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

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  • This work was supported by the National Basic Research Program of China (No. 2005CB623703), the National Natural Science Foundation of China (No. 51021063), the National High-Tech Research and Development Program of China (No. 2008AA030501), and the Program for New Century Excellent Talents in University of the Ministry of Education of China (No. NCET-08-0572).

Author to whom correspondence should be addressed. e-mail: zhoukechao@csu.edu.cn

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.

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