The Effect of Annealing on the Formation of Jexaferrite Absorber Coatings Produced by Thermal Spraying Technologies

Barium hexaferrites are well-known materials suitable for a variety of applications. In this contribution we present the possibility of applying spraying technologies in the preparation of electromagnetic absorbers based on barium hexaferrites. Thick barium hexaferrite coatings with the compositions BaFel2019 and BaCoTiFelOOl9were deposited by high-velocity oxygen-fuel (HVOF) and atmospheric plasma-spraying (APS) technologies. The coatings were prepared from pre-reacted powders of the desired composition. The crystallization of the hexaferrites was studied by X-ray powder diffractometry, electron microscopy, including microanalysis, which was also combined with thermal analyses and magnetic measurements. The as-deposited coatings were poorly crystallized because of the phase changes induced in the powders "in-flight" in the HVOF and the APS flames and during their quenchingwhile fonning the coating. The coatings produced by APS showed a higher crystallinity than those prepared with HVOF. The largest amount of the hexaferrite phase was obtained in the APS-sprayed BaCoTiFelOOl9coating. In general, the crystallinity of the coatings and their magnetic properties were improved by a subsequent annealing. Fig. I and Table 1 are presented as examples of the annealing studies. Fig. 1 shows the crystallization of the BaCoTiFelOOl9produced by APS followed by annealing. The evolution of the magnetic properties; of the BaFe12019prepared with HVOF depending on the annealing temperatures is presented in Table 1. Similar results were also achieved for the other coatings. Single-phase hexaferrite coatings were obtained at 900°C or at 1000°C, depending on the chemical composition and on the spraying technology. The absorption characteristics were calculated from the electromagnetic properties measured· at 0.4-60 GHz. For tile BaFe12019coatings with thicknesses of 0.15-0.4 mm an enhanced absorption was predicted for mm-waves, while for the BaCoTiFelOOl9coatings with thicknesses of 1-4 mm, enhanced absorption was predicted for microwaves.