Publications on High Temperature Nanoindentation in 2011

By AZoNano

Table of Contents

Introduction

The year 2011 proved that high temperature nanoindentation is a potential growth area in materials science. The function and operation of the NanoTest instrument from Micro Materials Ltd (MML) at temperatures up to 750°C has been well established and has also been recognized, implying that the challenge to the nanomechanical community is no longer in the acquisition of reliable data, but in the interpretation of the resulting data produced.

In 2011, numerous papers were published in the area of high temperature nanoindentation. This article will provide a summary of selected work from users of the MML NanoTest system, which was the only instrument to produce publications featuring data above 200°C. This article will précis experiments carried out at temperatures above 600°C only.

High Temperature Nanoindentation – The Importance of Isothermal Contact

N.M. Everitt, M.I. Davies & J.F. Smith

A major issue in high temperature nanoindentation is stability of the instrument and the need to reduce drift during testing. This is important for accuracy of hardness and modulus data, and also for long-duration creep data.

A key focus area in the last few years has been the evaluation of heat flow and stability during the indent itself, when the indenter material is brought into contact with the sample. It makes logical sense that the diamond must be heated as well as the sample in order to ensure isothermal contact and prevent unwanted system instability, and this paper demonstrates this.

Finite element analysis modelling was utilized to offer a qualitative view of how the thermal picture develops under a diamond indenter without controlled heating of the diamond. In the case of a low-conductivity sample such as fused silica, the thermal gradient below the indenter tip can be relatively insignificant, whereas with a high-conductivity sample such as gold, only a small region of the sample reaches thermal equilibrium with the tip. As a result, a very steep thermal gradient is formed in the sample.

Such a thermal gradient will cause heat flow between the sample and the indenter immediately after the indenter moves into the sample, causing unwanted contraction/ expansion of both during indentation, and thus inaccuracy in measurement.

The model results were validated by comparing results obtained by heating the indenter either indirectly by contact with the sample or utilizing a separate heater for the indenter (an isothermal contact method).

Figure 1a displays a nanoindentation curve obtained on a gold sample at 300°C, using a method where the heater is heated indirectly by prolonged contact with the sample prior to indentation. The curve seems to exhibit negative creep, with the unloading curve crossing the loading curve. This is due to instrument drift. Figure 1b shows how this can be prevented by heating the tip separately so that contact is isothermal.

Figure 1. Figures1a (left) shows a nanoindentation curve acquired on a gold sample at 300°C, using a method where the heater is indirectly heated by prolonged contact with the sample prior to indentation. The curve appears to exhibit negative creep, with the unloading curve crossing the loading curve. This is as a result of instrument drift. Figure 1b (right) shows how this can be avoided by heating the tip separately so that contact is isothermal.

Nanoindentation results were shown for experiments on fused silica at temperatures up to 600°C, and annealed gold at temperatures up to 300°C. The results showed that indentation without separate indenter heating produced unacceptable thermal perturbation in the system, whereas the isothermal contact method maintained acceptable thermal drift and displayed values of modulus and hardness that compared well with those in the literature.

Mechanical Properties of Solid Oxide Fuel Cell Glass-Ceramic Seal at High Temperatures

J. Milhans, D. Li et al

This group at Georgia Tech recently published NanoTest data describing the mechanical properties of solid oxide fuel cell glass-ceramic seal material, G18. Hardness, modulus and creep properties were investigated via depth-sensing nanoindentation at room temperature, and then at temperatures of 550, 650 and 750°C.

Results proved a decrease in modulus with increasing temperature, with considerable decrease above the glass transition temperature, while hardness generally decreased with increasing temperature as shown in Figure 2.

Figure 2. Hardness measurements show that aging the G18 sample for longer improved stability.

Creep data acquired over 120s at a maximum load of 120mN showed that creep increased with increasing temperature, but then decreased with further aging as shown in Figure 3.

Figure 3. High temperature creep data for G18 aged for 4 hours

The tip heating used by the NanoTest ensures superior instrument stability even at these very high temperatures, allowing such creep data to be obtained.

High Temperature Microcompression and Nanoindentation in Vacuum

S. Korte, R.J. Stearn, J. Wheeler, W.J. Clegg

Nanoindentation is now used commonly as a method of studying micropillar compression.

At high temperatures it is essential to test in an inert environment so as to minimise oxidation effects. Also, impurities in inert gases can cause problems so that testing in vacuum can also be desirable. NanoTest users in Cambridge have altered their instrument to allow it to be used in a vacuum chamber, allowing high temperature nanoindentation in a vacuum environment.

By carefully controlling the temperatures of the indenter tip and the sample, the group was able to carry out flat punch indentations of gold, a good thermal conductor, over several minutes at 665°C in vacuum.

This tip heating capability also allowed thermal stability to be once again re-established in site-specific microcompression experiments. This allowed compression of nickel superalloy micropillars up to sample temperatures of 630°C with minimal levels of oxidation after 48 h. Furthermore, the measured Youngs modulus, yield and flow stresses were consistent with literature data.

NanoTest capabilities that made this work possible include the following:

• The MML NanoTest uses an exclusive horizontal loading mechanism, meaning electronics and measurement hardware are free from the influence of heat convection. This, combined with the separate heating of both sample and indenter, ensure makes the NanoTest stand out as the only option for high temperature measurements.
• Patented PID loop control of the heating mechanisms ensures excellent temperature stability, enabling long duration creep tests.

About Micro Materials

Established in 1988, Micro Materials Ltd are manufacturers of the innovative NanoTest system, which offers unique nanomechanical test capability to materials researchers for the characterisation and optimisation of thin films, coatings and bulk materials. The current model, the NanoTest Vantage was launched on June 1^st 2011.

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

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