Densification, Microstructure and Mechanical Properties of Reactive Hot-Pressed ZrB₂-ZrC-Zr Cermets

Abstract

This study reviews densification behavior, microstructure and mechanical properties of the various composition ZrB₂-ZrC_0.6-Zr cermets prepared by reactive hot-pressing of Zr + B₄C powder mixtures between 1673 K and 2173 K. During reactive sintering, densification consisted of two parts, one resulted from the volume contraction due to the conversion of Zr + B₄C mixture into ZrB₂ and ZrC_0.6, other resulted from the shrinkage due to grain boundary diffusion and grain boundary migration that mostly contributed to the densification. Highly-dense ZrB₂-ZrC_0.6-Zr cermets were obtained for Zr + B₄C mixtures at equal to or greater than 1673 K. The resulting ZrB₂-ZrC_0.6-Zr cermets consisted of platelet-like ZrB₂ grains, equiaxed ZrC_0.6 grains and intergranular Zr. The ZrB₂ and ZrC_0.6 grains were coarsened with increase of the excess Zr amount and/or sintering temperature. On the other hand, the flexural strength and fracture toughness of the cermets strongly depended on the excess Zr amount and/or sintering temperature. In addition, the crack propagation behavior of the cermets was associated with the excess Zr amount. The cracks directly propagated in the samples with less than 16 vol% Zr, however, the multiple cracking behavior was observed for the samples with equal to or greater than 16 vol% Zr.