Effect of sintering atmosphere on the hardness of ThO2

Abstract The hardness and toughness of ThO 2 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 ThO 2 is H 0 = 10.5 ± 0.3 GPa for oxidizing conditions and H 0 = 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 ThO 2 sintered under reducing conditions. Furthermore, and of interest for nuclear fuel production, is the finding that ThO 2 sintered under reducing conditions is significantly easier to grind compared to material sintered under oxidizing conditions.