Activation of NdFeB magnetic material upon exposure to oxygen plasma

Abstract

In bonded NdFeB magnets, the fillers’ adhesion in the polymer matrix of composites is often insufficient, necessitating filler wettability modification before mixing with liquid polymers. Inductively coupled radiofrequency oxygen plasma was used to modify the surface wettability of commercial NdFeB flakes. A pronounced minimum in water contact angle (WCA) was observed after approximately 100 ms of treatment in plasma (50 Pa, 500 W). A deeper minimum was observed upon treatment in the flowing afterglow. The flux of oxygen atoms on the NdFeB flakes’ surface was ∼3 × 1023 m−2 s−1, and the WCA below 20° was observed after treating the samples in the afterglow for 0.05–0.5 s; corresponding O-atom doses 1022–1023 m−2. Larger doses caused a gradual loss of hydrophilicity, and an initial WCA of ∼75° was established after treating the samples in the afterglow for ∼40 s. This was attributed to the segregation and oxidation of iron on the NdFeB surface. Thorough XPS depth profiling revealed oxidation kinetics. The treatment in the glowing plasma caused a similar evolution, except that hydrophobicity was re-established after 1-s plasma treatment. The segregation of iron on the surface caused the formation of Fe<inf>2</inf>O<inf>3</inf>, and the intermediate layer toward the bulk consisted of Nd<inf>2</inf>O<inf>3</inf> dispersed in the Fe matrix.