Nanoindentation response analysis of thin film substrates-II: Strain hardening-softening oscillations in subsurface layer

Abstract

We have extracted stress-strain field (SSF) gradient and divergence rep-resentations from nanoindentation data sets of bulk solids often used as thin film substrates: bearing and tooling steels, silicon, glasses, and fused silica. Oscillations of the stress-strain field gradient and divergence induced in the subsurface layer by the nanoindentation have been revealed. The oscillations are especially prominent in single indentation tests at shallow penetration depths, h<100 nm, whereas they are concealed in the averaged datasets of 10 and more single tests. The amplitude of the SSF divergence oscillations decays as a sublinear power-law when the indenter approaches deeper atomic layers, with an exponent -0.9 for the steel and -0.8 for the fused silica. The oscillations are interpreted as alternating strain hardening-softening plastic deformation cycles induced in the subsurface layer under the indenter load.