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Effect of titania particles on the microstructure and properties of the epoxy resin coatings on sintered NdFeB permanent magnets
Journal of Magnetism and Magnetic Materials 355 (2014) 31–36
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Effect of titania particles on the microstructure and properties of the epoxy resin coatings on sintered NdFeB permanent magnets J.L. Xu n, Z.X. Huang, J.M. Luo, Z.C. Zhong n School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 July 2013 Received in revised form 20 November 2013 Available online 3 December 2013
The nanometer titania particles enhanced epoxy resin composite coatings were prepared on the sintered NdFeB permanent magnets by cathodic electrophoretic deposition. The effects of titania particle concentrations on the microstructure and properties of the epoxy coatings were investigated by surface and cross-sectional morphologies observation, surface roughness and microhardness measurement, H2SO4 solution immersion test, neutral salt spray test and magnetic properties measurement. The results showed that the thickness of epoxy coatings with and without the titania particles addition was about 40 μm. The titania particles could be uniformly dispersed and embedded in the epoxy matrix if the titania particles concentration was lower than 40 g/l. With increasing titania particle concentrations, the number of the particles embedded in the epoxy matrix increased and the surface roughness and microhardness of the composite coatings increased. At the same time, the weight loss of the coated samples immersed in H2SO4 solution decreased and the neutral salt spray time of the coated samples prolonged. It could be concluded that the titania particles did not change the thickness of the epoxy coatings and did not deteriorate the magnetic properties of NdFeB substrates, but could greatly improve the microhardness and corrosion resistance of the epoxy coatings. & 2013 Elsevier B.V. All rights reserved.
Keywords: Sintered NdFeB permanent magnet Epoxy resin Titania particle Cathodic electrophoretic deposition Corrosion resistance
1. Introduction Since the powder-sintered NdFeB (neodymium–iron–boron) rare earth permanent magnets were first reported in the 1980s, they have been widely applied in various different industrial fields due to their excellent magnetic properties such as high remanence, high coercivity and large maximum energy product [1–4]. These magnets have been applied in electronics, acoustics, microwave technology, auto industry, magnetic resonance imaging, wind power generators, new energy vehicles, magnetic separation technology biomedical uses, etc. However, the poor intrinsic corrosion resistance of the sintered NdFeB permanent magnets in humid environment has severely impeded and limited their further application and development in many fields [5–7]. In order to improve the corrosion resistance of the sintered NdFeB permanent magnets, many efforts have been made, including alloy elements additions [8–10] and surface coatings [11–15]. Although alloying can alter and improve the intrinsic corrosion resistance of NdFeB magnets, the improvement of corrosion resistance is always accompanied by a sacrifice of magnetic properties [9]. In industrial fields, the surface coating materials
n
Corresponding authors. Tel.: þ 86 791 83863034; fax: þ 86 791 86453203. E-mail addresses: [email protected] (J.L. Xu), [email protected] (Z.C. Zhong). 0304-8853/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2013.11.050
can be generally categorized into two groups: polymers [16–18] and metals [19–24]. Polymers, specially epoxy resin, are easy to apply and less costly, and their level of corrosion protection is much better than that of metals. However, it is well known that the polymers are susceptible to moisture attack and mechanical scratch. According to literatures [25–28], the mechanical properties, thermal stability and corrosion resistance of epoxy resin can be greatly improved using reinforcing nanofillers, such as TiO2, SiO2 and Ti powders. Therefore, in this paper, the nanometer TiO2 particles enhanced epoxy resin composite coatings were prepared on the sintered NdFeB permanent magnets by cathodic electrophoretic deposition, the effect of TiO2 particles on the microstructure and corrosion resistance of the epoxy resin coatings was investigated, and the mechanisms of corrosion resistance enhancement were analyzed and discussed. 2. Material and methods Commercially available powder-sintered NdFeB permanent magnet samples (banded 38SH, Jiangxi JLMAG Rare-Earth Co., Ltd. (China), in a demagnetized state) were used as the substrate materials in this experiment. The samples were cut into 18 mm 10 mm 2 mm, successively grinded with SiC abrasive paper from 100 # to 1200 #, and then ultrasonically cleaned in acetone and distilled water. The grinded samples were dipped into
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5% nitric acid solution for approximately 30 s at room temperature, and then rinsed with distilled water. The acid-washed samples were dipped into zinc phosphating solution at 50 1C for 10 min. At last, the phosphatized samples were treated by cathodic electrophoretic deposition in an aqueous electrolyte with the voltage of 60 V for 40 s by high frequency pulse electrophoresis power supply (KGY-5-02, Wenzhou Huihuang electropainting Co., Ltd. (China)). The aqueous electrolytes were prepared from the water soluble epoxy resin (90#, Wenzhou Huihuang electropainting Co., Ltd. (China)), distilled water and titania particles of 200 nm, in which the mass ratio of the water soluble epoxy resin to distilled water was 1:2 and the concentration of titania particles was about 0–50 g/l. During the cathodic electrophoretic deposition process, the NdFeB substrates were used as the cathode, while 304 stainless steel was used as the anode. The electrolytes were heated in the water bath and kept at 30 1C. After electrophoretic deposition, the samples were washed with distilled water and high temperature cured in a drying oven at 165 1C for 40 min. The surface and cross-sectional morphologies of the coated samples were observed by scanning electron microscopy (SEM, FEI Quanta 200, America). The surface roughness Ra of the samples was tested by a surface roughmeter (SJ-301, Mitutoyo, Japan). The average Ra value of each sample was obtained from 5 measurements at different positions. The HV microhardness of the coatings was measured using a microhardness tester (HVS-1000, Guangzhou, China) at a load of 50 g with a loading duration of 20 s. The average HV microhardness value of each sample was obtained from 5 measurements at different positions. The coated and uncoated NdFeB samples were immersed in 250 ml 0.5 mol/l H2SO4 solutions for 4 h at room temperature to measure the weight loss and evaluate their corrosion resistance. The neutral salt spray test (NSS) (RECE, Shanghai, China) was also performed to investigate the corrosion resistance of the coated samples. The test was carried out in a standard salt spray cabinet spraying NaCl solution (50 g/L, pH ¼6.5–7.2) at 3572 1C and the evolution of the corrosion phenomena was visually observed up to 600 h. The remanence (Br), intrinsic coercive force (Hcj) and large maximum energy product ((BH)max) of the uncoated and coated NdFeB magnets samples were measured by a hysteresis curve measuring instrument (NIM-10000H, China National Metrology Technology Development Co.) to analyze the effect of the coatings on the magnetic properties of NdFeB magnets.
