Effects of Buoyancy and Surface Roughness on Mechanical Characterization of Shipbuilding Steel by Immersion Instrumented Indentation

To develop the applications of nanoindentation in a liquid environment, a higher requirement has been presented for evaluating measured errors of micromechanical performance, particularly for the immersion indentation. In this study, a numerical investigation was conducted to explore the influence of buoyancy and surface roughness on the indentation responses of EH36 steel. The results show that a difference in indentation load–depth curves and mechanical properties with and without the buoyancy effect is observed as the indentation depth increases. The relative calculated errors of reduced modulus and indentation hardness are more than 25.35% and 1.92%, respectively. Meanwhile, as the surface roughness increases, a pronounced increase in the data scatter of indentation responses is observed, especially at shallow nanoindentations. The coupling effects of buoyancy and surface roughness on the deviation of indentation data exhibit a competitive relationship rather than a simple cumulative effect. The measured errors of reduced modulus at roughness values below 0.10 μm are predominantly affected by buoyancy during immersion indentations, while the surface roughness is the main factor in other cases. This study provides a comprehensive understanding of these factors and predicts the qualitative variations in micromechanical responses under assumed immersed conditions.