Effect of sintering atmosphere on the hardness of ThO₂

The hardness and toughness of ThO₂ sintered under reducing and oxidizing conditions has been investigated and, quite unexpectedly, a significant difference in hardness was observed for the entire range of porosities studied. Reducing conditions systematically yielded higher hardness values than oxidizing conditions. Extrapolated to zero porosity, the hardness for ThO₂ is H₀ = 10.5 ± 0.3 GPa for oxidizing conditions and H₀ = 12.4 ± 0.7 GPa for reducing conditions. Toughness values have been derived from Vickers indentations; differences in toughness were insignificant and only a single value is proposed: K_IC = 0.97 ± 0.12 MPa √m. The difference in hardness is attributed to the presence of point defects, also acting as color centers and causing grey coloration of ThO₂ sintered under reducing conditions. Furthermore, and of interest for nuclear fuel production, is the finding that ThO₂ sintered under reducing conditions is significantly easier to grind compared to material sintered under oxidizing conditions. - Highlights: • Reducing sintering conditions increase the hardness of polycrystalline ThO₂. • Systematic series of hardness versus porosity of ThO₂ were obtained. • Reducing sintering conditions improve grinding of ThO₂.