Insight into the spontaneous breakdown of ‘Toughened glass’: From nano- to macroscale

Instances of sudden breakdown of ‘toughened glass’ (TG) without apparent causes have grave consequences. Here we present results of investigations on two types of samples – pieces of a TG door that broke down spontaneously (STG) and pieces of a TG sheet broken down intentionally (ITG). In both cases, the broken pieces have the same morphology and were studied through X-ray Diffraction (XRD), X-ray Fluorescence Spectroscopy (XRF), X-ray Absorption Near-edge Spectroscopy (XANES) at Na K-edge, Fourier Transform Infrared Spectroscopy (FTIR), Twin Mercury Porosimetry (TMP), Atomic Pair Distribution Function (PDF) and Positron Annihilation Spectroscopy (PAS). XRD and PAS showed the same structure and high defect concentration for both the samples indicating no major difference in microstructure due to aging in STG. No trace of Ni or S could be observed from XRF, casting doubt on the presence of NiS inclusions. PAS studies indicated very large concentration of free volume cavities in the STG sample, implying that mechanical instability leading to the erosion of the structure and loss of rigidity is a result of atomic motions within the glass and the associated weakening of the molecular bonds. From XRF, a small depletion in Na content was seen in STG, which became much more apparent from low angle XANES, pointing to a surface depletion. From FTIR, a reduction in the Si–O–Si bonds and an enhancement in the O–H bonds could be identified indicating a conversion of the former to Si(OH)4. The bond distances of different pair of atoms were calculated from atomic Pair Distribution Function experiments. An increase in total porosity especially in larger pores (diameter ≥100–200 µm, from TMP) were observed in STG over ITG. Based on these results, we propose a simple model in which the Si–O network is visualized as suffering from destabilization due to the leaching out of the Na-ions from the STG over time, leading to the accumulation of large pores, increase of overall porosity and the final collapse of the structure.