3. Results and discussion Fig. 1 shows the effect of titania particle concentrations on the surface morphologies of the epoxy coatings on NdFeB magnets. The surface of the neat epoxy coating (without adding titania particles) was smooth and even with no impurities and no defects like pin holes, particles, orange peel, etc (seen in Fig. 1a). Some white particles embedded in the epoxy matrix could be observed due to the addition of titania particles into the electrolytes, and the number of particles increased with the increasing titania particle concentrations. When the concentration of the titania particles was lower than 40 g/l, the white particles embedded in the epoxy matrix exhibited uniform and dispersed distribution and the size of the while particles was about 200 nm, consistent with the size of the titania particles added into the electrolytes, which indicated that the white particles were the titania particles and they had excellent dispersion without any agglomeration. Unfortunately, when the titania particle concentration was up to 50 g/l, the white particles suffered serious agglomeration (seen in Fig. 1f) and some macroscopic particles defects and obviously uneven surface were observed. Therefore, the concentration of titania
particles should be controlled lower than 40 g/l to insure surface quality (a very important feature for a coated NdFeB product) of the composite coatings on the NdFeB magnets. The surface roughness of the coated samples was measured as a function of the titania particle concentrations, as shown in Fig. 2. It was clear that the surface roughness Ra values of the coated samples increased with the increasing titania particle concentrations. The surface roughness Ra value of the neat epoxy coating was only 0.05 μm, much lower than that of the phosphatized NdFeB substrate (0.94 μm), indicating that epoxy coating could cover the intrinsic defects like pores, loose and cracks of the powder-sintered NdFeB [29], which is beneficial to improve the corrosion resistance of the sintered NdFeB. The surface roughness Ra value of the coated sample with the 10 g/l titania particles had a little increase to 0.06 μm. By further increasing the titania particle concentrations, the surface roughness presented a linear increase and the Ra value was up to 0.5 μm for the coated sample with 40 g/l titania particles. Both the surface roughness Ra value and its deviation of the coated sample with 50 g/l titania particles were suddenly increased, indicating that surface uniformity of the coated sample deteriorated severely, which was consistent with its surface microcosmic and macroscopic morphology. According to Fig. 1, the increase of titania particle concentrations would increase the number of titania particles embedded in the epoxy resin matrix. The more titania particles embedded in the epoxy resin matrix inevitably increased the surface concave-convex level of the epoxy resin coatings, which resulted in the increase of surface roughness. Especially, the serious agglomeration and macroscopic particles defects resulted in the sudden increase of surface roughness for the coating with 50 g/l titania particle. In general, roughness had negative effects on the corrosion resistance of the materials. Higher roughness would lead to larger contact area of the materials to the corrosive mediums, which would accelerate the corrosion occurrence. In this experiment, the roughness of the epoxy resin coatings increased due to the addition of titania particles, but the corrosion resistance of the coatings was improved. The titania particles embedded in the epoxy resin matrix impeded the decrease of the coating thickness at the tips and edges, and impeded and delayed the aggressive mediums to permeate and pass through the coating and corrode the NdFeB substrate, which would be discussed in detail. Fig. 3 shows the cross-sectional morphologies of the neat epoxy coated sample and the sample coated with 40 g/l titania particles. It could be seen that both the coatings had a thickness of 40 μm, which illustrated that the titania particles had no effects on the thickness of the epoxy coatings on NdFeB magnets. Moreover, no apparent discontinuity between the coatings and substrates was observed, which indicated that both the coatings had good interfacial bonding to the NdFeB substrates. There were no apparent defects such as pores and cracks throughout the crosssection of the coatings, which was beneficial to improve the corrosion resistance of the NdFeB magnets. Effect of the titania particles on the microhardness of the epoxy coatings on NdFeB magnets can be seen in Fig. 4. The microhardness of the epoxy coatings increased with the increasing concentration of titania particles, nearly linear increase. The microhardness of neat epoxy coating was only 17.9 HV, while the microhardness of composite coating with 40 g/l titania particle was about 27.8 HV, 55% higher than that of the neat epoxy. The titania particles embedded in the epoxy matrix could hinder the movement of the molecular chain and play a particle dispersion enhancement role, which could greatly increase the microhardness of the epoxy coatings. The more titania particles embedded in the epoxy resin matrix, the higher microhardness of the composite coating. Therefore, the microhardness of the coatings increased with the increasing concentration of titania particles. By means of nanometer titania
J.L. Xu et al. / Journal of Magnetism and Magnetic Materials 355 (2014) 31–36
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Fig. 1. Surface morphologies of the coated NdFeB samples with the different concentrations of titania particles: (a) neat epoxy coating, (b) 10 g/l, (c) 20 g/l, (d) 30 g/l, (e) 40 g/l, and (f) 50 g/l.
