:: AZoNanotechnology Article
Nanomechanical Testing of Polymeric Materials using the Nanotest System -
Application Note by Micro Materials
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Topics Covered
Introduction
The NanoTest Advantage for
Polymer Testing
NanoTest Capability
Stability at High Temperature
Elevated Temperature
Nanoindentation Testing
High Speed, High Accuracy
Dynamic Mechanical Compliance Testing
Fluid Cell
Testing at High Strain Rate
Nano-Scratch and Nanowear Testing
Specifications
High Temperature
Testing
Dynamic Compliance Testing Module
Fluid Cell Package
High Strain Rate: Nano-Impact
Testing
Nano-Scratch/Nanowear Testing
Acknowledgements
Introduction
Thermal stability of the testing instrument is key to meaningful measurements
of the viscoelastic properties of time-dependent materials. At room temperature
the thermal drift of the NanoTest is very low, typically an order of magnitude less than
some other commercial systems.
The NanoTest Advantage for Polymer Testing
NanoTest offers the following advantages for polymer
testing
- Highest resolution measurements
- Flexibility of design and thermal stability
- Minimal thermal drift even at elevated temperatures
- Ultra-high strain rate tests
- Tests in fluid environment
NanoTest Capability
Capabilities of NanoTest include:
- Viscoelastic properties
- Nanotribology
- Elevated temperature nanoindentation
- Testing in fluid environment
- Ultra-high strain rate testing
- Ultra-low load tests
- Nano-scale fatigue testing
Stability at High Temperature
Elevated Temperature Nanoindentation Testing
The NanoTest advantage becomes more pronounced when testing at elevated
temperatures. This is due to the unique design for elevated temperature testing
that relies on separate heating (and active temperature control) of both probe
and sample ensuring no heat flow occurs during the indentation process.
The NanoTest is unique in this isothermal contact. As no significant
thermal drift occurs during elevated temperature measurements it becomes possible
to perform long-duration tests – such as indentation creep tests - at
elevated temperatures and observe how the properties of polymeric materials
change as they go through the glass transition temperature. This can be done
in a fully automated procedure at a more local scale and on thinner films than
by other methods such as DMA.
The localised approach enables more rapid heating/cooling than by heating
the entire sample chamber, and so thermal history/recrystallisation processes
can now be studied in detail at the nanoscale.
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Figure 1. Schematic of NanoTest hot stage showing
separate tip and sample heaters, figure courtesy of AJ Muir Wood, University of
Cambridge
As an example, the NanoTest elevated temperature testing capability has been used
to determine the variation in mechanical properties with temperature of a range
of PET films with different processing history and crystallinity. Figure 2 shows
the behaviour of an amorphous (non-heat set) sample.
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Figure 2. The variation in nanoindentation behaviour
(left) and (above) creep compliance with test temperature over the range
60-110กใC for an amorphous PET thin film
The nanomechanical properties of the film at 60°C were virtually the same
as at room temperature. Above 60°C clear changes in indentation response
were observed. A sharp decrease in mechanical properties was seen between 70°C
and 80°C consistent with the presence of a glass transition over this temperature
range in agreement with literature values for bulk materials. A further increase
in time-dependent deformation and drop in stiffness occurred on increasing the
temperature to 90°C. At 110°C a dramatic improvement in mechanical properties
was observed consistent with cold recrystallization.
High Speed, High Accuracy
Combinatorial testing is fast becoming a popular new route towards producing
new materials with interesting and unexpected properties. Rather than trying
to engineer perfect materials, in a combinatorial approach, many hundreds or
more are made at small scale.
Scientists at MIT have used the NanoTest to test the properties of polymeric materials where
each material had a different combination of 2 different monomers. Within 24
hours of automated testing (in a single continuous run) they had data on every
polymer in a 576-element array and could map the effects of the % of each monomer
on the properties of the material. This automated analysis of a large library
of acrylate-based materials demonstrated a range of mechanical properties affected
by composition in unexpected ways.
The authors noted that the absence of piezocrystal actuation in the load frame
actuation (present in some other nanoindentation systems) resulted in the highly
stable frame compliance and load/displacement signals necessary.
