Contrast-Enhanced Imaging of Carbon Fiber Composites using Hafnium Oxide Nanocrystals

Carbon fiber reinforced polymers (CFRPs) are of utmost importance in high-performance structural applications (e.g. aerospace, automotive, wind energy), but their behaviour is not fully understood. Damage propagation models need to take into account the type, geometry and orientation of a multitude of failure mechanisms and defects. Micro-Computed Tomography (micro-CT) data could provide the necessary input for these models, but the poor contrast between the carbon fibers and the polymer matrix does not allow automatic geometry extraction. To overcome this issue, hafnium oxide nanocrystals (HfO2 NCs) were introduced as CT contrast agents to the polymer matrix to provide the required contrast. To ensure a homogeneous and stable dispersion of the NCs in the epoxy resin, a bisphosphonic acid ligand consisting of ethylene glycol oligomer segments is used. The NCs do not significantly alter important parameters such as matrix stiffness, viscosity, glass transition temperature and curing time, enabling them to be implemented without having to alter current composite resin infusion methods. NC-doped CFRPs with 5 and 10 m% of HfO2 NCs added show a drastic improvement in CT contrast, allowing for segmentation of the carbon fiber tows and visualization of micrometer-scale cracks. The contrast-enhanced HfO2 NC-doped composites thereby enable the validation of damage models by accurate micro-CT data.