Influence of sinking-in and piling-up on the mechanical properties determination by indentation: A case study on rolled and DMLS stainless steel

The recent development of the instrumented indentation technique allows the determination of the mechanical properties of materials by analyzing the load–depth curve. In hard materials a “sinking-in” type deformation is observed during indentation and the contact area for such materials is usually calculated using the methodology proposed by Oliver and Pharr [1]. However, in softer materials, “piling-up” instead of “sinking-in” occurs along the edges of the residual indent and, in this case, the methodology proposed by Hochstetter et al. [2] is appropriate. In this work, the above methods proposed by Oliver and Pharr [1] and Hochstetter et al. [2] have been applied for characterizing the mechanical properties of austenitic stainless steel and another stainless steel consolidated using direct metal laser sintering, respectively. These two materials have shown different modes of deformation around the indent. While piling-up has occurred for the commercial stainless steel, sinking-in has been observed in the case of the laser sintered samples. It has been possible to obtain reasonable values of elastic modulus for both materials using the methods stated above. These methods have also been utilized to calculate the contact area during indentation for the purpose of hardness determination. As predicted by theory, the contact hardness value is higher than Martens hardness when sinking-in occurs and the same is lower in the case of piling-up. It has also been observed that the indentation size effect uniformly affects all hardness calculations. Finally, the findings of this analysis open the door to a question: What is the best hardness definition for characterizing the hardness behavior of a material?