The Mechanical Properties of Thin Polymer Films from Nanoindentation

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The Mechanical Properties of Thin Polymer Films from Nanoindentation F R Jones and S A Hayes Ceramics and Composites Laboratory, Department of Engineering Materials, The University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield, S1 3JD, UK. ABSTRACT The thermomechanical properties of thermoset polymeric materials are shown to be suitable for detecting relevant degradative processes in the matrix of a fibre composite. In space environments, the surface of the material will be modified first because of the diffusion of the active molecules. Nanoindentation has been shown to be capable of determining changes in viscoelastic properties of thin polymer films and at surfaces of bulk material. The Hysitron Triboscope has been used to characterise an epoxy resin and polycyanurate in dynamic and static scanning modes. Dynamic nanoindentation has been shown to provide a full description of the viscoelastic response. Progress has been made in analysing static indentation for time dependence.

INTRODUCTION Lightweight materials for space vehicles will by necessity need to be manufactured from fibre composite materials which employ high performance fibres such as high strength or high modulus carbon or high performance polymer fibres in a thermosetting matrix resin. The mechanical properties of thin films of the matrix resin may differ from those of the bulk resin casting because of a higher concentration of defects. In a fibre composite material where the fibre volume fraction is approximately 60%, the resin will tend to behave as a thin film rather than as a bulk material. In addition, to prevent mechanical damage to the reinforcing fibres during service, the surface of the structure will be resin rich. Thus, in space environments the degradation of the thermosetting resin is an issue which needs to be addressed, especially the changes in mechanical performance which will occur initially at the surface. Because of the diffusing nature of environments such as oxygen, ozone in their activated states or moisture, it is the properties of thin polymer films which need to be fully understood. The purpose of this paper is to demonstrate how the properties of thin polymer films can be estimated using a nanoindentation technique. This technique enables the mechanical properties of small volumes of materials to be determined. It is, therefore, practical to examine the change in mechanical performance of thin films at the surfaces of structural materials such as fibre composites. Nanoindentation is a technique whereby an indentor is placed in contact with a sample surface which is then loaded. The resistance to indentation and the depth of the indent is continuously monitored in order to obtain the load indentation curve. The analysis of this data requires a number of fitting procedures which enables the Young’s Modulus of the material to be determined. The procedures for analysing this data is generally based on that reported by Oliver and Pharr[1]. Most of the materials which have been studied are hard su

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