Influence of surface modifications on interfacial and mechanical properties in fibre-reinforced composites

Carbon fibres play a crucial role in fibre-reinforced composite materials. Properties of these composites are not solely determined by the fibre and matrix. In contrast, the interface between fibre and matrix also plays a pivotal role. The presence of sufficient van der Waals and hydrogen bonds between carbon fibre and matrix is vital for achieving strong interfacial adhesion during the composite fabrication. However, due to the chemical inertness and hydrophobic nature of carbon fibre surfaces, establishing effective interfacial bonding with the matrix poses a challenge. To address this, a range of methods has been developed to modify the carbon fibre surface and facilitate the formation of functional groups. Combined methods, such as sized carbon fibre and ozone treatment, have gained widespread usage in overcoming these challenges. In this study, a detailed examination was conducted to compare differences between ozonation of desized and sized carbon fibres by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), Raman spectroscopy and scanning electron microscopy (SEM). XPS results distinctly demonstrated the transformation of epoxy groups into hydroxyl and carboxyl functional groups following the ozonation process. Notably, in the 60 min ozone treatment with 5 L/min (ozone-5), carboxyl groups increased from 1.27 to 8.12 %. In addition, in ozone-5 conditions, hydroxyl groups increased by 28.37 %. An escalation in graphite regions was also identified during the ozonation of the desized carbon fibre sample. Conversely, there was no significant alteration observed in the Raman value of sized carbon fibre samples. Furthermore, mechanical test results for both desized and sized carbon fibres after ozonation were compared. Ozone treatment of carbon fibres, regardless of whether sizing is present or not, leads to an increase in the interlaminar shear strength (ILSS). ILSS values for sized carbon fibres were initially 41 MPa. However, following ozone treatment, ILSS values of sized carbon fibres increased to 64 MPa. The tensile strength of sized carbon fibres increased from 772 to 831 MPa. This behaviour can be attributed to the increased amount of functional groups detected by XPS, Raman spectroscopy and the increase in roughness. Graphical Abstract