Linking microstructures and performances of functional coatings and engineering materials through multiscale and correlative microscopy methods

Correlative characterisation workflows linking optical, electron and X-ray microscopy, enable researchers to chain multiple techniques, overcoming the limitations of any one technique. Applying correlative workflows create rich multimodal datasets covering chemical, mechanical and microstructural information across 2D and 3D, traversing length scales from macro to nanoscale. The application of correlative microscopy is becoming more common due to increased availability and improvements to techniques. Careful thought is required to prepare correlative workflows ensuring decisions do not have negative impacts on downstream processes e.g. destructive techniques should not limit further characterisation, if possible. For example, once a sample undergoes milling to prepare a cross section it is not possible to analyse the removed material. This study focusses on two specific applications/materials with differing origins, structures, chemistry and scale to demonstrate potential differences and similarities for users in very different fields. The two applications/materials are carbon-based perovskite solar cells, and thornback ray (Raja clavata) dermal denticle. The reason for selecting these two different materials is because they both have a thin functional layer which presents challenges for characterisation. The conductive layer of the carbon-based perovskite solar cell is around 20 microns, while the outer layer of the dermal denticle is around 30-50 microns. Carbon-based perovskite solar cells offer a low cost, efficient and stable design for photovoltaics. Within this EngD, these multilayered engineered devices have been characterised in 3D for the first time revealing the carbon flake orientations and porosity within the top mesoporous carbon layer and infiltration defects within the active layer. Three workflows of sample preparation and characterisation were designed and employed for these devices, and an optimised workflow was identified. With the carbon-based perovskite solar cell, the correlative workflow consisted of focused ion billing to prepare a pilar for X-ray microtomography, femtosecond laser machining for nano X-ray microscopy and focused ion beam tomography with energy dispersive X-ray spectroscopy. This work has the potential to become part of a feedback loop for researchers in the photovoltaics community, and other applications such as semiconductor technology who are at the cutting edge of pilot-scale manufacture. Dermal denticles of thornback ray comprise of a dentine protrusion with an enamel cap. They were characterised with a correlative workflow for the first time, analysing the chemical, mechanical and microstructural properties to understand how the enamel layer can perform the required function. The correlative workflow for the dermal denticle consisted of optical microscopy, X-ray microtomography, scanning electron microscopy with energy dispersive X-ray spectroscopy, mechanical testing with nanoindentation, in-situ nanoindentation during X-ray microscopy along with computational modelling. The results show the potential function, of enamel as providing a sacrificial layer to protect the dentin body, it can do so with chemical enrichment, mechanically enhanced properties, and an optimised microstructure for wear resistance. This study demonstrates the application and development of correlative workflows for two different applications showcasing challenges and similarities.