Nanotechnological applications of nonlinear surface acoustic waves: Brittle fracture

Nonlinear surface acoustic wave (SAW) pulses with shocks were used to initiate cracks in brittle materials such as single-crystal silicon, quartz crystals, and fused quartz. The strongly nonlinear elastic surface pulses were excited with pulsed laser radiation and detected with a continuous-wave laser employing the probe beam deflection method. These SAW pulses with finite amplitudes develop steep shock fronts and spikes during propagation with strains in the 10-3 - 10-2 range that break most chemical bonds. The resulting impulsive induction of dynamic fracture occurs without an artificial seed crack. Crack extension along the surface was detected by scanning force microscopy (SFM) and optical microscopy. The penetration of cracks into the solid was studied by the focused ion beam (FIB) technique in silicon and confocal laser microscopy (CLM) in transparent fused and crystalline quartz. In anisotropic crystals the initiation process launches cracks propagating mainly along the weakest cleavage planes of the crystal. In isotropic materials crack evolution depends on the temporal and spatial variation of the stress field. By taking into account the stress field of the elliptically polarized SAWs and the tendency of the crack tip to maintain pure tensile stress conditions, crack propagation in fused quartz can be explained