Abstract
Silicon has such versatile characteristics that the mechanical behavior and deformation mechanism under contact load are still unclear and hence are interesting and challenging issues. Based on combined study using molecular dynamics simulations and experiments of nanoindentation on Si(100), the versatile deformation modes, including high pressure phase transformation (HPPT), dislocation, median crack and surface crack, were found, and occurrence of multiple pop-in events in the load-indentation strain curves was reported. HPPTs are regard as the dominant deformation mode and even becomes the single deformation mode at a small indentation strain (0.107 in simulations), suggesting the presence of a defect-free region. Moreover, the one-to-one relationship between the pop-in events and the deformation modes is established. Three distinct mechanisms are identified to be responsible for the occurrence of multiple pop-in events in sequence. In the first mechanism, HPPTs from Si-I to Si-II and Si-I to bct5 induce the first pop-in event. The formation and extrusion of α-Si outside the indentation cavity are responsible for the subsequent pop-in event. And the major cracks on the surface induces the pop-in event at extreme high load. The observed dislocation burst and median crack beneath the transformation region produce no detectable pop-in events.
Highlights
IntroductionIn situ electrical measurements show a transformation from a semiconductor-like phase (pristine diamond cubic structure, Si-I) to a metallic phase[14, 15]
In situ electrical measurements show a transformation from a semiconductor-like phase to a metallic phase[14, 15]
The authors investigated the mechanism of initial plastic deformation with respect to the hold time under the maximum load, and suggested that plastic deformation is typically initiated by high pressure phase transformation (HPPT) or crystalline defect[24]
Summary
In situ electrical measurements show a transformation from a semiconductor-like phase (pristine diamond cubic structure, Si-I) to a metallic phase[14, 15]. The resulted metallic phase is generally accepted to be the β-Si phase, which is a body-centered-tetragonal structure with sixfold coordination, based on the phase transformation sequences observed in diamond-anvil cell (DAC) experiments[15]. This transformation may lead to an abrupt change in the load-displacement curve, which is called a ‘pop-in’ event. The MD simulations successfully predict the formation of the Si-II phase and show the detailed distribution, structural characteristics and phase transformation process of Si-II36, 39–46, which is almost impossible to achieve in present experimental conditions Another new body-centered-tetragonal high-pressure phase of bct[5] with fivefold coordination is predicted by the MD simulations[41, 42, 45]. MD simulations have become an indispensable approach to furthering our understanding of these processes at the atomic scale
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