180
7 Nano-Mechanical Properties of Solid Surfaces Obtained …
nano-indentation” to distinguish with the former usual test. Electrochemical nanoindentation can be achieved during a potentiostatic polarization of a solid electrode.
The present chapter deals with the nano-mechanical properties of bare metal
surface, bulk metal oxide surface, and thin oxide film on metal obtained by nanoindentation. We discuss the issues for determination of hardness from the measured
load-depth curve in addition to the indentation size effect on hardness. Furthermore,
we discuss the nano-mechanical properties of passive single crystal iron surfaces in
solution, obtained by electrochemical nano-indentation.
7.2 Fundamentals of Nano-Indentation
The nano-indentation apparatus combined with atomic force microscope (AFM) is
commercially available. The original AFM head (cantilever and detector) is replaced
by a three-plate capacitive force/displacement transducer with a shaft (tungsten rod)
on which an indenter tip is mounted. The transducer can generate the electrostatic
force used for the indentation and simultaneously measure the relationship between
load (force) and displacement of the indenter [1]. Since the transducer with the
indenter tip is mounted on the AFM, the same tip can serve as both indenter and
scanning probe, which allows the characterization of the sample surface prior to the
indentation and the immediate imaging after the load removal.
Figure 7.1 shows the geometries of diamond tips mostly employed for nanoindentation test. A Berkovich tip (see Fig. 7.1a) has a total included angle (A + B)
of 142.3° with a centerline-to-face angle (B) of 65.35°. The curvature radius of the
Berkovich tip is 100–200 nm. A cube-corner tip (see Fig. 7.1b) has a total included
angle (A + B) of 90° with a centerline-to-face angle (B) of 35.3°. The curvature
radius (40–60 nm) of the cube-corner tip is much smaller than that of the Berkovich
tip. The cube-corner tip because of sharp angle and high aspect ratio is specified to
ultra-thin films. A cono-spherical (conical) tip with a spherical end (see Fig. 7.1c)
has an included(cone) angle (C) of 140.6° or 90°, and the curvature radius (1–3 μm)
of the tip end is larger than that of the Berkovich tip. The indentation tip used for
electrochemical nano-indentation test on a solid electrode in liquid is mounted on a
long shaft which extends the tip away from the transducer to avoid the contact of the
transducer with liquid.
For nano-indentation test, the load is applied at a constant loading velocity vertically to a flat sample surface by an indenter tip, and simultaneously the displacement
of the sample surface in depth direction (depth-displacement) under the indenter is
measured as a function of load. After attaining a maximum load, the load is released
at a constant unloading velocity and the recovery in depth-displacement is measured
during unloading. After the indentation, the surface profile of the indent can be
observed from the atomic force microscopic (AFM) image measured by using the
same indenter tip. Figure 7.2 represents schematically a load-depth (displacement)
curve for nano-indentation test. During loading, the sample surface region of very
small volume under the indenter is elastically and then plastically deformed as the
7 Nano-Mechanical Properties of Solid Surfaces Obtained …
nano-indentation” to distinguish with the former usual test. Electrochemical nanoindentation can be achieved during a potentiostatic polarization of a solid electrode.
The present chapter deals with the nano-mechanical properties of bare metal
surface, bulk metal oxide surface, and thin oxide film on metal obtained by nanoindentation. We discuss the issues for determination of hardness from the measured
load-depth curve in addition to the indentation size effect on hardness. Furthermore,
we discuss the nano-mechanical properties of passive single crystal iron surfaces in
solution, obtained by electrochemical nano-indentation.
7.2 Fundamentals of Nano-Indentation
The nano-indentation apparatus combined with atomic force microscope (AFM) is
commercially available. The original AFM head (cantilever and detector) is replaced
by a three-plate capacitive force/displacement transducer with a shaft (tungsten rod)
on which an indenter tip is mounted. The transducer can generate the electrostatic
force used for the indentation and simultaneously measure the relationship between
load (force) and displacement of the indenter [1]. Since the transducer with the
indenter tip is mounted on the AFM, the same tip can serve as both indenter and
scanning probe, which allows the characterization of the sample surface prior to the
indentation and the immediate imaging after the load removal.
Figure 7.1 shows the geometries of diamond tips mostly employed for nanoindentation test. A Berkovich tip (see Fig. 7.1a) has a total included angle (A + B)
of 142.3° with a centerline-to-face angle (B) of 65.35°. The curvature radius of the
Berkovich tip is 100–200 nm. A cube-corner tip (see Fig. 7.1b) has a total included
angle (A + B) of 90° with a centerline-to-face angle (B) of 35.3°. The curvature
radius (40–60 nm) of the cube-corner tip is much smaller than that of the Berkovich
tip. The cube-corner tip because of sharp angle and high aspect ratio is specified to
ultra-thin films. A cono-spherical (conical) tip with a spherical end (see Fig. 7.1c)
has an included(cone) angle (C) of 140.6° or 90°, and the curvature radius (1–3 μm)
of the tip end is larger than that of the Berkovich tip. The indentation tip used for
electrochemical nano-indentation test on a solid electrode in liquid is mounted on a
long shaft which extends the tip away from the transducer to avoid the contact of the
transducer with liquid.
For nano-indentation test, the load is applied at a constant loading velocity vertically to a flat sample surface by an indenter tip, and simultaneously the displacement
of the sample surface in depth direction (depth-displacement) under the indenter is
measured as a function of load. After attaining a maximum load, the load is released
at a constant unloading velocity and the recovery in depth-displacement is measured
during unloading. After the indentation, the surface profile of the indent can be
observed from the atomic force microscopic (AFM) image measured by using the
same indenter tip. Figure 7.2 represents schematically a load-depth (displacement)
curve for nano-indentation test. During loading, the sample surface region of very
small volume under the indenter is elastically and then plastically deformed as the