Tribo-carbofilm evolution under suppressed oxidation in graphite-reinforced Al6061 composites processed by multi-pass friction stir processing

The development of a stable carbonaceous tribofilm at the sliding interface and its evolution under suppressed oxidation conditions represent a critical yet insufficiently explored dimension of tribological behavior in friction stir processed (FSP) aluminum matrix composites. This study investigates the tribo-carbofilm formation mechanism in graphite-reinforced Al6061 composites processed through multi-pass FSP (single and two passes), with systematic comparison under ambient and inert atmospheric (Ar) conditions. Al6061 alloy specimens reinforced with 0.67 wt.% graphite were processed using optimized FSP parameters (rotational speed: 1300 rpm; traverse speed: 0.3 mm/min; plunge depth: 1.3 mm). Five sample conditions were characterized: untreated alloy (O), single- and double-pass unreinforced FSP (T-1, T-2), and single- and double-pass graphite-reinforced FSP composites (T/Gr-1, T/Gr-2). The two-pass graphite-reinforced composite (T/Gr-2) demonstrated a significant improvement in microhardness (116.47 HV), nanohardness (3.97 GPa), and elastic modulus (108.83 GPa) relative to the untreated alloy. A 73.5% reduction in wear volume and a 28.8% reduction in the coefficient of friction were recorded for T/Gr-2 against the baseline. SEM-EDS analysis confirmed a marked suppression of tribo-oxidation (oxygen content reduced to approximately 5.9 at.% from approximately 30.6 at.% in the untreated condition). Raman spectroscopy and XPS data collectively established the formation of a well-ordered, graphitic tribo-carbofilm at the contact interface, mechanistically linked to shear-induced graphite exfoliation.