High temperature microcompression and nanoindentation in vacuum
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In small-scale testing at elevated temperatures, impurities in inert gases can pose problems so that testing in vacuum would be desirable. However, previous experiments have indicated difficulties with thermal stability and instrument noise. To investigate this, measurements of the temperature changes in a modified nanoindenter have been made and their influence on the displacement and load measurements is discussed. It is shown that controlling the temperatures of the indenter tip and the sample enabled flat punch indentations of gold, a good thermal conductor, to be carried out over several minutes at 665 °C in vacuum, as well as permitting thermal stability to be quickly re-established in site-specific microcompression experiments. This allowed compression of nickel superalloy micropillars up to sample temperatures of 630 °C with very low levels of oxidation after 48 h. Furthermore, the measured Young moduli, yield and flow stresses were consistent with literature data.
I. INTRODUCTION
Micromechanical testing, in particular nanoindentation and microcompression, has been used extensively to study local material properties or the behavior of materials only available at small scales, such as thin films. Both techniques are also being used to study effects of size on plasticity.1,2 The majority of experimental data collected to date is based on room temperature measurements. However, characterizing deformation requires the ability to test over as wide a range of temperatures as possible to enable the study of properties under operating conditions as well as the determination of activation energies and volumes. This is complicated by the difficulties encountered at elevated temperatures, such as oxidation of the sample, degradation of the tip material and geometry, thermal drift and heating of electronic components.3–5 Of those experiments carried out at high temperatures, most are carried out in air,3,5–8 resulting in either a limitation of the temperatures or materials tested. To avoid oxidation, testing can be carried out under protective atmosphere, such as argon.4,5,9,10 However, like in air, stable experiments are difficult to achieve with an unheated indenter tip, particularly in pyramidal indentation, where the changing contact area leads to a varying drift rate as the heating rate of the tip increases under load so that any correction of the drift component becomes difficult and requires a computational analysis involving modeling of the contact.5 Heating of both a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.268 J. Mater. Res., Vol. 27, No. 1, Jan 14, 2012
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indenter tip and sample can overcome this difficulty and very low drift rates have been reported in both air and argon once thermal equilibrium at the contact is achieved.4,11 However, in pyramidal indentation the extracted results are very sensitive to the shape of the indenter tip and significant changes in the indenter tip shape have indeed been found t
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