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Preparation of parylene-coated bonded NdFeB magnets
Accepted Manuscript Preparation of parylene-coated bonded NdFeB magnets Bin Ma, Aizhi Sun, Xuexu Gao, Xiaoqian Bao, Jiheng Li PII: DOI: Reference:
S0304-8853(18)31009-6 https://doi.org/10.1016/j.jmmm.2018.07.061 MAGMA 64167
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
6 April 2018 10 July 2018 20 July 2018
Please cite this article as: B. Ma, A. Sun, X. Gao, X. Bao, J. Li, Preparation of parylene-coated bonded NdFeB magnets, Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/j.jmmm.2018.07.061
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation of parylene-coated bonded NdFeB magnets Bin Maa, Aizhi Sunb,*, Xuexu Gaoa*, Xiaoqian Baoa, Jiheng Lia a
State key laboratory for advanced metals and materials, University of Science and Technology Beijing, Beijing 100083, PR China
b
Institute for advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, PR China
Corresponding author: Aizhi Sun. E-mail address: [email protected] Corresponding author: Xuexu Gao. E-mail address: [email protected] Abstract The anisotropic parylene-coated bonded NdFeB magnets with parylene C coating on the surface of NdFeB powders are prepared by warm compaction process. The parylene C coating is characterized by focused ion beam-scanning electron microscopy (FIB-SEM) and energy dispersive X-ray Spectroscopy (EDS), revealing a parylene C film of 300~500nm thickness on the surface of NdFeB powders. The parylene-coated NdFeB magnets show excellent magnetic properties of maximum energy product ((BH)max,104 kJ/m3), degree of alignment (DOA, 0.476) and density(6.17 g/cm3) than non-coated NdFeB magnets (60 kJ/m3, 0.337, 5.72 g/cm3). And with the increasing thickness of parylene C coating, (BH)max and density increase first, and then decrease while DOA keeps a growing trend all the time. The parylene-coated NdFeB magnets also exhibit better corrosion resistance according electrochemical test. The mechanisms for the improvement of magnetic properties and corrosion resistance are the good lubricating property and super chemical stability of parylene C coating on the surface of NdFeB powders. Key Word： bonded NdFeB magnets; parylene-coated; DOA; density; lubricating property.
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1. Introduction Since high-energy NdFeB magnets were first reported in 1983[1,2], a large number of research activities has been devoted to the improvement of intrinsic material properties and the development of fabrication techniques. The NdFeB-type magnets can roughly be classified in two categories, the sintered and bonded magnets. Sintered NdFeB magnet can either be produced by the conventional alloy casting, powder metallurgy method or by the hot deformation routes[3]. Differently, bonded NdFeB magnets, isotropic or anisotropic, are fabricated by first mixing magnetic powders with polymer binder and next compression or injection molding[4-8]. For bonded NdFeB magnets, magnetic powders and preparation process are important factors affecting magnetic properties. Many researches, therefore, have been made in improving magnetic powders and optimizing preparation process of bonded NdFeB magnets[9-14]. Unfortunately, the low density and DOA still limit bonded NdFeB magnets to be intermediate energy products. Up to now, increasing of density and DOA is still the hot topics of bonded NdFeB magnets. It is well known improvement of lubrication can decrease rotational resistance and frictional resistance of powders, and thereby increase the density and DOA of green compact[15,16]. There are two mainly methods to improve lubricating effect during compaction and orientation process: developing preparation process and improving lubricating property of magnetic powders. Warm compaction process, a developed process that improving the lubrication effect by liquid binder softened at elevated temperature rangeing from 100℃to 150℃, has been used in anisotropic bonded NdFeB magnet to increase density and DOA[17-23]. Another effective method to increase density and DOA is the surface lubricating treatment of NdFeB powders. Parylene coating, a surface coating polymer material that exhibits excellent properties of electrical insulation, toughness, corrosion resistance, low friction coefficient and so on[24-26], has been widely applied in surface modification of iron-based magnetic powders to improve lubricating properties of powders[27-31]. In this paper, surface lubricating treatment of parylene C on NdFeB powders and 2
warm compaction process are applied to improve lubricating effect during orientation and compaction process, and thereby increase magnetic properties of bonded NdFeB magnets. The (BH)max, DOA and density of parylene-coated NdFeB magnets (sample 2) compressed at 470MPa (104 kJ/m3, 0.476 and 6.17 g/cm3) are increased by 73.33%, 41.24%, 7.87% than non-coated NdFeB magnets (60 kJ/m3, 0.337, 5.72 g/cm3). And with the increasing thickness of parylene C coating, (BH)max and density increase first, and then decrease while DOA keeps a growing trend all the time. The corrosion resistance of bonded NdFeB magnets is obviously improved after NdFeB powders being coated with parylene C coating. The mechanisms of parylene C coating on the improvement of (BH)max, DOA, density and corrosion resistance of bonded NdFeB magnets is also discussed .
