High-Throughput Nanoindentation for Statistical and Spatial Property Determination
JOM, Vol. 70, No. 4, 2018
https://doi.org/10.1007/s11837-018-2752-0
2018 The Author(s). This article is an open access publication
BEYOND INDENTATION HARDNESS AND MODULUS: ADVANCES IN NANOINDENTATION TECHNIQUES: PART II
High-Throughput Nanoindentation for Statistical and Spatial
Property Determination
ERIC D. HINTSALA,1 UDE HANGEN,2 and DOUGLAS D. STAUFFER
1,3
1.—Bruker Nano Surfaces, Eden Prairie, MN, USA. 2.—Bruker Nano Surfaces, Aachen, Germany.
3.—e-mail:
Standard nanoindentation tests are ‘‘high throughput’’ compared to nearly all
other mechanical tests, such as tension or compression. However, the typical
rates of tens of tests per hour can be significantly improved. These higher
testing rates enable otherwise impractical studies requiring several thousands
of indents, such as high-resolution property mapping and detailed statistical
studies. However, care must be taken to avoid systematic errors in the measurement, including choosing of the indentation depth/spacing to avoid overlap of plastic zones, pileup, and influence of neighboring microstructural
features in the material being tested. Furthermore, since fast loading rates
are required, the strain rate sensitivity must also be considered. A review of
these effects is given, with the emphasis placed on making complimentary
standard nanoindentation measurements to address these issues. Experimental applications of the technique, including mapping of welds,
microstructures, and composites with varying length scales, along with
studying the effect of surface roughness on nominally homogeneous specimens, will be presented.
INTRODUCTION
Nanoindentation has been proven to be a powerful tool for exploring mechanical behavior at smalllength scales. This is due to the technique being
highly localized and only semi-destructive, while
simultaneously allowing extraction of a diverse set
of properties including elastic, plastic, and fracture.
In addition, the sample preparation requirements
are significantly less stringent than most other
mechanical testing techniques, and the procedures
are well established.1–5 However, standard nanoindentation testing requires several minutes per test
for tasks such as locating suitable areas, the sample
approach, drift correction, and retraction of the tip.
This makes certain applications, such as property
mapping by indentation grids6,7 or generation of
statistical data sets, extremely time consuming and,
in some cases, too slow to be practical. Various
technologies have been developed in recent years
that have greatly accelerated nanoindentation testing, with state-of-the-art speeds of up to 6 indents/
second representing at least two orders of magnitude improvement over standard quasi-static
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testing. This means that 10,000 indent maps can
be completed in less than an hour—for perspective,
this would generate a property map of
100 9 100 lm with 1-lm spacing. The speeds, resolution, scan size, and sample preparation requirements are comparable to a variety of SEM mapping
techniques, such as electron backscatter diffraction
(EBSD) and energy dispersive spectrometry (EDS).
The fact that they also give highly complementary
information means that correlated surveying of
microstructural features for their crystallographic,
chemical and mechanical properties provides
researchers with a powerful tool. Besides this,
statistical indentation techniques allow users to
quickly determine parameters of significance,
screen materials, and identify more global trends
from highly localized nanoindentation tests.8 In
addition, statistical data sets help combat factors
producing data outliers, such as surface roughness,9–12 which are a hindrance to accurate nanoindentation testing. Lastly, these techniques can also
be applied to systems with environmental control,
such as heating, controlled humidity, or even submerged specimens, providing additional variables to
(Published online February 14, 2018)
High-Throughput Nanoindentation for Statistical and Spatial Property Determination
495
Fig. 1. (a) Measured hardness of fused quartz after indenter tip area function calibration for three Berkovich indenters of varying tip radii. A
dynamic indentation mode with a 0.2-nm displacement amplitude is capable of measuring values for low penetration depths. (b) Comparison of
purely elastic and elastic–plastic indents in fused quartz, with inset schematics of the elastic zone (light) and the plastic zone (dark). During
elastic indentation in the Hertzian regime the average contact stress is less than that required to initiate plasticity. The elastic–plastic indentation
shows hysteresis in the load-depth curve. In this case a constant hardness is observed.
explore. To date, hardness mapping has been utilized to explore spatial variations in a variety of
materials including cement pastes,13 concrete,14
tooth enamel,15 metal matrix composites,6,7,15,16
intermetallics,17 metal alloys,6,18,19 and wood adhesive bonds.20
However, properly conducting high-speed nanoindentation and interpreting its results requires one
to consider various factors, such as indentation
spacing, strain rate effects, and indentation depth.
In addition, it can currently only be applied to
measuring hardness and elastic modulus because of
the restrictions on load function choice. Thus, highspeed indentation is not a replacement for standard
indentation techniques. Rather, the approach that
is advocated is that of a complementary technique
for standard indentation, where the standard testing protocol allows one to assess indentation size
effects,21–23 rate dependence,24 and spacing effects.
Here, we will review these key concepts first before
presenting example application data emphasizing
property-mapping techniques.
EXPERIMENTAL CONSIDERATIONS FOR
NANOINDENTATION MAPPING
To produce high-quality property maps, consideration of the stress field underneath the indenter tip
is critical. Not only is there potential for the damage
zones from individual tests to overlap and invalidate the results, but the ‘‘resolution’’ of the nanoindentation test is relevant when testing near
boundaries of features, such as grain and phase
boundaries, weld zones, composite interfaces, and
material gradients, for damage or composition. The
indenter resolution needs to be carefully defined, as
the stress field occurs in three dimensions and
consists of separately sized elastic and plastic zones.
A second consideration involves the necessity of
high loading rates, which may induce strain rate
sensitivity changes in the measured hardness. Both
subjects will be covered in the following two sections, ‘‘Indentation Spacing and Resolution’’ and
‘‘Strain Rate Sensitivity’’.
Indentation Spacing and Resolution
When mapping surface properties, the in-plane
spatial resolution is of primary concern; however,
defining this requires consideration of the full threedimensional shape of the indentation stress field or
the volume of material being tested. Since th (...truncated)
