QUANTITATIVE CHARACTERIZATION OF THE FRACTURE SURFACE OF SI SINGLE-CRYSTALS BY CONFOCAL MICROSCOPY

Experiments are conducted to study the dislocation nucleation conditio ns at the crack tip in {110}[110] oriented Si single crystals. Specime ns with surface cracks are first statically loaded at elevated tempera tures for a prolonged period of time to initiate and move dislocations away from the crack tip, then cooled down to room temperature and loa ded to fracture to measure the fracture toughness. Fractographic analy sis of the fracture surfaces is performed. Distinct wavy patterns on t he fracture surface at the initial cleavage crack front are observed, which is attributed to the existence of local mixed mode I/mode III st resses resulting from the inhomogeneous dislocation activity, Confocal microscopy is employed to quantify the fracture surface roughness. Th e results show that the increase of fracture toughness is directly ass ociated with the increased area of the rough surface, which is charact erized by the roughness number or the fractal dimension increment. Our results also demonstrate that dislocation nucleation can occur only a t discrete sites, The spacing between these dislocation nucleation sou rces is of the order of 1 mu m. A simple model is developed for the re lationship between the fracture toughness and the surface roughness pa rameters, which is in good agreement with the experimental results.