The work presented here is an effort to enhance instrumented indentation as a mechanical characterization tool by developing an electrical technique to measure in-situ contact area, and thus improve mechanical characterization at micro/nano-scales.
First, an electrical technique, electrical instrumented indentation , was developed and validated by indenting annealed Cu. An electrical coupled instrumented indentation system was designed and developed to enable reliable simultaneous electrical contact measurements during instrumented indentation. By indenting annealed Cu, the measured electrical contact current-voltage (I – V) curves were found to be nonlinear and asymmetric, posing significant challenge to obtain mechanical properties from electrical measurements. A new data analysis method was then proposed to obtain in-situ contact area and hardness from the measured contact I – V curves by relating the nonlinear contact I – V curves to the instantaneous contact area. The area under I – V curves were found to be correlated to the contact area from the Oliver-Pharr method at the initial unloading, and this correlation was a power-law function within the studied range. Using this relation, the in-situ contact area and hardness were obtained as a continuous function of applied force during instrumented indentation.
Second, a thorough investigation was conducted to determine the optimal data analysis methods for the developed electrical technique. The nonlinearity in the measured I – V curves posed significant challenge to inferring contact area and mechanical properties. To overcome this challenge, various I – V curve analysis methods were investigated for their abilities to infer in-situ contact area and hardness. Analyzing the resulting data from indentation of annealed Cu, the feasibility of each method was evaluated. 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. Error in the inferred contact area was systematic and primarily from the velocity dependence of the I – V response.
Finally, the electrical instrumented indentation technique was extended to characterize work-hardened Cu, a material that exhibits pile-up. The Oliver-Pharr method is known to overestimate the hardness of materials that pile-up, sometimes by as much as 60%. To apply the electrical technique to characterize materials that pile-up, it was hypothesized that materials with the same chemical constituent have the same relation between contact area and I – V curve. Thus, it is possible to characterize a material that piles-up using the relation between contact area and I – V curve calibrated on its counterpart that sinks-in via the Oliver-Pharr method. This hypothesis was tested by indenting both annealed and work-hardened Cu and imaging the residual indents. It was found that the relations between contact area and I – V curve for these two materials were sometimes as much as 10% different. This difference, on one hand, suggested that the electrical measurements are sensitive to laboratory environment (e.g. temperature and humidity) and sample surface condition (e.g. surface roughness and oxide layer). On the other hand, the moderate difference was encouraging for us to pursue the same electrical contact characteristics for materials with the same chemical constituent with reasonable experiment control. This will prove to be useful for important applications such as characterization of thin films. Furthermore, measurements of electrical contact responses between a conductive diamond indenter tip and the sample along with post-indentation imaging yielded in-situ contact area and hardness as a continuous function of applied force with the pile-up effect appropriately accounted for.
The electrical instrumented indentation technique developed in this dissertation was demonstrated to measure in-situ contact area as a continuous function of applied force during instrumented indentation, thus enhanced instrumented indentation for characterization of dynamic behavior of materials such as creep and fatigue. It also laid the foundation of a standalone technique that has the potential to overcome all the other limitations that the Oliver-Pharr method has. (Abstract shortened by UMI.)