Additive double-stir friction processing of Al₂O₃/AA6063 hybrid composites reinforced with coconut husks: Microstructure, mechanical and tribological performance

Coconut husks are an abundant agro-waste material often discarded in landfills and waterways, posing environmental challenges. While various valorization strategies have been explored, studies investigating the tribological, mechanical, and microstructural performance of coconut husk ash (CHA) as a hybrid reinforcement in alumina-aluminum matrix composites remain limited. This study addresses this gap by fabricating CHA/Al₂O₃/AA6063 hybrid composites with varying reinforcement ratios (2, 4, 5, 6, and 8 wt% CHA) using the additive double-stir friction processing technique. Elemental analysis of the CHA particulate revealed high concentrations of K₂O, CaO, and SiO₂, which contributed significantly to the formation of potassium-rich tribo-ceramic phases and surface glaze layers. The composites demonstrated progressive enhancements in hardness (from 72.43 to 91.46 BHN) and reductions in wear rate (from 0.00240 to 0.00136 mm³/Nm) with an increase in CHA weight percent. Conversely, the coefficient of friction rose from 0.31 to 0.49 across the same composition range. Microstructural analysis accounts for these findings, revealing increasing surface roughness, particle agglomeration, and tribo-glaze formation at higher CHA levels, consistent with the presence of KSiAlO₄ phases detected by XRD. Additionally, worn surface morphology indicated a wear mechanism transition from mild abrasive to mixed (abrasive, adhesive, and oxidative) with increasing CHA content. The findings highlight the dual functionality of coconut husk ash as both a mechanical and chemical modifier, suggesting its potential not only in automotive tribo-components but also in ceramic glaze formulations, thereby supporting sustainable material innovation in alignment with the United Nations SDG 9.