Enhanced Tribological Properties of Castor and Sesame Oil Mixture with Ascorbyl Palmitate for Boundary Lubrication

Vegetable oil blends significantly improve friction and wear performance compared to individual oils. However, the oxidizing nature of these blends remains a consistent challenge. Currently, both natural and synthetic antioxidants are recommended to address this issue. This study investigated the use of ascorbyl palmitate as a natural additive in a previously evaluated mixture of castor and sesame oils, focusing on its tribological performance in an AISI 4140/AISI 52100 tribopair. The chemical composition of the biolubricants was analyzed using Fourier-transform infrared spectroscopy (FTIR), while the physical properties such as density and kinematic viscosity were measured at various temperatures. To evaluate their suitability for tribological applications, their friction and wear performance were assessed using a ball-on-disk tribometer. The friction coefficient, coefficient of lubrication efficiency (CLE), and wear behavior (volume loss, wear rate, and wear mechanism) were analyzed. This study demonstrates a notable enhancement in the tribological properties of the binary mixture with varying ascorbyl palmitate concentrations (0.25, 0.5, and 0.75 wt.%). The addition of ascorbyl palmitate reduces wear by forming protective interfacial layers, resulting in a low friction coefficient of 0.07 and a 46% reduction in volume loss. Some concentrations of ascorbyl palmitate also mitigated the severity of the wear mechanism.