, energy-saving, reliable repeatability, and batch processing can be achieved. During the metallographic polishing process of coarse-grained tungsten, high-density dislocations will be introduced near the surface. These dislocations are in a thermodynamically metastable state. During the subsequent vacuum heat treatment process, they will recover to form stable nanocrystals with a grain size of 200 to 400 nm and a surface nanolayer thickness of 200 to 400 nm. In addition, there is no obvious interface between the nanocrystalline layer and the matrix structure, which is not easy to peel off and separate, and is expected to further improve the radiation resistance of tungsten materials.

Description

Technical Field

[0001] The invention belongs to the technical field of material surface nano-processing, and relates to a method for surface nano-processing of refractory metal tungsten. Background Art

[0002] Metal tungsten (W) is widely used in nuclear radiation shielding materials, such as the first wall material facing the plasma in fusion reactors, due to its high melting point, high thermal conductivity, high radiation resistance sputtering threshold and other characteristics. Although tungsten-based materials have been widely used, their radiation resistance still needs to be further improved.

[0003] Nanostructures can significantly improve the radiation resistance of materials. For example, nanoscale grain boundaries, as capture sites for radiation defects, can effectively absorb and disperse radiation defects, reduce the aggregation of radiation defects, and thus inhibit the generation of large-sized defect clusters such as voids and bubbles, thereby improving the radiation resistance of materials. However, the industrial production of pure tungsten usually adopts the bottom-up powder sintering method. Due to the limitation of the coarse size of the initial raw material powder and the further coarsening of the grain size during the subsequent high-temperature sintering process, the particle size of the tungsten material currently prepared is usually in the range of several microns to hundreds of microns, which seriously restricts the improvement of the material's radiation resistance. Therefore, it is urgent to develop surface nano-processing technology for tungsten materials.

[0004] The Chinese patent document “A device and method for nano-processing the surface of metal tungsten” with publication number CN108456854A discloses a technical solution: by ionizing helium gas and obtaining a low-voltage, high-current helium plasma, and bombarding the surface of the tungsten sample with helium ions, a nanostructure can be obtained on the tungsten surface after injecting a certain dose of helium ions. Although the above technical solution obtains a nanostructure on the tungsten surface, it forms a fluffy nano-hair structure, and this nano-tungsten hair also contains "helium bubbles", which poses a risk of insufficient bonding strength with the substrate. In addition, the Chinese patent document “A method for nano-processing the surface of a tungsten-copper alloy” with publication number CN103484806A discloses a technical solution: placing the tungsten-copper alloy in a supersonic particle bombardment device for surface nano-processing, and a nanocrystalline layer with a thickness of 0.3 to 1.2 mm can be obtained after the process. This patented technology uses high-energy hard particles to bombard the tungsten surface, thereby breaking up the coarse-grained tungsten to obtain nano-sized tungsten grains. This high-energy bombardment does achieve the purpose of nano-sizing, but it also introduces other defects, such as high-density dislocations, etc. These defects are thermodynamically metastable, that is, they will become significantly unstable at high temperatures, resulting in significant recrystallization and grain coarsening. Therefore, in order to obtain a thermodynamically stable surface nanolayer that is tightly bonded to the substrate, it is necessary to further develop the corresponding tungsten surface nano-sizing technology. Summary of the invention

[0005] One of the purposes of the present invention is to provide a simple and easy method for surface nano-processing to address the problem that the existing coarse-grained tungsten materials have insufficient radiation resistance. By combining mechanical polishing and vacuum heat treatment, a fine and high-temperature stable nanocrystalline layer can be controllably constructed on the surface of the coarse-grained tungsten, and the nanocrystalline layer is tightly bonded to the substrate.

[0006] To achieve the above object, the present invention adopts the following technical solution: a method for nano-crystallizing the surface of refractory metal tungsten, comprising the following steps:

[0007] S1. Grind the surface of coarse-grained tungsten particles to remove the oxide layer and obtain a smooth metallic glossy surface;

[0008] S2, mechanically polishing the surface of the coarse-grained tungsten particles after grinding at a set pressure to remove scratches, obtain a mirror-polished surface, and clean the surface;

[0009] S3, the cleaned coarse-grained tungsten particles are placed in a vacuum tube furnace for annealing heat treatment, the annealing temperature is 900-1100°C, the heating rate is 5-10°C / min, the holding time is 1-2 hours, and the vacuum degree is better than 5×10 -3 Pa, a nano-sized surface layer is formed from the surface of the coarse-grained tungsten particles to a range of 200 to 400 nm from the surface, and the grain size is 200 to 400 nm.

