Nanoindentation of Viscoelastic Materials

This dissertation, "Nanoindentation of Viscoelastic Materials" by Bin, Tang, 唐斌, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled: Nanoindentation of Viscoelastic Materials Submitted by Tang, Bin B Eng (Tongji University) for the degree of doctor of Philosophy at the university of Hong Kong in December 2005 The Oliver-Pharr method is the standard nanoindentation analysis method. The method assumes that during the unloading process the contact between the tip and the sample surface is purely elastic. Unfortunately, in many cases, this contact is far from purely elastic, and it has been found that viscoelastic effects may significantly influence the accuracy of the measurement by the Oliver-Pharr method. Although viscoelastic effects on stiffness measurement have been successfully taken into account in previous studies, viscoelastic effects on contact depth measurement and other effects caused by viscoelasticity behavior have still not been seriously considered. A systematic method for dealing with viscoelastic effects during nanoindentation measurement remains to be developed. In this thesis a systematic viscoelasticity correction method to resolve this problem is proposed. Viscoelastic effects on contact depth measurement, the load-drop and load-increase phenomenon caused by viscoelastic effects are taken fully into account in this method. The validity of the proposed method was verified on different types of soft material, including polypropylene (PP), amorphous selenium (a-Se) and the cortical bone of a mouse, and the results were found to be satisfactory. The nose phenomenon was also investigated. It was found that the nose may appear on the unloading portion of load-displacement curve when the holding time at peak load is short or the unloading rate is small, and when the peak load is high enough. It was found that relationship between the loads at which the nose appears and the unloading rate is linear. A linear viscoelasticity analysis performed to interpret this effect predicts that there is a linear variation between the nose load and the unloading rate, and the slope of such a linear variation is also shown to be proportional to the viscosity of the material. Therefore, by measuring the "nose-load," the viscosity of the material can be obtained. A critical assessment was also made of the surface-referencing technique, which has been developed to eliminate thermal drift effects. It was found that the surface-referencing technique may cause a problematic artificial drift due to the viscoelastic effects during the nanoindentation. A possible solution is proposed to overcome this problem. Nanoindentation was also adopted to measure the mechanical properties of amorphous selenium below glass transition. The viscoelasticity correction method is employed to measure the modulus. The measured Young's modulus exhibits a strong decreasing trend from about 10 GPa to 4.4 GPa as the temperature increases from about 302 K to 309 K. Two new methods are also proposed in this study to 12 measure the viscosity. The measured shear viscosity decreases from about 1 10 10 Pa-s to about 2 10 Pa-s in same range. The activation energy for the viscous creep process was found to be about 463 kJ/mol. DOI: 10.5353/th_b3655408 Subjects: Viscoelastic materials Nanotechnology