Quantitative imaging of nanoscale mechanical properties using hybrid nanoindentation and force modulation

In this article, we present a quantitative stiffness imaging technique and demonstrate its use to directly map the dynamic mechanical properties of ma terials with nanometer-scale lateral resolution. For the experiments, we us e a "hybrid" nanoindenter, coupling depth-sensing nanoindentation with scan ning probe imaging capabilities. Force modulation electronics have been add ed, enhancing instrument sensitivity and enabling measurements of time depe ndent materials properties (e.g., loss modulus and damping coefficient) not readily obtained with quasi-static indentation techniques. Tip-sample inte raction stiffness images are acquired by superimposing a sinusoidal force ( similar to1 muN) onto the quasi-static imaging force (1.5-2 muN), and recor ding the displacement amplitude and phase as the surface is scanned. Combin ing a dynamic model of the indenter (having known mass, damping coefficient , spring stiffness, resonance frequency, and modulation frequency) with the response of the tip-surface interaction, creates maps of complex stiffness . We demonstrate the use of this approach to obtain quantitative storage an d loss stiffness images of a fiber-epoxy composite, as well as directly det ermine the loss and storage moduli from the images using Hertzian contact m echanics. Moduli differences as small as 20% were resolved in the images at loads two orders of magnitude lower than with indentation, and were consis tent with measurements made using conventional quasi-static depth-sensing i ndentation techniques. (C) 2001 American Institute of Physics.