Effect of graphene nano-platelets coating on carbon fibers on the hygrothermal ageing driven degradation of carbon-fiber epoxy laminates

While spray coating of graphene nano-platelets (GNPs) on-to long carbon fibers (CF) is a viable strategy to enhance the strength of carbon fiber reinforced plastics (CFRP), the degradation of these laminates under hot and humid conditions remains unexamined. Here, Pristine and 0.4 % GNP coated CF (0.4GNP) laminates are manufactured using vacuum assisted resin transfer molding (VARTM) technique and subjected to hygrothermal ageing in de-ionized water at 75 °C for 30 days. Mechanical and structural characterization are performed after 15 and 30 days of hygrothermal ageing once the specimens were dried long enough to attain a saturation in mass. 0.4GNP laminate offered higher water resistance than Pristine laminate as 0.4 wt% non-functionalized GNP coated CFs were more hydrophobic than Pristine CF as quantified through contact angle-based measurements. Additionally, a thick carbonaceous hydrophobic interphase is formed between the CF and epoxy for 0.4GNP laminate which hinders water transport in composite. The thicker interphase also enhances the tensile strength and interlaminar shear-strength (ILSS) of 0.4GNP laminate in the unaged condition through enhanced CF—epoxy mechanical locking and epoxy strengthening in the vicinity of CF. However, both tensile strength and ILSS decreased with hygrothermal ageing time and the enhancement in strength achieved through GNP addition is completely lost after 15 days of hygrothermal ageing. Under tensile loading, the stress-strain plots showed a step-wise decrement in stress due to a ply-by-ply failure triggered by failure of the weakened inter-ply epoxy regions. In case of ILSS specimens the number of delaminations triggered increased considerably with hygrothermal ageing.