Development of In Situ Scanning Electron Microscope Nanomechanical Testing

In situ scanning electron microscope (SEM) mechanical testing offers a new level of insight to any number of micro and nano wear problems. Via both nanoindentation and nano scratch testing not only can mechanical data of materials be collected, but high-resolution SEM imaging and videos allow for analysis of wear and degradation mechanisms as they happen. In this project in situ SEM mechanical testing will be deployed across two key materials that are pivotal to both the semiconductor industry and the electric vehicle market.
Diamond like carbon (DLC) covers a group of amorphous carbon materials that are highly regarded for their excellent tribological properties. DLCs see high usage in areas where traditional lubrication may not be viable. DLCs are highly utilised in the process of producing semiconductors as a protective coating on equipment. For this reason, the nano level wear of DLCs is of critical interest. In this thesis Investigations into the interactions of small diamond conical asperities with DLC coatings has been conducted with both in situ SEM and ex situ nano scratch testing. Failure points of each coating were determined with 3d optical profiling offering topographical information of wear scars and failure events. Live testing observations combined with post-mortem analysis showed that all DLCs deposited upon silicon displayed significant tensile cracking regimes. Focused ion beam (FIB) cross sectioning then allowed for an observation of lateral and radial cracking in the substrate to be related back to deformation and cracking seen on the surface of the coating. The combination of these techniques allowed for a greater picture to be painted of the wear of a number of different DLCs across various substrates to be mapped and their failure mechanism observed.
In addition to this the fracture toughness of lithium-ion battery NMC 811 cathode active material secondary particles were investigated. Mechanical degradation of NMC across varying charge states has already been observed via a degree of intergranular cracking caused by primary particle shrinkage during de-lithiation. In situ SEM nano indentation was used to accurately target axial compression of NMC 811 secondary particles. Here it was found that with increasing charge state came a reduction in overall particle lateral diameter and a reduction in particle indentation strength. Increases in pre exiting intergranular cracking were also observed in much higher frequency in samples charged to 3.9V and 4.3V Li+/Li vs their pristine or lower charged counterparts. This effect was observable via both SEM imaging and the mechanical data provided by the indenter.