Effect of addition of esters of fatty acids on the microstructure and properties of sintered Nd–Fe–B magnets produced by PLP

• Application of internal lubricants in the PLP-produced magnets is studied.
• The alignment degree is enhanced by addition of esters of fatty acids.
• Critical concentrations of lubricants are determined.
• Exceeding the critical concentration decreases magnetic properties by increasing oxygen content.

High filling density of powders for production of sintered Nd–Fe–B magnets by the pressless process (PLP) impedes magnetic alignment. The latter can be enhanced by reduction of friction forces between powder particles. Thus, increase in the remanence and maximum energy product of the magnets by lubrication of powder particles is studied. Esters of fatty acids have been added in toluene or acetone in the course of grinding of Nd–Fe–B alloy in a vibratory mill. Coated by a thin layer of a lubricant powders have been aligned in pulsed magnetic field. It is shown that the remanence of sintered magnets has been increased by 5–7%. Lubricant concentration should not exceed critical values, which for the lubricants used varied between 2.0 wt% (ethyl butyrate) and 0.3 wt% (ethyl laurate). Otherwise, the complicated removal of lubricant residue leads to reaction of the latter with Nd-rich grain-boundary phase in the course of sintering and results in a sharp decrease in magnetic hysteresis properties. Addition of lubricating additives allows one to produce PLP-magnets with density exceeding 7.5 g/cm3, Br≥14 kG, Нс≥9 kOe and (BH)max≥45 MG Oe.