2. Experimental The anisotropic NdFeB powders uesd in this work are prepared by d-HDDR process, where (BH)max, Hcj, Br are 294 kJ/m3, 1084 kA/m and 1.33T, respectively. The NdFeB powders are first cleaned 2h in dilute solution with 5 vol% silane coupling agent (KH550) and 95 vol% alcohol using ultrasonic cleaner (BK-240B, China) and then washed three times in ethanol. These dried powders (60g) are placed in the cylinder that keeps rotating in deposition chamber. The schematic diagram of parylene coating process is shown in Fig.1. Parylene C powders, placed in vaporization chamber, are vaporized to dimeric gas at 180℃ and next cleaved into monomer at 680℃ in pyrolysis chamber. Monomer vapor, finally, is deposited on the surface of powders and polymerizes into Parylene C coating[19-21]. The thickness of parylene C coating is increased with the increasing Parylene C mass. In this work, parylene-coated NdFeB powders are defined as sample1, sample2, sample3 and sample4 with the increasing parylene C mass of 4.5g, 6.5g, 8.5g and 10.5g, respectively.
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Fig.1 Schematic diagram of parylene coating process. NdFeB powders are mixed with 2.5wt% epoxy resin and 0.5wt% silane coupling agent (KH550) in acetone. After volatilization of acetone, the blocky bonded magnetic powders are ground in mortar for 10 minutes. The ground powders are heated in mould cavity of 140℃ for 1minute, followed by compressed into cubic magnet with 7.0 × 7.0 ×7.0 mm3 at vertical pressure of 200~500 MPa and a horizontal magnetic field of 1.8T. Finally, the compressed magnets are demoulded and solidified in a drying oven at 140℃ for 60 min[23,31]. Parylene C powders and parylene coating machine are supplied by Penta Technology (Suzhou) Co. Ltd. The magnetic properties of bonded NdFeB magnets are measured by a hysteresis curve measuring instrument (NIM-2000, China). The NdFeB powders, parylene C coating and bonded NdFeB magnets are characterized by focused ion beam scanning electron microscopy (FIB-SEM, ZEISS Aurrga) coupled with energy dispersive X-ray Spectroscopy (EDS). A focused ion beam system is applied to analyze the thickness of parylene C coating on the surface of NdFeB powders. The degree of alignment (DOA) was used to describe the orientation effect of anisotropic bonded NdFeB magnets[13]. The actual density is measured by gravimetry on the basis of Archimedes method using distilled water as the medium.
3. Results and discussion Fig.2 shows SEM images of (a) NdFeB powders, (b) parylene-coated NdFeB powders and (c) EDS spectrum of selected area. It can be seen from Fig.2 that there 4
are many Nd2O3 spots (indicated by arrows) on the surface of NdFeB powders (a) while most of these Nd2O3 spots are covered by thin parylene C coating (b). According EDS spectrum in Fig.2 (c), parylene-coated NdFeB powder shows presence of peak corresponding to Cl, existing only in parylene C from Fig.1, and superstrong peak of C compared with NdFeB powder, which presents the surface coating of parylene C coating.
Fig.2 SEM images of (a) NdFeB powders, (b) parylene-coated NdFeB powders and (c) EDS spectrum of selected area. FIB-SEM images of parylene-coated NdFeB powder is shown in Fig.3(a) and (b)，where (b) is the enlarged view in selected areas of (a). It is obvious a thin coating (dark area), revealing thickness of about 300~500 nm, is coated on the surface of NdFeB powder (bright area). Fig.3(c) shows SEM-EDS line scan analysis of chlorine, neodymium and iron through parylene C coating interface. With the increasing distance from thin coating, the chemical element contents of Fe, Nd increase slowly at the interface and rapidly in NdFeB substrate, while Cl element shows the opposite trend, which further proves the thin coating of parylene C coating.
