טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentHerman David
SubjectCrack Path Selection in Brittle Single Crystals under
Combined Tensile and Shear Stresses
DepartmentDepartment of Materials Science and Engineering
Supervisor Dr. Dov Sherman
Full Thesis textFull thesis text - English Version


Abstract


We investigated the conditions required to deflect a crack from the cleavage plane by dynamic mixed-mode fracture experiments, where the shear stress, crack speed, and crystal structure are the governing parameters. Silicon crystal, which shows cleavage along the {110} and {111}, was used as a model material. The experiments were analyzed by finite element analysis (FEA), to compute the mode-mixity and to estimate crack speed by Freund equation of motion, calibrated by strain gage measurements. Fracture surfaces were analyzed by means of optical, AFM, and HR-SEM.

Deflection in silicon occurs on the micro (surface instabilities), meso (local deflection), and macro (path deflection) scales. Due to anisotropic thermal phonon emission, crack path selection in crystals depends on crack speed. At the lower crack speed regime, below ~2,000 m/s, deflection by the formation of steps on the  or deflection from  to (110) requires little shear. At higher crack speeds, mesoscale deflection, even under little shear, is observed. This is characterized by nanometer steps and corrugation, micrometer facets, and sub-millimeter out-of-plane faceting often observed in the absence of macroscopic in-plane deflection. At higher crack speeds, above ~3,600 m/s, crack propagation on the  is energetically unfavorable. This results in the  faceting in the out-of-plane direction to allow the crack to propagate along the . As a result, when the crack speed increases higher shear is required to macroscopically deflect the crack. Our work indicates that crack path selection, both at the meso and macro-scale, in single crystal silicon can vary between nearly perfect cleavage and nearly isotropic path selection due to competing forces tending to minimize shear energy, surface energy, and thermal phonon emission energy. This energy balance depends on the externally controlled crack speed, shear, and crystal structure (cleavage plane, crack front orientation, and direction of deflection).