Abstract
<title>Abstract</title> <p> Zinc dialkyldithiophosphate (ZDDP) is known as the most successful anti-wear additive in lubricating oils, thanks to the formation of a protective layer activated by high temperature and shear stress (noted as tribofilm). Understanding the initial growth of ZDDP tribofilm remains challenging, as few experimental approaches offer an <italic>in situ</italic> monitoring to explore their concurrent morphological and structural evolution. Here, we employed <italic>in lubro</italic> atomic force microscopy (AFM) to mimic how surface bumps interact at the microscopic level, promoting tribofilm formation. Tailored sliding protocols (comprising orthogonal and pyramid patterns) reveal a dynamic interplay during morphological evolution: early-formed tribopads exhibit viscous flow and coalescence while simultaneously undergoing abrasion and rupture. Complementing these observations, structural transitions were elucidated by probing tribopads of varying heights using state-of-the-art transmission electron microscopy, cryo-electron microscopy and Auger electron spectroscopy. Furthermore, our work reveals the chemical origin of the remarkably strong adhesion of the ZDDP tribofilm to the steel substrate, which is well supported by DFT simulations. Altogether, these results lead to a novel thermodynamic approach based on mixing entropy. This work bridges the gap between the different scales involved in the ZDDP tribofilm formation, offering a deeper understanding of how the atomistic reactivity of this additive contributes to wear prevention at the macroscopic scale. </p>