Nanoindentation response of diatom frustules

Diatom frustules have been identified as potential candidate materials for nanotechnology applications. Current research determined the mechanical properties of frustules of the centric diatom Coscinodiscus concinnus using Berkovich Nanoindentation, and conducted finite element analysis to study the nanoindentation response of diatom frustules, so as to evaluate diatom frustules for their engineering applications.; A series of nanoindentation tests were performed on the outer surfaces of frustules at various locations. Analysis of the indentations revealed that the stiffness and hardness values appear to be strongly dependent on the location of the indentation. The stiffness varied from 0.591 GPa to 2.768 GPa in the center and 0.347-2.446 GPa at locations away from the center. Similarly, the frustules hardness varied between 0.033-0.116 GPa in the center and between 0.076-0.12 GPa away from the center.; Another series of nanoindentation tests were performed on the frustules (positioned in both concave and convex orientations) at various locations to analyze the failure modes. It was found that the failure modes in each of the orientation were also drastically different. In convex orientation cracks initiated along the sharp edges of the indentation, followed by circular ring cracks, whereas in concave orientation only cracks along the sharp edges (corresponding to the three edges of the indenter) were revealed.; The finite element analysis of Berkovich Nanoindentation on diatom frustules was conducted using four different approaches so as to gain better insight into its mechanical response.; Sphere shell model under point load at the pole was first compared with Reissner's and Koiter's theories. The deformation of sphere shell was further studied by varying the point load position, and the contour plots of vertical displacement, max principal stress and direction were made and compared with experimental results.; The load displacement response of Berkovich nanoindentation was analyzed by varying the relative material properties in a 2D axial symmetrical FEA model. It was found that final depth, maximum load, and elastic recovery are linear with (E/Y)0.4, EY1/8, and ln(Y/E), respectively.; The effect of pore size and distribution on the material properties and load displacement curves were studied in 3D porous shell FEA models. It was found that the effective stiffness is linear with pore number and quadric with pore size. The above two relationships were simplified to a linear curve between the effective stiffness and the porosity of the frustules.; 3D solid and porous FEA models were defined by only one sixth of the indenter and frustules to simulate and compare the experimental load-displacement curves. It shows that the porous model has lower stiffness and yield stress than the solid model. The introduction of the pores into the models causes decrease in the effective stiffness of the shell.; An elastic plastic cracking (EPC) model was incorporated into ABAQUS by the user subroutine VUMAT to simulate the cracks during the indentation and to determine the fracture toughness of the frustule. Radial crack and ring crack during loading were observed in the EPC model, but the lateral cracks during unloading were not found. It was found that the fracture toughness (0.05 MPa m ) of the frustule is much lower than the values of solid silica glass or fused silica.; Current research on the mechanical properties and finite element analysis results of Berkovich Nanoindentation on diatom frustules are important in the evaluation of diatom frustules in their engineering applications. For example, knowledge of the effect of pore size and pore distribution on the material properties can help diatomists determine the ideal structure of the frustules to engineer the desired material properties in the intended applications. Determination of stiffness, hardness, and fracture toughness and their distribution on the frustule surfaces can help...