Fig. 2. Change in the surface roughness of the coated samples measured as a function of the titania particles concentration.
particles enhancement, not only the microhardness of the epoxy coatings could obtain great improvement, but also the mechanical scratch resistance, wear resistance, thermal decomposition temperature, and corrosion resistance could be improved partly [25– 28]. Fig. 5 shows the weight loss–immersion time curves of the uncoated and coated NdFeB magnets samples immersed in 0.5 mol/l H2SO4 solution for 4 h. The weight loss of the uncoated NdFeB magnet sample was much higher than that of the coated samples, and the weight loss of the coated NdFeB samples gradually decreased with increasing titania particle concentrations. When the uncoated NdFeB magnet was immersed in the H2SO4 solution, a large number of bubbles appeared immediately all over the uncoated NdFeB sample surface, indicating that NdFeB suffered a serious homogeneous corrosion. A few bubbles first generated at the tips and edges of the neat epoxy coated sample after being immersed in the H2SO4 solution for 40 min. The bubble generation time of the coated sample with 10 g/l, 20 g/l, 30 g/l and 40 g/l titania particles was about 60 min, 100 min, 180 and
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Fig. 3. Cross-sectional morphologies of the neat epoxy coated sample (a) and the coated sample with 40 g/l titania particles (b).
Fig. 4. Variation of microhardness with the concentration of titania particles.
Fig. 5. Weight loss–immersion time curves of the uncoated and coated NdFeB samples in 0.5 mol/l H2SO4 solution.
220 min, respectively. The bubble generation time was effectively prolonged through the addition of titania particles and the corrosion resistance of NdFeB was greatly improved. When immersed in the H2SO4 solution for 4 h, the average corrosive velocity of the uncoated NdFeB sample was about 13.86 g/dm2 h, one order of magnitude higher than that of the neat epoxy coated NdFeB sample with a value of 1.5 g/dm2 h and three orders of magnitude
higher than that of the 40 g/l titania particles coated NdFeB sample with a value of 0.03 g/dm2 h. According to the phenomena of the H2SO4 solution immersion test, corrosion of the coated samples first occurred at the tips and edges. The main reason was that the coating thickness at tips and edges of the coated samples was thinner than other places. The cross-sectional morphologies at edges of the neat epoxy coated sample and 40 g/l titania particles coated sample are shown in Fig. 6. It could be seen that the coating thickness at edges of the neat epoxy coated sample was much thinner than other places and the coating thickness at edges of the 40 g/l titania particles coated sample was a little thinner than other places. During the high temperature curing process, the neat epoxy coating at the tips and edges would transfer to the surface due to the existence of surface tension, resulting in the thickness at the tips and edges of the epoxy coating much thinner than other places. The addition of titania particles could make the particles embed in the epoxy matrix and impede the epoxy coating movement during high temperature curing. Therefore, the thickness at the tips and edges of the composite coatings decreased a little and they exhibited excellent corrosion resistance compared to the neat epoxy coating. In order to further investigate the effect of titania particles on the corrosion resistance of the coated NdFeB, the neutral salt spray test (NSS) was also employed. For the uncoated NdFeB sample, a layer of brown rust was found to be uniformly distributed over the sample surface after 1 h exposure, consistent with the literature [5]. The brown rust was initially observed at the tips and edges of the neat epoxy coated sample after 240 h exposure to the salt spray environment. The time of brown rust corrosion products to appear on the 40 g/l titania particles coated samples was after 600 h exposure to the salt spray environment, much longer than that of the neat epoxy coated sample. Fig. 7 shows the cross-sectional morphologies of the neat epoxy coated sample and 40 g/l titania particles coated sample after being exposed to the salt spray environment for 500 h. Many pores and river-like defects were detected throughout the cross-section of the neat epoxy coating, very different than Fig. 3a. However, no defects could be detected on the cross-sectional morphology of the 40 g/l titania particles coated sample after being exposed to the salt spray environment for 500 h, consistent with Fig. 3b. At the start of the neutral salt spray test, the neat epoxy coating protected the NdFeB substrate by acting as a barrier layer. As the test time prolonged, the water and Cl ions were able to gradually permeate into the neat epoxy coating, pass through the coating to the interface and corrode the substrate NdFeB at last. Therefore, many brown rusts were detected on the neat epoxy coated sample after 500 h exposure to the salt spray environment. Those pores and river-like defects detected in the neat epoxy coating
J.L. Xu et al. / Journal of Magnetism and Magnetic Materials 355 (2014) 31–36
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Fig. 6. Cross-sectional morphologies of edges of the neat epoxy coated sample (a) and 40 g/l titania particles coated sample (b).
Fig. 7. Cross-sectional morphologies of the neat epoxy coated sample (a) and 40 g/l titania particles coated sample (b) after exposure to salt spray environment for 500 h.
Table 1 Magnetic properties of the coated and uncoated NdFeB samples.
