Comparative study of tribomechanical properties of B ₄ C/Gr and SiC/Gr reinforced AA6061 hybrid nanocomposites via stir-ultrasonic-squeeze casting

This study presents a comparative evaluation of AA6061-based aluminium metal matrix hybrid nanocomposites reinforced with B4C/Gr and SiC/Gr nanoparticles, fabricated using a novel stir – ultrasonic – squeeze casting route. Five compositions – as cast AA6061 alloy (AA6061), AA6061 + 2 wt.% B4C + 2 wt.% Gr (AA2B2G), AA6061 + 3 wt.% B4C + 2 wt.% Gr (AA3B2G), AA6061 + 2 wt.% SiC +2 wt.% Gr, (AA2S2G), and AA6061 + 3 wt.% SiC +2 wt.% Gr (AA3S2G) – were synthesised. SiC – Gr composites exhibited superior grain refinement (minimum grain size of 35.49 µm), attributed to enhanced heterogeneous nucleation and favourable interfacial thermal characteristics. Hardness increased from 52.67 BHN (as-cast) to a maximum of 69.55 BHN for AA3S2G. Tensile properties showed similar improvement. Hall – Petch analysis revealed that variations in the slope coefficient (k) reflect interface-controlled dislocation blocking rather than grain refinement alone. Although B4C–Gr composites exhibited a higher Hall – Petch slope, SiC–Gr systems achieved higher absolute hardness and tensile strength due to improved dispersion and more effective interfacial load transfer. Tribological studies showed that SiC–Gr composites promote a stable, adherent graphite-rich tribofilm, resulting in lower friction and wear, whereas B4C–Gr systems exhibited tribofilm disruption due to interfacial debonding and third-body abrasion.