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Fig.3 FIB-SEM images of parylene-coated NdFeB powder, (b) is the enlarged view in selected areas of (a), and (c) SEM-EDS line scan analysis of parylene C coating interface. Fig.4 shows (BH)max (a), DOA (b), density (c), Br (e) and coercivity (f) of bonded NdFeB magnets under different pressure and corresponding 2nd quadrant demagnetization curves (d). With the increase of pressure, (BH)max, Br and density increase all the time but DOA shows a decreasing trend. Furthermore, it is clear that density plays a decisive role in determining (BH)max. Anyway, parylene-coated NdFeB magnets show much higher (BH)max, Br, DOA and density than non-coated NdFeB magnets. It should be noted, in this work, the thickness of parylene C coating on the surface of NdFeB powder is increased from sample 1 to sample 4. Accordingly, with the increasing thickness of parylene C coating, (BH)max, Br and density exhibit increasing trend to sample 2 and then decrease gradually whereas DOA keeps a growing trend. The (BH)max, Br, DOA and density of parylene-coated sample 2 compressed at 470MPa (104 kJ/m3, 0.803 T, 0.476, 6.17 g/cm3) are increased by 6
73.33%, 28.27%, 41.24%, 7.87% than non-coated sample (60 kJ/m3, 0.626 T, 0.337, 5.72 g/cm3) and 6.12%, 4.96%, -2.86%, 2.49% compared with coated sample 4 (98 kJ/m3, 0.767 T, 0.49 and 6.02 g/cm3), respectively. The coercivity of bonded magnets, seen from Fig.4 (f), remains almost unchanged no matter with the increase in pressure or the increase in thickness of parylene C coating. Fig.4 (d) displays corresponding 2nd quadrant demagnetization curves of non-coated sample, sample 2 and sample 4 at 470MPa, showing remanence (Br) of 0.626 T, 0.803 T, 0.767 T and coercivity (H) of 12.84 kOe, 12.58 kOe, 12.75 kOe, respectively. non-coated sample sample1 sample2 sample3 sample4 110
(b) 0.54
90 80
DOA
（ BH） max（ kJ/m3）
100
0.60
(a)
70
0.48 0.42
60 0.36 50 40
6.0
250
300 350 400 Pressure（ MPa）
450
0.30
500
(c)
200
250
300 350 400 Pressure（ MPa）
450
500
0.8
(d)
3
Density（ g/cm ）
6.2
200
5.6
0.4
Br（ T）
0.6
5.8
5.4 0.2 5.2 5.0
0.8
200
250
300 350 400 Pressure（ MPa）
450
500
-14
(e)
13.6
-10
-8 -6 H(kOe)
-4
-2
0.0 0
(f)
12.8
H(kOe)
0.7
Br（ T）
-12
0.6
12.0 11.2
0.5 10.4 0.4
200
250
300 350 400 Pressure（ MPa）
450
500
200
250
300 350 400 Pressure（ MPa）
450
500
Fig.4 (BH)max, DOA, density, Br and coercivity of bonded NdFeB magnets under different pressure and corresponding 2nd quadrant demagnetization curves. The mechanism for the improvement of DOA and density of parylene-coated NdFeB magnets is the good lubricating property of parylene-coated NdFeB powder, 7
resulting from low friction coefficient of parylene C, that reducing rotational resistance and frictional resistance between powders, which is proved in our previous study[31]. Additionally, the lubricating property is improved with the increasing thickness of parylene C coating, which is also the mutual proof of increasing DOA. Fig.5 displays backscattered electron (BSE) images of bonded NdFeB magnet: (a) non-coated sample, (b) coated sample 2. It can be seen from Fig.5 that sample 2 shows smaller volume fraction of holes (dark area) than non-coated sample, which corresponds to higher density. There are two mainly effects of parylene C coating on density of bonded NdFeB magnet. On one hand, the low friction coefficient of parylene C coating makes the excellent lubricating property of NdFeB powder, and thereby improves the density of bonded magnets. On the other hand, the lower density of parylene C coating results in decrease of density. Under these two aspects, finally, density reach the maximum at sample 2 with the increasing thickness of parylene C coating.
Fig.5 BSE SEM images of bonded NdFeB magnet: (a) non-coated sample, (b) coated sample 2. The corrosion resistance of NdFeB powders is characterized by degree of reaction with acid solution. Fig.6 shows PH-T curve of acid solution after putting non-coated, sample 2 and sample 4 NdFeB powders into it, where the initial PH is about 1.5. By comparing three PH-T curves in Fig.6, with the increasing thickness of parylene C coating, PH values decrease from non-coated sample to sample 4. And the ending time of reaction between acid solution and non-coated, sample 2, sample 4 NdFeB powders are 35min (PH=2.1 and disappearance of NdFeB powders), 20min (PH=1.7) and 14min (PH=1.6), respectively. It is difficult for parylene-coated NdFeB 8
powders to react with acid solution due to the super chemical stability of parylene coating on the surface of NdFeB powders, that effectively prevents the contact between NdFeB powders and H+ in acid solution, thereby improving the acid corrosion resistance of NdFeB powders.
