A Processing–Microstructure Correlation in ZrB<sub>2</sub>–SiC Composites Hot-pressed under a Load of 10 MPa

Abstract

Monolithic ZrB₂ ceramic and its composites, with 5 to 30 vol. % SiC, has been prepared by hot pressing at temperatures of 1700, 1850 and 2000℃, for 30 minutes under relatively low pressure of 10 MPa. Densification behavior of ZrB₂-based composites is improved by the addition of SiC particulates. The fracture surface of monolithic ZrB₂ ceramics shows a grained structure, with faceted ZrB₂ grains, as the fracture appears to spread prevalently along an intergranular path. The ZrB₂/ZrB₂ boundary interface is seemingly free of any secondary phases. The microstructure of ZrB₂–30 vol. % SiC composite, hot-pressed at 1700℃, is consistent with measured porosity for the sample that has ~8% open pores, nearly without closed pores. It seems that mechanical interlocking between ZrB₂ and SiC is an important mechanism for densification. In the microstructure of specimens consolidated at 1850℃, neck formation between ZrB₂ particles is visible. In contrast, relatively fully dense samples are obtained by hot-pressing at 2000℃. Intergranular SiC particles inside ZrB₂ grains show the occurrence of mass transfer among ZrB₂ particles, which in effect brings the elimination of pores to a fortunate ending. Efficient mixing of starting powders is very critical in order to achieve a fine-grained homogenous microstructure.