Compact test platform for in-situ indentation and scratching inside a scanning electron microscope (SEM)

In-situ materials tests have the advantage to link visual and sensor based information during a dynamic experiment. In this thesis, a compact indenter-scratch test device has been built at EPFL-LSRO and installed inside a Scanning Electron Microscope (SEM) at EMPA-Thun. This indenter has been designed having high resolution and long-range stick-slip piezoelectric actuators to position and test the samples. The combination of the SEM high magnification / high depth of focus images and the high resolution positioning system has permitted to identify and test very particular regions of interest. The output of the test is an associated video/image sequence with the Load – Penetration depth (P × h) graph. The stick-slip actuator is a special type of inertial drive. Its working principle uses a stick phase (static friction) to drive and a quick redraw movement to create the slip phase (using dynamic friction). The movement is thus a sequence of 300-400nm steps. The drawback is an inherent backlash (from 30-100nm) and a microvibration after each slip phase. A rather low driving force (≈ N range) is another known limitation of stick-slip actuators. The first contribution of this thesis has been to evaluate the impact of using stick-slip actuators to realize indentation and scratching. For this, different materials have been tested with Cube Corner and Berkovich tips in two driving modes – continuous and stick-slip mode. The comparison has been realized visually (videos, pictures) and through the P × h graph. Scratch test has also been performed in these two driving modes, but has been limited to visual comparison only. Results during indentation have not shown perceptible differences in the P × h graphs nor in the image obtained in both modes. The P × h curve overlap has been mainly congruent in both driving modes. This has been assessed in Fused Silica, GaAs and Zn-BMG. However, a scratch in AlCu, Zr-BMG and in GaAs has revealed the presence of a visual pattern, which has been related to the actuator's slip phase. The results are discussed in the text. The second contribution is the optimization of stick-slip actuators. The goal was to obtain a given driving force with a minimum amount of jump-back. Various solutions have been proposed and then validated through simulations and experiments. A reduction of the jump-back size by factor of four has been achieved. This is considered to be the demonstration of the promising potentialities of the proposed methods. The work also includes a design guide for a SEM environment as well as an overview of several tests that have been made using the in-situ indenter. Examples apart from nanoindentation are surface patterning capability and micropillars compressions tests. The thesis has successfully demonstrated that stick-slip actuators can be an option to build compact and SEM compatible devices for material characterization, and especially for a SEM-indenter.

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