Effect of tip shape on nanomechanical properties measurements using AFM

Atomic force microscopy (AFM)-based indentation has been widely used to understand mechanical properties in conjunction with surface topography and structure at the nano-scale. In this work, nanomechanical properties of three different specimens were determined using four different AFM probes with spherical and flat-ended tips and conical tips with rounded apexes, to provide useful information for probe selection and for better interpretation of force-indentation data. These probes were modeled as a sphere, flat punch, and hyperboloid, respectively, after careful characterization to determine the elastic modulus based on contact models. Polyacrylic acid, polyvinylidene fluoride, and styrene-butadiene rubber were used as specimens. The results showed that the probe with a flat-ended tip was prone to misalignment between the flat end of the tip and specimen surface, which caused that the force-indentation data could not be interpreted using the contact model due to imperfect contact. Also, hysteresis was consistently observed in force-indentation data for the probes with a spherical tip and conical tips with rounded apexes, likely due to friction. This further resulted in significant differences in the elastic moduli of the specimens as large as 22-100% as determined from extension and retraction curves. The elastic moduli approximated by the mean of those from the extension and retraction curves generally agree with those from the instrumented indentation. Considering the uncertainties associated with the modeling of tip shape and force and indentation measurements, the probe with relatively large tip radius (e.g., ∼30 nm) was recommended for more accurate measurement for the specimen with a few GPa elastic modulus. Furthermore, the difference between elastic moduli determined from extension and retraction curves was found to increase as the ratio of contact stiffness to flexural stiffness of the AFM probe decreased. The outcome of this work is expected to provide useful information for obtaining accurate mechanical property measurements using an AFM based on a better understanding of the interaction between AFM probe and specimen.