Loading Rate Effect on Nanohardness of Soda-Lime-Silica Glass

Abstract

To understand how hardness, the key design parameter for applications of brittle solids such as glass concerning contact deformation, is affected by loading rate variation, nanoindentation with a Berkovich tip was used to measure the nanohardness of a 330-lm-thick soda-lime-silica glass as a function of loading rate (1 to 1000 mNAEs À1 ). The results showed for the very first time that, with variations in the loading rate, there was a 6 to 9 pct increase in the nanohardness of glass up to a threshold loading rate (TLR), whereafter it did not appreciably increase with further increase in loading rate. Further, the nanohardness data showed an indentation size effect (ISE) that obeyed the Meyer's law. These observations were explained in terms of a strong shear stress component developed just beneath the nanoindenter and the related shear-induced deformation processes at local microstructural scale weak links. The significant or insignificant presence of shear-induced serrations in load depth plots and corresponding scanning electron microscopic evidence of a strong or mild presence of shear deformation bands in and around the nanoindentation cavity supported such a rationalization. Finally, a qualitative picture was developed for different deformation processes induced at various loading rates in glass.

An abnormal displacement change during holding period in nanoindentation tests on zirconia dental ceramic

“…The displacement change associated with the unknown process is comparatively small and has little effect on the displacement-time relation. Second, some studies were also made on brittle ceramics and glasses [6,12,16], but with a small loading rate (low peak load to avoid the cracking due to brittleness) and/or a short holding time (considering the insignificant creep deformation). This may restrain the amount of the displacement reduction.…”

Nanoindentation Response Analysis of Thin Film Substrates-II: Strain Hardening-Softening Oscillations in Subsurface Layer

“…The sample in the subsurface layer is expected to be rather heterogeneous having different sized nanograins, nanoparticles, dislocations within nanocrystallites, nanovoids, etc. This gives rise to many different specific plastic deformation mechanisms that might occur underneath the loaded indenter during the nanoindentation experiment: slipping, twinning, cold working, grain boundary sliding, strain bursts or flow serrations, shear band formation, stick and slip events, among others [8]- [10], [13], [14].For example, nanovoids can be frequently present, which then can be packed by the applied load during the indentation process. The process of packing is largely a plastic deformation causing a strain hardening effect, which is indicated by the positive divergence of the plastic SSF divergence (H''(h) > 0).…”

“…We associate these oscillations with alternating cycles of strain hardening and strain softening processes of the elastic-plastic deformation induced during the indentation experiment. The terms strain hardening and strain softening are used here in a general sense, encompassing many possible underlying specific plastic deformation mechanisms such as slipping, twinning, serration, [8]- [10] etc. (as opposed to the restricted, historical meaning of the terms as processes caused by the dislocation creation and movement in crystalline materials).…”

Contact Deformation of Alumina

“…The effect of these deformations are revealed by the occurrence of "multiple micro-pop-in" and "multiple micropop-out" events in the load-depth plots, followed by the formation of shear deformation bands and localized microcracking in and around the indentation cavity [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] and the formation of dislocation loops in the alumina sample. The detailed mechanisms of pop-in events which occur during indentation in ceramics are far from being well understood and may be strongly sensitive to the plane bearing the nanoindentation, nanoindenter tip radius, temperature and so forth, while the microscale hardness may [38][39][40][41] or may not be [42] sensitive to strain and loading rate variation.…”

“…Recently, significant effect of loading rate on hardness of glass [21][22][23][24] and alumina [25,26,43,44] was reported. The occurrence of the pop-in behaviour was also reported for glass [21][22][23][24], polycrystalline alumina [25,26,43,44], bulk metallic glass [27][28][29], sapphire [30], GaN [31], and ZnO [32].…”