In-Situ Synthesis ZrB₂ Based Ceramics By Spark Plasma Sintering

ZrB₂ is one of important ultra-high temperature ceramics, due to its high melting point and strong mechanical properties, and it is considered have broad application prospects in the aerospace, advanced nuclear power and other fields, but their strong covalent bonding and the low self-diffusion coefficients, results it is difficult to achieve fully dense by conventional sintering methods. Therefore, ZrB₂-based ceramics was fabricated by in-situ synthesis reaction combined with Spark Plasma Sintering method in order to achieve a certain density and good mechanical properties in this article.In this study, Zr, ZrO₂ and B4 C were used as a raw material to synthesis ZrB₂ powder by high-temperature solid-phase reaction in-situ plasma discharge occurs in a furnace and fired directly at 1900℃, successfully fabricated nearly fully dense ZrB₂ material. And in situ synthesis mechanism and densification behavior of the material were studied. The results show that Zr reacted to B4 C, and generated ZrB₂ and ZrC at 800℃. When the temperature reached 1400℃, material occur the second step of in-situ synthesis reaction, ZrC was reacted to ZrO₂ and B4 C and generated ZrB₂ and CO. Material start densification at about 1700℃, occured rapid volume contraction, when the temperature reaches 1900℃, the material has been substantially dense.When the starting material in ZrO₂ content of B4 C and increased by-product produced by the reaction of graphite with decreasing material by in-situ synthesized. However, due to the formation of excessive B2O3 in situ synthesis process and volatile in the high temperature sintering process, thus resulting in increased porosity of the material appear small amplitude, reducing the mechanical properties of the material.With the gradual increase in the Zr content of the raw material, the material appears a certain content of m-ZrO₂ phase which is not involved in the reaction. With the increase of Zr content, ZrC phase was significantly increased, and it will significantly reduce the sintering temperature of the material. The ZrO₂ appears in the ZrB₂ grain boundaries, limited the grain ZrB₂ grew up, changing the fracture mode of the material enhanced the mechanical properties of the material.At 1500 ℃, respectively, by a discharge plasma sintering and hot pressing successfully prepared porous ZrB₂-based ceramic. By plasma density and mechanical properties of the base ZrB₂ discharge sintering hot pressing sintering ceramics is higher than ZrB₂ prepared. Thermal shock resistance of the ceramic substrate and a porous ZrB₂ were characterized. After the material thermal shock temperature difference of 900 ℃, the material formed on the surface of the dense layer of ZrO₂, can effectively organize the material further oxidation. And due to dense surface oxide layer can effectively fill the surface of the open porosity and thermal stress cracking, so the material is not decreased thermal shock.