In high-pressure, high-temperature sliding contacts, WS2 nanoadditives react with the metal substrate to generate 100+ nm chemical tribofilms with a layered structure and excellent tribological properties. The friction, wear and micromechanical properties of WS2 tribofilms are compared with those of tribofilms formed by the zinc dialkyldithiophosphate (ZDDP) antiwear additive and ZDDP-organic friction modifier (OFM) mixture. Nanoindentation measurements showed that WS2 generates tribofilms with higher values of hardness and Young’s modulus than ZDDP and ZDDP+OFM, which explains its excellent antiwear properties. The friction performance of WS2 surpassed that of ZDDP+OFM. The striking reduction of boundary friction is credited to the layered structure of the WS2 tribofilm, with exfoliated /squashed WS2 nanoparticles that fill the gaps and cover the reacted tribofilm. In view of these results, WS2 proves to be a suitable candidate for the replacement of problematic lubricant additives currently in use
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