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-cer amics was investigated using both Vickers indentation and the notched beam technique (NBT). Five representative equations were applied and i t was found that for the variation of K(IC) with B2O3 content, the Law n and Fuller equation showed the best correspondence with the NBT. The values of fracture toughness obtained from the Lawn and Fuller equati on showed the same trend with B2O3 content as that determined by NBT, although the values from indentation were on average 33% lower. The de termination of absolute fracture toughness by indentation requires a c orrection factor which can be obtained by calibration using NBT. A sig nificant 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 t hat from indentation (0.89 MN m-3/2) is attributed to internal stresse s due to thermal expansion differences between the crystalline and res idual glass phases leading to additional microcrack toughening.