Fracture toughness of dental ceramics: comparison of bending and indentation method

Objectives: The aim of this study was to compare two fracture toughness methods, the bending method on notched specimens as reference and the indentation method as comparison. Potentialities and limitations of the indentation method were analyzed. Methods: Fracture toughness values were determined for seven dental ceramic materials on ‘single-edge-v-notched beams’ (bending method) as our standard method. Additionally indentation tests were done on the identical samples from the bending tests. The results were determined before and after annealing of the samples. With the reference fracture toughness results from the bending tests, the prefactor in the indentation test formula was individually adapted for each tested material. Results: The individual prefactors varied between 0.0122 and 0.0253 for the specimens before annealing and between 0.0150 and 0.0267 for the annealed specimens. Subsequently the differences between the K Ic-values calculated by the ISO draft (TC 206) direction and calculated by the modified formulae with the material specific prefactors were up to 48% for unannealed and up to 33% for annealed specimens, respectively. Significance: The indentation method is not an adequate tool to exactly determine the fracture toughness of an unknown ceramic material. This method can only be used for a first rough K Ic estimation.