Quantitatively Evaluating the Effect of Oxygen/Fuel Ratio on Fe 2+ Content in HVOF-Sprayed Ni-Zn Ferrite Coatings
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Deen Zhang and D.G. McCartney (Submitted August 25, 2008; in revised form February 15, 2009) This article quantitatively describes the dependence of Fe2+ content in Ni-Zn ferrite coatings on the oxygen-fuel (O/F) ratio in high velocity oxy-fuel (HVOF) spraying. Fe2+ is detrimental to the magnetic properties of ferrites and should be tightly controlled. Methods were developed for calibrating energy dispersive x-ray analysis and for calculating Fe2+ content. The results show that the atomic percentage of the Fe2+ decreased from 25 to 15 at.% by increasing the O/F ratio from 80 to 110% of the stoichiometric ratio. The effect of O/F ratio has also been observed on in flight particle temperature, microstructure, and deposition efficiency. Simulations on the chemical reactions during HVOF spraying demonstrated that Fe2+ was produced only by the reduction of Fe3+ and that oxygen and zinc losses by the decomposition of ZnO in fact increased the percentage of oxygen in the coatings.
Keywords
charge balance, energy dispersive x-ray (EDX) analysis, HVOF thermal spray, Ni-Zn ferrites, particle temperature
1. Introduction Spinel soft magnetic oxides such as (NiZn)Fe2O4 and (NiCo)Fe2O4 ferrites are of great interest to the electronics industry because of their high electrical resistivity and magnetic permeability. They find a broad range of applications in telecommunications, audio and video, power transformers, magnetic storage, microelectronics, magneto-optics, microwave devices, etc. The most common method for manufacturing ferrite components is by a powder metallurgy process. There could be a number of advantages in manufacturing ferrite components by using thermal spraying processes such as high velocity oxy-fuel (HVOF) or air plasma. Costly procedures in powder metallurgy could be replaced by the direct deposition of powder onto substrates. The chemical compositions, microstructure, and crystal structure of ferrites are extremely process sensitive; the combination of elevated temperature and gaseous atmosphere influence the magnetic properties (Ref 1). Dias and Moreira (Ref 2) reported that heating in air at 1400 C increases zinc concentration in the outer surface when compared with the center of sintered specimens. The same results were also observed by Hellicar and Sicignano Deen Zhang and D.G. McCartney, Department of Mechanical, Materials and Manufacturing Engineering, Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
(Ref 3). Van and Johnson (Ref 4) found the opposite case with zinc loss on the surface of samples. Other authors (Ref 5, 6) reported that zinc loss in sintered ferrites leads to variations in its lattice parameter, together with changes in Fe2+ ion concentration and grain growth patterns. Suh and Han (Ref 7) investigated the zinc loss phenomena by monitoring the weight change of Mn-Zn ferrites in air and in nitrogen at temperatures up to 1400 C. Weight change was not observed afte
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