Tribological Performance of ZnO Green Particles as Lubricating Oil Additives

ZnO particles, synthesized using a green method, were used as additives to enhance the tribological properties of lubricants. Polyphenolic extracts obtained from by-products of the winemaking process from two grape varieties, Barbera (red berry) and Moscato (white berry), were utilized as reducing agents in the synthesis of ZnO starting from two Zn salts (nitrate and acetate). The grape extracts were analysed for their polyphenolic profile. The ZnO particles were characterized by X-ray diffraction, SEM, FESEM, and FTIR. A ball-on-disk tribometer was used to study the tribological behaviour of the ZnO particles as oil additives in comparison to a reference base oil. Electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to characterise wear scars. Polyphenolic compounds were more abundant in the Moscato extracts than in the Barbera extracts. Although FTIR analyses evidenced differences in the region related to the stretching of carbonyl bonds, all kinds of ZnO particles were crystallised in their pure phase, as shown by the XRD patterns. Morphological analysis revealed that precursors significantly influenced particle size and shape, with acetate producing regular-shaped nanoparticles (50–200 nm) while nitrate produced pyramid-like particles (10–20 μm). Addition of ZnO to oil resulted in a more stable friction coefficient (COF) than the reference oil, with lower values obtained using ZnO particles obtained from acetate compared to nitrate. The addition of the ZnO particles derived from Barbera by-products lowered on average wear values compared to the pure lubricant oil.