Deformation mechanisms of nanoporous oxide glasses: Indentations and finite element simulation

Abstract

Nanoporous oxide glasses have a wide array of applications including medicine, sensor and optics, owing to their high thermal and chemical stability. However, the correlations between their topological structure, glass system and mechanical properties are still poorly understood. In this paper, we bridged this structure-composition-property gap by utilizing nanoindentation and finite element simulation on two series of sol-gel derived nanoporous glasses: phosphate (AP) and silicate (SA) glasses. The critical length of cracks 2a_c was found much smaller in the AP glasses (7.3 nm) than in the SA glasses (20.3 nm). As a result, the AP glasses with larger pores (6∼10 nm) were more easily crushed than the SA glasses with finer pores (1∼6 nm). Similar small physical activation volumes Ω (2∼3 nm³) have been measured in fully dense SiO₂ glass and the porous glasses with lower porosity (< 30 %) or smaller pore size (< 2 nm), indicating the catastrophic cracks initiate at the inhomogeneities of glass network, regardless of its porous structure or not. The linear dependence of Ω on pore size or porosity probably arise from the less constrained porous structure, and the shearing crack prefer to nucleate on the pore wall under compression. The deformation mechanisms of nanoporous glasses have been summarized and rationalize the evolution of brittle fracture behaviors in the SA and AP glasses. The hardness H and elastic modulus E of porous glasses were found still obey the classical Gibson & Ashby's model, which is valuable to develop novel porous glasses for future industrial applications.