- Email: [email protected]
Probing more than the surface
Probing more than the surface Nanoindentation is a powerful method for quantitative mechanical property determination ideally suited for measuring small scale and thin film materials. The unique and complimentary capabilities of atomic force microscopy (AFM) and nanoindentation in combination have enabled the development of nanomaterials research to be brought to the forefront in recent years. The more recent advancement of in-situ scanning probe microscopy (SPM) imaging, where the nanoindenter tip is simultaneously used as a 3D imaging device combined with nanoindentation has enabled a new wave of novel materials research to progress. Michelle E. Dickinson1*, Jeffrey P. Schirer2 1Chemical and Materials Engineering Department, The University of Auckland, Auckland, New Zealand 2Hysitron, Inc., Minneapolis, Minnesota, USA *E-mail: [email protected]
Since the 1800’s when Friedrich Mohs introduced a simple scratch
46
Measuring at the nano and micro level provides a unique set of
tester, the use of relative hardness as a way to characterize
challenges; however nanoindentation has progressed to become a
mechanical properties has evolved through Knoop, Vickers
relatively simple technique suitable for many material types. Typically
and Rockwell tests. These indentation tests have enabled the
force, displacement and time are recorded simultaneously while a
measurement of hardness of materials to become commonplace;
nanoindentation tip is pushed into the sample under a controlled
however, the advancement of miniaturized devices and nanoscale
load. The forces applied during nanoindentation can be as small as a
films has necessitated mechanical property measurements at
few nanoNewtons or as large as several Newtons enabling a range of
a much smaller scale. This led to the development of macro-
size scales to be studied. Nanoindentation tests are output as a load-
and eventually nano-indenters1 and enable testing of a variety
displacement curve as shown in Fig.1a which can be analyzed using
of materials including low-k-dielectrics, diamond-like-carbon
well defined equations2,3 to calculate the mechanical properties of
coatings, composites, polymers and biomaterials. Nanoindentation
the sample. The shape of the load-displacement curve can provide
and SPM imaging are inherently complimentary techniques
crucial information as to how the material responds showing ductility
providing valuable material information at the nanoscale and the
or brittleness, dislocation motion and creep behavior. For more
TriboScope™ (Hysitron, Inc.) was the first commercial product to
quantitative calculations, the stiffness can be deduced in many
provide these techniques in combination.
materials by measuring the slope of the tangent line at the initial point
JULY-AUGUST 2009 | VOLUME 12 | NUMBER 7-8
ISSN:1369 7021 © Elsevier Ltd 2009
Probing more than the surface
APPLICATIONS
of unloading enabling hardness and modulus to be calculated. Due
force (similar to contact mode AFM), and any changes in this due to
to the small volume sampled during nanoindentation, the test can be
topography or surface roughness are monitored through the movement
highly localized to a specific microstructural feature or within a thin film
of the z-piezo and recorded. This force change as a function of position
on a substrate.
results in a map of the surface which can be carried out pre- and postindentation enabling position control and confirmation that the tests
When working with nanostructured materials, the samples of interest are typically scattered over a substrate. Ensuring that the
were accurately placed (Fig. 1c). The advantages of using the same tip
particle of interest is tested independently of the substrate has proved
to image the surface and mechanically test the sample are plentiful,
difficult due to the particle size falling below the optical microscope
and include position accuracy, time saving, data location confirmation
resolution. The primary issues arising when testing such small structures
and local surface roughness measurements. There are many examples of the importance of this combination
includes the requirement of test position accuracy and determination of test location after the measurement. AFM is one method of scanning
of mechanical testing and in-situ imaging. A recent study pushes
the topography of the surface to identify nanoscale structures, and
the boundaries of in-situ SPM imaging and enables the mechanical
TriboScope systems can be used on an AFM platform to enable
properties of an individual ZnO nanorod to be measured (Fig. 2).
nanoindentation testing and AFM scanning. However, a more efficient
ZnO nanostructures show a promising future for applications in
and accurate technique was developed using the principles of AFM
optoelectronic, electromechanical and electrochemical nanodevices4-6
known as in-situ SPM imaging which is standard on the TriboIndenter™
due to their unique semiconducting and piezoelectric properties.
and Ubi™ platforms (Hysitron, Inc.).
