Strain rate sensitivity in nanoindentation creep of hard materials

This paper examines the strain rate sensitivity of the hardness ν_H in relation to the strain rate sensitivity of the flow stress (ν_σ) in hard solids when there is friction between the indenter and specimen. Finite element analysis is used to simulate indentation creep of von Mises solids with a range of hardness/modulus ratios (H/E*) and coefficients of friction, μ, for indenter–specimen contact. We find that, although the level of H is affected by friction, the ratio ν_H/ν_σ as a function of H/E* remains nearly unchanged. Measurements indicate that ν_H = 0.015 ± 0.02 for fused silica, from which, based on the present analysis, ν_σ ≈ 0.022 and from which an activation volume of 0.13 nm³ can be estimated for plastic deformation.