Abstract

The purpose of this work was to explore the possibility of measuring properties of viscoelastic materials by nanoindentation. Indentation is not a common method for determining properties of viscoelastic materials and nanoindentation is a very new, state-of-the art technology. Therefore, this research is one of very few works in this area. This study includes nanoindentation experiments on viscoelastic materials, determining bulk properties of the same materials by conventional rheological techniques, suggesting of physical models to measure properties of viscoelastic solids and viscoelastic liquids by nanoindentation and numerical simulations of the nanoindentation process. The experimental part includes nanoindentation tests of viscoelastic solids and viscoelastic liquids and comparing measured local properties with the bulk ones. The bulk properties were measured with SAOS and Torsion tests. For this investigation polybutadiene was selected as an example of a viscoelastic liquid and silicon cross-linked rubber as a viscoelastic solid. It was found that the local properties of solid polymers vary widely. However, by averaging data collected from various locations, the bulk properties can be determined accurately for the viscoelastic solids. For the physical modeling we validated the Sneddon & Sakai model of indentation of viscoelastic solids and suggested a model for the indentation of viscoelastic liquids, based on Stoke's theory of a potential flow around a sphere. Also nanoindentation of a viscous liquid was simulated using the FLUENT commercial code. It was found that for the indentation of viscoelastic materials with dominantly viscous properties, the indentation model developed from Stoke's theory gives realistic values for shear forces, but predicts a smaller than actual compression force, acting on the surface of the indenter. The comparison of the results of the mentioned above different approaches allowed us to draw conclusions about the advantages and limitation of the technology and theoretical analysis of nanoindentation and its application in rheometry. It was shown that nanoindentation can be successfully used for investigation of viscoelastic materials. Because of its unique abilities nanoindentation will become an irreplaceable tool in such areas as the testing of thin films, study of materials in a transient states, and biomedical research. However, there are number of technical and theoretical issues that need to be addressed. We outlined issues that need to be resolved and suggested direction for further research and development. Among them are: selection of a proper model to simulate behavior of particular viscoelastic material, further improvement of indentation control and data acquisition system, manufacturing new indenters of optimum shape and material