Illustrative analysis of load-displacement curves in nanoindentation

The nature of the elastic unloading after an elastic-plastic contact with a conical or Berkovich indenter is studied. Three representative specimens having different mechanical properties were tested. Finite-element results for the pressure distribution beneath the indenter during unloading suggest that the effective indenter is in fact very closely approximated by a sphere in the case of fused silica (a material with a relatively low value of E/H) and a more uniform pressure distribution in the case of silicon and sapphire (materials with higher values of E/H). The proposed reason for these observations is the extent and influence of an elastic enclave directly beneath the indenter as revealed by finite-element analysis. The results also show that the pressure distribution retains its form during the entire unloading. The work seeks to provide a physical reason for the value of the fitting exponent m as used in popular nanoindentation data analysis procedures.