Self-adjusting unique nanoscale contact resistance of a single alumina grain

This work evaluates the nanohardness of a single alumina grain for a coarse grain alumina ceramic of ∼10 μ m grain size. The results reveal that the nanoscale contact deformation resistance of the single grain has a unique self-adjusting characteristic. It increases in response to enhancement in the externally applied load. The nanoscale contact deformation resistance of a single alumina grain is determined by controlled nano-indentation experiments. The corresponding load versus depth plots are carefully analysed to identify the critical load at which the very first burst of incipient nanoscale plasticity is initiated. To avoid any spurious effect from neighbouring grain boundaries the nano-indentations experiments are deliberately carried out with only single grains. A range of ultra low loads that span from 1000 to 12 000 μ N is used for this purpose. Both partial unload and load controlled nano-indentation experiments are performed with a Berkovich indenter on single alumina grains. The indenter has a tip radius of 150 nm. The results show for the very first time that a mild indentation size effect exists even in single grain hardness at nanoscale. In addition the intrinsic nanoscale contact deformation resistance increases as the externally applied load is enhanced. The way it increases follows an empirical power law. These results are analysed in terms of the dislocation loop radius, critical resolved shear stress and the maximum shear stress that is generated just underneath the indenter.