Fig.6 PH-T curve of acid solution after putting NdFeB powders into it, where the initial PH is about 1.5. To further investigate the corrosion resistance of parylene-coated NdFeB magnet, the electrochemical test was employed. Fig.7 shows the potentiodynamic polarization curves of bonded NdFeB magnets for non-coated sample, sample 2 and sample 4 in 3.5 wt % NaCl solution. The results of the potentiodynamic test are summarized in Table 2. As can be seen from Fig.7, the polarization curve of parylene-coated sample moves left and the Ecorr is shifted positively compared with non-coated sample. The corrosion potential (Ecorr) of non-coated sample, sample 2, sample 4 is about -0.966V, -0.838V, -0.703V, while corrosion current density (Icorr) is 2.16×10-5 A/cm2, 6.46×10-6 A/cm2, 1.83×10-7 A/cm2, rspectively. The Icorr of sample 2 decreased one order of magnitude than that of non-coated sample while sample 4 decreased about two orders of the magnitude. The smaller current density represents the inhibition of anode and cathode reactions, indicating better corrosion resistance. From the information above, it can be concluded that the corrosion resistance of NdFeB magnets is obviously improved after NdFeB powders being coated with parylene C coating.
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Fig.7 Tafel polarization curves of bonded NdFeB magnet for non-coated sample, sample 2 and sample 4. Table 1 Corrosion potential (Ecorr) and corrosion current density (Icorr) of samples. Samples
Ecorr（V）
Icorr（A/cm2）
sample 4
-0.703
1.83×10-7
sample 2
-0.838
6.46×10-6
non-coated sample
-0.966
2.16×10-5
4. Conclusions The anisotropic bonded NdFeB magnets with parylene C coating on the surface of NdFeB powders are prepared by warm compaction process. The parylene C coating, synthesized on the surface of NdFeB powder by chemical vapor deposition polymerization (CVDP), is characterized by FIB-SEM EDS analysis, revealing the thickness of 300~500nm. The parylene-coated NdFeB magnets show excellent magnetic properties than non-coated NdFeB magnets. The (BH)max, DOA and density of parylene-coated sample 2 compressed at 470MPa (104 kJ/m3, 0.476 and 6.17 g/cm3) are increased by 73.33%, 41.24%, 7.87% than non-coated sample (60 kJ/m3, 0.337, 5.72 g/cm3) and 6.12%, -2.86%, 2.49% compared with coated sample 4 (98 kJ/m3, 0.49 and 6.02 g/cm3), respectively. And with the increasing thickness of parylene C coating, (BH)max and density increase first and then decrease gradually 10
whereas DOA keeps a growing trend. The reason for the improvement of DOA and density of parylene-coated NdFeB magnets is the good lubricating property of parylene-coated NdFeB powder that resulting from low friction coefficient of parylene C coating. The parylene-coated NdFeB magnets also exhibit better corrosion resistance for the super chemical stability of parylene C coating on the surface of NdFeB powders.
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Figures and tables captions Fig.1 Schematic diagram of parylene coating process. Fig.2 SEM images of (a) NdFeB powders, (b) parylene-coated NdFeB powders and (c) EDS spectrum of selected area. Fig.3 FIB-SEM images of parylene-coated NdFeB powder, (b) is the enlarged view in selected areas of (a), and (c) SEM-EDS line scan analysis of parylene C coating interface. Fig.4 (BH)max, DOA, density, Br and coercivity of bonded NdFeB magnets under different pressure and corresponding 2nd quadrant demagnetization curves. Fig.5 BSE SEM images of bonded NdFeB magnet: (a) non-coated sample, (b) coated sample 2. Fig.6 PH-T curve of acid solution after putting NdFeB powders into it, where the initial PH is about 1.5. Fig.7 Tafel polarization curves of bonded NdFeB magnet for non-coated sample, sample 2 and sample 4. Tab.1 Corrosion potential (Ecorr) and corrosion current density (Icorr) of samples.
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Research Highlights Parylene C is coated on the surface of NdFeB powders by CVDP. The lubricity of NdFeB powders is improved by surface coating of parylene C. The (BH)max DOA, density of magnets are increased by 73.33%, 41.24%, 7.87%. The parylene-coated NdFeB magnets exhibit better corrosion resistance.
