The development of more sustainable lubricants with tailored tribological performance is a crucial activity in the pursuit of net zero greenhouse gas emissions. Modern lubricants consist of a mixture of base oils, which are most often nonpolar hydrocarbons, blended with several different types of additives, which can be polar organic molecules, polymers, or organometallic compounds. Probably the most important of these additives is zinc dialkyldithiophosphate (ZDDP), which was originally developed as an antioxidant, but is now used primarily for its remarkable antiwear performance. ZDDP reduces wear by forming protective 'tribofilms' on rubbing steel surfaces. Despite decades of research and development to find a more environmentally-friendly alternative, ZDDP remains ubiquitous in engine lubricants. To rationally design antiwear additives with improved performance, a molecular-level understanding of the tribofilm formation and wear-reduction mechanisms of ZDDP are first required. The tribofilms formed by ZDDP on steel substrates are mainly composed of zinc and iron polyphosphates. These are formed by nucleophilic substitution reactions at the P atoms, which result in the formation of the P-O-P chains. This reaction is dramatically accelerated once some of the alkyl or aryl groups (that are added to ensure solubility in the base oil) are removed through C-O cleavage. Therefore, ZDDPs containing different alkyl and aryl groups show greatly varying tribofilm growth and antiwear performance; however, the underlying molecular mechanisms for these differences in macroscale behavior remain unclear.