[0010] As a further improvement of the treatment method of surface nano-crystallization of refractory metal tungsten:

[0011] Preferably, in the coarse-grained tungsten particles, the size of the mother crystal is greater than 10 μm, and the size of the sub-crystal is greater than 1 μm.

[0012] Preferably, in step S1, sandpaper with a mesh size of 400-2000 is used to grind the surface of the coarse-grained tungsten particles.

[0013] Preferably, when sandpaper is used to polish the surface of the coarse-grained tungsten particles in step S1, the mesh sizes of the sandpaper used are increased successively.

[0014] Preferably, the sandpaper is SiC sandpaper, and when used to grind the surface of coarse-grained tungsten particles, the mesh size of the SiC sandpaper increases gradually.

[0015] Preferably, in step S2, polishing paste is coated on the wet flannel cloth, and a pressure of 5 to 50 N is applied to polish the surface of the coarse-grained tungsten particles, and the mesh number of the polishing medium in the polishing paste is 4000 to 10000.

[0016] Preferably, the coarse-grained tungsten particles are polished using polishing pastes with successively increasing polishing medium mesh sizes.

[0017] Preferably, during the process of polishing the surface of the coarse-grained tungsten particles in step S2, the pressure applied is gradually reduced.

[0018] Preferably, the polishing medium is made of diamond.

[0019] Preferably, during the process of polishing the surface of the coarse-grained tungsten particles in step S2, the applied pressure decreases gradually, and the polishing time at each pressure is 20-30 minutes, and finally the scratches are completely removed to obtain a smooth surface.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1) The present invention adopts a method of mechanical polishing combined with vacuum heat treatment to form a fine nanocrystalline layer on the surface of a coarse-grained tungsten substrate.

[0022] In the prior art, grinding and polishing are only used to remove oxide layers, oil stains, etc. as cleaning functions. However, this patent uses mechanical polishing with applied stress to introduce high-density dislocations in the near-surface layer of coarse-grained tungsten. These dislocations are thermodynamically metastable. In the subsequent high-temperature 900-1100-degree vacuum annealing and vacuum heat treatment, recovery occurs, driving the dislocations to rearrange and evolve into dense nanocrystals. The whole process is to achieve surface nano-crystalization by introducing dislocations and then regulating dislocations. There are the following two superior effects: First, dislocations belong to defects of the tungsten matrix itself. Nanocrystals formed by prefabricated dislocations and recovery dislocations are tightly combined with the tungsten matrix and do not form a heterogeneous interface. Therefore, the bonding strength between the nanocrystal layer and the matrix is ​​high; second, dislocations belong to thermodynamically metastable structures. High-temperature annealing allows them to recover and form nanocrystals, which is a thermodynamic adaptation process. Therefore, the formed nanocrystals have higher thermal stability. The stable surface nanocrystal layer finally constructed is expected to improve the radiation resistance of the material.

[0023] This patent introduces dislocations first, and then regulates the dislocation rearrangement to form a surface nanocrystalline layer. The initial mother crystal and subcrystal size will affect the introduced dislocation density, etc. The size of the mother crystal in the coarse-grained tungsten particles is greater than 10μm, and the size of the subcrystal is greater than 1μm, which is conducive to the introduction of dislocations.

[0024] 2) The Chinese invention patent document "Preparation method of tungsten / copper layered composite material based on nano-tungsten sheet surface" with publication number CN108570703A discloses a method of nano-tungsten sheet surface by anodic oxidation, which is actually to convert tungsten into fluffy tungsten oxide first, and then convert tungsten oxide into tungsten with nanoporous structure by hydrogen reduction in the third step. The steamer process is a chemical reaction process: tungsten (oxidation) → fluffy tungsten oxide (reduction) → porous tungsten. This is completely different from the technical principle of the above-mentioned application.

