A comparison of nanoindentation pile-up in bulk materials and thin films

During nanoindentation testing, there are many issues that need to be considered if high-quality data are to be obtained when testing both bulk and thin film materials. For soft materials, one of the main issues in determining mechanical properties based on the Oliver and Pharr method is the accuracy of the determined contact area due to the pile-up around the indenter leading to a significant increase in the contact area. During nanoindentation tests for both thin films and bulk materials, the deformation mechanisms and, therefore, the governing dislocation nucleation and propagation events are complex. Hence, the volume of the pile-up is not always proportional to the indentation load and its shape can vary. Therefore, an accurate measurement of the Young's modulus and hardness requires the determination of the contact area using another technique such as atomic force microscopy (AFM) or scanning electron microscopy (SEM) images. In this study, AFM images obtained using the indenter tip after the main indentation cycle was completed were analysed to measure the pile-up heights and widths obtained in bulk materials (copper, gold and aluminium), and the results were compared to those from their respective thin films under similar indentation conditions. It was observed that the amount of pile-up that appeared in the thin films was considerably higher than in the bulk materials. Thin films with low hardness values deposited on harder substrates show a different plastic response under the indenter. During the indentation tests, the harder substrate does not deform to the same extent as the softer deposited coating and consequently it has an extreme effect on the degree of pile-up formation for the thin film.