Synergistic Tribofilm Evolution and Multi-Scale Surface Transformation in Hybrid CuO–Al₂O₃ Nano-Lubricated Diesel Cylinder liner Interfaces

Tribological losses at the cylinder liner–piston ring interface significantly affect the efficiency and durability of diesel engines, necessitating advanced lubrication strategies. This study presents a mechanism-driven investigation of hybrid CuO–Al₂O₃ nano-lubricants to elucidate their synergistic role in wear reduction and surface stabilization. A comprehensive multi-scale characterization framework integrating wear analysis, Keyence microscopy, FESEM, AFM, and X-ray diffraction was employed to establish correlations between macroscopic tribological behavior and micro/nano-scale surface transformations. The results reveal a distinct transition from material removal-dominated wear under base oil lubrication to tribofilm-mediated surface adaptation in nano-lubricated systems. The hybrid nano-lubricant exhibited enhanced surface protection characterized by tribofilm formation and interfacial stabilization. A quantitative synergy assessment was introduced to evaluate nanoparticle interactions, demonstrating that the hybrid system outperforms the individual CuO and Al₂O₃ nano-lubricants. Multi-scale analysis revealed reduced surface roughness, improved morphology, decreased Fe exposure, and enhanced crystallographic stability, collectively supporting the formation of a stable and continuous tribofilm. The enhanced performance is attributed to the coupled action of Al₂O₃ (mechanical load support) and CuO (tribochemical activity), resulting in a self-adaptive, multi-layer tribofilm that stabilizes the sliding interface. This study provides a unified mechanistic framework for hybrid nano-lubrication and highlights its potential for improving fuel efficiency and extending component life in engine applications.