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Table 1. Corrosion potential (Ecorr) and corrosion current density (Icorr) of samples. Samples
Ecorr（V）
Icorr（A/cm2）
sample 4
-0.703
1.83×10-7
sample 2
-0.838
6.46×10-6
non-coated sample
-0.966
2.16×10-5
17
18
19
20
21
22
23
24
25
26
27
28
29
S0304-8853(18)31009-6 https://doi.org/10.1016/j.jmmm.2018.07.061 MAGMA 64167
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
6 April 2018 10 July 2018 20 July 2018
Please cite this article as: B. Ma, A. Sun, X. Gao, X. Bao, J. Li, Preparation of parylene-coated bonded NdFeB magnets, Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/j.jmmm.2018.07.061
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation of parylene-coated bonded NdFeB magnets Bin Maa, Aizhi Sunb,*, Xuexu Gaoa*, Xiaoqian Baoa, Jiheng Lia a
State key laboratory for advanced metals and materials, University of Science and Technology Beijing, Beijing 100083, PR China
b
Institute for advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, PR China
Corresponding author: Aizhi Sun. E-mail address: [email protected] Corresponding author: Xuexu Gao. E-mail address: [email protected] Abstract The anisotropic parylene-coated bonded NdFeB magnets with parylene C coating on the surface of NdFeB powders are prepared by warm compaction process. The parylene C coating is characterized by focused ion beam-scanning electron microscopy (FIB-SEM) and energy dispersive X-ray Spectroscopy (EDS), revealing a parylene C film of 300~500nm thickness on the surface of NdFeB powders. The parylene-coated NdFeB magnets show excellent magnetic properties of maximum energy product ((BH)max,104 kJ/m3), degree of alignment (DOA, 0.476) and density(6.17 g/cm3) than non-coated NdFeB magnets (60 kJ/m3, 0.337, 5.72 g/cm3). And with the increasing thickness of parylene C coating, (BH)max and density increase first, and then decrease while DOA keeps a growing trend all the time. The parylene-coated NdFeB magnets also exhibit better corrosion resistance according electrochemical test. The mechanisms for the improvement of magnetic properties and corrosion resistance are the good lubricating property and super chemical stability of parylene C coating on the surface of NdFeB powders. Key Word： bonded NdFeB magnets; parylene-coated; DOA; density; lubricating property.
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1. Introduction Since high-energy NdFeB magnets were first reported in 1983[1,2], a large number of research activities has been devoted to the improvement of intrinsic material properties and the development of fabrication techniques. The NdFeB-type magnets can roughly be classified in two categories, the sintered and bonded magnets. Sintered NdFeB magnet can either be produced by the conventional alloy casting, powder metallurgy method or by the hot deformation routes[3]. Differently, bonded NdFeB magnets, isotropic or anisotropic, are fabricated by first mixing magnetic powders with polymer binder and next compression or injection molding[4-8]. For bonded NdFeB magnets, magnetic powders and preparation process are important factors affecting magnetic properties. Many researches, therefore, have been made in improving magnetic powders and optimizing preparation process of bonded NdFeB magnets[9-14]. Unfortunately, the low density and DOA still limit bonded NdFeB magnets to be intermediate energy products. Up to now, increasing of density and DOA is still the hot topics of bonded NdFeB magnets. It is well known improvement of lubrication can decrease rotational resistance and frictional resistance of powders, and thereby increase the density and DOA of green compact[15,16]. There are two mainly methods to improve lubricating effect during compaction and orientation process: developing preparation process and improving lubricating property of magnetic powders. Warm compaction process, a developed process that improving the lubrication effect by liquid binder softened at elevated temperature rangeing from 100℃to 150℃, has been used in anisotropic bonded NdFeB magnet to increase density and DOA[17-23]. Another effective method to increase density and DOA is the surface lubricating treatment of NdFeB powders. Parylene coating, a surface coating polymer material that exhibits excellent properties of electrical insulation, toughness, corrosion resistance, low friction coefficient and so on[24-26], has been widely applied in surface modification of iron-based magnetic powders to improve lubricating properties of powders[27-31]. In this paper, surface lubricating treatment of parylene C on NdFeB powders and 2
warm compaction process are applied to improve lubricating effect during orientation and compaction process, and thereby increase magnetic properties of bonded NdFeB magnets. The (BH)max, DOA and density of parylene-coated NdFeB magnets (sample 2) compressed at 470MPa (104 kJ/m3, 0.476 and 6.17 g/cm3) are increased by 73.33%, 41.24%, 7.87% than non-coated NdFeB magnets (60 kJ/m3, 0.337, 5.72 g/cm3). And with the increasing thickness of parylene C coating, (BH)max and density increase first, and then decrease while DOA keeps a growing trend all the time. The corrosion resistance of bonded NdFeB magnets is obviously improved after NdFeB powders being coated with parylene C coating. The mechanisms of parylene C coating on the improvement of (BH)max, DOA, density and corrosion resistance of bonded NdFeB magnets is also discussed .
