A calibration round-robin protocol for nanoindentation measurements of thin film coatings

Nanoindentation has become a common technique for measuring hardness and elastic-plastic properties of materials, including coatings and thin films. In recent years, different nanoindenter instruments have been commercialized and used for this purpose. Every instrument is equipped with his own analysis software for the derivation of the hardness from the raw data. These data are mostly analyzed through the Oliver and Pharr method. In all cases, the calibration of compliance and area function are mandatory. The present work illustrates and describes a calibration procedure and an analysis approach of the raw data carried out in TiN and DLC coatings, for height different nanoindentation instruments, through several round robin experiments. Three different indenters were used: Berkovich, cube corner, spherical. It was clearly shown that the use of these common procedures consistently limited the hardness data spreading compared to same measurements performed using the instrument specific procedures. The following key recommendation for nanoindentation calibration must be followed: a) set a cut-off upper value for the penetration depth below which measurements must be considered unreliable, b) perform stiffness measurement specifically for each instrument/indenter couple, c) use a function, rather than a single value, for the stiffness, d) use a unique protocol and software of raw data analysis to limiting the data spread related to the instruments, and making the hardness data intercomparable.