Simulation and analysis of nanoindentation investigating indenter tip geometry and porous microstructure using finite element methods (FEM)

Abstract

Nanoindentation is a widely used technique to detect the mechanical properties of materials at the nanoscale. However, nanoindentation is influenced by various factors such as indenter tip geometry and the microstructure of the investigated material. Understanding the impact of these factors is essential for accurate material property characterisation.The thesis is divided into two parts, focusing on the investigation of nanoindentation simulations on indenter tip geometry and porous microstructure using the finite element method. The study aims to understand the influence of the various tip geometries and bluntness on the nanoindentation response and the mechanical behaviour of the material with different pore sizes. The indenter tip geometry study was performed to understand the material behaviour using theBerkovich and conical indenters with various bluntness in nanoindentation. The Berkovich and its equivalent conical indenter had bluntness of 1, 5, 15 and 20nm. A cylindrical 3D model with a half-space was developed based on the literature studies. The model was developed using the local mesh refinement in the region of interest below the indenter tip to balance the computational efficiency and accuracy of the results. The nanoindentation response such as force-displacementcurves and hardness results were produced to compare all the indenter variations together with the ideal geometries of a tetrahedron and a cone.The study on the porous media investigates the mechanical properties of the material depending on the different pore sizes. The 3D model from the indenter tip geometry study was modified into a cuboid, where infinite elements were used to represent the half-space of the specimen. An automated pore generation mechanism was used to create porous microstructure within the specimen with a volume fraction of 0.1. The spherical pores were created in four different sizes 150, 200, 300 and 400nm. The distribution of the pores within the specimen was madestochastically to create multiple possible microstructure configurations. In total, 100 simulations were conducted for each pore size. The simulations were performed using a conical indenter for simplification, where mechanical properties such as reduced stiffness were calculated for each simulation. As a part of the study on the porous microstructure, a mesh convergence study was performed on the non-porous 3D model with infinite elements.The important findings from the study include the presence of numerical errors as a result of the numerical simulations when even the ideal indenter geometries require a minimal displacement for the converged hardness results. From the microstructure study, it was proved that the larger pores lead to lower stiffness values; however, the stiffness has larger variations due to various possible microstructure configurations. It was found that smaller pores of 150 and 200nm show stiffness near that of a homogeneous material, differing from Königsberger et al.’s[25] critical diameters of 32 to 48nm.The thesis contributes to the understanding of the influence of the blunted indenters on the mechanical response of the material and investigates the material behaviour of the microstructure with spherical pores.