Uncoated NdFeB 40 g/l Titania particles coated NdFeB Variation (%)
Br (kGs)
Hcj (kOe)
(BH)max (MGsOs)
12.51 12.50 0.08
22.57 23.41 3.7
38.58 38.43 0.4
must be the corrosive channels formed during the neutral salt spray test. The titania particles uniformly dispersed and embedded in the epoxy matrix could effectively impede and delay the water and Cl ions to permeate and pass through the coating. Therefore, no obvious corrosive channels could be detected in the 40 g/l titania particles coating (seen in Fig. 7b), and there were no brown rusts on the sample after 500 h exposure to the salt spray environment. In industries, electroplated Ni or NiCuNi coatings were generally applied to improve the corrosion resistance of NdFeB magnets, but these coatings deteriorated the magnetic properties and even the coercive force reduced by more than 10% [30]. Chen et al. [24] reported that there was a loss of coercive force by 24% and remanence by 16% due to the preparation of electroless Ni–P coatings on NdFeB magnets. Therefore, the effect of titania particles enhanced epoxy coatings on magnetic properties of NdFeB magnets should be explored with necessity. Table 1 shows the magnetic properties of 40 g/l titania particles coated and uncoated NdFeB samples. The remanence (Br) value and large maximum energy product ((BH)max) value had few changes before and after deposition of the coating, and the intrinsic
coercive force (Hcj) had a 3.7% increase after deposition of the coating, which might be related to the semiconductor characteristic of the titania particles. Therefore, it can be concluded that the titania particles enhanced epoxy coatings do not deteriorate the magnetic properties of NdFeB magnets.
4. Conclusions (1) The titania particles were uniformly dispersed and embedded in the epoxy matrix when the titania particles concentration was lower than 40 g/l, while with further increasing the concentration, the titania particles suffered serious agglomeration and macroscopic particles defect appeared. (2) The titania particles did not change the thickness of the epoxy coatings, both the thickness of the neat epoxy coating and the titania particles enhanced, epoxy coating were about 40 μm. With increasing titania particle concentrations, the surface roughness and the microhardness of the composite coatings increased. (3) The weight loss of the coated NdFeB samples immersed in H2SO4 solution was much lower than that of the uncoated NdFeB sample and with increasing titania particle concentrations, the weight loss of the coated NdFeB samples decreased. In the NSS test, the time taken by brown rust corrosion products to appear on titania particles enhanced epoxy coatings was much longer than that of the neat epoxy coating. (4) The titania particles embedded in the epoxy matrix impeded the decrease of the coating thickness at the tips and edges, and
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J.L. Xu et al. / Journal of Magnetism and Magnetic Materials 355 (2014) 31–36
impeded and delayed the aggressive mediums to permeate and pass through the coating and corrode the NdFeB substrate. Therefore, the addition of the titania particles could greatly improve the corrosion resistance of the epoxy coatings and did not deteriorate the magnetic properties of NdFeB substrates.
Acknowledgments The authors gratefully acknowledge the financial support of the project from the National Science and Technology Key Support Projects of China (Grant no. 2012BAE02B01), the Major Science and Technology Special Projects of Jiangxi Province (Grant no. 2010AZX00200), the Science and Technology Plan Projects of Jiangxi Province (Industrial field) (Grant no. 20121BBE50001), and the Science and Technology Plan Projects of Department of Education of Jiangxi Province (Grant no. KJ201109132281). References [1] J.W. Kim, S.H. Kim, S.Y. Song, Y.D. Kim, Nd–Fe–B permanent magnets fabricated by low temperature sintering process, J. Alloys Compd. 551 (2013) 180–184. [2] O.G. Eisch, M.A. Willard, E. Brück, C.H. Chen, S.G. Sankar, J.P. Liu, Magnetic materials and devices for the 21st century: stronger, lighter, and more energy efficient, Adv. Mater. 23 (2011) 821–842. [3] B.E. Davies, R.S. Mottram, I.R. Harris, Recent developments in the sintering of NdFeB, Mater. Chem. Phys. 67 (2001) 272–281. [4] M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto, Y. Matsuura, Permanent magnet materials based on the rare earth–iron–boron tetragonal compounds, IEEE Trans. Magn. 55 (1984) 1584–1589. [5] H.H. Man, H.C. Man, L.K. Leung, Corrosion protection of NdFeB magnets by surface coatings – Part I: salt spray test, J. Magn. Magn. Mater. 152 (1996) 40–46. [6] K. Tokuhara, S. Hirosawa, Corrosion resistance of Nd–Fe–B sintered magnets, J. Appl. Phys. 69 (1991) 5521–5523. [7] G.L. Yan, P.J. McGuiness, J.P.G. Farr, I.R. Harris, Environmental degradation of NdFeB magnets, J. Alloys Compd. 478 (2009) 188–192. [8] A.A. El-Moneim, A. Gebert, M. Uhlemann, O. Gutfleisch, L. Schultz, The influence of Co and Ga additions on the corrosion behavior of nanocrystalline NdFeB magnets, Corros. Sci. 44 (2002) 1857–1874. [9] W. Fernengel, W. Rodewald, R. Blank, P. Schrey, M. Katter, B. Wall, The influence of Co on the corrosion resistance of sintered Nd–Fe–B magnets, J. Magn. Magn. Mater. 196–197 (1999) 288–290. [10] R.K. Murakami, H.R. Rechenberg, A.C. Neiva, F.P. Missell, V. Villas-Boas, Effect of Ti and C additions on structural and magnetic properties of (Pr,Nd)–Fe–B nanocrystalline magnetic materials, J. Magn. Magn. Mater. 320 (2008) e65–e68.
