Effect of dysprosium infiltration by magnetron sputtering on properties and microstructure of sintered NdFeB

A layer of metal Dy was deposited on the surface of NdFeB by magnetron sputtering. After high temperature diffusion, Dy diffuses into the magnet and displaces Nd in the magnet to form a uniform and continuous Nd rich phase. The (Nd, Dy) 2Fe14b phase is formed on the surface of the main phase grain, and the diffusion depth is about 350 μM. The coercivity increases by 52.1% and 32.2% respectively for the 10 mm thick Dy free and low Dy content magnets with 1% Dy (mass fraction). The results show that the coercivity of Nd-Fe-B increases significantly and the remanence of Nd-Fe-B does not decrease.

Since its appearance in the 1980s, sintered NdFeB has been favored for its superior magnetic properties and high cost performance. However, the temperature stability of sintered NdFeB produced at present is poor, which results in the application range of sintered NdFeB being greatly reduced. Increasing the coercivity of sintered NdFeB is a way to improve its high temperature stability. Secondly, the coercivity of sintered NdFeB is far from its theoretical value (6368ka / M) [2], which shows that the coercivity of sintered NdFeB still has a lot of room to improve.
In order to improve the coercivity and temperature stability, the commonly used method is to add heavy rare earth elements Dy or TB to improve the magnetocrystalline anisotropy field of the main phase grains. However, there are two problems in the traditional method of adding heavy rare earth elements: first, the production cost is greatly increased; second, antiferromagnetic coupling is formed between HRE and Fe, resulting in a significant reduction of remanence and magnetic energy product. Grain boundary diffusion (GBD) is a hot research topic in recent years. Heavy rare earth metals or rare earth compounds are wrapped on the surface of bulk or powder magnets by various processes for GBD.
There are many ways to achieve grain boundary diffusion, which can be divided into dry process and wet process. The dry process includes double alloy process [3,4], thermal evaporation process [7,8] and magnetron sputtering, while the wet process includes surface coating process [5,6], electrophoretic deposition, etc. By means of high temperature diffusion, Dy or TB diffuses into the edge of the main phase grain along the grain boundary. At the edge of the main phase grain, Dy or TB preferentially displaces Nd to form higher anisotropy
At the same time, the defects at the grain boundary are reduced, which effectively reduces the formation of reversal domain [10], significantly improves the coercivity of the magnet, and has no obvious effect on the remanence and magnetic energy product.
In this paper, the magnetron sputtering coating method is used to compare a Dy free magnet and a low Dy content magnet. A layer of rare earth film is deposited on the magnet. Combined with the grain boundary diffusion technology, the purpose of improving the performance of the magnet is achieved.

Experimental method

In the experiment, a commercial sintered NdFeB 1# (PR, nd) 31fe67b0.96al0.4co0.3cu0.15 (mass fraction, the same below)) and a sintered NdFeB 2# (PR, nd)) 30.6febalb1Dy0.13al0.5co1.1tb0.11cu0.15 with low Dy content were used to cut the magnet into a cylinder with the size of Ø10 × 10mm. All samples were deposited with Dy layer by magnetron sputtering. Before coating, the magnet needs to be cleaned: after removing the surface grease in the degreasing solution at 80 ℃, pickling with 3% dilute nitric acid solution for 1 min and ultrasonic water for 2 min. The coating equipment is a self-made magnetron sputtering system in the laboratory, and the workpiece is turned over by a manual manipulator to realize the completely uniform coating of the block. The working pressure of the coating is 0.1pA, the power of the target is 100W, and the two targets work at the same time. After coating, the samples were put into a vacuum annealing furnace for diffusion heat treatment. The samples were treated at 900 ℃ for 0.5h-16h, and then tempered at 500 ℃ for 2h.
The demagnetization curves of the samples after diffusion were measured by permanent magnet material measurement system (nim2000) at room temperature, and the changes of magnetic properties before and after diffusion were analyzed. After mechanical polishing, the cross-section and surface microstructure of the magnet were observed under feiquanta feg250 field emission scanning electron microscope (FESEM), and the selected area was analyzed by EDs. D8advanced XRD of Bruker AXS company in Germany was used to compare the structural changes of the samples, and gda750hp of spectroma company in Germany was used to analyze the element distribution along the depth after diffusion.

Results and discussion

Microstructure and composition changes of Dy free magnets

Fig. 1a and Fig. 1b show the cross-section microstructure of commercial sintered NdFeB-1 magnets without Dy before and after Dy infiltration. In Figure 1a, the silver white area is the Dy film deposited by magnetron sputtering, and the dark gray area is the magnet substrate. In order to clearly observe the composition changes before and after Dy infiltration, line scan analysis was carried out along the depth direction in Fig. 1a and Fig.1b, respectively. The results of composition analysis correspond to Fig.1c and Fig.1D. It is obvious that Dy diffuses into the matrix after high temperature infiltration.

