A new method to evaluate the cleavage energy of brittle crystals

Seminars
15.06.2017
14:30
אודיטוריום ע"ש דיוויד וואנג, קומה 3, בנין דליה מידן
Hagit Cohen

Hagit Cohen
Department of Materials Science and Engineering, Technion, Haifa

We developed a new method for stable, displacement controlled crack propagation in brittle crystals. The method was inspired by Obreimoff’s experiment. It is aiming at evaluating the cleavage energy at initiation for very slow cracks, and requires small specimens. It is low-cost and easy to operate. Since the method generates stable crack, crack propagation occurs in cycles of initiation, propagation and arrest. Each cycle can be used to extract a measurement of the cleavage energy, so each specimen produces multiple results.
The general idea of the method is to use the thermal expansion coefficient mismatch between the specimen and a pin inserted into a hole drilled in the specimen as the driving force to initiate and propagate the crack. Silicon crystal specimens were chosen as a model material, and aluminum pin was chosen due to its high thermal expansion coefficient. During experiment, the assembly is being heated up slowly by few tens of centigrade until the crack propagates towards the end of the specimen. Thermocouple and high resolution camera are being used to measure the temperature and crack length, and finite element analysis was employed to compute the calibration function and to calculate the cleavage energy using the results obtained from the experiments.
Second major goal using this method was to perform tensile experiment under UHV conditions and surface scans by STM conditions, which will serve to detect the atomistic events along the crack surface, which is still debated in brittle crystals.

The method and its complexities as well as preliminary results of the cleavage energy for the (110)[110] cleavage system will be presented and discussed.

Advisor: Prof. Dov Sherman.