MEAM Ph.D. Thesis Defense: “Enabling Ultra-Low Viscosity Lubricants Through Fundamental Understanding of ZDDPs Anti-Wear Additives and their Tribofilm Growth Mechanisms: An In-Situ Study”

Lubricants with low viscosity have the potential to improve fuel efficiency in engines due to friction reduction. However, a reduction in viscosity increases the likelihood of wear. Zinc dialkyldithiophosphate (ZDDP), the most widely used antiwear additive in engine oils, has been extensively studied over the last few decades. ZDDP forms surface-bound tribofilms at sliding contacts that prevent surface wear. Recent studies reveal that mechanochemical reactions drive tribofilm growth via stress and temperature. However, the individual effects of shear stress, compressive stress, and temperature on tribofilm growth are not yet fully understood.

In this study, we investigate the kinetic of ZDDP tribofilm formation on both microscale and nanoscale. In the microscale, we studied the driving factors separately by using different compositions of a high-viscosity, high-traction fluid mixed with a polyalphaolefin (PAO) basestock with a ball-on-disc tribometer in the elastohydrodynamic lubrication regime. In addition, we presented a new approach, the strip analysis method, to deconvolute the mechanisms under one single contact more effectively and efficiently. In the nanoscale, we used an atomic force microscope (AFM) to simulate a single asperity contact sliding on an iron oxide surface with droplets containing alkylated naphthalene (AN), PAO, and ZDDP. This enables us to investigate the mechanochemistry of ZDDP tribofilm formation in the boundary lubrication regime with a well-controlled contact.