Influence of sinking-in and piling-up on the mechanical properties determination by indentation: A case study on rolled and DMLS stainless steel P.P. Bandyopadhyay a,n , D. Chicot b , C.S. Kumar a , X. Decoopman b , J. Lesage b a Department of Mechanical Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721302, India b University Lille Nord de France, USTL, LML, CNRS-UMR 8107, F-59650 Villeneuve d’Ascq, France article info Article history: Received 27 January 2013 Received in revised form 27 March 2013 Accepted 28 March 2013 Available online 9 April 2013 Keywords: Hardness measurement Electron microscopy Steel Mechanical characterization abstract 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? & 2013 Elsevier B.V. All rights reserved. 1. Introduction Layer manufacturing (also known as rapid prototyping or RP) techniques are utilized to fabricate, layer-by-layer, objects from 3- D solid models, created using computer aided design (CAD) tools. There are several commercially available machines which utilize different building methods, e.g., 3-D printing, fused deposition modeling, selective laser sintering (SLS) or direct metal laser sintering (DMLS), and 3-D laser cladding. In the beginning, RP was mostly used for the fabrication of prototypes made from polymers as demonstration aids and inspection tools. Nowadays, research activities also focus on production of functional parts directly from metals and ceramics using these techniques [3]. In this process a thin layer of powder is first placed onto the part- building table. A laser beam is scanned onto the powder bed to form a solidified/sintered layer in a preselected region conforming to the final part geometry. In other words, the sintered layer is actually a slice of the final product. The powder in other areas is not sintered and acts as support. Next, the part building table is fed downward so that another powder layer can be placed above the sintered layer. Thus, the downfeed is equal to a layer thickness which is around 0.02–0.1 mm. Then another layer is sintered. The cycle is repeated until the 3-D part is complete. The fabrication chamber is closed and the process is performed in an inert atmosphere (nitrogen or argon) to avoid oxidation [3]. This technique is suitable for producing either porous or fully dense products by appropriate choice of process parameters. The present work addresses depth sensing indentation based measurement of mechanical properties of a sample processed using DMLS. Dewidar et al.[4] have investigated on the properties like compressive strength and elastic modulus of highly porous 316 L stainless steel fabricated using SLS for biomedical application. The compressive strength and the corresponding elastic modulus have been found in the 21–32 MPa and 26–43 GPa range, respectively. The porosity has been in the 40–50 vol% range and the properties have been found to decline with porosity. The sample porosity has been varied using various power levels, scan speeds and hatching distances. These parts are apparently suitable for biomedical applications. In another study, Uzunsoy and Chang [5] have Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A 0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.03.081 n Corresponding author. Tel.: +91 3222 282950; fax: +91 3222 282700. E-mail addresses: ppb@mech.iitkgp.ernet.in, partha.bandyopadhyay@gmail.com (P.P. Bandyopadhyay). Materials Science & Engineering A 576 (2013) 126–133