Development of NiZn-Ferrite Coatings forElectromagnetic Applications

Ferrites are iron-containing, ceramics which exhibit magnetic properties. They are non-conducting and therefore employed in high frequency applications. Most ferrites possess a spinel crystal structure and one of the most common is (Ni,Zn)1Fe2O4. Bulk ferrites are typically made by conventional ceramic powder processing. However, manufacture of thick coatings is of growing interest and deposition by thermal spraying is potentially attractive. This paper reports work on the development of (Ni,Zn)1Fe2O4 based powder feedstock of specific composition and coating deposition by both air plasma (APS) and high velocity oxy-fuel (HVOF) spraying. Powders and coatings were analysed by X-ray diffraction, optical and scanning electron microscopy so that phases present, microstructure and phase composition could be investigated. The influence of process parameters on coating development was studied for both APS and HVOF sprayed deposits. Particular attention was paid to degradation of the spinel crystal structure, the formation of iron oxide phases and elemental loss during spraying. The results obtained show that a degree of decomposition of the ferrite material occurred with the loss of zinc and formation of wustite (FeO). The Zn loss is very dependent on the surface-to-volume ratio of the powder used in spray deposition and this is a more important factor than particle temperature or residence time in a high temperature environment.