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Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Effect of titania particles on the microstructure and properties of the epoxy resin coatings on sintered NdFeB permanent magnets J.L. Xu n, Z.X. Huang, J.M. Luo, Z.C. Zhong n School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 July 2013 Received in revised form 20 November 2013 Available online 3 December 2013
The nanometer titania particles enhanced epoxy resin composite coatings were prepared on the sintered NdFeB permanent magnets by cathodic electrophoretic deposition. The effects of titania particle concentrations on the microstructure and properties of the epoxy coatings were investigated by surface and cross-sectional morphologies observation, surface roughness and microhardness measurement, H2SO4 solution immersion test, neutral salt spray test and magnetic properties measurement. The results showed that the thickness of epoxy coatings with and without the titania particles addition was about 40 μm. The titania particles could be uniformly dispersed and embedded in the epoxy matrix if the titania particles concentration was lower than 40 g/l. With increasing titania particle concentrations, the number of the particles embedded in the epoxy matrix increased and the surface roughness and microhardness of the composite coatings increased. At the same time, the weight loss of the coated samples immersed in H2SO4 solution decreased and the neutral salt spray time of the coated samples prolonged. It could be concluded that the titania particles did not change the thickness of the epoxy coatings and did not deteriorate the magnetic properties of NdFeB substrates, but could greatly improve the microhardness and corrosion resistance of the epoxy coatings. & 2013 Elsevier B.V. All rights reserved.
Keywords: Sintered NdFeB permanent magnet Epoxy resin Titania particle Cathodic electrophoretic deposition Corrosion resistance
1. Introduction Since the powder-sintered NdFeB (neodymium–iron–boron) rare earth permanent magnets were first reported in the 1980s, they have been widely applied in various different industrial fields due to their excellent magnetic properties such as high remanence, high coercivity and large maximum energy product [1–4]. These magnets have been applied in electronics, acoustics, microwave technology, auto industry, magnetic resonance imaging, wind power generators, new energy vehicles, magnetic separation technology biomedical uses, etc. However, the poor intrinsic corrosion resistance of the sintered NdFeB permanent magnets in humid environment has severely impeded and limited their further application and development in many fields [5–7]. In order to improve the corrosion resistance of the sintered NdFeB permanent magnets, many efforts have been made, including alloy elements additions [8–10] and surface coatings [11–15]. Although alloying can alter and improve the intrinsic corrosion resistance of NdFeB magnets, the improvement of corrosion resistance is always accompanied by a sacrifice of magnetic properties [9]. In industrial fields, the surface coating materials
n
Corresponding authors. Tel.: þ 86 791 83863034; fax: þ 86 791 86453203. E-mail addresses: [email protected] (J.L. Xu), [email protected] (Z.C. Zhong). 0304-8853/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2013.11.050
can be generally categorized into two groups: polymers [16–18] and metals [19–24]. Polymers, specially epoxy resin, are easy to apply and less costly, and their level of corrosion protection is much better than that of metals. However, it is well known that the polymers are susceptible to moisture attack and mechanical scratch. According to literatures [25–28], the mechanical properties, thermal stability and corrosion resistance of epoxy resin can be greatly improved using reinforcing nanofillers, such as TiO2, SiO2 and Ti powders. Therefore, in this paper, the nanometer TiO2 particles enhanced epoxy resin composite coatings were prepared on the sintered NdFeB permanent magnets by cathodic electrophoretic deposition, the effect of TiO2 particles on the microstructure and corrosion resistance of the epoxy resin coatings was investigated, and the mechanisms of corrosion resistance enhancement were analyzed and discussed. 2. Material and methods Commercially available powder-sintered NdFeB permanent magnet samples (banded 38SH, Jiangxi JLMAG Rare-Earth Co., Ltd. (China), in a demagnetized state) were used as the substrate materials in this experiment. The samples were cut into 18 mm 10 mm 2 mm, successively grinded with SiC abrasive paper from 100 # to 1200 #, and then ultrasonically cleaned in acetone and distilled water. The grinded samples were dipped into
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5% nitric acid solution for approximately 30 s at room temperature, and then rinsed with distilled water. The acid-washed samples were dipped into zinc phosphating solution at 50 1C for 10 min. At last, the phosphatized samples were treated by cathodic electrophoretic deposition in an aqueous electrolyte with the voltage of 60 V for 40 s by high frequency pulse electrophoresis power supply (KGY-5-02, Wenzhou Huihuang electropainting Co., Ltd. (China)). The aqueous electrolytes were prepared from the water soluble epoxy resin (90#, Wenzhou Huihuang electropainting Co., Ltd. (China)), distilled water and titania particles of 200 nm, in which the mass ratio of the water soluble epoxy resin to distilled water was 1:2 and the concentration of titania particles was about 0–50 g/l. During the cathodic electrophoretic deposition process, the NdFeB substrates were used as the cathode, while 304 stainless steel was used as the anode. The electrolytes were heated in the water bath and kept at 30 1C. After electrophoretic deposition, the samples were washed with distilled water and high temperature cured in a drying oven at 165 1C for 40 min. The surface and cross-sectional morphologies of the coated samples were observed by scanning electron microscopy (SEM, FEI Quanta 200, America). The surface roughness Ra of the samples was tested by a surface roughmeter (SJ-301, Mitutoyo, Japan). The average Ra value of each sample was obtained from 5 measurements at different positions. The HV microhardness of the coatings was measured using a microhardness tester (HVS-1000, Guangzhou, China) at a load of 50 g with a loading duration of 20 s. The average HV microhardness value of each sample was obtained from 5 measurements at different positions. The coated and uncoated NdFeB samples were immersed in 250 ml 0.5 mol/l H2SO4 solutions for 4 h at room temperature to measure the weight loss and evaluate their corrosion resistance. The neutral salt spray test (NSS) (RECE, Shanghai, China) was also performed to investigate the corrosion resistance of the coated samples. The test was carried out in a standard salt spray cabinet spraying NaCl solution (50 g/L, pH ¼6.5–7.2) at 3572 1C and the evolution of the corrosion phenomena was visually observed up to 600 h. The remanence (Br), intrinsic coercive force (Hcj) and large maximum energy product ((BH)max) of the uncoated and coated NdFeB magnets samples were measured by a hysteresis curve measuring instrument (NIM-10000H, China National Metrology Technology Development Co.) to analyze the effect of the coatings on the magnetic properties of NdFeB magnets.
