Challenges to Accurate Evaluation of Bulk Hardness from Nanoindentation Testing at Low Indent Depths

Estimations of bulk hardness from nanoindentation data are frequently subject to considerable uncertainties, due to indentation size effects, potential pileup effects, and potential influence of surface quality or test methods. In this study, we examined materials science principles of nanoindentation test methods to enable accurate prediction of bulk hardness for a series of high purity Fe and Fe alloys containing 3-25 wt. %Cr. These materials were tested in as-annealed and thermally aged (100-900 hours at 475 oC for 14-25 wt. %Cr) conditions to produce Cr-rich a’ precipitates of varying size and density (bulk Vickers hardness values of ~50 to 350 VHN). Nanoindentation performed with a Berkovich indenter using constant strain rate (0.05 to 0.5 /s) and constant loading rate test methods provided comparable bulk equivalent hardness (H0) extracted by Nix-Gao model, indicating a weak strain rate sensitivity of the investigated materials at room temperature. Results from electropolished and fine mechanically polished samples were found to give comparable measured hardness. Conversely, material pileup adjacent to the indented area produced a 7-20% correction to the indent contact area. The Nix-Gao fitted nanoindentation H0 after pileup corrections agreed quantitatively better with the bulk Vickers hardness than the uncorrected H0 values. The model-predicted (dispersed barrier hardening superposition) and measured strength values agreed for aged Fe18Cr, indicating that Nix-Gao model combined with pileup corrections significantly improved the accuracy of hardness evaluation by nanoindentation. Analytic and experimental analyses demonstrate that inappropriate methods such as hardness ratios or changes at a reference depth (applied in many prior nanoindentation studies) can cause quantitative errors in bulk hardness estimates as large as 60% due to indentation size effects.