Processing and protection of rare earth permanent magnet particulate for bonded magnet applications
Rapid solidification of novel mixed rare earth-iron-boron, MRE2 Fe14B (MRE = Nd, Y, Dy; currently), magnet alloys via high pressure gas atomization (HPGA) have produced similar properties and structures as closely related alloys produced by melt spinning (MS) at low wheel speeds. Recent additions of titanium carbide and zirconium to the permanent magnet (PM) alloy design in HPGA powder (using He atomization gas) have made it possible to achieve highly refined microstructures with magnetic properties approaching melt spun particulate at cooling rates of 105-106K/s. By producing HPGA powders with the desirable qualities of melt spun ribbon, the need for crushing ribbon was eliminated in bonded magnet fabrication. The spherical geometry of HPGA powders is more ideal for processing of bonded permanent magnets since higher loading fractions can be obtained during compression and injection molding. This increased volume loading of spherical PM powder can be predicted to yield a higher maximum energy product (BH)max for bonded magnets in high performance applications.;Passivation of RE-containing powder is warranted for the large-scale manufacturing of bonded magnets in applications with increased temperature and exposure to humidity. Irreversible magnetic losses due to oxidation and corrosion of particulates is a known drawback of RE-Fe-B based alloys during further processing, e.g. injection molding, as well as during use as a bonded magnet. To counteract these effects, a modified gas atomization chamber allowed for a novel approach to in situ passivation of solidified particle surfaces through injection of a reactive gas, nitrogen trifluoride (NF3). The ability to control surface chemistry during atomization processing of fine spherical RE-Fe-B powders produced advantages over current processing methodologies. In particular, the capability to coat particles while "in flight" may eliminate the need for post atomization treatment, otherwise a necessary step for oxidation and corrosion resistance. Stability of these thin films was attributed to the reduction of each RE's respective oxide during processing; recognizing that fluoride compounds exhibit a slightly higher (negative) free energy driving force for formation. Formation of RE-type fluorides on the surface was evidenced through x-ray photoelectron spectroscopy (XPS). Concurrent research with auger electron spectroscopy has been attempted to accurately quantify the depth of fluoride formation in order to grasp the extent of fluorination reactions with spherical and flake particulate. Gas fusion analysis on coated powders (dia. <45mum) from an optimized experiment indicated an as-atomized oxygen concentration of 343ppm, where typical, nonpassivated RE atomized alloys exhibit an average of 1800ppm oxygen. Thermogravimetric analysis (TGA) on the same powder revealed a decreased rate of oxidation at elevated temperatures up to 300°C, compared to similar uncoated powder.