Fracture toughness of CaO-P2O5-B2O3 glasses and glass-ceramics determined by indentation

The fracture toughness, (K _IC) of CaO-P₂O₅-B₂O₃ glasses and glass-ceramics was investigated using both Vickers indentation and the notched beam technique (NBT). Five representative equations were applied and it was found that for the variation of K _IC with B₂O₃ content, the Lawn and Fuller equation showed the best correspondence with the NBT. The values of fracture toughness obtained from the Lawn and Fuller equation showed the same trend with B₂O₃ content as that determined by NBT, although the values from indentation were on average 33% lower. The determination of absolute fracture toughness by indentation requires a correction factor which can be obtained by calibration using NBT. A significant increase in K _IC occurred after a 37CaO-37P₂O₅-20B₂O₃-6Al₂O₃ (mol%) glass was converted to a glass-ceramic. The much higher K _IC for the glass-ceramic measured by NBT (1.32 MN m^–3/2) compared with that from indentation (0.89 MN m^–3/2) is attributed to internal stresses due to thermal expansion differences between the crystalline and residual glass phases leading to additional microcrack toughening.