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 due to their unique combination of light-weight, yet high-strength behaviour. However, their behaviour and damage mechanisms are 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 needed as input. 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 was used. The NCs did 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 showed a drastic improvement in CT contrast, allowing for segmentation of the carbon fiber tows and visualization of micrometer-scale cracks. The contrast-enhanced NC-doped composites thereby enable the validation of damage models by accurate micro-CT data.