The effects of the pressing step on the microstructure and aging of NdFeB bonded magnets E.A. Périgo a, ⁎, M.F. de Campos b , R.N. Faria c , F.J.G. Landgraf a, d a Institute for Technological Research (IPT), São Paulo, SP 05508-901, Brazil b EEIMVR-Federal Fluminense University, Volta Redonda, RJ 27255-125, Brazil c Nuclear and Energy Research Institute (IPEN), São Paulo, SP 05508-000, Brazil d University of São Paulo, São Paulo, SP 05508-901, Brazil abstract article info Article history: Received 16 January 2012 Received in revised form 13 February 2012 Accepted 8 March 2012 Available online 14 March 2012 Keywords: NdFeB Aging Magnetic properties Rietveld analysis The effects of the compaction step on the (micro)structural features and aging behavior of polymer coated NdFeB-based bonded magnets is reported. Due to the fracture of the material during pressing, it is estimated an increase of at least 14% in the particles' area which is not coated. Such uncoated surfaces, when exposed to the environment, reduce the magnetic performance of the magnets aged/cured in air by 19% in the conditions evaluated in this investigation. Furthermore, XRD results interpreted by Rietveld analyses show a lattice parameter change in the tetragonal structure of the hard magnetic phase after pressing. Such change varies as a function of the height of the compacted part and it is ascribed to macro-elastic stress arising from the pressure distribution in the magnet. An aging/curing step during 24 h is able to relief such macro-elastic stress. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The RE 2 TM 14 B (RE_Nd or Pr and TM_Fe or Co) phase, also iden- tified as Φ, possesses the highest maximum energy product (BH max ) among the available hard magnetic materials. Commercially, it can be found in magnets of mainly two classes: sintered or bonded. The former is produced by the usual powder metallurgy techniques [1,2] and the densification is obtained by a liquid phase sintering process promoted by the intergranular RE-rich phase. Concerning the latter, the parts are usually prepared from rapidly solidified alloys frequently coated with a compound responsible for the mechanical strength of the piece after pressing and aging (also known as curing if the period of time exposed to a given temperature is short) [3]. Analyzing the bonded magnets' production process in more details, it is possible to verify that the optimization of the starting material fabrication has already been reported [4] whereas the most favorable curing conditions are usually defined as a function of the compound used on the magnetic particles. On the other hand, a key processing parameter whose effects on the NdFeB magnets' micro- structure which has not been investigated in details is the compaction step. For the preparation of soft metallic magnetic parts by powder metallurgy, the compaction pressure (P) must be as high as possible to achieve a high magnetic permeability, magnetic polarization and low magnetic losses. Such property combination is possible due to the partial densification achieved by plastic deformation of the metal during pressing (typically ranging from 80% to 90% of the metal theoretical density). In bonded magnets P must also be high since remanence is proportional to density [5], but the Φ phase pre- sents a plastic deformation degree much inferior compared to that of pure iron for instance, and a fracture should be expected. The se- quence pressing →(possible) plastic deformation →(possible) fracture will play a significant role on the (micro)structural features and, conse- quently, on the aging magnitude of NdFeB-based particles which consti- tute bonded magnets. Furthermore, as the commercial applications of magnets are frequently exposed to temperatures above 120 °C, it is necessary to investigate the aging and accelerated trials are done in higher temperatures. A correlation processing – microstructure – aging comprises the outline of this work. 2. Experimental The MQP-B powder was used as the stating material. Isotropic cylinders with height to diameter ratio of ~ 1 were prepared compact- ing the particles at 800 MPa at room temperature. No additional polymer, except the epoxy resin (density of about 600 kg m −3 ) already existent on the surface of the particles, has been added before pressing. Next, pieces were aged at 200 °C during 1 h or 24 h under air or vacuum (pressure below 10 −2 mbar). In order to separate the effects of the compaction pressure, the powders have also been exposed to the same aging conditions (only in air) to those of the magnets. It is worth mentioning that the aging step also acts as a Powder Technology 224 (2012) 291–296 ⁎ Corresponding author. Tel.: + 55 11 3767 4211; fax: + 55 11 3767 4037. E-mail address: eaperigo@ieee.org (E.A. Périgo). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2012.03.010 Contents lists available at SciVerse ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec