In recent years, there has been much debate about the best way to perform elevated temperature nanoindentation tests, with thermal drift, tip erosion and noisefloor taking pride of place in the list of issues which can obstruct such experiments. Anton Paar TriTec has been at the forefront of this development work, which has resulted in the launch of its High Temperature Ultra Nanoindentation Tester (UNHT3 HTV).
Earlier work [1-2] has demonstrated that, besides oxidation, thermal drift is one of the key issues that will cause error in elevated temperature tests, with drift rates tending to increase with an increase in temperature. Solving this issue in the UNHT3 HTV has taken an important development and has needed many modifications to account for all possible variables.
Based on Anton Paar’s longstanding experience in nanoindentation, the heart of the UNHT3 HTV is based on the greatly successful and patented Ultra Nanoindentation Tester (UNHT3) which has already established itself as an instrument with unparalleled measurement stability [3-4].
Figure 1. Schematic representation of the UNHT3 HTV system.
Its measurement head has been optimized in order to obtain high temperature operation and incorporated with a patent-pending sample stage which enables measurements to be carried out at any temperature within the operating range with the highest possible thermal stability.
As shown schematically in Figure 1, the measuring head, optical video microscope and sample stage are mounted in a high vacuum chamber capable of being pumped down to 10-7 mbar using a turbomolecular secondary pump and a primary pump. The two key advantages of operating within a vacuum are:
(i) Removal of the effects of oxidation, meaning that it is possible to test sample materials without their surface mechanical properties becoming modified by the development of oxides. In addition, it is also possible to use indenter materials which would otherwise be inadapted to an oxidizing environment: for instance, diamond is the indenter material of choice at room temperature but it oxidizes above ~400 °C, and then softens and becomes easily blunted thus making it virtually useless for nanoindentation.
(ii) Limiting heat losses by convection within the chamber, thus significantly aiding thermal stabilization.
The main disadvantage of vacuum operation is that the working of the valves and pumps will introduce extra mechanical noise into the measurements and hence, specific actions have been taken in order to minimize this as best possible, including the following:
(a) Materials choice: the frame’s internal construction has been optimized by using a blend of aluminum, cast iron and stainless steel which allows optimum mechanical damping.
(b) Incorporation of a vacuum buffer linked between the backing valve and the secondary pump to allow operation over many hours without the requirement for the primary pump. This can maintain a 10-6 mbar vacuum for over 10 hours.
(c) Use of a 5-axis magnetically-levitated turbomolecular pump with low friction bearings to minimize mechanical vibration.
(d) Anti vibration: the whole vacuum chamber is mounted on a 4-point anti-vibration table which uses active compressed air to “float” the chamber, thus eliminating the majority of vibrational noise.
(e) Stiffening the springs of the UNHT3 HTV measuring head providing a spring constant of 6 Nmm-1 (as compared to 3 Nmm-1 with a standard UNHT3) thus maintaining an acceptable noisefloor and compensating for the extra mass of the indenter and reference.
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This information has been sourced, reviewed and adapted from materials provided by Anton Paar GmbH.
For more information on this source, please visit Anton Paar GmbH.