Augmented instrumented indentation using nonlinear electrical contact current-voltage curves

An electrical technique was recently developed to measure the in situ contact area continuously during instrumented indentation by simultaneously monitoring electrical contact response between a conductive indenter tip and a conductive sample. This technique has the potential to overcome limitations of the Oliver-Pharr method caused by the lack of a direct contact area measurement. However, the electrical contact current-voltage (I-V) curves measured from the technique were nonlinear, posing a significant challenge to inferring accurate in situ contact areas. To overcome this challenge and extend the electrical technique to more applications, various I-V curve analysis methods were investigated for their abilities to infer in situ contact area and hardness. Annealed Cu was indented using both linear and exponential loading tests. When analyzing the resulting data, the feasibility of each method was evaluated and the optimal methods to calculate the in situ contact area and hardness were determined. It was found that a simple summation of the absolute values of area under I-V curves or the area under I-V curves at positive voltages yielded the most robust area measure, whereas error in the inferred contact area was systematic and primarily from velocity dependence of the I-V response.