Enabling Ultra-Low Viscosity Lubricants Through Fundamental Understanding of ZDDPs Anti-Wear Additives and their Tribofilm Growth Mechanisms: An In-Situ StudyLu Fang Robert W. Carpick Low-viscosity lubricants can improve the efficiency of vehicles and machinery by reducing viscous losses. However, this increases the likelihood of wear. Zinc dialkyldithiophosphate (ZDDP) is an antiwear (AW) additive universally used in vehicle lubricants to generate tribofilms that prevent wear. While highly effective, new AW additives are needed for future-generation vehicle technologies. Recent studies revealed that ZDDP tribofilms growth was driven mechanochemically by stress and temperature. However, the independent effects of different stress components were not determined. Furthermore, gaps in understanding the tribofilm growth mechanisms at the macroscale and nanoscale exist. In this dissertation, we study the fundamental mechanisms of ZDDP tribofilm growth at both scales, providing understanding to guide the rational design of next-generation AW additives. At the macroscale, we developed a stress-controlled mechanochemical reactor to independently control shear and compressive stresses. We then devised a new approach, the contact strip analysis method, that takes advantage of the inhomogeneous stress distribution of the contact to spatially resolve the stress-dependence of tribofilm growth shear stress strongly promotes tribofilm growth, while compressive modestly stress inhibits it. We conclude that a bond-breaking reaction is the rate-limiting step in tribofilm growth. An extended Eyring model captures the growth behavior, permitting the activation energy and activation volumes to be determined. At the nanoscale, we used atomic force microscopy (AFM) single-asperity contacts to observe growth in the boundary lubrication regime. Since this requires extreme conditions that can cause tip wear, we tested different tip materials, finding that TiN-coated probes off the most reliable performance. Using these probes for long-duration tests, three growth regimes were discovered: slow linear growth with low friction, fast linear growth with high friction, and fast removal with low friction. These results indicate a sulfide layer forms first, followed by the polyphosphate film. Both regimes follow the Eyring model, again permitting the extraction of kinetic parameters. We attribute the fast removal regime to ZDDP depletion in the oil via oxidation.