Dynamic Mechanical Compliance Testing
The NanoTest dynamic compliance testing module includes a lock-in
amplifier and sample oscillation system to vibrate a sample and allow the compliance
to be measured on a continuous basis. It can be thought as a nanoscale analogue
of dynamic mechanical analysis (DMA). After collecting the raw phase angle data
with spherical or pyramidal indenters, it is analysed with a 4-element linear
viscoelastic model to determine loss and storage modulus, indentation complex
modulus and tan delta which are indicative of energy damping in the surface/near
surface of the material. The example below shows an excellent fit of the model
to experimental data on an epoxy sample. A value for tan delta of 0.017 was
determined in good agreement with bulk DMA values.
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Figure 3. Variation in phase signal with indentation
depth for three repeat tests on an epoxy sample. The reproducibility of the data
and its fit to the 4-element linear viscoelastic model used in the analysis is
good and produces a value of tan delta of 0.017.
Fluid Cell
The mechanical properties of biological and polymeric samples often vary considerably
when in a fluid environment compared to the usual dry testing conditions. If
we wish to understand their properties and behaviour in fluid media it is therefore
highly desirable to test under these conditions rather than to attempt to infer
from measurements on dry (or 50% relative humidity) samples. To meet this need,
the testing capability of the NanoTest
has been extended by the development of a fluid cell allowing nanoindentation, nano-scratch and nanowear
testing of samples fully immersed in fluids.
For example, nylon (PA6) can swell by 7-9% at saturation. The NanoTest
fluid cell has been used to investigate how its nanomechanical properties (primarily
elastic modulus and creep compliance) are affected by the test medium. Typical
indentation curves for a low molecular weight PA6 sample are shown for dry (~50%
relative humidity) and after immersion in deionised water for several hours
are shown in figure 4. There is a decrease in elastic modulus of about 67% after
24 hours immersion (figure 5).
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Figure 4. Typical nanoindentation curves dry and wet for
low MW PA6 using a Berkovich indenter loading at 0.2 mN/s to a peak load of 5
mN. Holding periods at peak load and 90% unloading allow investigation of
viscoelastic response.
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Figure 5. Effect of test environment on elastic modulus
of PA6 after >24 hr immersion.
Testing at High Strain Rate
Materials show differences in mechanical behaviour at high and low strain rates.
The NanoTest is unique amongst indentation systems in having the
(patent protected) ability to produce ultra-fast, high strain rate indentations
and can be used to study material behaviour at strain rates far in excess of
those on any other instrument.
This is possible due to the pendulum geometry that enables the probe to be
accelerated to produce high energy impacts in a fraction of a second. With the
aid of a fast DAQ system (up to 500000 Hz possible) all the probe displacement-time
data is captured and can be analysed to produce dynamic hardness and viscoelastic
property information. Dynamic hardness is defined (after Tabor) as energy per
unit volume and has units of pressure just as conventional hardness.
As an example the high-strain indentation behaviour of commercial low-density
polyethylene [LDPE], polycarbonate [PC] and polytetrafluoroethylene [PTFE] polymers
is shown in the Figure 6. The probe (a diamond indenter in this case) bounces
on the surface of all three polymers before the energy is dissipated but there
are clear differences in how this occurs. PC shows essentially elastic behaviour,
LDPE shows rubber-like behaviour and PTFE damps out the impact energy very effectively.
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Figure 6. The damping ability of the PTFE material is shown
by the lack of recoil (energy absorption).
In addition to single impacts the nano-impact
module can be used to investigate differences in fatigue due to repetitive impact.
Differences in impact behaviour have been correlated to differences in ductility
on nanocomposites. In the example below (figure 7) there are clear differences
in deformation due to the repetitive impacting, with PTFE in particular showing
dramatic continuing deformation.
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Figure 7. Deformation due to multiple impact on PTFE, LDPE
and PC at 0.14 Hz, applied load 2 mN accelerated from 14 µm in 40 ms,
with 3 µm impact probe.
