Effects of thermoplastic coating on interfacial interactions in advanced engineering composites for aerospace applications

Delamination due to an inferior adhesion between reinforcement material and matrix in carbon fiber-reinforced thermoplastic (CFRTP) composites is a crucial problem to be solved. To this end, this study aims to overcome poor wettability between reinforcing phase, i.e., carbon fiber (CF), and thermoplastic matrix, i.e., polyetherether ketone (PEEK). Herein, CF’s surface was tailored by application of different polymeric sizing agents which have different chemical structures. Morphology and topology analyses were performed by Scanning Electron Microscope and 3D laser scanning, respectively. Later, a variety of wettability results were obtained by the sessile drop method used in Contact Angle (CA) measurements for CFs throughout application of each sizing agent applied by dip coating. Sizing materials were designed such that the chemical structure of CF’s surface could exhibit compatibility with the matrix itself. Consequently, complete wettability (CA: 0°) was achieved for CFs sized by HPEEK (CF/hydroxylated PEEK (HPEEK)) and the surface free energy (SFE) of CF was enhanced from 5.43 to 72.8 mJ/m2 while the SFE of the PEEK matrix is 40.1 mJ/m2. Moreover, sizing by HPEEK improved the average surface roughness of CF by 32% which enables optimized adhesion. Afterward, repetitive tensile tests were carried out to observe effects of improved interfacial interlocking on the mechanical properties of the final CFRTP composite. Stress–strain curves revealed that the tensile strength of CFRTP improved from 473 to 508 MPa through the sizing of CF by HPEEK whereas pristine PEEK has a much smaller tensile strength (98 MPa) than the aforementioned CF-reinforced composites.