Indentation size effect at the initiation of plasticity for ceramics and metals

In nanoindentation, the plasticity size effect has been observed for several years, where a higher hardness is measured as contact size decreases. For spherical indenters, Lim and Chaudhri (1999 Phil. Mag. 79 2979) first showed that the entire flow curve appears at higher contact pressures for smaller radius indenters in copper. However, few papers have reported the initial yield size effect due to the difficulty in defining the yield point. Recently, Spary et al (2006 Phil. Mag. 86 5581) demonstrated that the initial yield strength of metals increases linearly with inverse cube root of indenter radius, by nanoindentation together with finite elemental modelling. Here, we use a clear method to determine the onset of plasticity in spherical nanoindentation without the uncertainties of modelling. This enables us to measure the yield pressure of tungsten metal and a series of ceramics with a high degree of accuracy and over a large range of indenter radii (hundreds of nanometres to several tens of micrometres). Our data of all ceramics and metals show clearly that there is a significant yield strength enhancement, which is inversely proportional to the cube root of the indenter radius. Normalizing the data for each material by its yield pressure for an infinite radius indenter, we find that the data for metals and ceramics fall on lines of different slope, indicating that material parameters influence the indentation yield strength size effect as well as the geometrical size effect.