The a.c. indentation technique and its applications

Abstract

An a.c. microindentation technique, namely indenting with a small displacement modulation superimposed on an otherwise linear indenter motion, will be introduced. The working principle and theory will also be illustrated by using a mechanical model to simulate the indenter behavior. Other than being as capable as conventional indentation, the a.c. technique acquires the unloading slope simultaneously and continuously with the penetration depth and applied load during an entire indentation process. With this extra information, the conversion between the total depth and plastic depth can be executed right after a single indentation, and in turn the hardness as well as contact modulus depth profiles can be calculated. This is in contrast to the conventional indentation technique where a group of indentations associated with different maximum loads are required in order to achieve the same purpose. Furthermore, it also avoids the subjectivity in the selection of the fitting portions from the unloading stages of indentation loading curves to extract the unloading slopes as well as the plastic penetration depths. Another important advantage of using this a.c. technique is the high sensitivity in detecting the indenter/ surface contact. This advantage is very useful in the determination of the origins of penetrations depths as well as in the investigation of the evolution of the indenter contact area during unloading, and both issues are very crucial in the microhardness calculations. The strain rate effect on the hardness measurements of a 1 μm thick Al-2% Si coating has been demonstrated by using the a.c. technique. As the indenter loading speed increases from 2.5-10 nm s-1, the measured hardness of the coating can be increased from ~20 to ~80% depending on the penetration depth, and the shallower the penetration depth the larger the increment is. However, the contact modulus depth profiles remain unchanged for all the indentation rates. © 1993.