Assessing hardness and fracture toughness in sintered zinc oxide ceramics through indentation technique

Micro hardness of sintered zinc oxide (ZnO) ceramics made from two different particle sizes was measured in a relatively wider range of applied test load (0.2-49 N). Experimentally measured apparent hardness data were analyzed to understand the indentation size effect (ISE) in the full range of load. Meyer's law was successfully utilized to describe the ISE behavior though it was unable to generate true hardness data. Load independent true hardness was assessed from the measured diagonal of the indented impression and the test load using the modified PSR model. True hardness numbers determined with this empirical method vary within 1.50-1.83 GPa depending on grain size of the sintered products. Indentation fracture toughness was measured with slightly higher test load of 9.8-49 N. Nature of crack emanated at the corners of the indent was identified as radial median type. The quantity P/C3/2 was observed to be independent on applied load for all the samples with experimental scatter. Fracture toughness calculated from the length of the indentation induced cracks exhibits dependence on grain size. Maximum fracture toughness of 2.50±0.30 MPa m1/2 was observed for the ground ZnO sample sintered at 1300 °C for 1 h duration.