Fully dense ZrB2 ceramics by borothermal reduction with ultra-fine ZrO2 and solid solution

The effects of ZrO2 particle size (55 nm and 113 nm) and borothermal reduction routes (borothermal reduction with water-washing (BRW) and in situ 5 mol% TaB2 solid solution, BRS) on synthesis and densification of ZrB2 were investigated. Irrespective of reduction routes, the use of finer ZrO2 powders as raw materials resulted in finer ZrB2 powders. Compared to the powders derived from BRS, the powders derived from BRW had smaller particle size with higher oxygen content, especially the powders synthesized with finer ZrO2. Irrespective of ZrO2 particle size, the oxygen contents of ZrB2 powders prepared by the BRS route were similar. Because of the high oxygen content, the ZrB2 ceramics synthesized by BRW with finer ZrO2 demonstrated the lowest relative density (90.5%), which resulted in the lowest Vickers' hardness (14.2 +/- 0.9 GPa). Due to the low oxygen content and small particle size of ZrB2 powders, fully dense ZrB2 ceramics (relative density: 99.6%) with highest Vickers' hardness (16.0 +/- 0.2 GPa) were achieved by BRS with finer ZrO2 powders.