Fracture toughness of Co4Sb12 and In0.1Co4Sb12 thermoelectric skutterudites evaluated by three methods

► Indium interstitials improve thermoelectric properties of Co4Sb12 skutterudites. ► Fracture toughness is not degraded by indium interstitial doping. ► Indentation fracture toughness test is inaccurate for Co4Sb12 skutterudites. ► Fracture toughness of Co4Sb12 skutterudites is as low as 0.5MPa√m. ► Low fracture toughness raises concerns over durability of thermoelectric modules. Interstitially filled skutterudites are a promising class of state-of-the-art thermoelectric materials. Although thermoelectrics are exposed to significant thermal stresses, little information is known about the fracture toughness of interstitially doped skutterudites. This work explores the fracture toughness of undoped Co4Sb12 and indium doped In0.1Co4Sb12 skutterudites using three methods: (1) Vickers indentation fracture (VIF), (2) Vickers indent crack opening displacement (COD), and (3) single-edge vee-notched bend (SEVNB) in 4-point flexure. Indium addition to the icosahedral void-sites is verified by an observed increase in the crystal lattice parameter and strongly enhanced thermoelectric properties in the indium-doped samples. Fracture toughness values for Co4Sb12 and interstitially doped In0.1Co4Sb12 were found to be identical using both the COD and SEVNB methods indicating no interstitial embrittlement occurs due to indium void-site filling. Furthermore, it was found that there is no significant extrinsic toughening by crack bridging or other mechanisms and the toughness was insensitive to grain size variations. Fracture toughness values derived from the Vickers indentation fracture (VIF) method did not agree with the other two methods and it is recommended that the VIF method be avoided. The results indicate that the fracture toughness of skutterudites may be, at least in some cases, significantly lower (∼0.5MPa√m) than previously reported and there may be concern over the durability of skutterudite-based power-producing thermoelectric modules if care is not taken to ensure adequate toughness.