Influence of glass topology and medium range order on the deformation mechanisms in borosilicate glasses, a multiple length scale approach

点击翻译按钮获取中文摘要

In this project, the influence of topology and medium range order on the mechanical properties of the NBS1 and NBS2 type borosilicate glasses is studied following a multiple length scale approach. For NBS1, the degree of polymerisation depends on the pressure during glass preparation. The glass state can be further modified by local treatments like laser or ion irradiation. In NBS2, the medium range order of the glass can be changed in the bulk as well as on the local scale, despite the fact that the basic glass forming entities remain the same. After processing, the topology of the glass is characterised by Infrared and Raman spectroscopy. Moreover, by using small additions of structure indicator ions, such as Mn2+ or Cr3+, it is possible to detect structural changes by fluorescence microscopy.In conjunction with the preparation and characterisation of the different glass states, their mechanical properties are studied in a multi-scale approach. With in-situ micro-cantilever deformation experiments and pillar compression tests in the SEM, as well as the nanoindentation and impact testing methods, the elastic, plastic and fracture properties of the materials are assessed over a range of length scales and deformation conditions. Plastic flow in the glass and densification effects will be studied by indentation, as cracking can be suppressed, due to the confinement provided by the surrounding material. By changing the indenter or the specimen geometry (Berkovich, cube-cornerindenter or micropillars and small glass spheres), the effect of the stress state or sample volume on the deformation mechanism is studied. In this context, micropillars provide a test in which the hydrostatic stress component is minimised. Using microcantilevers, the maximum bending strength and, in the case of notched cantilevers, the local fracture toughness of glasses with different topology is also accessible. By varying the strain rate and test temperature in impact and indentation testing, the dynamics of the fracture and plastic deformation mechanisms in the different glass structures is quantified. After deformation, the activated densification and flow mechanisms are characterised by spectroscopy and microscopy (AFM, SEM and TEM) of the plastically deformed volumes.Of particular interest are the structural parameters which govern the length scales in the deformation and damage behaviour of the glasses. By comparison of the observed deformation mechanisms with the results on bulk metallic glasses or other glass systems, general concepts for the understanding of localised deformation and fracture behaviour in glasses will be obtained.