Nanoscopic plastic deformation in mono-crystalline silicon is especially important in the mechanism understanding of nano-tribology and nano-machining. Although the phase transformation of silicon has received extensive attention during the past decade but there is little literature discussing in depth the possibility of predicting the onset of plastic deformation of silicon associated with these mechanical processes.
This study aims to establish a convenient stress criterion for an accurate prediction of the onset of nanoscopic plastic deformation with the aid of the molecular dynamics analysis. Two models were used to investigate the stress states at the initial phase tranformation of diamond cubic silicon to its _-silicon. One consists of a diamond cubic silicon workpiece of dimensions 10.3nm × 10.3nm × 6.5nm, indented with a hemispherical diamond indenter of radius 2.1nm and the other is a block of diamond cubic silicon of dimensions 10.3nm × 10.3nm × 6.0nm, subjected to uniaxial compression.
Though experimental results show diamond cubic silicon transforms into _-silicon under hydrostatic conditions, there were also experimental evidence that the transition pressure is lowered by the application of non-hydrostatic stress. In addition, there were also suggestions that phase transformations during indentation may be induced by shear (bond bending) rather than compression (bond shortening), and therefore the difference in pressures obtained from indentation experiments as compared to purely hydrostatic conditions in diamond anvil studies. In order to clarify all such discrepancies and determine what stress condition has the greatest influence and best describes the phase transformation, several stress criteria such as the octahedral shear stress, maximum shear stress, maximum principal stress and hydrostatic stress are examined. The stresses are then matched with the shape and size of the transformed zone to determine a stress criterion for the onset prediction of phase transformation and hence nanoscopic plastic deformation.
It was shown that the nanoscopic plastic deformation of diamond cubic silicon occurs when there is phase transformation of diamond cubic silicon to _-silicon. Before this critical state, diamond cubic silicon will recover elastically when the external load is removed. It was found that the stress criterion required for the phase transformation of diamond cubic (100) silicon to _-silicon is the maximum principal stress of 10 MPa in the  direction. This stress criterion works well in predicting the phase change in the case of indentation and uniaxial compression of silicon on the nanometer scale.
L C Zhang
School of Aerospace, Mechanical and Mechatronic Engineering, University of Sydney
J07, Faculty of Engineering, University of Sydney, Sydney, NSW 2006 AUSTRALIA
Phone: 61 2 9351 2835 Fax: 61 2 9351 7060