Understanding the mechanical properties is crucial for predicting the failure mechanics when using these structures in such devices. The
In-situ SPM nanoindentation imaging is a technique which uses a piezo or closed loop scanner to raster scan the indenter tip across the
difficulty with testing these nanorods and nanotubes is they are usually
sample surface while monitoring the force between the tip and the
freestanding and cannot be held easily with a clamping or mounting
sample (Fig. 1b). The system is set to scan at a predefined constant
mechanism. The low loads used by SPM imaging provide a suitable
(a)
(b)
(c)
Fig.1 a) Schematic of nanoindenter capable of indentation and raster scanning to produce a topographical image. b) Example of topographic image created using in-situ SPM imaging showing sample surface after nanoindentation test with residual indent impression visible. c) Example load-displacement curve from nanoindentation test showing measurements required for mechanical property calculation.
JULY-AUGUST 2009 | VOLUME 12 | NUMBER 7-8
47
APPLICATIONS
Probing more than the surface
(a)
(b)
(c)
Fig. 2 a) Force-displacement curve from nanoindentation test on ZnO nanorod. b) In-situ SPM image of ZnO nanorod before and c) after indentation test, residual indents are visible along rod axis (shown by arrows).
solution for such structures without the need for a gripping device
of scatter due to different components within the sample being
which may damage these delicate samples. The shape and size of the
tested. There is great difficulty in extrapolating out which data point
rod are easily measured from the gradient (imaging setpoint error)
correlates to which structural component and typically this would
image created (Fig 2b) and the desired nanoindentation test locations
have been carried out using an external Scanning Electron Microscope
can be defined. The same indentation tip is then used to indent the
(SEM) or AFM to analyze the sample surface and attempt to find the
nanorod along its axis resulting in a load-displacement curve for each
residual impressions. This has been compared to searching for a needle
test7.
in a haystack due to the very small size of the indents in comparison
Fig 2a shows one of the curves obtained from an indentation on
the rod and the small indent penetration depth of 33 nm and low force
to the whole sample and is very time consuming. In-situ SPM
of 100 μN can be seen. Information from this curve can be used to
imaging solved this issue by knowing the exact location that the tests
obtain hardness and modulus values; however this data is only useful
were made.
if there is certainty that the center of the nanorod itself was tested
during cell differentiation8. The wood structure consists of hollow
errors. The in-situ SPM imaging technique can easily scan the surface
tubular cells which are of interest in industry where moisture resistant
again after the mechanical test portion and provides an image clearly
treatments and strengthening fillers are commonly impregnated into
showing the residual impression of each of the seven indentation
the structure9. The cell walls add strength and toughness and the
measurements taken along the rod’s axis. This is just one example of
hollow intracellular spaces contribute to ductility and compliance.
how the combination of in-situ SPM imaging with the nanoscale force
Testing the mechanical properties of such heterogeneous materials
and displacement capabilities of the nanoindenter allows miniature
without an imaging technique is usually carried out by running a
structures such as individual nanorods to be tested.
series of indentation tests in a line, spacing the indents evenly apart
Results from mechanical testing of heterogeneous materials such as biological materials and composites usually have a large amount
48
Wood is a natural composite consisting of several layers formed
as deviation from this location can result in edge and substrate effect
JULY-AUGUST 2009 | VOLUME 12 | NUMBER 7-8
across the sample surface. This leads to a wide range of hardness and modulus measurements due to indenting different structures within
Probing more than the surface
APPLICATIONS
(a)
(b)
(c)
Fig. 3 a) Plot showing results from nanoindentation test on wood cross section from line profile (red) and targeted location (green) tests. b) In-situ SPM image of sample indicating position of indents for each test. c) In-situ SPM image showing residual indents after test.
