Characterization of (HfTiZrTaM)B₂ (M=Mn/Mo/W/Cr) high entropy diboride ceramics with five/six components prepared via powder metallurgy

This study shows the possibility of synthesizing five/six constituent high entropy (HfTiZrTaM)B₂ (M=Mn/Mo/W/Cr) diboride ceramics via mechanical alloying (MA) assisted spark plasma sintering (SPS). Firstly, metal borides as starting materials were synthesized in-house via mechanochemical synthesis (MCS) and leaching using optimum conditions. Secondly, metal borides were mixed in equimolar ratios to consist of five or six components, and then MA was used to hybridize the mixture for 6 h. SPS (2000 °C, 30 MPa) was used to convert the hybridized powders into densified structures. The compositional, microstructural, physical, mechanical and thermal analyses were conducted. Based on the results, the main HEB phase and low amount of (Hf, Zr) oxides were detected. Also, there was a secondary phase in the (HfTiZrTaCrW)B₂ composition. Approximately 8.19 g/cm³ and 6.72 g/cm³ were found as the highest and the lowest Archimedes’ densities for the (HfTiZrTaCrW)B₂ and (HfTiZrTaCrMo)B₂ samples, respectively. (HfTiZrTaCrMn)B₂ had the lowest hardness value nearly 22 GPa, and (HfTiZrTaCrW)B₂ had the highest hardness value at about 24 GPa. (HfTiZrTaCrW)B₂ had the highest wear rate (∼6.85x10⁻⁶ mm³/Nm), and (HfTiZrTaCrMo)B₂ had the lowest wear rate (∼5.95x10⁻⁷ mm³/Nm).