Doping Zironium Diboride with Rare-Earth Compounds for Extreme Environments

Zirconium diboride (ZrB₂) is an ultra-high temperature ceramic with a high melting point and good oxidation resistance, making it of interest for use in extreme environments. When used as a phase pure material, evaporation of the previously protective surface B₂O₃ glass at intermediate temperatures (~1300°C+) limits use in extreme environments. Silicon carbide is often added to increase the useful operating temperature to 1700°C by the formation of a protective SiO₂ glass at the surface, but is limited by the active oxidation that occurs above 1700°C. The addition of rare earth compounds (REs) to ZrB₂ is of interest due to high melting points, contribution to the formation of oxygen diffusion limiting zirconate phases, and increased emittance. Here, we present results of ZrB₂ doped with REs to maintain thermal and mechanical stability above 1700°C via the formation of a stable oxide layer and reduced overall temperature of the specimen from increased radiative heat losses. Oxides and borides of both samarium and dysprosium are synthesized via co-precipitation from their respective nitrates. During the pressureless sintering process, a reducing environment is used to remove the native oxide layer on the surface of the ZrB₂, which affects the stability of the added REs. This is mitigated by the use of the RE-boride phases. The properties of the resulting RE-doped ZrB₂ is investigated before and after oxyacetylene torch testing, with a focus on the effects of addition of specific RE compounds. Future efforts aim to integrate RE-doping into existing work of fabricating carbon fiber reinforced ZrB₂ via the additive manufacturing technique of material extrusion.