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Title: Corrosion protection of Nd–Fe–B magnets by bismaleimide coating
Authors: Cheng, FT
Man, HC 
Chan, WM
Cheng, CW
Chan, WO
Issue Date: 15-Apr-1999
Source: Journal of applied physics, 15 Apr. 1999, v. 85, no. 8, p. 5690-5692
Abstract: Samples of Nd–Fe–B were coated with bismaleimide (BMI), a resin which is well known for its desirable barrier, mechanical, and thermal properties but has not been used as a coating material. The performance of BMI-coated samples (average coating thickness ~15 μm) was compared with commercial epoxy-coated samples (average coating thickness ~20 μm) in a series of tests. In the copper-accelerated acetic acid–salt spray test, the epoxy-coated samples failed badly (damaged area >50%) within 20 h while the BMI-coated samples showed no sign of failure up to 200 h. Using the cross-cut tape test for adhesion strength (ASTM D 3359), the commercial epoxy coatings suffered 25% area detachment while for the BMI coatings, no detachment was observed. The scratch resistance of the BMI coatings, measured in terms of scratch depth in the pin scratch test, was about 1.5 that of the commercial epoxy coatings, and the Vickers hardness was higher by a factor of about 2.5. When magnetized under the same magnetizing field, there was no significant difference in the magnetization obtained among bare, epoxy-coated, and BMI-coated samples. These results suggest that BMI coatings are excellent for protection of the Nd–Fe–B magnets against corrosion and function better than the commonly used epoxy resin.
Keywords: Permanent magnets
Neodymium alloys
Iron alloys
Boron alloys
Corrosion protection
Corrosion protective coatings
Polymer films
Corrosion testing
Wear resistance
Vickers hardness
Ferromagnetic materials
Publisher: American Institute of Physics
Journal: Journal of applied physics 
ISSN: 0021-8979
EISSN: 1089-7550
DOI: 10.1063/1.369842
Rights: © 1999 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in F. T. Cheng et al., J. Appl. Phys. 85, 5690 (1999) and may be found at
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