Effect of processing parameters on the magnetic properties and microstructures of molybdenum permalloy compacts made by powder metallurgy Zhangming Zhang, Wei Xu, Ting Guo, Yinzhu Jiang, Mi Yan ⇑ State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China article info Article history: Received 28 January 2013 Received in revised form 14 January 2014 Accepted 17 January 2014 Available online 26 January 2014 Keywords: Molybdenum permalloy powder cores Compaction Annealing treatment Effective magnetic permeability Insulation layer Core loss abstract Effects of compaction and annealing process on the magnetic properties and microstructures of molybdenum permalloy (MP) powder cores have been investigated. MP compacts, with density as high as 92% of the theoretical value, were obtained under 1800 MPa compaction pressure. The MP powder cores show an enhanced effective magnetic permeability of 160 after post-annealing at 690 °C, which is attributed to the relief of internal stress rather than the phase transformation evidenced by the XRD analysis. However, higher annealing temperature destroys the insulating layer, resulting in the drop of the electrical resistivity, the effective magnetic permeability as well as the frequency stability. The results show that the samples compacted at 1800 MPa and annealed at 690 °C exhibit excellent magnetic prop- erties, with core loss of 780 mw/cm 3 (100 kHz, 100 mT) and effective magnetic permeability of 160 whose frequency stability is up to 1 MHz. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Molybdenum permalloy (MP) is one of the excellent soft magnetic materials, the ribbon cores of which have been widely applied due to its high magnetic permeability, low magneto crystalline anisotropy, and nearly zero magnetostriction [1,2]. However, MP ribbon cores are limited to the toroidally wound or stacked types, which can only be used in a low frequency range due to the high electrical conductivity. In contrast to ribbon cores, powder cores are more suitable to be produced in various shapes and applied in higher frequencies [3]. Similar to the other soft magnetic composites, the MP powder cores are produced through powder metallurgical route, including the consolidation of surface-insulated MP powders and annealing treatment. The insulation layer on MP powders is extremely important, ensuring the enough resistivity and thus high frequency applications. Some researchers have reported iron phosphate–silane [4], MgO [5] and Al 2 O 3 [6] insulation coatings for the iron-based soft magnetic composites. But the phosphate insulation layer will dis- appear after annealing. MgO and Al 2 O 3 insulation possesses higher thermal stability, but the preparation, either evaporation or sol–gel method, is unstable, making it hard to get uniform insulation layer. Besides, those methods are too complicated to get practical applications. For Fe-based powder cores, phosphoric acid (PA) is mostly applied to the insulation process due to the effective reaction between iron and PA. However, PA is not useful for MP powders since the reaction between nickel and PA for generating the effective insulation layer is insufficient. Therefore, new kind of insulation layer should be designed for MP powder cores. Besides, in order to relieve the stress generated in the compaction process, the powder cores should be annealed between 550 °C and 775 °C. Therefore, the insulation layer should also be thermal stable in such temperature range [7]. It is believed that high compact density, perfect insulation layer among particles and little internal stress are favorable for the powder cores to get good magnetic properties [8,9]. In the present work, the MP powder particles were insulated by high-temperature resistant chromic passivation coating to ensure the enough insulation and increase the temperature stability. The influence of compaction pressure and annealing temperature on the microstructures and magnetic properties of the compacts was studied in detail. 2. Experimental The gas atomized MP powders for fabrication of MP powder core are supplied by Advanced Technology & Materials Company (AT&M). The chemical composition and particle size distribution are listed in Table1. http://dx.doi.org/10.1016/j.jallcom.2014.01.123 0925-8388/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: mse_yanmi@zju.edu.cn (M. Yan). Journal of Alloys and Compounds 594 (2014) 153–157 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom