High temperature nanoindentation - the importance of isothermal contact

The suitability of high temperature techniques was investigated by finite element analysis modelling to give 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 is relatively diffuse, whereas with a high-conductivity sample such as gold, most of the sample is able to equilibrate at the set temperature, leading to a very steep thermal gradient in the volume of material that must accommodate the deformation. However, in both cases indentation is occurring in material that is at a lower, and unknown, temperature than the bulk sample. The results of the model are validated by comparing results obtained by heating the indenter either indirectly by contact with the sample or utilising a separate heater for the indenter (an isothermal contact method). Nanoindentation results are presented for experiments using a cubic boron nitride Berkovich indenter on both fused silica, at temperatures up to 600 degrees C, and annealed gold at temperatures up to 300 degrees C. Indentation without separate indenter heating tended to produce unacceptable thermal perturbation in the system, whereas the isothermal contact method maintained acceptable thermal drift and produced values of modulus and hardness that compared well with those in the literature.