[0025] The invention adopts simple two-step mechanical polishing and vacuum heat treatment, has simple process, short time consumption, low energy consumption, reliable repeatability, and can realize batch processing. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The scanning electron microscope characterization diagram of the product of Example 1-3, wherein Figure 1 (a) and 1(b) are respectively a top view and a cross-sectional side view of the scanning electron microscope photograph of the surface nanocrystalline layer in Example 1; Figure 1 (c) and 1(d) are respectively a top view and a cross-sectional side view of the SEM photograph of the surface nanocrystalline layer in Example 2; Figure 1 (e) and 1(f) are the top view and cross-sectional side view of the scanning electron microscope photograph of the surface nanocrystalline layer in Example 3, respectively.

[0027] Figure 2 This is a transmission electron microscope photo of the surface nanocrystals obtained in Example 1, wherein Figure 2 (a) is a bright field image of a transmission electron microscope. Figure 2 (b) is a high-resolution image of a subgrain boundary. Figure 2 (c) shows the distribution of dislocations at subgrain boundaries. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present invention.

[0029] Example 1

[0030] This embodiment provides a method for nano-crystallizing the surface of refractory metal tungsten, which specifically includes the following steps:

[0031] S1. Take coarse-grained tungsten particles with a size of 20 mm (length) × 20 mm (width) × 2 mm (thickness), and use SiC sandpaper with mesh sizes of 400, 800, 1200, and 2000 to polish the surface of the coarse-grained tungsten particles to remove the surface oxide layer and obtain a smooth metallic glossy surface;

[0032] S2, then apply a polishing paste with a particle size of W2.5 (mesh number of 4000) on the wet flannel, apply a pressure of 50N, and polish the surface of the coarse-grained tungsten particles for 30 minutes; then change to a polishing paste with a particle size of W0.5 (mesh number of 10000 mesh), apply 20N, 10N, and 5N pressures in turn, and polish for 30 minutes each, and finally wash with ethanol to obtain mirror-polished coarse-grained tungsten particles;

[0033] S3. Then, the cleaned coarse-grained tungsten particles are placed in a vacuum tube furnace and heated to 900°C at a heating rate of 10°C / min and kept at this temperature for 2 hours. The vacuum degree is maintained at 5×10 -3 Pa or more, after annealing is completed, a surface nanocrystalline layer is obtained to achieve surface nanocrystalization.

[0034] Example 2

[0035] This embodiment provides a method for nano-crystallizing the surface of refractory metal tungsten, which specifically includes the following steps:

[0036] S1. Take coarse-grained tungsten particles with a size of 20 mm (length) × 20 mm (width) × 2 mm (thickness), and use SiC sandpaper with mesh sizes of 400, 800, 1200, and 2000 to polish the surface of the coarse-grained tungsten particles to remove the surface oxide layer and obtain a smooth metallic glossy surface;

[0037] S2, then apply W2.5 (mesh number is 4000) polishing paste on the wet flannel, apply 50N pressure, and polish the surface of the coarse-grained tungsten particles for 30 minutes; then change to W0.5 (mesh number is 10000 mesh) polishing paste for polishing, apply 20N, 10N, and 5N pressures for polishing for 30 minutes each, and finally wash with ethanol to obtain mirror-polished coarse-grained tungsten particles;

[0038] S3. Then, the cleaned coarse-grained tungsten particles are placed in a vacuum tube furnace, heated to 1100°C at a heating rate of 10°C / min, and kept at this temperature for 1 hour. The vacuum degree is maintained at 5×10 -3 Pa or more, after annealing is completed, a surface nanocrystalline layer is obtained to achieve surface nanocrystalization.

[0039] Example 3

[0040] This embodiment provides a method for nano-crystallizing the surface of refractory metal tungsten, which specifically includes the following steps:

[0041] S1. Take coarse-grained tungsten particles with a size of 20 mm (length) × 20 mm (width) × 2 mm (thickness), and use SiC sandpaper of 400, 800, 1200, and 2000 mesh specifications to grind the surface of the coarse-grained tungsten particles to obtain a smooth metallic glossy surface;

[0042] S2, then apply a polishing paste with a particle size of W2.5 (mesh number of 4000) on the wet flannel, apply a pressure of 50N, and polish the surface of the coarse-grained tungsten particles for 30 minutes; then change to a polishing paste with a particle size of W0.5 (mesh number of 10000 mesh), apply a pressure of 20N, 10N, and 5N for 30 minutes each, and finally wash with ethanol to obtain mirror-polished coarse-grained tungsten particles;

[0043] S3. Then, the cleaned coarse-grained tungsten particles are placed in a vacuum tube furnace, heated to 1000°C at a heating rate of 5°C / min, and kept at this temperature for 1 hour. The vacuum degree is maintained at 5×10 -3 Pa or more, after annealing is completed, a surface nanocrystalline layer is obtained to achieve surface nanocrystalization.

