Importance of surface preparation on the nano-indentation stress-strain curves measured in metals
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In this work, we investigated experimentally the various factors influencing the extraction of indentation stress-strain curves from spherical nanoindentation on metal samples using two different tip radii. In particular, we focused on the effects of (i) the surface preparation techniques used, (ii) the presence of a surface oxide layer, and (iii) the occurrence of pop-ins at the elastic-plastic transition on our newly developed data analysis methods for extracting reliable indentation stress-strain curves. Rough mechanical polishing was shown to introduce a large scatter in the measured indentation yield strengths, whereas electropolishing or vibropolishing produced consistent results reflective of the pristine sample. The data analysis techniques used were able to discard the portions of the raw data affected by a thin oxide layer, present on most metal surfaces, and yield reasonable indentation stress-strain curves. Experiments with different indenter tip radii on annealed and cold-worked samples indicated that pop-ins are caused by delayed nucleation of dislocations in the sample under the indenter.
I. INTRODUCTION
Nanoindentation, with its high resolution load and depth sensing capabilities, is being used increasingly to characterize the local mechanical behavior in a broad range of materials systems with heterogeneous microstructures.1–4 There have been numerous attempts at the analysis of the raw load-displacement data measured by this technique to extract meaningful material properties such as hardness and modulus, mostly using sharp indenters.5,6 Historically, most of the prior data analysis methods3,5–7 have focused on the unloading segments (after some amount of elastic-plastic loading) that are believed to be purely elastic. There has also been considerable interest in the literature in extracting indentation stressstrain curves from the measured nanoindentation loaddisplacement data using both analytical8–13 and finite element14–18 methods. Recently, we have developed and validated new data analysis procedures for spherical nanoindentation19–21 that transform the entire loaddisplacement dataset, including both the loading and the unloading segments, into much more meaningful indentation stress-strain curves. These new data analysis procedures, because of their ability to analyze the initial loading segments, can potentially lead to much improved methods for the characterization of the local mechanical response at the indentation site. Sample surface a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0137
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 3, Mar 2009 Downloaded: 14 Jul 2014
preparation is expected to play a dramatically increased role in these new methods, especially for any properties extracted from the initial loading segment of the indentation dataset. Understanding the precise role of surface preparation on the measured nanoindentation data has been made difficult by several factors, including: (i) presence of a hi
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