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

Abstract

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.