Design and manufacture of Nd–Fe–B thick coatings by the thermal spray process

Permanent magnetic coatings show potential in micro scale applications such as micro motors and micro generators. Nd–Fe–B magnetic coatings of average thickness 50μm were produced via the flame spray method where the behavior of Nd–Fe–B particles in the thermal spray process was studied through single splat formation. There are generally five types of Nd–Fe–B splat morphology; which correspond to different solidification routes and varying degree of splashing. Microstructures of coating cross sections exhibit features such as cracks and porosity. The microstructural features were related to the physical properties and the brittle nature of the rare-earth alloy feedstock. Cross sections of the coatings also exhibited the presence of two distinct phases: Nd-rich and Fe-rich regions, which have been validated by EDS analysis. Metastable phase formation and decomposition followed by non-equilibrium solidification of the molten droplet prior to impact have been suggested to cause phase separation and were also identified in the formation of single splats. Hardness tests further confirmed the two distinct phases as Nd-rich and Fe-rich areas. ► Five types of splat morphology were identified for flame sprayed Nd–Fe–B. ► Standoff distances affect splat size and morphology. ► Phase separation by metastable phase formation and non-equilibrium solidification. ► Coatings have poor cohesion where they failed to build up beyond 50μm.