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By combining weak-beam dark-field imaging with tomography, we have been able to reconstruct the three-dimensional structure of dislocation arrays in GaN. With a mixture of both threading and in-plane dislocations, owing to plastic relaxation of the film, we look at how well each dislocation is reconstructed and what limits are imposed by way of dislocation density and material anisotropy.

Many metal on metal joint failures have been associated with adverse local tissue reactions due to the response of the body to wear debris and corrosion products, released from the bearing surface or the taper interface. Both a carbonaceous layer and an oxide film are reported on the CoCrMo metal joints, which play important roles in the wear resistance of the material. However, there is a lack of quantitative data on the structure and distribution of the carbonaceous layer and the oxide film. The current work provides a detailed investigation of the surface damage on retrieved CoCrMo taper interfaces. In addition, systematic differences in the starting surface structure of biomedical CoCrMo were studied through a comparison of a standard mechanical polished (MP) and an electropolished (EP) surface after reciprocating test. Site-specific FIB/TEM cross-sections show the evidence of the carbonaceous layer and the oxide film on both CoCrMo taper interfaces and MP, EP CoCrMo, ranging from 5 -500 nm depending on the location. The amorphous carbonaceous layer exhibits a π * peak in the EEL spectra, with trace of Ca and N. The oxide film on the taper interfaces has a porous texture and HRTEM indicates chromium oxide nano-crystals in an amorphous background, however, only a very thin oxide film (~ 2 nm) exists on MP and EP CoCrMo after reciprocating test.

A FIB lift-out sample was made from a wear-resistant carbon coating deposited by high power impulse magnetron sputtering (HIPIMS) with Mo and W. TEM analysis found columnar grains extending the whole ∼1800 nm thick film. Within the grains, the carbon was found to be organised into clusters showing some onion-like structure, with amorphous material between them; energy dispersive X-ray spectroscopy (EDS) found these clusters to be Mo- and W-rich in a later, thinner sample of the same material. Electron energy-loss spectroscopy (EELS) showed no difference in C-K edge, implying the bonding type to be the same in cluster and matrix. These clusters were arranged into stripes parallel to the film plane, of spacing 7-8 nm; there was a modulation in spacing between clusters within these stripes that produced a second, coarser set of striations of spacing ∼37 nm.

Weak-beam dark field tomography of dislocations has so far been done in a heuristic way with little theoretical analysis of how to optimise the technique. In this paper a more quantitative analysis of the limits of WBDF tomography is summarised. It can be shown that the object reconstructed from WBDF images by a conventional tomography algorithm is the locus of dislocation images laid down as central slices at the appropriate tilt angles, as would be intuitively expected. By comparison with projections of the displacement field, a reconstruction from dark-field images gives a better indication of the dislocation's path than a projection of the displacement field itself. Reconstruction is performed on a simulated tilt series of stacking fault images; the resulting structure follows the correct plane but is a 3D array of rods, not a flat plane. This is produced by the 'movement' of dynamical fringes in the stacking fault images as the sample is tilted and is an example of WBDF images being suboptimal for tomography. Finally, the influence of misalignment between the tilt axis and systematic row is modelled, and is shown to degrade the reconstruction, especially in the out-of-plane direction. This misalignment changes the excitation error with tilt, which introduces another variable to the reconstruction process and breaks the requirement that the images be a straight-ray projection of a monotonic function of the required parameter.

After previous work producing a successful 3D tomographic reconstruction of dislocations in GaN from conventional weak-beam dark-field (WBDF) images, we have reconstructed a cascade of dislocations in deformed and annealed silicon to a comparable standard using the more experimentally straightforward technique of STEM annular dark-field imaging (STEM ADF). In this mode, image contrast was much more consistent over the specimen tilt range than in conventional weak-beam dark-field imaging. Automatic acquisition software could thus restore the correct dislocation array to the field of view at each tilt angle, though manual focusing was still required. Reconstruction was carried out by sequential iterative reconstruction technique using FEI's Inspect3D software. Dislocations were distributed non-uniformly along cascades, with sparse areas between denser clumps in which individual dislocations of in-plane image width 24 nm could be distinguished in images and reconstruction. Denser areas showed more complicated stacking-fault contrast, hampering tomographic reconstruction. The general three-dimensional form of the denser areas was reproduced well, showing the dislocation array to be planar and not parallel to the foil surfaces.