Microplasticity and Fracture Behaviour of PbO-B₂O₃ Glasses

Abstract

It has been found that both the Vickers hardness and the elastic constants of PbO-B₂O₃ glasses increase with increasing PbO content up to their maximum values at about 42 mole% PbO and then decrease. It has also been shown that, unlike the Vickers hardness and elastic modulus, the fracture toughness and the related fracture energy of the same glasses appear to decrease monotonically with increasing PbO content. In the present study, indentation and fracture behaviours of PbO-B₂O₃ glasses containing 20.3 to 60.0 mole% PbO were examined on the basis of plastic flow theories for glass. The flow stress was calculated from the hardness and Young's modulus data according to Marsh's theory of indentation. The calculated results indicated that, although the flow stress itself varied with PbO content in a similar manner as the hardness and elastic modulus, the ratio σ_y/E, where σ_y is the flow stress and E is the Young's modulus, was found to decrease monotonically with PbO content. This variation of σ_y/E could be explained well by the plastic flow theory for oxide glasses proposed by Argon, which predicts a relationship between theoretical flow stress and glass transition temperature. The fracture energy variation with PbO content was interpreted in terms of plastic deformation which may occur at the crack tip. By assuming that glass behaves as a non-work-hardening solid and that failure occurs when the plastic zone at the crack tip reaches a critical size, fracture energy behaviour of the present glasses could be reasonably interpreted.