Enhancement and evaluation of PAN-derived carbon fibres and resulting composites by active screen plasma surface modification

Liang, Yana (2022). Enhancement and evaluation of PAN-derived carbon fibres and resulting composites by active screen plasma surface modification. University of Birmingham. Ph.D.

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Abstract

Owing to their excellent properties, such as outstanding mechanical properties, high strength to weight ratio, high thermal stability and corrosion resistance, carbon fibres (CFs) have become materials of choice for the reinforcements of high-performance composites. However, their chemical inertness and low surface free energy, which is attributed to their high density of graphitic planes on the surface, limit the interfacial adhesion of CFs to the matrix. This poses a critical challenge in further improving the properties of CFs reinforced composites to meet the requirement arising from demanding applications.

To this end, numerous surface modification methods have been developed to improve interfacial adhesion of CFs to the matrix in composites, such as chemical treatment, electrochemical treatment, plasma treatment and polymer/nano particles coating. However, most of these treatments either lead to environmental concerns due to the use of toxic chemicals or reduce more or less the mechanical properties of CFs due to surface damage.

Plasma treatment is highlighted as an environmentally-friendly, economic and adaptable process for surface treatment of CFs with less impact on the fibre strength as compared with other methods. However, conventional direct plasma still damages the fibre surface due to ions bombardment. Therefore, a more advanced plasma technology, active screen plasma (ASP), has been developed to avoid the undesirable effects associated with conventional direct plasma treatment such as arcing, edge effect and hollow-cathode induced damage.

In this project, advanced active screen plasma technology has been developed to modify PAN-derived CF surfaces. The ASP modified CF surfaces were fully characterised by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and dynamic vapour sorption (DVS) to understand the response of CF surface to the active screen plasma. Single fibre tensile tests were carried out to directly study the effect of ASP treatment on single fibre tensile strength. Nano-indentation enabled single fibre push-out testing method, which was further improved by purposely designing the sample holder and using a conical indenter, was adopted to evaluate the interfacial adhesion between the ASP modified CF and the epoxy resin. Interlaminar shear strength, flexural strength and fracture toughness of ASP modified CF/epoxy composites were also evaluated.

The results have demonstrated that the advanced ASP technology is capable of modifying the CF surface without causing any surface damage or tensile strength degradation. Unlike conventional plasma treatments, the ASP treatments can reduce the structural disorder of the CF surfaces and increase the surface crystallite size. Moreover, the ASP treatments can lead to an increased single fibre tensile strength. This is mainly because the post-plasma nature of the ASP technology can effectively eliminate ion bombardment induced degradation while providing radicals necessary for surface modification. Changes have also been found in the chemical composition and wettability of the ASP modified fibre surfaces. Furthermore, a more than 30% improvement was found in the interfacial shear strength (IFSS) between the modified CFs and the epoxy substrate as disclosed by push-out tests. The mechanical properties (shear strength, flexural strength and fracture toughness) of the composites reinforced with ASP modified CFs have been improved as well.

Based on the experimental results, the potential mechanisms involved in the interaction between the CF surface and the active screen plasma, significantly increased single fibre tensile strength, effectively improved interfacial shear strength and enhanced mechanical properties of CF-reinforced composites have been discussed.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):