Anomalous hydraulic fluid absorption by carbon fiber/PEKK composites: Physical and mechanical aspects

Abstract Carbon fiber (CF)/polyetherketoneketone (PEKK) composites are exposed to Skydrol, a hydraulic fluid made of phosphate esters widely used in the aviation field. The present study investigates Skydrol absorption of CF/PEKK composite layups and the associated PEKK matrix. A significant unexpected increase of Skydrol uptake is observed for cross‐ply composites (0.50%) compared to unidirectional ones (0.06%), independently of ply number. The use of ‘model’ fluids like water and ethanol allows to identify the origin of this anomalous Skydrol absorption in the case of cross‐ply layup. We propose that the latter involves an imbibition process governed by the fluid surface tension and the presence of submicronic cavities. SEM imaging of composite cross‐section after ion beam polishing confirms fiber‐matrix submicronic debonding in the interlaminar region in the cross‐ply composite. SEM–EDX confirms the presence of Skydrol into the submicronic cavities. Despite this anomalous absorption, off‐axis tensile testing as well as ILSS testing show no significant impact of Skydrol on CF/PEKK mechanical properties with less than 10% difference compared to the initial values. Highlights Absorption by diffusion of Skydrol in neat PEKK and CF/PEKK composites Link between fluid surface tension and anomalous Skydrol absorption Evidence of submicronic cavities in CF/PEKK composites by means of SEM–EDX Low impact of Skydrol on CF/PEKK mechanical properties