Criticalities of iterative calibration procedures for indentation testing machines in the nano-range

The modern manufacturing industry requires to achieve thorough multiscale mechanical characterisation to qualify processes and materials. Amongst the few available methods, Instrumented Indentation Test (IIT) outstands for its capabilities of estimating hardness, Young’s modulus, stress-strains curve, creep and relaxation behaviour by means of a non-destructive procedure which analyses the force applied to the sample and the displacement of the indenter with respect to the sample surface continuously acquired during the application of a loading/unloading indentation cycle. Traceability of the characterisation is ensured by calibration procedures described in the ISO 14577-2. In particular, the calibration of the frame compliance and the indenter area function are critical as they have been proven major contributor to the mechanical characterisation measurement uncertainty. Iterative methods, that do not require the indenter area function to be independently calibrated by means of high-resolution microscopes, e.g. AFM, are largely exploited in both academia and industry. Several calibration recipes are available according to the standard and reference literature. These include different reference materials, algorithms, and testing procedures, in terms of force range and number of tests. However, literature lacks detailed works aimed at reporting the performances of the several available methods combining the possible parameters admissible for the standard. Furthermore, approaches for estimating the measurement uncertainty of this calibration are undefined in both literature and the standard. This work, exploiting a Monte Carlo approach for evaluating calibration uncertainty, discusses the results of the comparison of these calibration methods in the nano-range. Shortcomings of the standard due to the effect of calibration materials and algorithms are highlighted and discussed.