Modern Techniques for High Temperature Nanoindentation

Table of Contents

Introduction
Why Test at Higher Temperatures?
The Challenges of High Temperature
NanoTest Xtreme
Nuclear Reactors and Material Testing
Thermal Barrier Coating Bond Coats

Introduction

Nanoindentation is a well-established technique that is used for testing the hardness of extremely small volumes of materials, and sometimes even single crystals, using a pyramidal or conical indenter tip to deform the material.

With the introduction of commercially available high-temperature testing equipment, nanomechanical testing methods have been refined over time.

Why Test at Higher Temperatures?

Temperature-dependent changes such as creep processes and dislocation movement are experienced by many materials. High Temperature Nanoindentation offers a robust method to study mechanical properties at high temperatures, which is not possible with traditional mechanical testing techniques.

The following are examples of temperature-dependent material’s mechanical properties examined by high-temperature nanoindentation technology [8, 9]:

  • Glass transitions and recrystallization in bulk metallic glasses and polymers
  • Phase transitions on semi-conductors such as Ge and Si
  • Oxide-scale formation in TiAl and FeAl intermetallics
  • Creep in Al alloys
  • Superelasticity in NiTi

The Challenges of High Temperature

Measurements taken in high-temperature conditions pose unique challenges to obtain reliable data. These challenges include:

  • Appreciation of the relationship between non-elastic deformation and elastic contact mechanics at high temperature
  • Minimization of thermal gradients so that thermal drift does not impact the raw data
  • Oxidation of the sample and the indentation probe/tip
  • Calibration/validation of the method and instrument using reference materials at the appropriate testing temperature

In 2000, Smith and Zheng became the first researchers to perform NanoTesting above room temperature using a commercial nanoindentation system [7]. Resistive heaters and control thermocouples with horizontal loading were used by these early systems to allow vertical convection that removed heat from the displacement sensor.

In order to reduce any thermal gradient, the sample and the indenter were heated separately to achieve an isothermal contact. The sample’s temperature should be matched with the tip’s temperature. This temperature matching is important to prevent heat flow upon contact.

NanoTest Xtreme

Micro Materials has recently launched the NanoTest Xtreme instrument, which has extended the range of temperature within which measurements can be performed. The instrument is designed to conduct measurements at temperatures between -100 and 1000°C without oxidation or frosting.

This large temperature range is achieved with to the instrument’s unique design features including:

  • Active indenter heating to match the temperature of the tip and sample
  • High thermal stability across its operating temperature range
  • Horizontal configuration enables heat to flow away from the displacement sensors

These features make it possible to examine a wide range of materials under different experimental conditions (high vacuum <1×10-5 mbar, low oxygen conditions and inert gas atmosphere backfill). The NanoTest Xtreme has a load range of 10 µN to 500 mN and can be used in all standard NanoTest techniques such as Nanoindentation, Nano-fretting, Nano-scratch and wear and Nano-impact.

The following are examples of applications that demand the high-temperature backfill inert atmosphere capabilities of the Nano Test Xtreme:

  • Tool coatings for rapid machining
  • High temperatures for aerospace engine components
  • Low temperatures, low oxygen for satellite development
  • High temperatures for power station steam pipes
  • The impact of cold on weld repairs in oil/gas pipelines
  • Irradiation effects in nuclear reactor cladding at reactor operating temperatures

Nuclear Reactors and Material Testing

Tungsten testing conducted by Dr David Armstrong and Professor Steve Roberts at Oxford University Department of Materials [9, 10] is one recent example of nanoindentation research. In this case, the research is aimed toward tungsten-rhenium alloys that are suitable for use in nuclear fusion reactors.

So far, one of the main outcomes is the ability to perform micromechanical testing of tungsten alloys at reactor-specific temperatures of about 750 °C. One of the key issues when 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 that could operate within a high vacuum chamber at 1×10- 6 mbar at 750 °C was required. Micro Materials’ NanoTest Xtreme not only enhanced the experimental design, but also provided precise measurements that agreed with the values given by other techniques.

Data from high temperature experiments on Tungsten. Data courtesy of DEG Armstrong University of Oxford.

The group then compared the performance of a commercial tungsten-rhenium alloy was then compared with some helium-doped tungsten-rhenium alloys. At room temperature, the undoped material exhibited a hardness of 6.5 GPa that dropped off as it was heated to 300 °C and stabilized at approximately 3 GPa.

At room temperature, the helium-doped tungsten exhibited a high hardness of over 10 GPa, which was retained up to 200 °C. While this rapidly reduced at higher temperature, it still remained harder than the undoped material. The increased hardness of the helium-modified alloy was retained on cooling, and according to Armstrong, helium had been intercalated or trapped within the alloy [9, 10].

Thermal Barrier Coating Bond Coats

Headed by Prof Sandra Korte-Kerzel, a research group at Aachen has recently presented a study where it used a NanoTest integrated into a customized vacuum chamber design to test a thermal barrier coating bond coat for aerospace applications at up to 1000°C.

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.

James Gibson, Post-Doc in Professor Korte-Kerzel’s group

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.”

By operating in vacuum and thinning down the sample to 200 µm to improve thermal transfer, the team was able to acquire creep and hardness data on the CoNiCrAlY bond coat and the underlying CMSX-4 superalloy at temperatures up to 1000 °C.

Comparison with literature data was very promising: the bond coat follows the (limited) literature data in terms of creep and hardness exponent. Going forward, the mechanical properties of these bond coats and the validity of nanoindentation creep in these materials will be further investigated.

References

  1. Poon, D. Rittel, G. Ravichandran, An analysis of nanoindentation in linearly elastic solids, International Journal of Solids and Structures 45 (2008) 6018–6033
  2. I. Bulychev, V.P. Alekhin, et al., Determining Young’s modulus from the indenter penetration diagram, Zavodskaya Laboratoriya, 1975, 41 (9), 1137–1140
  3. C. Oliver and G.M. Pharr, Measurement of hardness and elastic modulus by instrumented indentation: Advances in understanding and refinements to methodology, J. Materials Research., Vol. 19, No. 1, 2004, pp 3-20
  4. C. Oliver and G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Materials Research., Vol. 7, No. 6, 1992, pp 1564-1583
  5. M. Wheeler, D.E.J. Armstrong, W. Heinz, R. Schwaiger, High temperature nanoindentation: The state of the art and future challenges, Current Opinion in Solid State and Materials Science: Recent Advances in Nanoindentation, Volume 19, Issue 6, December 2015, Pages 354–366
  6. Anthony C. Fischer-Cripps, Nanoindentation: Mechanical Engineering Series, Edition 2, Springer Science & Business Media, 2004, ISBN 0387220453, 9780387220451, 264 pages
  7. F. Smith and S. Zheng, High temperature nanoscale mechanical property measurements, Surf. Eng., 2000, 16, 143–146
  8. A. Schuh, J. K. Mason & A. C. Lund, Quantitative insight into dislocation nucleation from high-temperature nanoindentation experiments, Nature Materials 4, 617 – 621 (2005)
  9. J. Harris, B. D. Beake and D. E. J. Armstrong, Extreme nanomechanics: vacuum nanoindentation and nanotribology to 950 °C, Tribology 2015 Vol. 9 No.4, pp174-180
  10. The mission for fusion and fission, Materials World magazine, 1 Feb 2015
  11. Korte, et al., High temperature microcompression and nanoindentation in vacuum. Journal of Materials Research, 2012. 27: p. 167- 176

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

For more information on this source, please visit Micro Materials.

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