טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentSagi Hagit
SubjectA Method for Stable Crack Propagation in Brittle Crystals
DepartmentDepartment of Materials Science and Engineering
Supervisors Dr. Dov Sherman
Professor Rachman Chaim


Abstract

This research presents the development of a new method to cleave brittle single crystals. This method is used to propagate a crack in a stable manner under pure Mode I. The method was developed to fulfil two main goals: The first is to evaluate accurately the cleavage energy at initiation for slow cracks and the second is to detect atomistic events along the crack front using the Scanning Tunnelling Microscope.

This method is low-cost and easy to operate, and a big advantage of it is the small specimen size. The specimen is thin and rectangular, cut from available commercial silicon wafers. Here, single crystal silicon served as a model material for the development of the method. The main idea of the method is to insert a conic aluminium pin into an identical drilled conic hole in the specimen, and to heat them together on top of an electrical heating stage. An atomistic sharp precrack was generated in the specimen, which is identical to a natural crack in the material. Thus, while heating up the assembly by several tens of degrees Celsius, the thermal expansion coefficient mismatch between the silicon and the aluminium generates sufficient driving force to initiate and propagate the crack. The crack propagates in stable manner, meaning, by cycles of initiation, short propagation and arrest, while exposing visible marking lines of the arrested fronts on the crack surface. Each one of these fronts indicates the following initiation site of the crack and thus can be used for the evaluation of the cleavage energy. Thus, a single experiment can produce several data points.

Finite Element Analysis was used to evaluate the cleavage energy by supplying the variables obtained from the lab experiments, e.g., the temperature and crack length. The former was measured by two thermocouples and the latter by a high-resolution camera.

Experiments were carried out to evaluate the cleavage energy of single crystal silicon, however, further investigation is required to obtain accurate results. The evaluated cleavage energy values obtained in this research are still challenging, probably due to lack of information of the temperature field measurements during the experiments. This new system was developed for about two and a half years, and it is still not completed and further improvements are necessary.

Cleaving the specimen under Ultra High Vacuum conditions and scanning the crack's surface using Scanning Tunnelling Microscope, was performed to detect atomistic events along the crack front. The purpose was to search for the crack's propagation mechanisms at the atomistic scale, whether it propagates by kinking mechanisms, or by a straight line, since propagation mechanisms of a crack in brittle materials is still controversial in the field of fracture mechanics. In the framework of this research, experiments were successfully performed inside the Scanning Tunnelling Microscope chamber, however the atomistic kinking mechanisms were not detected due to insufficient vacuum level.

The new method indeed works, and the crack propagates stably. However, further refinements are needed to overcome some difficulties that occurred during the investigation.