3. Results and discussion Fig. 1 shows the effect of titania particle concentrations on the surface morphologies of the epoxy coatings on NdFeB magnets. The surface of the neat epoxy coating (without adding titania particles) was smooth and even with no impurities and no defects like pin holes, particles, orange peel, etc (seen in Fig. 1a). Some white particles embedded in the epoxy matrix could be observed due to the addition of titania particles into the electrolytes, and the number of particles increased with the increasing titania particle concentrations. When the concentration of the titania particles was lower than 40 g/l, the white particles embedded in the epoxy matrix exhibited uniform and dispersed distribution and the size of the while particles was about 200 nm, consistent with the size of the titania particles added into the electrolytes, which indicated that the white particles were the titania particles and they had excellent dispersion without any agglomeration. Unfortunately, when the titania particle concentration was up to 50 g/l, the white particles suffered serious agglomeration (seen in Fig. 1f) and some macroscopic particles defects and obviously uneven surface were observed. Therefore, the concentration of titania
particles should be controlled lower than 40 g/l to insure surface quality (a very important feature for a coated NdFeB product) of the composite coatings on the NdFeB magnets. The surface roughness of the coated samples was measured as a function of the titania particle concentrations, as shown in Fig. 2. It was clear that the surface roughness Ra values of the coated samples increased with the increasing titania particle concentrations. The surface roughness Ra value of the neat epoxy coating was only 0.05 μm, much lower than that of the phosphatized NdFeB substrate (0.94 μm), indicating that epoxy coating could cover the intrinsic defects like pores, loose and cracks of the powder-sintered NdFeB [29], which is beneficial to improve the corrosion resistance of the sintered NdFeB. The surface roughness Ra value of the coated sample with the 10 g/l titania particles had a little increase to 0.06 μm. By further increasing the titania particle concentrations, the surface roughness presented a linear increase and the Ra value was up to 0.5 μm for the coated sample with 40 g/l titania particles. Both the surface roughness Ra value and its deviation of the coated sample with 50 g/l titania particles were suddenly increased, indicating that surface uniformity of the coated sample deteriorated severely, which was consistent with its surface microcosmic and macroscopic morphology. According to Fig. 1, the increase of titania particle concentrations would increase the number of titania particles embedded in the epoxy resin matrix. The more titania particles embedded in the epoxy resin matrix inevitably increased the surface concave-convex level of the epoxy resin coatings, which resulted in the increase of surface roughness. Especially, the serious agglomeration and macroscopic particles defects resulted in the sudden increase of surface roughness for the coating with 50 g/l titania particle. In general, roughness had negative effects on the corrosion resistance of the materials. Higher roughness would lead to larger contact area of the materials to the corrosive mediums, which would accelerate the corrosion occurrence. In this experiment, the roughness of the epoxy resin coatings increased due to the addition of titania particles, but the corrosion resistance of the coatings was improved. The titania particles embedded in the epoxy resin matrix impeded the decrease of the coating thickness at the tips and edges, and impeded and delayed the aggressive mediums to permeate and pass through the coating and corrode the NdFeB substrate, which would be discussed in detail. Fig. 3 shows the cross-sectional morphologies of the neat epoxy coated sample and the sample coated with 40 g/l titania particles. It could be seen that both the coatings had a thickness of 40 μm, which illustrated that the titania particles had no effects on the thickness of the epoxy coatings on NdFeB magnets. Moreover, no apparent discontinuity between the coatings and substrates was observed, which indicated that both the coatings had good interfacial bonding to the NdFeB substrates. There were no apparent defects such as pores and cracks throughout the crosssection of the coatings, which was beneficial to improve the corrosion resistance of the NdFeB magnets. Effect of the titania particles on the microhardness of the epoxy coatings on NdFeB magnets can be seen in Fig. 4. The microhardness of the epoxy coatings increased with the increasing concentration of titania particles, nearly linear increase. The microhardness of neat epoxy coating was only 17.9 HV, while the microhardness of composite coating with 40 g/l titania particle was about 27.8 HV, 55% higher than that of the neat epoxy. The titania particles embedded in the epoxy matrix could hinder the movement of the molecular chain and play a particle dispersion enhancement role, which could greatly increase the microhardness of the epoxy coatings. The more titania particles embedded in the epoxy resin matrix, the higher microhardness of the composite coating. Therefore, the microhardness of the coatings increased with the increasing concentration of titania particles. By means of nanometer titania
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Fig. 1. Surface morphologies of the coated NdFeB samples with the different concentrations of titania particles: (a) neat epoxy coating, (b) 10 g/l, (c) 20 g/l, (d) 30 g/l, (e) 40 g/l, and (f) 50 g/l.
