Preparation and evaluation of tungsten tips relative to diamond for nanoindentation of soft materials

A simplified electrochemical etching technique has been developed to produce conical tungsten tips with radii of curvature ranging between 0.05 and 3 μm. These tips were characterized using scanning electron microscopy. Following characterization, the tungsten tips were mounted into tip holders and used in nanoindentation experiments on bulk aluminum, polystyrene, and polyethylene samples. Calculated modulus values for the polymers, based upon indentation data and known tip radii, agreed very well with modulus obtained using more macroscopic techniques (e.g., stress-strain apparatus). In the case of aluminum, calculated modulus was far lower than expected due to tip deformation during indentation. Calculated hardness values for the polymers tended to rise with indentation depth due to pressure effects that lead to densification and/or phase transformation. These results were also compared to modulus values, on the same three samples, obtained using a diamond tip with a comparable radius of curvature. For all of the samples the diamond tip data yielded larger modulus and hardness values than did the tungsten data. The diamond-based modulus and hardness values for the polymers were larger than expected due to greater adhesion between tip and sample relative to tungsten, which resulted in a larger than expected contact area. Using a new analytical method that accounts for differences in adhesion between the tip and sample, the modulus calculated using diamond and tungsten indenters was found to agree.