2. Experimental The anisotropic NdFeB powders uesd in this work are prepared by d-HDDR process, where (BH)max, Hcj, Br are 294 kJ/m3, 1084 kA/m and 1.33T, respectively. The NdFeB powders are first cleaned 2h in dilute solution with 5 vol% silane coupling agent (KH550) and 95 vol% alcohol using ultrasonic cleaner (BK-240B, China) and then washed three times in ethanol. These dried powders (60g) are placed in the cylinder that keeps rotating in deposition chamber. The schematic diagram of parylene coating process is shown in Fig.1. Parylene C powders, placed in vaporization chamber, are vaporized to dimeric gas at 180℃ and next cleaved into monomer at 680℃ in pyrolysis chamber. Monomer vapor, finally, is deposited on the surface of powders and polymerizes into Parylene C coating[19-21]. The thickness of parylene C coating is increased with the increasing Parylene C mass. In this work, parylene-coated NdFeB powders are defined as sample1, sample2, sample3 and sample4 with the increasing parylene C mass of 4.5g, 6.5g, 8.5g and 10.5g, respectively.
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Fig.1 Schematic diagram of parylene coating process. NdFeB powders are mixed with 2.5wt% epoxy resin and 0.5wt% silane coupling agent (KH550) in acetone. After volatilization of acetone, the blocky bonded magnetic powders are ground in mortar for 10 minutes. The ground powders are heated in mould cavity of 140℃ for 1minute, followed by compressed into cubic magnet with 7.0 × 7.0 ×7.0 mm3 at vertical pressure of 200~500 MPa and a horizontal magnetic field of 1.8T. Finally, the compressed magnets are demoulded and solidified in a drying oven at 140℃ for 60 min[23,31]. Parylene C powders and parylene coating machine are supplied by Penta Technology (Suzhou) Co. Ltd. The magnetic properties of bonded NdFeB magnets are measured by a hysteresis curve measuring instrument (NIM-2000, China). The NdFeB powders, parylene C coating and bonded NdFeB magnets are characterized by focused ion beam scanning electron microscopy (FIB-SEM, ZEISS Aurrga) coupled with energy dispersive X-ray Spectroscopy (EDS). A focused ion beam system is applied to analyze the thickness of parylene C coating on the surface of NdFeB powders. The degree of alignment (DOA) was used to describe the orientation effect of anisotropic bonded NdFeB magnets[13]. The actual density is measured by gravimetry on the basis of Archimedes method using distilled water as the medium.
3. Results and discussion Fig.2 shows SEM images of (a) NdFeB powders, (b) parylene-coated NdFeB powders and (c) EDS spectrum of selected area. It can be seen from Fig.2 that there 4
are many Nd2O3 spots (indicated by arrows) on the surface of NdFeB powders (a) while most of these Nd2O3 spots are covered by thin parylene C coating (b). According EDS spectrum in Fig.2 (c), parylene-coated NdFeB powder shows presence of peak corresponding to Cl, existing only in parylene C from Fig.1, and superstrong peak of C compared with NdFeB powder, which presents the surface coating of parylene C coating.
Fig.2 SEM images of (a) NdFeB powders, (b) parylene-coated NdFeB powders and (c) EDS spectrum of selected area. FIB-SEM images of parylene-coated NdFeB powder is shown in Fig.3(a) and (b)，where (b) is the enlarged view in selected areas of (a). It is obvious a thin coating (dark area), revealing thickness of about 300~500 nm, is coated on the surface of NdFeB powder (bright area). Fig.3(c) shows SEM-EDS line scan analysis of chlorine, neodymium and iron through parylene C coating interface. With the increasing distance from thin coating, the chemical element contents of Fe, Nd increase slowly at the interface and rapidly in NdFeB substrate, while Cl element shows the opposite trend, which further proves the thin coating of parylene C coating.
