Effect of optimal aging treatment on magnetic performance and mechanical properties of sintered Nd-Fe-B permanent magnets

The magnetic performance and mechanical properties including hardness, brittleness, fracture toughness and strength characteristics of the as-sintered and the optimal aged Nd-Fe-B magnets were examined in this work. A new method of Vickers hardness indentation combined with acoustic emission was used to test the brittleness of the magnets.The results show that the magnetic properties of the magnets could be improved through aging treatment, especially the intrinsic coercive force. But it is accompanied by a decrease of strength and fracture toughness. Theoretical calculation confirms that acoustic emission energy accumulated count value could be used to characterize the material brittleness. The bending fracture morphologies of the as-sintered and the optimal aged Nd Fe B magnets were investigated with the emphasis on the relationship between mechanical properties and microstructure using a field emission scanning electron microscopy(FE-SEM). The research results indicate that the intergranular fracture is the primary fracture mechanism for both as-sintered and optimal aged Nd Fe B magnets. Aging treatment changes the morphology and distribution of the Nd-rich phases, reducing the sliding resistance between Nd_2Fe_(14)B main crystal grains and lowers the grain boundary strength, which is the main reason for the strength and fracture toughness decrease of the aged Nd-Fe-B magnets.

Surface resistivity of carbonaceous fiber/PTFE antistatic coatings

PAN(Polyacrylonitrile)-based carbonaceous fibers were prepared at the heat treatment temperature(HTT)range of 650 to 900 oC.The relationships among HTT,carbon content and volume resistivity of the carbonaceous fibers were investigated.The carbonaceous fibers/PTFE(Polytetrafluoroethylene)antistatic coatings were prepared by the spraying technology and the effects of carbonaceous fibers and pigments on surface resistivity of the coatings were systematically discussed.Micrographs provide insight into the antistatic mechanism of the coating.The results show that carbon content of the carbonaceous fibers increases from 68.8% to 74.8%(mass fraction)and the volume resistivity decreases drastically from 1.94×103 to 8.27×10-2 Ω·cm.The surface resistivity of the antistatic coating is adjustable between 105 and 108 Ω to fit the different antistatic materials.Static is dissipated by a conductive network of short fibers and the tunneling effect between the neighboring fibers and conductive pigments.Conductive pigments make the conductive network more perfect and improve the antistatic ability,but insulating pigments acting as barriers for the formation of conductive channel increases the surface resistivity of the coatings.The influence of pigments on the surface resistivity drops gradually with the decrease of the carbonaceous fibers volume resistivity.