Microstructure and composition changes of low Dy magnets after Dy infiltration

In order to observe the diffusion of Dy at the grain boundary of magnets with different Dy contents, the microstructure of the cross section before and after the diffusion of Dy at the grain boundary of magnets with low Dy content and the distribution of Dy, Nd and Fe were compared, as shown in Fig. 2. After grain boundary diffusion treatment, Dy diffuses along the depth direction, which is consistent with the results in Fig. 1. At 0-10 μm away from the surface, the content of Dy decreased obviously, and then decreased slowly.

Performance change of the magnet before and after Dy infiltration

Fig. 3 shows the relation curve of magnetic properties with diffusion time after grain boundary diffusion of low Dy and non Dy sintered NdFeB magnets. The corresponding optimal diffusion properties are listed in Table 1. Although the thickness of the magnet is up to 10 mm, the coercivity of the two sintered NdFeB is significantly improved after grain boundary diffusion. With the increase of diffusion time, the increase of coercivity tends to be smooth. The coercivity of the Dy free 1 # magnet increases from 12.30koe to 18.71koe by 52.1%, and the remanence decreases from 13.87kgs to 13.60kgs by 1.9%. However, the coercivity of low Dy 2{ magnets increases from 16.39 to 21.66 kOe by 32.2%, and the remanence decreases slightly from 13.68 kgs to 13.14 kgs. Obviously, under the same conditions, the effect of Dy permeating without Dy magnet is better. Although the atomic magnetic moment of Dy is higher than that of Fe, the ferromagnetic coupling between Dy and FE results in the decrease of MS in the main phase and the decrease of remanence and maximum magnetic energy product.
Table.1 magnetic properties of two kinds of magnets without diffusion and after grain boundary diffusion

Samples B/kGs Hcj /kOe ( BH) max /MGOe Dy/%
1#untreated 13. 87 12. 30 46. 20 0
1#Dy diffused 13. 60 18. 71 45. 17 1. 13
2#untreated 13. 68 16. 39 44. 74 0. 136
2#Dy diffused 13.14 21.66 42.49 1.27

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Figure.1 No Dy magnet 1# microstructure before diffusion (a) and line scan (c); after diffusion (b) and line scan (d)

Element distribution along depth after Dy infiltration

Figure 4 shows the distribution of Nd and Dy elements with depth after diffusion of the two magnets. According to the surface of 100 μm, 200 μm and 300 μm, the Dy content of 1 ﹥ magnet is 9.7%, 6.2% and 1.8% respectively, and the Dy content of 2 ﹥ magnet is 9.0%, 5.0% and 1.5% respectively. The results of GD-OES element depth distribution show that the Dy content in the non Dy magnet 1 # is higher than that in the low Dy magnet 2 #, and Dy may diffuse more easily in the non Dy magnet. This may be due to the existence of re rich corner phase in the grain boundary phase of Dy containing magnets, which hinders the diffusion of Dy in the grain boundary.
Figure 5 shows the backscatter SEM micrographs of the original state and grain boundary diffusion for 10h. By analyzing the volume fraction of Nd rich phase with image analysis software, it can be seen that the volume fraction of Nd rich phase in 1 # original state is 6.0%, after 10 h diffusion, the Nd rich phase increases to 7.0%, and the volume fraction of Nd rich phase in 2 # original state is 6.24%, after 10 h diffusion, the Nd rich phase increases to 19.17%. Obviously, with the grain boundary diffusion of Dy, the Nd rich phase increases, and the continuous and uniform grain boundary phase gradually forms between the main phase grains Therefore, the magnetic coupling effect of hard magnetic grains is effectively suppressed. At the same time, the tempering treatment in grain boundary diffusion makes the grain boundary of the main phase flat and regular, which makes it difficult for the diamagnetic domain to nucleate.

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Figure 2 Low Dy content magnet 2# before diffusion microstructure (a) and line scan (c) and after diffusion (b) and line scan (d)

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Fig.3 demagnetization curve of the magnet and the relationship between coercivity and diffusion time of Dy magnet by magnetron sputtering
Fig. 6 shows the scanning electron microscope micrographs at 150 μm away from the surface of 1 ᦇ and 2 ᦇ magnets respectively after 10 h Dy infiltration. It is obvious that there is a light gray shell structure about 1 μm ~ 2 μm thick at the edge of the main phase grain, which is consistent with the description in reference [12]. It can be concluded that Dy element diffuses through the grain boundary and enters into the edge of the main phase to replace Nd element in the edge of the main phase to form (Nd, Dy) 2Fe14B. Because the anisotropic field ha of Dy2Fe14B is 15.0t, which is 7.4t higher than that of Nd2Fe14B [13], the coercivity of the magnet forming this shell core structure is effectively improved. In addition, the Nd element is replaced and discharged from the main phase grains, and more Nd rich phases are formed at the grain boundaries, which further supports the phenomenon of increasing Nd rich phases diffused at the grain boundaries.

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Fig. 4 content distribution of Nd and Dy elements along the depth direction of the magnet

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Fig. 5 micrographs of Dy free and Dy containing magnets after grain boundary diffusion
In order to further determine the composition difference between the light gray shell structure and other areas, spot scanning was carried out at different positions. The five spot scanning positions are shown in Fig. 6a, and the composition is listed in Table 2. It can be seen that there is enrichment of Dy element in the shell structure at the edge of the main phase grain, the Dy content is about 5.2% ~ 6.1%, and there is no Dy element in the grain.

XRD analysis before and after diffusion

In order to understand the change of the grain orientation of the magnet before and after diffusion, the grain orientation degree of 1 # and 2 # magnets before and after Dysprosium infiltration was analyzed by XRD, and the results are shown in Fig. 7. It can be seen from Fig. 7 that after diffusion, the degree of crystal orientation of both magnets increases at (006), indicating that Dy preferentially diffuses at (006). At the same time, it is noticed that all diffraction peaks shift to the right, which is due to the preferential substitution of Nd atoms at the edge of the main phase grains by Dy atoms during the grain boundary diffusion process, forming (nd, Dy) 2Fe14B shell phase [15]. Because the lattice constant of Dy2Fe14B is smaller than that of the main phase Nd2Fe14B, according to the blager diffraction formula, the crystal plane spacing D decreases and the θ angle increases, which leads to the change of the diffraction peak position in the XRD pattern Offset.
Table 2 contents of elements at different positions /%

Position Element content
Nd Pr Fe Dy Al
Site 1 43. 64 17. 93 26. 66 2. 73 1. 04
Site 2 18. 70 5. 21 69. 34 5. 21 0. 48
Site 3 22. 18 5. 41 70. 79 0 0. 48
Site 4 21. 31 6. 02 66. 00 6. 13 0. 48
Site 5 47. 89 19. 8 24. 21 0 0. 57

Discussion

At present, there are many methods to optimize the microstructure and improve the coercivity of magnets by grain boundary diffusion, such as double alloy method, hot evaporation method, immersion method and so on. Compared with these methods, the magnetron sputtering method has several advantages: (1) the film substrate adhesion is good, and the film is compact. The kinetic energy of magnetron sputtered atoms is 10-100 times higher than that of thermally evaporated atoms, so the diffusion layer with better adhesion and denser film can be produced, which is more conducive to the diffusion of Dy. (2) The film thickness can be controlled accurately.
Compared with the thermal evaporation and immersion method, the deposition rate of magnetron sputtering is constant, the quantitative addition of Dy can be realized, and the effective utilization rate of rare earth elements is improved.

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Fig. 6 microstructure (a, c) and line scan (B, d) of 1 # and 2 # magnets after Dy infiltration (about 150 μm from the surface)

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Fig. 7 XRD results before and after 1 # and 2 # magnet diffusion
During the diffusion process of Dy atoms attached to the surface of the magnet at high temperature, there are two diffusion modes: grain boundary diffusion and lattice diffusion. Dy diffuses into the magnet along the grain boundary and displaces with Nd2Fe14B. The (NdDy) 2Fe14B shell with higher anisotropy field is formed at the edge of Nd2Fe14B, the excess Nd atoms are expelled from the grain boundary, and the continuous and uniform Nd rich grain boundary phase is formed at the grain boundary, which reduces the exchange coupling between the hard magnetic phases, reduces the original grain boundary defects, and inhibits the nucleation and growth of reverse domains. All these factors affect the properties of the magnet and make the coercivity of the magnet increase significantly I’m trying to improve.

Conclusion

The results show that the film / substrate bonding force of the magnet infiltrated with Dy by magnetron sputtering is better, which is more conducive to the diffusion of Dy in the magnet.

  • 1. The results show that the coercivity of sintered NdFeB magnets with low Dy content increases from 16.39 to 21.66 and that of non Dy magnets from 12.30 to 18.71.
  • 2. Compared with the microstructure before and after Dy infiltration, a more continuous and uniform Nd rich phase structure is formed after Dy infiltration, which effectively inhibits the magnetic coupling effect of the main phase grains, which is a major reason for the improvement of the coercivity of the magnet.
  • 3. The shell structure of (ndDy) 2Fe14B is formed at the edge of the main phase grain after Dy permeating, and the anisotropy field is higher than that of Nd2Fe14B, which is another reason to improve the coercivity of the magnet.
  • 4. The results of element depth analysis of glow discharge emission spectrum show that the diffusion depth of Dy is more than 350 μm, and the magnet without Dy is more conducive to the diffusion of Dy.

Author:ZHANG Li-jiao1,MAO Shou-dong,YANG Li-jing,HE Yan-lin1,SONG Zhen-lun,XU Cheng
Source: China Permanent Magnet Manufacturer – www.ymagnet.com

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