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Reliability of fracture toughness test geometries at the micron-scale

Focus: The research focused on testing the reliability of various novel fracture toughness test geometries at the small length scales using in-situ fracture tests in the SEM.

With continuous device miniaturization and use of thin films and nanoscaled structures, various novel fracture toughness testing geometries have been developed to test materials directly at small length scales.

Since ASTM like standards do not exist for samples at micro and nano-scaled dimensions, it is important to validate the different geometries by measurement of KIC in a well known sample. Si (100) single crystal wafers were used as model systems to quantitatively compare the fracture toughness values obtained from single cantilever, clamped beam and double cantilever bend and pillar splitting geometries. Factors like notch tip radius, FIB micromachining damage and aspect of the specimen were also considered.

It was found that all four geometries yield a fracture toughness of 0.8 MPam1/2 on an average, close to values observed from macro-scale tests. This ruled out both geometry as well as size effect for fracture tests at the micron-scale. Suitable test geometries have been further applied to elevated temperature testing of Si and other brittle oxide thin films.

Fig Caption: Four micro-scale geometries chosen for the study (a) single cantilever bending (b) clamped beam bending (c) double cantilever bending and (d) pillar splitting

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