The field of nanomechanical testing has long evolved beyond the evaluation of materials under ambient laboratory conditions. Since 2000 researchers have been pushing to extend the possibilities of the technique, in an effort to make the measurements more realistic to the conditions the materials see in service.
Testing at high and low temperatures, high strain rates, in liquids, under gas and under controlled humidity are all now possible.
Professor Steve Roberts (left) and Dr David Armstrong with their new NanoTest vacuum system.
Testing at high temperature brings many challenges, the main one being oxidation of the sample and the indenter, both of which must be heated to maintain stability during experiments. This also allows a very clearly defined test temperature. While gas purging has been used effectively to reduce oxidation to an acceptable level, the 'holy grail' of high temperature nanoindentation was always to be able to do it under vacuum.
Professor Steve Roberts and his group at Oxford Materials worked with Micro Materials Ltd (MML) to produce a dedicated nanoindentation instrument, which achieves vacuum levels of 10-6 Torr. The instrument platform itself has been designed to be fully vacuum compatible. The instrument has a useable load range of 10ìN - 500mN, and can currently reach temperatures between +750°C and -30°C. There are plans to extend the temperature range down to -150°C.
The instrument also features a piezoelectric nanopositioning stage, which allows the indenter to be used in AFM mode, so that small test elements such as microcantilevers can be located, imaged and tested, to an accuracy of 3nm, over the full range of temperatures.
The instrument, which is to have a full commercial launch by Micro Materials Ltd in the coming months, has successfully completed its initial trials, and is now (March 2012) being used by Dr Dave Armstrong and Dr Ben Britton for their first experiments.
Excellent pioneering work was done in this field at The Gordon Laboratory at Cambridge University, again using a Micro Materials NanoTest instrument. The method used there was to place a standard instrument inside a custom built vacuum chamber, which produced some ground-breaking publications.
Professor Roberts and his research group work predominantly on developing Materials for Fusion & Fission Power and are part of the Materials Department at the University of Oxford, UK.