Measuring Poisson's ratio during nanoindentation via lateral contact stiffness; issues of slip and plasticity

TL;DR

This paper introduces a method to measure Poisson's ratio during nanoindentation using lateral contact stiffness, addressing challenges like slip and plasticity to improve accuracy at small scales.

Contribution

It presents a novel technique for in-situ Poisson's ratio measurement during nanoindentation with lateral load application, highlighting issues and solutions for slip and plasticity effects.

Findings

Poisson's ratio measured as 0.16 for fused silica

Poisson's ratio measured as 0.34 for polycrystalline aluminium

Identified slip and plasticity as key factors affecting measurement accuracy

Abstract

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.