Developing high temperature nano-mechanical testing
Researcher: Dr Vineet Bhakhri
Supervisors: Dr Finn Giuliani
Funding: Centre for Advanced Structural Ceramics
He is currently involved in development and calibration of small scale testing techniques such as nanoindentation and micro-pillar compression. The aim is to extend their use to high temperatures to determine the fundamental plastic deformation properties of hard materials at elevated temperatures. Initial work has been centred on high temperature nanoindentation of TiN and TiAlN. Addition of Al to TiN significantly improves its high temperature performance. Indentation hardness data from these ceramic materials is further analysed to extract the fundamental deformation parameters at 293K to 623K. The measured activation volume was estimated to be of the order of 0.75×b3 for both TiN and Ti0.44Al0.56N (b is the Burgers’ vector). The calculated activation energies were in the range of 0.76 eV for TiN and 3.0 eV for Ti0.44Al0.56N and are typical of lattice-controlled dislocation glide mechanism. TEM work is ongoing to observe and understand the differences in deformation behaviour of these materials.
The use of a sharp tip Berkovich indenter during indentation tests results in non-uniform deformation and invokes high strain-gradient in deformed material in the plastic zone beneath it. Micro-pillar uni-axial compression testing provides a way to study the mechanical behaviour of materials in a much simpler stress state. This technique is employed to measure the grain boundary strength in the SiC ceramic system. This type of testing makes use of micron-sized pillar geometries, fabricated by focussed ion-beam milling, in range of microstructures and compositions in polycrystalline SiC. Each micro-pillar contained a grain boundary orientated as close as possible to the direction of maximum shear stress, and was subjected to compression using a flat-punch indenter tip at ambient temperature. Preliminary findings suggest that single-crystal 4H-SiC display higher fracture strength compared to its polycrystalline counterpart.
In the near future plan this technique will be calibrated for high-temperature measurements. Further analysis of the grain boundary structure and phases present will be carried out by transmission electron microscopy and shall be correlated with the measurements of the grain boundary strength.