Nanoindentation analysis as a two-dimensional tool for mapping the mechanical properties of complex surfaces
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Nanoindentation analysis as a two-dimensional tool for mapping the mechanical properties of complex surfaces Nicholas X. Randalla) CSM Instruments, Needham, Massachusetts 02494
Matthieu Vandammeb) and Franz-Josef Ulm Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 30 July 2008; accepted 11 December 2008)
Instrumented indentation (referred to as nanoindentation at low loads and low depths) has now become established for the single point characterization of hardness and elastic modulus of both bulk and coated materials. This makes it a good technique for measuring mechanical properties of homogeneous materials. However, many composite materials are composed of material phases that cannot be examined in bulk form ex situ (e.g., carbides in a ferrous matrix, calcium silicate hydrates in cements, etc.). The requirement for in situ analysis and characterization of chemically complex phases obviates conventional mechanical testing of large specimens representative of these material components. This paper will focus on new developments in the way that nanoindentation can be used as a two-dimensional mapping tool for examining the properties of constituent phases independently of each other. This approach relies on large arrays of nanoindentations (known as grid indentation) and statistical analysis of the resulting data.
I. INTRODUCTION
The nanoindentation method for assessing mechanical properties at low loads and shallow depths is well established for the characterization of thin films as well as bulk materials. The depth-sensing indentation method produces a load-displacement curve from which quantitative property values (commonly hardness and elastic modulus) can be calculated using a variety of approaches.1–3 Although nanoindentation has proved to be a powerful technique for measuring the properties of homogeneous, monolithic material systems, little work has been done in optimizing the method for use on heterogeneous (composite) materials, of which most solids are made up. It has already been shown that nanoindentation can be a valid tool for measuring the mechanical properties of small grains (grain size < 10 mm) within a composite matrix,4 but such measurements have significant disadvantages, namely: (i) A compromise always has to be made between having sufficient indentation depth to overcome surface roughness (most microstructures are polished in some way), but shallow enough depth to measure grain-only properties. a)
Address all correspondence to this author. e-mail: [email protected] b) Present address: Universite´ Paris-Est, Ecole des Ponts, UR Navier, 77455 Champs-sur-Marne, France DOI: 10.1557/JMR.2009.0149 J. Mater. Res., Vol. 24, No. 3, Mar 2009
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(ii) Most microstructures are mechanically polished, a process that can significantly work-harden the material in the near-surface region (particularly in metals)
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