Strength and fracture surface energy of phase-separated glasses

Flexural strength and fracture surface energy were determined for lead borate glasses whose compositions lie in the immiscible region of the PbO-B₂O₃ system. The microstructural characterization indicated that the glasses are typical particulate composites which consist of two immiscible phases. For the glasses whose microstructure consists of PbO-rich particles/B₂O₃-rich matrix (B₂O₃-rich side of the miscibility gap), the fracture surface energy was found to decrease with increasing second-phase particles. To explain this behaviour, a crack propagation model in a brittle composite containing penetrable particles was proposed. On the other hand, for the glasses whose microstructure consists of B₂O₃-rich particles/PbO-rich matrix (PbO-rich side of the miscibility gap), an increase in fracture surface energy with volume fraction of dispersed particles was observed. This phenomenon could be best explained by Lange-Evans theory of fracture in brittle composites containing impenetrable particles. It was concluded that, when the critical crack size in a non-dispersed host glass is much larger than the particle size, the crack size in particulate composites is invariant with microstructure and also that the variation of strength results entirely from the variation of fracture toughness.