the material without identifying which structure was tested. The red
where the indent number on the image correlates to the same indent
data in Fig 3a shows results from a line profile along a wood structure
number on the plot in Fig. 3a. Confirmation of the location of each test
in which 14 indents were made 1.3 μm apart. This data has a large
can be carried out by locating the residual indent impression left on
amount of scatter and the results range from 5.1 GPa to 10.2 GPa
the cell wall after testing (Fig. 3c). The numbered position data enables
with no indication as to which part of the wood substructure has
not only the properties of the cell wall to be obtained, but also shows
been tested. From this line profile it is difficult to distinguish the cell
differences in the properties of the wall between neighboring cells.
wall properties from that of the intracellular matter, although use
Without this it would be much more difficult to interpret any scatter
of an SEM or AFM post-test to determine this may be possible. The
in the data as cell to cell differentiation or to position the indents
importance of the in-situ SPM imaging technique is easily shown in
accurately on different cells.
Fig. 3b where the cross section of the same wood sample is taken.
Nanocomposites are a new class of composites consisting of
The red dots across the image show the actual positions from the line
particle filled polymers with at least one dimension of the particle
profile test and provide visualization that half of the tests were carried
in the nanometer range10. They typically have a unique combination
out inside the cell core and half across the cell wall explaining the
of properties such as fire resistance and increased stiffness and
variation in results. The power of the in-situ SPM technique is really
toughness while also having low production costs11,12. This makes them
shown by using the image to identify and select the exact locations
ideal materials for the automotive and construction industry where
of interest for testing rather than just measuring properties in a line.
the combination of low cost and improved performance is always
The mechanical properties of the cell wall itself are of interest in most
desired. One question that often arises with nanocomposites is how
wood research, and the hollow cell core is typically empty space of
the properties of the individual particle affects the bulk composite
no mechanical interest. To ensure efficient use of time and relevant
properties and how the particles deform under load when constrained
data, test locations only along the cell wall can be defined as shown
under a polymer matrix. Typically particles used in nanocomposites
in green in Fig. 3b. The results from this targeted approach are shown
are clay based, although sometimes glass particles are used. This
JULY-AUGUST 2009 | VOLUME 12 | NUMBER 7-8
49
APPLICATIONS
Probing more than the surface
(a)
(b)
Fig. 4 In-situ SPM image of two nanocomposite samples showing a) brittle cracking, b) particle breakdown, after 3000 μN indentation load applied to each particle in polymer matrix.
composition results in a brittle particle within a ductile matrix, and the failure mechanism of the particle can relate to energy absorption and thus toughness of the bulk composite material. The strength of in-situ SPM imaging has already been shown as a useful technique for indent
⎛E⎞ Kc = α ⎜ ⎟ ⎝ H⎠
1
2
⎛Pmax⎞ ⎜ 3⎟ ⎜ c 2⎟ ⎝ ⎠
where α is a tip shape constant, E is elastic modulus and H is
positioning, however the technique can also be used to visualize the
hardness, both of which are calculated from the nanoindentation test.
type of failure mechanism as well as calculate fracture toughness in
This unique way of measuring fracture toughness of a small particle
some particles.
while confined in a soft matrix is a simple yet powerful technique for
Fig. 4 shows SPM images of two different particles within a polymer matrix after one indentation test at the same load. Comparison of the two shows distinct differences in the failure mechanisms between the
advanced materials where both nanoscale properties and microscale properties are of interest. The development of new nanoscale functional materials has been
particles with one particle showing brittle behavior in the form of cracks
rapid over the past decade and with the introduction of in-situ SPM
initiating from the corners of the indenter tip and the other particle
imaging and nanoindentation the mechanical characterization of such
showing catastrophic failure and breaking up of the particle structure.
