Subsurface mechanical defects

Applications. We have performed, with the system described above, high-resolution, thermal-wave imaging on many different materials. We have detected and Imaged subsurface mechanical defects such as microcracks and voids, grain boundaries, grains, and dislocations, and dopant regions and lattice variations in crystals. 

Figure 3 Examples of subsurface mechanical defects in a Si device. The electron micrograph (a) shows no defect features. The thermal-wave image (b) shows a subsurface network of microcracks in the lower half of the device and a subsurface delamination or chip-out in the top center.

There is evidence supporting both of these. However, in view of our earlier remarks we can postulate another more general mechanism, which should apply at temperatures where defect mobility in the surface and subsurface layers of a catalyst is significant (i.e., at temperatures >0 25 of the catalyst). Both reactants are assumed to be chemisorbed, but the reaction step is now initialed by the defect appearing at an appropriate point underneath the other reactant. One then obtains the rather naive picture of the process as one by which the active reactant pushes the other off the surface as a product molecule. This would certainly provide more favourable steric conditions for the reaction path. Let us exanaine one or two catalytic reactions on this basis. 

CF cracks are always initiated at the surface, unless there are near-surface defects that act as stress concentration sites and facilitate subsurface crack initiation. Crack initiation takes place independently of fatigue limit in air as it can be decreased or eliminated through the increase of dissolution rates at anodic sites. Localized corrosion such as pitting favors fatigue crack initiation through stress concentration and a local acidic environment. The two main mechanisms of CF are anodic slip dissolution and HE (80). 

Photothermal imaging has also found applications in the study of practical industrial materials, including fibers, ceramics, and composites, where the depth dependence of the recovered image may be useful in identifying surface features due to mechanical or oxidative processes, or subsurface defects and delamination. 

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