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Fig.3 FIB-SEM images of parylene-coated NdFeB powder, (b) is the enlarged view in selected areas of (a), and (c) SEM-EDS line scan analysis of parylene C coating interface. Fig.4 shows (BH)max (a), DOA (b), density (c), Br (e) and coercivity (f) of bonded NdFeB magnets under different pressure and corresponding 2nd quadrant demagnetization curves (d). With the increase of pressure, (BH)max, Br and density increase all the time but DOA shows a decreasing trend. Furthermore, it is clear that density plays a decisive role in determining (BH)max. Anyway, parylene-coated NdFeB magnets show much higher (BH)max, Br, DOA and density than non-coated NdFeB magnets. It should be noted, in this work, the thickness of parylene C coating on the surface of NdFeB powder is increased from sample 1 to sample 4. Accordingly, with the increasing thickness of parylene C coating, (BH)max, Br and density exhibit increasing trend to sample 2 and then decrease gradually whereas DOA keeps a growing trend. The (BH)max, Br, DOA and density of parylene-coated sample 2 compressed at 470MPa (104 kJ/m3, 0.803 T, 0.476, 6.17 g/cm3) are increased by 6
73.33%, 28.27%, 41.24%, 7.87% than non-coated sample (60 kJ/m3, 0.626 T, 0.337, 5.72 g/cm3) and 6.12%, 4.96%, -2.86%, 2.49% compared with coated sample 4 (98 kJ/m3, 0.767 T, 0.49 and 6.02 g/cm3), respectively. The coercivity of bonded magnets, seen from Fig.4 (f), remains almost unchanged no matter with the increase in pressure or the increase in thickness of parylene C coating. Fig.4 (d) displays corresponding 2nd quadrant demagnetization curves of non-coated sample, sample 2 and sample 4 at 470MPa, showing remanence (Br) of 0.626 T, 0.803 T, 0.767 T and coercivity (H) of 12.84 kOe, 12.58 kOe, 12.75 kOe, respectively. non-coated sample sample1 sample2 sample3 sample4 110
(b) 0.54
90 80
DOA
（ BH） max（ kJ/m3）
100
0.60
(a)
70
0.48 0.42
60 0.36 50 40
6.0
250
300 350 400 Pressure（ MPa）
450
0.30
500
(c)
200
250
300 350 400 Pressure（ MPa）
450
500
0.8
(d)
3
Density（ g/cm ）
6.2
200
5.6
0.4
Br（ T）
0.6
5.8
5.4 0.2 5.2 5.0
0.8
200
250
300 350 400 Pressure（ MPa）
450
500
-14
(e)
13.6
-10
-8 -6 H(kOe)
-4
-2
0.0 0
(f)
12.8
H(kOe)
0.7
Br（ T）
-12
0.6
12.0 11.2
0.5 10.4 0.4
200
250
300 350 400 Pressure（ MPa）
450
500
200
250
300 350 400 Pressure（ MPa）
450
500
Fig.4 (BH)max, DOA, density, Br and coercivity of bonded NdFeB magnets under different pressure and corresponding 2nd quadrant demagnetization curves. The mechanism for the improvement of DOA and density of parylene-coated NdFeB magnets is the good lubricating property of parylene-coated NdFeB powder, 7
resulting from low friction coefficient of parylene C, that reducing rotational resistance and frictional resistance between powders, which is proved in our previous study[31]. Additionally, the lubricating property is improved with the increasing thickness of parylene C coating, which is also the mutual proof of increasing DOA. Fig.5 displays backscattered electron (BSE) images of bonded NdFeB magnet: (a) non-coated sample, (b) coated sample 2. It can be seen from Fig.5 that sample 2 shows smaller volume fraction of holes (dark area) than non-coated sample, which corresponds to higher density. There are two mainly effects of parylene C coating on density of bonded NdFeB magnet. On one hand, the low friction coefficient of parylene C coating makes the excellent lubricating property of NdFeB powder, and thereby improves the density of bonded magnets. On the other hand, the lower density of parylene C coating results in decrease of density. Under these two aspects, finally, density reach the maximum at sample 2 with the increasing thickness of parylene C coating.
Fig.5 BSE SEM images of bonded NdFeB magnet: (a) non-coated sample, (b) coated sample 2. The corrosion resistance of NdFeB powders is characterized by degree of reaction with acid solution. Fig.6 shows PH-T curve of acid solution after putting non-coated, sample 2 and sample 4 NdFeB powders into it, where the initial PH is about 1.5. By comparing three PH-T curves in Fig.6, with the increasing thickness of parylene C coating, PH values decrease from non-coated sample to sample 4. And the ending time of reaction between acid solution and non-coated, sample 2, sample 4 NdFeB powders are 35min (PH=2.1 and disappearance of NdFeB powders), 20min (PH=1.7) and 14min (PH=1.6), respectively. It is difficult for parylene-coated NdFeB 8
powders to react with acid solution due to the super chemical stability of parylene coating on the surface of NdFeB powders, that effectively prevents the contact between NdFeB powders and H+ in acid solution, thereby improving the acid corrosion resistance of NdFeB powders.
