Micrometeorite impact characteristics
Four impact sites are of particular interest from the cratering phenomena observed from optical surfaces of tray B08. The impact on the uncoated calcium fluoride (B8-2) occurred near to the edge of the sample holder. The impact crater was ~1mm diameter with a spallation zone diameter of ~5.5mm. The substrate cleaved in two directions from the impact site at an angle of ~75° to the opposite sides of the sample, breaking the sample into 3 pieces, as shown in Figure 24.
Although other samples had impact craters of this size with large spallation diameters and small fractures, this was the only sample which showed evidence of complete substrate fracture, showing the fragile and brittle nature of calcium fluoride as a substrate material, whilst remaining functional. The crater itself consists of finely shattered material, as shown in Figure 25, and contains no visible remaining impact debris.
|Figure 24, CaF2 Impact crater||Figure 25, Localized CaF2 impact site|
A large impact feature on an exposed PbTe/ZnS coated Ge substrate (B8-37) occurred close to the edge of the substrate and holder interface with a spallation diameter of ~4.5 mm (Figure 26). The sample consisted of a 1mm thick Germanium substrate coated with a lead telluride/zinc sulphide based multilayer. Delamination as a result of the impact has occurred around the localized periphery of the spallation zone (Figure 27), extending the impact area by approximately 1mm. The impact has not induced any further coating damage beyond the localized delamination area, or added stress to the surrounding coating material so validating the adherence integrity of the coating.
|Figure 26, B8/37 Impact feature||Figure 27, B8/37 Localized impact site|
An antireflection coated silicon sample (B8-47) received a small impact directly at the edge position between the specimen and its holder (Figure 28), producing an ejecta spray pattern of molten aluminium which back-reflected across the sample surface. The component comprised a 1mm thick silicon substrate coated with a single antireflection layer of silicon monoxide. Photographs obtained by scanning electron microscopy in Figure 29 show the detailed impact damage sustained in the collision. This SEM was taken at 175x magnification at an angle of 30° and shows the nature of the aluminium surface-deposit ejected out of the impact area. The figure also shows the high granularity of the deposited material, which gives a visual indication of the impact force distribution of material ejected.
|Figure 28, B8/47 Edge position impact||Figure 29, B8/47 impact site 175x|
The impact at this site exposed the edges of crystal cleavage planes running parallel to the surface of the substrate. The surface fragmented at the crater edge without damaging the coating around the periphery of the impact site.
Sample B8-25 received three impact craters over a small localized area of the exposed aperture. The sample comprised a germanium/silicon monoxide multilayer coating deposited on a 1 mm thick sapphire substrate. Observing two of the sites by optical microscopy (Figure 30 & 31) showed that two different types of impact damage had been sustained. Two of the sites were similar in form with uniform concentric rings extending out at various radii from the centre. The other site was larger in size and not so concentrically uniform. The various colours observed are reflections from different thicknesses of layer materials, broken by radial fractures in the concentric rings.
|Figure 30, B8/25 Ge/SiO on Al2O3||Figure 31, B8/25 Ge/SiO on Al2O3|
SEM micrographs obtained from these sites (Figure 32 & 33) show the nature and form of two different types of impact cratering.
|Figure 32, B8/25 Ge/SiO on Al2O3||Figure 33, B8/25 Ge/SiO on Al2O3|
SEM Figure 32 shows a region of the original uncoated substrate material around the centre of the site. A small ring of very fine debris, and the uniform circular nature of the first concentric ring, indicate a possible shock-wave extending from the centre radially upon impact. The coating material, fragmenting radially on the first elevated concentric ring, produced the different colours observed optically. Material further away from the centre appears less fragmented and more molten, possibly due to the radial propagation of heat from the centre. Further magnification from the centre of the impact sites show possible residual micrometeorite debris.
Figure 33 is a larger impact site on that same sample magnified by 190x. Here the impact has produced a central circular area of the original uncoated substrate material, surrounded by which is a uniform inner concentric ring of coating material shaped from a possible shock-wave extending radially on impact from the centre of the site. Material beyond the interface between the uncoated substrate and coating has fragmented in a non-uniform single concentric area. Possible impact debris is visible at the centre of the impact crater.
As a result of these impacts, no further delamination of the coating material has occurred beyond the immediate periphery of the impacted area. This could have been produced by either induced stress in the coating, or by general mechanical fatigue. Subsequently the effects of these impacts have produced no significant degradation on either the performance, usage or environmental adhesion integrity of the coatings or substrate materials.
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