Nano-Scratch and Nanowear Testing
Nanotribological testing of polymeric materials is performed using the nano-scratch and nanowear
module of the NanoTest system. In addition to measuring the critical load to
failure of polymer coatings the technique has found application in fundamental
studies of scratch resistance at small scale.
It has been found that the scratch resistance is a strong function of processing
history. As an example, Figure 8 shows typical scratch and postscratch traces
on three different heat set PET thin films. These were 1) undrawn (0% crystallinity),
2) uniaxially drawn (33 % crystallinity) and 3) biaxially drawn (50% crystallinity).
The draw process induces crystallinity and orientational changes which alter
the mechanical properties.
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Figure 8. Correlation between the variation in elastic
recovery during scratching (dark blue) and the polymer's H/E ratio (light
blue = 10 x H/Er). The insets show typical scratch and post-scratch traces for
500 µN scratch load with a 3 µm diamond indenter at 1 µm/s.
Ultra-low load (20µN) nanoindentation was used to determine the mechanical properties
of the films. As figure 8 shows there is a 1:1 correspondance between the H/E
ratio and the degree of recovery during scratching. The on-load scratch depths
are rather similar but the recovery ratio differs dramatically with crystallinity
in the thin films.
Specifications
High Temperature Testing
The hot stage, heated indenter and thermal control system operates to 500°C
(option to 750°C). Separate heating (and active temperature control) of
both probe and sample ensuring no heat flow occurs during the indentation process.
Minimal instrumental thermal drift at elevated temperatures allows indentation
creep tests at elevated temperatures and determination of properties through
the glass transition temperature.
Dynamic Compliance Testing Module
For the investigation of storage and loss moduli, and tan delta. Oscillation
frequency range 0.1Hz to 250Hz (larger ranges optional). Frequency sweep capability.
Amplitude of oscillation typically sub-nm to 50 nm (larger ranges optional).
Optimised computer control of the lock in amplifier for setting the gain, time
constant, frequency and amplitude.
Fluid Cell Package
The fluid cell package includes an indenter adapter, liquid cell software and
the liquid cell itself. In comparison to other methods (such as DMA) it enables
more highly localised measurements of mechanical properties and testing of thinner
and more heterogeneous samples. Flow cell option required for controlled fluid
exchange during experiments. The fluid cell package provides information on
a materials performance to the influence of fluids in its service environment.
This option finds great application in friction, wear and lubrication studies
as well as mechanical property response to changes in the relative humidity
of an environment.
High Strain Rate: Nano-Impact Testing
The nano-impact module includes two distinct impact testing modes
as standard.
Sample oscillation mode:
Piezoelectric oscillation system, signal generator, amplifier and software
for control and data analysis allowing both impact and contact fatigue tests
to be performed depending on the magnitude of the static load. Frequency range
1-500 Hz.
Pendulum impulse impact mode:
Pendulum impulse using an A/C solenoid to produce very high strain rate
indentations (nanoimpacts). Single and repetitive impacts. Dynamic hardness
is determined from analysis of single impacts and fatigue behaviour from multiple
impacts.
Nano-Scratch/Nanowear Testing
For progressive load scratches, 3-pass multi-pass (where second scan is ramp)
and longer multi-pass friction and wear tests.
Wide range of robust friction probes with different force constant
available.
Choice of spheroconical diamond scratch probes with end radii 0.7-200 µm.
Easy and quick probe exchange (~ 1 min) – fully modular with nanoindentation and nano-impact
modules. No recalibration necessary on switching between nanoindentation and nanoscratch
modules. Robust - no danger of damage to loading head springs during scratching.
Acknowledgements
Research groups at MIT, SUNY at Stonybrook and NPL are thanked for their on-going
collaboration with Micro Materials. In particular Dr Nigel Jennett and Dr John
Nunn at NPL (fast DAQ system, high strain rate), Prof Krystyn Van Vliet's
group at MIT (fluid cell and high strain rate) and Prof Raman Singh's
group at SUNY (dynamic compliance testing).
A complete set of references is available be referring to the
source document.
Source: "Nanomechanical Testing of Polymeric Materials" Application
Note by Micro Materials Ltd
For more information on this source please visit Micro
Materials
Date Added: Jul 26, 2008
|
Micro Materials
