Fracture toughness and fatigue investigations of polycrystalline silicon

The fracture toughness of micromachined polycrystalline silicon samples, pre-cracked with an indenter or notched using a focused ion beam (FIB) machine, were tested using either bending or tensile loading. Fracture mechanics approaches were applied to determine the fracture toughness from these results. For the pre-cracked specimens tested by tensile loading, a fracture toughness value of KI,crit equals 0.86 MP(root)a derived. The FIB notched specimens had higher fracture toughness values, probably due to the influence of the notch tip radius and the FIB process. In addition, fatigue investigations of un-notched tensile specimens were performed using tensile cyclic loading with frequencies of 50, 200 and 1000 Hz. A reduction in the tensile strength from 1.10 GPa to 0.75 GPa after 108 cycles was detected while no influence of the test frequency on the fatigue behavior was observed.

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