Compared to non-biodegradable synthetic fibres, natural fibres have been used as potential reinforcement in composite manufacturing industries to reduce the overall carbon emission in the environment and to produce lightweight automotive, marine and aircraft parts. In recent years, natural fibre composites such as napier, jute, hemp, and flax have garnered many researchers and manufacturing companies' attention as a potential substitute for synthetic fibres because of their cost-effectiveness, environmental sustainability, and biodegradation qualities. In this study, moisture absorption effects using distilled water and alkaline solution and thermal ageing were conducted on napier, kenaf, hemp, and jute reinforced epoxy composites. The particulate form of the natural fibres was mixed with epoxy resin to fabricate napier, kenaf, hemp and jute fibre-filled epoxy composites. Moisture exposure tests using distilled water and alkaline solutions were conducted over 720 h to determine the composites' critical time conditions. Moisture absorption by distilled water and alkaline solution both indicated initial linear absorption curve before saturated conditions. Scratch tests were then performed to investigate the scratch resistance properties of the natural fibre-filled epoxy composites. The scratch resistance properties investigated were critical normal load, coefficient of friction, penetration depth, fracture toughness and scratch hardness. The scratch resistance properties of the natural fibre reinforced epoxy composites decreased as the exposure time to environmental conditions increased. However, napier fibre-filled epoxy composite had the best scratch resistance properties compared to kenaf, hemp and jute fibre reinforced epoxy composites after exposure to distilled water, alkaline solution and thermal ageing. This study opined that exposure to environmental conditions such as moisture and heat affected the natural fibre reinforced epoxy composites' tribological properties over a long time.
