Investigations on indentation size effects using a new measure of hardness

초록

Several microindentation researches have demonstrated that indentation to decreasing penetration depth results in an increase in hardness, which is known as the indentation size effect (ISE). The ISE become severe in nanoindentation regime because various influencing factors come into active at very shallow contacts. Additionally, the general analysis on nanoindentation curve enlarges hardness ambiguities due to its limitations of implying elastic/plastic deformation characteristics and disregarding material pile-up or sink-in around contacts. Thus we tried to investigate intrinsic indentation size effect in crystalline solids by observing remnant nanoindents with an atomic force microscope. Such influencing factors as surface roughness, indenter blunting, and curve analysis limitations can be removed by adopting the direct observation technique. We approximated a contact bound as the peak of pile-up region around an impression and analyzed the actual contact area from radial differentiations of the remnant indent. The load-off hardness representing only plastic deformation is calculated by dividing the load with the newly measured contact area. A dependency of the load-off hardness on the penetration depth was investigated for various materials such as (100) tungsten monocrystal, polycrystalline copper, and gold thin film. A contribution of the material pile-up to the load-off hardness was clearly modified but more significant hardness increase appeared at very shallow indentation regime due to severe elastic recovery. These load-off hardness variations are explained with the existing strain gradient plasticity models.