Load effects on the phase transformation of single-crystal silicon during nanoindentation tests

Depth-sensing nanoindentation tests were made on single-crystal silicon wafers at various loads using a sharp Berkovich indenter, and the resulting indents were studied using transmission electron microscope and selected area diffraction techniques. The results indicated that the shape of the unloading parts of the load-displacement curves was affected by indentation load. The geometry and the size of the phase transformation region were also dependent on the indentation load. A strong correlation between the indentation load and the microstructure change of silicon was confirmed. A small load (∼20 mN) leads to a complete amorphous indent after unloading, whereas a big load (∼50 mN) produces a mixture of amorphous and nano-crystalline structure around the indent. The critical load for this transition to occur was approximately 30 mN. These results provide information for ductile regime machining technologies of silicon parts.