materials has been made possible. The simplicity of the technique
In addition to the nature of the failure within each particle, the crack
combined with the power of quantitative mechanical property
lengths in Fig. 4a can be measured and used to calculate the fracture
measurement has enabled novel nanostructured and smart materials
toughness of the particle13.
to progress as well as provide a fundamental understanding of how
Using a sharp indentation tip, radial cracking can occur after a
the material properties at this small scale can be independent of
critical load has been reached due to stress concentrations from the tip
those more defined bulk parameters. With the speed of technical
edges14,15. Taking the maximum indentation load (Pmax) and measuring
developments in nanoindentation technology and the continual
the crack length (c) in the resultant image, the fracture toughness (Kc)
progression to better force and displacement resolutions, the future of
of the material can be estimated using:
nanomechanical testing is endless.
REFERENCES 1. Fischer-Cripps, A.C., Nanoindentation, Springer, New York, (2004), 21
10. Alexandre, M., Dubois, P., Mat. Sci. and Eng. R (2000) 28, 1
3. Oliver, W.C., Pharr, G.M., J. Mater. Res. (1992) 7, 1564
11. Jordan, J., et al., Mat. Sci. and Eng. A (2005) 393, 1
4. Law, M., et al., Nature Materials (2005) 4, 455 5. Yang, R., et al., Nature Nanotechnology (2009) 4, 34
12. Zhang, S., Ali, N., Nanocomposite Thin Films and Coatings, Imperial College Press, London (2006) 281
6. Kang, B.S., et al., Sensors (2006) 6, 643
13. Fischer-Cripps, A.C., Nanoindentation, Springer, New York, (2004), 208
7.
14. Volinsky, A., et al., Thin Solid Films, (2003) 429, 201
Li, X., et al., Nano Letters (2003) 3, 1495
8. Koponen, S., et al., Wood Sci. and Tech. (2004) 25, 1432
50
9. Gindl, W., et al., Appl. Phys. A (2004) 79, 1432
2. Doerner, M.F., Nix, W.D., J. Mater. Res (1986) 1, 601
JULY-AUGUST 2009 | VOLUME 12 | NUMBER 7-8
15. Lawn, B., Wilshaw, R., J. Mat. Sci, (1975) 10, 1049
Since the 1800’s when Friedrich Mohs introduced a simple scratch
46
Measuring at the nano and micro level provides a unique set of
tester, the use of relative hardness as a way to characterize
challenges; however nanoindentation has progressed to become a
mechanical properties has evolved through Knoop, Vickers
relatively simple technique suitable for many material types. Typically
and Rockwell tests. These indentation tests have enabled the
force, displacement and time are recorded simultaneously while a
measurement of hardness of materials to become commonplace;
nanoindentation tip is pushed into the sample under a controlled
however, the advancement of miniaturized devices and nanoscale
load. The forces applied during nanoindentation can be as small as a
films has necessitated mechanical property measurements at
few nanoNewtons or as large as several Newtons enabling a range of
a much smaller scale. This led to the development of macro-
size scales to be studied. Nanoindentation tests are output as a load-
and eventually nano-indenters1 and enable testing of a variety
displacement curve as shown in Fig.1a which can be analyzed using
of materials including low-k-dielectrics, diamond-like-carbon
well defined equations2,3 to calculate the mechanical properties of
coatings, composites, polymers and biomaterials. Nanoindentation
the sample. The shape of the load-displacement curve can provide
and SPM imaging are inherently complimentary techniques
crucial information as to how the material responds showing ductility
providing valuable material information at the nanoscale and the
or brittleness, dislocation motion and creep behavior. For more
TriboScope™ (Hysitron, Inc.) was the first commercial product to
quantitative calculations, the stiffness can be deduced in many
provide these techniques in combination.
