Repurposed carbon fibre as a polyamide 66 reinforcement in gear applications: Correlation of composition and properties

The study investigates the use of repurposed milled carbon fibre (mCF) as reinforcement for polyamide 66 (PA66) in gear applications, addressing environmental and cost concerns of virgin carbon fibres. Neat PA66 and PA66 composites reinforced with mCF, glass fibres (GF), and carbon fibres (CF), with and without polytetrafluoroethylene (PTFE), were injection moulded and evaluated for microstructure (fibre length), thermal, mechanical, surface, and tribological properties, as well as gear performance under VDI 2736 guidelines. CF reinforced composites showed the highest modulus and tensile strength, followed by mCF and GF. PTFE reduced modulus and strength in binary composites. All reinforced composites significantly lowered the coefficient of friction (COF) and wear rate compared to neat PA66, with mCF showing the most notable improvements. PTFE slightly improved tribological performance only for GF (wear) and CF (COF) composites. In gear testing, binary composites outperformed neat PA66, with CF performing best, followed by mCF and GF. Ternary composites had slightly lower performance than their binary equivalents. Correlation analysis showed that gear performance is closely linked to structural integrity. Failure analysis revealed higher crack susceptibility in mCF reinforced gears due to shorter fibre length. The findings highlight mCF reinforced PA66 as a sustainable, cost-effective material for durable polymer gears.