Tribological analysis of ZDDP enhanced synthetic-based neem seed oil blends and its performance evaluation in a real-life load bearing application

The study examines the tribological characteristics of various oil blends developed from the incorporation of ZDDP additive at various proportions (0.10–3 wt.%) in a 75% Neem seed oil (NSO) − 25% polyalphaolefin (PAO6) base oil blend. The tribological characteristics of these formulations were evaluated using a four-ball tester and its effectiveness are compared with SAE20W40 to determine the ideal ZDDP concentration. The blend with 1 wt.% ZDDP (NPZ4) exhibited superior performance, resulting in a 21.5% reduction in coefficient of friction and an 11% decrease in wear scar diameter when compared to the base formulation. The wear scar morphology and elemental analysis validated the formation of a stable Zn–P–S-rich tribofilm, which significantly improved wear protection. The further analysis of the lubrication regime using minimum film thickness and lambda (λ) ratio indicated that the blend NPZ4 works within the boundary-to-mixed lubrication regime (hmin = 155.42 nm, λ = 0.51), nearing the performance characteristics of SAE20W40. The selected ideal blend was then subjected to further experimental analysis in hydrodynamic plain journal bearing at different speeds while maintaining a constant load. The pressure profile trend for all oil samples was consistent across all situations, with the mineral oil displaying a predominant profile, with the ideal blend demonstrated superior characteristics compared to the base oil sample. The application of two-way ANOVA for statistical validation showed a significant impact of lubricant formulation and operating speed on bearing pressure performance, with a R² value of 95.1%.