materials by measuring the slope of the tangent line at the initial point
JULY-AUGUST 2009 | VOLUME 12 | NUMBER 7-8
ISSN:1369 7021 © Elsevier Ltd 2009
Probing more than the surface
APPLICATIONS
of unloading enabling hardness and modulus to be calculated. Due
force (similar to contact mode AFM), and any changes in this due to
to the small volume sampled during nanoindentation, the test can be
topography or surface roughness are monitored through the movement
highly localized to a specific microstructural feature or within a thin film
of the z-piezo and recorded. This force change as a function of position
on a substrate.
results in a map of the surface which can be carried out pre- and postindentation enabling position control and confirmation that the tests
When working with nanostructured materials, the samples of interest are typically scattered over a substrate. Ensuring that the
were accurately placed (Fig. 1c). The advantages of using the same tip
particle of interest is tested independently of the substrate has proved
to image the surface and mechanically test the sample are plentiful,
difficult due to the particle size falling below the optical microscope
and include position accuracy, time saving, data location confirmation
resolution. The primary issues arising when testing such small structures
and local surface roughness measurements. There are many examples of the importance of this combination
includes the requirement of test position accuracy and determination of test location after the measurement. AFM is one method of scanning
of mechanical testing and in-situ imaging. A recent study pushes
the topography of the surface to identify nanoscale structures, and
the boundaries of in-situ SPM imaging and enables the mechanical
TriboScope systems can be used on an AFM platform to enable
properties of an individual ZnO nanorod to be measured (Fig. 2).
nanoindentation testing and AFM scanning. However, a more efficient
ZnO nanostructures show a promising future for applications in
and accurate technique was developed using the principles of AFM
optoelectronic, electromechanical and electrochemical nanodevices4-6
known as in-situ SPM imaging which is standard on the TriboIndenter™
due to their unique semiconducting and piezoelectric properties.
and Ubi™ platforms (Hysitron, Inc.).
Understanding the mechanical properties is crucial for predicting the failure mechanics when using these structures in such devices. The
In-situ SPM nanoindentation imaging is a technique which uses a piezo or closed loop scanner to raster scan the indenter tip across the
difficulty with testing these nanorods and nanotubes is they are usually
sample surface while monitoring the force between the tip and the
freestanding and cannot be held easily with a clamping or mounting
sample (Fig. 1b). The system is set to scan at a predefined constant
mechanism. The low loads used by SPM imaging provide a suitable
(a)
(b)
(c)
Fig.1 a) Schematic of nanoindenter capable of indentation and raster scanning to produce a topographical image. b) Example of topographic image created using in-situ SPM imaging showing sample surface after nanoindentation test with residual indent impression visible. c) Example load-displacement curve from nanoindentation test showing measurements required for mechanical property calculation.
JULY-AUGUST 2009 | VOLUME 12 | NUMBER 7-8
47
APPLICATIONS
Probing more than the surface
(a)
(b)
(c)
Fig. 2 a) Force-displacement curve from nanoindentation test on ZnO nanorod. b) In-situ SPM image of ZnO nanorod before and c) after indentation test, residual indents are visible along rod axis (shown by arrows).
solution for such structures without the need for a gripping device
of scatter due to different components within the sample being
which may damage these delicate samples. The shape and size of the
tested. There is great difficulty in extrapolating out which data point
rod are easily measured from the gradient (imaging setpoint error)
correlates to which structural component and typically this would
image created (Fig 2b) and the desired nanoindentation test locations
have been carried out using an external Scanning Electron Microscope
can be defined. The same indentation tip is then used to indent the
(SEM) or AFM to analyze the sample surface and attempt to find the
nanorod along its axis resulting in a load-displacement curve for each
residual impressions. This has been compared to searching for a needle
test7.
