Nanoindentation is a well-known methodology for testing the hardness of very small volumes of material, and sometimes even single crystals, by using a conical or pyramidal indenter tip to deform the material.
Over time nanomechanical testing techniques have been refined with the introduction of commercially available high temperature testing equipment.
Why test at higher temperatures?
Many materials can experience temperature dependent changes such as dislocation movement and creep processes. High Temperature Nanoindentation provides a robust technique to examine mechanical properties at elevated temperatures, not possible with conventional mechanical testing methods.
Examples of temperature-dependent material’s mechanical properties studied by high-temperature nanoindentation technology [8, 9] include:
- Phase transitions on semi-conductors such as Si and Ge
- Glass transitions and recrystallization in polymers and bulk metallic glasses
- Superelasticity in NiTi
- Creep in Al alloys
- Oxide-scale formation in FeAl and TiAl intermetallics
The Challenges of High Temperature
Measurements taken under high temperature conditions present special challenges for reliable data to be obtained. These include:
- Minimisation of thermal gradients so that thermal drift does not affect the raw data
- Appreciation of the relationship between elastic contact mechanics and non-elastic deformation at high temperature
- Oxidation of the indentation probe/tip and the sample
- Calibration/validation of the instrument and method using reference materials at the appropriate testing temperature
Smith and Zheng were the first researchers to conduct NanoTesting above room temperature using a commercial nanoindentation system in 2000 . These early systems used resistive heaters and control thermocouples with horizontal loading to permit vertical convection that removed heat from the displacement sensor.
To minimise any thermal gradient, both the indenter and the sample were heated independently to attain an isothermal contact. It is essential to match the temperature of the sample with the temperature of the tip. Temperature matching is essential to avoid any heat flow upon contact.
The recently introduced NanoTest Xtreme instrument from Micro Materials has extended the temperature range within which measurements can be made. The instrument can perform measurements at temperatures between -100 and 1000°C without frosting or oxidation.
This large temperature range is made possible by the unique design features of the instrument including:
- High thermal stability across its operating temperature range.
- Horizontal configuration allowing heat to flow away from the displacement sensors.
- Active indenter heating to match tip and sample temperature
These features allow a vast range of materials to be examined under a range of experimental conditions (high vacuum <1×10-5 mbar, inert gas atmosphere backfill and low oxygen conditions). The NanoTest Xtreme has a load range of 10 µN to 500 mN and is ideal for use in all standard NanoTest techniques including Nano-fretting, Nanoindentation, Nano-scratch and wear and Nano-impact. Examples of applications requiring the high temperature backfill inert atmosphere capabilities of the Nano Test Xtreme include:
- High temperatures for aerospace engine components
- Tool coatings for rapid machining
- Low oxygen, low temperatures for satellite development
- Irradiation effects in nuclear reactor cladding at reactor operating temperatures
- The impact of cold on weld repairs in gas/oil pipelines
- High temperatures for power station steam pipes
Nuclear Reactors and Material Testing
A recent example of nanoindentation research is the tungsten testing conducted by Professor Steve Roberts and Dr David Armstrong at Oxford University Department of Materials [9, 10]. Here the research is directed toward tungsten-rhenium alloys suitable for nuclear fusion reactor use.
One of the key outcomes so far is the ability to carry out micromechanical testing of tungsten alloys at reactor-specific temperatures of around 750°C. One of the main issues with testing tungsten at high temperatures is the oxidation that begins from 400°C and the sublimation (solid to gas phase transition) of the oxide at 700°C.
A nanoindentation instrument was required that could operate within a high vacuum chamber at 1×10-6 mbar at temperatures of 750°C. The NanoTest Xtreme from Micro Materials was able to enhance the experimental design and provide precise measurements that agreed with values provided by other methods.
Armstrong’s group compared the performance of commercial tungsten-rhenium alloy and some that had been doped with helium. The undoped material showed a hardness of 6.5GPa at room temperature, which dropped off as it was heated to 300°C and stabilised at about 3GPa.
The helium-doped tungsten showed a high hardness of more than 10GPa at room temperature, which was retained up to 200°C; this decreased rapidly at higher temperature but still remained harder than the undoped material. The elevated hardness of the helium-modified alloy was retained on cooling and Armstrong believed helium had been trapped or intercalated within the alloy [9, 10].
Thermal Barrier Coating Bond Coats
A group at Aachen headed by Prof Sandra Korte-Kerzel recently presented work where they had used a NanoTest integrated into their own custom vacuum chamber design to test a thermal barrier coating bond coat for aerospace applications at temperatures up to 1000°C.
James Gibson, post-doc in Professor Korte-Kerzel’s group commented on the research they are conducting.
“At the IMM, one of our avenues of research is the investigation of high-temperature materials. Bond coats, in this case CoNiCrAlY, are an interesting challenge as their small size makes traditional mechanical testing very difficult. Additionally, we would like to test as close to the ‘real’ applications – be that traditional bond coatings or novel new blade repairs – in order to have our data be as relevant as possible.
These size considerations lead us directly to nanoindentation, but the bond coat application temperatures push the boundaries of the capability of current nanoindentation techniques. To this end, we’ve employed the MicroMaterials system, which I believe is the only system capable of reaching these extreme temperatures. Furthermore, the thermal stability of the system allows us to obtain near drift-free data, essential for accurate measurements.
We have upgraded the system in order to carry out these tests, which included the use of the vacuum chamber to prevent oxidation of the sample.”
The vacuum modification was carried out by Prof. Dr. Sanda Korte-Kerzel, outlined in ‘High temperature microcompression and nanoindentation in vacuum’, JMR 27 2012. To quote
“This was achieved by placing the entire hardware into a vacuum chamber… extending the electrical connections to the external controllers, using vacuum compatible motors to drive the sample stage and adapting the existing heaters for the sample and indenter to operate in vacuum using NiCr or FeCrAl wire.”
Operating in vacuum, and thinning down the sample to 200µm in order to maximise thermal transfer, the team were able to obtain hardness and creep data on the CoNiCrAlY bond coat and the underlying CMSX-4 superalloy up to 1000?C. Comparison with literature data is very promising: the bond coat follows the (limited) literature data in both hardness and creep exponent. Future work will further investigate the mechanical properties of these bond coats as well as the validity of nanoindentation creep in these materials.
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