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Differentiating and Quantifying Material Phases via Nanoindenter-Based Techniques (invited presentation)

O. Warren
Hysitron Inc., US

Keywords: nano, characterization


There now exist numerous nanoindenter-based techniques that can differentiate individual material phases of a heterogeneous material, and in some cases, the differentiation can be done quantitatively in terms of the mechanical properties of each phase. The simplest approach is to make an array of indents and analyze the corresponding set of force vs. displacement curves to determine the spatial distribution of elastic modulus and hardness. However, at the maximum of each load cycle, this quantitative method of material phase differentiation generally involves a substantial contact area between the indenter and the sample, which hampers spatial resolution. It is possible to achieve much higher spatial resolution by carrying out true force modulation at a fixed indenter oscillation frequency while using the indenter to image the surface of the sample in a scanning probe microscopy fashion. The force modulation amplitude is kept low enough relative to the quasistatic imaging force that the indenter always maintains repulsive contact with the sample. Lock-in detection is used to monitor the amplitude and the phase of the oscillating component of the indenter displacement. Spatially-correlated images of topography and local storage and loss components of complex modulus (at the fixed oscillation frequency) can be achieved assuming extended Hertzian contact mechanics and a known radius of curvature of the indenter (the discrepancy between different methods of radius of curvature determination is now understood and will be discussed). Such sets of images can serve as high-resolution roadmaps for detailed dynamic mechanical analysis over a range of oscillation frequencies at each local point of interest. Nanoindenter-based true force modulation imaging is particularly useful for heterogeneous polymers such as phase-segregated polymer blends. The increasingly hybrid nature of nanoindenter-based techniques offers even further ways of differentiating material phases, including differentiating them on the basis of diffraction pattern evolution during in-situ mechanical testing in a transmission electron microscope.
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