Effect of the continuous stiffness measurement on the mechanical properties extracted using spherical nanoindentation

The effects of the superimposed oscillating load while using the continuous stiffness measurement (CSM) technique on the extracted indentation stress–strain (ISS) curves were systematically investigated using spherical nanoindentation tests performed on a variety of samples, using probes of 1 and 100 μm radii. Particular care was afforded to the corrections that were needed on the load, displacement and stiffness signals to account for the effect of the superimposed oscillations. A decrease in the oscillation frequency from the commonly used 45 Hz caused the measured stiffness values to decrease. This resulted in questionable high strain hardening values in the ISS curves. The noise levels in the recorded stiffness signal were significantly higher while using an oscillation amplitude of 1 nm. It was concluded that these observations are artifacts of the machine dynamic control systems and do not represent the material’s true behavior. The CMS signal with a 45 Hz oscillation frequency and 2 nm amplitude appears to produce most reasonable values of stiffness values and is therefore the best choice for reliably extracting ISS curves for the materials studied.