in a haystack due to the very small size of the indents in comparison
Fig 2a shows one of the curves obtained from an indentation on
the rod and the small indent penetration depth of 33 nm and low force
to the whole sample and is very time consuming. In-situ SPM
of 100 μN can be seen. Information from this curve can be used to
imaging solved this issue by knowing the exact location that the tests
obtain hardness and modulus values; however this data is only useful
were made.
if there is certainty that the center of the nanorod itself was tested
during cell differentiation8. The wood structure consists of hollow
errors. The in-situ SPM imaging technique can easily scan the surface
tubular cells which are of interest in industry where moisture resistant
again after the mechanical test portion and provides an image clearly
treatments and strengthening fillers are commonly impregnated into
showing the residual impression of each of the seven indentation
the structure9. The cell walls add strength and toughness and the
measurements taken along the rod’s axis. This is just one example of
hollow intracellular spaces contribute to ductility and compliance.
how the combination of in-situ SPM imaging with the nanoscale force
Testing the mechanical properties of such heterogeneous materials
and displacement capabilities of the nanoindenter allows miniature
without an imaging technique is usually carried out by running a
structures such as individual nanorods to be tested.
series of indentation tests in a line, spacing the indents evenly apart
Results from mechanical testing of heterogeneous materials such as biological materials and composites usually have a large amount
48
Wood is a natural composite consisting of several layers formed
as deviation from this location can result in edge and substrate effect
JULY-AUGUST 2009 | VOLUME 12 | NUMBER 7-8
across the sample surface. This leads to a wide range of hardness and modulus measurements due to indenting different structures within
Probing more than the surface
APPLICATIONS
(a)
(b)
(c)
Fig. 3 a) Plot showing results from nanoindentation test on wood cross section from line profile (red) and targeted location (green) tests. b) In-situ SPM image of sample indicating position of indents for each test. c) In-situ SPM image showing residual indents after test.
the material without identifying which structure was tested. The red
where the indent number on the image correlates to the same indent
data in Fig 3a shows results from a line profile along a wood structure
number on the plot in Fig. 3a. Confirmation of the location of each test
in which 14 indents were made 1.3 μm apart. This data has a large
can be carried out by locating the residual indent impression left on
amount of scatter and the results range from 5.1 GPa to 10.2 GPa
the cell wall after testing (Fig. 3c). The numbered position data enables
with no indication as to which part of the wood substructure has
not only the properties of the cell wall to be obtained, but also shows
been tested. From this line profile it is difficult to distinguish the cell
differences in the properties of the wall between neighboring cells.
wall properties from that of the intracellular matter, although use
Without this it would be much more difficult to interpret any scatter
of an SEM or AFM post-test to determine this may be possible. The
in the data as cell to cell differentiation or to position the indents
importance of the in-situ SPM imaging technique is easily shown in
accurately on different cells.
Fig. 3b where the cross section of the same wood sample is taken.
Nanocomposites are a new class of composites consisting of
The red dots across the image show the actual positions from the line
particle filled polymers with at least one dimension of the particle
profile test and provide visualization that half of the tests were carried
in the nanometer range10. They typically have a unique combination
out inside the cell core and half across the cell wall explaining the
of properties such as fire resistance and increased stiffness and
variation in results. The power of the in-situ SPM technique is really
toughness while also having low production costs11,12. This makes them
shown by using the image to identify and select the exact locations
ideal materials for the automotive and construction industry where
of interest for testing rather than just measuring properties in a line.
the combination of low cost and improved performance is always
The mechanical properties of the cell wall itself are of interest in most
desired. One question that often arises with nanocomposites is how
wood research, and the hollow cell core is typically empty space of
the properties of the individual particle affects the bulk composite
no mechanical interest. To ensure efficient use of time and relevant
properties and how the particles deform under load when constrained
data, test locations only along the cell wall can be defined as shown
under a polymer matrix. Typically particles used in nanocomposites
in green in Fig. 3b. The results from this targeted approach are shown
are clay based, although sometimes glass particles are used. This
JULY-AUGUST 2009 | VOLUME 12 | NUMBER 7-8
49
APPLICATIONS
Probing more than the surface
(a)
(b)
Fig. 4 In-situ SPM image of two nanocomposite samples showing a) brittle cracking, b) particle breakdown, after 3000 μN indentation load applied to each particle in polymer matrix.
