Synergistic Effects of Alumina and Graphite Reinforcement on Microstructural Evolution and Tribological Performance of Friction Stir Processed Copper based Composites

Hybrid metal matrix composites of copper reinforced with varying proportions of alumina and graphite particles were fabricated through two-pass friction stir processing. Five compositions spanning 100% Al₂O₃ to 100% Gr were investigated through microstructural, mechanical, and tribological characterization. Dynamic recrystallization generated refined equiaxed grain structures with progressive refinement as graphite content increased, attributed to enhanced Zener pinning. Vickers hardness measurements revealed compositional dependence, with pure graphite achieving maximum hardness of approximately 140 HV—surpassing pure alumina—demonstrating that grain refinement through self-lubricating particle pinning equals ceramic-based strengthening. Pin-on-disc testing documented 80% wear reduction in pure graphite relative to unprocessed copper. The 50Al₂O₃-50Gr balanced composition achieved optimal performance through synergistic effects: near-maximum hardness, 70% wear reduction, and significantly reduced friction coefficients. Wear mechanism analysis established progressive transitions from abrasive wear in alumina samples toward boundary lubrication in graphite-enriched materials, confirmed through wear surface morphology and debris characterization. The investigation demonstrates that carefully engineered hybrid reinforcement proportions deliver superior tribological performance through synergistic interactions between ceramic strengthening and graphite lubrication, providing guidance for designing advanced copper-based composites for bearing and wear-resistant applications.