Inconsistent nanoindentation test hardness using different Berkovich indenters

Abstract

When testing the nanoindentation hardness of the same sample with different Berkovich indenters, inconsistencies arise even when the frame stiffness and indenter tip area functions are regularly calibrated. This phenomenon has not been fully understood, making it challenging to accurately test and compare material hardness data from different laboratories. To address this issue, incremental test hardness caused by indenter tip defects and indentation size effects (ISE) is quantified. The ISE correction method proposed in this study is based on the Nix-Gao model for nanoindentation load-indentation depth (P-h) curves, and an area function equivalent modeling method is used to reproduce the geometry of Berkovich indenters in finite element (FE) analysis. Although the test hardness using different Berkovich indenters is inconsistent, their intrinsic hardness remains consistent. The proposed ISE correction method is validated by FE simulation results for A508-3 and 316 L steel that match perfectly with the ISE-corrected experimental P-h curves. The analysis of load increments shows that tip defects affect the test result through statistically stored dislocation (SSD) and geometrically necessary dislocation (GND). Indenter tip defects mainly affect the nanoindentation test results through GND. The smaller the indenter tip radius, the higher the GNDs, and the higher the test hardness. The different effects of indenter tip defects on GND lead to inconsistent hardness measurements using different Berkovich indenters.