טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentMiri Markovich
SubjectDynamic Crack Propagation along {111} Si Under Bending
DepartmentDepartment of Materials Science and Engineering
Supervisor Dr. Sherman Dov
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

Investigation of dynamic crack propagation in silicon along the {111} cleavage plane is described in this thesis. Controlled experiments under three point bending (3PB) were conducted on thin rectangular pieces. The velocity of crack propagation was controlled by the initial notch length which was introduced prior to the experiment. This velocity was measured at the bottom of the specimen using the "potential drop" technique.

Recent studies on dynamic fracture of crystalline brittle materials show similar behavior to amorphous materials. The crack is stable at low velocities and becomes unstable at a critical velocity which varies between different materials. When the crack becomes unstable, it deflects from the original plane and the fracture surface becomes rough. Most of the controlled fracture experiments conducted until now on crystalline materials use tensile experiments. In a single experiment the crack is propagating in one plane and one direction and the information which can be drawn from the experiment is limited. In our fracture experiments, a controlled bending system is used. This loading system enables crack propagation on one plane at all directions in single fracture experiment.

The results obtained in our experiments are essentially different from recent fracture experiments.  The fracture surface shows new forms of instabilities. In regions where the crack traveled fast, smooth surface is obtained. As the crack approaches a slow velocity, corrugated instabilities are formed. The velocity obtained through calculations was 1045 [m/sec]. The peak of the wave advances in a straight line in the [211] direction which is the zone axis of (0-11) and (-111) planes. AFM images indicate these instabilities to be composed of atomistic steps of (0-11) and (-111) planes.

The stability of fracture in a single crystal relies on the fracture energy which varies between planes and directions. A dependency exists between the fracture energy and the velocity of the crack which varies in different planes and directions, as well as due to different levels of phonon emission. The crack would prefer to travel in a low fracture energy system. When this energy increases to a critical value due to phonon emission, the crack will divert to a different cleavage plane which enables energy consumption.