  Although Poisson's ratio, v, is a quantity of great importance in materials
design, presently, very few methods are available to measure it at small
(micrometre) scales and those typically involve intensive geometric refinement
of the sample. In this work we show how Poisson's ratio may be measured during
nanoindentation experiments by means of a second load actuator oriented
orthogonally to the primary vertical load axis. We apply a small oscillating
lateral load to Berkovich and sphero-conical indenter tips during
nanoindentation into fused silica and metallic samples. This enables the
elastic lateral contact stiffness to be measured as a function of depth in a
manner analogous to continuous stiffness measurements in conventional
indentation experiments. The stiffness may in turn be used to calculate v.
During constant strain rate experiments, we extract values of v = 0.16 for
fused silica and v = 0.34 for polycrystalline aluminium over indentation depths
of several hundred nanometres. We highlight two critical issues that must be
addressed to ensure accurate measurement of v during indentation; partial slip
and additional plasticity brought about through the added shear under
tangential loading. We study these separate phenomena in fused silica and
single crystal nickel, suggesting that interfacial slip is favoured in harder,
brittle materials, while plasticity is dominant in more ductile materials. We
offer some suggestions on how to mitigate these problems based on tip geometry
and loading protocols.
