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Nanoindentation tests are performed on single-crystal silicon wafers using a Berkovich indenter and maximum indentation loads of 30 mN, 40 mN, and 70 mN. The microstructural evolutions of the indented specimens are examined using transmission electron microscopy and selected area diffraction techniques. The results show that the unloading curve of the specimen indented to a maximum load of 30 mN has a smooth profile, whereas those of the specimens indented to 40 mN or 70 mN have a pop-out feature. The hardness and Young's modulus of the silicon specimens reduce with an increasing indentation load, and have values of 15.8 GPa and 182 GPa, respectively, under the highest indentation load of 70 mN. In addition, a strong correlation is observed between the indentation load and the microstructural change in the indentation affected area of the silicon specimens. Specifically, a completely amorphous phase is induced within the indentation zone in the specimen indented to a maximum load of 30 mN, whereas a mixed structure comprising amorphous phase and nanocrystalline phase is found in the indentation zones in the specimens loaded to 40 mN and 70 mN. The microstructural observations imply that the load-dependent nature of the unloading curves is related to the occurrence of different phase transformation mechanisms under different indentation loads.

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