Nanoindentation Of Silicate Glasses:Insights From Molecular Dynamics Simulations

Silicate glasses is the most commonly and widely used glasses in both daily life and specific industrial application due to its good optical,mechanical and chemical properties.However,during the process of manufacturing,processing and daily usage,it is often in contact with sharp objects,which will expand the internal defects and significantly reduce its practical strength.The indentation method is a reliable test method for simulating deformation of glass by contact with a sharp object.However,the plastic deformation mechanism and microstructure evolution mechanism of glass during the indentation process are still unclear.In this project,we applied molecular dynamics simulations to model the nanoindentation process of three typical silicate glasses（NS3:75%Si O₂-25%Na₂O,NBS2:50%Si O₂-25%Na₂O-25%B₂O₃,N-BK7:74.5%Si O₂-10.2%Na₂O-10.2%B₂O₃-5.1%K₂O）.Exploring the macroscopic response and microstructure evolution of glass during indentation process at the atomic level.The specific research contents are as follows:Firstly,we investigate the plastic deformation mechanism of NS3 glass under indentation,using a sharp indenter to simulate nanoindentation with a depth of 25 nm.By analyzing the macroscopic response of NS3 glass under indentation,we find that there is an obvious pile-up area on the glass surface after unloading.In order to better study the microstructure changes of glass under indentation,we propose a new analysis method to quantitatively explore the evolution law of glass structure with indentation distance.The analysis results show that the change of atomic local coordination numbers decay exponentially with the indentation distance.Moreover,it is preliminarily concluded that the pile-up region near the indentation is caused by shear flow,while the lift-up region is caused by density changes.Next,we explore the deformation mechanisms of NBS2 and N-BK7 glasses under indentation using the same simulation method as NS3 glass.By analyzing the macroscopic response of NS3,NBS2 and N-BK7 glasses under indentation,we investigate that the height of the plie-up region after loading to maximum depth is related to the densification of the glass,mainly related to the degree of connectivity of the network in the glass structure.By analyzing the deformation mechanism of glass after indentation,we find that shear flow and densification mechanisms coexist during the plastic deformation process of glass.The plie-up region near the indentation is primarily originated from shear flow,while the lift-up part（which is relatively far away from the indent）is mainly caused by densification.This constitutes the first microscopic explanation for the lift-up region observed in recent experiments.The analysis of the microstructure evolution of the unloaded glass shows that the change of the local environment of the network modifier is more flexible than that of the network former,which is the structural origin of the shear flow.The change of Si-O-Si bond angle and the formation of boron-oxygen tetrahedron（[BO₄]）are the structural origins of volume densification,the former plays a dominant role in N-BK7 glass,while the latter plays a dominant role in NBS2 glass.Finally,we investigate the effect of B₂O₃ on glasses under nanoindentation.It’s find that B₂O₃can improve the stability of silicon oxide tetrahedral[SO₄]structure,decrease the stability between silicon-oxygen tetrahedra connected by bridging oxygen,suppress shear strain,and promote the formation of non-bridging oxygen,resulting in a more disordered and heterogeneous network structure.In summary,the macroscopic response,plastic deformation mechanism and microstructural evolution of silicate glasses under nanoindentation have been systematically studied in this thesis.The findings of this study provide a theoretical basis for improving the mechanical properties of silicate glasses,thus making it possible for the development of ultra-high-strength glass.