Fracture toughness measurements of some dental core ceramics: a methodologic study.

Fracture toughness is regarded as an important property of dental ceramics. The most widely used methods for fracture toughness (KIc) determination are based on assessment of cracks created by hardness indentations. Different formulas have been developed for KIc calculations and all these methods and formulas include empirical factors based on pure ceramics, i.e. non-composite ceramics. These factors may, however, vary for a specified method for materials with different and complex structure. An important question is whether the various proposed methods and formulas lead to approximately the same numerical KIc values or at least to the same ranking of materials. The aim of this work was to compare two indentation methods and various formulas for calculation of KIc values when used on four commercial composite dental ceramics. The two applied methods and the different formulas showed substantial differences in the obtained values for one and the same material and a different ranking of various materials. It is unknown which method gives the most correct KIc values for these ceramic materials.