X-ray photo-electron spectroscopic studies of cryogenic and plasma surface-treated z-pins

The mechanical properties of composite materials are primarily dependent on the load transmission between their components. Especially, the fracture performance of a fibre-reinforced polymer made of a preimpregnated fibres (prepreg) with a three-dimensional z-pin reinforcement can be influenced by the interface characteristics between pins and laminate. X-ray photo-electron spectroscopic (XPS) analysis and scanning electron microscopic (SEM) studies of untreated, cryogenic and plasma surface-treated z-pins were carried out to determine the effect of surface treatments on chemical composition, formation of functional groups and topography of pin surface. Pullout tests were conducted to investigate the impact of a surface treatment on the bridging forces, which have significant influence on delamination toughness of a z-pinned laminate. It was pointed out that a cryogenic treatment cannot lead to an increase of oxygen or nitrogen functional groups at the z-pin surface. Nevertheless, the pullout forces can be increased which are caused by an increased surface roughness. However, with a plasma treatment, an oxygen and nitrogen functionalization can be reached. An increase of the O/C ratio with all plasma treatments can be determined. Summarizing the results, it can be shown that oxygen functional groups can be generated by a plasma treatment. These groups are able to establish covalent bonds between z-pin surface and bulk material, which can lead to a better load transmission between the pins and laminate and thus to higher fracture properties. The increased pullout forces result from a combination of improved adhesion between pins and laminate and increased interlocking effects, due to the higher surface roughness after plasma treatment.