Ultra-microindentations have been performed on {110} surfaces of silicon single crystals in the temperature range 20–600°C. The indents formed were observed with transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The hardness determined from the measured load-displacement curves after correcting for the elastic recovery is almost independent of temperature up to 500°C, as is found for the Vickers hardness in the literature. The TEM images, electron diffraction patterns, SEM images and their variations with temperature are investigated in connection with high-pressure studies and plasticity data. A conclusion consistent with every observation is that below 500°C, the temperature-insensitive hardness is determined by the transformation to the metallic β-tin phase, which amorphizes or nanocrystallizes during unloading while, above 500°C, plastic deformation due to dislocation activity causes temperature-dependent hardness.

Power laws are omnipresent and actively studied in many scientific fields, including plasticity of materials. Here, we report the power-law statistics in the second and subsequent pop-in magnitudes during load-controlled nanoindentation testing, whereas the first pop-in is characterized by Gaussian-like statistics with a well-defined average value. The transition from Gaussian-like to power-law is due to the change in the deformation mechanism from dislocation nucleation to dislocation network evolution in the sharp-indenter induced abruptly decaying stress and dislocation density fields. Based on nanoindentation testing on the (100) and (111) surfaces of body-centered cubic (BCC) iron and the (100) surface of face-centered cubic (FCC) copper, the scaling exponents of the power laws were determined to be 5.6, 3.9, and 6.4, respectively. These power-law exponents are much higher than those typically observed in micro-pillar plasticity (1.0–1.8), suggesting that the nanoindentation plasticity belongs to a different universality class than the micro-pillar plasticity.