Microstructural Evolution of a Nickel Ferrite–Copper Alloy Cermet During Sintering and High‐Temperature Oxidation

Abstract

The microstructural evolution during sintering and high‐temperature oxidation of a cermet comprised of (wt%) a 40/60 ratio of an alloy of composition 80% Cu/20% Ni and a ceramic of composition 90% NiFe2O4/10% NiO has been studied as a function of processing temperature. The porosity of the cermet reduced significantly without undue coarsening of the microstructure during sintering up to the melting temperature of the alloy phase (Tm ≈ 1160°C), whereas at sintering temperatures higher than Tm, the ferrite rapidly coarsened with little additional reduction in porosity, and the alloy phase formed a continuous interconnected network while partially extruding from the composite. Upon oxidation in air at temperatures between 850°C and 1000°C, the cermet developed a multilayered oxidized scale, comprising a thin copper‐rich outer oxide layer formed via external oxidation and a thicker zone of internal oxidation. The external oxide scale transformed from CuO to Cu2O with increasing oxidation temperature. Out‐diffusion of the copper to the externally oxidized layer led to significant porosity at the internal interface between oxide and unoxidized cermet.