composition results in a brittle particle within a ductile matrix, and the failure mechanism of the particle can relate to energy absorption and thus toughness of the bulk composite material. The strength of in-situ SPM imaging has already been shown as a useful technique for indent
⎛E⎞ Kc = α ⎜ ⎟ ⎝ H⎠
1
2
⎛Pmax⎞ ⎜ 3⎟ ⎜ c 2⎟ ⎝ ⎠
where α is a tip shape constant, E is elastic modulus and H is
positioning, however the technique can also be used to visualize the
hardness, both of which are calculated from the nanoindentation test.
type of failure mechanism as well as calculate fracture toughness in
This unique way of measuring fracture toughness of a small particle
some particles.
while confined in a soft matrix is a simple yet powerful technique for
Fig. 4 shows SPM images of two different particles within a polymer matrix after one indentation test at the same load. Comparison of the two shows distinct differences in the failure mechanisms between the
advanced materials where both nanoscale properties and microscale properties are of interest. The development of new nanoscale functional materials has been
particles with one particle showing brittle behavior in the form of cracks
rapid over the past decade and with the introduction of in-situ SPM
initiating from the corners of the indenter tip and the other particle
imaging and nanoindentation the mechanical characterization of such
showing catastrophic failure and breaking up of the particle structure.
materials has been made possible. The simplicity of the technique
In addition to the nature of the failure within each particle, the crack
combined with the power of quantitative mechanical property
lengths in Fig. 4a can be measured and used to calculate the fracture
measurement has enabled novel nanostructured and smart materials
toughness of the particle13.
to progress as well as provide a fundamental understanding of how
Using a sharp indentation tip, radial cracking can occur after a
the material properties at this small scale can be independent of
critical load has been reached due to stress concentrations from the tip
those more defined bulk parameters. With the speed of technical
edges14,15. Taking the maximum indentation load (Pmax) and measuring
developments in nanoindentation technology and the continual
the crack length (c) in the resultant image, the fracture toughness (Kc)
progression to better force and displacement resolutions, the future of
of the material can be estimated using:
nanomechanical testing is endless.
REFERENCES 1. Fischer-Cripps, A.C., Nanoindentation, Springer, New York, (2004), 21
10. Alexandre, M., Dubois, P., Mat. Sci. and Eng. R (2000) 28, 1
3. Oliver, W.C., Pharr, G.M., J. Mater. Res. (1992) 7, 1564
11. Jordan, J., et al., Mat. Sci. and Eng. A (2005) 393, 1
4. Law, M., et al., Nature Materials (2005) 4, 455 5. Yang, R., et al., Nature Nanotechnology (2009) 4, 34
12. Zhang, S., Ali, N., Nanocomposite Thin Films and Coatings, Imperial College Press, London (2006) 281
6. Kang, B.S., et al., Sensors (2006) 6, 643
13. Fischer-Cripps, A.C., Nanoindentation, Springer, New York, (2004), 208
7.
14. Volinsky, A., et al., Thin Solid Films, (2003) 429, 201
Li, X., et al., Nano Letters (2003) 3, 1495
8. Koponen, S., et al., Wood Sci. and Tech. (2004) 25, 1432
50
9. Gindl, W., et al., Appl. Phys. A (2004) 79, 1432
2. Doerner, M.F., Nix, W.D., J. Mater. Res (1986) 1, 601
JULY-AUGUST 2009 | VOLUME 12 | NUMBER 7-8
15. Lawn, B., Wilshaw, R., J. Mat. Sci, (1975) 10, 1049