The tribological effect of treatment oil in for restoring degraded Automotive Transmission Fluid (ATF)

Automatic Transmission Fluid (ATF) plays a vital role in lubrication, cooling, and ensuring smooth gear shifting in vehicle transmission systems. Typically, ATF is recommended for replacement every 25,000 to 40,000 kilometres, depending on manufacturer specifications and driving conditions. However, ATF can deteriorate much earlier, sometimes before 20,000 kilometres, under extreme operating conditions such as high temperatures, heavy loads, or frequent stop-and-go driving. This early degradation occurs due to base oil oxidation, additive depletion, and accumulation of wear metals, which disrupt boundary film formation and increase friction and wear. This early degradation leads to increased friction, wear, and reduced performance, often resulting in poor gear shifting, abnormal noise, and seal deterioration. To address these issues, treatment oils are introduced as a supplementary additive package. The additive system in ATF, consisting of anti-wear, antioxidant, detergent, dispersant, and extreme-pressure components, plays a critical role in maintaining tribo-film stability and protecting metal surfaces under sliding contact. The treatment is formulated to restore the degraded properties of used ATF by replenishing depleted additives, enhancing lubrication performance, reducing the coefficient of friction, minimizing wear, and improving overall tribological behaviour compared to used ATF. The effectiveness and compatibility of treatment oil in improving tribological performance for used ATFs remain uncertain. This study aims to characterise the physicochemical properties of commercial ATFs and treatment oil, assess the effectiveness of treatment oil in improving the quality of used ATFs, and evaluate the tribological performance of the treated fluids. The treatment involved mixing 4 vol.% of treatment oil into used ATFs, followed by ultrasonication. The samples were analysed according to ASTM standards, which are elemental analysis (ASTM D6595), viscosity and Viscosity Index (VI) measurements (ASTM D445 and D2270), Fourier Transform Infrared Spectroscopy (FTIR) (ASTM E1252), and four-ball tribological tests (ASTM D4172 and D2783). The results shows significant degradation of used ATF in terms of viscosity reduction, additive depletion, and weakened tribological performance due to oxidation and service exposure. Viscosity decreased from 45.6 cSt (fresh ATF) to 36.8 cSt at 40°C and partially recovered to 34.7 cSt after treatment. Elemental analysis revealed depletion of Zn, P, and Ca in the used ATF, while these elements reappeared after treatment. FTIR supported the restoration of functional groups associated with anti-wear and antioxidant additives. Tribological performance improved with up to 18% reduction in friction coefficient and 77% decrease in wear scar diameter compared to untreated used ATF, demonstrating enhanced lubrication and film strength. These findings demonstrate that treatment oil can partially recover degraded ATF by improving boundary film formation, load-carrying capacity, and overall tribological performance, providing new insight into additive-based rejuvenation strategies for used transmission fluids. In general, the treatment oil improved the performance of degraded ATF and could extend fluids last longer.