Slip distance model for the indentation size effect at the initiation of plasticity in ceramics and metals

The indentation size effect, in which the contact pressure increases as the size of contact decreases, has been observed for many years for both spherical and pointed indenters. The concept of strain gradient plasticity has been used to describe this phenomenon, but it is often necessary to introduce a material length scale l* to fit experimental data. Here we present a theory based on the concept of dislocation slip distance, which naturally generates the scaling and incorporates the material parameters that influence the size effect. We compare this model with experimental data for a range of ceramics and tungsten metal and show that the yield strain and Burgers vector are the important material parameters in the indentation size effect.