Fig.6 PH-T curve of acid solution after putting NdFeB powders into it, where the initial PH is about 1.5. To further investigate the corrosion resistance of parylene-coated NdFeB magnet, the electrochemical test was employed. Fig.7 shows the potentiodynamic polarization curves of bonded NdFeB magnets for non-coated sample, sample 2 and sample 4 in 3.5 wt % NaCl solution. The results of the potentiodynamic test are summarized in Table 2. As can be seen from Fig.7, the polarization curve of parylene-coated sample moves left and the Ecorr is shifted positively compared with non-coated sample. The corrosion potential (Ecorr) of non-coated sample, sample 2, sample 4 is about -0.966V, -0.838V, -0.703V, while corrosion current density (Icorr) is 2.16×10-5 A/cm2, 6.46×10-6 A/cm2, 1.83×10-7 A/cm2, rspectively. The Icorr of sample 2 decreased one order of magnitude than that of non-coated sample while sample 4 decreased about two orders of the magnitude. The smaller current density represents the inhibition of anode and cathode reactions, indicating better corrosion resistance. From the information above, it can be concluded that the corrosion resistance of NdFeB magnets is obviously improved after NdFeB powders being coated with parylene C coating.
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Fig.7 Tafel polarization curves of bonded NdFeB magnet for non-coated sample, sample 2 and sample 4. Table 1 Corrosion potential (Ecorr) and corrosion current density (Icorr) of samples. Samples
Ecorr（V）
Icorr（A/cm2）
sample 4
-0.703
1.83×10-7
sample 2
-0.838
6.46×10-6
non-coated sample
-0.966
2.16×10-5
4. Conclusions The anisotropic bonded NdFeB magnets with parylene C coating on the surface of NdFeB powders are prepared by warm compaction process. The parylene C coating, synthesized on the surface of NdFeB powder by chemical vapor deposition polymerization (CVDP), is characterized by FIB-SEM EDS analysis, revealing the thickness of 300~500nm. The parylene-coated NdFeB magnets show excellent magnetic properties than non-coated NdFeB magnets. The (BH)max, DOA and density of parylene-coated sample 2 compressed at 470MPa (104 kJ/m3, 0.476 and 6.17 g/cm3) are increased by 73.33%, 41.24%, 7.87% than non-coated sample (60 kJ/m3, 0.337, 5.72 g/cm3) and 6.12%, -2.86%, 2.49% compared with coated sample 4 (98 kJ/m3, 0.49 and 6.02 g/cm3), respectively. And with the increasing thickness of parylene C coating, (BH)max and density increase first and then decrease gradually 10
whereas DOA keeps a growing trend. The reason for the improvement of DOA and density of parylene-coated NdFeB magnets is the good lubricating property of parylene-coated NdFeB powder that resulting from low friction coefficient of parylene C coating. The parylene-coated NdFeB magnets also exhibit better corrosion resistance for the super chemical stability of parylene C coating on the surface of NdFeB powders.
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Figures and tables captions Fig.1 Schematic diagram of parylene coating process. Fig.2 SEM images of (a) NdFeB powders, (b) parylene-coated NdFeB powders and (c) EDS spectrum of selected area. Fig.3 FIB-SEM images of parylene-coated NdFeB powder, (b) is the enlarged view in selected areas of (a), and (c) SEM-EDS line scan analysis of parylene C coating interface. Fig.4 (BH)max, DOA, density, Br and coercivity of bonded NdFeB magnets under different pressure and corresponding 2nd quadrant demagnetization curves. Fig.5 BSE SEM images of bonded NdFeB magnet: (a) non-coated sample, (b) coated sample 2. Fig.6 PH-T curve of acid solution after putting NdFeB powders into it, where the initial PH is about 1.5. Fig.7 Tafel polarization curves of bonded NdFeB magnet for non-coated sample, sample 2 and sample 4. Tab.1 Corrosion potential (Ecorr) and corrosion current density (Icorr) of samples.
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Research Highlights Parylene C is coated on the surface of NdFeB powders by CVDP. The lubricity of NdFeB powders is improved by surface coating of parylene C. The (BH)max DOA, density of magnets are increased by 73.33%, 41.24%, 7.87%. The parylene-coated NdFeB magnets exhibit better corrosion resistance.
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Table 1. Corrosion potential (Ecorr) and corrosion current density (Icorr) of samples. Samples
Ecorr（V）
Icorr（A/cm2）
sample 4
-0.703
1.83×10-7
sample 2
-0.838
6.46×10-6
non-coated sample
-0.966
2.16×10-5
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19
20
21
22
23
24
25
26
27
28
29