Area function calibration in nanoindentation using the hardness instead of Young's modulus of fused silica as a reference value

For nanoindentation hardness and Young's modulus measurements the area function of the indenter has to be calibrated carefully since direct measurement of the contact area is not possible. Most often this is done by making a series of indentations into a material with known Young's modulus, e.g. fused silica. In this paper it will be shown that this method may lead to significant deviations in hardness values found over the lifetime of the indenter due to successive rounding of the tip. Additionally, depth-dependent hardness values may apparently be found which would falsify all hardness determinations. Using instead the hardness of fused silica as a reference value these problems will be avoided. Additionally, the calibration becomes independent of material constants such as the Young's modulus of the diamond tip and the Poisson ratios of both indenter and sample, which enter into the conventional calibration method. Finally, this paper discusses whether the indentation modulus determined by nanoindentation and the Young's modulus determined by, for example, tensile testing, are quantitatively equal. This is a precondition for the traceability of the Young's modulus method often mentioned as its main advantage.