Biomedical Engineering Reference
In-Depth Information
for HAp ceramics. By contrast, in nanoindentation testing the size of the residual impres-
sions are often only a few microns and this makes it very difficult to obtain a direct measure
using optical techniques. In nanoindentation a set load (in the mN range) is applied to the
indenter in contact with the specimen. As the load is applied the penetration depth is mea-
sured (nm range). At maximum load the area of contact is determined by the depth of the
impression and the known angle or radius of the indenter. The result is a continuous mea-
surement of the load and displacement (Figure 2.12b) as a function of time that yields con-
tact pressure or hardness as well as Young's modulus from the shape of the unloading curve
using software based on the model and the diamond indenter type (pointed, i.e., Berkovich,
Vickers, Knoop, or spherical) (Field and Swain 1995). Likewise different types of loading
and unloading methods can be used to extract desired properties as a function of depth of
penetration (Fischer-Cripps 2002; Gan and Ben-Nissan 1997a, 1997b; Fischer-Cripps 2006).
In the analysis of nanoindentation the elastic modulus is obtained from the contact stiff-
ness
S
, which is the slope of the unloading portion of an indentation load-displacement
curve at maximum load (Figure 2.12b):
d
d
P
h
2
π
S
=
=
E
*
A
(2.13)
with
A
the contact area at maximum load and
E
* is the combined elastic modulus of the
indenter and specimen expressed as:
(
)
+
(
)
2
2
1
−
1
−
ν
ν
1
i
s
*
=
(2.14)
E
E
E
i
s
where
E
i
,
v
i
and
E
s
,
v
s
are Young's modulus and Poisson's ratio of the indenter and speci-
men, respectively. Equation 2.13 is valid for elastic contacts of axis-symmetric indenters
(i.e., spherical, conical, and cylindrical punches). The hardness or mean contact pressure
for indentation is obtained from the maximum load,
P
, over the projected contact area,
A
:
P
=
H
(2.15)
The contact area,
A
, is determined by the indenter geometry and the contact depth (Fischer-
Cripps 2002).
Nanoindentation not only provides a comprehensive assessment of hardness and
Young's modulus of the coating but shows the elastic-plastic response from the loading
and unloading curves based on the coating-substrate combination (i.e., soft/hard and
rigid/compliant coating on soft/hard and rigid/compliant substrate). Furthermore, many
tests can be performed in selected regions or in specific areas of interest that may show
local variations in properties. Nanoindenters have been used to measure residual stress,
coating adhesion from the load at which delamination occurs (taken from the pop-in
that corresponds to a plateau or discontinuity in the load-displacement curve) and also
function as scratch testers depending on the equipment capability (Fischer-Cripps 2002).
Similarly, creep and viscoelastic behavior can be examined (Fischer Cripps 2004; Latella
2008) for softer materials and is particularly relevant in bone, dental composites/resins,
and biological tissue studies.
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