6 AFM Nanoindentation Method: Geometrical Effects of the Indenter Tip

Abstract. Atomic force microscopy (AFM) nanoindentation is presently not that widespread for the study of mechanical properties of materials at the nanoscale. “Nanoindenter ” machines have greater accuracy and are presently more standardized. However, AFM could provide interesting features such as imaging the indentation impression right after the application of load. The AFM has, in fact, become an increasingly popular tool for characterizing surfaces and thin films of many different types of materials and recent developments have led to the utilization of the AFM as a nanoindentation device, increasing the accuracy of this machine. In this work a new method for nanoindentation via AFM is proposed. It allows hardness mea-surement with standard sharp AFM probes and a simultaneous high-resolution imaging (which is not achievable with standard indenters – Cube Corner and Berkovich). How the shape of the indenter and the tip radius of curvature affect the hardness measurement at the nanoscale is herein analyzed with three different approaches: experiments, numerical simulations, and theo-retical models. In particular the effect of the tip radius of curvature, which is not negligible for the real indenters, has been considered both in the nature of the indentation process, and in the practice of imaging via AFM. A complete theoretical model has been developed and it includes the effect of the tip radius of curvature as well as the variable corner angle. Through this model we have been able to define a correction factor C that permits us to evaluate the actual hardness of the material, once measured the actual geometry of the tip