Influence of tribo-carbofilm formation on the performance of CNTs-reinforced Al6061 composites processed via multi-pass FSP

Al6061 alloy is an environmentally friendly engineering material extensively used in the aviation, construction, and shipbuilding industries; however, its tribo-mechanical performance is relatively limited and can be enhanced by grain refinement induced by the friction stir process (FSP). The current study investigates the influence of carbon-rich tribofilm formation on the tribo-mechanical performance of carbon-nanotube (CNT)-reinforced Al6061 composites processed via multi-pass FSP. Raman spectroscopy confirmed the retention of structurally modified carbon in the nugget zone due to FSP-induced plastic strain, evidenced by a progressive increase in I_D/I_G ratio (raw CNT ∼ 0.74; T/CNT-2 ∼ 0.87), which was further supported by scanning electron microscopy. Crucially, no Al₄C₃ interfacial phase was detected by XRD, confirming that CNTs retained structural integrity as lubricating reinforcements. The CNT-reinforced composite (T/CNT-2 ∼ 123.57 HV) exhibited a substantial enhancement in hardness as compared to the unprocessed sample (O ∼ 62.87 HV), primarily due to crystallite size refinement (from O ∼ 47 nm to T/CNT-2 ∼ 24 nm). Tribological investigations revealed a significant reduction in the coefficient of friction (COF) from 0.80 for the untreated sample to 0.51 for the CNT-reinforced metal matrix composite (MMC), accompanied by a substantial reduction in wear volume from 0.2427 × 10⁻³ mm³ to 0.0531 × 10⁻³ mm³. The improved tribological behaviour is associated with the in situ formation of a carbon-rich lubricating tribofilm developed from CNT reinforcement. Scanning electron microscopy confirmed adhesive wear as the dominant wear mechanism, with a cracked tribolayer developed due to cyclic stress during fretting wear. The CNTs-reinforced composite demonstrated the lowest frictional force with minimum energy dissipation of 5.1 × 10⁻⁴ J/cycle compared to 8 × 10⁻⁴ J/cycle for the untreated sample (∼ 36% reduction). The improved performance demonstratre the development of self-lubricating, carbon-rich tribofilm, which enhances the tribo-mechanical performance of Al6061 composites for wear-resistant applications.