Abstract
Fracture toughness (KIc) for polycrystals of Ni-Zn ferrite (grain size; 22μm) and Ba hexaferrite (grain size; 3.5μm) was measured at room temperature and elevated temperatures up to 1000°C. Room-temperature KIc's determined by the controlled surface flaw (CSF) technique for Ni-Zn ferrite and Ba ferrite were 1.50±0.12 and 2.97±0.16MN/m3/2, respectively. In the indentation fracture (IF) technique, different KIc values were obtained, depending on the equations proposed to evaluate KIc and indentation load for testing. There was a difference in fracture mode at room temperature between the two ferrites transgranular fracture with relatively smooth surface in Ni-Zn ferrite, and intergranular fracture or cleavage of a specific plane in each grain with rough surface on a grainsizescale in Ba ferrite. There were also differences in KIc value and fracture behavior at elevated temperatures between the two ferrites. KIc of Ba ferrite decreased linearly with increasing temperature up to 1000°C, which appeared as if temperature dependence of KIc reflected only that of Young's modulus. However, tip of flaw introduced by Knoop indenter blunted above 800°C as inferred from the facts that the fracture stress increased and that the fracture often initiated nott from the introduced surface flaw but from an exaggerated grain at those temperatures. For Ni-Zn ferrite, the blunting of flaw tip appeared above 600°C, with gradual increase in KIc to 800°C. Above 900°C, KIc and fracture stress decreased and the fracture initiated not at a flaw tip but at grain boundary near the flaw tip, showing intergranular fracture throughout the fracture surface. At those temperatures, fracture often initiated from an exaggerated grain as in Ba ferrite. However, it seemed that the fracture from a large grain in Ba ferrite may be due to dislocation pile-up or cleavage in the large grain rather than the decreasing strength in grain boundary with increasing temperature as in Ni-Zn ferrite. Bending strength of Ni-Zn ferrite without an introduced flaw had a maximum at -700°C and then decreased with increasing temperature from 900°C with intergranular fracture, corresponding to the KIc versus temperature relationship.
