The continuous stiffness measurement technique in nanoindentation intrinsically modifies the strength of the sample

Abstract

The continuous stiffness measurement (CSM) method is a well-established technique for obtaining elastic modulus and hardness data continuously during a nanoindentation process. The applicability of this technique is based on the assumption that the material properties of the specimen being tested are not affected by the imposed oscillatory excitation of the indenter. In this study, nanoindentation experiments on aluminium with a Berkovich tip show that nanometric oscillations of the CSM can lead to significant softening even though the indents made are micron-sized. The amount of softening is similar to that observed from macroscopic indentation tests with simultaneous ultrasonic excitation of similar displacement/amplitude ratios. Electron microscopy analyses reveal subgrain formation under the CSM nanoindents, which is also a feature of ultrasonically deformed bulk aluminium. The Oliver–Pharr and CSM method of hardness measurement are found to be erroneous with the CSM mode switched on.

Keywords:

• nanoindentation
• hardness
• aluminium
• continuous stiffness measurement

Acknowledgments

The work described in this paper was supported by grants from the Research Grants Council (Project No. 7159/10 E) as well as from the University Grants Committee (Project No. SEG-HKU06) of the Hong Kong Special Administrative Region.