Fig. 2. Change in the surface roughness of the coated samples measured as a function of the titania particles concentration.
particles enhancement, not only the microhardness of the epoxy coatings could obtain great improvement, but also the mechanical scratch resistance, wear resistance, thermal decomposition temperature, and corrosion resistance could be improved partly [25– 28]. Fig. 5 shows the weight loss–immersion time curves of the uncoated and coated NdFeB magnets samples immersed in 0.5 mol/l H2SO4 solution for 4 h. The weight loss of the uncoated NdFeB magnet sample was much higher than that of the coated samples, and the weight loss of the coated NdFeB samples gradually decreased with increasing titania particle concentrations. When the uncoated NdFeB magnet was immersed in the H2SO4 solution, a large number of bubbles appeared immediately all over the uncoated NdFeB sample surface, indicating that NdFeB suffered a serious homogeneous corrosion. A few bubbles first generated at the tips and edges of the neat epoxy coated sample after being immersed in the H2SO4 solution for 40 min. The bubble generation time of the coated sample with 10 g/l, 20 g/l, 30 g/l and 40 g/l titania particles was about 60 min, 100 min, 180 and
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Fig. 3. Cross-sectional morphologies of the neat epoxy coated sample (a) and the coated sample with 40 g/l titania particles (b).
Fig. 4. Variation of microhardness with the concentration of titania particles.
Fig. 5. Weight loss–immersion time curves of the uncoated and coated NdFeB samples in 0.5 mol/l H2SO4 solution.
220 min, respectively. The bubble generation time was effectively prolonged through the addition of titania particles and the corrosion resistance of NdFeB was greatly improved. When immersed in the H2SO4 solution for 4 h, the average corrosive velocity of the uncoated NdFeB sample was about 13.86 g/dm2 h, one order of magnitude higher than that of the neat epoxy coated NdFeB sample with a value of 1.5 g/dm2 h and three orders of magnitude
higher than that of the 40 g/l titania particles coated NdFeB sample with a value of 0.03 g/dm2 h. According to the phenomena of the H2SO4 solution immersion test, corrosion of the coated samples first occurred at the tips and edges. The main reason was that the coating thickness at tips and edges of the coated samples was thinner than other places. The cross-sectional morphologies at edges of the neat epoxy coated sample and 40 g/l titania particles coated sample are shown in Fig. 6. It could be seen that the coating thickness at edges of the neat epoxy coated sample was much thinner than other places and the coating thickness at edges of the 40 g/l titania particles coated sample was a little thinner than other places. During the high temperature curing process, the neat epoxy coating at the tips and edges would transfer to the surface due to the existence of surface tension, resulting in the thickness at the tips and edges of the epoxy coating much thinner than other places. The addition of titania particles could make the particles embed in the epoxy matrix and impede the epoxy coating movement during high temperature curing. Therefore, the thickness at the tips and edges of the composite coatings decreased a little and they exhibited excellent corrosion resistance compared to the neat epoxy coating. In order to further investigate the effect of titania particles on the corrosion resistance of the coated NdFeB, the neutral salt spray test (NSS) was also employed. For the uncoated NdFeB sample, a layer of brown rust was found to be uniformly distributed over the sample surface after 1 h exposure, consistent with the literature [5]. The brown rust was initially observed at the tips and edges of the neat epoxy coated sample after 240 h exposure to the salt spray environment. The time of brown rust corrosion products to appear on the 40 g/l titania particles coated samples was after 600 h exposure to the salt spray environment, much longer than that of the neat epoxy coated sample. Fig. 7 shows the cross-sectional morphologies of the neat epoxy coated sample and 40 g/l titania particles coated sample after being exposed to the salt spray environment for 500 h. Many pores and river-like defects were detected throughout the cross-section of the neat epoxy coating, very different than Fig. 3a. However, no defects could be detected on the cross-sectional morphology of the 40 g/l titania particles coated sample after being exposed to the salt spray environment for 500 h, consistent with Fig. 3b. At the start of the neutral salt spray test, the neat epoxy coating protected the NdFeB substrate by acting as a barrier layer. As the test time prolonged, the water and Cl ions were able to gradually permeate into the neat epoxy coating, pass through the coating to the interface and corrode the substrate NdFeB at last. Therefore, many brown rusts were detected on the neat epoxy coated sample after 500 h exposure to the salt spray environment. Those pores and river-like defects detected in the neat epoxy coating
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Fig. 6. Cross-sectional morphologies of edges of the neat epoxy coated sample (a) and 40 g/l titania particles coated sample (b).
Fig. 7. Cross-sectional morphologies of the neat epoxy coated sample (a) and 40 g/l titania particles coated sample (b) after exposure to salt spray environment for 500 h.
Table 1 Magnetic properties of the coated and uncoated NdFeB samples.
