On the mechanical behavior of the sintered Nd-Fe-B permanent magnet during diamond scratching

Abstract

This study evaluates the mechanical behavior of a sintered Nd-Fe-B permanent magnet submitted to scratch tests considering its magnetic anisotropy. The elastic properties of the magnet and their phases were measured using indentation techniques. The scratch tests were performed under constant and varying the normal force (Fn) in the range of 0 to 100 mN, monitoring the tangential force (Ft), the coefficient of friction (COF) and the maximum penetration depth (hmax). The scratch morphologies were analyzed on a scanning electron microscope (SEM) and an optical non-contact profilometer. Results showed that the hardness of the faces parallel and perpendicular to the c-axis had similar values whereas the Nd2Fe14B1 and Nd-rich phases presented different hardness and elastic modulus values. The scratch morphologies presented mainly plastic deformation under lower Fn, while increasing Fn led to pile-up formation, microcracks and grain replacement in the groove. Fluctuations in Ft and COF were observed during scratching, which can be attributed to the presence of hard and soft phases in the microstructure of the Nd-Fe-B. Specific energy remained within 20–90 J/mm3 and it raised with the increase of hmax. By increasing Fn, the progressive change of the machining mechanism from microgrooving to microploughing and finally microcracking was observed. Under Fn ≥ 60 mN there were replacements of Nd2Fe14B1 grains, bulging of Nd-rich phase, microcrack formation and grain pull-out. The penetration depths for ductile-brittle transition for faces parallel and perpendicular to the c-axis were around 271 nm and 250 nm, respectively. The magnetic anisotropy did not affect the mechanical behavior of Nd-Fe-B. A case study about abrasive machining of Nd-Fe-B confirms the mechanisms of material removal pointed out in scratching trials. This study provides insights into the fundamental mechanisms of material removal of an Nd-Fe-B permanent magnet, which contributes to enhancing the abrasive machining of magnet components.