Using Nanoindentation for Nano Puncture Resistance

Conventional Puncture Tests are usually used to determine the penetration characteristics of a wide range of protective materials such as membrane, foils, or films. They are extensively used in medical and food industries on applications ranging from rubber gloves and needles to food packaging. The test is carried out similar to that of a compressive test. A probe, with multiple tip selection, uses an increasing load until the material is penetrated and the load is recorded at failure. These traditional tests were typically executed at higher loads, Newton and above, with poor resolution and load control.

Importance of Nanoindentation Puncture Test

With a lot of conventional mechanical tests (puncture, hardness, compression, yield strength etc.), the present day’s quality control environments with sophisticated sensitive materials now require greater reliability and precision. Conventional mechanical instrumentation does not provide the required sensitive load control and resolution. Moreover, several instruments are required to carry out a variety of mechanical tests which can now be carried out on a single system. Many tests, including puncture tests, can now be tested with precise resolution at Nano through Macro controlled loads with just a single instrument.

Measurement Objective

In this application, the Nanovea Mechanical Tester, in Nanoindentation mode, is used to study the puncture resistance of an aluminum foil sample with the help of a cylindrical flat tip indenter. In order to secure thin film and foil samples, a custom sample holder was designed.

Nanovea Mechanical Tester

Measurement Principle

Nanoindentation is based on the standards for instrumented indentation, ISO 14577 and ASTM E2546. It uses a method that was already established, in which an indenter tip with a known geometry is driven into a particular site of the material to be tested, by applying an increasing normal load. After reaching a preset maximum value, the normal load is reduced until total relaxation takes place. The load which is applied by a piezo actuator is measured in a controlled loop with a high-sensitivity load cell. Throughout the experiment, the position of the indenter with respect to the sample surface is precisely monitored with the high precision capacitive sensor.

The resulting load/displacement curves give data that is specific to the mechanical nature of the material being examined. Quantitative hardness and modulus values for such data are calculated using established models. Nanoindentation is particularly suitable for load and penetration depth measurements at nanometer scales and has the following specifications:

. . .
Maximum displacement (Dual Range) : 50 µm or 250 µm
Depth Resolution (Theoretical) : 0.003 nm
Depth Resolution (Noise Level) : 0.05 nm
Maximum force : 400 mN
Load Resolution (Theoretical) : 0.03 µN
Load Resolution (Noise Floor) : 1.5 µN

Analysis of Indentation Curve

Hardness and elastic modulus are determined through load/displacement curve according to the ASTM E2546 (ISO 14577) as shown in the example below.

Analysis of Indentation Curve

Hardness

The hardness is established from the maximum load, Pmax divided by the projected contact area, Ac:

Hardness

Young’s Modulus

The reduced modulus, Er, is given by:

Young’s Modulus

Which can be calculated having derived S and Ac from the indentation curve using the area function, Ac being the projected contact area. The Young’s modulus, E, can then be obtained from:

Here, Ei is the Young’s modulus, νi is the Poisson coefficient of the indenter, and ν the Poisson coefficient of the tested sample.

How are These Calculated?

A power-law fit through the upper 1/3 to 1/2 of the unloading data intersects the depth axis at ht. The stiffness, S, can be calculated from by the slope of this line. The contact depth, hc, is then calculated as:

The indenter area function can be evaluated to calculate the contact Area Ac. This function will rely on the geometry of the diamond and at low loads by an area correction.

For the ideal Berkovich and Vickers indenters, the area function is Ac = 24.5 hc2; for Cube Corner indenter, the area function is Ac = 2.60 hc2; for Spherical indenter, the area function is Ac = 2πRhc where R is the radius of the indenter. The elastic components, as mentioned before, can be modeled as springs of elastic constant E, given the formula:

σ = Eε

where σ is the stress, E is the elastic modulus of the material, and ε is the strain that takes place under the given stress, similar to Hooke's Law. The viscous components can be modeled as dashpots such that the stress-strain rate relationship can be given as,

where σ is the stress, η is the viscosity of the material, and dε/dt is the time derivative of strain.

As the analysis is highly dependent on the selected model, Nanovea offers the tool to collect the data of displacement versus depth during the creep time. The maximum creep displacement versus the maximum depth of indent and the average speed of creep in nm/s is given by the software. Creep may be best examined when loading is quicker. The spherical tip might be a more suitable choice.

Other Possible Tests Include the Following:

Stress-Strain & Yield Stress, Puncture Resistance, Compression Strength, Fracture Toughness, Fatigue testing, and many more.

Test Conditions

. .
Applied Force (mN) 400
Loading rate (mN/min) 600
Unloading rate (mN/min) 600
Indenter type Cylindrical Flat

Conclusion

To summarize, it was shown how the Nanovea Mechanical Tester, in Nanoindentation mode, offers a precise measurement of puncture resistance and has a superior benefit in comparison to conventional instruments. Other tests possible with the same system, for instance, depth versus load also enable creep, hardness, and elastic modulus to be tested with a sharp Berkovich diamond or during a more compressive test with a flat or sphere indenter. Fatigue could be tested with multiple cycle loading, and yield strength can be tested with increasing load multiple cycles.

By using the controlled XY stage, the same instrument can be used to measure marring and scratch properties or to gauge coefficient of friction of these film samples. Multi-pass wear is also a common test as it provides valuable information on the long-term feasibility. Examining the coatings on foils and films could also be done using the lower load range of the nano module.

Nanovea

This information has been sourced, reviewed and adapted from materials provided by Nanovea.

For more information on this source, please visit Nanovea.

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