1983 Volume 91 Issue 1052 Pages 149-156
Mn-Zn ferrite crystals were fractured along the main low-index planes, (100), (110) and (111) to obtain fracture toughness (KIc) for each plane. Controlled surface flaw (CSF) technique and indentation fracture (IF) technique were used. In the CSF technique, microplastic zones around Vickers impressions were identified by slip traces. Removal of the zones by surface grinding resulted in elimination of residual stress. In the IF technique, the proposed expressions for KIc evaluation and effects of surface treatment were discussed. The plane of lowest toughness was (110) with KIc of 1.14±0.06MN/m3/2 in CSF technique. For IF techique, there was a difference in KIc between chemically and mechanically polished surfaces: KIc=0.73±0.02 MN/m3/2 for the former, KIc=1.09±0.09MN/m3/2 for the latter (indentation load: 100gf to 300gf, crack to impression ratio: 3 to 4). Etching after indentation resulted in underestimation of KIc because of crack growth. It was difficult to fracture the crystal along a (100) plane without causing fractures along the other planes (mainly (110)) both in the CSF and IF techniques. Only small loads (≤50gf) could yield Palmqvist-type cracks along (100). For a (111) plane, fractures were necessarily deviated from the intended planes in the IF technique. KIc for near (111) was obtained by CSF technique to be 1.40±0.21MN/m3/2. The lowest toughness plane of the spinel ferrite was (110), which was inconsistent with the prediction from the single crystal elastic anisotropy. Furthermore, this plane was a slip plane at higher temperatures (>1000°C). These results were different from those reported on MgAl2O4.