[0044] In order to further verify that the formation of surface nanocrystals is caused by dislocation recovery, we performed transmission electron microscopy characterization on the products of Examples 1-3. Figure 1 The scanning electron microscope characterization diagram of the product of Example 1-3, wherein Figure 1 (a) and 1(b) are respectively a top view and a cross-sectional side view of the scanning electron microscope photograph of the surface nanocrystalline layer in Example 1; Figure 1 (c) and 1(d) are respectively a top view and a cross-sectional side view of the SEM photograph of the surface nanocrystalline layer in Example 2; Figure 1 (e) and (f) are respectively the top view and the cross-sectional side view of the SEM photograph of the surface nanocrystalline layer in Example 3. Figure 1 It can be seen that a dense, non-villi-structured nano-surface layer is obtained within a range of 200 to 400 nm from the surface of the coarse-grained tungsten particles, and the grain size is 200 to 400 nm.

[0045] Figure 2 is a representative transmission electron microscope photo of the product of Example 1, wherein Figure 2 (a) is a bright field image of a transmission electron microscope. Figure 2 (b) is a high-resolution image of a subgrain boundary. Figure 2 (c) shows the distribution of dislocations at subgrain boundaries. The results show that the nanocrystalline grain boundaries formed on the surface are small-angle subgrains, proving that they evolved from cellular structures formed by dislocation recovery, and these cellular structures no longer contain thermodynamically unstable high-density dislocations and are more stable at high temperatures.

[0046] Those skilled in the art should understand that the above are only some specific embodiments of the present invention, rather than all embodiments. It should be noted that for those of ordinary skill in the art, many modifications and improvements can be made, and all modifications or improvements that do not exceed the scope of protection of the present invention should be regarded as the scope of protection of the present invention.

Claims

1. A method for nano-crystallizing the surface of refractory metal tungsten, wherein the metal tungsten is used as a substrate, characterized in that: The steps include: S1. Grind the surface of the coarse-crystalline tungsten particles to remove the oxide layer and obtain a smooth metallic glossy surface; the specific steps of grinding are: use sandpaper with a mesh number of 400-2000 to grind the surface of the coarse-crystalline tungsten particles, and the mesh number of the sandpaper used in the grinding process increases successively; S2, apply W2.5 polishing paste on the wet flannel, apply 50 N pressure, and polish the surface of the coarse-grained tungsten particles for 30 minutes; then change to W0.5 polishing paste for polishing, apply 20 N, 10 N, and 5 N pressures for 30 minutes each, and wash to obtain mirror-polished coarse-grained tungsten particles; S3, the cleaned coarse-grained tungsten particles are placed in a vacuum tube furnace for annealing heat treatment, the annealing temperature is 900~1100℃, the heating rate is 5~10℃ / min, the holding time is 1~2 hours, and the vacuum degree is better than 5×10 -3 Pa, a nano-sized surface layer is formed from the surface of the coarse-grained tungsten particles to the range of 200~400 nm from the surface, and the grain size is 200~400 nm.

2. The method for surface nano-processing of refractory metal tungsten according to claim 1, characterized in that: The size of the mother crystal in the coarse-grained tungsten particles is greater than 10 μm, and the size of the sub-crystal is greater than 1 μm.

3. The method for surface nano-processing of refractory metal tungsten according to claim 1, characterized in that: The sandpaper is SiC sandpaper. When used to grind the surface of coarse-grained tungsten particles, the mesh size of the SiC sandpaper increases gradually.

4. The method for surface nano-processing of refractory metal tungsten according to claim 1, characterized in that: The material of the polishing medium in the polishing paste is diamond.

5. The method for surface nano-processing of refractory metal tungsten according to claim 1, characterized in that: During the process of polishing the surface of the coarse-grained tungsten particles in step S2, the applied pressure is gradually reduced, and the polishing time is 20-30 minutes at each pressure, and finally the scratches are completely removed to obtain a smooth surface.

Images