Uncoated NdFeB 40 g/l Titania particles coated NdFeB Variation (%)
Br (kGs)
Hcj (kOe)
(BH)max (MGsOs)
12.51 12.50 0.08
22.57 23.41 3.7
38.58 38.43 0.4
must be the corrosive channels formed during the neutral salt spray test. The titania particles uniformly dispersed and embedded in the epoxy matrix could effectively impede and delay the water and Cl ions to permeate and pass through the coating. Therefore, no obvious corrosive channels could be detected in the 40 g/l titania particles coating (seen in Fig. 7b), and there were no brown rusts on the sample after 500 h exposure to the salt spray environment. In industries, electroplated Ni or NiCuNi coatings were generally applied to improve the corrosion resistance of NdFeB magnets, but these coatings deteriorated the magnetic properties and even the coercive force reduced by more than 10% [30]. Chen et al. [24] reported that there was a loss of coercive force by 24% and remanence by 16% due to the preparation of electroless Ni–P coatings on NdFeB magnets. Therefore, the effect of titania particles enhanced epoxy coatings on magnetic properties of NdFeB magnets should be explored with necessity. Table 1 shows the magnetic properties of 40 g/l titania particles coated and uncoated NdFeB samples. The remanence (Br) value and large maximum energy product ((BH)max) value had few changes before and after deposition of the coating, and the intrinsic
coercive force (Hcj) had a 3.7% increase after deposition of the coating, which might be related to the semiconductor characteristic of the titania particles. Therefore, it can be concluded that the titania particles enhanced epoxy coatings do not deteriorate the magnetic properties of NdFeB magnets.
4. Conclusions (1) The titania particles were uniformly dispersed and embedded in the epoxy matrix when the titania particles concentration was lower than 40 g/l, while with further increasing the concentration, the titania particles suffered serious agglomeration and macroscopic particles defect appeared. (2) The titania particles did not change the thickness of the epoxy coatings, both the thickness of the neat epoxy coating and the titania particles enhanced, epoxy coating were about 40 μm. With increasing titania particle concentrations, the surface roughness and the microhardness of the composite coatings increased. (3) The weight loss of the coated NdFeB samples immersed in H2SO4 solution was much lower than that of the uncoated NdFeB sample and with increasing titania particle concentrations, the weight loss of the coated NdFeB samples decreased. In the NSS test, the time taken by brown rust corrosion products to appear on titania particles enhanced epoxy coatings was much longer than that of the neat epoxy coating. (4) The titania particles embedded in the epoxy matrix impeded the decrease of the coating thickness at the tips and edges, and
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impeded and delayed the aggressive mediums to permeate and pass through the coating and corrode the NdFeB substrate. Therefore, the addition of the titania particles could greatly improve the corrosion resistance of the epoxy coatings and did not deteriorate the magnetic properties of NdFeB substrates.
Acknowledgments The authors gratefully acknowledge the financial support of the project from the National Science and Technology Key Support Projects of China (Grant no. 2012BAE02B01), the Major Science and Technology Special Projects of Jiangxi Province (Grant no. 2010AZX00200), the Science and Technology Plan Projects of Jiangxi Province (Industrial field) (Grant no. 20121BBE50001), and the Science and Technology Plan Projects of Department of Education of Jiangxi Province (Grant no. KJ201109132281). References [1] J.W. Kim, S.H. Kim, S.Y. Song, Y.D. Kim, Nd–Fe–B permanent magnets fabricated by low temperature sintering process, J. Alloys Compd. 551 (2013) 180–184. [2] O.G. Eisch, M.A. Willard, E. Brück, C.H. Chen, S.G. Sankar, J.P. Liu, Magnetic materials and devices for the 21st century: stronger, lighter, and more energy efficient, Adv. Mater. 23 (2011) 821–842. [3] B.E. Davies, R.S. Mottram, I.R. Harris, Recent developments in the sintering of NdFeB, Mater. Chem. Phys. 67 (2001) 272–281. [4] M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto, Y. Matsuura, Permanent magnet materials based on the rare earth–iron–boron tetragonal compounds, IEEE Trans. Magn. 55 (1984) 1584–1589. [5] H.H. Man, H.C. Man, L.K. Leung, Corrosion protection of NdFeB magnets by surface coatings – Part I: salt spray test, J. Magn. Magn. Mater. 152 (1996) 40–46. [6] K. Tokuhara, S. Hirosawa, Corrosion resistance of Nd–Fe–B sintered magnets, J. Appl. Phys. 69 (1991) 5521–5523. [7] G.L. Yan, P.J. McGuiness, J.P.G. Farr, I.R. Harris, Environmental degradation of NdFeB magnets, J. Alloys Compd. 478 (2009) 188–192. [8] A.A. El-Moneim, A. Gebert, M. Uhlemann, O. Gutfleisch, L. Schultz, The influence of Co and Ga additions on the corrosion behavior of nanocrystalline NdFeB magnets, Corros. Sci. 44 (2002) 1857–1874. [9] W. Fernengel, W. Rodewald, R. Blank, P. Schrey, M. Katter, B. Wall, The influence of Co on the corrosion resistance of sintered Nd–Fe–B magnets, J. Magn. Magn. Mater. 196–197 (1999) 288–290. [10] R.K. Murakami, H.R. Rechenberg, A.C. Neiva, F.P. Missell, V. Villas-Boas, Effect of Ti and C additions on structural and magnetic properties of (Pr,Nd)–Fe–B nanocrystalline magnetic materials, J. Magn. Magn. Mater. 320 (2008) e65–e68.
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