Experimental and numerical investigation on mechanical and tribological behaviours of epoxy composites incorporating flax fibre and graphene nanoplatelet

Experimental and numerical investigation on mechanical and tribological behaviours of epoxy composites incorporating flax fibre and graphene nanoplatelet

Polymer tribology gained great attention over recent decades, due to their advantages over other materials such as ceramics and metals, including but not limited to their lightness in weight, corrosion resistance, maintenance free status and inexpensiveness. In fact, the majority of polymer in their pristine form have a high coefficient of friction and low wear resistance, so they need to be modified to meet the requirements of such tribological applications like bushings and bearings. This study focuses on investigating the tribological and mechanical behaviours of eco-epoxy composites incorporating graphene nanoplatelets (GNPs) and flax fibre. The adhesive wear tests were conducted in a dry contact condition using block on ring (BOR) machine, covering the mild and severe wear regions. Scanning electron microscope (SEM) technique was used to study the fracture surfaces of tensile samples and worn surfaces of wear samples. First, an intensive characterisation of the effect of the GNPs filler to resin ratio on the mechanical and tribological properties of the epoxy matrix was evaluated. The additions of GNPs ranged from 0 wt. % to 4.5 wt. %. The results showed that GNPs addition has improved epoxy's tribological behaviours, indicated by a significant reduction in the coefficient of friction (COF), specific wear rate (Ws) and the interface temperature. SEM observations showed a stable transfer film in the case of GNPs additions. Concerning the mechanical properties, GNPs improved the stiffness and the hardness of the epoxy matrix; however, the mechanical strength and fracture strain deteriorated. A correlation study between the mechanical and tribological behaviours considering the effect of applied loads was established and the results revealed that the applied load plays an important role in this relation. The effect of the various volume fraction (Vf) of flax fibre on the adhesive wear performance of epoxy was investigated. The results indicated that the Vf increase has the greatest influence on epoxy's wear resistance. Flax/epoxy composite exhibited the best wear performance at the mild wear region, i.e., 15-30 N of the applied load. Whilst, at the severe wear region (45-60 N), it exhibited low wear resistance, mainly because of the epoxy's low capacity to dissipate the heat at the rubbing zone. On the other hand, inclusion of GNPs resulted in higher wear resistance at both the mild and severe wear regions. This improvement highlighted the benefits of using GNPs as this has a high capability to improve polymers' thermal conductivity and GNPs act as solid lubricant fillers at the same time. SEM investigations of the worn surfaces observed a transfer film in presence of GNPs. Ultimately, incorporating GNPs and flax fibres together into the epoxy matrix showed reasonable tribological behaviours with high mechanical performance. One of the main objectives of this study was assessing the impact of the geometrical change of the block on ring (BOR) configuration on the wear process, using a FE approach. A two dimensional finite element (FE) model of BOR configuration was developed and validated with the experimental investigations, with less than ±15% error. Results revealed the geometrical change plays a crucial role in the wear process, leading to highly concentrated stress at a specific region on the wear profile. Accordingly, the wear profile was examined in terms of two main zones: the normal zone, subjected to the contact pressure; and the critical zone, where the mechanical stress was highly concentrated. Interestingly, SEM observations were consistent with this classification. Effects of GNPs and flax fibre were studied at both normal and critical zones.